Experimental determination of the temperature dependence of oxygen-isotope fractionation between water and chitinous head capsules of chironomid larvae

Oxygen-isotope values of invertebrate cuticle preserved in lake sediments have been used in palaeoenvironmental reconstructions, generally with the assumption that fractionation of oxygen isotopes between cuticle and water (alpha(cuticle-H2O)) is independent of temperature. We cultured chironomid larvae in the laboratory with labelled oxygen-isotope water and across a range of closely controlled temperatures from 5 to 25 degrees C in order to test the hypothesis that fractionation of oxygen isotopes between chironomid head capsules and water (alpha(chironomid-H2O)) is independent of temperature. Results indicate that the hypothesis can be rejected, and that alpha(chironomid-H2O) decreases with increasing temperature. The scatter in the data suggests that further experiments are needed to verify the relationship. However, these results indicate that temperature-dependence of alpha(chironomid-H2O) should be considered when chironomid delta O-18 is used as a paleoenvironmental proxy, especially in cases where data from chironomids are combined with oxygen-isotope values from other materials for which fractionation is temperature dependent, such as calcite, in order to derive reconstructions of past water temperature.