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Rock varnish is a manganiferous dark coating accreted on subaerially exposed rocks in drylands. It often contains a layered microstratigraphy that records past wetness variations. Varnish samples from latest Pleistocene and Holocene geomorphic features in the Lake Turkana basin, East Africa display a regionally replicable microstratigraphy record of Holocene millennial-scale wetness variability and a broad interval of wetter conditions during the African Humid Period (AHP). Three major wet pulses in the varnish record occurred during the generally wet interval of the early Holocene (11.5–8.5 ka) when the lake attained its maximum high stand (MHS) at 455–460 m. A >23 m drop from the MHS occurred between 8.5 and 8 ka. Subsequently two additional wet pulses occurred during the early to middle Holocene (8–5 ka) when the lake occupied its secondary high stand at 445 m. Collectively, these five wet phases represent an extended wet interval coincident with the AHP in the region. One moderate wet phase occurred during the subsequent climatic transition from the humid to arid regime (5–4.3 ka) after the lake level dropped rapidly from 445 m to <405 m. Five minor wet phases took place during the overall arid period of the late Holocene (4.3–0 ka) when the lake level oscillated below 405 m. These findings indicate that the AHP terminated rapidly around 5 ka in the Turkana basin in terms of lake level drop, but the regional shift in relative humidity from the AHP mode to its present-day condition lagged for about 700 years until 4.3 ka, hinting at a gradual phasing out in terms of moisture condition. These findings further suggest that Lake Turkana overflowed intermittently into the Nile drainage system through its topographic sill at 455–460 m during the early Holocene and has become a closed-basin lake thereafter for the past 8 ky. [ABSTRACT FROM AUTHOR]

The Antarctic ice sheet blankets >99% of the continent and limits our ability to study how subglacial geology and topography have evolved through time. Ice-rafted dropstones derived from the Antarctic subglacial continental interior at different times during the late Cenozoic provide valuable thermal history proxies to understand this geologic history. We applied multiple thermochronometers covering a range of closure temperatures (60-800 °C) to 10 dropstones collected during Integrated Ocean Drilling Program (IODP) Expedition 318 in order to explore the subglacial geology and thermal and exhumation history of the Wilkes Subglacial Basin. The Wilkes Subglacial Basin is a key target for study because ice-sheet models show it was an area of ice-sheet retreat that significantly contributed to sea-level rise during past warm periods. Depositional ages of dropstones range from early Oligocene to late Pleistocene and have zircon U-Pb or 40Ar/39Ar ages indicating sources from the Mertz shear zone, Adélie craton, Ferrar large igneous province, and Millen schist belt. Dropstones from the Mertz shear zone and Adélie craton experienced three cooling periods (1700-1500 Ma; 500-280 Ma; 34-0 Ma) and two periods of extremely slow cooling rates (1500-500 Ma; 280-34 Ma). Low-temperature thermochronometers from seven of the dropstones record cooling during the Paleozoic, potentially recording the Ross or Pan-African orogenies, and during the Mesozoic, potentially recording late Paleozoic to Mesozoic rifting. These dropstones then resided within ~500 m of the surface since the late Paleozoic and early Mesozoic. In contrast, two dropstones deposited during the mid-Pliocene, one from the Mertz shear zone and one from Adélie craton, show evidence for localized post-Eocene glacial erosion of =2 km. [ABSTRACT FROM AUTHOR]

The Picabo volcanic field is one of the key silicic volcanic fields in the time-transgressive track of the Yellowstone hotspot. The Picabo volcanic field is also one of the most poorly defined volcanic fields along the track of the Yellowstone hotspot. Determining the age and areal extent of the Picabo volcanic field ignimbrites is one of the primary objectives of this study. In our effort to correlate ignimbrites within the Picabo volcanic field as well as identify those from the neighboring Twin Falls and Heise volcanic fields, we present new petrographic, 40Ar/39Ar, and paleomagnetic data. With these data, we correlated several ignimbrites within the Picabo volcanic field. In some cases, we correlate units previously thought to be in the Picabo volcanic field to older volcanic fields. This includes the Picabo Tuff, which we suggest originates from the Twin Falls volcanic field rather from its namesake volcanic field. The first and best documented major silicic eruption of the volcanic field, the Arbon Valley Tuff, is also the largest ignimbrite in the Picabo volcanic field. There is disagreement as to whether the Arbon Valley Tuff is the result of a single ignimbrite eruption or multiple eruptions. We previously have suggested that the Arbon Valley Tuff is the result of two eruptions, one at 10.41 ± 0.01 Ma and the other at 10.22 ± 0.01 Ma (Anders et al., 2014). Those combining radiometric dates into a single eruption age report ages of 10.2 Ma, 10.27 ± 0.01 Ma, 10.34 ± 0.03 Ma, and 10.44 ± 0.27 Ma. We also suggest the final eruption of the Picabo volcanic field was the tuff of American Falls dated at 7.58 ± 0.02 Ma. Estimates of the location of Picabo volcanic field have been used to mark a major change in the migration rate of the Yellowstone-Snake River Plain silicic volcanic system. Based on our new data, we found only minor changes of the boundaries of the Picabo volcanic field from previous studies. Using the age of the Arbon Valley Tuff (10.41 Ma), we calculated an extension-corrected migration rate of 2.27 ± 0.2 cm/yr between the position of the Picabo volcanic field and that of the Yellowstone volcanic field over the past ~10 m.y. This estimate is close to the extension corrected 2.38 ± 0.21 cm/yr value based on the migration of the hotspot deformation field. These rates are consistent with independent estimates of North American plate velocity over the past 10 m.y. and therefore consistent with a fixed reference frame for the Yellowstone hotspot. These results stand in contrast with several recent models for the evolution of the Yellowstone-Snake River Plain volcanic system. We also discovered a new ignimbrite from the Heise volcanic field, the 4.37 ± 0.08 Ma tuff of Birch Creek Sinks, in core from the U.S. Geological Survey (USGS) borehole 2-2A, which now represents the youngest outflow ignimbrite of the Heise volcanic field. Although recently, several intracaldera ignimbrites younger than 4 Ma have been identified in the volcanic field, the age range of outflow ignimbrites from the Heise volcanic field is now extended from 6.66 Ma to at least 4.37 Ma. [ABSTRACT FROM AUTHOR]

