High-resolution ontogenetic stable isotope (δ18O and δ13C) analysis of accretionary carbonate bivalve shells can provide subannual records of their environments. These records present an opportunity to better understand the seasonal variation of past climates and environments. Characterizing the seasonality of past environments is especially helpful in resolving subtle changes during intervals of climate change or environmental perturbations such as during extinction events. Late Cretaceous and early Paleogene bivalve shells of Lahillia larseni from Seymour Island, Antarctica, were isotopically sampled at a high ontogenetic resolution in order to characterize the seasonality of this unstable environment for a period of ~1.3 Myr across the Cretaceous-Paleogene (K-Pg) extinction interval. Ontogenetic δ18O profiles support an austral spring or summer season of growth for L. larseni, with the magnitude of seasonal temperature variation experienced during the growing season ranging from 1.6 ± 0.3°C to 9.5 ± 0.5°C (1σ). The ontogenetic δ13C profiles record isotopic variations in the bottom water dissolved inorganic carbon (DIC) reservoir resulting from the seasonal activity of primary producers. Interannual variations in the primary productivity signal are interpreted as possible evidence for a second bloom of productivity in late summer in years with an additional flux of nutrients, and for the delayed onset of primary productivity in years with winter sea ice. Abnormally low δ13C values in ontogenetic carbon isotope profiles with large seasonal variation provide evidence for the seasonal cycling of biogenic methane production and oxidation occurring during a ~180 kyr interval across the K-Pg boundary. Stratigraphic trends reveal a sudden 6.8 ± 1.3°C (1σ) warming event across the K-Pg boundary, with a duration of ~100 kyr, similar to previous estimates of warming across the K-Pg. This study, which presents the highest resolution ontogenetic sampling of L. larseni shells, provides new insights into the seasonality of an unstable paleoenvironment with no modern analogue, and highlights the potential for changes in subannual variability to destabilize an environment during an interval of extinction.