Isotope paleontology: growth and composition of extant calcareous species

Isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms (especially temperature), and to obtain clues about life history and modes of growth. In calcareous fossils, oxygen isotopes are mainly used in the former, and carbon isotopes in the latter. However, since physical surroundings and organism response are intimately associated, both types of information are contained in each of the isotopic signals. To explore the potential of isotope paleontology, and to provide a basis for reconstruction, a broad range of extant organisms has been studied, taking the pioneering work of Epstein and associates as a starting point. Results are summarized for a representative sampling of these studies, with emphasis on work at the laboratories of the authors, from the mid-seventies to the present. The organisms considered are nannoplankton, benthic algae, planktonic and benthic deep-sea foraminifera, “larger” foraminifera, sponges, corals, bryozoans, polychaetes, arthropods, bivalves, gastropods, cephalopods, and vertebrates (fish otoliths). The survey broadly suggests that, regarding oxygen isotopes, materials tend to be precipitated close to equilibrium with the surrounding seawater (as postulated by Urey), and that for carbon isotopes disequilibrium is the rule. Life spans, growth rates, differential seasonal growth, and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts. In detail, the interpretation of isotope records of individual shells is quite complicated, and simple models will fail to give satisfactory results in many or most cases.