(Table 1) Stable isotope data of chemoherm carbonates at Hydrate Ridge on the Cascadia continental margin

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as -48.1 per mill PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The d18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 per mill PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 per mill PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.在卡斯卡迪亚（Cascadia）大陆边缘的水合物脊（Hydrate Ridge），发现了两座自由向上生长至大洋水体中的活跃化学沉积丘（chemoherm）——尖峰丘与东南海丘化学沉积丘。这类由微生物介导形成的碳酸盐建造，可高出海底数十米，呈现尖峰状形态且侧翼陡峭。所采集的岩样均为纯碳酸盐岩，以文石（aragonite）为主要矿物组分。根据组构与矿物学特征，可区分出不同类型的自生文石。详细的形貌观察与岩相学研究明确证实，微生物参与了该碳酸盐岩的形成过程。隐晶质与纤维状文石的组构可被归类为凝块状（thrombolitic）。微晶文石中保存的微生物化石丝状体，表明微生物与碳酸盐岩形成过程存在密切关联。样品的碳同位素值显著亏损¹³C（最低可达-48.1‰ PDB），该特征表明甲烷是该碳酸盐岩形成的主要碳源。形成碳酸盐岩沉淀的富甲烷流体，最可能源自海底模拟反射层（bottom-simulating reflector, BSR）下方的气藏，并通过断裂系统向上运移。该文石质化学沉积丘碳酸盐岩的δ¹⁸O值（最高可达5.0‰ PDB）远高于由环境海水形成文石的理论平衡同位素值（约3.5‰ PDB）。初步估算表明其形成于冰期海水，但也需考虑其他潜在影响因素。基于原位观测与该碳酸盐岩的详细岩相学研究，本文提出了这类化学沉积丘碳酸盐岩的沉淀成因概念模型。该模型阐释了附着于碳酸盐岩表面的古菌与硫酸盐还原菌菌群的功能：该菌群通过厌氧甲烷氧化（anaerobic oxidation of methane, AOM）过程，使得化学沉积丘能够在含氧海水环绕的海底上方形成并沉积。生长在化学沉积丘表面的贝氏硫菌（Beggiatoa）菌席，可在有氧环境中氧化由硫酸盐依赖型厌氧甲烷氧化过程产生的硫化物。贝氏硫菌与下方微生物菌群的接触界面，代表了上层含氧水体与发生碳酸盐岩形成的缺氧微环境之间的分界。