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Ice loss in the Southern Hemisphere has been greatest over the past 30 years in West Antarctica. The high sensitivity of this region to climate change has motivated geologists to examine marine sedimentary records for evidence of past episodes of West Antarctic Ice Sheet (WAIS) instability. Sediments accumulating in the Scotia Sea are useful to examine for this purpose because they receive iceberg-rafted debris (IBRD) sourced from the Pacific- and Atlantic-facing sectors of West Antarctica. Here we report on the sedimentology and provenance of the oldest of three cm-scale coarse-grained layers recovered from this sea at International Ocean Discovery Program Site U1538. These layers are preserved in opal-rich sediments deposited ∼1.2 Ma during a relatively warm regional climate. Our microCT-based analysis of the layer's in-situ fabric confirms its ice-rafted origin. We further infer that it is the product of an intense but short-lived episode of IBRD deposition. Based on the petrography of its sand fraction and the Phanerozoic 40Ar/39Ar ages of hornblende and mica it contains, we conclude that the IBRD it contains was likely sourced from the Weddell Sea and/or Amundsen Sea embayment(s) of West Antarctica. We attribute the high concentrations of IBRD in these layers to "dirty" icebergs calved from the WAIS following its retreat inland from its modern grounding line. These layers also sit at the top of a ∼366-m thick Pliocene and early Pleistocene sequence that is much more dropstone-rich than its overlying sediments. We speculate this fact may reflect that WAIS mass-balance was highly dynamic during the ∼41-kyr (inter)glacial world.

Patterns of variability in Pleistocene magnetic susceptibility (k) from deep-sea sediment cores from the Scotia Sea show a striking similarity to patterns of dust flux recorded in the EPICA Dronning Maud Land (EDML) ice core.  Antarctic marine k records broadly reflect the interplay of lithogenic sediment provenance, biological productivity, sediment transport processes, and post-depositional diagenesis. Here we explore the origin of the Scotia Sea k record via a detailed rock magnetic study across the transition from MIS 6 to MIS 5. We analyzed bulk sediment and grain size separates in order to construct magnetic signatures of iceberg rafted debris (IBRD), sortable silt, and eolian input. The MIS 6-MIS 5 transition consists of three lithologies, a high k silty-clay-rich diatomaceous mud deposited during the glacial interval, an IBRD-rich but low k silty clay that marks the onset of deglaciation, and a low k diatomaceous ooze in which IBRD decreases forward through time. The high k glacial sediment is characterized by multi-domain hysteresis parameters, low χARM/χ values, S ratios near 1, and thermomagnetic curves indicative of low-Ti titanomagnetite. The absence of k peaks in the IBRD-rich silty-clay and IBRD rich diatomaceous ooze likely reflects the weakly magnetic lithogenic detritus supplied by Weddell Sea Embayment (WSE) ice streams, such as sandstone, quartzite, metasedimentary lithologies, phyllite and schist observed in lateral moraines adjacent to ice streams of the eastern WSE. The deglacial interval is characterized by elevated MR/MS, χARM/χ, and HIRM values, and decreased S-ratios in the bulk sediment, suggesting a greater proportion of high coercivity minerals such as hematite or goethite in the iron oxide assemblage. Preliminary data from grain size separates indicates that the clay mass fraction is > 0.5 in all three lithologies. Clay is also the dominant size fraction in the EDML ice core dust, with particle sizes generally < 5 μm. The Scotia Sea clay fraction k values are a factor 1.5 to 5 weaker than the silt fraction k values, and therefore are not the main carrier of the bulk k signal. The rock magnetic signatures of Scotia Sea sediment will be compared to those of terrestrial till and bedrock from the WSE, and to those of potential dust sources in South America to identify the sediment sources and environmental processes responsible for the k signal.

