MIDDLE HOLOCENE SEA SURFACE TEMPERATURE OSCILLATIONS RESOLVED BY STABLE ISOTOPE AND GROWTH INCREMENT ANALYSES OF PERUVIAN BIVALVES

A sea surface temperature (SST) proxy with monthly temporal resolution was developed using a combination of growth increment and stable isotope analyses of marine bivalves from the north coast of Peru. The intertidal species Anadara tuberculosa, Chione subrugosa, and Protothaca ecuatoriana exhibit daily incremental growth, while the subtidal species Trachycardium procerum exhibits subdaily incremental growth. 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 which were correlated with observed SST and salinity in order to test the accuracy of growth increment counts and determine an accurate oxygen isotope temperature equation. Subsequently, a single temperature equation was applied to Middle Holocene (6490 ± 90 B.P. to 4930 ± 80 B.P.) specimens of A. tuberculosa, C. subrugosa, P. ecuatoriana, and T. procerum excavated from shell middens at the Siches (4.5°S) and Ostra Base Camp (9°S) sites. The SST time series (two to four years in duration) 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 flux and anaerobic respiration during exposure at low tide, accurately record ambient water conditions. Ancient specimens of T. procerum from the Ostra Base Camp have isotopic values that differ from modern values. Some C. subrugosa, A. tuberculosa, and P. ecuatoriana from the Siches and Early Honda components (6490 ± 90 B.P. to 4930 ± 80 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 modern climate regime at 4.5°S, which may result from an elevated mean SST prior to 4930 ± 80 B.P.