SCLEROCHRONOLOGICAL RECORDS OF CARBONATE ISOTOPES FROM ANTARCTIC BIVALVES REVEAL CLIMATE WARMING AND VARIATIONS IN SEASONAL PRODUCTIVITY ACROSS THE K-PG BOUNDARY

High-resolution ontogenetic stable isotope (δ¹⁸O and δ¹³C) analysis of accretionary carbonate bivalve shells can provide subannual records of the climates and environments in which they lived. These subannual paleorecords can provide a better understanding of mass extinction events, which are often accompanied by intervals of rapid climate change and environmental perturbations. 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 an unstable environment for a period of ~1.3 Myr across the Cretaceous-Paleogene (K-Pg) extinction interval.

If all of the δ¹⁸O variation is interpreted in terms of temperature, the ontogenetic δ¹⁸O profiles of L. larseni indicate that it had an austral spring or summer season of growth, with the magnitude of temperature variation experienced during the growing season ranging from 1.6 ± 0.3°C to 9.5 ± 0.5°C (1σ). A sudden warming event occurs in the ~100 kyr across the K-Pg boundary, with average temperatures increasing 6.8 ± 1.3°C (1σ). The lower-amplitude ontogenetic δ¹³C profiles of L. larseni (<2-3‰ variation) record seasonal variations in the dissolved inorganic carbon reservoir of the ocean as a result of the activity of primary producers. Interannual variations in these ontogenetic δ¹³C profiles are interpreted to be possible evidence for a second bloom of productivity in years with a late summer influx of additional nutrients, and for the delayed onset of primary productivity in years with winter sea ice. Higher-amplitude ontogenetic δ¹³C profiles with values as low as -21.6‰ provide evidence for the seasonal cycling of methanogenesis and methane oxidation occurring during a ~180 kyr interval across the K-Pg boundary. This study, which presents the highest resolution ontogenetic sampling of L. larseni shells, provides new insights into the seasonality of an unstable paleoenvironment and highlights the potential for changes in subannual variability to destabilize an environment during an interval of extinction.