Interactions between Fluid Flow, Geochemistry and Dibgeochemical Processes at Methane Seeps

Since their first discovery 20 years ago, numerous cold seeps have been found, mainly along active and passive continental margins. Methane gas plays a key role in the global carbon cycle and, as a highly potent greenhouse gas, in the control of the Earth's climate. In diffusion-controlled sediments, methane is completely oxidized within the sediment column and does not reach the overlying water. In contrast, at cold seeps, methane is transported to the sediment surface by advective forces. For geologic reasons, fluid advection and the respective methane flux are spatially and temporarily variable. Microbial communities of methane-oxidizing archaea in syntrophy with sulfate-reducing bacteria have been found to play a key role in consuming methane and thus controlling methane efflux from the sediments. Anaerobic oxidation of methane leads to the production of dissolved inorganic carbon and subsequent precipitation of carbonate, representing a permanent sink for methane-derived carbon. The metabolic pathway and the role of the different groups of methanotrophic archaea and sulfate-reducing bacteria that are involved in the anaerobic oxidation of methane remain poorly understood. There seem to be site-specific differences in the composition and function of the microbial communities. The upwelling of methane- and sulfide-rich fluids supports rich benthic communities of sulfur-oxidizing bacterial mats and abundant macrofauna species harboring thiotrophic or methanotrophic symbionts. Variation in fluid flow, and thus methane supply and hydrogen sulfide concentrations, are key factors controlling the occurrence and community structure of benthic communities.

自20年前首次被发现以来，全球已发现大量冷泉（cold seeps），它们主要分布于活动大陆边缘与被动大陆边缘沿线。甲烷（methane）在全球碳循环中发挥关键作用，同时作为一种强效温室气体，对地球气候调控具有重要影响。在扩散控制型沉积物（diffusion-controlled sediments）中，甲烷会在沉积物柱内被完全氧化，无法抵达上覆水体。与之相反，在冷泉环境中，甲烷通过平流作用力（advective forces）被输送至沉积物表层。受地质作用影响，流体平流（fluid advection）与对应的甲烷通量（methane flux）存在空间与时间上的动态变化。研究发现，与硫酸盐还原菌（sulfate-reducing bacteria）互营共生的甲烷氧化古菌（methane-oxidizing archaea）微生物群落，在消耗甲烷并进而调控沉积物甲烷逸出的过程中发挥关键作用。甲烷厌氧氧化（anaerobic oxidation of methane）会生成溶解无机碳，并随后引发碳酸盐沉积，这一过程构成了甲烷来源碳的永久汇。参与甲烷厌氧氧化的各类甲烷营养古菌（methanotrophic archaea）与硫酸盐还原菌的代谢途径及其功能，目前仍鲜为人知。微生物群落的组成与功能似乎存在位点特异性差异。富含甲烷与硫化物的流体上涌，孕育了丰富的底栖群落：包括硫氧化细菌席，以及携带有化能硫营养或甲烷营养共生体的大量大型底栖动物物种。流体流动的变化，进而甲烷供给与硫化氢浓度的变化，是调控底栖群落存在与否及其群落结构的关键因素。