Middle Permian astronomically forced upwelling in the Yangtze carbonate platform: Implications for organic matter preservation and benthic biomass

Abstract

The continental shelf and slope, with upwelling systems, are usually accompanied by the formation of oxygen minimum zones, thereby stimulating marine paleoproductivity and promoting organic matter (OM) enrichment. However, due to a more oxygen-enriched environment, a distinct paleoecological characteristic, and the uncertainty of upwelling effects, it remains ambiguous whether a similar mechanism of OM preservation occurs in the adjacent carbonate platform. Here, limestone–marl alternations (LMAs) that developed in the Yangtze Carbonate Platform (YCP) during the Middle Permian were selected as the research object and compared with the chert–mudstone alternations developed on the Lower Yangtze continental shelf (LYCS). Both sets of rhythmites belong to the contemporaneous products of the Permian Chert Event in South China.Considering that the Middle Permian rhythmites in South China contain the Milankovitch record and that upwelling was controlled by astronomically forced monsoons, this study investigated the impact of upwelling on the carbonate platform by adopting astronomical cycle research. Based on GR data, a framework of astronomical cycles was established. The suboxic to anoxic conditions of the YCP are supported by evidence from syngenetic pyrite framboids, Ce/Ce* ranges, and UEF–MoEF covariation. Upwelling geochemical indicators and the detection of a pivotal (∼210-kyr) astronomical cycle regarding upwelling indicate that the YCP was affected by upwelling. The paleoproductivity/OM/fossil-related data exhibit this ∼210-kyr astronomical cycle, and the correspondence between the converted AMs determines that upwelling enhanced marine primary paleoproductivity and OM preservation. However, the intensities of short eccentricity cycles exceed those of the ∼210 kyr cycle in the time series analyses of data from the nearshore YCP. Additionally, the covariations between the upwelling proxy and total fossil content confirm the impact of upwelling on biomass of benthic organisms. Therefore, the (monsoon-driven) upwelling system in the continental shelf can extend to the carbonate platform. After reaching the platform, under the control of a (an) (astronomically forced) climatic–oceanic system, limited upwelling can stimulate primary productivity and marine OM preservation; however, upwelling reduces biomass in the bottom water. In the nearshore side of the platform, detrital dilution likely replaces upwelling as the primary stimulus for productivity. Moreover, this study reveals and explains the ∼200-kyr cycles in the Permian strata of South China for the first time.