Abstract Tropical rainforests have strongly influenced Earth systems throughout their evolutionary history. Plant taxa found in the Fotan Group of Fujian Province indicate that an Asian tropical rainforest prevailed in southeastern China during the Miocene. Abundant insect-bearing ambers co-occur with diagnostic plant fossils in outcrop. Fotan Group fossils demonstrate that highly diverse tropical ecosystems existed during the early Neogene in southeastern China. This study reports a robust 40Ar/39Ar age of 14.7 ± 0.4 Ma for the Fotan fossils. This age constrains interpretations of 1) when tropical rainforest ecosystems developed in China, 2) whether the Fotan flora temporally overlapped with the Asian tropical forest in India, and 3) whether the Middle Miocene Climatic Optimum triggered expansion of tropical rainforests in Asia. A precise age for Fotan units provides important constraints on the biogeographic history of the field area and paleographic models linking Asian and Indian biota. Highlights • We report robust 40Ar/39Ar age for a Neogene tropical rainforest in China. • Our study allows to understand plant expansion and the Middle Miocene Climatic Optimum. • Our age data indicate a biodiversity hotspot existed in southeastern China at 14.7 Ma. [ABSTRACT FROM AUTHOR]

Monogenetic volcanism in the Maniototo Basin in the Dunedin Volcanic Group was unusual because the eroded lavas, plugs and dikes comprise spatially and temporally restricted silica-saturated and silica-undersaturated magmas. 40Ar/39Ar dating coupled with existing data and field relationships indicate that volcanism was focused at ∼11 Ma. Major and trace element data show the volcanic rocks to have mafic compositions and little evidence for fractionation and/or crustal contamination, but to have been variably altered. The volcanic rocks comprise an alkaline group and an alkaline to subalkaline transitional group, with one intermediate unit. With the exception of one flow, the transitional group has more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb than the alkaline group, as well as smaller Ti and K primitive mantle-normalised anomalies. Overall, the different magma compositions cannot be related to varying degrees of melting of equivalent mantle sources. Trace element ratios indicate that residual Ti and K-bearing mantle phases (amphibole, phlogopite) probably played a major role in the source of alkaline group but a smaller role in the transitional group. Therefore, the varying compositions of the Maniototo volcanic rocks relates to melting of a heterogeneous lithospheric mantle, or/and varying degrees of interaction with mantle hydrous metasomes as the magmas ascended. [ABSTRACT FROM AUTHOR]

Geochemical provenance studies of glacial-marine sediments provide a powerful approach to describe subglacial geology, sediment transport pathways, and past ice sheet dynamics. The marine-based West Antarctic Ice Sheet (WAIS) is considered highly vulnerable to ocean warming and sea level rise that is likely to cause its rapid and irreversible retreat. Studies of its past response to climate change are hence essential for projecting its future behaviour. The application of radiogenic and trace element provenance studies for past ice sheet reconstructions requires surveying the geographic variability of geochemical compositions of glaciomarine sediments. In this study, we characterize the provenance of the detrital fraction of 67 Late Holocene marine sediment samples collected off the Pacific margin of West Antarctica (60°W to 160°W), including 40 Ar/ 39 Ar ages of individual hornblende and biotite grains (>150 μm), as well as Sr and Nd isotope and trace element composition of the fine-grained (<63 μm) sediment fraction. Overall, this approach allows differentiating West Antarctica into five source regions: the Antarctic Peninsula, Bellingshausen Sea, Amundsen Sea, Wrigley Gulf-Hobbs Coast and Sulzberger Bay. Minor geochemical variability is found within each individual sector due to local variability in onland geology. 40 Ar/ 39 Ar ages of iceberg-rafted hornblende and biotite grains record primarily Carboniferous to Lates Quaternary ages (~0 to 380 Ma), with a notable age peak of ~100 Ma, associated with plutonic intrusions or deformation events during the mid-Cretaceous. Permian-Jurassic 40 Ar/ 39 Ar ages are widespread in the Amundsen Sea sector, marking episodes of large-volume magmatism along the long-lived continental margin. Metasedimentary rocks and Late Cenozoic alkali basalts in West Antarctica cannot be detected using detrital hornblende and biotite 40 Ar/ 39 Ar ages due to the absence or small grain-size (i.e. <150 μm) of these minerals in such rocks. These sources can however be readily recognized by their fine-grained geochemical composition. In addition, geographic trends in the provenance from proximal to distal sites provide insights into major sediment transport pathways. While the transport of fine-grained detritus follows bathymetric cross-shelf troughs, the distribution of iceberg-rafted grains shows influence by transport in the Antarctic Coastal Current. Our study provides the first systematic geochemical characterisation of sediment provenance off West Antarctica, and highlights the importance of combining multiple provenance approaches in different size fractions of glacial-marine sediments, and paves the way to investigate past WAIS dynamics. [ABSTRACT FROM AUTHOR]

