Body size variability of North Atlantic benthic fauna driven by bottom-water temperature and oxygen during late Quaternary glacial-interglacial cycles

Organism body size is a critical aspect of marine ecosystems and is influenced by climate change on seasonal to geologic time scales. Recent integration of mechanistic models of metabolism, laboratory experiments, and fossil records has opened a new avenue for understanding the roles of thermal sensitivity and hypoxia tolerance in body-size evolution. Here we explore climatic factors driving intraspecific body size variability of benthic ostracods in the central and eastern North Atlantic Ocean. We analyzed over 300 adult shell sizes of multiple ostracod species in the genus Krithe at Sites Chain 84-24-4PC (42°N, 33°W, 3427 m water depth) for the past ~50,000 years and IODP U1588 (37°N, 9°W, 1139 m water depth) for the past ~700,000 years. Chain 84-24-4PC and U1588 are predominantly influenced by North Atlantic Deep Water (NADW) and Mediterranean Outflow Water (MOW) today, respectively. Results show that size reduction corresponded to up to 5 °C deglacial warming during the interval 22-14 ka (MIS 2-1) at the Chain 84-24-4PC core site. Even more striking, size varies 60-70% during major glacial-interglacial transitions (MIS6-5, MIS12-11, and MIS16-15) at Site U1588. The differences observed in the magnitude of size reduction between the two sites are likely influenced by the varying ranges of temperature and, potentially, oxygen variability at their respective water depths. We discuss the potential of using body size changes to reconstruct variability in temperature and oxygen across glacial-interglacial cycles.