Origin and Evolution of Natural Diversity;Proceedings


Experimental Study of the Use of the Stable Isotopic Composition of Calcareous Microfossils in Shallow Marine Sediments for Reconstructing Paleoenvironment, and a Comparison with the MART Index

Ishimura, Toyoho;Dick, Matthew H.;Takashima, Reishi;Hirose, Masato;Gautam, Pitambar;Nishi, Hiroshi;Tsunogai, Urumu

Permalink : http://hdl.handle.net/2115/38446
KEYWORDS : Calcareous microfossil;Carbon;Oxygen;Shallow-water environment;Stable isotope;MART analysis


For reconstruction of paleoenvironments using the isotopic compositions of shallow-water microfossils, especially benthic microfossils at high latitudes, difficulties arise due to variation in environmental factors, including seasonal temperature, depth, water currents, and habitat. In this study, we applied a microscale technique to determine stable carbon and oxygen isotopic ratios in shallow-water calcareous benthic microfossils (foraminifera and bryozoans), in order to choose suitable species for reconstructing paleoenvironmental changes in a Pleistocene shallow marine setting. We compared isotopic trends in Cibicides refulgens, a shallow water epibenthic foraminifer, with the mean annual range of temperature (MART) based on bryozoan analyses. We found no strong trend in δ13C composition in the foraminifers from stratigraphically sequential sediment samples of the Soebetsu Sandstone Member (~0.8 Ma) of the Pleistocene Setana Formation, but did find δ18O isotopic fluctuation among samples that was larger than the range of isotopic variation among individual foraminifera. MART values were almost identical from two strata showing a large difference in δ18O composition. We interpret our results to mean that the δ18O isotopic composition of benthic foraminifera was affected mainly by global and/or regional environmental changes rather than local temperature variation on the seafloor, in contrast to MART analysis, which reflects only annual range in local temperature. Further studies are needed on possible correlations between MART values and microscale isotopic determinations for calcareous microfossils in high-latitude shallow-water paleoenvironments.