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The fate of organic matter during early diagenesis is a key concern for many oceanographic and limnological studies because the early diagenesis process is driven by the mineralization of organic matter associated with the interactions of this matter with the surrounding water, leading to a series of physical, biological and chemical changes in the sediments (Berner, 1980;Hedges et al., 2001). These changes normally include aerobic respiration, nitrate reduction, metal (Fe, Mn) oxide reduction, sulfate reduction and methane generation, forming a series of redox reaction zones with increasing sediment burial depths (Hesse & Schacht, 2011) and some special authigenic minerals, such as strawberry pyrite and glendonite. Typically, the products of the early diagenetic stage form from the shallow sediment layers to the water-sediment interface and are able to record the geochemical processes of the pore water and even of the bottom water; additionally, these products usually undergo element migration and thereby produce unique geological information. Thus, understanding the early diagenesis of sediments is of great significance for obtaining information on ambient porewater and even bottom water to establish connections with the coeval environmental changes and further test or enhance the reliability of the derived sedimentary process information.Traditional nitrogen isotope analysis assumes that δ 15 N TN mainly represents the nitrogen isotope of the organic fraction. However, modern ocean sediment studies show that the proportion of inorganic nitrogen to total nitrogen in Arctic Ocean sediments and East Atlantic sediments is quite high, resulting in a 6‰ deviation of δ 15 N TN from δ 15 N org (Freudenthal et al., 2001;Schubert & Calvert, 2001); N inorg accounts for approximately 13%-100% (Emmer & Thunell, 2000) and 15%-50% (Holtvoeth et al., 2003) of TN in sediments from the Santa Barbara and Congo Basins, respectively; the proportion of N inorg to TN in South China Sea sediments is as high as 71% on average (Li & Jia, 2011). These investigations have led to a growing concern about the limitations of using

The Devonian–Carboniferous (D–C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo‐ocean environment of different depositional facies, we studied a shallow‐water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well‐preserved succession spanning the D–C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ15N excursion (~−3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium‐bearing waters. Negative excursions in the δ13Ccarb and δ13Corg values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered 13C‐depleted anoxic water masses. Decreased δ34S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water‐column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ13Corg values associated with maximal △13C. The integrated δ15N‐δ13C‐δ34S data suggest that significant ocean‐redox variation was recorded in South China during the D–C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.