Biological and environmental effects of Late Cretaceous benthic foraminiferal stable isotope compositions
Revue de Paleobiologie, 25, (2), .
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Stable isotope ratios of d18O and d13C measured on the calcite of foraminiferal tests are widely used as proxies to reconstruct the paleoceanographic conditions of the Cretaceous and the Cenozoic. In particular oxygen isotope ratios are one of the most important proxies to estimate paleotemperatures of marine sediments for the last 100 Ma (e.g., HUBER et al., 2002 ; LEA, 2003). Most of these applications, however, have been restricted to the Cenozoic since the availability of pristine benthic foraminiferal tests and knowledge of the ecological requirements of different species are generally limited for older time periods. Furthermore, the complexity of factors influencing the stable isotope composition of benthic foraminiferal tests (carbonate growth out of thermodynamic equilibrium, vital effects, pore water d13C gradients, and age and origin of the respective water mass ; e.g., WEFER & BERGER, 1991, MACKENSEN & BICKERT, 1999) demonstrates the necessity of studies on living specimens from ecosystems with well-known environmental parameters. For the fossil record, however, comparable information is still missing, since environmental studies on stable isotopes demand excellent preservation of foraminiferal tests. Nevertheless, stable isotope studies are commonly used to reconstruct the paleoceanographic and paleoclimatic evolution also for the Cretaceous. But until now, little information exists on the potential influence of microhabitat and vital effects on the stable isotope signal of Cretaceous benthic foraminifera. For this reason, different species of Late Cretaceous benthic foraminifera from ODP (Ocean Drilling Program) Hole 690C were investigated based on the pristine preservation of foraminiferal calcite (e.g., BARRERA & HUBER, 1990). Hole 690C was drilled during ODP Leg 113 in a water depth of 2914 m on the southwestern flank of Maud Rise in the eastern Weddell Sea, southern South Atlantic. Studied sediments are composed of calcareous chalks and oozes representing the uppermost Campanian and Maastrichtian (Shipboard Scientific Party, 1990). In total, nine samples that are decided to represent the best preserved benthic foraminiferal shells were selected for stable isotope analyses. These samples are located within a time interval with very stable environmental conditions to avoid an overprint of the original interspecific isotope signatures by strong environmental changes and isotope excursions. Sediment was disintegrated in distilled water and washed over a 63 µm mesh. Between 2 and 13 individuals of the biggest size-fraction possible were measured for stable isotopes, to make sure that the isotopic signatures of adult specimens were investigated. Stable isotopes were measured with a Finnigan MAT 251 mass spectrometer at the Leibniz-Labor (Kiel), coupled on-line to the Carbo-Kiel device I (external precision better than 0.07 ‰ and 0.04 ‰ for oxygen and carbon, respectively).
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