Reconstructing Holocene Kelp Ecosystems through Stable Isotope Analysis of Archaeological Shell Material

Kelp forests represent one of the most diverse and productive ecosystems on Earth, yet our understanding of the processes regulating their structure and dynamics is constrained by the contemporary focus of most previous studies. The poor preservation of marine algae in archaeological or fossil deposits accounts for part of this oversight. One possible method to overcome this is stable isotope analysis of the remains of kelp consumers (e.g., echinoids, gastropods, etc.), which are a common part of Holocene middens. The stable isotope compositions of these remains are related to the diets of the consumers and can serve as proxies for the types of primary producers present when the animals were living. Recovery of this proxy from shell material extracted from archaeological middens could be used to identify shifts in the abundance of different marine producers within coastal ecosystems and provide a way to gauge the significance of kelp ecosystems in the past.

We have initiated a two-phase project that involves analysis of shell material from recent and Holocene communities to assess the utility of this method. Phase I involved collection of shell material from different species of marine grazers (Strongylocentrotus, Tegula) and sessile filter-feeders (Mytilus) from coastal sites in central California with and without kelp. The stable isotope composition of the organic matrix extracted from these shells was then analyzed to identify isotopic markers for kelp and other marine algae. Results from this project will be carried over to Phase II using shell material from collected from a nearly complete trans-Holocene (10,000 calendar years BP to ca. AD 1820) record of shell remains from San Miguel Island. The stable isotope composition of these remains will be used to reconstruct changes in the diets of these consumers over time and establish whether sampled consumers lived in forested or deforested habitats.