INFLUENCE OF EARLY DIAGENESIS ON FORAMINIFERAL SHELL CHEMISTRY AND ISOTOPE SIGNATURES: RESULTS FROM THE TANZANIA DRILLING PROJECT

Uncertainties in reconstructing palaeotemperatures and CO₂-levels in the Cretaceous mainly arise from various degrees of diagenetic overprinting of most Cretaceous sediments. In the Tanzania Drilling Project exceptionally well preserved foraminiferal shells have been retrieved from Cenomanian and Turonian sediments with low carbonate and high clay content. This allows for a comparison of the primary shell material of individual species concerning their mineralogy, chemistry and stable isotopes. It further offers the unique opportunity to compare pristine shells that are glassy with slightly re-crystallized shells that are still well preserved and hollow, but appear more “frosty” or less translucent. The influence of these first processes of diagenetic alteration on the shell geochemistry and isotope signatures is studied by applying a combination of light microscope imaging, scanning electron microscope analyses, cathodoluminescence and microprobe observations. To exclude variation due to environmental factors or vital effects we compare specimens from the same sample and within the same species. Cathodoluminescence studies confirmed the presence of aragonitic shells for a number of species, testifying to the excellent preservation of the material. Their isotopic composition shows a considerable offset as compared to other benthic species. There is also a large difference in oxygen isotopes between benthic and planktic foraminifera. Slightly re-crystallized shells of benthic foraminifera show isotope values that fall within the range of variability of the species. However, the presence of cement can lead to a decrease in δ¹⁸O and becomes evident from bright orange linings under the cathodoluminescence microscope and from a rough shell surface under the SEM. Microprobe analysis reveals that manganese and iron increase in the cemented areas while strontium decreases. A similar trend is observed by comparing the shell chemistry of glassy and less translucent specimens of the same species, indicating some alteration of the primary shell material during re-crystallization.