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The microfossil record contains abundant, diverse, and well‐preserved fossils spanning multiple trophic levels from primary producers to apex predators. In addition, microfossils often constitute and are preserved in high abundances alongside continuous high‐resolution geochemical proxy records. These characteristics mean that microfossils can provide valuable context for understanding the modern climate and biodiversity crises by allowing for the interrogation of spatiotemporal scales well beyond what is available in neo‐ecological research. Here, we formalize a research framework of "micropaleoecology," which builds on a holistic understanding of global change from the environment to ecosystem level. Location: Global. Time period: Neoproterozoic‐Phanerozoic. Taxa studied: Fossilizing organisms/molecules. Our framework seeks to integrate geochemical proxy records with microfossil records and metrics, and draws on mechanistic models and systems‐level statistical analyses to integrate disparate records. Using multiple proxies and mechanistic mathematical frameworks extends analysis beyond traditional correlation‐based studies of paleoecological associations and builds a greater understanding of past ecosystem dynamics. The goal of micropaleoecology is to investigate how environmental changes impact the component and emergent properties of ecosystems through the integration of multi‐trophic level body fossil records (primarily using microfossils, and incorporating additional macrofossil data where possible) with contemporaneous environmental (biogeochemical, geochemical, and sedimentological) records. Micropaleoecology, with its focus on integrating ecological metrics within the context of paleontological records, facilitates a deeper understanding of the response of ecosystems across time and space to better prepare for a future Earth under threat from anthropogenic climate change. [ABSTRACT FROM AUTHOR]

We document an apparent downward displacement of the Matuyama‐Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well‐defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe2+ enrichment below the depth of SO42− depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO42− depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe2+ enrichment below the depth of SO42− depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe2+ production exceeds the production of H2S by SO42− reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence‐bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe2+ accumulation below the SO42− reduction depth. Plain Language Summary: Sediment on the seafloor is an archive of Earth history, including past climate variations, ice sheet dynamics, and geomagnetic field variations. The lattermost include a history of when the Earth's magnetic field flipped upside down in the past so that compass needles would have pointed to the South Pole instead of the North Pole. Scientists have a good understanding of when these flips happened and, once identified in sedimentary layers, the flips can be used to identify time horizons—which are useful for dating other events recorded by the sediment. In this study, we present two magnetic reconstructions from nearby locations that initially recorded the same geomagnetic signals. After deposition, one of the magnetic records changed as a result of chemical reactions that modified the magnetic minerals in the sediment, making it difficult to correlate to the geomagnetic time scale. We have a nearby record with minimal chemical modifications so we could identify the chemical reactions were likely responsible for the changes. This observation will provide additional insights into what signals to look for when studying similar complications from other locations in the world. Key Points: The last magnetic reversal at Scotia Sea Site U1538 is offset downward by ∼20 m compared to the more reliable Site U1537 recordFerrous iron enrichment below the sulfate reduction depth is observed at Site U1538Anomalous magnetic minerals and porewater differences reveal a geochemical setting conducive to chemical remanent magnetization acquisition [ABSTRACT FROM AUTHOR]

On glacial‐interglacial time scales, changes in the Earth's orbital configuration control climate seasonality and mean conditions. Tropical coral skeletons can be sampled at a sufficient resolution to reconstruct past seasonality. Here, last deglacial Porites skeletons from Integrated Ocean Drilling Program Expedition 310 to Tahiti are investigated and, supported by a modern calibration, monthly resolved time series in geochemical proxies (Sr/Ca, δ18O, δ13C) are constructed. For most of the deglaciation, Sr/Ca seasonality was similar to modern (0.139 ± 0.010 mmol mol−1; 2.8 ± 0.2°C) reflecting the small change in insolation seasonality. However, during the Younger Dryas, high values in Sr/Ca seasonality (0.171 ± 0.017 mmol mol−1; 3.4 ± 0.3°C) suggest a reduced mixed layer depth and enhanced influence of the South Pacific Subtropical Gyre due to South Pacific Convergence Zone (SPCZ) inactivity. Furthermore, high amplitudes in Younger Dryas skeletal δ18O (0.40 ± 0.22 ‰) and δ13C (0.86 ± 0.22 ‰) seasonality compared to modern (δ18O = 0.29 ± 0.08 ‰; δ13C = 0.27 ± 0.08 ‰) point to elevated winter‐summer discrepancies in rainfall and runoff. Mean coral Sr/Ca variability suggests an influence of Northern Hemisphere climate events, such as the Younger Dryas cooling (+0.134 ± 0.012 mmol mol−1;−2.6 ± 0.2°C), or the Bølling–Allerød warming (+0.032 ± 0.040 mmol mol−1; −0.6 ± 0.4°C). Deglacial mean coral Δδ18O (δ18Oseawater contribution to skeletal δ18O), corrected for the ice volume effect, was elevated pointing to more saline, thus dryer conditions, likely due to a northward migration of the SPCZ. Seasonal cycles in coral δ13C were likely caused by variations in linear extension rates that were reduced during the last deglaciation (1.00 ± 0.6 cm year−1) compared to today (1.6 ± 0.3 cm year−1). Plain Language Summary: Changes in the Earth's movement around the sun are the primary cause of changes in the annual cycle and annual mean of environmental conditions during the last glacial‐interglacial periods. Distinct from most climate archives, the annual bands of coral skeletons can be sampled at a monthly resolution that allows for reconstructions of past seasonal cycles. In this study, the chemical composition of coral skeletons from the genus Porites that grew during the last deglaciation at Tahiti (∼15,000–9,000 years ago) was analyzed to infer seasonal variations and mean conditions in past sea surface temperature and salinity. Prior to application, the methods were successfully calibrated using modern coral skeletons from Tahiti. Results showed that sea surface temperature seasonality was similar to today during most of the deglaciation. However, for the Younger Dryas, a particular cold interval, a marked increase in sea surface temperature seasonality suggests that Tahiti was at that time located outside the South Pacific Convergence Zone, a rain band extending across the tropical Southwest Pacific. Furthermore, coral reconstructions revealed that, although distant, North Atlantic last deglacial climate events influenced mean climate conditions at Tahiti. In addition, we show that last‐deglacial coral growth rates were reduced compared to today. Key Points: Last deglacial changes in Tahiti coral Sr/Ca suggest an influence of Northern‐Hemisphere climate events on the tropical South PacificVariations in last deglacial Tahitian climate were modulated by meridional migrations of the South Pacific Convergence ZoneLast deglacial Porites corals revealed reduced linear extension rates (1.0 ± 0.6 cm year−1) compared to today (1.6 ± 0.3 cm year−1) [ABSTRACT FROM AUTHOR]

This study characterizes the microstructure and mineralogy of 132 (ODP sample), 1000 and 1880 million‐year‐old chert samples. By using ultra‐small‐angle X‐ray scattering (USAXS), wide‐angle X‐ray scattering and other techniques, the preservation of organic matter (OM) in these samples is studied. The scarce microstructural data reported on chert contrast with many studies addressing porosity evolution in other sedimentary rocks. The aim of this work is to solve the distribution of OM and silica in chert by characterizing samples before and after combustion to pinpoint the OM distribution inside the porous silica matrix. The samples are predominantly composed of alpha quartz and show increasing crystallite sizes up to 33 ± 5 nm (1σ standard deviation or SD). In older samples, low water abundances (∼0.03%) suggest progressive dehydration. (U)SAXS data reveal a porous matrix that evolves over geological time, including, from younger to older samples, (1) a decreasing pore volume down to 1%, (2) greater pore sizes hosting OM, (3) decreasing specific surface area values from younger (9.3 ± 0.1 m2 g−1) to older samples (0.63 ± 0.07 m2 g−1, 1σ SD) and (4) a lower background intensity correlated to decreasing hydrogen abundances. The pore‐volume distributions (PVDs) show that pores ranging from 4 to 100 nm accumulate the greater volume fraction of OM. Raman data show aromatic organic clusters up to 20 nm in older samples. Raman and PVD data suggest that OM is located mostly in mesopores. Observed structural changes, silica–OM interactions and the hydrophobicity of the OM could explain the OM preservation in chert. [ABSTRACT FROM AUTHOR]

Sedimentary beds of alternating red and green colour are commonly interpreted to reflect orbitally‐forced cyclic climatic, syn‐depositional conditions, although colour changes caused by post‐depositional fluids are also documented. Results from IODP Hole U1502A marine sediments in the South China Sea exemplify post‐depositional reducing fluid–rock interactions that locally changed the sediment colour from red to green. Petrographic, rock magnetic and paleomagnetic data on cores show that the red colouration originates from an early, basin‐wide, pervasive diagenetic oxidation event (forming haematite), whereas the green colouration results from subsequent fluid‐driven reduction (forming pyrrhotite‐magnetite). The dense sulfidic stockwork in the basaltic basement underlying these sediments was the likely source of reducing fluids. Drilling deep holes into marine basin basements can thus provide useful information on fluid transfer from the basement to the overlying sedimentary layers. [ABSTRACT FROM AUTHOR]

