Not up in smoke: Lipid and phytolith evidence for the function of combustion features at CA-ALA-11, a San Francisco Bay area shellmound

Abstract

Ash and black char samples from seven combustion features at CA-ALA-11, an Early to Middle Period (ca. 2500 cal BCE to 585 cal CE) shellmound site on the San Francisco Bay shoreline, were analyzed for lipid, isotope, and phytolith content. Three features were intermingled with human burials and four were from nearby contexts not directly associated with human remains. Unlike more fragile biomolecules and floral remains, lipids and phytoliths can survive exposure to high temperatures. Together, these techniques supply independent and complementary lines of data for considering past cultural practices, local ecology, and post-depositional contributions. Our results shed light on the function and content of several combustion features, highlighting the untapped potential of such applications in the archaeology of California and elsewhere in North America. We focus on two combustion features with very different purposes. One appears to be the burnt remains of a basket (or possibly, a woven mat) coated with bitumen. This feature was associated with a burial and the basket or other woven object may have been burned as part of the funerary ceremony. Another feature, not directly associated with a burial, was composed of burned oyster shells and layered with leaves from a broadleaf tree—seemingly the remains of an ancient cooking feature for baking/steaming shellfish. Though small, this study demonstrates that analysis of sediments from combustion features can provide behavioral and ecological insights while avoiding destructive analysis of artifacts or human remains. We conclude with simple recommendations for integrating phytolith and lipid analysis of combustion features in future archaeological projects.