Past interglacial climates with smaller ice sheets offer analogs for ice sheet response to future warming and contributions to sea level rise; however, well-dated geologic records from formerly ice-free areas are rare. Here we report that subglacial sediment from the Camp Century ice core preserves direct evidence that northwestern Greenland was ice free during the Marine Isotope Stage (MIS) 11 interglacial. Luminescence dating shows that sediment just beneath the ice sheet was deposited by flowing water in an ice-free environment 416 ± 38 thousand years ago. Provenance analyses and cosmogenic nuclide data and calculations suggest the sediment was reworked from local materials and exposed at the surface <16 thousand years before deposition. Ice sheet modeling indicates that ice-free conditions at Camp Century require at least 1.4 meters of sea level equivalent contribution from the Greenland Ice Sheet. [ABSTRACT FROM AUTHOR]

Lead isotope ratios have been determined in multiple feldspar grains from hand samples of impact melt rock at the Manicouagan and Sudbury impact structures in Canada. The results reveal an extreme range of isotope values. This indicates that melt sheets are not homogeneous with respect to Pb at the millimeter scale. Such heterogeneity is significantly larger than that seen in non-impact-generated igneous rocks. Individual Pb isotope ratios of feldspars from Martian shergottites show a large range in 206Pb/204Pb values within one sample, more similar to the terrestrial impact melt sheets than to nonimpact igneous rocks. We suggest crystallization from impact melt sheets rather than volcanic sources as a petrogenetic model for some of the Martian shergottites. [ABSTRACT FROM AUTHOR]

Tills from moraines adjacent to major ice streams of the Weddell Sea Embayment contain distinct detrital zircon (n = 5304) and K-bearing mineral age populations (n = 323) that, when combined with pebble composition data, can be used to better understand Antarctica's subglacial geology and ice sheet history. Till representing the Institute, Foundation, Academy, Recovery and Slessor Ice Streams each have distinct detrital zircon age populations. Detrital Ar-Ar ages are mostly younger than zircon ages, and distinctive populations include 270--300 Ma (Institute), 170--190 Ma (Foundation), and 1200--1400 Ma (Recovery), which are not easily explained by known outcrops. Pebble fractions of the Foundation and Academy tills are most diverse with up to >40% exotic erratics. The southern side of the Recovery Glacier has fossiliferous limestone erratics. Mixing models created using a nonlinear squares curve-fitting approach were developed to evaluate contributors of zircons to Foundation Ice Stream till. These model results and pebble lithology data both indicate that unexposed (subglacial) bedrock is mixed with exposed rocks to produce the observed till. Notably, the model required limited local Patuxent Formation input to the Foundation till's zircon population. Our data suggest that sandstones underlie the Foundation Ice Stream and Recovery Glacier troughs, which has a bearing on basal ice flow conditions and results in geological controls on ice stream location. This geo- and thermo-chronological characterization of the ice streams will enable ice-rafted debris in Weddell Sea marine sediments to be traced back to its sources and interpreted in terms of ice stream dynamics. [ABSTRACT FROM AUTHOR]

Creeping faults are difficult to assess for seismic hazard because they may participate in rupture even though they likely cannot nucleate large earthquakes. The creeping central section of the San Andreas fault in California (USA) has not participated in a historical large earthquake; however, earthquake ruptures nucleating in the locked northern and southern sections may propagate through the creeping section. We used biomarker thermal maturity and K/Ar dating on samples from the San Andreas Fault Observatory at Depth to look for evidence of earthquakes. Biomarkers show evidence of many earthquakes with displacements >1.5 m in and near a 3.5-m-wide patch of the fault. We show that K/Ar ages decrease with thermal maturity, and partial resetting occurs during coseismic heating. Therefore, measured ages provide a maximum constraint on earthquake age, and the youngest earthquakes here are younger than 3 Ma. Our results demonstrate that creeping faults may host large earthquakes over longer time scales. [ABSTRACT FROM AUTHOR]

