A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea
A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea
复制标题
南海西北部冷泉甲烷衍生碳循环的定量评估
DOI：
10.3390/min10030256
复制
发表时间：
2020-03-01
期刊：
影响因子：
2.5
通讯作者：
Chen, Duofu
中科院分区：
文献类型：
--
作者：
Feng, Junxi;Li, Niu;Chen, Duofu
沿着大陆边缘广泛分布的冷泉是海水溶解碳的重要来源。然而，很少有人知道的甲烷周转和渗漏对底层海水在中国南海（SCS）的冷渗漏可能产生的影响。本文介绍了从南海西北部4个冷泉区采集的6个推压岩心、1个活塞岩心和3个钻孔以及15个底层水样品的海底观测和孔隙水数据。硫酸盐-甲烷过渡区的深度一般较浅，约为7至<0.5 mbsf（海底以下米）。反应输运模拟结果表明，甲烷动力学是高度可变的，由于上升气体的运输和溶解。溶解的甲烷主要被SMTZ的甲烷厌氧氧化（AOM）消耗，并被其下方的天然气水合物形成所捕获，深度积分AOM速率范围为59.0和591 mmol m−2 yr−1。底水溶解无机碳（DIC）的δ 13 C和Δ 14 C值表明，在冷泉区，13 C和14 C贫化的化石碳向底水排放。根据双端元估计，冷渗漏流体可能贡献16-26%的底层海水DIC，并可能对长期深海碳循环产生影响。研究结果表明，在冷泉系统中，甲烷相关的碳储量具有高度的异质性，这可能取决于采样点到冷泉中心的距离。据我们所知，这是第一个定量研究冷泉流体的贡献，以底层水碳储层的南海，并可能有助于了解在南海烃类渗漏的动力学和环境影响。
Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to <0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m−2 yr−1. The δ13C and Δ14C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of 13C- and 14C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS.