Marine Reservoir Age Determinations from Shell-Charcoal Pair

Archaeological shell mounds (or middens) are another important archive of past climate change. Shell mounds are stratified accumulations of sediment, marine shells, animal and human bones, artifacts, ash, rock, and soil. We are analyzing the geochemistry of shell mounds along coastal California to assess changes in coastal upwelling and salinity over the past several thousand years. Radiocarbon dating of closely associated charcoal and shell provides a measure of the reservoir age in the waters in which the shells precipitated. These reservoir ages in turn reflect variations in the upwelling of slowly circulating deep water masses along the California coast. Our preliminary studies indicate that the reservoir ages along coastal California varied significantly throughout the Holocene, and appear to be related to changes in freshwater inflow and precipitation in California.

Seasonal Paleosalinity Records From Archaeological Shellmounds

The Ingram laboratory and colleagues at UC Berkeley have been engaged in ongoing research into the environmental history of the San Francisco Bay and its watershed for a number of years, using multiple proxy records, including geochemical analysis of calcite, to infer changes in this key estuarine system. The estuary’s fresh and saltwater mixing components differ systematically in d 18 O and, consequently, estuarine w ater d 18 O is a reliable indicator of its salinity. Since biogenic carbonates such as mollusk shell and foraminiferal tests record the d 18 O of the water from which they are deposited, isotopic analyses of these materials play a key role in our paleoclimate reconstruction work.

Superimposed on the secular change is a strong intra-annual variation typical of watersheds subject to highly seasonal precipitation and runoff regimes. The seasonal shift in the estuary’s salinity field is a key feature of the estuarine environment. We are currently beginning to study the history of this phenomenon through geochemical analysis of incrementally deposited calcite in fossil mussel (Mytilus trossulus) shells recovered from Native American shellmound from the shores of San Francisco Bay . We have begun analyses of Mg/Ca of the mussel shells to determine water temperature, in addition to the oxygen isotope composition (these two proxies should allow us to determine both temperature and salinity).

PUBLICATIONS:

Kennett, D.J., Ingram, B.L., Southon, J.R., and Wise, K. (2002) Differences in 14C age between stratigraphically associated charcoal and marine shell from the Archaic period site of Kilometer 4, Southern Peru: old wood or old water? Radiocarbon V. 44, p. 53-58.

Ingram, B. L. (1998) Differences in radiocarbon age between shell and charcoal from a Holocene shellmound in northern California, Quaternary Research V. 49, p. 102-110.

Erlandson, J., Tveskov, M., Kennett, D. and Ingram, B. L. (1997) Further evidence for a terminal Pleistocene occupation of Daisy Cave, San Miguel Island, California, Current Research in the Pleistocene.

Kennett, D. J., Ingram, B. L., Erlandson, J. M., and Walker, P. (1997) Evidence for temporal fluctuations of marine radiocarbon reservoir ages in the Santa Barbara Channel, Southern California, J. Archaeol. Science V. 24, p. 1051-1059.

Erlandson, J. M., Kennett, D. J., Ingram, B. L., Guthrie, D. A., Morris, D. P., Tveskov, M. A., West, G. J., and Walker, P. L. (1996) An archaelogical and paleontological chronology for Daisy Cave (CA-SMI-261), San Miguel Island, California, Radiocarbon. V. 38, p. 355-373.

Ingram, B. L., and Southon, J. R. (1996) Reservoir ages in Eastern Pacific coastal and estuarine waters, Radiocarbon 38, 573-582.