New insights into the carbon cycle and depositional models of the Eocene saline lake, Jianghan basin, China

Abstract

Geochemical characteristics and sedimentary processes of sediments, especially mixed sediments, have long been hot topics of research in basin systems, which can be used to infer runoff, evaporation, productivity, precipitation patterns of the paleoenvironment, as well as records of a diverse range of climate-related signs. However, compared with shallow marine mixed siliciclastic-carbonate sediments, the study of lacustrine mixed sedimentary systems in combination with geochemical information is relatively limited. Also, model-based study on the reconstruction of the lacustrine carbon biogeochemical cycle is defective, especially in saline lacustrine environments. In this work, a total of 26 mixed siliciclastic–carbonate sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, major and minor element test, and isotope measurements (e.g., δ13Ccarb, δ18Ocarb, and δ34SCAS). Two types of mixed sediments were generated in the overall inflow/outflow balance of water within the saline basin during the Eocene: siliciclastic-dominated mixed sediment (SDMS) and carbonate-dominated mixed sediment (CDMS). The δ13Ccarb values of SDMS and CDMS samples range from −7.13‰ to −4.52‰ (average −5.82‰) and −9.00‰ to −6.11‰ (average −7.48‰), respectively. The δ18Ocarb values of SDMS and CDMS samples range from −10.26‰ to −1.40‰ (average −4.73‰) and −3.80‰ to −0.96‰ (average −1.55‰), respectively. In addition, combining the geochemical parameters of sedimentation rate, Mn2+ content in carbonate lattice ([Mncarb]), carbonate mass fraction, and [Mg2+]/[Ca2+] ratio, we established a lacustrine δ13C variation model at the sediment-water interface (SWI) with the aim of accurately reflecting δ13Ccarb changes during carbonate formation. Based on the sedimentary processes and geochemical characteristics, we established two depositional models for these two types of mixed sediments in a saline lacustrine basin in the overall inflow/outflow balance of water. Model I illustrates SDMS sedimentary processes with a relative low salinity, low FluxDIC-O, high water inflow, and the organic carbon oxidation is dominated by Fe (III) reduction. Model II demonstrates CDMS sedimentary processes with a high salinity, high FluxDIC-O, high water outflow, and the organic carbon oxidation is dominated by sulfate reduction.