Authigenic Pyrite And Gypsum From ODP Leg 204 Sediments On Hydrate Ridge And Their Significances

As a stable end product of sulfate reduction and anaerobic methane oxidation occurring in gas hydrate-bearing sediment, authigenic pyrite has been recovered approaching the sulfate-methane interface (SMI) and plays a major role for finding detailed evidences about the origin and implication of the"Paleo-SMI"buried in deeper sedimentation through time as well as investigating large methane releasing history and further indicating gas hydrate stability depositing. Besides, an unusual type of authigenic gypsum has been found in surface of this sediment environment, which may methane-related likewise.Under the research and analysis of abundances, fluxes and stable sulfur isotopes of pyrite and gypsum from Sites 1245 and 1252 sediment cores recovered by Ocean Drilling Program (ODP) Leg 204 on Hydrate Ridge, the results show that: (1) Adjacency to modern SMI, discordant variation of sulfur isotopic composition profiles with depth between dissolved sulfide and pyrite suggest that pyrite formation has been nearly ceased though sulfate reduction is ongoing now, presumably because of the lack of dissolved iron; (2) Under the modern SMI, dissolved sulfide diffuses downward into the H2S-Fe2+ transition and reacts with dissolved iron diffusing from deeper sediment to produce new pyrite now, which will mix with original pyrite formed in sulfate reduction zone before and then buried to this transition; (3) In the deeper sediment below the modern SMI, synthesizing the downhole discontinuous variation in pyrite abundances and fluxes, which may indicate that the dynamic evolution of methane fluid flux and/or gas hydrate destabilized state through geologic history, as well as sulfur isotopes and oxygen isotope stratigraphy from Martinson et al. (1987) will supply some geochemical and paleoceanographic evidences for"Paleo-SMI"identification; (4) The difference of tectonic background and evolution between Sites 1245 and 1252 makes different effects on gas hydrate stability and methane releasing process, and then is marked in difference of pyrite abundances and fluxes between these two Sites; (5) Authigenic gypsum crystallization may go through in terms of the episodic growth sequence (tabular mono-crystalline→tabular bi-crystalline→cluster aggregate), which may related to sulfide re-oxidization and/or disproportionation during sulfate reduction and may have little relevant to gas hydrate occurrence and methane leakage.