Estimation of primary productivity in the Toarcian Tethys — A novel approach based on TOC, reduced sulphur and manganese contents

Abstract

About 60 samples from the non-bioturbated Úrkút Manganese Ore Formation (Bakony Mountains, Hungary), representing the early Toarcian anoxic event, were studied for: (1) TOC content, Rock-Eval parameters and kerogen δ13C; (2) S content and δ34S and (3) Al, Ca, Mn, Si and Ti contents. Using the example of the formation, a novel approach for estimating paleoproductivity is presented, taking into account reduced sulphur and manganese contents, which reflect the amount of organic carbon lost during early burial. Drastic changes in Mn content (between 0.7% and 37%) during deposition of the formation resulted in very variable early diagenetic processes, with sulphate reduction dominating in low-Mn sediments, and Mn reduction dominating in high-Mn sediments. The amounts of reduced sulphur and manganese allowed calculation of the original organic carbon (TOCor) content throughout the formation. Based on stratigraphic variations in chemical composition, changes in the rate of sedimentation were assessed for the three main members of the formation. Next, using the formula describing the relationship between measured productivity, rate of sedimentation and carbon flux reaching the sea-floor, developed by Suess (1980), past productivity was calculated. The values obtained, lower than those prevailing in present-day upwelling systems, are probably underestimated because of uncertainties of chronostratigraphic and water-depth data. However, relative differences between productivity values obtained for the three members are considered as realistic and suggest an increase in productivity during deposition of the formation. This increase is not related to stratigraphic variations in Mn content and is accompanied by an increase in the growth of calcareous plankton.