Precipitation behaviors of Cu and Fe in dislocated Floating-zone-grown silicon crystals are investigated by means of transmission electron microscopy (TEM) and the electron-beam-induced-current (EBIC) technique. Cu precipitation on dislocations is affected significantly by the cooling rate of a specimen after contamination. Cu develops precipitate colonies at some special sites on dislocations and does not decorate other parts of dislocations if the specimen is cooled slowly. These preferential precipitation sites are suggested to be non-dissociated edge-type dislocation segments. The fast cooling of a specimen leads to that Cu precipitates on all of dislocations. Fe decorates all of dislocations uniformly, irrespective of the cooling rate of a specimen.

The occurrence of seep carbonates is one of the characteristic features for cold seep sites at continental margins. The carbonates documented the venting history of methane-rich fluid. Compared to the chemoherm carbonates and carbonate pavements which formed on the sediment-water interface, chimney-like seep carbonates precipitated around fluid conduits below the sediment-water interface therefore better recording information of the past fluid flow and composition. Here the chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were studied to understand the origin and nature of the venting fluids and their potential relationship with gas hydrate deposits underneath the seafloor. Based on the occurrence, morphology, petrology, mineralogy and C-and O-isotope compositions, combined with present and past bottom water temperatures and the timing of methane release events, the oxygen isotopic fractionation between calcite and water were used to estimate the equilibrium δ18 O values of the precipitating fluids. The δ 13 C values ranging from −56.33‰ to −42.70‰ V-PDB and thus clearly show that the studied chimneys were mainly derived from biogenic methane oxidation. The calculated equilibrium δ 18 O values of the precipitating fluids ranged from 1.9‰  0.3‰ to 0.6‰  0.3‰ V-SMOW, with an average of 1.4‰  0.3‰ V-SMOW which is heavier than those of seawater even at the last glacial maximum. It is considered that the formation of chimney-like carbonates was closely related to methane hydrate dissociation in the area. The methane hydrates contributed as much as 45.7% of water to the venting fluids. It is suggested that the climate and environmental changes (e.g. sea-level lowering, down-cutting canyons and mass wasting) are the major mechanisms maybe responsible for the destabilization of methane hydrates in the study area. The extensive occurrence of seep carbonates indicates that a large amount of the methane released from methane hydrate dissociation has been effectively captured and sequestered by microbial anaerobic oxidation of methane (AOM) before it escapes into the water column.South China Sea, seep carbonate chimneys, carbon and oxygen isotopes, gas hydrate deposits, methane-rich fluids Citation:

No abstract

No abstract

No abstract

No abstract

No abstract

No abstract