Marcus Gutjahr

The Weddell Sea influences global ocean circulation and sea level through modulating Antarctic Bottom Water formation and Antarctic Ice Sheet discharges, but the history of ice‐sheet dynamics in this region remains poorly constrained due to limited well‐dated geological records. To provide a new geochemical perspective on ice‐sheet dynamics since the Last Glacial Maximum (LGM), this study presents paired authigenic and detrital Pb isotope records from sediment core PS1599‐3 recovered from the continental slope north of the Filchner Trough. Comparison with Antarctic Pb isotope provinces indicates that both detrital and authigenic Pb signatures were possibly sourced from a Shackleton Range‐like region in the south‐eastern Weddell Sea sector of East Antarctica. This suggests that dominant upstream drainage sources remained broadly stable throughout the last ~23 ka. We interpret the downcore Pb isotope variations to primarily reflect changes in the delivery of subglacial weathering‐derived Pb associated with grounding‐line dynamics in the Filchner Trough. Authigenic ²⁰⁶ Pb/ ²⁰⁴ Pb ratios show a sharp increase at ~23 ka, followed by two decreases at ~20 and 11–9 ka. We suggest that a re‐advance of grounded ice near the LGM was followed by initial retreat at ~20 ka and further grounding‐line retreat or reorganization during the Early Holocene. These findings highlight the use of authigenic Pb isotopes as potential tracers for reconstructing polar ice‐sheet dynamics and meltwater influences.

Improving our understanding of Antarctic glacial dynamics is essential for accurately predicting ice sheet behavior and its contribution to global sea level rise under future climate scenarios. The Weddell Sea Embayment, which channels ice from East Antarctica, West Antarctica, and the Antarctic Peninsula into the Filchner‐Ronne Ice Shelf, is a critical region for such investigations. Yet, considerable uncertainty remains about the ice sheet evolution since the last glacial period, which limits the ability to robustly constrain earth system models across deglacial intervals. To address this, we present new marine geological evidence from Hughes Trough, near the calving front of the modern Ronne Ice Shelf. Geomorphic evidence suggests that grounded ice existed here during the last glacial period, with neodymium (Nd) and lead (Pb) isotope compositions of subglacially‐derived sediments suggesting ice originated from either East Antarctica or a province in West Antarctica that is relatively unradiogenic isotopically. Radiocarbon dating of grounding line‐proximal sediments using the ramped pyrolysis‐oxidation suggests that local ice retreat may have already commenced by 16.8 cal ka BP. Radiogenic isotopes preserve evidence of ice stream provenance and the progressive establishment of oceanic connectivity between Hughes and Ronne Troughs, followed by linkage to Filchner Trough. The development of inter‐trough seaways facilitated oceanic circulation and sediment transport throughout the embayment. However, a subsequent temporary re‐isolation of the Ronne and Filchner sectors occurred during the early‐Holocene, associated with glacial readvance in the Filchner Trough and ice stream reconfiguration.

Lead (Pb) and its isotopes are known to be released incongruently during early chemical weathering in continental settings. Incongruent weathering implies that a chemical weathering induced continental runoff trace metal isotope signature is not identical to bulk rock isotopic compositions. The incongruent release of Pb can mostly be ascribed to preferential chemical weathering of less weathering resistant accessory uranium and thorium-rich mineral phases present that are most abundant in differentiated continental crust. If this continental crust is ancient, these accessory mineral phases contain present-day Pb isotopic signatures that are in places extremely radiogenic, as well as substantially different from bulk rock Pb isotopic compositions. Several studies that investigated the Pb isotopic runoff evolution in the Labrador Sea, NW Atlantic and Arctic Beaufort Sea already reported very radiogenic Pb isotopic runoff signatures in these marine basins bordering the Laurentide Ice Sheet (LIS) during key time intervals of the last deglaciation. These earlier results require the existence of very radiogenic Pb isotopic freshwater signatures inland North America that were generated during incipient post-glacial chemical weathering reactions in response to the retreat of the LIS during the last deglaciation.We targeted subarctic Lake Melville in central Labrador aiming to resolve how the Pb specific chemical weathering signature changed in response to deglacial warming, in an initially subglacial setting that transitioned to completely ice-free conditions in the early Holocene. Lake Melville is a fjord‑like subarctic estuary in central Labrador that receives most of its freshwater and sediment from the Churchill River and other major tributaries draining a large early to mid-Proterozoic shield. We analysed two sediment cores from central Lake Melville that together archived the ambient dissolved Pb isotope signature over the past 13 ka. Our authigenic Pb isotope records are complemented by associated bulk detrital Pb isotope compositions, enabling us to compare the dissolved Pb isotope signature in the lake with corresponding sedimentary signatures. The lake was covered by the LIS until about 10.3 ka BP, yet still located in an ice-proximal setting until 8.5 ka BP. The region Labrador-Québec was ice free after ca. 5.7 ka BP.The most striking result of our record is the observation of (i) very radiogenic authigenic Pb isotope compositions throughout that are (ii) much elevated relative to the associated detrital compositions, which are rather unradiogenic. Very invariant Pb isotopic signatures observed until 10.5 ka BP confirm the suggested subglacial lacustrine sedimentary setting in the oldest section. The subsequent deglaciation witnessed most variable compositions, with most radiogenic compositions seen at ~8.2 ka BP. The record becomes substantially smoother after ~6 ka BP when the catchment area was no longer influenced by direct glacial runoff. While the detrital compositions suggest some geographic variability in sediment sourcing, the authigenic Pb isotopic compositions are not following these detrital signatures. Our results highlight the unique geological setting that make authigenic Pb isotopes in proximal North American sediment cores a sensitive proxy for for the detection of elevated deglacial runoff fluxes in circum-North American marine basins.

