Exploring the Paleoceanographic Utility of Intertidal Mussels Using Paired Geochemical and Microstructural Proxies

Molluscan accretionary skeletons offer seasonally resolved archives of sea surface temperature (SST) and rainfall when shell chemistry and microstructure are examined in tandem. Bivalve shells are particularly advantageous for reconstructing past nearshore marine environments, but SST and rainfall interpretations remain dependent on our understanding of shell structure and growth rate, among other physiological factors. In order to investigate this potential archive, we analyze and compare trace elemental ratios (Mg/Ca, Ba/Ca) and microstructural features in modern Mytilus californianus (mussel) shells from Bodega Bay, CA; we chose a well-studied taxon abundant in both intertidal environments and archaeological middens to inform future paleoenvironmental studies. Additionally, we selected a location with daily instrumental SST and rainfall records for the past 20 years. In our specimens, we identify distinct geochemical extremes experienced by individual mussels over multiple growing seasons, highlighting the variability among individuals. We suggest that intra-individual variability within one site is a product of ontogenetic age and intertidal position; specimens between 60 and 100 mm record SST conditions most closely in their shell chemistry and specimens from a high intertidal position (+ 3 m) exhibit Mg/Ca profiles with greater variability (2-8 mmol/mol). We also find that ontogenetic Ba/Ca profiles are characterized by low background variability (~ 0.4 mmol/mol) with extreme peaks, a pattern consistent with profiles generated by previous Mytilus studies. Trace element profiles will be paired with a microstructural examination of each shell along the same ontogenetic profile analyzed for geochemistry. We predict that, when evident, disturbance lines are more likely to pair with Ba/Ca peaks, denoting extreme rainfall events, and wide accretionary increments are more likely to occur during Mg/Ca minima (~ 2 mmol/mol), denoting cooler upwelling months at Bodega Bay. Interpreted SST extrema and rainfall events can be corroborated by the instrumental records. These paired geochemical-microstructural proxies can then be applied to archaeological shells to track changes in nearshore sea surface conditions and identify storm events at sites with longstanding human occupation.