ABSTRACT

Analysis of the stable isotopic composition of a Late Eocene whale, Basilosaurus cetoides, from Wayne County, Mississippi, provided oxygen isotopic values for cetacean bone phosphate, carbonate cement, and structural carbonate. The least-squares regression comparing cetacean phosphate to seawater oxygen isotopic composition (Yoshida and Miyazaki, 1991) suggests either that Gulf

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Coastal waters of the Late Eocene were isotopically 'heavier' than Pleistocene glacial maxima, or that B. cetoides incorporated 'heavier' body water than modern cetaceans.

Stable-isotopic analysis of tooth enamel samples from two modern dolphins, Tursiops truncatus, from the Gulf of Mexico produced ¹⁸O values of 19.799 and 20.111 0/00, which are also 'heavier' than data for other modern whales and dolphins (Yoshida and Miyazaki, 1991).

Comparison of these modern dolphin data to oxygen-isotopic values for seawater from the modern Gulf of Mexico should allow us to determine if the anomalously heavy cetacean data are caused by some local fractionation effect in the northeastern Gulf, causing ¹⁸O-enriched surface waters. Alternatively, comparison of these data to previously-published phosphate isotopic values of Miocene cetaceans (Barrick and others, 1993) may indicate a long-term shift in the isotopic composition of cetacean phosphates since the Late Eocene. This could have been caused by increased exchange of environmental water and body water, with modern cetaceans retaining isotopically 'lighter' metabolic water, whereas their Archaeocete ancestors had less exchange between environmental water and body water during phosphate formation, resulting in 'heavier' tooth enamel and phosphates.

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