Extension of carbon flux estimation to oxic sediments based on sulphur geochemistry and analysis of benthic foraminiferal assemblages: A case history from the Eocene of Hungary

The amount and type of organic matter (OM), chemical and mineralogical compositions, and the amount and isotopic ratio of sulphur were measured and benthic foraminiferal assemblages were analysed in a 70 m thick core section comprising marine marls from the Eocene of Hungary. On the basis of Benthic Foraminiferal Oxygen Index (BFOI), the studied sediments were deposited under a range of anoxic to oxic bottom water conditions. The amount of Corg mineralised during early burial was calculated in two steps. First, carbon mineralised by sulphate reduction was calculated from the amount of sedimentary sulphur, corrected for escaped/oxidised H2S by taking into account the isotopic composition of sulphur. Then, on analogy of recent sediments which are deposited with a similar rate of sedimentation and beneath bottom waters of similar O2 content, carbon loss by aerobic respiration was estimated. A good stratigraphic control and knowledge of variation of mineralogical composition along the section permitted to estimate past variation of local organic carbon flux (Cflux) from calculated original Corg contents. During deposition of the studied strata, Cflux first decreased from 1.16 to 0.41 g Corg/m2/y, then it progressively increased to 1.81 g Corg/m2/y. Changes in local Cflux are explained by changes in water depth, flux of terrestrial organic components and availability of nutrients liberated by alteration of products of the coeval pyroclastic activity.