SEDIMENTARY INORGANIC CARBON ISOTOPE VALUES SUGGEST BENTHIC ANOXIA BECOMES MORE COMMON ACROSS THE KPB ON SEYMOUR ISLAND, ANTARCTICA

Seymour Island, on the Antarctic Peninsula, preserves the most southerly outcrop exposure of the Cretaceous-Paleogene Boundary (KPB). Previous paleontological research there has argued for a complex, multi-causal KPB extinction (Tobin et al., 2012; Petersen et al., 2016; Tobin, 2017) or for an extinction driven solely by bolide impact (Witts et al., 2016; Whittle et al., 2019). Arguments for a complex extinction typically attribute the pre-KPB extinctions to some level of anoxia in the benthic environment, which has been observed in sedimentary redox proxies (Schoepfer et al., 2017) and sclerochronological isotope sampling of bivalves (Hall et al., 2018; Mohr et al., 2020).

We present sedimentary inorganic δ¹³C data derived from the deepest portions of shallow (~10 cm) cores drilled for paleomagnetic analysis from well-cemented strata (Tobin et al., 2012; Schoepfer et al., 2017). Samples were pulverized in a process that likely averages multiple carbonate cements. The resulting δ¹³C values are bimodally distributed around -20‰ and 0‰ (VPDB) albeit with substantial variation. The lower values are likely consistent with early diagenetic cements derived from dissolved inorganic carbon (DIC) sourced largely from organic matter, or possibly methanogenesis for the lowest values. Values around 0‰ (VPDB) are likely derived from early cements consistent with oxygenated benthic DIC, while the few higher values may require later diagenetic fluids.

The samples span 1150m of stratigraphy, with the KPB placed about 60m below the top of the sampled section. In the lower 850m of the section δ¹³C values fluctuate sharply between extremes, but the upper 300m of stratigraphy has fairly consistent δ¹³C values (-18±3‰) with two exceptions, one point at the KPB and one 60m below the boundary. This interval of consistently low δ¹³C values spanning the KPB may result from sustained periods of low benthic oxygen, in part consistent with prior work. However, the δ¹³C data is not well correlated with other anoxia proxies from the same materials. Additionally, analyzed samples were available only where paleomagnetic samples could be acquired, which might result in sampling artifacts or diagenetic biases over this interval. Future work targeting this problem specifically may allow more insight into the cause of this data distribution.