A latitudinal gradient of seasonal temperature variation recorded in oyster shells from the coastal waters of France and The Netherlands

Abstract

Cathodoluminescence (CL) microscopy of the foliated calcite shell hinge sections of live-collected oyster Crassostrea gigas collected at seven locations along a latitudinal gradient from the Netherlands in the North Sea to the Atlantic coast of France, revealed variations in luminescence that were attributable to seasonal variations in calcification of the hinge. Photomicrographs of hinge sections and luminescence profiles were analyzed to define a micro-sampling strategy that was adopted to drill the hinge samples to determine their isotopic composition. Reconstructed seasonal seawater temperatures determined from the stable oxygen isotope (δ18O) composition along growth profiles from 32 oyster shell hinges showed distinct seasonal isotopic cycles that were compared with in situ measured seawater temperatures and salinities at each location. Comparison of the amplitude of the (δ18O) signal and the annual maximum and minimum seawater temperatures demonstrated that C. gigas shells from several locations provided a reliable record of seasonal seawater temperature variation. The exception to this was oysters from the Netherlands and northern France where winter growth rates at low temperatures were slow so that insufficient shell was deposited to allow adequate spatial resolution of sampling and this resulted in time-averaging of the reconstructed seawater temperatures and an overestimation of winter seawater temperature. A potential variability in δ18Ow–salinity relationship at low salinities could also explain the high difference between measured and predicted seawater temperatures in Dutch areas. The finding that latitudinal differences in oyster hinge growth rates and/or changes in the δ18Ow–salinity relationship can result in bias of the seawater temperature deduced from the stable isotopic composition of the hinge should be taken into account when reconstructing latitudinal gradients in seawater temperature.