Influence of Sedimentary Environment Evolution on Fingerprint Characteristics of Methane Isotopes: A Case Study From Hangzhou Bay

To better understand the depositional constraints on the fingerprint characteristics of methane isotopes, we present a set of carbon/hydrogen isotopic data for CH4, CO2, pore water, carbonate, and total organic carbon along a 70-m sedimentary core. The sedimentary facies (Units I, II, and III from upper to bottom) suggested depositional environments of the present estuary, shallow marine, and floodplain-estuary. Calculations suggested that around 86% of methane was produced through the CO2 reduction pathway. In this pathway, the hydrogen in CH4 is from ambient water, while the carbon is from dissolved inorganic carbon. Correspondingly, our study showed that the values of dD(CH4) displayed similar trends with those of dD(H2O) and Cl- concentrations along the depth profiles. The low dD(CH4) below 44.5 m corresponded to low dD(H2O) and low salinity during the cold and low-sea-level period. The values of d(13)C(CH4) generally synchronously changed with those of d(13)C(CO2). The variation trends of d(13)C(CH4) and d(13)C(CO2) were the same with d(13)C(carbonate) from 10 to 70 m depth but decoupled above 10 m. The values of d(13)C(CH4) in Units II and III were correlated with the d(13)C(carbonates), which is related to the sedimentary processes. But decoupling of low values of d(13)C(CH4) and d(13)C(CO2) from d(13)C(carbonates) in Unit I may be related to preferential microbial consumption of labile compounds with light carbon isotopic compositions, such as lipids. In short, the variations of d(13)C(CH4) and dD(CH4) were largely related to the sedimentary process, which controlled the isotopic compositions of both water and carbonate.