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The strength of the geomagnetic field has decreased rapidly over the past two centuries, coinciding with an increasing field asymmetry due to the growth of the South Atlantic Anomaly. The underlying processes causing the decrease are debated, which has led to speculation that the field is about to reverse. Here, we present a geomagnetic field model based on indirect observations over the past 9,000 y and identify potential ancient analogs. The model is constructed using a probabilistic approach that addresses problems with age uncertainties and smoothing of sedimentary data that have hampered previous attempts. We find evidence for recurrent hemispherical field asymmetries, related to quasiperiodic millennial-scale variations in the dipole moment. Our reconstruction indicates that minima in the dipole moment tend to coincide with geomagnetic field anomalies, similar to the South Atlantic Anomaly. We propose that the period around 600 BCE, characterized by a strongly asymmetric field, could provide an analog to the present-day field. The analogy implies that the South Atlantic Anomaly will likely disappear in next few hundred years, accompanied by a return to a more symmetric field configuration and possibly, a strengthening of the axial dipole field.

Significance Atlantic multidecadal sea surface temperature variability (AMV) strongly influences the Northern Hemisphere’s climate, including the Arctic. Here using a well-dated annually laminated lake sediment core, we show that the AMV exerts a strong influence on High-Arctic climate during the instrumental period (past ∼150 y) through atmospheric teleconnection. This highly resolved climate archive is then used to produce the first AMV reconstruction spanning the last ∼3 millennia at unprecedented temporal resolution. Our terrestrial record is significantly correlated to several sea surface temperature proxies in the Atlantic, highlighting the reliability of this record as an annual tracer of the AMV. The results show that the current warmth in sea surface temperature is unseen in the context of the past ∼3 millennia., Global warming due to anthropogenic factors can be amplified or dampened by natural climate oscillations, especially those involving sea surface temperatures (SSTs) in the North Atlantic which vary on a multidecadal scale (Atlantic multidecadal variability, AMV). Because the instrumental record of AMV is short, long-term behavior of AMV is unknown, but climatic teleconnections to regions beyond the North Atlantic offer the prospect of reconstructing AMV from high-resolution records elsewhere. Annually resolved titanium from an annually laminated sedimentary record from Ellesmere Island, Canada, shows that the record is strongly influenced by AMV via atmospheric circulation anomalies. Significant correlations between this High-Arctic proxy and other highly resolved Atlantic SST proxies demonstrate that it shares the multidecadal variability seen in the Atlantic. Our record provides a reconstruction of AMV for the past ∼3 millennia at an unprecedented time resolution, indicating North Atlantic SSTs were coldest from ∼1400–1800 CE, while current SSTs are the warmest in the past ∼2,900 y.

Approximately 74 ka, Toba caldera in Sumatra, Indonesia, erupted in one of the most catastrophic supereruptions in Earth's history. Resurgent uplift of the caldera floor raised Samosir Island 700 m above Lake Toba, exposing valuable lake sediments. To constrain sediment chronology, we collected 173 discrete paleomagnetic 8 cm3 cubes and 15 radiocarbon samples from six sections across the island. Bulk organic 14C ages provide an initial chronostratigraphic framework ranging from ~12 to 46 ka. Natural and laboratory magnetizations were studied using alternating field demagnetization. A generally well-defined primary magnetization is isolated using principal component analysis. Comparison of inclination, and to a lesser degree declination, across independently dated sections suggests paleomagnetic secular variation (PSV) is recorded. Average inclination of −6° is more negative than a geocentric axial dipole would predict, but consistent with an eastward extension of the negative inclination anomaly observed in the western equatorial Pacific. The 14C- and PSV-derived age model constrains resurgent uplift, confirming faster uplift rates to the east and slower rates to the west, while suggesting that fault blocks moved differentially from each other within a generally trapdoor-type configuration.

Sediment records from deep-drilling projects such as those carried out by the International Continental Scientific Drilling Program are often tens to hundreds of meters in length. To ensure the complete recovery of a stratigraphic section, a basin is usually cored multiple times in adjacent holes so that gaps between sequential cores, poorly recovered sections, or intervals affected by disturbance can be bridged or replaced with sediments from another hole. Stratigraphic correlation, the alignment of stratigraphically-equivalent horizons in cores from different holes in a common-depth scale, and splice generation, the integration of the most-representative core sections into a composite-stratigraphic section, are essential steps in this process to both evaluate and synthesize the recovered-sediment record and focus the scientific analyses. However, these undertakings can be complex and are inherently subjective, making the need for the development of a single robust stratigraphic section early in the project critical to its success. Despite this, the steps between core recovery and on-splice data generation are rarely published in sufficient detail to allow reconstruction, or refinement, of the composited record at a later date. To increase the transparency of how the composite record is created, and to provide a template for future projects, we detail the step-by-step approaches and decisions involved in generating the composite-depth scale and complete-stratigraphic splice following recovery of sediments from Lake Junín, Peru. We first explain the details and nuances of different drilling-depth scales before describing how we integrated different physical property records to generate the composite-depth scale and complete-stratigraphic splice. Here, we show that due to the complex stratigraphy in the Lake Junín sediments, high-resolution line-scan images of the cores offer millimeter-scale precision for construction of the primary-stratigraphic splice at a resolution not afforded by other physical property data. Finally, through comparison of the spliced record to physical-property records acquired in situ on the borehole, we demonstrate that the stratigraphic splice is an accurate representation of the sediment accumulated in the Lake Junín basin.

