Assessing methane cycling in the seep sediments of the mid-Okinawa Trough: Insights from pore-water geochemistry and numerical modeling
Assessing methane cycling in the seep sediments of the mid-Okinawa Trough: Insights from pore-water geochemistry and numerical modeling
复制标题
评估冲绳海槽中部渗流沉积物中的甲烷循环：孔隙水地球化学和数值模拟的见解
DOI：
10.1016/j.oregeorev.2020.103909
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发表时间：
2021-01-15
影响因子：
3.3
通讯作者：
Zhai，Bin
中科院分区：
文献类型：
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作者：
Xu，Cuiling;Wu，Nengyou;Zhai，Bin
冷渗漏中富含甲烷的流体很可能结晶为气体水合物，并成为海水碳的重要来源。在这项研究中，我们分析了孔隙水的组成，包括CH 4，溶解无机碳（DIC），Cl−，Br−，SO 42 −，Na+，Mg 2+，Ca 2+，Sr 2+和NH 4+，以及δ 13 CDIC，δ 13 CCH 4，从断层陡坎和穹隆状构造（DS）中提取的两个重力岩心和六个遥控潜水器（ROV）视频引导推送岩心的δ DCH 4和δ DCH 4值位于冲绳海槽中段的西坡。此外，一个反应-传输模型被应用到量化的甲烷通量和相关的生态地球化学过程。在断层崖和穹隆构造上分别识别出生物甲烷（δ 13 CCH 4 ~ -70‰ V-PDB）和热成因甲烷（δ 13 CCH 4 = −40‰ ~ −56‰ V-PDB）的活跃渗漏。甲烷的渗流是由上升气体的输运和溶解控制的，而不是由粘土脱水或天然气水合物解离引起的流体平流。在R3-C2、R4-C4和R6-C1站点的高甲烷浓度和浅的硫酸盐-甲烷过渡区（SMTZs;在0.1和0.4 mbsf之间）表明，在四个研究DS中的三个处有强烈的甲烷渗漏（最高气体溶解速率为6450、1475和515 mmol m− 2 yr −1）。位于沿着断层崖的GC 08站点显示出中度甲烷渗漏; SMTZ位于~2.5 mbsf处，甲烷厌氧氧化速率（AOM）为130 mmol m− 2 yr −1。从深部运移的甲烷主要被AOM消耗。然而，约12%-66%的甲烷从两个最强烈的渗漏点释放到水柱中。高镁方解石（在所有站点）和文石（仅在站点R3-C2）的沉淀固定了DIC的27%-50%（平均= 39%）。因此，排放到水柱中的碳已经减少。在研究区域，面积加权海底甲烷和DIC通量分别为~30和20 mmol m− 2 yr −1。总的来说，它们相当于有机碳埋藏率的7%-14%，这表明沉积物不能简单地被视为稳定的碳汇，因为它们为水柱提供甲烷和一定量的DIC。我们的研究结果有助于了解潜艇冷渗漏系统中碳循环的持续努力的结果。
The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH4, dissolved inorganic carbon (DIC), Cl−, Br−, SO42−, Na+, Mg2+, Ca2+, Sr2+, and NH4+, and the δ13CDIC, δ13CCH4, and δDCH4values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13CCH4~ –70‰ V-PDB) and thermogenic (δ13CCH4= −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5 mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH4and DIC fluxes are ~30 and 20 mmol m−2yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems.