Organic matter accumulation mechanism under global/regional warming: Insight from the Late Barremian calcareous shales in the Qiangtang Basin (Tibet)

Abstract

The Late Barremian (Early Cretaceous) is a global/regional warming interval. Deciphering the mechanism of organic matter (OM) accumulation in this interval could cast lights on OM accumulation mechanism under global/regional warming. This study presented high-resolution geochemical data of the Late Barremian marls and calcareous shales in the Qiangtang Basin to unravel the effect of paleoclimate and hydrothermal activity on OM enrichment, and to further discuss OM enrichment mechanism under warming climates. Our work revealed that the Late Barremian calcareous shales in the Qiangtang Basin showed high total organic carbon contents and were deposited under an anoxic-euxinic condition, high bioproductivity, and relatively stable high-salinity water mass. Hydrothermal activity was intense during this calcareous shale interval deposition. Hydrothermal fluids not only released reducing gas to increase the reducibility of water column, but also provided sufficient nutrients to enhance bioproductivity, thus facilitating the preservation and accumulation of OM. A climate transition from cold-arid to warm-humid occurred at the onset of calcareous shale deposition. A warming climate evidences in the late Barremian succession in the Qiangtang Basin suggested at least a regional warming event. The OM enrichment in the Late Barremian calcareous shales in the Qiangtang Basin was caused by the combined effect of regional (warming climate) and local (hydrothermal activity) factors. Accelerated hydrological cycle under warming climates promoted fluvial delivery of nutrients to sedimentary basins, and thus flourished bioproductivity. Local factors (e.g., hydrothermal activity and water-column stratification) facilitated OM accumulation and preservation under a warming setting.