墨西哥湾北部上陆坡Green Canyon 140冷泉活动在自生碳酸盐岩中的地球化学记录

Cold seep activities recorded by geochemical characteristics of authigenic carbonates from Green Canyon 140,Gulf of Mexico

墨西哥湾北部上陆坡Green Canyon 140产出的冷泉碳酸盐岩可以分为结核结壳、生物碎屑结壳和块状结壳三类。三类结壳均以高镁方解石和文石为主要矿物,另外,结核结壳含少量白云石,生物碎屑结壳含少量低镁方解石。团粒、凝块、草莓状黄铁矿和生物钻孔等生物成因沉积组构发育,且含有以有孔虫和双壳类为主的生物碎屑。三类结壳的碳同位素值变化较大,^δ13C值为-36.1‰～5.1‰,指示了产甲烷残余CO2、海水源碳或热成因甲烷的混合碳源特征。结核结壳胶结的生物壳^14C年龄为46.5～25.8 ka BP,生物碎屑结壳为17.6～11.7 ka BP,块状结壳为-1.2 ka BP,显示了该地区冷泉活动的间歇性特征。

Seep carbonates collected from Green Canyon block 140, Gulf of Mexico, could be divided into three types： nodule-rich carbonates, bioclast-rich carbonates and massive carbonates. XRD investigation shows that high Mg-calcite and aragonite are the dominate minerals. Additionally, nodule-rich carbonates contain minor amounts of dolomite and bioclast-rich carbonates include some low Mg-calcite. Peloids, clotted microfabric and pyrite framboids are developed in carbonates and suggest a genesis linked with bacterial activities. The carbonates exhibit a large variation in ^δ13 values （-36.1‰ to 5.1‰）, suggesting multiple carbon sources that include thermogenic methane, seawater and residual CO2 from methanogenesis. Shells cemented in nodule-rich carbonates show ^14C ages between 46.5 ka and 25.8 ka BR bioclast-rich carbonates formed from 17.6 ka to 11.7 ka BE and massive carbonates have ^14C ages of 1.2 ka BR suggesting that seep activity was discontinuous.