Three volcanic arcs have been the source of New Zealand's volcanic activity since the Neogene: Northland arc, Coromandel Volcanic Zone (CVZ) and Taupō Volcanic Zone (TVZ). The eruption chronology for the Quaternary, sourced by the TVZ, is well studied and established, whereas the volcanic evolution of the precursor arc systems, like the CVZ (central activity c. 18 to 2 Ma), is poorly known due to limited accessibility to, or identification of, onshore volcanic deposits and their sources. Here, we investigate the marine tephra record of the Neogene, mostly sourced by the CVZ, of cores from IODP Exp. 375 (Sites U1520 and U1526), ODP Leg 181 (Sites 1123, 1124 and 1125), IODP Leg 329 (Site U1371) and DSDP Leg 90 (Site 594) offshore of New Zealand. In total, we identify 306 primary tephra layers in the marine sediments. Multi‐approach age models (e.g. biostratigraphy, zircon ages) are used in combination with geochemical fingerprinting (major and trace element compositions) and the stratigraphic context of each marine tephra layer to establish 168 tie‐lines between marine tephra layers from different holes and sites. Following this approach, we identify 208 explosive volcanic events in the Neogene between c. 17.5 and 2.6 Ma. This is the first comprehensive study of New Zealand's Neogene explosive volcanism established from tephrochronostratigraphic studies, which reveals continuous volcanic activity between c. 12 and 2.6 Ma with an abrupt compositional change at c. 4.5 Ma, potentially associated with the transition from CVZ to TVZ. Plain Language Summary: Since 18 Ma, volcanic activity in New Zealand is dominantly sourced by the Coromandel Volcanic Zone (CVZ). Most caldera systems of the CVZ identified so far are located on Coromandel Peninsula in the NW of North Island, New Zealand, but studies of the CVZ are rare mainly due to the limited accessibility of its volcanic deposits, as well as missing stratigraphic continuity between different outcrops and the volcanic source. Here, our ocean drilling tephra record—mainly volcanic ash from explosive eruptions, distributed and falling out over the ocean—has a great potential to reveal the eruption history of the CVZ because it is preserved in marine sediments in a nearly undisturbed stratigraphic context. We analyzed ∼400 marine tephra layers from multiple ocean sediment cores off the coast of New Zealand for their geochemical glass compositions and identified 306 as largely undisturbed ash deposits. These primary ash deposits correspond to a total number of 208 Neogene volcanic events. Different dating methods result in a continuous marine tephra record for the last 12 Ma, equivalent to a unique and most complete eruptive history for the CVZ. This enables us to further unravel changes in the composition of the associated magmas with time. Key Points: New Zealand's Neogene explosive volcanism based on the marine tephra recordGeochemical fingerprinting of marine tephra layers across the study area to establish volcanic eventsInsights into geochemical variations with time, repose times and spatiotemporal distribution [ABSTRACT FROM AUTHOR]

The Cascadia subduction megathrust off the Pacific Northwest follows an "end member" seismogenic behavior, producing large (up to moment magnitude 9) but infrequent (every several hundred years) earthquakes and tsunamis. Crustal deformation associated with the ongoing plate convergence has been characterized by land‐based geodetic observations, but the state of locking across the full breadth of the seismogenic fault is poorly constrained. We report results of offshore monitoring of borehole fluid pressure, as a proxy for formation volumetric strain, at a site ∼20 km landward of the Cascadia subduction deformation front since 2010. The multi‐depth pressure records were plagued by hydrologic noise, but noise at the deepest monitoring level (303 m sub‐seafloor) abated in 2015. Subsequently, including at the times of regional large earthquakes that caused significant dynamic stressing, no persistent pressure transients are present above a threshold of 0.08 kPa imposed by unremovable oceanographic signals, corresponding to a strain detection limit of ∼16 nanostrain. Simple dislocation models using local megathrust geometry suggest a resolvable slip of <1 cm along a trench‐normal corridor beneath the borehole for a range of slip‐patch dimensions. A large slip patch can be well resolved even at considerable along‐strike distances from the borehole; for instance, ∼10 cm slip is detectable over a 200‐km strike range for a slip‐patch radius of ∼50 km. This high sensitivity for detecting slip, along with the lack of observed events, stands in stark contrast to observations at other subduction zones, and suggests that the Northern Cascadia megathrust is most likely fully locked. Plain Language Summary: Subduction thrust faults can produce Earth's largest earthquakes and tsunamis. Estimating the seismic and tsunami potential requires knowing how complete the fault locking is. The Cascadia subduction zone is known to have hosted very large earthquakes, but with the existing measurements of deformation being on land, the state of locking of the shallow sub‐marine portion of the fault remains unclear. Addressing this question requires offshore deformation measurements directly above the shallow fault. In this study, we report results of formation fluid pressure measurement—as a volumetric strain indicator—from a deep‐ocean borehole off Vancouver Island near the deformation front of Cascadia. While data from shallow monitoring depths suffer from noise likely caused by formation gas dynamics, the record from the deepest level (300 m beneath the seafloor) has been sufficiently quiet since mid‐2015 to allow detection of any slip‐triggered pressure variations. Lack of such variations, together with deformation modeling, limits local slip events to be <1 cm. This suggests that the slip deficit of the plate convergence is being accumulated at a substantial rate and it may be released in the next megathrust earthquake. This contrasts with other subduction margins where slip‐related pressure transients have been frequently detected. Key Points: 8‐year formation pressure records from northern Cascadia suggest a lack of tectonic transients despite occasional large dynamic stressingNoise level of the record defines a detection threshold of 0.08 kPa, equivalent to 16 nanostrain for shallow accretionary prism sedimentDeformation modeling shows that the data would resolve local slip down to <1 cm, and thus suggests a fully locked state of the megathrust [ABSTRACT FROM AUTHOR]

The Eocene‐Oligocene transition (EOT) marks the shift from greenhouse to icehouse conditions at 34 Ma, when a permanent ice sheet developed on Antarctica. Climate modeling studies have recently assessed the drivers of the transition globally. Here we revisit those experiments for a detailed study of the southern high latitudes in comparison to the growing number of mean annual sea surface temperature (SST) and mean air temperature (MAT) proxy reconstructions, allowing us to assess proxy‐model temperature agreement and refine estimates for the magnitude of the pCO2 forcing of the EOT. We compile and update published proxy temperature records on and around Antarctica for the late Eocene (38–34 Ma) and early Oligocene (34–30 Ma). Compiled SST proxies cool by up to 3°C and MAT by up to 4°C between the timeslices. Proxy data were compared to previous climate model simulations representing pre‐ and post‐EOT, typically forced with a halving of pCO2. We scaled the model outputs to identify the magnitude of pCO2 change needed to drive a commensurate change in temperature to best fit the temperature proxies. The multi‐model ensemble needs a 30 or 33% decrease in pCO2, to best fit MAT or SST proxies respectively. These proxy‐model intercomparisons identify declining pCO2 as the primary forcing of EOT cooling, with a magnitude (200 or 243 ppmv) approaching that of the pCO2 proxies (150 ppmv). However individual model estimates span a decrease of 66–375 ppmv, thus proxy‐model uncertainties are dominated by model divergence. Plain Language Summary: Antarctica was once a continent with little to no ice on it. Around 34 million years ago Antarctica developed its first permanent ice sheet as temperatures cooled. To evaluate how much cooling occurred on and around Antarctica, we compiled evidence from the molecules left behind by ancient organisms, that carry information about temperature, as reported previously in the literature. We then compared the ancient evidence to climate model experiments which allows us to test cause and effect. Cooling is thought to be caused by a drop in carbon dioxide concentrations in the atmosphere, and we tested how much carbon dioxide levels would need to drop to explain the cooling found. Our estimates are similar to independent evidence from marine organisms for carbon dioxide concentrations. Key Points: Air temperatures at the margins of the Antarctic continent dropped by 0°C–4°C across the Eocene‐Oligocene transition (EOT)Southern high latitude sea surface temperatures cooled by 0°C–3°CBest fit to the proxy surface air temperatures from CO2‐only runs suggest a 30% decrease in pCO2 across the EOT [ABSTRACT FROM AUTHOR]

The influence of a warming‐induced reduction in Arctic sea ice extent (SIE) on the South Asian monsoon (SoAM) is not well understood. The mid‐Piacenzian Warm Period (MPWP, 3.264–3.025 million years ago, Ma), the most recent warm climate event when the Earth's CO2 levels were similar to the present, is considered an analog for future global warming. We present a multi‐proxy high‐resolution record of SoAM variability during the late Pliocene (∼3.4–2.7 Ma) using denitrification, productivity, weathering, and terrestrial influx proxies from the eastern Arabian Sea (International Ocean Discovery Program Expedition 355). The proxy‐based SoAM reconstruction is independently verified using a hybrid model‐proxy data set (Oscillayers). We identify two distinct intervals of monsoon intensification—during the MPWP and at 2.9 Ma. The SoAM variability appears to result from an interplay between dynamic (Insolation, Arctic SIE variability, and southern tropical Indian Ocean surface temperature variability due to Indonesian Throughflow changes) and thermodynamic effects (global temperature change due to pCO2 variability). Using a late Pliocene Arctic spring sea ice concentration reconstruction, we find that lower (higher) Arctic SIE leads to stronger (weaker) late Pliocene SoAM via asymmetric interhemispheric energy export and through modulating jet stream flow and meridional circulation. Our finding of strengthened SoAM during the MPWP corroborates model results and implies that more intense precipitation is expected over South Asia with continued climate warming. Plain Language Summary: Ongoing climate warming is causing the Arctic sea ice extent (SIE) to decline at an alarming rate, which may influence the South Asian monsoon (SoAM). The late Pliocene Mid Piacenzian Warm Period (MPWP, ∼3 million years ago) is considered the nearest analog to modern climate, as it had similar atmospheric CO2 concentrations and continental configuration. Thus, this past warm interval may provide insights into what might be expected with continued climate warming. Here we reconstruct late Pliocene SoAM variability using a combined proxy data‐model approach and explore forcing factors, including linkages with Arctic SIE. We find that the SoAM intensified significantly during the MPWP and at 2.9 Ma. Further, we observed that lower Arctic SIE strengthens the SoAM, whereas higher Arctic SIE weakens the SoAM. Our study helps to understand how the SoAM may behave in the present scenario of global warming and how the Arctic SIE may influence it. Key Points: Two distinct intervals of monsoon intensification are found during the mid‐Piacenzian Warm Period and at 2.9 MaLower (higher) Arctic sea ice extent leads to a stronger (weaker) Pliocene South Asian monsoonResults suggest more intense precipitation over South Asia with continued climate warming [ABSTRACT FROM AUTHOR]

