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...,为更好地明晰甲烷同位素指纹特征的沉积约束机制，本研究报道了一套采自中国杭州湾70米沉积岩芯的甲烷（CH₄）、二氧化碳（CO₂）、孔隙水、碳酸盐以及总有机碳（Total Organic Carbon, TOC）的碳/氢同位素数据。 该岩芯的沉积相（自上至下划分为I、II、III单元）指示了现代河口、浅海以及泛滥平原-河口的沉积环境。δD_CH₄的数值变化趋势与δD_H₂O及Cl⁻浓度的垂向剖面特征相似；δ¹³C_CH₄的数值整体上与δ¹³C_CO₂的变化同步。在10 m至70 m深度区间内，δ¹³C_CH₄与δ¹³C_CO₂的变化趋势与δ¹³C_carbonate保持一致，但在10 m以上深度则出现解耦。 计算结果表明，约86%的甲烷通过二氧化碳还原途径生成，该途径中甲烷的氢来源于环境水，碳则来自溶解无机碳。本研究中较低的δD_C[...] 1. 气体含量与同位素组成分析 对装载10 mL沉积物的玻璃瓶顶空内的甲烷与二氧化碳浓度，采用配备热导检测器（Thermal Conductivity Detector, TCD）的气相色谱仪（Gas Chromatograph-Thermal Conductivity Detector, GC-TCD，赛默飞世尔科技）及Pora Plot Q色谱柱（30 m × 0.32 mm × 20 μm）进行测定。柱温箱、气化室及热导检测器的温度分别设置为60℃、150℃与200℃；热导检测器的灯丝电压为10 V，参比气与补充气（氦气）的流速分别为12 mL/min与10 mL/min。采用超高纯氦气（Ultra High Purity, UHP, 99.999%）作为载气，流速为3 mL/min，进样体积为50 μL，分流比为10:1。测定精度优于3%。顶空浓度以顶空体积百分比（v/v）形式报告，沉积物每升的气体含量（mmol）通过理想气体定律计算得到（Li et al., 2019）。 甲烷的碳、氢同位素组成以及二氧化碳的碳同位素组成[...]