GENERAL CONSIDERATIONS ON ISOTOPIC PALEOTEMPERATURE DETERMINATIONS AND ANALYSES ON JURASSIC AMMONITES

Paleotemperature analyses have been carried out on the aragonitic outer shells and septa of three Jurassic ammonites. Postmortal alterations can be recongnized easily by the degree of conversion of original aragonite to secondary calcite. The amount of secondary calcite is used as an indication of an isotopic exchange. It often proves difficult, however, to distinguish primary from secondary calcite in calcitic fossils. Therefore, calcitic fossils should only be used for paleotemperature determinations, if they are associated with aragonitic fossils, for instance ammonites, showing a good preservation of the more unstable aragonite. © 1969.