Abstract: The fossils of the Cretaceous Jehol Biota are magnificent, exquisitely preserved and extraordinarily diverse. Strata that may be correlative with the classic Jehol localities are known over much of Central and Eastern China, as well as Korea, Japan, Mongolia and Siberia. It has been previously hypothesized based on fossil assemblages that the Jehol Biota progressively expanded from the classic Jehol area, with younger representatives covering progressively larger geographic ranges. It has been interpreted that the youngest Jehol Biota bearing layers are in the East Anhui province of South China. Here we present robust 40Ar/39Ar dates of 129.0±0.2Ma for one rhyolite sample from the lower Longwanshan Formation and 127.1±1.3Ma and 128.0±0.9Ma for two volcanic samples from the upper Honghuaqiao Formation from the outcrops of Jehol fossils in East Anhui. Our age results indicate that fossils from these formations are time-equivalent to the second phase fossils from the lower Yixian Formation in the western Liaoning, NE China, and the deposition of these beds is at least 6Ma older than previously estimated. Therefore our results suggest that the hypothesis of the Jehol radiation and migration should be reevaluated. [Copyright &y& Elsevier]

Abstract: We present new data for the age of the Mono Lake Excursion at its type locality. Using the (U-Th)/He system on allanite, we dated Wilson Creek Ash 15 (Lajoie, 1968) to 38.7±1.2ka (2 SE). The new age for this ash supports the hypothesis (Kent et al., 2002; Zimmerman et al., 2006) that the Mono Lake Excursion is coincident with, and probably the same event as, the Laschamp Geomagnetic Excursion (40.4±2ka), an event that shares similar magnetic characteristics with the excursion identified at Mono Lake. We also estimate an allanite magma residence time of slightly less than 30ka based on 230Th/238U disequilibrium and the (U-Th)/He-based eruption age. [Copyright &y& Elsevier]

Abstract: The type section silts of the late Pleistocene Wilson Creek Formation at Mono Lake contain outsized clasts, dominantly well-rounded pebbles and cobbles of Sierran lithologies. Lithic grains >425μm show a similar pattern of variability as the >10mm clasts visible in the type section, with decreasing absolute abundance in southern and eastern outcrops. The largest concentrations of ice-rafted debris (IRD) occur at 67–57ka and 46–32ka, with strong millennial-scale variability, while little IRD is found during the last glacial maximum and deglaciation. Stratigraphic evidence for high lake level during high IRD intervals, and a lack of geomorphic evidence for coincidence of lake and glaciers, strongly suggests that rafting was by shore ice rather than icebergs. Correspondence of carbonate flux and IRD implies that both were mainly controlled by freshwater input, rather than disparate non-climatic controls. Conversely, the lack of IRD during the last glacial maximum and deglacial highstands may relate to secondary controls such as perennial ice cover or sediment supply. High IRD at Mono Lake corresponds to low glacial flour flux in Owens Lake, both correlative to high warm-season insolation. High-resolution, extra-basinal correlation of the millennial peaks awaits greatly improved age models for both records. [Copyright &y& Elsevier]

The explosive rhyolitic eruptions that define the track of the Snake River Plain--Yellowstone volcanism have produced a large volume of tephra found in late Miocene and younger basin-fill sediments throughout the western United States. Here we use 40Ar/39Ar isotopic dating, paleomagnetic analysis, major- and trace-element geochemistry, and standard optical techniques to establish regional tephra correlations. We focus on tephra deposits in three Neogene basins in spatially separated areas--Grand Valley, in eastern Idaho; Jackson Hole, in northwestern Wyoming; and the Granite Mountains area, in central Wyoming. These basins have experienced relatively continuous deposition from the late Miocene to the Holocene. We found tephra layers that directly tie the stratigraphy between all three basins. Using these correlations we found that basins experienced discrete pulses of extension separated by long periods of relative quiescence, the dates of which are staggered between basins. An early pulse of extension occurred in Grand Valley and Jackson Hole between 10.3 Ma and 16.33 Ma with a second pulse initiating between 4.54 Ma and 2.09 Ma. The Granite Mountains basin experienced a single pulse of extension sometime between 11.14 Ma and 6.75 Ma. These pulses of accelerated extension, along with evidence of similar pulses in other basins, present a pattern of west-to-east migration that we suggest is related to the Yellowstone hotspot. The later pulse of activity in Grand Valley and Jackson Hole corresponds to the migration of the North American Plate over the tail of the Yellowstone hotspot. We speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location, where the Columbia River Plateau Flood Basalt Province initiated in southeastern Oregon, to its outermost edge under central Wyoming. Our results are consistent with this model of a plume head, though not unique to it. The results of our study also indicate that previous suggestions that the rate of Snake River Plain explosive volcanism has decreased by a factor of 2 or 3 since emplacement of the middle Miocene Trapper Creek tufts likely underestimate post-Trapper Creek eruption rates. We have discovered a large number of previously unidentified post-middle Miocene major eruptive events, both as ash-flow tufts and as vitric air-fall tuffs. Recalculation to include these newly discovered events results in a rate of major eruptions that is fairly uniform until ca. 4.54 Ma. However, there is a substantial gap in major silicic eruptions in the interval between 4.54 Ma and 2.09 Ma, which we call the "post-Heise eruptive gap." With the exception of this gap, the rate of major eruptions on the Snake River Plain has been roughly constant since inception of the eastern Snake River Plain-Yellowstone volcanic track between 16 Ma and 17 Ma. [ABSTRACT FROM AUTHOR]

