High-resolution stable isotope (δ18O and δ13C) sclerochronology of accretionary carbonate bivalve shells can provide subannual environmental records useful for understanding intervals of extinction, which are commonly periods of rapid change and instability. Here, we present results from high-resolution serial sampling of Lahillia larseni bivalve shells across the Cretaceous-Paleogene boundary (KPB) on Seymour Island, Antarctica. These data highlight two intervals of anomalous δ18O and δ13C values that coincide with condensed fossil last occurrences: one at the KPB and one at an apparent extinction event 150 k.y. earlier. We interpret these two intervals to represent periods of both climate warming, as indicated by lower δ18O, and seasonal anoxia or euxinia, as evidenced by anomalously low (−21.6‰ to −3.0‰ VPDB [Vienna Peedee belemnite]) δ13C values with high (2‰ to 19‰ in magnitude) seasonal variation. Low-oxygen conditions may have acted as a kill mechanism at the earlier extinction interval and possibly prolonged recovery from the KPB extinction.