218-12 New Geochemical Analysis of Well-Preserved Cliff Outcrops of Seymour Island, Antarctica Provide Insights into Paleoenvironments Prior to the End Cretaceous Mass Extinction

Abstract:

Seymour Island, located in the Weddell Sea of the NE Antarctic Peninsula, has an expanded stratigraphic record of the uppermost Cretaceous (Maastrichtian) through the lowermost Paleogene. Specifically, the López de Bertodano Formation (LBF), a shallow marine clastic shelf deposit, has been subject to decades of studies investigating the End Cretaceous Mass Extinction (ECME; ~66 Ma), including analyses of well-preserved macro- and microfossil records, and sedimentological and geochemical data. A major challenge to this work, especially for sedimentary geochemistry, is that a combination of intense freeze-thaw weathering at the surface and a deeper permafrost layer make acquisition of fresh samples difficult. Weathering affects geochemical data quality and obscures primary sedimentological features, particularly in unconsolidated mudstones. To address this shortcoming, the NSF funded CSI: Antarctica expedition aboard the R/V Sikuliaq sampled sediments from a cliff exposure along the southern shore of Seymour Island, where rapid erosion outpaces freeze-thaw weathering. New geochemical data from this exposure can evaluate the fidelity of prior studies and provide new insights about the environmental conditions leading up to and through the ECME.

In January/February 2026, we measured and sampled approximately 215 meters of stratigraphy spanning the Cretaceous-Paleogene Boundary (KPB). Bulk sediment samples (n=170) were sub-sampled for major and trace element concentration (XRF, ICP-MS) and subjected to grain characterization through thin section and grain size analyses. Preliminary XRF analyses reveal several intervals with elevated redox-sensitive trace element (V, U, As) concentrations indicative of anoxic bottom water conditions. Two of these intervals correspond to the glauconitic horizons at the base (~40 m; ~20% glauconite) and top (~200 m; ~30%) of “Unit 9” of the LBF, the latter of which is coincident with the KPB. Here, we hypothesize that anoxic conditions are indicative of ferruginous conditions due to the abundant iron-rich glauconite. Redox-sensitive trace element enrichments are present at a concretionary (~65% carbonate) horizon within “Unit 9” (~110m). We hypothesize that this horizon is a cold methane seep, with methanogenesis near the sediment-water interface reflecting low-oxygen conditions in sedimentary porewater.  

The coincidence of iron-rich glauconite (suggesting an abundant supply of reactive ferrous Fe) with evidence for low-oxygen conditions and methanogenesis is consistent with low seawater sulfate concentrations during the Maastrichtian at Seymour Island. Future work includes further grain characterization and coupled ICP-MS data.

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