Ice loss in the Southern Hemisphere has been greatest over the past 30 years in West Antarctica. The high sensitivity of this region to climate change has motivated geologists to examine marine sedimentary records for evidence of past episodes of West Antarctic Ice Sheet (WAIS) instability. Sediments accumulating in the Scotia Sea are useful to examine for this purpose because they receive iceberg‐rafted debris (IBRD) sourced from the Pacific‐ and Atlantic‐facing sectors of West Antarctica. Here we report on the sedimentology and provenance of the oldest of three cm‐scale coarse‐grained layers recovered from this sea at International Ocean Discovery Program Site U1538. These layers are preserved in opal‐rich sediments deposited ∼1.2 Ma during a relatively warm regional climate. Our microCT‐based analysis of the layer's in‐situ fabric confirms its ice‐rafted origin. We further infer that it is the product of an intense but short‐lived episode of IBRD deposition. Based on the petrography of its sand fraction and the Phanerozoic 40Ar/39Ar ages of hornblende and mica it contains, we conclude that the IBRD it contains was likely sourced from the Weddell Sea and/or Amundsen Sea embayment(s) of West Antarctica. We attribute the high concentrations of IBRD in these layers to "dirty" icebergs calved from the WAIS following its retreat inland from its modern grounding line. These layers also sit at the top of a ∼366‐m thick Pliocene and early Pleistocene sequence that is much more dropstone‐rich than its overlying sediments. We speculate this fact may reflect that WAIS mass‐balance was highly dynamic during the ∼41‐kyr (inter)glacial world. Plain Language Summary: If the West Antarctic Ice Sheet melted completely it would raise global sea‐level by ∼4.3 m and displace hundreds of millions of people currently living below high‐tide lines. This concern has motivated scientists to examine Antarctic‐proximal marine sediments for evidence of elevated iceberg‐rafted debris (IBRD) deposition indicative of ice‐sheet instability during the warmer than present geological past. Marine archives recovered recently by deep‐sea drilling of the Scotia Sea (aka. Iceberg Alley) by the International Ocean Discovery Program are ideal to examine for this purpose because many icebergs calved from Antarctica (and the IBRD they contain) end up here. To date, nothing is known, however, about IBRD deposition in Iceberg Alley prior to the Last Glacial Maximum nor its regional source(s). We show that three IBRD layers indicative of intense centennial‐paced episodes of iceberg calving to Iceberg Alley ∼1.2 million years ago were likely sourced from West Antarctica. These layers sit at the top of a ∼366‐m sequence that is more IBRD‐rich than its overlying sediments. We speculate that these observations reflect that West Antarctic Ice Sheet retreat was more frequent during the early Pleistocene than the last 1.2 million years. Key Points: We present the first provenance data generated for Pleistocene‐aged iceberg‐rafted debris deposited in Iceberg AlleyWe conclude that prominent iceberg‐rafted debris layers deposited at Pirie Basin Site U1538 ∼1.2 Ma were sourced from West AntarcticaThey represent intense suborbitally‐paced episodes of iceberg discharge from tidewater glaciers, most likely in the Weddell Embayment [ABSTRACT FROM AUTHOR]

The northern Hikurangi subduction margin hosts slow slip events (SSEs), which are exceptionally shallow (<15 km). The sedimentary sequence on the incoming plate is therefore representative of the shallow fault material where the SSEs will take place once they enter the subduction zone. Knowledge about the frictional behavior of these sediments is required to know which lithologies are capable of hosting SSEs, and what mechanisms are causing them. Frictional behavior is material specific and depends on sliding velocity, but it is unknown how these natural sediments behave at plate‐rate velocities. We performed laboratory shearing experiments testing the major lithologies sampled during International Ocean Discovery Program (IODP) Expedition 375, at velocities ranging from the plate convergence rate at the Hikurangi margin (5 cm/year), up to those characteristics of the shallow SSEs (160 and 530 cm/year), under simulated in‐situ as well as standardized conditions. We find that the calcite‐rich pelagic sediments are relatively strong and display the velocity‐weakening frictional behavior required for slip events, whereas other lithologies are weaker and show velocity‐neutral to velocity‐strengthening friction. We observe spontaneous laboratory SSEs in the calcareous pelagic sediments, which show partial locking in between sliding events, consistent with the interpretation of SSEs within the spectrum of slow to fast earthquakes. For the Hikurangi margin, our results suggest that SSE occurrence requires the stronger carbonate‐rich unit to be incorporated into the plate‐boundary fault zone, which we suggest occurs because the rough incoming plate introduces geometrical complexity into the fault zone. Plain Language Summary: In the Hikurangi subduction zone, located offshore the east coast of the North Island of New Zealand, the movement between the downgoing and overriding tectonic plates can occur as slow slip events (SSEs). During SSEs the two plates move relative to each other, a process in many aspects similar to ordinary earthquakes, except SSEs take weeks instead of seconds and no ground‐shaking can be felt on the surface. SSEs occur in many subduction zones worldwide, but at the Hikurangi margin they occur shallow, relatively often and close to many mostly land‐based GPS stations that track the movement of the two plates, which makes them ideal to study. A recent research expedition (IODP Expedition 375) drilled the stack of sediments going into the subduction zone to learn more about the circumstances that control where and when SSEs occur. In this paper, we test all the major different sediment types going into the subduction fault zone to find out which one of them is responsible for the SSEs. Using a laboratory device that mimics the subduction zone, we find that sediments containing the mineral calcite cause slip events that are similar to the SSEs observed in the Hikurangi margin. Key Points: Spontaneous laboratory slow slip events (SSEs) suggest the shallow Hikurangi SSEs are favored in carbonate‐rich sedimentsRate‐and‐state friction and critical stiffness theory can partly explain SSEs, but they can occur under velocity‐strengthening conditionsSSE generation in Hikurangi is probably related to heterogeneity in the plate‐boundary fault zone [ABSTRACT FROM AUTHOR]

Micro‐organisms are known to change fluid flow and permeability processes in subsurface environments, but this has only been demonstrated for coarse‐grained sediments and fractures. For fine‐grained sediments (mudstones), little is known about the effects of micro‐organisms on hydromechanical properties. Here, we investigated the influence of micro‐organisms on the porosity, permeability, and compressibility of fine‐grained sediments. We performed resedimentation experiments with and without micro‐organisms added to two reconstituted, fine‐grained sediment samples. These sediments were collected from the Ursa and Brazos‐Trinity Basins in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Micro‐organisms caused a systematic, yet small increase in compression index for both sediments. Changes to permeability caused by micro‐organisms, while relatively minor, were greater for the Ursa sediment than the Brazos‐Trinity sediment. Additionally, the effect of micro‐organisms on permeability is greater at higher porosities and lower vertical effective stresses. Differences in permeability behavior between the two sediments are likely due to differences in sediment properties and nutrients for microbial growth. We therefore suggest that the effectiveness of micro‐organisms at altering fluid flow in fine‐grained sediments is dependent on burial depth (porosity as a function of vertical effective stress) and the grain size, pore and pore throat size, and specific surface area of a sediment. Characterizing the effects of micro‐organisms on the hydromechanical properties of fine‐grained sediments can further our understanding of the controls on pore pressure near the sediment–water interface in marine environments and aid in bioclogging practices around contaminated sites in terrestrial environments. Plain Language Summary: Micro‐organisms dwell in the pore space between sediments (porosity) all across Earth's surface. This could potentially affect the rate at which porosity is lost as clay‐rich sediments are buried (compressibility) and the ease at which fluids flow through clay‐size sediments (permeability). In this paper, we investigate if and how micro‐organisms change the compressibility and permeability of fine‐grained sediments (sediments dominated by clay‐size grains). To do this, we experimentally compress these fine‐grained sediments in the vertical direction with micro‐organisms added and without micro‐organisms added (control experiment) while measuring sediment porosity, compressibility, and permeability. We find that micro‐organisms cause a small increase in the rate of porosity loss during compression and a small decrease in permeability. However, the amount of permeability decrease caused by micro‐organisms is dependent on multiple properties of the clay‐size sediment. These findings can be used to help understand how water pressures in ocean sediments can become elevated, which could lead to damage of seafloor infrastructure, or benefit geotechnical engineering practices that use micro‐organisms to prevent fluid flow around contaminated sediments. Key Points: We experimentally determine the effects of micro‐organisms on the compression and permeability behavior of fine‐grained sedimentsMicro‐organisms cause a small increase in compression index and small decrease in permeability for fine‐grained sedimentsThe degree to which micro‐organisms decrease permeability in fine‐grained sediments is controlled by multiple sediment properties [ABSTRACT FROM AUTHOR]

Deep ocean circulation in the Southern Hemisphere plays a central role in global ocean overturning circulation and determines ocean carbon sink variability on multimillion‐year timescales. For this reason, it is important to understand how deep currents that originate in the Southern Ocean responded to past climate fluctuations and changing temperature patterns in the Southern Hemisphere oceans. To investigate this feedback mechanism, we analyze deep‐sea sediment cores formed under the influence of bottom currents from the Mentelle Basin (International Ocean Discovery Program Sites U1513, U1514, and U1516), in the Southeastern Indian Ocean. We show that bottom current activity intensified in the Australian‐Antarctic Basin during the cooling intervals of the late Miocene (ca. 7.8–4.8 Ma) and late Pliocene (ca. 3.59–3.2 Ma). At those intervals, bulk grain‐sizes and terrigenous input increased, via current winnowing. Ferromanganese nodules and stratigraphic hiatuses are also observed within these periods, indicative of intense sediment erosion at the seafloor. Sediment accumulation and preservation in the basin was particularly low at the deep Site U1514 (3,838 m water depth), suggesting that enhanced current transport and sediment erosion occurred preferentially within the deepest parts of the basin. We suggest that bottom water fluxes in the Australian‐Antarctic Basin intensified during late Miocene and Pliocene cooling intervals, likely boosted by renewed Australian‐Antarctic Discordance spreading since the Miocene. Key Points: Deep‐water transport in the Australian–Antarctic Basin intensified during Neogene cooling intervalsIntervals of winnowing favored the preservation of coarser terrigenous packages, the formation of hiatuses and ferromanganese nodulesRenewed rifting in the Australian–Antarctic Discordance in the Miocene may have boosted bottom water fluxes into the Mentelle Basin [ABSTRACT FROM AUTHOR]

