Oxygen isotope fluctuations in a modern North Sea oyster (Crassostrea gigas) compared with annual variations in seawater temperature: Implications for palaeoclimate studies

A total of 181 oxygen isotope values from sequential samples of the left shell of a modern Pacific Oyster (Crassostrea gigas) that lived on a sub-tidal oyster bank in the List Basin (North Sea, Germany) shows periodically varying values between + 1.3‰ and −2.5‰. In order to test whether these δ18O fluctuations reflect seawater temperature changes, the isotope values of the shell were compared to actual seawater temperature variations from the region. C. gigas serves as an excellent proxy for temperature of palaeoseawater and the results show that the examined oyster precipitated its shell in δ18O equilibrium with the ambient seawater. A cessation of the oyster shell calcification starts at water temperatures below 6 °C, at lower temperatures than previously thought for Crassostrea. For palaeoclimate investigations the termination of shell production is important because the lowest temperatures might not be represented in the shell. Higher water temperatures are therefore potentially over-represented in time spans when the shell grows faster and mean oxygen isotope values may potentially bias the mean seawater temperatures of annual variations towards higher temperatures.

Research Highlights
► δ18O from modern oyster shell reflects annual seawater temperature variations.
► Shell precipitation occurs close to isotopic equilibrium.
► Shell growth stops at temperatures of 6 °C or below.
► Large, homogenized samples tend to overestimate palaeotemperatures.