Comparing oxygen isotope records of Silurian calcite and phosphate -: δ18O compositions of brachiopods and conodonts

Oxygen isotope compositions of Silurian calcitic brachiopod shells from Gotland are compared with those of phosphatic inarticulate brachiopod shells and conodonts from identical stratigraphic horizons. The delta(18)O(phosphate) values of inarticulate brachiopod shells vary widely (13.0 to 17.5 parts per thousand V-SMOW) and are significantly lower than those of coeval conodont samples (17.5 to 19.5 parts per thousand V-SMOW). Conodont samples from individual beds have homogenous delta(18)O(phosphate) values with variations close to the analytical precision (+/-0.2 parts per thousand). Together with a significant correlation of conodont delta(18)O(phosphate) and articulate brachiopod shell delta(18)O(carbonate) values (r = 0.85), this observation suggests that primary marine oxygen isotope compositions are better preserved in conodont apatite than in inarticulate brachiopod shell apatite. However, with decreasing delta(18)O(carbonate) values, the delta(18)O(phosphate) values of conodonts are higher than expected for theoretical isotopic equilibrium with brachiopod calcite. Different habitats of conodonts and brachiopods cannot account for the observed offset in delta(18)O because both organisms were thriving on a broad shallow marine carbonate platform. Whereas delta(18)O(phosphate) values of Silurian conodonts are close to delta(18)O(phosphate) values of modern and Mesozoic tropical biogenic apatites, most Silurian calcitic brachiopod shells from Gotland are depleted in O-18 when compared to modern or Mesozoic brachiopods. We therefore suggest that Silurian conodonts from Gotland record paleotemperature and delta(18)O of Silurian sea water more faithfully than coeval calcitic brachiopod shells. Copyright (C) 2000 Elsevier Science Ltd.