Petrographic and major element investigations of carbonates from seven drill cores recovered during IODP Expedition 357 on the Atlantis Massif (AM) provide information on the genesis of carbonate minerals in the oceanic lithosphere. Textural sequences and mineralogical assemblages reveal three distinct types of carbonate occurrences in ultramafic rocks that are controlled by (a) fluid composition and flow, (b) temperature, and (c) the presence of mafic intrusions. The first occurrence of carbonate consists of different generations of calcite that formed syn‐ to post‐ serpentinization. These calcites formed at temperatures between 30 and 184°C (based on clumped isotope thermometry) and from a fluid influenced by interaction with mafic intrusions. The second occurrence consists of magnesite, dolomite, calcite, and aragonite veins that also formed syn‐ to post serpentinization. These carbonates formed at temperatures between ambient and 196°C, from fluids with highly variable FeO, MnO, and SrO composition and Mg/Ca ratios, but overall high CO2 and moderate SiO2 concentrations. High FeO (3.3 wt%) and MnO (7.3 wt%) contents indicate high temperatures, high water/rock ratios, and low oxygen fugacity for both carbonate assemblages. The third occurrence consists solely of aragonite veins formed at low temperatures (<9°C) within the uplifted serpentinized peridotites. Major element analyses suggest that aragonite precipitated from seawater, which experienced little interaction with the basement. Combining these results, we propose a model that positions different carbonate occurrences in a conceptual framework considering mafic domains in the peridotites and fluid heterogeneities during progressive exhumation and alteration of the AM. Plain Language Summary: Carbonates form in diverse geological settings within the continental and oceanic lithosphere. They present a safe and permanent geological storage for CO2, which is an important greenhouse gas. Thus, understanding the mechanism that drives carbonate formation will help to find a solution to stabilize atmospheric CO2 and reduce global warming. This study uses petrographic and chemical investigations of drilled lithosphere samples from the Atlantis Massif to create a model for carbonate formation. The Atlantis Massif is located in the Atlantic Ocean close to the Mid‐Atlantic‐Ridge. It is characterized by a high amount of mantle rocks exposed at the seafloor. Mantle rocks react in contact with seawater to form serpentine and create an environment that leads to the transformation of CO2 into carbonates. Our results demonstrate a high variability in carbonate chemistry as well as timing and temperature of formation within the Atlantis Massif. It shows that carbonate formation not only depends on the chemistry of the fluid from which it precipitates but also on the physical properties of the fluid. Furthermore, the study shows that Mg‐rich carbonate (magnesite) formation can occur in an oceanic setting, which is predicted but rarely observed in natural settings. Key Points: Carbonate precipitation is controlled by fluid composition and flow, temperature, and the presence of mafic intrusionsMagnesite and dolomite formed at high Mg/Ca ratios and zones of channelized flow in serpentinized peridotites at the Atlantis MassifCO2 capture is limited in the shallower crust, concentrated around Lost City, but may be more extended in deeper circulation [ABSTRACT FROM AUTHOR]

We collected a suite of core top samples during R/V Sonne Cruise SO257 in May 2017 along the southwestern front of the Indo‐Pacific Warm Pool (IPWP) to monitor the variability of Southern Hemisphere tropical and subtropical sea surface hydrology and to assess temperature and salinity reconstructions with data sets reflecting conditions in the post‐monsoonal season. In our core top samples, a steep increase in planktic δ18O, associated with a decrease in sea surface temperature (SST), indicates that the southwestern front of the IPWP is located between 23° and 24°S during austral fall. We additionally reconstructed SST, sea surface salinity ,and δ18O seawater (δ18Osw) over the last 450 kyr in two sediment successions located within and beyond the monsoonal rain belt. Our records show that SST was highly coherent and phase‐locked with atmospheric pCO2 during the last 450 kyr. The regional differences in the δ18Osw records reveal that the Western Australian Margin north of 15°S remained seasonally under the influence of IPWP water masses, even during glacials. The temporal variability in upper ocean hydrology along the Western Australian Margin is not directly coupled to local monsoonal precipitation, but is strongly affected by advective mixing of Indonesian Throughflow derived water masses. Key Points: Southwest front of modern Indo‐Pacific Warm Pool (IPWP) during austral fall is located between 23° and 24°SWestern Australian Margin north of 15°S remained seasonally influenced by IPWP throughout past 450 kyrUpper ocean hydrology off Western Australia represents an integrated signal of monsoonal precipitation and advective mixing [ABSTRACT FROM AUTHOR]

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered during IODP Expedition 357 on the Atlantis Massif (AM) was examined to characterize carbon sources and the fate of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in seawater during long‐lived hydrothermal circulation and serpentinization. Carbon isotopes reveal three stages of carbonate formation, starting at least 38,000 yr ago: (a) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50°C–190°C); (b) carbonate vein formation related to high and focused fluid fluxes still at high temperatures (30°C–200°C); and (c) seawater circulation leading to cold carbonate precipitation controlled by late, brittle fractures during uplift, and unroofing of the oceanic core complex. Our study reveals three main DIC sources in the system: (a) DIC from abiotic hydrothermal degradation of dissolved organic matter (OM); (b) DIC from seawater; and (c) DIC from mantle‐derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (∼800 ppm) of total non‐carbonate carbon (NCC) and 13C‐depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favor the transport and incorporation of marine DOC in serpentinites and that carbonates record isotopic signals of OM decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the AM and suggests that serpentinites may be an important sink of DOC from seawater. Plain Language Summary: Carbon can be found in higher concentrations within the atmosphere, terrestrial biosphere, oceans, and lithosphere. It is an essential element for various abiotic and biotic reactions and is in the form of CO2 and CH4, an important greenhouse gas. The transfer of carbon between different reservoirs is not well constrained. This study uses samples recovered by drilling the lithosphere at the Atlantis Massif (AM) and chemical investigations to evaluate carbon sources, speciation, and transformations. The AM is located in the Atlantic Ocean close to the Mid‐Atlantic‐Ridge and comprises a high amount of mantle rocks exposed at the seafloor. Mantle rocks react in contact with water to form serpentine and create an environment that promotes the transformation of CO2 into carbonates. Our results demonstrate that the oceanic lithosphere is a significant reservoir of organic compounds and inorganic carbonates and that carbonates record the oxidation of organic matter over long periods. Our study can help to evaluate the potential of mantle rocks to store carbon and reduce CO2 emission. However, it may also provide information to evaluate the potential for life within the lithosphere, not only on Earth but also on other planets where serpentinization processes occur. Key Points: Serpentinization is associated with dispersed carbonate and multiple generations of veins formed at temperatures from 5°C to 200°CThe system has multiple carbon sources: inorganic and organic carbon from seawater, mantle‐derived carbon, and minor in situ productionAbiotic hydrothermal degradation of dissolved organic matter contributes to carbon cycling in peridotite‐hosted hydrothermal systems [ABSTRACT FROM AUTHOR]

Characterization of the porosity evolution across the sedimentary section entering subduction zones and accreted sediments provide valuable information for understanding the deformation history at accretionary margins and the physico‐chemical processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged, and cored the reference section on the incoming plate (Site U1520) and across the active Pāpaku thrust (Site U1518), which is a <30° westward‐dipping splay fault in the frontal accretionary wedge at the north Hikurangi margin in a region where tsunami earthquakes and recurrent slow slip events are documented. We observe strong variations of physical properties across the thrust fault. In particular, interstitial porosity increases by ∼10% through the fault zone while it exponentially decreases with depth, showing lower values in the hanging‐wall (HW; on average, ∼36%) than in the footwall (∼42%). Based on comparisons of porosity with the compaction curve at Site U1518 with that of reference Site U1520, we infer an overcompaction in the HW and a nearly normal compaction in the footwall of the Pāpaku thrust. We suggest that the porosity pattern across the thrust infers differences in the maximum burial depth of sediments, characterized by an overcompacted HW that has been uplifted, thrusted, and concomitantly tectonically eroded above the footwall. Porosity data indicate hydrostatic conditions around the thrust fault contrasting with commonly assumed excess pore pressure at the plate interface. Key Points: Strong contrasts of physical properties including porosity occur across the Pāpaku thrust fault at IODP Site U1518The hanging‐wall (HW) is overcompacted and shows lower interstitial porosity and higher P‐wave velocity and resistivity than the footwallPorosity suggests differences in maximum burial depth with an uplifted, thrusted, and concomitantly eroded HW above the footwall [ABSTRACT FROM AUTHOR]

