杭州湾沉积物中硫酸盐-甲烷转换带内的碳循环

Carbon cycle within the sulfate-methane transition zone in the marine sediments of Hangzhou Bay

杭州湾海底沉积物中蕴藏着大量的浅层生物气,作为温室气体CH4的重要载体,研究其甲烷厌氧氧化(AOM)及相关碳循环过程,对正确评估浅层生物气的生态环境效应具有重要的科学意义。通过对YS6孔柱状沉积物孔隙水、顶空气等地球化学参数的测试分析,基于质量平衡和碳同位素质量平衡原理,利用"箱式模型"定量研究了硫酸盐-甲烷转换带(SMTZ)内的碳循环过程。结果表明:SMTZ位于海底约6~8 mbsf沉积层,其内部碳循环过程除了包含有机质的硫酸盐还原(OSR)、AOM和碳酸盐沉淀(CP)反应外,还隐藏存在"AOM生成的溶解无机碳(DIC)"产甲烷反应(CR),反应速率分别为9.14、7.42、4.36、2.72 mmol·m^-2·a^-1,而有机质降解产甲烷反应(ME)未发生。各反应对SMTZ内孔隙水DIC的补充贡献率为OSR>AOM>ME,而对DIC的消耗贡献率CP>CR。深部含甲烷沉积层向上扩散而来的CH4并不是驱动SMTZ内部SO42-还原的唯一电子供体,CR和OSR反应亦是导致进入SMTZ内硫酸盐扩散通量大于甲烷的重要因素,且SMTZ下边缘沉积层出现明显的13CH4亏损亦与CR反应有关。本研究认为,定量评估海底沉积物中AOM作用的相对强弱时,SMTZ内可能存在的"隐藏的"产甲烷作用(如CR、ME等)不能忽视。

Large amount of shallow biogenic gas occurs in the marine sediments of Hangzhou Bay.As an important greenhouse gas and carbon carrier,methane and its anaerobic oxidation(AOM)and carbon cycle within the sulfate-methane transition zone(SMTZ)in marine sediments are of great significance for accurately assessment of the eco-environmental effects.Based on the test results and geochemical parameters,such as those from pore water and headspace gas in the YS6 sediment cores,following the principles of mass conservation and carbon isotope mass conservation,the internal carbon cycle in SMTZ for the YS6 was quantitatively studied with the“box model”.It is found that the SMTZ occurs in the 6~8 mbsf sediment layer.In addition to organoclastic sulfate reduction(OSR),AOM and carbonate precipitation(CP),there are concealed methanogenesis by carbon dioxide reduction of DIC produced from AOM(CR).However,methanogenesis from organic matter degradation(ME)almost not observed in the SMTZ-internal carbon cycling.The reaction rates of OSR,AOM,CP,CR and ME were 9.14 mmol·m^−2·yr^−1,7.42 mmol·m^−2·yr^−1,4.36 mmol·m^−2·yr^−1,2.72 mmol·m−2·yr^−1 and 0.00 mmol·m^−2·yr^−1,respectively.The contribution rate of each reaction to pore water DIC in SMTZ was in an order of OSR>AOM>ME(ME=0),while the consumption rate was CP>CR.Methane diffused upward from deeper methane zone was not the only electron donor to drive the internal sulfate reduction(SR)in SMTZ.CR and OSR were also the important factors for sulfate flux into SMTZ to be greater than methan,and the obvious 13C-depletion of methane in the lower border of SMTZ was also related to the CR.When quantitatively evaluating the relative strength of AOM in marine sediments,the“cryptic”methanogenesis(such as CR,ME,etc.)in SMTZ cannot be ignored.