We present a paleomagnetic study of two Holocene lake sediment sequences from Northwest Spitsbergen on Svalbard, Norway. Core HAP-212 from Lake Hajeren preserves a rare combination of a well-defined Characteristic Remanent Magnetization (ChRM) and robust age control defined by 21 terrestrial macrofossil-derived radiocarbon dates—providing a unique opportunity to assess the stratigraphic potential of High Arctic Holocene paleomagnetic secular variation (PSV) close to 80° N. Core AMP-112 from Hakluytvatnet, which is also well dated, is a poor magnetic recorder and cannot be used for geomagnetic reconstruction. Comparison of the HAP-212 PSV record with poorly dated regional records show similar directional features, suggesting that PSV can be used for chronological synchronization. Comparison of the HAP-212 PSV with well dated records ∼1800 km to the southwest and southeast show that these records are in-phase at millennial timescales, within dating and magnetic uncertainties, indicating that a consistent geomagnetic signal is recorded at these temporal and spatial scales. These findings support the notion that PSV, when optimally recorded, can be used to import chronologies from lower latitudes to the High Arctic, where low organic content, hard water effects, lack of datable material, and other difficulties often preclude more conventional radiocarbon dating.

Two > 5-m-long sediment cores from Cascade Lake (68.38° N, 154.60° W), Arctic Alaska, were analyzed to quantify their paleomagnetic properties over the past 21,000 years. Alternating-field demagnetization of the natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization, and hysteresis experiments reveal a strong, well-defined characteristic remanent magnetization carried by a low coercivity magnetic component that increases up core. Maximum angular deviation values average 210Pb and 14C were used to correlate the major inclination features of the resulting paleomagnetic secular variation (PSV) record with those of other regional PSV records, including two geomagnetic field models and the longer series from Burial Lake, located 200 km to the west. Following around 6 ka (cal BP), the ages of PSV fluctuations in Cascade Lake begin to diverge from those of the regional records, reaching a maximum offset of about 2000 years at around 4 ka. Several correlated cryptotephra ages from this section (reported in a companion paper by Davies et al., this volume) support the regional PSV-based chronology and indicate that some of the 14C ages at Cascade Lake are variably too old.

This is a pre-copyedited, author-produced version of an article accepted for publication in Geophysical Journal International following peer review. The version of record Velle, J.H., Walczak, M.H., Reilly, B.T., St-Onge, G., Stoner, J.S., Fallon, S.J., Mix, A.C., Belanger, C.L. & Forwick, M. (2022). High resolution inclination records from the Gulf of Alaska, IODP Expedition 341 Sites U1418 and U1419. Geophysical Journal International, 229(1), 345-358 is available online at https://doi.org/10.1093/gji/ggab479. International Ocean Drilling Program (IODP) Expedition 341 recovered sediments from the south Alaska continental slope that preserves a well resolved and dated inclination record over most of the past ∼43 000 yr. The Site U1419 chronology is among the highest resolution in the world, constrained by 173 radiocarbon dates, providing the ability to study Palaeomagnetic Secular Variation (PSV) on centennial to millennial timescales. This record has an exceptionally expanded late Pleistocene sedimentary record with sedimentation rates commonly exceeding 100 cm kyr–1, while also preserving a lower resolution Holocene PSV record at the top. Natural and laboratory-induced magnetic remanences of U1419 u-channels from the 112-m-long spliced record were studied using stepwise AF demagnetization. Hysteresis loops were obtained on 95 and IRM acquisition curves on 9 discrete samples to facilitate magnetic domain state, coercivity and magnetic mineralogical determinations. Due to complexities related to lithology, magnetic mineralogy, and depositional and post-depositional processes, Site U1419 sediments are not suitable for palaeointensity studies and declination could not be robustly reconstructed. Progressive (titano-)magnetite dissolution with depth results in decreasing NRM intensity and signal-to-noise that is exacerbated at higher demagnetization steps. As a result, inclination measured after the 20 mT AF demagnetization step provides the most reliable directional record. Inclination appears to be well resolved with removal of just a few intervals influenced by depositional and/or sampling and coring deformation. The shipboard inclination stack from nearby IODP Site U1418, on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC, verifies centennial to millennial scale variations in inclination observed in U1419. Comparisons with other independently dated records from the NE Pacific and western North America suggest that these sites likely capture regional geomagnetic variability. As such, this new high-resolution and well-dated inclination record, especially robust between 15 and 30 cal kyr BP, offers new geomagnetic insights and a regional correlation tool to explore this generally understudied part of the world