Continuous sedimentary records of paleomagnetic directional variability and relative paleointensity (RPI) provide valuable information on the evolution of the geodynamo while also facilitating stratigraphic correlation and age control. While the Quaternary RPI record has received much attention, Pliocene records are relatively rare. Here, a u‐channel paleomagnetic study from Integrated Ocean Drilling Program (IODP) Site U1396 in the Caribbean Sea refines the shipboard‐derived polarity stratigraphy and generates an RPI and directional record extending back 4.5 Ma. Rock magnetic data reveal changes in magnetic coercivity around 2.1 Ma that influences the quality of the paleomagnetic record; while the older record is well‐resolved and passes RPI quality criteria, much of the younger section does not. To facilitate the development of the RPI record, spike noise associated with discrete intervals of volcanogenic sediments are filtered from the data set. The resulting record passes RPI reliability criteria between 0–0.6 Ma and 2.1–4.5 Ma and represents the highest resolution RPI record extending into the early Pliocene. We use this record to refine the existing benthic δ18O chronology and open the door to high‐resolution RPI chronostratigraphies during the Pliocene. Although we find no evidence for a previously observed increase in magnetic field intensity after ∼4 million years ago, we do observe an asymmetrical form to RPI in the normal polarity intervals of the Gauss chron. We also find a polarity bias in inclination that cannot be simply explained by a drill string overprint, suggesting polarity‐driven field asymmetries that are particularly pronounced during the Pliocene. Plain Language Summary: As sediments slowly accumulate on the ocean floor past variations in magnetic field intensity and direction are recorded. While we have a relatively good record of intensity variations over the last 2–3 million years, we know little about earlier time intervals. Here, we reconstruct past magnetic (or paleomagnetic) variations over parts of the last 4.5 million years using a sediment core from the Caribbean Sea collected by the Integrated Ocean Drilling Program. A deterioration in the quality of the paleomagnetic signal occurred about 2.1 million years ago, however, prior to that time, the sediments preserve a high fidelity paleomagnetic record at a temporal resolution higher than any previously obtained. Good agreement to existing data from 2.1 to 3 million years ago builds confidence in the quality of our new observations. Although we find no evidence for a previously observed change in magnetic field intensity at around 4 million years ago, differences in mean inclination during normal as compared to reverse polarity intervals are observed, suggesting field asymmetries during this time interval. The unparalleled quality and length of our new record means we can now begin to define characteristic geomagnetic field behavior during times rarely observed. Key Points: Rock and paleomagnetic filtering provide a more robust estimate of relative paleointensity (RPI) that is used to refine the Integrated Ocean Drilling Program Site (IODP) U1396 δ18O chronostratigraphyIODP Site U1396 provides the longest, highest‐resolution, independently dated record of RPI to establish characteristic Pliocene behaviorPlio‐Pleistocene inclination anomaly contains a strong polarity bias and RPI asymmetry may be a characteristic feature of the Gauss chron [ABSTRACT FROM AUTHOR]

Global warming is most pronounced in the Arctic as evident from the massive sea ice loss during the past few decades. The Mid‐Pliocene Warm Period (MPWP), 3.264 – 3.025 million years ago with similar CO2 levels, is the nearest analogue for understanding the impacts of future global warming. High‐resolution studies of relative nutrient utilization and productivity from the Atlantic‐Arctic Gateway (AAG) can provide insight into the nutrient availability governed by stratification strength during past warm climates. Here, we present relative nutrient utilization and productivity variability during the MPWP using sediments collected during the Ocean Drilling Program (ODP) Leg 151 from Fram Strait, AAG. We find that the relative nutrient utilization was high (low) implying stronger (weaker) stratification during warm (cold) periods during the MPWP. Stronger stratification inhibits the nutrient influx from intermediate water depths into the surface leading to higher utilization of available nutrients. It existed during warm periods likely due to enhanced summer sea ice melt and river discharge from the hinterland. As a consequence, the freshened surface layer could have stored more heat and accelerated the sea ice melt further implying that in the present‐day warm scenario, stronger stratification and upper layer freshening may lead to more sea ice melt in the Arctic Ocean. Key Points: First high‐resolution relative nutrient utilization record during Late Pliocene from ArcticRelative nutrient utilization is related to stratification at the study siteStronger (weaker) stratification found during warm (cold) periods of the Late Pliocene [ABSTRACT FROM AUTHOR]

We constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle. Plain Language Summary: Using geophysical images captured from the inside of boreholes drilled across the Hikurangi Subduction Margin, an area that experiences slow earthquakes, we describe variability in the direction of modern day maximum horizontal tectonic forces (stress) at this collisional plate boundary. Changes in the direction of maximum horizontal stress occur as you move from the plate boundary toward the onshore region of New Zealand's North Island. We suggest a range of possible tectonic and geological processes that either individually or in concert may explain our observed stress direction variations. This includes changing tectonic regimes as you move away from the plate boundary, topography, and effects on the stress field caused by the presence of subducting seamounts, and changing stress conditions related to the intermittent activity of slow earthquakes. Key Points: Maximum horizontal stress directions vary in a transect across the Hikurangi Subduction MarginStress orientations suggest a change occurs moving from offshore to onshore associated with changing dominant tectonic regimeOffshore stress variation may be caused by a number of specific tectonic and geological causes [ABSTRACT FROM AUTHOR]

Hosting both tsunami earthquakes and slow slip events at shallow depth, the northern Hikurangi margin has motivated strong research efforts over the last two decades to better understand the relation between fluid pressure and fault mechanical behavior. Recently, IODP Expeditions 372 and 375 drilled, cored, and logged the basin entering this subduction zone providing a unique opportunity to characterize initial hydrogeological and petrophysical properties that drive seismic hazards along the margin. We inventory bound and interstitial fluid contents and assess the compaction state of the heterogeneous incoming sequence by correcting shipboard total connected porosity from the bound water content that typically characterizes clay‐rich sediments. By combining mercury injection capillary pressure, nuclear magnetic resonance, and nitrogen gas adsorption, we further document the evolution of pore structure with increasing depth. We also compare porosity data at the logging and sample scales. Both logging and laboratory data evidence contrasted porosity, pore structure, and fluid distribution across the entering section. Shallow macroporous siliciclastic units show very low bound water content and undergo compaction‐induced dewatering as they are normally consolidated during burial. In contrast, mesoporous volcaniclastic Hikurangi Plateau facies are characterized by high potential for bound water release from smectite, zeolite, and opal dehydration at greater depths. They mostly exhibit very low permeability except potentially in uncemented highly porous intervals where the décollement may initiate. Macroporous to mesoporous pelagic carbonates sandwiched in between exhibit mixed behavior and may also host the future décollement, similarly to the southern part of the margin. Key Points: The Northern Hikurangi margin input section is characterized by contrasted fluid distribution, pore structure, and potential for excess pore pressure buildupIt is essential to use corrected porosity instead of apparent porosity in clay‐rich sediments and altered oceanic crustThe future décollement may develop in the lower part of pelagic carbonates or at the top of volcaniclastic facies of the Hikurangi Plateau [ABSTRACT FROM AUTHOR]

IODP Site U1437 is located in the Izu rear‐arc region, approximately 330 km west of the Izu‐Bonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6–15 Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]‐type, Rift‐type, and volcanic front [VF]‐type). RASC‐type has the most enriched Nd and Hf isotope and fluid‐immobile trace element ratios and dominates from 9 to 6 Ma. Rift‐type, dominant from 15 to 9 Ma, is similar to VF‐type in Nd‐Hf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VF‐type. Less common and randomly distributed VF‐type sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15 Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a melt‐rich slab component generated at ~125 km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial post‐Shikoku basin magmatism in the Izu rear‐arc generated Rift‐type magmas for about 6 million years before the distinctive RASC‐type magmatism began, which then became increasingly enriched. Plain Language Summary: We explore the history of magma genesis in the Izu rear‐arc, Japan, by studying the geochemistry of volcaniclastic sediments that were obtained by ocean drilling. This paper presents their Sr‐Pb‐Nd‐Hf isotope ratios and numerical models of magma genesis. We identify the presence of three geochemical types that change through time. Rift‐type formed soon after cessation of seafloor spreading in the Shikoku Basin at ~15 Ma and came from a more fertile mantle source with a less hydrous slab component than at the contemporaneous volcanic front. This type is most like basalts from rifts behind the current volcanic front and was unknown before drilling. RASC‐type dominates from 9 to 6 Ma and has more enriched Nd and Hf isotope and fluid‐immobile trace element ratios that are like those of rocks dredged from the coeval rear‐arc seamount chains. VF‐type is uncommon, randomly dispersed, and interpreted as derived from the distal volcanic front. The results confirm that the arc front has migrated trenchward since the Miocene. Key Points: The geochemistry of the volcaniclastics and hyaloclastites of IODP Site U1437 is reportedTwo magma types and one magma type of distal origin existed in the Izu rear‐arc after spreading in the Shikoku Basin endedThe initial post backarc basin magmatism generated magmas similar to Quaternary Rift‐type and evolved to rear‐arc seamount chain type [ABSTRACT FROM AUTHOR]

Pre‐Quaternary paleoclimate studies in Australia mainly focus on terrestrial records from the southeastern part of the continent. IODP Expedition 356 drilled on the northwestern Australian shelf, yielding Miocene‐Pleistocene paleoclimate records in an area where climate archives are scarce. Postexpedition research revealed a dry‐to‐humid transition across the latest Miocene and early Pliocene (start of the "Humid Interval"). However, the complex tectonic history of the area makes these interpretations challenging. In this study, we investigate late Miocene to early Pliocene sediment cores from two sites that are only 100 km apart but situated in two adjacent basins (Northern Carnarvon and Roebuck Basins). Combining lithofacies study, time series analysis of potassium content (K wt%), and calcareous nannofossil abundance counts (N/g), this work disentangles the complex interplay between basin evolution and climate change between 6.1 and 4 Ma. Overall, the investigated proxies show high correlation between both sites, except during 6.1–5.7 Ma. During this interval, Site U1463 records a gradual increase in K wt%, correlated with basin deepening, whereas Site U1464 records an abrupt rise in K wt% at ~6 Ma. We explain this diachronicity by differential basin subsidence. The tectonic interplay with our paleorecords makes it difficult to pinpoint the exact onset of the "Humid Interval," but we conclude that K wt% and coccolith abundances at Site U1464 indicate that a fluvial deposition system was already established since at least 6 Ma. This age is consistent with data supporting a southward movement of the Intertropical Convergence Zone rain belt at ~7 Ma. Key Points: Neighboring sites on the NW Australian shelf record different late Miocene sedimentation patterns due to diachronous basin subsidenceA well‐developed fluvial deposition system was established in NW Australia since at least 6 MaAstronomically tuned age models confirm synchronicity of early Pliocene coccolithophore productivity and shifts in species composition [ABSTRACT FROM AUTHOR]