The Southern Ocean plays a central role in the global climate system by regulating large-scale circulation, facilitating interbasin exchange, absorbing large amounts of anthropogenic heat and carbon and influencing Antarctic Ice Sheet stability. Yet, observational data, including rare earth element measurements such as neodymium (Nd) isotopes and samarium (Sm), have so far remained sparse in the Indian sector and along the East Antarctic continental margin, thereby limiting our understanding of circulation, water mass transformation, and sediment-ocean interactions in a changing climate.Water masses in the Southern Ocean are traditionally characterized using hydrographic parameters (e.g., potential temperature, salinity, neutral density). During physico-chemical weathering along the East Antarctic margin, old continental crust supplies a distinctly unradiogenic neodymium isotope signature (low εNd) to regional shelf waters that interact with more radiogenic Antarctic Circumpolar Current waters (higher εNd) further north. This isotopic difference makes neodymium isotopes an especially powerful tracer of regional circulation and mixing along the East Antarctic continental margin. We present the first high-resolution dataset of dissolved εNd, together with Nd and Sm concentrations, from the Indian sector of the Southern Ocean. Gridded water column samples in conjunction with bottom water samples extracted from multicorer sediment supernatants, collected during expedition EASI-2 onboard RV Polarstern (Dec 2023–Feb 2024), provide a meridional transect from the Denman Glacier front (~66°S) to ~45°S along 100°E. Combined with hydrographic observations, this dataset provides a detailed framework for examining water mass structure, mixing, and regional boundary fluxes along the transect.Away from direct Antarctic continental influences, the εNd distributions show largely conservative behavior in intermediate to deep waters and allow clear identification of major Southern Ocean water masses. A striking feature is the persistence of a remnant North Atlantic Deep Water εNd signal within lower Circumpolar Deep Water, highlighting long-range interbasin connectivity. Near the Denman Glacier, warm and radiogenic modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf, evident in both physical properties and εNd signatures below ~400 meters water depth. As seen in earlier studies, a pronounced mCDW tongue was observed to reach close to the Denman Glacier front, with associated high basal melt rates evident from potential temperature and salinity in sampled local East Antarctic shelf waters.Nd and Sm concentrations increase linearly with depth north of the Polar Front, but exhibit substantial enrichment south of the front, reflecting deep-water upwelling, biogenic scavenging, and a latitudinally gradual boundary exchange. Pronounced variations in εNd and rare earth element concentrations in bottom waters point to substantial benthic additions in the southern reaches of the transect driven by weathering inputs from ambient terrigenous sediments, whereas particle-related scavenging appears to dominate offshore.This study closes a critical observational gap in the Indian sector of the Southern Ocean and provides new constraints on the present-day circulation, water mass structure, and the influence of Antarctic crustal and benthic Nd additions, while demonstrating the value of εNd as a tracer in modern and paleoceanographic contexts.