Calving events of Petermann Glacier, northwest Greenland, in 2010 and 2012 reduced the length of its ice tongue by c. 25 km, allowing exploration of newly uncovered seafloor during the Petermann 2015 Expedition. This article presents the results of foraminiferal analysis and environmental data from thirteen surface sediment samples in northern Nares Strait and Petermann Fjord, including beneath the modern ice tongue. This is the first study of living foraminifera beneath an arctic ice tongue and the first modern foraminiferal data from this area. Modern assemblages were studied to constrain species environmental preferences and to improve paleoenvironmental interpretations of foraminiferal assemblages. Sub–ice tongue assemblages differed greatly from those at all other sites, with very low faunal abundances and being dominated by agglutinated fauna, likely reflecting low food supply under the ice tongue. Fjord fauna were comprised of 80 percent or more calcareous species. Notably, Elphidium clavatum is absent beneath the ice tongue although it is dominant in the fjord. Increasing primary productivity associated with the transition to mobile sea ice, diminishing influence of the Petermann Glacier meltwater with distance from the grounding line, and increased influence of south-flowing currents in Nares Strait are the important controls on the faunal assemblages.

International audience; 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.

High-resolution palaeomagnetic records preserved in sediments (especially those that are well dated) provide valuable continuous information on past changes in Earth’s magnetic field. These data are essential for us to develop better understanding on the dynamics and causes of geomagnetic changes at various time scales. In this study, we conducted palaeomagnetic analyses on continuous u-channel samples collected from well-dated late Quaternary sediment sequences cored in the west Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Natural remanent magnetisations (NRM) as well as a suite of laboratory-induced magnetisations of the samples were measured at 1-cm interval resolution on a superconducting rock magnetometer before and after stepwise alternating field (AF) demagnetisation. NRM demagnetization data of the samples reveal a very stable and well-defined primary magnetisation component. Chronology of the studied cores is well constrained and tied to the polar ice cores as well as the absolutely dated Asian speleothem records. Average sedimentation rates of the studied cores range between ~10 cm/kyr to over 70 cm/kyr. Relative palaeointensity (RPI) records reconstructed from these sediments, when placed on the acquired age models, correlate well with other global and regional RPI records on time scales of ~10 kyr or longer. RPI features recorded at higher sedimentation rate sites appear slightly younger (a few hundreds to a couple of thousand years), possibly due to effects of the sediment magnetisation lock-in process. These Iberian Margin RPI records also show common millennial to multi-millennial scale variabilities, especially after deconvolution and correction of the lock-in induced age offset.

Deriving paleo-perspectives on polar systems in so-called “last ice” regions of North Greenland and the High Canadian Arctic have been traditionally challenged by logistical/accessibility issues and paleo proxy (including chronology) limitations. Sea-ice retreat and proxy development are changing this paradigm, allowing the region to be mapped, materials collected, and paleo-records developed that provide new insights on the evolution of the region. Here we report on continued progress from the joint US/Swedish 2015 Petermann Expedition to North Greenland and the joint US/Canadian 2012 Sawtooth Lake Expedition to Ellesmere Island, where new developments in physical properties and chronology are changing our understanding of the region. Computed tomography, X-Ray fluorescence, ice-rafted debris counts, and the magnetic properties of specific particle size fractions constrain changes in depositional processes and sediment sources providing info on glacial retreat and advance and other environmental changes. While an improved understanding of the geomagnetic field supported by radiocarbon dating enables regional magnetic synchronization allowing Holocene ice sheet and environmental dynamics to be placed in the context of High Arctic climate evolution.