The abundance and composition of modern phytoplankton are primarily related to equator‐to‐pole temperature gradients and global ocean circulation, which in turn determine the availability of nutrients in the photic zone. The nutricline is found at greater depths in warm, tropical waters, whereas more vigorous surface mixing in higher latitudes (seasonally) enhances nutrient availability and primary productivity. Ocean temperatures were ~7°C higher during the middle Miocene Climatic Optimum (MCO; ~16.9–14.7 million years ago, Ma), which was followed by Antarctic glaciation and global cooling during the middle Miocene Climate transition (MMCT; 14.7–13.8 Ma). Four decades ago, Haq (1980, https://doi.org.10.2307/1485353) already related migration patterns of low‐latitude versus high‐latitude calcareous nannoplankton in the Atlantic Ocean to major climatic fluctuations during the Miocene. Here, we detail and discuss the macroevolutionary patterns and processes across the middle Miocene (~16.5–11 Ma) at five deep sea sites on a north‐south transect in the Atlantic Ocean (57°N to 28°S). We show that the major cooling step toward the modern "icehouse" world impacted coccolithophore communities at all latitudes. Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the MMCT, we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids. Global cooling and related increased meridional overturning circulation are implicated as likely forcings for this macroevolutionary step toward establishing modern coccolithophore communities that are dominated by eurythermal and eurytrophic species such as Emiliania huxleyi. Plain Language Summary: How will marine plankton communities respond to a much warmer world than today? How fast or slow would such changes be? We can learn valuable lessons from the fossil record of coccolithophores that represent a prominent phytoplankton group in both past and modern oceans. Changes in the composition of fossil assemblages show that species tracked past climate change on submillion year scales in the Atlantic Ocean. When ocean temperatures were ~7°C warmer during the middle Miocene (~16 million years ago), tropical species reached into the midlatitudes (42°N) of the North Atlantic. When the Antarctic ice sheet grew larger and the oceans cooled after ~14.7 million years ago, a group known as the reticulofenestrids diversified and became the most common everywhere, also in the tropics. Apparently, this group maintained high fitness in an "icehouse" world with greater latitudinal temperature contrasts, more vigorous ocean circulation, and higher nutrient availability in the photic zone. Indeed, their cosmopolitan distribution implies broad temperature and other environmental tolerances, similar to the group's latest descendant, the modern species Emiliania huxleyi that also displays a wide genetic and ecophenotypic diversity. Key Points: A major cooling step during the middle Miocene impacted phytoplankton communities across high and low latitudes in the Atlantic OceanOnset of "icehouse world" saw an increase in abundance and diversification of alkenone‐producing haptophyte algaeThe middle Miocene Climate Transition was a significant step toward establishing modern ocean circulation and phytoplankton communities [ABSTRACT FROM AUTHOR]

In the IODP 372A proposal, working hypotheses used with respect to actively deforming gas hydrate‐bearing landslides were based on an initial calculation considering gas hydrate to occur in fresh water. This initial inaccuracy leads to predict the base of methane hydrate stability zone (MHSZ) to be at around 162 m below the seafloor (mbsf) at site U1517 while it is expected to be between 85 and 128 mbsf. It is most interesting that important negative chloride anomalies were measured well below the theoretical MHSZ without direct evidence of the presence of gas hydrate. These anomalies are usually a strong indication of gas hydrate occurrence. This initial inaccurate hypothesis provides the opportunity to evaluate the effectiveness of the indirect salinity detection method to quantify gas hydrate. Key Points: The IODP U1517 site was selected based on an initial hypothesis neglecting the effect of salt on the methane hydrate stability lawReported chloride anomalies are deeper than the methane hydrate stability zone at site U1517Predicted methane hydrate (up to 68%) is not confirmed by direct evidence of the presence of gas hydrate nor by indirect seismic velocity [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) is used to infer past changes in the state of the climate system. Here we use a combination of newly generated and published organic paleothermometer records, together with novel high‐resolution benthic foraminiferal δ18O stratigraphy, from four sites in the midlatitude North Atlantic (41–58°N) to reconstruct the long‐term evolution of the latitudinal SST gradient during the Pliocene and early Pleistocene (4.0 to 2.4 Myr), the last time atmospheric CO2 reached concentrations above 400 ppmv. We demonstrate that the latitudinal SST gradient in the North Atlantic nearly collapsed twice during this period. We conclude that the latitudinal SST gradient in the midlatitude North Atlantic has two end‐members: a maximum as existing at present and a minimum that existed during certain periods of the (late) Pliocene. Our results suggest that the 400‐ppmv Pliocene world was more dynamic than currently thought. Key Points: New SST records spanning Plio/Pleistocene from the North AtlanticReveal episodic collapse latitudinal SST gradient North AtlanticWe speculate that collapse is related to changes in Gateways [ABSTRACT FROM AUTHOR]

We investigate chronology and age uncertainty for the middle to upper Pleistocene lower Bengal Fan using a novel age‐depth modeling approach that factors lithostratigraphic, magnetostratigraphic, biostratigraphic, cyclostratigraphic, and seismic stratigraphic constraints, based on results from the International Ocean Discovery Program Expedition 354 Bengal Fan and analysis of the GeoB97‐020/027 seismic line. The initial chronostratigraphic framework is established using regionally extensive hemipelagic sediment units, and only age‐depth models of fan deposits that respect the superposition of channel‐levee systems between sites are accepted. In doing so, we reconstruct signals of regional sediment accumulation rate and lithogenic sediment input through the perspective of a two‐dimensional ~320 km transect at 8°N that are consistent with more distal and more ambiguous regional records. This chronology allows us to discuss the depositional history of the middle to upper Pleistocene lower Bengal Fan within the context of sea level, climate, and tectonic controls. We hypothesize, based on the timing of accumulation rate changes, that progradation and intensification of the Bengal Fan's channel‐levee system at 8°N was largely driven by increases in sea level amplitude during this time. However, it is also possible this progradation was influenced by changes in Pleistocene climate and increased Himalayan erosion rates, driving greater sediment flux to the fan. Plain Language Summary: Deep sea fans are sediment deposits in the ocean that often form near river systems offshore continental margins. The largest of these, the Bengal Fan in the northern Indian Ocean, contains the most complete record of materials eroded from the Himalayan Mountains and can be used to study the climate and tectonic history of the region. Sediments are moved from the river mouth to the fan in a series of ever‐changing channels that distribute sediments across the fan surface, making it impossible to obtain a complete and continuous record of Himalayan erosion at any one location. International Ocean Discovery Program Expedition 354 drilled a series of seven locations in a transect across the fan to capture a more complete record of where sediment was deposited over the last 1.25 Myr, a time characterized by major changes in Earth's climate system. Here we discuss statistics of sediment deposition from a computer model constrained by observations from those seven sites. The results indicate that the Bengal Fan grew rapidly during a time when global sea level changes, caused by the growth and decay of continental ice sheets, became more intense. Key Points: A novel regional chronostratigraphic model is used to investigate evolution of the lower Bengal Fan over the last 1.25 MyrSeven IODP site chronologies are constrained by lithostratigraphic, magnetostratigraphic, biostratigraphic, cyclostratigraphic, and seismic stratigraphic constraintsGrowth and intensification of channel‐levee systems at 8°N correspond to increases in the amplitude of Pleistocene sea level changes [ABSTRACT FROM AUTHOR]

To learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly <1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly "pseudo‐single‐domain" behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank. Plain Language Summary: Critical evidence documenting Earth's evolution has been provided by measuring the magnetic field generated by a magnetized ocean crust. Most of this field comes from the uppermost crust produced by seafloor volcanic eruptions. However, sometimes important contributions come from the lower crust that is not commonly exposed on the seafloor. To better understand these lower crustal contributions, rock samples were collected via ocean drilling at Atlantis Bank in the Indian Ocean. Previous magnetic work on this type of rock has mostly been limited to measurements that provide an average of all magnetic minerals present. In this study, we perform more detailed analyses and are able to link certain magnetic behaviors with specific populations of the magnetic mineral magnetite. In one form, magnetite is present in relatively large (tens of microns) oxide minerals with fine‐scale compositional variations. Magnetite is also present as tiny particles enclosed within other minerals (plagioclase and pyroxene and/or olivine). Because rock layers containing the large oxides are relatively close to the seafloor at Atlantis Bank, and because they contain a large quantity of magnetite, they will contribute most strongly to any magnetic field measurements made close to the seafloor. Key Points: Magnetite in Atlantis Bank gabbros is present as single‐domain inclusions in silicate minerals and as intergrowths in coarse oxidesFine‐scale lamellar intergrowths in the coarse oxides give rise to pseudo‐single‐domain‐like behavior and likely dominates near‐bottom anomaliesThermomagnetic data are consistent with high‐temperature hydrothermal alteration in the uppermost 200–500 m [ABSTRACT FROM AUTHOR]

The International Ocean Discovery Program Expedition 350 drilled between two Izu rear‐arc seamount chains at Site U1437 and recovered the first complete succession of rear‐arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear‐arc seamounts came into existence at this site, were recovered in the deepest part of the hole (∼15–16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear‐arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear‐arc seamount chains. It is suggested that following the opening of the Shikoku back‐arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at ~ 9 Ma by the start of rear‐arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back‐arc Basin opening. [ABSTRACT FROM AUTHOR]

Recognition of coring‐induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (Kmin) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring‐induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2) Kmin axes of sediments with upward‐arching beds are deflected out of the splitting face of the working half, and (3) suck‐in sediments are characterized by vertical Kmax axes. These deformation‐dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low‐coercivity, vertical, drilling‐induced overprint, we observed a high‐coercivity component that is likely a composite of the primary magnetization with a demagnetization‐resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring‐induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies. Key Points: Deformation‐dependent AMS patterns are artifacts of coring‐induced disturbance in poorly lithified sediment cores, IODP Expedition 362Primary paleomagnetic record is preserved in sediments with normal AMS fabric but overprinted by drilling in those with distorted AMSAMS measurements offer a potential tool for identifying core deformation in soft sediments and verifying fidelity of paleomagnetic record [ABSTRACT FROM AUTHOR]

