MUSSEL CASTS AS INDICATORS OF PAST CLIMATES: AN EXAMPLE FROM AN ARCHAEOLOGICAL SITE IN NORTHERN SAN FRANCISCO BAY, CALIFORNIA, USA

Evidence of marine food resources such as bivalves are a common dietary remains at bay margin occupation sites. Prehistoric site CA-MRN-254, located on the Dominican University of California campus in Marin County, California, provides a record of occupancy spanning the past 1,800 years. An important food source was the quiet- water dwelling blue mussel (Mytilus trossulus), and less commonly the intertidal California mussel (M. californianus). In several cultural horizons, endocasts associated with the mussel Modiolus sp. were found, although no shell fragments were identifiable as such. These casts are thought to have formed soon after death when the shells filled with sediment and were collected along with living mussels and all were roasted before consumption. Those found to contain sediment were discarded. Radiocarbon dating of charcoal found with the endocasts suggests the mussel casts formed between circa A.D. 190-720. Thin-sections from several of these casts were analyzed for their sediment characteristics and microfossil content. Fine-grained sediment and pelletal mud indicate that these molluscs probably occupied a quiet inner bay depositional environment prior to harvest. Benthic foraminifers characteristic of estuaries (Ammonia tepida) were present in very small numbers. A diverse diatom flora of 34 species was also recovered, including taxa characteristic of freshwater to marine salinities, indicating the influence of local rivers and tidal conditions that brought marine waters to the collection site. For most of the archaeological record, the diatom assemblage is indicative of a cool thermal regime similar to present conditions, but slightly warmer climatic conditions occurred at ca. A.D. 680 as evidenced by the rare presence of Triceratium dubium and Thalassionema nitzschioides. Both of these taxa are indicative of normal marine conditions that were possibly warmer than present, coincident with the beginning of the Medieval Climate Anomaly, a period during which the volume of water flowing from the Sacramento River into San Francisco Bay was lower. These results demonstrate that mussel endocasts have great potential for providing paleoenvironmental information at maritime archaeological sites.