New Paleomagnetic Secular Variation (PSV) data from Fish Lake, Utah, USA, along with previously published regional records, allow us to build an independently dated Western North America PSV stack (WNAM17) from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties. In February 2014, we recovered a composite 11-m-long sediment record from Fish Lake, Utah that retrieved glacially influenced and post-glacial sediments. Computed tomography (CT) scans and magnetic susceptibility were used to build a precise depth scale, while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age-control. Glacially influenced sediments have well-defined characteristic remanent magnetizations with all maximum angular deviation values

Holocene records of lacustrine primary production are commonly used to reconstruct past changes in environmental and climatic conditions. While several methods exist to infer paleoproductivity trends, few studies to date have applied multiple geochemical indices in the same core sequence from Arctic lakes to evaluate their fidelity and sensitivity to specific climate variables over long (Holocene length) timescales. In this study, we evaluate sub-century to millennial-scale fluctuations in paleoproductivity over the Holocene using geochemical (biogenic opal and sedimentary chlorin) analyses of sediments from Burial Lake in the western Brooks Range, Alaska. Large fluctuations in opal and related proxies occur at millennial timescales over the last 10,000 years. We interpret the changes in opal to result from variability in diatom productivity, which is indirectly mediated by climate primarily through changes in the duration of the ice-free growing season and the availability of limiting nutrients at this oligotrophic, tundra lake. Comparison of the opal and sedimentary chlorin record, which is correlated with TOC, shows contrasting patterns on both short (century to multi-century) and relatively long (millennial) time scales. The concentration of opal far exceeds that of TOC and variations in sediment dry bulk density, driven by changes in the accumulation of opal, are likely responsible in part for the variations in sedimentary chlorin. Further, C/N ratio values indicate a mixed algal-terrestrial source of sedimentary organic matter. This result highlights the complexity in the climatic interpretation of sedimentary chlorin as an index of whole lake production, because the signal is prone to dilution/concentration from opal and also reflects a combination of aquatic and terrestrial production. Time series analysis of the productivity records indicates the presence of a significant ∼1500-yr oscillation in opal concentration, which has been found in North Atlantic Ocean proxy records and numerous other marine and terrestrial paleorecords. Comparison of diatom productivity against a sea-ice inferred reconstruction of the Arctic Oscillation (AO) from the Beaufort Sea (Darby et al., 2012) shows that periods of reduced productivity at Burial Lake coincide with inferred positive phases of the AO (AO+). Combined with modern observations of sea ice extent and meteorological data, we hypothesize that AO + conditions and a strengthened polar jet correspond with a shortened ice-free growing season, a decrease in the availability of limiting nutrients, and lower levels of diatom production at Burial Lake. Comparison of the spectral properties between opal and the AO reconstruction reveal similar millennial scale variations with ∼1500-yr variability during the middle Holocene that transition to ∼1000-yr variability during the late Holocene. In light of these findings, we suggest the possibility that millennial variations in diatom productivity observed in the Burial Lake record are related to millennial variability in high-latitude atmospheric circulation similar to the AO. These results shed light on the sensitivity of aquatic ecosystems in northern Alaska to changes in the duration of the ice-free growing season, the availability of limiting nutrients for phytoplankton growth, and Arctic-wide atmospheric circulation dynamics over the Holocene on millennial timescales.

Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the Matuyama-Brunhes (M-B) polarity reversal ∼771 thousand years ago. Our high-precision ^(40)Ar/^(39)Ar ages constrain several rapid and short-lived changes in field orientation up to 33,000 years prior to the M-B reversal. These changes are similar to ones identified in other less well-dated lava flows in Maui, Chile and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000-year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial-to-millennial scale polarity changes in the presence of a strongly weakened dipole field.

Accepted manuscript version. Published version available at https://doi.org/10.1130/G39904.1. Fertilization of the ocean by eolian dust and icebergs is an effective mechanism to enhance primary productivity. In particular, high-nutrient, low-chlorophyll (HNLC) areas where phytoplankton growth is critically iron-limited, such as the subarctic Pacific Ocean and the Southern Ocean, are proposed to respond to increases in bioavailable Fe supply with enhanced phytoplankton productivity and carbon export to the seafloor. While Fe-fertilization from dust is widely acknowledged to explain a higher export production during glacial periods in the Southern Ocean, paleoceanographic records supporting links between productivity and eolian dust and/or icebergs in the North Pacific are scarce. By combining independent proxies indicative of ice-sheet dynamics and ocean productivity from a single marine sedimentary record (Integrated Ocean Drilling Program [IODP] Site U1417), we present a comprehensive data set of phytoplankton response to different fertilization mechanisms in the subarctic northeast Pacific between 1.5 and 0.5 Ma, including the Mid Pleistocene Transition. Importantly, the timing of the fertilization events is more strongly controlled by local ice-sheet extent than by glacial-interglacial climate variability. Our findings indicate that fertilization by glacigenic debris results in productivity events in HNLC areas adjacent to ice sheets, and that this mechanism may represent an important, yet rarely considered, driver of phytoplankton growth.