Patterns of variability in Pleistocene magnetic susceptibility (k) from deep-sea sediment cores from the Scotia Sea show a striking similarity to patterns of dust flux recorded in the EPICA Dronning Maud Land (EDML) ice core.  Antarctic marine k records broadly reflect the interplay of lithogenic sediment provenance, biological productivity, sediment transport processes, and post-depositional diagenesis. Here we explore the origin of the Scotia Sea k record via a detailed rock magnetic study across the transition from MIS 6 to MIS 5. We analyzed bulk sediment and grain size separates in order to construct magnetic signatures of iceberg rafted debris (IBRD), sortable silt, and eolian input. The MIS 6-MIS 5 transition consists of three lithologies, a high k silty-clay-rich diatomaceous mud deposited during the glacial interval, an IBRD-rich but low k silty clay that marks the onset of deglaciation, and a low k diatomaceous ooze in which IBRD decreases forward through time. The high k glacial sediment is characterized by multi-domain hysteresis parameters, low χARM/χ values, S ratios near 1, and thermomagnetic curves indicative of low-Ti titanomagnetite. The absence of k peaks in the IBRD-rich silty-clay and IBRD rich diatomaceous ooze likely reflects the weakly magnetic lithogenic detritus supplied by Weddell Sea Embayment (WSE) ice streams, such as sandstone, quartzite, metasedimentary lithologies, phyllite and schist observed in lateral moraines adjacent to ice streams of the eastern WSE. The deglacial interval is characterized by elevated MR/MS, χARM/χ, and HIRM values, and decreased S-ratios in the bulk sediment, suggesting a greater proportion of high coercivity minerals such as hematite or goethite in the iron oxide assemblage. Preliminary data from grain size separates indicates that the clay mass fraction is > 0.5 in all three lithologies. Clay is also the dominant size fraction in the EDML ice core dust, with particle sizes generally < 5 μm. The Scotia Sea clay fraction k values are a factor 1.5 to 5 weaker than the silt fraction k values, and therefore are not the main carrier of the bulk k signal. The rock magnetic signatures of Scotia Sea sediment will be compared to those of terrestrial till and bedrock from the WSE, and to those of potential dust sources in South America to identify the sediment sources and environmental processes responsible for the k signal.

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

We investigate the amplitude and frequency of directional geomagnetic change since 15 ka in the Northern North Atlantic (∼67°N) using five "ultra‐high" resolution continental shelf sediment cores deposited at rates greater than 1 m/kyr. The ages of these cores are constrained by 103 radiocarbon dates with reservoir ages assessed through tephra correlation to terrestrial archives. Our study aims to address many of the uncertainties that are common in sedimentary paleomagnetic studies, including signal attenuation in low to moderate resolution archives and difficulty to demonstrate reproducibility in higher resolution archives. The "ultra‐high" accumulation rates of our cores reduce "lock‐in" and smoothing uncertainties associated with magnetic acquisition processes. Abundant radiocarbon dates along with an objective alignment algorithm provide a test of signal reproducibility at sub‐millennial timescales. The paleomagnetic secular variation (PSV) signal, evaluated as individual records and as a new stack (GREENICE15k), validates prior results, but provides stronger geochronological constraints, demonstrates a reproducible PSV signal and amplitude, and extends through the abrupt Bølling–Allerød and Younger Dryas climate transitions of the latest Pleistocene. While broadly consistent with time‐varying spherical harmonic models and varve dated records from Northern Europe, we demonstrate greater variance and higher amplitudes—particularly at sub‐millennial timescales. This robust variability on centennial timescales is rarely observed or discussed, but is likely important to our understanding of some of the most intriguing aspects of the geodynamo. Plain Language Summary: Our study used mud from the seafloor to investigate changes in Earth's magnetic field in the Northern North Atlantic region over the past 15 thousand years. Magnetic minerals eroded from rocks and deposited on the seafloor can preserve a record of the geomagnetic field, like little compass needles suspended in mud. We obtained sediment cores from areas near land with very high accumulation rates, which are rare in the modern ocean, that can record short magnetic field variations that occur on the timescale of centuries. We used radiocarbon dating to determine the ages of the sediment samples and a new alignment algorithm to verify the accuracy and consistency of the magnetic signal we observed. Understanding these short geomagnetic field variations is challenging, but it is key to unlocking many mysteries about the past and future of Earth's magnetic field. Key Points: Sediment cores with ultra‐high accumulation rates are used to study directional changes in the Northern North Atlantic's geomagnetic fieldStudy addresses uncertainties in sedimentary paleomagnetic research, such as signal attenuation, chronology, and reproducibilityStudy reveals robust variability on centennial timescales with greater variance than generally observed over the last 15,000 years [ABSTRACT FROM AUTHOR]

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