Organic matter cycling across the sulfate-methane transition zone of the Santa Barbara Basin, California Borderland
Organic matter cycling across the sulfate-methane transition zone of the Santa Barbara Basin, California Borderland
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DOI：
10.1016/j.gca.2015.12.022
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发表时间：
2016-03
影响因子：
5
通讯作者：
T. Komada;D. Burdige;Huan Li;C. Magen;J. Chanton;A. K. Cada
中科院分区：
文献类型：
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作者：
T. Komada;D. Burdige;Huan Li;C. Magen;J. Chanton;A. K. Cada
硫酸盐-甲烷过渡区（SMTZ）中硫酸盐（SO42−）的消耗通常被认为仅仅是由于甲烷的厌氧氧化（AOM）。然而，最近的研究表明，进入SMTZ的SO42−通量超过了甲烷（CH4）通量，从而挑战了这一概念模型。在SMTZ中，有机碎屑SO42−还原（oSR）与AOM的共同发生被假设是导致这种通量不平衡的原因，但缺乏确凿的证据。为了解决这一知识空白，我们研究了加州边境地区圣巴巴拉盆地富有机质沉积物中SMTZ的有机质循环，并利用大块溶质剖面和选定碳库的Δ14C和δ13C值研究了该区域oSR的发生情况。我们还测试了一个假设，即SMTZ不仅作为CH4的氧化锋，而且还作为SMTZ下方产生并向上迁移的溶解有机碳（DOC）的氧化锋。基于反应化学计量学和相关碳通量Δ14C和δ13C值的SMTZ质量平衡计算表明，SMTZ中总SO42−还原的~ 35-45%是通过oSR发生的，其余归因于AOM。净DOC产量的δ13C值与大块POC降解部分的δ13C值不同，表明孔隙水DOC在可代谢POC池中是一个组成独特的部分。在450厘米处向上扩散的DOC几乎不含14c，并且含有低水平的短链有机酸。放射性碳质量平衡表明，这些预老化的（可能是难熔的）DOC中的bbb30 %从SMTZ内或其下的孔隙水中被去除。虽然SMTZ似乎不是主要的净DOC氧化锋，但这些结果表明，DOC动力学为这些地下沉积物中的有机质加工提供了独特的见解。
Consumption of sulfate (SO42−) in the sulfate-methane transition zone (SMTZ) has often been considered to be due solely to anaerobic oxidation of methane (AOM). However, recent studies show SO42−fluxes into the SMTZ that exceed methane (CH4) fluxes, thereby challenging this conceptual model. Co-occurrence of organoclastic SO42−reduction (oSR) with AOM in the SMTZ has been hypothesized to be the cause for this flux imbalance, but conclusive evidence is lacking. To address this knowledge gap, we investigated organic matter cycling in the SMTZ of the organic-rich sediments of the Santa Barbara Basin, California Borderland, and examined the occurrence of oSR within this zone using bulk solute profiles and Δ14C and δ13C values of selected carbon pools. We also tested the hypothesis that the SMTZ acts as an oxidation front not just for CH4, but also for dissolved organic carbon (DOC) that is produced below the SMTZ and migrates upward. Mass balance calculations for the SMTZ based on reaction stoichiometry and Δ14C and δ13C values of associated carbon fluxes indicate that ∼35–45% of total SO42−reduction in the SMTZ occurs via oSR, with the remainder attributable to AOM. The δ13C value of net DOC production is distinct from that of the fraction of bulk POC undergoing degradation, suggesting that pore-water DOC represents a compositionally unique slice of the metabolizable POC pool. DOC diffusing upward at 450 cm is virtually free of14C and contain low levels of short-chain organic acids. Radiocarbon mass balance shows that >30% of this pre-aged, and presumably refractory, DOC is removed from the pore waters within or immediately below the SMTZ. Although the SMTZ does not appear to be a major net DOC oxidation front, these results show that DOC dynamics provide unique insights into organic matter processing in these subsurface sediments.