Stable isotope profiles of Serripes groenlandicus shells. II. Occurrence in Alaskan Coastal Water in south St. Lawrence Island, northern Bering Sea

Abstract

Three bivalve shells (Serripes groenlandicus, Greenland cockle), collected alive in the south of St. Lawrence Island, have been analyzed for the stable oxygen and carbon isotopic composition using a sequential method to trace the variation of hydrographic conditions (i.e., temperature, salinity, and oxygen isotope) of ambient seawater during the life-spans of these organisms. The delta O-18 profiles of three shells exhibit a cyclic pattern interpreted as seasonal variation of Bering Shelf Water, which is the dominant water mass at the collection site. Discernible and distinct light delta O-18 peaks occurred in the summer of 1991, suggesting unusual hydrographic conditions. These are attributed to the migration of Alaskan Coastal Water into the south of St. Lawrence Island during the summer. The changed oxygen isotopic compositions of seawater between 1990 and 1993 in the study area supports the idea that the flow of Alaskan Coastal Water, intensified by the enhanced freshwater discharges from the Alaskan coast, can be migrated westward into the south of St. Lawrence Island. The delta C-13 profiles also show apparent cyclicity, reflecting the variation of available carbon isotopes in relation to photosynthesis and oxidation of organic matter. A seasonal bloom of phytoplankton and consequent oxidation of organic matter is the possible cause of the noticeably light delta C-13 peak in 1992, not the influx of terrestrial carbon or metabolically derived carbon. The warm Alaskan Coastal Water, characterized by low oxygen isotopic composition, is clearly recorded in the shell isotope profiles, which provides a record of the hydrographic variation of the ambient seawater.