We use isothermal and temperature dependent in-field and magnetic remanence methods together with electron microscopy to characterize different sieved size fractions from terrestrial sediments collected in Iceland and southern Greenland. The magnetic fraction of Greenland silts (3–63 μm) and sands (>63 μm) is primarily composed of near-stoichiometric magnetite that may be oxidized in the finer clay (

Computed Tomography (CT) of sediment cores allows for high resolution images, three dimensional volumes, and down core profiles. These quantitative data are generated through the attenuation of X-rays, which are sensitive to sediment density and atomic number, and are stored in pixels as relative grayscale values or Hounsfield units (HU). We present a suite of MATLABTM tools specifically designed for routine sediment core analysis as a means to standardize and better quantify the products of CT data collected on medical CT scanners. SedCT uses a graphical interface to process Digital Imaging and Communications in Medicine (DICOM) files, stitch overlapping scanned intervals, and create down core HU profiles in a manner robust to normal coring imperfections. Utilizing a random sampling technique, SedCT reduces data size and allows for quick processing on typical laptop computers. SedCTimage uses a graphical interface to create quality tiff files of CT slices that are scaled to a user defined HU range, preserving the quantitative nature of CT images and easily allowing for comparison between sediment cores with different HU means and variance. These tools are presented along with examples from lacustrine and marine sediment cores to highlight the robustness and quantitative nature of this method.

Columbia Glacier in Prince William Sound, Alaska, has retreated ∼20 km in the past three decades. We use marine sediment records to document the Columbia Glacier advance and retreat history over the past 1.6 k.y. in an effort to place its recent retreat in the context of the Common Era (C.E.). A change in magnetic mineralogy coincided with a shift in sediment geochemistry ca. 0.9 ka. This provenance change documents the advance of Columbia Glacier across a fault, resulting in glacial erosion of mafic rocks near the coast; this agrees with the timing of ice advance reconstructed using dendrochronology. Our marine provenance records show that Columbia Glacier remained advanced south of this fault into the 21st century. Columbia Glacier has now retreated north of this fault, making its recent retreat unprecedented since before ca. 0.9 ka. Southern Alaska temperatures have now warmed to pre–0.9 ka levels, based on tree-ring and reanalysis data. We show with glacier model simulations that the warming between C.E. 1910 and 1980, that includes anthropogenic forcing, was sufficient to trigger the recent retreat of Columbia Glacier from its extended position of the past 0.9 k.y., consistent with our data-driven assessment of the relationship between regional climate change and glacier extent. We conclude that the recent retreat of Columbia Glacier is a response to climate change rather than part of a natural internal tidewater-glacier oscillation.

Calving cousins Walczak et al. report that increases in Pacific Ocean ventilation and periods of rapid production of icebergs from the Cordilleran Ice Sheet during the last glacial period preceded episodic iceberg discharges into the Atlantic Ocean (see the Perspective by Jaeger and Shevenell). Marine sediments from the Gulf of Alaska show that increases in vertical mixing of the ocean there correspond with intense iceberg calving from the ice sheet that covered much of high-latitude western North America and that these changes occurred before the analogous Heinrich events in the North Atlantic. Thus, these Pacific climate system reorganizations may have been an early part of a cascade of dynamic climate events with global repercussions. Science , this issue p. 716 ; see also p. 662

Over the last decade, two major calving events of the Petermann Ice Tongue in Northwest Greenland have led to speculation on its future stability and contribution to further Greenland Ice Sheet mass loss. However, it has been unclear if these events are anomalous or typical within the context of limited historical observations. We extend the historical record of the floating ice tongue using the stratigraphy of Petermann Fjord sediments to provide a longer-term perspective. Computed tomography (CT) scans, X-Ray Fluorescence (XRF) scans, Ice-Rafted Debris (IRD) counts, and the magnetic properties of specific particle size fractions constrain changes in depositional processes and sediment sources at our core sites, allowing for reconstructions of past behavior of the Petermann Ice Tongue. Radiocarbon dating of foraminifera, 210Pb, and paleomagnetic secular variation (PSV) provide age control and help to address uncertainties in radiocarbon reservoir ages. A floating ice tongue in Petermann Fjord formed in late glacial time as Petermann Glacier retreated from an advanced grounded position. This paleo-ice tongue broke-up during the early Holocene when high northern latitude summer insolation was higher than present. After gradual regrowth of the ice tongue associated with regional cooling, the ice tongue reached its historical extent only within the last millennium. Little or no ice tongue was present for nearly 5000 years during the middle Holocene, when decadal mean regional temperatures are estimated to be 0.8–2.9 °C higher than preindustrial (1750 CE) and seasonal sea-ice in the Lincoln Sea was reduced. This pre-historical behavior shows that recent anthropogenic warming may already be in the range of ice tongue instability and future projected warming increases the risk of ice tongue break-up by the mid-21st Century.

