CLUMPED ISOTOPE PALEOTHERMOMETRY AND MOLLUSK ASSEMBLAGE-BASED PALEOTHERMOMETRY YIELD CONSISTENT ESTIMATES FOR LATE PLEISTOCENE MARINE TERRACES OF THE SOUTHERN CALIFORNIA BIGHT

Pleistocene marine terraces along the California coast preserve mollusk assemblages that have been studied for over 150 years. This record has been especially important for documenting geographic range shifts of species in response to climate changes, and early application of oxygen isotope paleothermometry found general consistency between paleotemperature estimates based on δ¹⁸O of mollusk shells and paleotemperature estimates based on the modern-day geographic distributions of co-occurring species. However, further progress in understanding this record has been inhibited by persistent uncertainties concerning (1) the formation ages of individual terraces and (2) the relative contributions of temperature and salinity variations to variation in δ¹⁸O values. Here we report new paleotemperature estimates for well-dated California terrace assemblages from clumped isotope paleothermometry, which is independent of salinity. We analyzed 6 recent and 26 fossil shells of the intertidal to subtotal gastropod Callianax biplicata, which is ubiquitous in both modern and ancient mollusk assemblages in southern California. Modern shells from 3 sites yield mean growth temperature estimates well within observed local annual temperature ranges. Fossil shells from nearby terraces yield temperatures that are indistinguishable from modern for the last full interglacial ~125,000 years ago (Marine Isotope Stage 5e) and on average ~2.5˚C cooler than modern for the Marine Isotope Stage 5a interglacial ~80,000 years ago. Seawater δ¹⁸O values calculated from clumped isotope temperature estimates and shell δ¹⁸O values are generally indistinguishable from modern seawater δ¹⁸O values in the Southern California Bight. We use these seawater δ¹⁸O values to extend our dataset by estimating paleotemperatures from more than 1,000 bulk δ¹⁸O measurements of 80 additional C. biplicata individuals. Our results confirm that isotopic paleotemperature estimates and paleotemperature estimates based on the modern-day geographic distributions of co-occurring species are generally consistent across multiple sites representing multiple Pleistocene interglacials. These findings bolster evidence that the thermal tolerance ranges of marine species are generally stable on ~100,000-year timescales.