Abstract: The transfer of terrigenous sediments from Antarctica to the Southern Ocean results from glaciological processes and subsequent transport by ocean currents. Establishing the link between the composition of circumpolar sediments and the geology of Antarctica can provide important insights into past ice sheet and ocean current dynamics. Here we document the variability of Antarctic sediment sources using 40Ar/39Ar ages of individual detrital hornblende grains and bulk (<63 m) Sm/Nd isotope systematics from glacio-marine sediments from 29 cores surrounding Antarctica. High 143Nd/144Nd ratios and associated young Sm/Nd-model ages characterize sediments proximal to West Antarctica, while lower 143Nd/144Nd ratios and correspondingly older Sm/Nd-model ages are found in samples nearby East Antarctica. Detrital hornblende grains in West Antarctic sediments typically have 40Ar/39Ar ages younger than 200 Ma while East Antarctic hornblende grains yield 40Ar/39Ar ages that cluster in a predominant population centered on ∼500 Ma, reflecting the widespread late Neoproterozoic–early Cambrian (Pan-African and Ross) orogenies that affected East Antarctica. An exception comes from sediments adjacent to Wilkes Land, where abundant Mesoproterozoic and Paleoproterozoic hornblende grains suggest that this portion of the East Antarctic craton was largely buffered from Pan-African metamorphism. The geochemical characteristics of sediment sources are thus consistent with the geology of rock outcrops around the Antarctic perimeter. The combined Sm–Nd data and 40Ar/39Ar ages of circum-Antarctic glacio-marine sediments outline several geographic sectors with distinct provenance signals, thereby providing a base map for fingerprinting sediment sources from the Antarctic margin. [Copyright &y& Elsevier]

Abstract: Benthic foraminiferal δ 13C suggests that there was a net shift of isotopically light metabolic CO2 from the upper ocean into the deep ocean during the last glacial period. According to the ‘CaCO3 compensation’ hypothesis, this should have caused a transient drop in deep ocean CO3 2− that was eventually reversed by seafloor dissolution of CaCO3. The resulting increase in whole-ocean pH may have had a significant impact on atmospheric CO2, compounding any decrease that was due to the initial vertical CO2 shift. The opposite hypothetically occurred during deglaciation, when CO2 was returned to the upper ocean (and atmosphere) and deep ocean CO3 2− temporarily increased, followed by excess burial of CaCO3 and a drop in whole-ocean pH. The deep sea record of CaCO3 preservation appears to reflect these processes, with the largest excursion during deglaciation (as expected), but various factors make quantification of deep sea paleo-CO3 2− difficult. Here we reconstruct deep equatorial Pacific CO3 2− over the last glacial–interglacial cycle using benthic foraminiferal Zn/Ca, which is strongly affected by saturation state during calcite precipitation. Our data are in agreement with the CaCO3 compensation theory, including glacial CO3 2− concentrations similar to (or slightly lower than) today, and a Termination I CO3 2− peak of ∼25–30 μmol kg−1. The deglacial CO3 2− rise precedes ice sheet melting, consistent with the timing of the atmospheric CO2 rise. A later portion of the peak could reflect removal of CO2 from the atmosphere–ocean system due to boreal forest regrowth. CaCO3 compensation alone may explain more than one third of the atmospheric CO2 lowering during glacial times. [Copyright &y& Elsevier]

High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187Os/188Os, it has 186Os/188Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic 186Os/188Os, as well as radiogenic 187Os/188Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have 187Os/188Os ratios that are much higher than average UCC, but 186Os/188Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower 186Os/188Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the 187Os/188Os of seawater can change as sources and weathering conditions change, yet seawater 186Os/188Os varies only minimally. [Copyright &y& Elsevier]

The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth's formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ~0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful "age" from a measured "date" or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity. [ABSTRACT FROM AUTHOR]

The marine-based West Antarctic Ice Sheet (WAIS) is considered the most unstable part of the Antarctic Ice Sheet, with particular vulnerability in the Amundsen Sea sector where glaciers are melting at an alarming rate. Far-field sea-level data and ice-sheet models have pointed towards at least one major WAIS disintegration during the Late Quaternary, but direct evidence for past collapse(s) from ice-proximal geological archives remains elusive. In order to facilitate geochemical and mineralogical tracing of the two most important glaciers draining into the Amundsen Sea, i.e. Pine Island Glacier (PIG) and Thwaites Glacier (TG), we here provide the first multi-proxy provenance analysis of 26 seafloor surface sediment samples from Pine Island Bay. Our data show that the fingerprints of detritus delivered by PIG and TG are clearly distinct near the ice-shelf fronts of both ice-stream systems for all grain sizes and proxies investigated. Glacial detritus delivered by PIG is characterised by low ε Nd values (~−9), high 87Sr/86Sr ratios (~0.728), low smectite content (<10%), and hornblende and biotite grains with Late Permian to Jurassic (170–270 Ma) cooling ages. In contrast, glacigenic detritus delivered by TG is characterised by higher ε Nd values (~−4), lower 87Sr/86Sr ratios (0.714), higher smectite (20%) and kaolinite content (37%), biotite and hornblende grains with 40Ar/39Ar cooling ages of <40 Ma and ~115 Ma, and high content of mafic minerals. The geochemical and mineralogical fingerprints for PIG and TG reported here provide novel insights into sub-ice geology and allow us to trace both drainage systems in the geological past, under environmental conditions more similar to those envisioned in the next 50 to 100 years. [ABSTRACT FROM AUTHOR]