We present new sedimentological and environmental magnetic records spanning ∼3.2–2.2 Ma, during the intensification of Northern Hemisphere glaciation, from North Atlantic Integrated Ocean Drilling Program Site U1307 on Eirik Drift. Our new datasets and their high-fidelity age control demonstrate that while inland glaciers – and potentially also at times restricted iceberg-calving margins – have likely existed on southern Greenland since at least ∼3.2 Ma, persistent and extensive iceberg-calving glacial margins were only established in this region at 2.72 Ma, ∼300 kyr later than in northeastern and eastern Greenland. Despite a dramatic increase in Greenland-sourced ice-rafted debris deposition on Eirik Drift at this time, contemporaneous changes in the bulk magnetic properties of Site U1307 sediments, and a reduction in sediment accumulation rates, suggest a decrease in the delivery of Greenland-sourced glaciofluvial silt to our study site. We attribute these changes to a shift in depositional regime from bottom-current-dominated to glacial-IRD-dominated between ∼2.9 and 2.7 Ma, in response to a change in the depth of the flow path of the Western Boundary Undercurrent relative to our study site.

We make fundamental observations of the particle size variability of magnetic properties from 71 core tops that span the southern Greenland and Norwegian Seas. These data provide the first detailed regional characterization of how bulk magnetic properties vary with sediment texture, sediment source, and sediment transport. Magnetic susceptibility (MS) and hysteresis parameters were measured on the bulk sediment and the five constituent sediment particle size fractions (clay, fine, medium, and coarse silt, and sand). The median MS value of the medium silt size fraction is ~3‐5 times higher than that of the sand and clay size fractions and results in a strong sensitivity of bulk MS to sediment texture. Hysteresis properties of the clay size fraction are relatively homogeneous and contrast that silt and sand size fractions which show regional differences across the study area. These coarser fractions are more transport limited and using medium silt hysteresis measurements and low temperature MS behavior we establish three endmembers that effectively explain the variability observed across the region. We model the response of bulk magnetic properties to changes in sediment texture and suggest that variations in sediment source are required to explain the bulk magnetic property variability observed in cores across the southern Greenland and Norwegian Seas. These findings imply that sediment source has a greater influence on driving bulk magnetic property variability across this region than has previously been assumed. Plain Language Summary Sediments from the bottom of the ocean can tell us about past environmental conditions (climate, ocean circulation, ice sheet growth and retreat) that we cannot obtain by other means. Naturally occurring iron‐bearing minerals in the sediment are strongly sensitive to changing environmental conditions and can be rapidly and non‐destructively characterized using by a suite of magnetic property measurements. To better understand these records we split bulk sediments recovered from 71 locations across the Norwegian and Greenland Seas into five different grain‐size ranges and made the same magnetic measurements on each size fraction and the bulk sediment. The silt‐size fractions contain several times the amount of magnetic material than the clay‐ or sand‐size fractions resulting in a strong dependence of bulk properties on the presence of clay, silt, and sand. Sediment source also strongly influences silt and bulk magnetic properties and by sampling widely across the study region we present new evidence for source‐driven variability that was not previously known. These findings suggest that we need to consider how changes in sediment source and sediment grain‐size might affect interpretations of bulk sediment magnetic properties. These new insights can provide detailed new information about Earth's past.

Today's knowledge about paleomagnetic secular variations in (southern) Africa is extremely limited. Here, we derive paleomagnetic records from independently radiocarbon dated sediment records from three maars in Madagascar: Andraikiba and Amparihibe are terrestrial maars located on the main island of Madagascar and on a small island in the Northwest, whereas Crater Lake is a maar which has an open connection to the Indian Ocean. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination and declination patterns in two of the archives for the past 2,500 years (except for the last 400 cal BP). These new data are the first allowing to test the reliability of previous records which often suffer from low resolution and other obstacles. This will further allow to start to distinguish between robustly confirmed paleomagnetic secular variation data for this region from potentially problematic data. Considering a much lower resolution and a shorter covered time interval of archaeomagnetic data from La Reunion, South Africa, Botswana and Zimbabwe the temporal successions of maxima and minima reveal a coherent picture. Although slightly shifted in time, similarities also exist to the global geomagnetic field reconstruction model SHAWQ2k ( Campuzano et al., 2019 ) which includes the above mentioned data. Surprisingly also similarities, though only in declination, are observed to the CALS3K.4 model ( Korte and Constable, 2011 ) which is used as Northern Hemisphere biased model example. In contrast to this, no declination similarities but remarkable inclination similarities are observed to a lake record from Lake Malawi. An analog inclination pattern is also observed in a record from the Makran Accretionary Wedge which is ∼5,000 km to the north of the investigated sites. Interestingly the spatial distribution of archives showing these inclination similarities resembles the spatial distribution of inclination anomalies detected in model predictions. PSV similarities over such a large area are suggestive of a large-scale core dynamic origin independent of westward drift of non-dipole field components often associated with PSV records. This study emphasizes the potential of maar lakes in Madagascar for paleo reconstructions but also suggests that shallow marine and marine-brackish systems should be avoided if possible when trying to expand the (South) African paleomagnetic database.

