Growth Increment Analysis of Marine Bivalves from the North
This study aids in developing a sea surface temperature (SST) proxy with monthly temporal resolution using a combination of growth increment and stable isotope analyses of marine bivalves from the north coast of Peru. Faunal assemblages from the Siches and Ostra Base Camp archaeological sites contain shells of warm-tropical mollusks that currently live farther north in Ecuador. The presence of warm-tropical species in these sites and others as far south as 10"s latitude and dating prior to 5730 cal yr B.P. indicates a stable warm-water regime in the eastern tropical Pacific which subsequently changes to a modern temperate-water regime after 5730 cal yr B.P. The presence of a warm-water regime may preclude the modern Walker Circulation and ENSO-scale (El Nino and Southern Oscillation) climate variability in the region. A high-temporal-resolution SST proxy can quantify interseasonal and interannual variability along the littoral of northern Peru in order to assess environmental conditions prior to 5730 cal yr B.P. The intertidal species Chione subrugosa and Protothaca ecuatoriana exhibit daily growth periods, while the subtidal species Trachycardium procerum and the mangrove swamp species A. tuberculosa exhibit subdaily growth periods. Incremental growth in these species enabled a time correlation of stable oxygen isotope samples drilled from the shells. Modern specimens of A. tuberculosa, C. subrugosa, and T. procerum provided discrete time series (two to four years in duration) of stable oxygen isotope ratios that were correlated with recorded ocean temperature and salinity. Middle Holocene (7420 cal yr B.P. to 5650 cal yr B.P.) specimens of A. tuberculosa, C. subrugosa, P. ecuatoriana, and T. procerum recovered from shell middens at the Siches (4.5"s) and Ostra Base Camp (9"s) sites provided oxygen isotope ratio time series two to four years in duration. The oxygen isotope ratio time series constructed from the ancient shells float within the date range provided by radiocarbon-dated contexts from which the shells were excavated. Results suggest that subtidal species, which lie beyond the influence of freshwater influx accurately record ambient water conditions. Furthermore, ancient specimens of T. procerum from the Ostra Base Camp (6660-6640 cal yr B.P.) have oxygen isotope ratios that differ from those of modern T. procerum. Some C. subrugosa, A. tuberculosa, and P. ecuatoriana from the Siches and Early Honda components (7420 cal yr B.P. to 5650 cal yr B.P.) of the Siches site indicate multiple freshening events (e.g. increased precipitation or river discharge) per annual cycle. These events imply a climate regime wetter than the modem climate regime at 4S0S, which may result from a warmer mean SST.