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of synthetic and natural reference materials for both 40Ar/39Ar and U-Pb techniques that permit both inter-laboratory and inter-technique comparisons. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein provides a potential natural Paleozoic reference for these two widely used radioisotopic systems. Here we report results for both radioisotopic systems, examining the suitability of this tonstein as a geochronologic reference. Sanidine crystals from the Fire Clay and co-irradiated monitors from eight irradiation positions were divided into eleven 40Ar/39Ar experiments. Single-grain sanidine 40Ar/39Ar analyses (n = 263) of the simplest 9 experiments have internal 2σ uncertainties at the ±1 Myr level (±0.3%), with a range of dates between ~315 and ~317 Ma (~1% precision), similar to the observed dispersion in the Fish Canyon sanidine monitor dates. Forty-one U-Pb analyses by the CA-ID-TIMS method on carefully selected single Fire Clay tonstein zircons have produced 206Pb/238U dates with an average 2σ precision of ±0.23 Myr (0.14%). Our Fire Clay preferred mean 40Ar/39Ar date of 315.36 ± 0.10 Ma (±1.10 Ma: fully propagated 2σ uncertainty, relative to a Fish Canyon age of 28.201 Ma) is consistent with our weighted mean 206Pb/238U zircon date of 314.629 ± 0.039 Ma (±0.35 Ma: fully propagated 2σ uncertainty; n = 27). The good single-crystal reproducibility of the sanidine data and the overall consistency between the two chronometers suggest that the tonstein holds promise as a Paleozoic age reference material. • Potential Paleozoic (Carboniferous) monitor standard for 40Ar/39Ar dating • Zircon selection based on morphology effectively screens out xenocrystic component. • 206Pb/238U zircon date of 314.629 ± 0.039 Ma (±0.35 Ma: 2σ total uncertainty) • Reproducible individual 40Ar/39Ar sanidine dates are within 1% of mean age. • 40Ar/39Ar sanidine date: 315.36 ± 0.10 Ma (±1.10 Ma: 2σ total, FCT: 28.201 Ma) [ABSTRACT FROM AUTHOR]

The terrestrial Yanliao Biota in northeastern China represents a Jurassic biodiversity hotspot. Abundant and diverse fossils include the oldest known representatives of several groups of plants, invertebrates, and vertebrates that thrived in a forest ecosystem under a humid and warm climate. Since the early 21st century, discoveries associated with the Yanliao Biota have significantly improved our understanding of Jurassic terrestrial ecosystems. However, a robust chronostratigraphic framework is required to calibrate the biota with key evolutionary events and to improve understanding of possible links between evolution and tectonism. In this paper, we report novel 40Ar/39Ar ages for two tuff layers (161.4 ± 0.8 Ma and 161.6 ± 0.6 Ma) positioned directly above the less-studied but salamander fossil-rich beds in Reshuitang, near Lingyuan, Liaoning Province, China. These new age data improve knowledge of the timing of diversification of early salamanders. We also review all reported age data and summarize significant fossils from classic Yanliao outcrops. Our study indicates that the Reshuitang fossil beds were deposited slightly later than the well-known Daohugou fossil beds situated ∼15 km away in Inner Mongolia and somewhat earlier than the Linglongta fossil beds in western Liaoning Province. These findings advance understanding of the Jurassic world during an episode of pronounced climatic warming. • We provide detailed age calibration for the Jurassic Yanliao Biota. • Our study allows us to understand Jurassic terrestrial ecosystems. • A robust chrono-stratigraphy reconstructs the link between life and climate. [ABSTRACT FROM AUTHOR]

Abstract: Regional climate-driven hydrological changes are accompanied by salinity changes in closed basin lakes. We have investigated acid leachable Li, along with other leachable ions including Mg, Ca and Sr, as geochemical proxies of salinity in lake sediments in the Mono Basin, California. All the elements in the acid leachable suite show a strong correlation with paleo-lake level estimates based on physical and stratigraphic evidence. The CaCO3 content of lake sediments, which has been shown to be a reliable proxy for lake level changes in the Mono basin and the adjoining Owens Lake basin, corresponds well with our acid-leachable proxy data. [Copyright &y& Elsevier]

A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, 40Ar/39Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8°C between 14.07 ± 0.05 Ma and 13.85 ± 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition. [ABSTRACT FROM AUTHOR]

