Externally or biologically controlled proxy records from bivalve shells of the Dutch Wadden Sea

Field experiments on different bivalve species have been conducted in the Dutch Wadden Sea near the Netherlands Institute for Sea Research (NIOZ) to investigate the environmental and biological controls on the fractionation pattern of different stable isotope systems (18O/16O, 13C/12C, 44Ca/^{42}Ca), and their reliability as geochemical proxies for high-resolution palaeoclimate reconstructions. Various individuals of the blue mussel, Mytilus edulis and juveniles of the ocean quahog, Arctica islandica were collected in spring 2005 and June 2006, respectively. All samples were individually marked, measured and placed either in nets or sand-filled containers. Individuals were re-measured every month to obtain precise time series of changing growth rate. Both set-ups are accompanied by in situ measurements of environmental conditions (e.g. seawater temperature, salinity) which can give a clue when and which factor in the field is most important for shell growth. Furthermore, the results obtained from field experiments have the advantage that they incorporate the short-term environmental variability. Our study is completed using samples from growth experiments on young Arcitca's which were also performed at the NIOZ laboratory. These individuals were cultured in five temperature controlled basins, ranging from 1 to 12° Celcius. Here, temperature should be resolvable as the dominating effect on the growth and on the isotopic composition of similar sized specimens. However, preliminary Ca isotope data on A. islandica suggest only a slight effect of temperature and a stronger trend related to growth rate. Seeing what this yields is particularly important in retrospective studies on basis of geological shell material. This is a contribution to EuroCLIMATE project 04 ECLIM FP08 CASIOPEIA.