Belemnites have been regarded in the past as one of the most suitable fossil groups for paleotemperature determinations. We have investigated the isotopic composition of carbon and oxygen of about 200 carefully selected belemnite samples. We have made special efforts to elucidate questions of preservation of primary isotopic composition and of possible metabolic effects. The two most striking results of our measurements are: (1) The existence of variations of as much as l4‰ in δC13 and 5‰ in δO18 within single belemnite specimens have been found. (2) The aragonite samples, taken from conotheca and septa of Megateutbis giganteus (v. Schloth.), show a broader variation than calcite. In two examples, in which a sequence of five or six septa could be prepared separately, a correlation exists between very light δC13 and δO18 values, indicating—at least partially—a metabolic source of the CO2.

The relevance of these results for studying paleotemperatures based on belemnites is discussed.

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