Bivalve Biomineralization in a Changing Climate: Investigating Marine Mussel Calcification Patterns over Seasons to Millennia

The California mussel (Mytilus californianus) is an ecologically and culturally important bivalve mollusc species that provides a habitat for hundreds of other taxa throughout rocky intertidal environments along the Pacific Coast of North America. Mytilus californianus is a well-studied organism in the fields of coastal archaeology and marine ecology, but an improved understanding of its shell structure, life-history traits, and calcification patterns could provide valuable insights paleo-seasonality or paleoceanography of Holocene coastal environments. Here, I investigated shell characteristics of M. californianus across a variety of spatial and temporal scales in order to assess and optimize its utility as a biogenic archive. By applying morphometrics, optical microscopy, and geochemical data-synthesis methods to modern, historic, and Holocene M. californianus shells collected throughout the California Current System, I established relationships between growth banding and seasonality, documented the influence of micro- habitat on shell growth, characterized stable isotopic variability over seasonal, ontogenetic, and millennial scales, and identified regional differences in calcification patterns. I present evidence for rapidly shifting calcification patterns in northern California mussels from the early 2000s to 2020, including thinner inner calcite layers and lowered contrast between dark-light growth bands. Shifting shell characteristics were correlated with greater temperature extremes, heightened temperature variability, and more intense upwelling in the past few years relative to the early 2000s. Northern populations of M. californianus preferentially grow their shells during stable, moderate periods (mean monthly temperature near 13°C) with low variability (< 5 C° seasonal temperature range), as indicated by calcite-rich light bands. I then expanded the geographic and temporal scope to characterize geochemical variability of M. californianus shells throughout southern California over millennial scales. By synthesizing the ample published stable isotope records from archaeological mussel shells of the Channel Islands, California, I found that individual analysis of complete stable isotope profiles of many shells offers more valuable paleobiological and paleo-seasonal insights, including an δ18O-inferred annual seawater temperature range of ~ 5°C in the Channel Islands. Collectively, stable isotope data from mussels revealed both millennial-scale isotopic variability indicative of Holocene warm-cool oscillations and an east-west temperature gradient characteristic of the modern-day Channel Islands. However, M. californianus shells exhibited highly variable intra-individual δ18O values depending on micro-environment and ontogeny, complicating the use of aggregated stable oxygen and carbon isotope data from many individuals as an annually resolved climate proxy. Finally, I investigated changes in shell morphology, growth banding, and microstructure over the twentieth century in M. californianus shells from the southern portion of the California Current System (Santa Barbara through Baja California Sur). Shell thickness, morphology, the percentage of calcite-rich light bands, and the contrast between dark-light banding remained unchanged throughout all study sites in southern California and the Baja California peninsula. It is likely that southern populations of M. californianus are adapted to warmer conditions, while mussels from regions farther north in the California Current System may be more susceptible to warming waters. Mytilus californianus is a complex yet useful archive of environmental variability. Its shell records its environment over seasonal to millennial scales, depending on a variety of factors such as ontogenetic age, tidal position, and the sampling technique. I conclude that M. californianus can serve as a valuable record when multiple approaches are applied in tandem: analysis of the growth band pattern, ontogenetic stable isotope profiles, shell morphometrics, and microstructural imaging in one individual, and then across multiple individuals from many sites through time