Presenter: Veronica Vriesman (UC Davis)

Description:
Understanding past coastal climate variability is valuable for contextualizing modern changes in coastal settings, yet existing paleoceanographic records are primarily from offshore marine sediments and may not represent the dynamic coastal environment. A potential archive of climate variability for coastal California, USA is the intertidal mussel species Mytilus californianus. Archaeologists have collected copious stable isotope (ẟ18O and ẟ13C) data from 14C-dated M. californianus shells to study human history. When analyzed together, these data produce windows into an 8800-year-long record of Holocene isotopic variability for the southern California coast, although these data have not been studied comprehensively as a regional climate archive before. Here, we synthesize 6200 ẟ18O and ẟ13C data points from 12 published studies of M. californianus shells to characterize shell isotopic variability across ontogenetic, geographic, seasonal, and millennial scales. We aim to evaluate whether M. californianus, an intertidal bivalve mollusc: (1) records broad regional Holocene climate signals (e.g., warm/cool periods in southern California, freshwater flood events in Santa Barbara Basin, etc.) and (2) contributes novel information about Holocene seasonality and climate variability for coastal southern California. Data from M. californianus stable isotope profiles with five or more subsamples per shell show that this species records millennial scale climate patterns, freshwater flood events, seasonality (~5°C annual range), long-term warming trends (an overall 0.52‰ decline in shell ẟ18O over the past 8800 years), and local scale oceanographic variability (e.g., a warmer coastal mainland shell ẟ18O signature of -0.27‰ relative to the offshore  Channel Islands cooler ẟ18O shell signature of 0.32‰). While this species is a promising coastal archive, we emphasize the importance of a large sample size and high-resolution subsampling to accurately infer broader climate patterns.