TURONIAN PRISTINE FORAMINIFERA FROM TANZANIA – SYSTEMATIC STABLE ISOTOPES AND SHELL CARBONATE GEOCHEMISTRY

Upper Cretaceous sediments recovered during the Tanzania Drilling Project from 2007 to 2009 include long intervals with exceptionally well preserved planktic and benthic foraminifera providing an excellent sample base for generating stable carbon and oxygen isotopic records as well as other geochemical data. Our study examines inter-species variation in stable isotopic values and their relation to the shell carbonate geochemistry and preservation by applying a combination of light-microscope and SEM imaging with microprobe and cathodoluminescence-based geochemical analyses. Our goal is to evaluate which species are best suited for studies of stable isotopes and temperature reconstructions in the Cretaceous. We present results from a Turonian sequence with relatively stable environmental conditions as inferred from minimal variation in intra-specific d¹³C and d¹⁸O values across the section. The studied material contains abundant and diverse benthic foraminifer assemblages that indicate an outer shelf to upper slope paleodepth. We performed single species d¹³C and d¹⁸O measurements for 18 calcareous benthic and 10 planktic species. Cross plots of d¹³C and d¹⁸O show distinct offsets between clusters of data points for individual species, reflecting a combination of differences in habitat and disequilibrium fractionation due to metabolic effects or an infaunal mode of life. Offsets in d¹⁸O between two groups of trochospiral planktic foraminifera apparently reflect water depth-related differences in temperature during their lifetime. An expected offset is observed between benthic species with aragonitic versus calcitic tests, but systematic offsets among aragonitic species indicate that micro-habitat or vital effects also influence measured values. The lowest d¹³C values among the calcitic benthic foraminifera were observed for species of the genus Lenticulina. Because we did not observe differences in their preservation relative to other benthic species, the isotope results suggest a strong influence of metabolic effects on isotopic fractionation for Lenticulina species. Among the calcitic benthic species studied the two species Berthelina berthelini and Lingulogavelinella convexa appear to have close-to-equilibrium values and are preferred for paleoenvironmental reconstructions.