We present the description and interpretation of a James Ross Island Volcanic Group (JRIVG) succession around the Cerro Domo and Ventisqueros Mesa area. Our aim is to reconstruct the volcanic evolution and provide new palaeoenvironmental information for the northwest sector of James Ross Island, northern Antarctic Peninsula. We present representative stratigraphic logs that are combined with new chronological constraints provided by 40Ar/39Ar ages and 87Sr/86Sr isotope analyses that demonstrate a record of the late Miocene to earliest Pleistocene. The first volcanic activity registered is associated with tuff cone deposits that resulted from marine hydromagmatic eruptions, close to 6.18 ± 0.14 Ma. Subsequently, lodgement till deposits record a grounded glacial advance. Overlying the glacigenic deposits, fossil-bearing sediments record a marine or glacimarine environment following the ice sheet retreat, and an 87Sr/86Sr age of 6.02 (+0.09, −0.09) Ma dates ice-poor conditions. A lava-fed delta system, which yields an age of 5.37 ± 0.12 Ma, prograded over the previous deposits. Stacked pillow lavas, curvy columnar-jointed basalts and subaerial tabular lavas, with a 4.9 ± 0.10 Ma age, were emplaced after the lava-fed delta deposition. Another lava-fed delta located to the north of the previously volcanic units yielded an age of 3.57 ± 0.19 Ma. A glacial pavement carved into the top of these deposits records a second ice advance in the area. The final volcanic activity emplaced Cerro Domo, a large volcanic edifice which was the centre of explosive hydromagmatic eruptions. The presence of pristine, articulated bivalves in life position within the lower volcanic deposits of Cerro Domo, together with an 87Sr/86Sr age of 3.16 (+0.59, −0.43) Ma, are strong indicators of a marine environment and suggest ice-poor "interglacial" conditions at this time. Dykes cutting the volcanic deposits of Cerro Domo and considered coeval with the eruptions afford ages of 2.62 ± 0.08 and 2.72 ± 0.09 Ma. Our results provide new insights on the volcanic and glaciation history of James Ross Island, as well as supporting marine, ice-poor conditions around ~6 Ma, and near or right before the Plio-Pleistocene boundary, between ~3.2 and ~2.6 Ma. • New stratigraphic and chronologic data for Antarctic Peninsula volcanism is presented. • Volcanic activity is recorded from late Miocene to earliest Pleistocene. • Volcanic activity includes lava-fed deltas and explosive hydromagmatic eruptions. • Glacial and marine environments occurred in close association with volcanic activity. • Ice-poor conditions occurred during late Miocene and near Plio-Pleistocene boundary. [ABSTRACT FROM AUTHOR]

• Probability distributions allow interpretation of overdispersed apatite helium data. • First quartile/minimum dates of apatite He data best represent skewed distributions. • Rapid exhumation near Beardmore Glacier started around Eocene-Oligocene transition. • Threshold benthic oxygen isotope value may correlate with enhanced glacial erosion. Apatite (U-Th)/He thermochronology has the potential to reconstruct records of erosional exhumation that are critical to understanding interactions between climate, tectonics, and the cryosphere at high latitudes on million-year timescales. However this approach is often hindered by the problem of intrasample single-grain date dispersion. Here we present an extensive new apatite (U-Th)/He dataset (n = 361) from the central Transantarctic Mountains of East Antarctica between 160°E to 170°W and 84 to 86°S, and show that apparently uninterpretable data in most samples are a reflection of inadequate sampling of skewed date distributions. We outline a workflow for interpreting such dispersed data and demonstrate that geologically meaningful age interpretations are possible in the case of rapidly cooled samples, despite the wide array of potential causes for date dispersion. We show that for samples and compilations with a large number of single-grain analyses (n > ∼25), the youngest probability distribution peak represents the most likely time of fast cooling through the apatite (U-Th)/He closure temperature. When fewer grains are analyzed, the youngest peak is represented best by the minimum date or first quartile date, depending on sample size. Using this workflow, we show that since the latest Eocene, up to 8.8 km of exhumation occurred to incise the deepest point of the Beardmore Glacier trough. Rapid incision began at c. 37-34 Ma (at the latest by 34±3 Ma), coinciding with or slightly preceding the initiation of Antarctic glaciation at the Eocene-Oligocene transition, and contributed to at least 2.6 km of exhumation within the first 3-6 million years, at an apparent exhumation rate of no less than 0.4 mm/a. [ABSTRACT FROM AUTHOR]

