USING QGIS AND OXYGEN ISOTOPE RATIOS TO ASSESS SEASONAL TO MULTI-DECADAL VARIATION IN THE BIVALVE, ASTARTE BOREALIS, FROM THE BALTIC SEA

Marine bivalves are the longest known living non-colonial animals on the planet today, reaching lifespans in excess of 500 years. Such long-lived species typically inhabit cold-water environments making them important archives for reconstructing high-latitude climate at seasonal, multi-year, decadal, and centennial timescales. Astarte borealis is found in the northern Pacific, Atlantic, and Arctic Oceans and lives for several decades and possibly longer. Thus, A. borealis is potentially an important archive for paleoclimate reconstruction. This study tests the hypothesis that slow shell growth occurs in winter and that this species grows for decades. Oxygen isotope ratios (δ¹⁸O) from two individuals from the Baltic sea were analyzed using secondary ion mass spectrometry (SIMS; WiscSIMS lab at UW-Madison) instead of conventional isotope ratio mass spectrometry because of their small size (height ~12 mm). SIMS δ¹⁸O analyses (10-mm-diameter spots) were targeted along traverses of shell cross-sections cut along the maximum axis of growth. We used the QGIS freeware and a Georeferencer plug-in to map the position of each δ¹⁸O analysis (n=452, 132) onto images of the shell cross-sections that show growth banding. We conclude that the highest δ¹⁸O values are associated with the dark increments (periods of slow growth) representing annual growth checks formed during winter months, and the specimens are ~25 years old.