Carbon and oxygen isotopic analyses were performed on several species of Holocene articulate brachiopods from various locations in the Caribbean Sea and in the Atlantic and Pacific Oceans, from salinities ranging between 30 and 38^pmil and over a temperature range from 4 to 28°C (39 to 82°F). The ^dgr¹⁸O of articulate brachiopods are systematically related to the ^dgr¹⁸O and temperatures of the ambient waters. Lowenstam in 1961 concluded that articulate brachiopods secrete calcium carbonate (low magnesium calcite) in isotopic equilibrium with the surrounding waters. The data compiled in this study, along with that of Lowenstam, closely approximate the equilibrium calcite-water

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line. Deviation from apparent equilibrium was most evident for samples from a few cold-water sites. Intraspecific isotopic variations in several warmer-water assemblages were up to 2^pmil, and isotopic variation within individual specimens as high as 1.5^pmil were noted. Articulate brachiopods were not separable at any taxonomic level on the basis of ranges in isotopic values; the two extant orders of brachiopods, the Terebratulida and the Rhynchonellida, as well as the suborder Thecideidina, had essentially equivalent isotopic ranges at any given locality. Thus, there is no evidence for significant taxonomic control of oxygen isotope fractionation among articulate brachiopods. No correlation appears to exist between ^dgr¹⁸O and ^dgr¹³C values.

Our results suggest strongly that isotopic data from small populations or communities of well-preserved articulate brachiopods can be used in paleogeographic and paleo-oceanographic reconstructions.

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