The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. International Ocean Discovery Program Expedition 361 drilled sites along the eastern margin of southern Africa that potentially provide archives of rainfall on the continent as well as dispersal in the Agulhas Current. We used standard methods to remove carbonate and ferromanganese oxides and Stokes settling to isolate the clay fractions. In comparison to most previous studies that aimed to extract the detrital signal from marine sediments, we additionally applied a cation exchange wash using CsCl as a final step in the sample preparation. The motivation behind the extra step, not frequently applied, is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure. Either would alter the composition of the detrital fraction if no cation exchange was applied. Moreover, using CsCl will provide an additional measure of the cation exchange capacity (CEC) of the samples. However, no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments. Here, we explore the effects of removing and replacing adsorbed cations on the clay surfaces with Cs+, conducting measurements of the chemical compositions, and radiogenic isotopes on a set of eight clay sample pairs. Both sets of samples underwent the same full leaching procedure except that one batch was treated with a final CsCl wash step. In this study, organic matter was not leached because sediments at IODP Site U1478 have relatively low organic content. However, in general, we recommend including that step in the leaching procedure. As expected, significant portions of elements with high concentrations in seawater were replaced by Cs+ (2SD 2.8%.) from the wash, including 75% of the sodium and approximately 25% of the calcium, 10% of the magnesium, and 8% of the potassium. Trace metals such as Sr and Nd, whose isotopes are used for provenance studies, are also found to be in lower concentrations in the samples after the exchange wash. The exchange wash affected the radiogenic isotope compositions of the samples. Neodymium isotope ratios are slightly less radiogenic in all the washed samples. Strontium and Pb isotopes showed significant deviations to either more or less radiogenic values in different samples. The radiogenic isotopes from the CsCl-treated fractions gave more consistent correlations with each other, and we suggest this treatment offers a superior measure of provenance. Although we observed changes in the isotope ratios, the general trend in the data and hence the overall provenance interpretations remained the same. However, the chemical compositions are significantly different. We conclude that a leaching protocol including a cation exchange wash (e.g. CsCl) is useful for revealing the terrestrial fingerprint. CEC could, with further calibration efforts, be useful as a terrestrial chemical weathering proxy. • Application of CsCl-wash leads to significant change in geochemical signature of the sediment. • Elements in high concentrations in seawater are replaced by Cs+ from the wash. • Cation exchange wash reveals a truer terrigenous signal. • The cation exchange capacity of the sediment may provide additional environmental information. [ABSTRACT FROM AUTHOR]

At least seven late Pliocene tills cap plateaus (mesetas) south of Lago Viedma, just east of the Andes in Argentine Patagonia. Chronologic constraints on the tills are provided by 40Ar/39Ar ages and magnetic polarities on associated basalt flows and sediments. The tills were deposited by piedmont glaciers that reached at least 80 km east of the crest of the Andes and flowed on a low-relief surface sloping gently downward in that direction. The oldest of the tills is about 3.6 Ma old. Glacial deposits dating to the Pliocene-Pleistocene transition are present at least 40 km beyond the east limit of the Pliocene tills at Lago Viedma, and tills of similar age occur at Condor Cliff in the Río Santa Cruz valley to the southeast. A sequence of at least seven Early Pleistocene (2.1–1.1 Ma) tills is present between basalt flows in the Cerro del Fraile meseta south of Lago Argentino. The glaciers that deposited these Early Pleistocene tills reached far beyond the Marine Isotope Stage 2 limit in the Río Santa Cruz valley. Based on positions, extents, and ages of the un-deformed, basalt-capped mesetas flanking Lago Viedma, we conclude that the topography in this area was profoundly changed during the Pleistocene – the low to moderate relief Pliocene surface was deeply incised by glaciers that became increasingly confined to, and flowed within, troughs. The valley floors today are up to 1350 m below the late Pliocene surface. • The Patagonian Ice Sheet advanced over the western Argentine steppe many times during the late Pliocene. • The steppe east of the southern Andes during the late Pliocene was an elevated surface of low to moderate relief. • The late Pliocene steppe was elevated and slightly tilted during the Quaternary, but not otherwise deformed. • Pleistocene glaciers deeply incised the Pliocene land surface, forming the deep valleys of the present landscape. [ABSTRACT FROM AUTHOR]

In the past decade, marine geophysical observations have led to the discovery of thin channels at the base of oceanic plates with anomalous physical properties that indicate the presence of low-degree partial melts. However, mantle melts are buoyant and should migrate toward the surface. We show abundant observations of widespread intraplate magmatism on the Cocos Plate where a thin partial melt channel was imaged at the lithosphere-asthenosphere boundary. We combine existing geophysical, geochemical, and seafloor drilling results with seismic reflection data and radiometric dating of drill cores to constrain the origin, distribution, and timing of this magmatism. Our synthesis indicates that the sublithospheric channel is a regionally extensive (>100,000 km 2 ) and long-lived feature that originated from the Galápagos Plume more than 20 Ma ago, supplying melt for multiple magmatic events and persisting today. Plume-fed melt channels may be widespread and long-lived sources for intraplate magmatism and mantle metasomatism.

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO 2 levels., (© 2022. The Author(s).)

The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today 1 ; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled 2 . An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian-Atlantic Southern Ocean (0-50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to 'escape' into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the 'southern escape' and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the '100-kyr world', in which glacial-interglacial cycles occur at roughly 100,000-year periods.

Evidence from high-sedimentation-rate South Atlantic deep-sea cores indicates that global and Southern Ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice-sheet growth and retreat, respectively. No consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility that ocean circulation triggered some millenial climate changes. At the major glacial-interglacial transitions, the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets.