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, carbonates, and total organic carbon (TOC) along a 70-m sedimentary core from Hangzhou Bay, China. The sedimentary facies (Units I, II, and III from upper to bottom) suggested depositional environments of the present estuary, shallow marine, and floodplain-estuary. The values of δDCH4 displayed similar trends with those of δDH2O and Cl- concentrations along the depth profiles. The values of δ13CCH4 generally synchronously changed with those of δ13CCO2. The variation trends of δ13CCH4 and δ13CCO2 were the same with δ13Ccarbonate from 10 m to 70 m depth but decoupled above 10 m. Calculations suggested that about 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. In our study, the low δDC...,
1. Gas content and isotopic composition analyses
Methane and carbon dioxide concentrations in the headspace of the glass vials containing 10-mL sediment were measured by a gas chromatograph coupled with thermal conductivity (GC-TCD, Thermo) and a Pora Plot Q column (30 m × 0.32 × 20 μm ). The temperatures of the column oven, the gasification chamber, and the thermal conductivity detector (TCD) were set at 60, 150, and 200°C, respectively. The filament voltage of TCD was 10 V, and the flow rate of the reference gas and the make-up gas (Helium) were 12 and 10 mL/min, respectively. Helium (UHP, 99.999%) was used as the carrier gas with a flow rate of 3 mL/min, with an injection volume of 50 μL and a split ratio of 10:1. The precision was less than 3%. Headspace levels are reported as % of headspace (v/v), and the gas content (mmol) per liter of sediment was calculated with the ideal gas law (Li et al., 2019).
The isotopic compositions (carbon and hydrogen of the CH4, carbon of the CO2) we...,