Characteristics of Authigenic Pyrite and its Sulfur Isotopes Influenced by Methane Seep--Taking the Core A at Site 79 of the Middle Okinawa Trough as an Example

Objective Authigenic pyrite often develops extensively in marine sediments,which is an important product of sulfate reduction in an anoxic environment.It has a specific appearance and complicated sulfur isotopic properties,and acts as important evidence of methane seep in marine sediments.Strong AOM（anaerobic oxidation of methane）activity has developed in the Okinawa Trough.

Geochemistry and genesis of authigenic pyrite from gas hydrate accumulation system in the Qilian Mountain permafrost, Qinghai, northwest China

A type of authigenic pyrites that fully fill or semi-fill the rock fractures of drillholes with gas hydrate anomalies are found in the Qilian Mountain permafrost; this type of pyrite is known as ＂fracture-filling＂ pyrite. The occurrence of ＂fracture-filling＂ pyrite has a certain similarity with that of the hydrate found in this region, and the pyrite is generally concentrated in the lower part of the hydrate layer or the hydrate anomaly layer. The morphology, trace elements, rare earth elements, and sulfur isotope analyses of samples from drillhole DK-6 indicate that the ＂fracture-filling＂ pyrites are dominated by cubic ones mainly aligned in a step-like fashion along the surfaces of rock fractures and are associated with a circular structure, lower Co/Ni and Sr/Ba, lower ZREE, higher LREE, significant Eu negative anomalies, and 634ScDT positive bias. In terms of the pyrites＇ unique crys- tal morphology and geochemical characteristics and their relationship with the hydrate layers or abnormal layers, they are closely related with the accumulation system of the gas hydrate in the Qilian Mountain permafrost. As climate change is an important factor in affecting the stability of the gas hydrate, formation of fracture-filling pyrites is most likely closely related to the secondary change of the metastable gas hydrate under the regional climate warming. The distribution intensity of these py- rites indicates that when the gas hydrate stability zone （GHSZ） is narrowing, the hydrate decomposition at the bottom of the GHSZ is stronger than that at the top of the GHSZ, whereas the hydrate decomposition within the GHSZ is relatively weak. Thus, the zone between the shallowest and the deepest distribution of the fracture-filling pyrite recorded the largest possible original GHSZ.

Characteristics of authigenic pyrite and its sulfur isotopes influenced by methane seep at Core A, Site 79 of the middle Okinawa Trough

The anaerobic oxidation of methane （AOM） has strongly developed at Core A, Site 79 of the middle Okinawa Trough, East China Sea, and a large amount of authigenic pyrite is preserved in the surface sediment. In this study, we analyze the characteristics of the authigenic pyrite and its sulfur isotopic values. The authigenic pyrite is stripy and tubular, and there were foraminifera compartments filled with pyrite. The pyrite is extracted using chromium reduction, and the values of δ348 are found to lie between -41.20‰ and 8.92‰ V-CDT. The bulk pyrite tends to be more enriched in 348 with increasing depth. Particularly, the 834S value of the pyrite lies between -32.73%o and -41.20‰ V-CDT above 278 cmbsf, but it quickly increases below this depth （-21.49%o-8.92‰ V-CDT）. At the same time, the total sulfur content of the pyrite shows an abrupt increase above 100 cmbsf but is otherwise stable between 1.04% and 0.55% below 100 cmbsf. The stable and negative values of 8345 and the decreasing values of total sulfur above 278 cmbsf indicate reduced AOM activities in 17.18-5.3 ka. In addition, the increasing δ34S and pyrite content indicate strong AOM development and methane seep below 278 cmbsf in 18.8-17.18 ka. In particular, the highest positive value of δ34S occurring in 18.78 ka indicates the most intense AOM activity. The shallow sul- fate-methane interface （SMI） and high methane flux below marine sediments also strongly support this activity.