OXYGEN ISOTOPE RATIOS MEASURED BY HIGH-PRECISION SIMS TO ASSESS SEASONALITY AND LIFE HISTORY OF BALTIC SEA ASTARTE BOREALIS (BIVALVIA)

The bivalve Astarte borealis (Schumacher, 1817) is an important component of the biomass in arctic-boreal seas and oceans and has a relatively long lifespan, living for several decades. Therefore, it is potentially a useful recorder of environmental information. Our recent sclerochronologic studies of this relatively small bivalve species in the Baltic and White Seas have found some populations exhibiting easily identifiable annual growth increments, while annual increments in other populations are less clear. This morphological difference in shell growth records is likely related to local environmental conditions. Conventional micromilling of specimens from the White Sea, Russia, for oxygen isotope analysis allowed us to identify the seasonal timing of annual growth checks. Because of the smaller size and narrower or more complex growth increments of specimens from the Baltic Sea, micromilling these shells was not possible. Here, we present a high-resolution submonthly resolved record of oxygen isotope variability in a specimen from the Baltic Sea using secondary ion mass spectrometry (SIMS; WiscSIMS lab at UW-Madison). Oxygen isotope ratios are quasi-sinusoidal, ranging from −6 to −3‰ (VPDB) across >20 years of growth (shell height = 12.5 mm). Amplitudes range from 2-3‰ and are consistent with what we would expect given the 12-15°C seasonal water temperature variability reported in an earlier nearby study. Annual growth checks occur during cool months. Comparison with water monitoring data will allow us to better understand the life history and environmental/ecological records contained in this new bio-archive.