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 ? 34 S are found to lie between ?41.20‰ and 8.92‰ V-CDT. The bulk pyrite tends to be more enriched in 34 S with increasing depth. Particularly, the ? 34 S value of the pyrite lies between ?32.73‰ and ?41.20‰ V-CDT above 278 cmbsf, but it quickly increases below this depth(?21.49‰–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 ? 34 S and the decreasing values of total sulfur above 278 cmbsf indicate reduced AOM activities in 17.18–5.3 ka. In addition, the increasing ? 34 S 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 ? 34 S occurring in 18.78 ka indicates the most intense AOM activity. The shallow sulfate-methane interface(SMI) and high methane flux below marine sediments also strongly support this activity.

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.