The Southern Ocean’s frontal system controls nutrient availability and sea-ice extent in the region, thus participating in the regulation of biological productivity and the export of carbon to the deep ocean. Sedimentary evidence indicates that latitudinal migrations of frontal boundaries accompanied glacial-interglacial transitions, with implications for spatial patterns of export production. Here, we present new biostratigraphic, geochemical, and sedimentological results of a sediment core transect along 100°E from the Antarctic coast to the Southeast Indian Ridge from the expedition PS140 of R/V Polarstern to address Pleistocene dynamics of oceanic fronts within the Antarctic Circumpolar Current, changes in sea surface water temperatures, sea-ice extent, and surface water productivity. A profile of eight piston cores was retrieved, comprising three sediment cores from the Seasonal Ice Zone (SIZ) and Permanent Open Ocean Zone (POOZ) south of the Antarctic Polar Front, and five sediment cores from the Polar Frontal Zone (PFZ) to the Subantarctic Zone (SAZ). Our stratigraphic framework is based on a combination of lithostratigraphic correlations, benthic foraminiferal oxygen-isotope data, and the tuning of dust records from our cores to Antarctic reference records. We also obtained planktic radiocarbon dates for younger sections. We use XRF-scanning-based element ratios, magnetic susceptibility, GRAPE densities, and bulk inorganic geochemistry data to reconstruct changes in export production and terrigenous sediment delivery via dust and ice transport. Major lithologies in the SIZ and POOZ are mainly diatom oozes with minor terrigenous components. The multiproxy data from the PFZ to the SAZ indicate a transition from biosiliceous to calcareous sediments. The northernmost cores of the transect are characterized by alternating sequences of foraminifera-bearing nannofossil ooze and diatom ooze. These alterations are likely derived from lateral migrations of the subantarctic frontal system and may reflect the frontal movement during glacial-interglacial changes. Our findings serve as the basis for ongoing and upcoming studies that will generate complementary paleoceanographic and paleoclimatic information in this to date poorly studied region of the Southern Ocean.

The Southern Ocean (SO) is of major importance in shaping climate transitions due to its substantial potential in storing carbon in the deep ocean and its release to the atmosphere most dominantly during glacial terminations. Through wind driven upwelling of deep waters and high latitude deep water formation, the SO acts as a gateway between the surface ocean and its interior. With the Antarctic Circumpolar Current (ACC), the world’s largest current system, the SO connects all three major basins of the global ocean and therefore integrates and responds to climate signals across the globe. Additionally, the SO exerts a major influence on the Antarctic Ice Sheet and partly controls its mass balance.The evolution of deep-water formation and export, as well as its interplay with the ACC and the Antarctic Ice Sheet are important factors that are still poorly constrained. We present a largely isotope geochemical based high-resolution multi-proxy reconstruction of IODP Site U1537 to examine these interplays in the southern Scotia Sea. The Scotia Sea is a key area in the SO, where newly formed well ventilated Weddell Sea Deep Water (WSDW) is admixed into and entrained underneath the ACC.Sedimentation in this area is mainly modulated by the strong ocean currents, as seen by extremely high sediment focusing throughout. Detrital neodymium (Nd) as well as authigenic and detrital lead (Pb) isotope compositions in Southern Ocean sediments provide insights into sediment sources, which can be clearly identified due to the distinct crustal ages of East and West Antarctica and its surrounding areas. Sediments at Site U1537 are dominantly sourced from the Antarctic Peninsula and the Weddell Sea region. The sediment provenance investigations are additionally complemented by K’-Ar analyses on the

Palaeoceanographic records spanning the Oligocene provide insights into Antarctic ice-sheet dynamics in a world warmer-than-today, allowing us to improve future climate projections linked to global warming. Here, we present a high-resolution multi-proxy record from ODP Site 689 (Maud Rise, Southern Ocean), to investigate the short-term Antarctic ice-sheet variability during the late Oligocene. Our oxygen isotope composition of sea water record reflects large, obliquity-paced ice volume fluctuations from 26.2 to 25.2 Ma. These fluctuations would have resulted in changes in the types of rocks eroded as the Antarctic ice-sheet advanced/retreated. Indeed, the radiogenic isotopic compositions of neodymium and lead co-vary with the oxygen isotopes for most of the record and identify changes in sediment provenance and weathering rates linked to advances and retreats of the Antarctic ice-sheet. At the same time, the stable incongruent lead isotope signal observed confirms the presence of a major East Antarctic ice-sheet during the late Oligocene.

The Last Interglacial (128–113 ka B.P.) provides key insights into climate-carbon interactions under warm, high-CO₂ conditions. While Antarctic temperature-driven Southern Ocean ventilation has been proposed to modulate atmospheric CO₂, role of Southern Hemisphere westerlies (SHW) remains unclear. Here we synthesize 15 Antarctic and 5 subarctic Pacific export productivity records over two glacial-interglacial cycles. We find that Antarctic and subarctic productivity generally tracked Antarctic temperature and atmospheric CO₂, but decoupled during Marine Isotope Stage 5, notably the Last Interglacial: high Antarctic productivity and CO₂ persisted despite reduced subarctic productivity and over 6 °C Antarctica cooling. We suggest that poleward-intensified SHW strengthened upwelling of Circumpolar Deep Water, sustaining Antarctic productivity and atmospheric CO₂, while altered Southern Ocean deep-water supply to the subarctic Pacific likely suppressed upwelling, limiting CO₂ release. These findings identify SHW as key in polar ocean dynamics and carbon cycling, with implications for future climate under projected SHW intensification.