Experimental calibration and field investigation of the oxygen isotopic fractionation between biogenic aragonite and water

Marine molluscs have long been recognised as potential records of palaeocli mate change using the patterns and differences in the stable isotopic compo sition of the carbonate shells. The aim of this study is to improve the rob ustness of this approach for aragonitic molluscs by completing the first ex perimental calibration of the fractionation between water and biogenic arag onite. Fractionation factors were calibrated by growing specimens of the fr eshwater mollusc Lymnaea peregra under controlled conditions of water tempe rature and isotopic composition. Fifteen populations of L. peregra were maintained at constant temperature a nd isotopic conditions for five months (at five different temperatures and using three different water compositions). Water samples and temperature me asurements were taken regularly throughout the experiment. The temperature dependence of the fractionation factor, between 8 and 24 degrees C, is give n by: 1000 ln alpha = 16.74 x (1000T(-1)) - 26.39 (T in Kelvin) and the relationship between temperature (T), delta(18)O(carb) and delta(18 )O(wat) is given by: T = 21.36 - 4.83 x (delta(+degrees)O(carb) - delta(+degrees)O(wat)) (T is in degrees C, delta(18)O(carb) is with respect to Vienna Pee Dee Bele mnite (PDB), the International Atomic Energy Agency (IAEA) replacement stan dard for PDB, and delta(18)O(wat) is with respect to Vienna standard mean o cean water (VSMOW)). The outcome of the controlled experiment is compared with previous studies on synthetic, and biogenic, calcite and aragonite from field and laboratory investigations. These comparisons suggest that although a vital offset exi sts between the fractionation of isotopes in synthetic and biogenic aragoni te for molluscs in general, there is no vital effect that is specific eithe r to freshwater, or to individual, genera. Therefore, the calibrated relati onship may be used for any freshwater or marine mollusc to derive palaeotem peratures providing the isotopic composition of the environmental water can be reliably constrained. Copyright (C) 1999 John Wiley & Sons, Ltd.