Toarcian Jurassic Oceanic Anoxic Event Evidence From Foraminifera and Ostracods Isotopic Evidence

Stable isotopes on foraminifera and ostracods for interpreting incidence of the Toarcian Oceanic Anoxic Event in Westernmost Tethys: Role of water stagnation and productivity

Author:

Matías Reolid

Abstract:

The analysis of δ13C and δ18O from whole rock and the shells of selected foraminifera (Lenticulina and Dentalina) and bairdioid ostracods from Lower Toarcian of the South Iberian Palaeomargin (Western Tethys) is presented with the aim of improving knowledge of the processes and the environmental effects of the Toarcian Oceanic Anoxic Event. In addition, isotopic data are compared with geochemical redox and palaeoproductivity proxies. The microhabitat affected δ13C values, δ13CLenticulina generally being lower than δ13CDentalina due to more depleted 13C values for sediment pore-water in the deep infaunal microhabitat. The lowest values of δ13C (lower part of Serpentinum Zone) happen during suboxic conditions, as indicated by redox proxies, low diversity and abundance of foraminifera and higher TOC values. The fine-grained, organic rich sediments allow for conditions favouring pore-water dissolved inorganic carbon that is depleted in 13C with respect to that of the bottom sea-water, particularly during the suboxic conditions. The δ13C of potential deep infauna (Lenticulina) reflects the oxygen restricted conditions better than shallow infauna (Dentalina) and whole sediment. Regarding δ18O, values from bulk rock present stronger fluctuations and lower values than δ18ODentalina and δ18OLenticulina. The stratigraphic differences between δ18ODentalina and δ18OLenticulina correspond to vital effects, since no important fluctuations in temperature occurred in the bottom sea-water, as deduced from the absence of peaks and stratigraphic trends in the interval studied. The δ18O values do not allow us to infer temperature changes related to the Toarcian Oceanic Anoxic Event in this part of the palaeomargin.