The potential of marine bivalve Spisula sachalinensis as a marine temperature record

Abstract

• The potential of Spisula sachalinensis as a high resolution temperature indicator • δ 18 O inversely correlates to seasonal ocean temperature but δ 13 C is less cyclical • Annual growth lines are formed during the warmest part of the year • Annual and non-annual growth lines are distinguishable via stable isotope analysis • Archaeological S. sachalinensis offer a valuable regional palaeoclimate record This paper presents a four year subannual isotope marine temperature record using modern Spisula sachalinensis specimens from Tomakomai (Hokkaido's Pacific coast, Japan). This species is commonly found in pre- and protohistoric shell middens and faunal assemblages from around the Seas of Japan and Okhotsk, so has significant potential as an indicator of past marine and, by inference, climatic conditions. However, previous sclerochronological research on the species’ growth has shown significant geographical variation in growth pattern, rendering palaeoclimatic interpretation difficult. To address this issue, this study applied sequential isotopic analysis to two sectioned modern valves, providing a proxy for sea surface temperature (SST) and allowing the direct identification of seasonality during shell growth. The sequences span four years of growth, as confirmed by visually identified growth patterns and the oxygen stable isotope results, which show clear annual cycles in δ 18 O. δ 13 C seasonality is less clear, but shows a weak inverse correlation with temperature potentially relating to primary productivity. Annual growth lines show that shell growth occurs during both warmer and cooler SSTs, but is more rapid during the cooler seasons. This is consistent with warm-season growth minima seen in shells from Hakodate Bay by Kato and Hamai (1975) , but not with their suggestion that it is associated with shells growing at the southern limits of their distribution. Comparison to average local SST shows that δ 18 O-derived temperature falls within the expected range, but, contrary to expectations given preferential cool-season shell growth, appears biased towards warmer temperatures. Factors that could contribute to this are discussed. Overall, stable isotope analysis of Spisula sachalinensis is considered a useful complement to macro/microscopic sclerochronological research in building a holistic picture of shell growth, and has significant potential as a high resolution proxy for palaeoenvironmental studies of past SST.