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

Abstract

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 δ 34S are found to lie between -41.20‰ and 8.92‰ V-CDT. The bulk pyrite tends to be more enriched in 34S with increasing depth. Particularly, the δ 34S 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 δ 34S and the decreasing values of total sulfur above 278 cmbsf indicate reduced AOM activities in 17.18–5.3 ka. In addition, the increasing δ 34S 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 δ 34S 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.