International Conference on Gas Hydrates (ICGH) (6th : 2008)

AUTHIGENIC PYRITES AND THEIR STABLE SULFUR ISOTOPES IN SEDIMENTS FROM IODP 311 ON CASCADIA MARGIN, NORTHEASTERN PACIFIC Wang, Jiasheng; Chen, Qi; Wei, Qing; Wang, Xiaoqin; Li, Qing; Gao, Yuya

Abstract

In order to understand the response of authigenic minerals to the gas hydrate geo-system, various authigenic pyrites were picked out under Zeiss Microscope and their S isotopes were analyzed later from 652 sediments samples at intervals of about 1.5m recovered from all 5 sites of Integrated Ocean Drilling Program (IODP) Expedition 311 on Cascadia Margin, northeastern Pacific. SEM photos of picked pyrites exhibit various aggregation features mainly in forms of strawberry, pillar/rod and dumbbell in sizes from 200 m to 1000m. Typical cubic pyrite crystals could be seen under smaller scale SEM photos. Most δ34S values in Site U1325 at the west deeper water location of IODP 311 show negative values low to -33.964‰ CDT, distinctly contrasted to the δ34S in Site U1329 at the east shallower location having much more positive values up to 28.29‰ CDT. At the cold venting position assigned as Site U1328 the δ34S values show strong negative values in the upper part of sediments column above 135 mbsf (meter below sea floor), increasing gradually with the depth from -35.83‰ CDT to -1.32‰ CDT, and then display many positive excursions up to 32.49‰ CDT below 135 mbsf, which is significantly distinguished from the values in nearby non-cold venting Site U1327 having much less positive excursions in the lower part of column below 110 mbsf. In all sites a general negative δ34S excursion occur in the upper part of sediments columns above 30~35 mbsf except in Site U1328 having more depth, indicating the potential current sulfate methane interface (SMI) activity zones. Distinct positive δ34S excursions up to the highest δ34S value 53.65‰ CDT from strawberry pyrites aggregations might indicate that sulfide products by AOM probably inherit completely the sulfate having high δ34S value and no sulfate was left after AOM at a high methane flux under gas hydrate geological background.

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