Ashley T. Townsend

In this study, we present a mix of full‐depth profiles and near‐bottom observations of dissolved ²³⁰ Th and ²³² Th from the Adélie Land margin in East Antarctica, a key site of Antarctic Bottom Water (AABW) formation. We show that the distribution of dissolved Th isotopes in this region is governed by a combination of reversible scavenging, conservative transport and strong lithogenic inputs, especially on the continental shelf. A model incorporating both reversible scavenging and the upwelling of Circumpolar Deep Water successfully reproduces observed ²³⁰ Th _xs distributions in the upper and mid‐water column. However, deviations at depth point to additional processes, such as enhanced scavenging near the seafloor, and inputs of ²³² Th from benthic and/or suspended sediment dissolution. Exceptionally high inputs of ²³² Th to bottom waters on the Adélie shelf, likely sourced from glaciogenic sediments near the Mertz Glacier, are observed. This lithogenic inputs impart a distinct elevated ²³² Th/ ²³⁰ Th _xs signature to shelf waters, which appears to be largely removed via scavenging before entering the abyssal ocean within the AABW. In contrast, the upper 200 m of the water column shows offshore transport of shelf‐derived ²³² Th within Winter Water (WW). Based on the strong correlation between ²³² Th/ ²³⁰ Th _xs and dissolved manganese (dMn) in Shelf Water samples, we estimate ∼75% of the dMn observed in WW is sourced from the dissolution of shelf sediment. As WW is progressively mixed into overlying surface waters, this source likely acts as a “slow release” pathway of dMn to the euphotic zone, potentially supporting biological productivity.

Greater ocean export productivity fueled by enhanced delivery of iron to the Sub‐Antarctic Zone (SAZ) of the Southern Ocean is hypothesized to play an important role in reducing atmospheric CO ₂ during Pleistocene glaciations. However, spatial variability in iron input and nutrient availability complicates quantifying the integrated impact of iron‐fertilization on carbon export. We reconstruct preserved carbon export and iron input across the central Indian sector of the Southern Ocean with a latitudinal transect of sediment cores. ²³⁰ Th‐normalized fluxes of organic and inorganic productivity proxies are compared to bulk sedimentary redox‐sensitive trace metal concentrations to investigate preservation bias related to glacial‐interglacial changes in bottom water oxygenation. Across the last 28 ka, a strong relationship is observed between iron fluxes and export productivity across the SAZ, suggesting enhanced iron delivery during the Last Glacial Maximum (LGM) strengthened the biological carbon pump. Changes in bottom water oxygenation influenced the preservation of organic‐based productivity proxies, while excess barium fluxes reliably record carbon export. The southern SAZ (44.7°S) received ∼8× more iron during the LGM than the central SAZ (42.3°S). This pronounced iron flux gradient suggests terrigenous inputs from the Kerguelen Plateau entrained in the Antarctic Circumpolar Current delivered greater volumes of iron to the southern SAZ, relative to the northern SAZ dominated by aeolian input. These results, alongside previous studies, demonstrate iron fertilization increased export productivity across large swathes of the SAZ during the LGM. However, spatial heterogeneity in iron supply led to regional differences in the magnitude of glacial‐interglacial shifts in productivity.

Components such as organic carbon (C_org), carbonate, opal or barite are valuable paleoceanographic proxies, enabling the reconstruction of past changes in carbon cycling under various climate regimes, particularly when ²³⁰Th‐normalization is used to reconstruct particulate vertical rain rates. However, these components can be affected by poor preservation in the sediment. Barite is generally better preserved compared to other productivity proxies, but its estimates using “excess barium” (xsBa) calculations or Ba/Al ratios may not always be reliable, especially near the continental margin. This study presents a multi‐proxy investigation of surface sediments from the East Antarctic margin to assess the robustness of xsBa as a paleo‐productivity proxy. ²³⁰Th‐normalized fluxes of C_org and opal record consistent variations within the sediment, while xsBa appears to be affected by different processes. Results suggest that xsBa may not robustly reflect marine barite in sediment close to the Antarctic Ice Sheet, because (a) the authigenic signal is overprinted by large lithogenic inputs; and (b) other mineral phases (e.g., Fe oxides) may be important Ba carrier phases in the region. ²³⁰Th‐normalized Ba concentrations in the oxide‐free sediment fraction, which was reductively leached to remove Fe‐Mn oxides, aligns with the other biogenic fluxes, implying that specific leaching for barite isolation might be required in sediment proximal to the Antarctic Ice Sheet, rather than using the standard “xsBa” calculation or Ba/Al ratios. This study gives new insights into the sedimentary cycle of Ba in East Antarctic sediment and confirms the importance of using multi‐proxy records to perform paleo‐reconstructions.

