臺灣西南海域永安海脊冷泉址沉積物之反差性黃鐵礦組織

A record of temporal cold-seep activities associated with enhanced anaerobic oxidation of methane in the Yung-An Ridge sediments offshore southwestern Taiwan was marked by the occurrence of high pyrite concentrations and local formation of carbonate nodules in sediments from the upper and lower sections of long piston core MD052911. Electron microscopic analysis of the cored sediments indicate that pyrite has a variety of microtextures including framboidal and irregular aggregates of fine-grained crystals, patches of disseminated anhedral to euhedral grains, and anhedral grains enclosed within microfissures of phyllosilicate cleavage. In addition, high percentages of pyrite framboids showing prominent overgrowth and pyrite neoformation within pellets of ferric smectite occur in these sediments. These diversified pyrite growth textures can occur in a single specimen and have no systematic correlation with burial depth. These imply subtle differences in geochemical environment over short distances, in accordance with a migration mode of methane dominated by slow seepage and/or diffusion. In contrast, the middle-section sediments contain much smaller amounts of pyrite with scanty framboidal aggregates and anhedral grains due to a lack of intense anaerobic oxidation of methane.