Chemical and isotopic compositions and age of carbonates from the Lost City hydrothermal field, Mid-Atlantic Ridge

Two genetically different types of authigenic carbonate mounds are studied: (1) from an active hydrothermal field related to serpentinite protrusions in a zone of intersection of a transform fracture zone with the Mid-Atlantic Ridge, (2) from an active field of methane seepings in the Dnieper canyon of the Black sea. General geochemical conditions, under which authigenic carbonate formation occurs within these two fields, were found. They include: presence of reduced H2S, H2, and CH4 gases at absence of free oxygen; high alkalinity of waters participating in carbonate formation; similarity of textural and structural features of authigenic aragonite, which represents the initial carbonate mineral of the mounds; paragenesis of aragonite with sulfide minerals; close relation of carbonate mounds with communities of sulfate-reducing and methane-oxidizing microorganisms. A new mechanism of formation of hydrothermal authigenic carbonates is suggested. It implies their microbial sulfate reduction over hydrogen from fluid in the subsurface mixing zone of hydrothermal solution and adjacent seawater.

本研究针对两类成因各异的自生碳酸盐岩丘（authigenic carbonate mounds）展开分析：其一为大西洋中脊（Mid-Atlantic Ridge）与转换断裂带（transform fracture zone）交汇区域内、与蛇纹岩凸起（serpentinite protrusions）相关的活动热液场，其二为黑海第聂伯峡谷中的活动甲烷渗漏场。研究明确了两类场域中自生碳酸盐形成的共性地球化学条件，具体包括：存在还原态硫化氢（H₂S）、氢气（H₂）与甲烷（CH₄）气体且无游离氧；参与碳酸盐形成的水体碱度显著偏高；作为岩丘初始碳酸盐矿物的文石（aragonite），其结构构造特征具有高度相似性；文石与硫化物矿物共生；碳酸盐岩丘与硫酸盐还原微生物（sulfate-reducing microorganisms）及甲烷氧化菌（methane-oxidizing microorganisms）群落存在紧密关联。本研究提出了一种全新的热液自生碳酸盐形成机制：即在热液流体与邻近海水的地下混合带中，微生物借助流体中的氢气完成硫酸盐还原反应，进而形成自生碳酸盐。