A 30.5 m sediment core was recovered from the coastal lake Eilandvlei (EV13), which represents a unique high-resolution record of environmental change for southern Africa. For the establishment of a robust chronology, special emphasis was placed on the calibration of radiocarbon ( 14 C) ages obtained from the dating of different material. However, the reliability of 14 C ages can be problematic since coastal lakes interact with different source pools providing 14 C-depleted (“old”) carbon thus causing reservoir effects. The origin of old carbon affecting the EV13 samples was most likely sourced from the Indian Ocean. Two pre-bomb marine molluscan shells were therefore analysed to determine the regional marine reservoir offset (ΔR), with obtained ΔR values of 134 ± 38 and 161 ± 38 14 C yrs providing the first available data for the south coast of South Africa. However, the application of the resulting average ΔR mean = 148 ± 27 14 C yrs for the calibration of the entire EV13 record underestimates the variable reservoir effects throughout the Holocene. These were possibly caused by past changes in the connectivity between the present lake system and the ocean as well as a varying degree of upwelling in this area. To solve this problem, three sample pairs (each consisting of wood fragments and bulk organic sediment from the same core depth) were dated to calculate the variable past reservoir effects. This approach provided a median basal age of 8920 +200 / -250 cal BP. Palaeomagnetic secular variation stratigraphy was used to corroborate the chronology for the topmost 1.5 m of the record (past millennium), thus providing the first Holocene sediment based inclination and declination data from South Africa.

IODP Expedition 323 recovered six complete and replicate records of Brunhes-Chron paleomagnetic field variability (0–780,000 years BP) in 2820 m core depth below sea floor (CSF) of deep-sea sediments. On shipboard, we made more than 220,000 paleomagnetic measurements on the recovered sediments. Since then, we have u-channel sampled more than 300 m of Brunhes Chron sediments to corroborate our shipboard measurements and improve our paleomagnetic and rock magnetic understanding of these sediments. Several intervals of distinctive paleomagnetic secular variation (PSV) have been identified that appear to be correlatable among sites 1343, 1344, and 1345. One magnetic field excursion is recorded in sediments of sites 1339, 1343, 1344, and 1345. We identify this to be excursion 7α/Iceland Basin Event (192,000 years BP), which is also seen in the high-latitude North Atlantic Ocean ( Channell et al., 1997 ). We have verified in u-channels the placement of the Brunhes/Matuyama boundary (780,000 years BP) at sites 1341 and 1343. Finally, we have developed a medium-quality relative paleointensity record for these sediments that is correlatable among the sites, even though it is still biased by large-amplitude environmental variability. On the basis of these observations we have built a magnetic chronostratigraphy of Expedition 323 sediments suitable for regional correlation and dating over the last 1 million years, and compared this with oxygen-isotope chronostratigraphy from sites U1339 and U1345.

Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has been often limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ∼100 m drill core from Lake Junin, Peru. The results form the foundation of an age-depth model spanning ∼700 kyrs. High uranium concentrations (0.3–4 ppm) of these sediments allow us to date smaller amounts of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest factor limiting the success of previous U-Th dating efforts in other lake basins. Despite this advantage, the dates from 174 analyses on 55 bulk carbonate samples reveal significant scatter that cannot be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and δ 234 Uinitial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. Alongside simulations of the isotopic evolution of our samples, we find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. Examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that geologic context is equally as important as the accuracy and precision of analytical measurements. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junin is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates.