Distinct differences were observed in geochemical signatures in sediments from two sites drilled in the upper plate of the Costa Rica margin during Integrated Ocean Drilling Program (IODP) Expedition 334. The upper 80 m at Site U1379, located on the outer shelf, shows pore water non‐steady state conditions characteristic of a declining methane flux. These contrast with analyses of the upper sediment layers at the middle slope site (U1378) that reflect steady state conditions. Distinct carbonate‐rich horizons up to 11 meters thick were recovered between 63 and 310 meters below seafloor at Site U1379 but were not found at Site U1378. The carbonates and dissolved inorganic carbon from Site U1379 have a depleted carbon stable isotope signal (up to −25‰) that indicates anaerobic methane oxidation. This inference is further supported by distinct δ34S‐pyrite and magnetic susceptibility records that reveal fluctuations of the sulfate‐methane transition in response to methane flux variations. Tectonic reconstructions of this margin document a marked subsidence event after arrival of the Cocos Ridge, 2.2 ± 0.2 million years ago (Ma), followed by increased sedimentation rates and uplift. As the seafloor at Site U1379 rose from ~2,000 m to the present water depth of ~126 m, the site moved out of the gas hydrate stability zone at ~1.1 Ma, triggering upward methane advection, methane oxidation, and the onset of massive carbonate formation. Younger carbonate occurrences and the non‐steady state pore water profiles at Site U1379 reflect continued episodic venting likely modulated by changes in the underlying methane reservoir. Plain Language Summary: Several carbonate‐rich layers, up to 11 m in thickness, were recovered in sediments collected at Site U1379 in the outer shelf of the Costa Rica margin, offshore the Osa Peninsula. These layers coincide with minima in the magnetic mineral assemblages. To investigate the processes and mechanisms that led to the formation of these carbonates, we used geochemical and sedimentological analyses of pore water and solid phase samples collected at two sites (U1378 and U1379) during the Integrated Ocean Drilling Program (IODP) Expedition 334. Our results, in the context of available dating and tectonic reconstructions, indicate that the discrete carbonate layers formed near the seafloor driven by anaerobic oxidation of methane, which was released from deeper sediments. The methane discharge events can be related to the subsidence and uplift history of the margin following the subduction of the Cocos Ridge beneath the Caribbean plate. Methane discharge at Site U1379 began ~1.1 Ma, when the ongoing margin uplift brought this site to a depth shallower than that where gas hydrates are stable, breaking a permeability barrier that had kept methane trapped within the sediment. Since then, methane continued to be released in discrete events modulated by characteristics of the gas reservoir. Key Points: Authigenic carbonate formation documents episodic methane ventingFluid release events respond to uplift and subsidence off Costa RicaDiagenetic changes impact sedimentary magnetic signal in non‐steady state system [ABSTRACT FROM AUTHOR]

Non‐skeletal carbonate grains are classically interpreted to form in shallow, tropical environments. Peloids deposited in deep, subtropical marine conditions are poorly studied. IODP site U1460 on the subtropical Carnarvon Ramp (Southwest Shelf of Australia) recovered a nearly continuous Pliocene to Recent record of outer shelf and slope sediments. The relative abundance of peloids varies between 0% and 67% of the fine to medium sand fraction, and contributes on average ~4% of all grains. The origin and composition of these peloids were investigated using scanning electron microscopy equipped with an energy‐dispersive X‐ray spectrometer, light microscopy, X‐ray diffraction and stable isotope analysis. The peloids have a uniform size and shape and are interpreted as faecal pellets. They are mainly composed of skeletal fragments such as ascidian spicules, planktic foraminifera and sponge spicules in a mud‐sized matrix containing abundant coccolith plates. Mineralogical analysis show that the pellets consist of aragonite, calcite and dolomite. The pellets have an identical mineralogical composition and skeletal assemblage as the surrounding matrix, indicating that they have formed in situ. They occur more abundantly during interglacials when the site was situated in deeper waters below the swell wave base, presumably because the pellets were protected from disintegration and therefore available for cementation. The presence of framboidal pyrite within the pellets indicates bacterial sulphate reduction (BSR). The reduction of iron by hydrogen sulphide produced during BSR decreases the pH and likely explains the observed aragonite dissolution. Aragonite dissolution likely increases the alkalinity, and in consequence causes the precipitation of calcite and dolomite cements. It is suggested here that pellets are hardened due to this early cementation close to the sea floor increasing the potential for preservation in the fossil record. [ABSTRACT FROM AUTHOR]

International Ocean Discovery program (IODP) Site U1438 is located within the Amami‐Sankaku Basin, ~50 km SW of the Kyushu‐Palau Ridge and ~500 km SE of the present‐day volcanic front of the Kyushu‐Ryukyu arc. Thirty‐eight rhyolitic tephra layers (1–15 cm thick) deposited over the last 4 Myr were recovered at this site. Representative ash layers dominated by unaltered, colorless glass shards reaching 100–300 μm in size, in association with rare pyroxene, amphibole, biotite, and zircon, were sampled for isotopic analysis. The isotopic compositions of the tephra have a narrow range in 87Sr/86Sr (0.704 to 0.706), 206Pb/204Pb (18.32 to 18.46), 207Pb/204Pb (15.57 to 15.62), and 208Pb/204Pb (38.46 to 38.75) and are more variable in εNd (−3.5 to +4.8) and εHf (+2.0 to +13.3). This record indicates the tephra were not sourced from the Izu‐Bonin‐Mariana arc or from Central Japan but derived from felsic volcanism from the Kyushu‐Ryukyu arc. The 4.8‐km‐deep Amami‐Sankaku Basin was located up to 600–900 km from the Kyushu‐Ryukyu arc over the last 4 Ma, which is significantly more distal than other drill sites from which extensive tephra layers sourced from SW Japan have been recovered. Site U1438 tephra are thus likely related to widely distributed (M > 6) rhyolitic eruptions. We propose the unique tephra record and high‐precision isotope analysis of recovered tephra from Site U1438 can be used as an important marker to identify submerged calderas within the Ryukyu arc and/or constrain the history of activity (>1 Ma) of major calderas‐forming eruptions within the Kyushu‐Ryukyu arc. Key Points: IODP Site U1438 records a unique 4‐Ma history of rhyolite eruptions from the Kyushu‐Ryukyu arcExplosive (>6 M) eruptions at 4 Ma, 2–2.5 Ma and <1 Ma in southwest Japan recorded in deep ocean basin (Amami‐Sankaku Basin)Radiogenic isotopes of tephra might be key to distinguish the source of eruptive products [ABSTRACT FROM AUTHOR]

Subseafloor sulfate concentrations typically decrease with depth as this electron acceptor is consumed by respiring microorganisms. However, studies show that seawater can flow through hydraulically conductive basalt to deliver sulfate upwards into deeply buried overlying sediments. Our previous work on IODP Site C0012A (Nankai Trough, Japan) revealed that recirculation of sulfate through the subducting Philippine Sea Plate stimulated microbial activity near the sediment–basement interface (SBI). Here, we describe the microbial ecology, phylogeny, and energetic requirements of population of aero‐tolerant sulfate‐reducing bacteria in the deep subseafloor. We identified dissimilatory sulfite reductase gene (dsr) sequences 93% related to oxygen‐tolerant Desulfovibrionales species across all reaction zones while no SRB were detected in drilling fluid control samples. Pore fluid chemistry revealed low concentrations of methane (<0.25 mM), while hydrogen levels were consistent with active bacterial sulfate reduction (0.51–1.52 nM). Solid phase total organic carbon (TOC) was also considerably low in these subseafloor sediments. Our results reveal the phylogenetic diversity, potential function, and physiological tolerance of a community of sulfate‐reducing bacteria living at ~480 m below subducting seafloor. [ABSTRACT FROM AUTHOR]

Carbonate veins in the igneous basement and in the lithified sedimentary cover of the Cocos Ridge at International Ocean Discovery Program (IODP) Hole 344‐U1414A reveal the hydrologic system and fluid‐rock interactions. IODP Hole 344‐U1414A was drilled on the northern flank of the Cocos Ridge and is situated 1 km seaward from the Middle America Trench offshore Costa Rica. Isotopic and elemental compositions were analyzed to constrain the fluid source of the carbonate veins and to reveal the thermal history of Hole 344‐U1414A. The formation temperatures (oxygen isotope thermometer) of the carbonate veins in the lithified sedimentary rocks range from 70 to 92 °C and in the basalt from 32 to 82 °C. 87Sr/86Sr ratios of the veins in the altered Cocos Ridge basalt range between 0.707307 and 0.708729. The higher ratios are similar to seawater strontium ratios in the Neogene. 87Sr/86Sr ratios lower 0.7084 indicate exchange of Sr with the igneous host rock. The calcite veins hosted by the sedimentary rocks are showing more primitive 87Sr/86Sr ratios <0.706396. The isotopic compositions indicate seawater, modified into a hydrothermal fluid by subsequent heating, as the main fluid source. Low‐temperature alteration and the presence of a high‐temperature fluid resulted in different carbonate precipitates forming up to several cm thick veins. The geochemical data combined with age data of the sedimentary rocks suggest intraplate seamount volcanism in the area between the Galapagos hot spot and the Cocos Island as an additional heating source, after the formation of the Cocos Ridge at the Galapagos hot spot. Key Points: Oxygen and strontium isotope ratios of carbonate veins suggest seawater and hydrothermally modified seawater as fluid sourceIsotopic and elemental compositions reveal fluid‐rock interactions in the Cocos Ridge basalt and in the lithified sedimentary rocksIntraplate seamount volcanism in the area between the Galapagos hot spot and the Cocos Island acted as an additional heating source [ABSTRACT FROM AUTHOR]

