Depth profiles of dissolved sulfide and sulfate in the pore water of hydrocarbon seep sediments offshore Pakistan

The interaction between fluid seepage, bottom water redox, and chemosynthetic communities was studied at cold seeps across one of the world's largest oxygen minimum zones (OMZ) located at the Makran convergent continental margin. Push cores were obtained from seeps within and below the core-OMZ with a remotely operated vehicle. Extracted sediment pore water was analyzed for sulfide and sulfate concentrations. Depending on oxygen availability in the bottom water, seeps were either colonized by microbial mats or by mats and macrofauna. The latter, including ampharetid polychaetes and vesicomyid clams, occurred in distinct benthic habitats, which were arranged in a concentric fashion around gas orifices. At most sites colonized by microbial mats, hydrogen sulfide was exported into the bottom water. Where macrofauna was widely abundant, hydrogen sulfide was retained within the sediment.

Numerical modeling of pore water profiles was performed in order to assess rates of fluid advection and bioirrigation. While the magnitude of upward fluid flow decreased from 11 cm yr**-1 to <1 cm yr**-1 and the sulfate/methane transition (SMT) deepened with increasing distance from the central gas orifice, the fluxes of sulfate into the SMT did not significantly differ (6.6-9.3 mol m**-2 yr**-1). Depth-integrated rates of bioirrigation increased from 120 cm yr**-1 in the central habitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr**-1 in the habitat of large and few small vesicomyid clams. These results reveal that chemosynthetic macrofauna inhabiting the outer seep habitats below the core-OMZ efficiently bioirrigate and thus transport sulfate down into the upper 10 to 15 cm of the sediment. In this way the animals deal with the lower upward flux of methane in outer habitats by stimulating rates of anaerobic oxidation of methane (AOM) with sulfate high enough to provide hydrogen sulfide for chemosynthesis. Through bioirrigation, macrofauna engineer their geochemical environment and fuel upward sulfide flux via AOM. Furthermore, due to the introduction of oxygenated bottom water into the sediment via bioirrigation, the depth of the sulfide sink gradually deepens towards outer habitats. We therefore suggest that - in addition to the oxygen levels in the water column, which determine whether macrofaunal communities can develop or not - it is the depth of the SMT and thus of sulfide production that determines which chemosynthetic communities are able to exploit the sulfide at depth. We hypothesize that large vesicomyid clams, by efficiently expanding the sulfate zone down into the sediment, could cut off smaller or less mobile organisms, as e.g. small clams and sulfur bacteria, from the sulfide source.

本研究于位于马克兰汇聚大陆边缘的全球最大氧最小值区（Oxygen Minimum Zone, OMZ）之一的冷渗口区域，开展了流体渗漏、底层水氧化还原反应与化能合成群落之间相互作用的研究。研究人员借助遥控水下机器人（Remotely Operated Vehicle, ROV），从核心氧最小值区及其下方的渗口处采集了推入式沉积物岩芯。对提取的沉积物孔隙水进行了硫化物与硫酸盐浓度分析。根据底层水的含氧水平，渗口区域要么仅被微生物席（microbial mats）定植，要么同时被微生物席与大型底栖生物（macrofauna）定植。其中大型底栖生物包括蛰龙介科多毛类（ampharetid polychaetes）与囊螂科蛤类（vesicomyid clams），它们分布于围绕气体喷口呈同心环状排列的独特底栖生境中。在多数仅被微生物席定植的区域，硫化氢被排放至底层水中；而在大型底栖生物大量分布的区域，硫化氢则被束缚在沉积物内部。

研究人员对孔隙水剖面开展数值模拟，以评估流体平流速率与生物灌溉（bioirrigation）速率。尽管随着距中央气体喷口距离的增加，向上流体流动的强度从11 cm yr⁻¹降至<1 cm yr⁻¹，且硫酸盐-甲烷转换带（sulfate/methane transition, SMT）的深度逐渐加深，但输入至SMT的硫酸盐通量并未出现显著差异（6.6~9.3 mol m⁻² yr⁻¹）。按深度积分的生物灌溉速率从中央生境（以微生物席与稀疏大型底栖生物为特征）的120 cm yr⁻¹，升至以大型且少量小型囊螂科蛤类为优势的生境中的297 cm yr⁻¹。上述结果表明，栖息于核心氧最小值区下方外围渗口生境的化能合成大型底栖生物可高效进行生物灌溉，从而将硫酸盐输送至沉积物上层10~15 cm处。借此，这些生物可通过利用硫酸盐刺激甲烷厌氧氧化作用（anaerobic oxidation of methane, AOM）的速率，来应对外围生境中较低的甲烷向上通量，且该氧化速率足以产生化能合成所需的硫化氢。通过生物灌溉作用，大型底栖生物可改造自身所处的地球化学环境，并通过甲烷厌氧氧化作用提升硫化氢的向上通量。此外，由于生物灌溉作用将含氧底层水引入沉积物，硫化物汇的深度逐渐向着外围生境加深。因此我们提出：除了决定大型底栖生物群落能否形成的水柱含氧水平之外，硫酸盐-甲烷转换带的深度（以及由此决定的硫化物生成深度）才是决定何种化能合成群落能够利用深部硫化物的关键因素。我们进一步假设：大型囊螂科蛤类可通过将硫酸盐生成带高效向下扩展至沉积物深部，从而将小型或移动性较弱的生物（如小型蛤类与硫细菌）与硫化物源隔离开来。