While modeling efforts have furthered our understanding of marine iron biogeochemistry and its influence on carbon sequestration, observations of dissolved iron (dFe) and its relationship to physical, chemical and biological processes in the ocean are needed to both validate and inform model parameterization. Where iron comes from, how it is transported and recycled, and where iron removal takes place are critical mechanisms that need to be understood to assess the relationship between iron availability and primary production. To this end, hydrographic and trace metal observations across the GO‐SHIP section SR3, south of Tasmania, Australia, have been analyzed in tandem with the novel application of an optimum multiparameter analysis. From the trace‐metal distribution south of Australia, key differences in the drivers of dFe between oceanographic zones of the Southern Ocean were identified. In the subtropical zone, sources of dFe were attributed to waters advected off the continental shelf, and to recirculated modified mode and intermediate water‐masses of the Tasman Outflow. In the subantarctic zone, the seasonal replenishment of dFe in Antarctic surface and mode waters appears to be sustained by iron recycling in the underlying mode and intermediate waters. In the southern zone, the dFe distribution is likely driven by dissolution and scavenging by high concentrations of particles along the Antarctic continental shelf and slope entrained in high salinity shelf water. This approach to trace metal analysis may prove useful in future transects for identifying key mechanisms driving marine dissolved trace metal distributions.

Manganese (Mn) is an essential element involved in photosynthesis, yet its concentrations in Southern Ocean open waters are very low, arising from biological uptake and limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources. In this study, we investigated the potential export of Mn‐rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, biological uptake decreased dissolved Mn concentrations in surface waters north of the South Antarctic Circumpolar Current Front (−1), limiting its export. Conversely, mixing between Mn‐rich Antarctic Bottom Waters and Mn‐depleted Lower Circumpolar Deep Waters combined with oxidative and scavenging processes led to a decrease in dissolved Mn concentrations within bottom waters, with distance from the coast. Particulate Mn concentrations also showed a decreasing trend with distance from the coast. A comparison with other Antarctic coastal regions suggests this bottom water Mn removal may be widespread and that East Antarctica may be characterized by lower Mn concentrations compared to other regions. Still, subsurface dissolved Mn maxima (0.3–0.6 nmol kg⁻¹) represented a potential reservoir for surface waters. We hypothesize that these high subsurface values result from external sources near the shelf. Overall, these results suggest that the moderate lateral export of trace metal‐enriched waters contributes to the extremely low and potentially limiting Mn concentrations previously reported further north in this Southern Ocean region.

The contamination of the marine environment surrounding coastal Antarctic research stations remains insufficiently understood in terms of its extent, persistence, and characteristics. We investigated the presence of contaminants in marine sediments near Casey Station, located in the Windmill Islands of East Antarctica, during the period spanning from 1997 to 2015. Metals, hydrocarbons, PBDEs, PCBs, and nutrients were measured in sediments at anthropogenically disturbed sites, including the wastewater outfall, the wharf area, two former waste disposal sites, and various control locations. Sampling was carried out at three spatial scales: Locations, which were generally kilometres apart and formed the primary scale for comparison; Sites, which were 100 meters apart within each location; and Plots, which were 10 meters apart within each site. Consistently higher concentrations of most contaminants, and in some cases nutrients, were observed at disturbed locations. Some locations also exhibited an increase in contaminant concentrations over time. The spatial distribution of sediment properties (such as grain size and organic matter) and contaminants displayed intricate patterns of variation. Variation in grain size depended on the size category, with fine grains (e.g., <63 μm) showing the greatest variation at the Location scale, while coarse grains exhibited minimal variation at this scale. Contaminant levels demonstrated significant differences between Locations, accounting for approximately 55% of the overall variation for metals, while the variation within the 10-meter scale generally exceeded that within the 100-meter scale. Residual variation among replicate samples was also very high, demonstrating the need for adequate replication in studies of sediments and contaminants around stations. Some contaminants exceeded international guidelines for sediment quality, including metals, hydrocarbons, and PCBs. We conclude that Antarctic research stations such as Casey are likely to pose a moderate level of long-term ecological risk to local marine ecosystems through marine pollution. However, contamination is expected to be confined to areas in close proximity to the stations, although its extent and concentration are anticipated to increase with time. Raising awareness of the contamination risks associated with Antarctic stations and implementing monitoring programs for marine environments adjacent to these stations can contribute to informed decision-making and the improvement of environmental management practices in Antarctica.