Abstract: Modern seafloor hydrothermal systems are unique environments in which many of the Earth's reservoirs, including the hydrosphere, biosphere, and geosphere, dynamically interact. Analysis of spatially constrained sulfur isotope compositions from fluids and hydrothermal precipitates within the discharge zone of a volcanogenic system can be used to trace the interactions between the various isotopically distinct sulfur reservoirs that result in the formation of hydrothermal massive sulfide deposits. Here we present in situ sulfur isotope results from laterally and vertically constrained euhedral hydrothermal pyrite from the Iheya North hydrothermal system in the Okinawa Trough, which was investigated during the Integrated Ocean Drilling Program Expedition 331. Hydrothermal pyrite at the North Big Chimney yields δ34S values of ~+11.9 ± 1.1‰ (1σ), which are near identical to the δ34S composition of the vent fluid. Outward, ~150 and ~450 m from North Big Chimney, hydrothermal pyrite within drill core yields δ34S equal to +10.9 ± 1.3‰ (1σ) and +7.0 ± 3.8‰ (1σ), respectively, showing a shift in isotopic composition away from the main vent site. This evolution to a lighter and more scattered isotopic signature of hydrothermal pyrite (which is easily identifiable from biogenic pyrite) is interpreted to indicate that the hydrothermal fluid leached sulfides (formed previously by biogenic processes) from the surrounding sedimentary strata. As the most significant metal enrichments (Fe, Zn, Cu, Bi, Tl, and Cd) are associated with samples that contain average hydrothermal pyrite δ34S values similar to δ34S of the vent fluid, we demonstrate that sulfur isotopes can vector toward metals in seafloor massive sulfide deposits. [ABSTRACT FROM AUTHOR]

Abstract: The inorganic portion of tuffaceous mud and mudstone in an oceanic island arc can be mostly volcanic in origin. Consequently, a large volume of submarine volcaniclastic material is as extremely fine‐grained as products of subaerial eruptions (<100 µm). Using results of IODP Expedition 350 in the Izu rear arc, we show that such material can accumulate at high rates (12–20 cm/k.y.) within 13 km of the nearest seamount summit and scores of km behind the volcanic front. The geochemistry of bulk, acid‐leached mud, and its discrete vitriclasts, shows that >75% of the mud is volcanic, and that most of it was derived from proximal rear arc volcanic sources. It faithfully preserves integrated igneous geochemical information about arc evolution in much the same way that terrigenous shales track the evolution of continental crust. In addition, their high sedimentation rate enables high resolution study of climate cycles, including the effects of Pleistocene glaciation on the behavior of the Kuroshio Current in the Shikoku Basin south of Japan. [ABSTRACT FROM AUTHOR]

Abstract: Provenance studies of widely distributed tephras, integrated within a well‐defined temporal framework, are important to deduce systematic changes in the source, scale, distribution, and changes in regional explosive volcanism. Here, we establish a robust tephrochronostratigraphy for a total of 157 marine tephra layers collected during IODP Expedition 352. We infer at least three major phases of highly explosive volcanism during Oligocene to Pleistocene time. Provenance analysis based on glass composition assigns 56 of the tephras to a Japan source, including correlations with 12 major and widespread tephra layers resulting from individual eruptions in Kyushu, Central Japan, and North Japan between 115 ka and 3.5 Ma. The remaining 101 tephras are assigned to four source regions along the Izu‐Bonin arc. One, exclusively assigned to the Oligocene age, is proximal to the Bonin Ridge islands; two reflect eruptions within the volcanic front and back‐arc of the central Izu‐Bonin arc, and a fourth region corresponds to the Northern Izu‐Bonin arc source. First‐order volume estimates imply eruptive magnitudes ranging from 6.3 to 7.6 for Japan‐related eruptions and between 5.5 and 6.5 for IBM eruptions. Our results suggest tephras between 30 and 22 Ma reflect a subtly different Izu‐Bonin chemical signature compared to the recent arc. After a ∼9 Ma gap in eruption, tephra supply from the Izu‐Bonin arc predominated from 15 to 5 Ma, and finally a subequal mixture of tephra sources from the (palaeo)Honshu and Izu‐Bonin arcs occured within the last ∼5 Ma. [ABSTRACT FROM AUTHOR]

Strength and slip behaviors of subduction megathrusts are controlled by the physical properties and stress state not only of the fault zones themselves but also the adjacent wall rocks in overriding and subducting plates. In the Nankai Trough, a 3 km deep riser borehole at Integrated Ocean Drilling Program Site C0002 was drilled into the Kumano fore-arc basin and the underlying accretionary prism. We quantify the full in situ stress tensor and pore pressure at Site C0002 by integrating drilling data, geophysical logging, and downhole measurements using empirical relations that describe sediment compaction behavior as a function of loading path. The Kumano Basin is loaded in uniaxial vertical strain conditions, whereas the prism below is loaded with greater differential stress, has a modestly elevated pore pressure, and lies in a strike-slip faulting regime. This can be reconciled with slip on the megathrust fault if the horizontal stresses vary with time or depth and/or if shear stress along the megathrust is low. [ABSTRACT FROM AUTHOR]

The paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off-Kumano region of the Nankai Trough and penetrate the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP expeditions 338 and 348: Nankai Trough seismogenic zone experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42-70°C at 1200-1300 mbsf, 44-100°C at 1600-2400 mbsf, and 56-115°C at 2600-3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (∼60-150°C/km) recorded at the hanging- and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (∼30-40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate. [ABSTRACT FROM AUTHOR]

This study presents a method of two-dimensional scanning electron microscope image analysis that directly quantifies microporosity abundance in clay-rich, fine-grained sediments. The method is novel in that it is specifically designed to circumvent the challenge to porosity quantification posed by mineral surface charging and topographical artifacts created during Ar-ion cross-section polishing. It utilizes the finding that differences in circularity values can be used to distinguish micropores from blemishes in a thresholded image. This method is powerful because it is fast and provides a direct microporosity estimation technique to augment or replace experimental data. The pore size range to which the method is applicable is clear and can be selected depending on the application of the analysis. When used appropriately, the method can be implemented on microporous sediments and sedimentary rock in general. The method is developed using marine muds of Pliocene and Miocene ages from the Nankai margin (burial depths from approximately 200 to 1100 m). The close match between imaging-derived microporosity and bulk N2 microporosity measurements shows that porosity in these young and relatively shallowly buried sediments is dominated by pores of sizes that can be imaged by scanning electron microscopy. In Kumano, forearc basin sediments of the Nankai Trough, results of this method show a significant increase in microporosity with burial depth, probably due to microporosity preservation during compaction and possibly early volcanic ash diagenesis. [ABSTRACT FROM AUTHOR]

During the Integrated Ocean Drilling Program ( IODP) Expedition 338, several methods were tested for the extraction of interstitial water in consolidated, low-porosity deep-sea sediments from Site C0002 in the Kumano Basin. On the basis of those tests, we propose a modified ground rock interstitial normative determination ( GRIND) method of extraction of interstitial water. In separate runs of the new method, sediment samples were ground in a ball mill with either ultrapure water or a solution of HNO3. The interstitial water was then extracted with a conventional squeezer. Sufficient solution was extracted by this method to analyse most major and a few minor components of interstitial water that were comparable to those previously reported for samples extracted by the conventional squeezing method. The new method requires much smaller amounts of sediment than that of the conventional method and will be useful for analysis of samples recovered during super-deep drilling programmes. [ABSTRACT FROM AUTHOR]

Oxygenation of the ocean is presumed to be an important factor stimulating the evolution of multicellular animals. The appearance of the Ediacaran-type biota ( ca 575 Ma) was assigned to the aftermath of the Gaskiers glaciation ( ca 580 Ma), when substantial oceanic oxygenation is believed to have started. However, several lines of evidence reveal that at least sponges evolved before this oxygenation. For understanding the first stage of animal evolution, we propose the hypothesis that Dissolved Organic Carbon (DOC) Stimulated the evolution for Animal Multicellularity (DOXAM). Recent geochemical studies of the Ediacaran sedimentary sequences have indicated that a substantial DOC mass was developed in the stratified ocean after the Marinoan glaciation (655-635 Ma), and this was supported by the inorganic and organic carbon isotope profiles of the Doushantuo Formation in South China. The DOC mass was an oxygen consumer in the water column; however, it could have provided a food source for filter-feeding animals such as sponges and cnidarians, and established a primitive food-web. Such an ecological structure is recognized in modern deep-sea coral mounds. Results from the integrated ocean drilling program (IODP) Expedition 307 for a mound in northeastern Atlantic suggested that organic carbon suspended around the density boundary in the water column is the key feature to feed the heterotrophic deep-sea coral community. Our hypothesis is consistent with the fact that the two most primitive animal phyla (Porifera and Cnidaria) are filter feeders. The evolution of filter feeding ecosystems removed the DOC mass and may have contributed to ocean oxygenation in the terminal Neoproterozoic when animal evolution passed into the second stage, with the appearance of bilaterians. [ABSTRACT FROM AUTHOR]

Multichannel seismic studies performed at fast‐spreading mid‐ocean ridges revealed the presence of a thin (tens to hundreds of meters high), narrow (< 1–2 km wide) axial melt lens (AML) in the mid‐crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program (IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast‐spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML (Teagle et al., 2006; Koepke et al., 2008). This can be regarded as rosetta stone to answer long‐standing questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the mid‐crust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz‐bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low‐degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization (Erdmann et al., 2015; Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high‐viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high‐Cl apatite core indicates assimilation of high‐Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater‐derived fluids at a high temperature (Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater‐derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids (Zhang et al., 2017c). [ABSTRACT FROM AUTHOR]