International audience; The Last Interglacial (LIG) represents an invaluable case study to investigate the response of components of the Earth system to global warming. However, the scarcity of absolute age constraints in most archives leads to extensive use of various stratigraphic alignments to different reference chronologies. This feature sets limitations to the accuracy of the stratigraphic assignment of the climatic sequence of events across the globe during the LIG. Here, we review the strengths and limitations of the methods that are commonly used to date or develop chronologies in various climatic archives for the time span (∼140–100 ka) encompassing the penultimate deglaciation, the LIG and the glacial inception. Climatic hypotheses underlying record alignment strategies and the interpretation of tracers are explicitly described. Quantitative estimates of the associated absolute and relative age uncertainties are provided.Recommendations are subsequently formulated on how best to define absolute and relative chronologies. Future climato-stratigraphic alignments should provide (1) a clear statement of climate hypotheses involved, (2) a detailed understanding of environmental parameters controlling selected tracers and (3) a careful evaluation of the synchronicity of aligned paleoclimatic records. We underscore the need to (1) systematically report quantitative estimates of relative and absolute age uncertainties, (2) assess the coherence of chronologies when comparing different records, and (3) integrate these uncertainties in paleoclimatic interpretations and comparisons with climate simulations.Finally, we provide a sequence of major climatic events with associated age uncertainties for the period 140–105 ka, which should serve as a new benchmark to disentangle mechanisms of the Earth system's response to orbital forcing and evaluate transient climate simulations.

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼ 2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼ 100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Patton, Jason ... et. al., In order to investigate the possibility of a long-term paleoseismic history from offshore sedimentary records in Sumatra, we collected 144 deep-sea sediment cores in the trench and in lower slope piggyback basins of the Sumatra accretionary prism. We used multibeam bathymetry and seismic reflection data to develop an understanding of catchment basins, turbidity current pathways, and depositional styles, as well as to precisely locate our gravity cores, piston cores, Kasten cores, and multicores. We use detailed physical property data, including computed tomographic X-ray, gamma density, magnetic susceptibility, grain-size analysis, faunal analysis, and smear slides, to evaluate the turbidite stratigraphy and sedimentology at each site. We use radiocarbon age control for piggyback basin sites above the carbonate compensation depth, and use 210Pb and 137Cs to evaluate the timing of the most recent sedimentary deposits. Using well-log correlation methods and radiometric age control, we test for potential correlations between isolated sites in piggyback basins and the trench. [...]

Environmental magnetism and radiocarbon dating of Burial Lake sediments constrain the timing and magnitude of regional aeolian deposition for the Noatak region of western Arctic Alaska for the last ∼37,000 years. Burial Lake (68.43°N, 159.17°W, 21.5 m water depth) is optimally located to monitor regional dust deposition because it is perched above local drainage and isolated from glacial processes. Cores collected in the summer of 2010 were studied through the application of magnetizations and progressive alternating field (AF) demagnetization of u-channel samples, with additional data provided by computed tomography (CT) derived density, hysteresis measurements, isothermal remanent magnetization (IRM) acquisition experiments, organic carbon content, biogenic silica, physical grain size, radiocarbon dating of wood, seeds, and plant macrofossils, point source magnetic susceptibility, and X-ray fluorescence (XRF). With similar magnetic properties to regional Alaskan loess deposits, low coercivity, highly magnetic material deposited during the late-Pleistocene contrasts with a high coercivity, weakly magnetic component found throughout the record, consistent with locally-derived detritus. The relative proportion of low coercivity to high coercivity magnetic material, defined by the S-Ratios, is used to reconstruct the regional input of dust to the basin over time. A four-fold decrease in the low coercivity component through the deglacial transition is interpreted to reflect diminished dust input to the region. Comparisons with potential sources of dust show that the timing of deposition in Burial Lake is largely consistent with general aridity, lack of vegetative cover, and increased windiness, rather than glacial advances or retreats. The influence from subaerial exposure of continental shelves cannot be ruled out as a significant far-field source of dust to interior Alaska during the Last Glacial Maximum (LGM), but is unlikely to have been the sole source, or to have contributed to increased dust in both the early and late Holocene.

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic in 2004 recovered a 175-m Quaternary sedimentary section at IODP Site U1307, located on the northern part of the Eirik Drift, south of Greenland. The seafloor at Site U1307, at 2575 m water depth, is situated within the water-depth range of the present-day main axis of the Western Boundary Under Current (WBUC) but lies on the NW side of the drift crest and is partly shielded from it. The uppermost 120 meters of sediment sequence were sampled with u-channels along a composite section which combines the two holes drilled at this site. An age model was obtained by identification of polarity reversals back to the Reunion Subchron ( ∼ 2.1 Ma ) and then refined in the 0–1.5 Ma interval by correlation of the relative paleointensity (RPI) record to the PISO paleointensity stack. The Site U1307 relative paleointensity (RPI) record is compared with the PISO and EPAPIS (Pacific) stacks, neighboring IODP sites from Eirik Drift (Sites U1305 and U1306), and with the Ocean Drilling Program (ODP) Site 984 record that extends beyond PISO. Magnetite grain-size and concentration proxies are compared to Sites U1305 and U1306 records, in the context of the past evolution of the WBUC.