Elemental sulfur(ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study, solid ES is discovered in sediments near the Jiulong Methane Reef in the northern South China Sea by scanning electron microscopy and Raman spectroscopy. Combining the morphology and distribution of ES, pyrite concentrations, and sulfur isotopes, we conclude that:(1) solid ES coexists with pyrite microcrystals and sulfide(oxyhydr)oxides as well as clay minerals, and they are mainly distributed on the surface of mineral aggregates;(2) ES mainly occurs within and near the sulfate-methane transition zone(SMTZ) despite little morphological diversity;(3) ES formation might be related to hydrogen sulfide oxidation and is therefore linked with fluctuations in the SMTZ. Within the SMTZ, hydrogen sulfide is produced and pyrite precipitates because of enhanced anaerobic oxidation of methane coupled with dissimilatory sulfate reduction. This enhances the efficiency of the inorganic sulfur cycle and provides favorable conditions for ES formation. The discovery of solid ES in sediments near the Jiulong Methane Reef suggests an important relationship with SMTZ fluctuations that could have implications for the evolution of methane hydrate in the South China Sea.
LIN QiWANG JiaShengFU ShaoYingLU HongFengBU QingTaoLIN RongXiaoSUN Fei
The geochemistry and paleogeography of the Doushantuo succession in the Shennongjia area have been insufficiently studied. Here, we report on the carbon and oxygen isotope compositions of Ediacaran Doushantuo cap carbonates from four sections（Longxi, Muyu, Yazikou and Songluo） in the Shennongjia area. A large C-isotopic gradient（~5‰） between the Longxi and Songluo sections, here identified for the first time, is inferred to have been dynamically maintained by photosynthesis in surface waters and anaerobic oxidation of dissolved organic carbon（DOC） in deep waters. Spatial variation in C-isotope chemostratigraphy among the four sections was related to Marinoan post-glacial sea-level elevation changes. At Longxi, a positive δ^（13）C_（carb） shift below the horizon containing tepee-like structure resulted from intense photosynthesis during early regression. A negative δ^（13）C_（carb） shift within the tepee-horizon was triggered by sulfate reduction and freshwater mixing with ^（13）C-depleted dissolved inorganic carbon（DIC） during late regression. A positive δ^（13）C_（carb） excursion in the uppermost part of the cap carbonate was related to enhance primary productivity and organic matter burial during early transgression. At Muyu, the carbon isotopic variation tendency, which is similar to that at Longxi, may have been mildly influenced by the surface water environment. At Songluo, the positive δ^（13）C_（carb） excursion, up to-4‰ in the lower part of the cap carbonate, was probably associated with methanogenesis in deep waters during late transgression to early regression and subsequently disappeared due to decomposition of methane hydrate during late regression. At Yazikou, the consistently stable δ^（13）C_（carb） values around-4‰ indicate that the cap carbonate may have deposited at intermediate water depths. As evidenced by diagnostic sedimentary characteristics of the study sections, the palaeogeographic framework of the Shennongjia area exhibited deep
Guangzhe WangJiasheng WangZhou WangCan ChenJunxia Yang
甲烷厌氧氧化作用（Anaerobic Oxidation of Methane,简称AOM）是海洋沉积物早期成岩过程中常见的地质作用,记录了沉积物埋藏压实期间海水下渗与沉积流体上涌过程中发生的硫酸盐还原和甲烷厌氧氧化等生物地球化学反应,形成了一套独特的自生矿物.通过实例分析东北太平洋IODP311航次U1328站位和南海北部陆坡GMGS2航次08站位的钻孔岩芯沉积物中自生矿物的类型和稳定碳、氧、硫同位素组成等,在U1328站位浅表层及约219m深度位置和在GMGS2-08站位15mbsf、60mbsf和84mbsf位置发现了AOM成因的自生碳酸盐类矿物,其δ13C值分别低至-41.50‰（VPDB）和-57.89‰（VPDB）,并且出现了富集的黄铁矿及其正偏硫稳定同位素组成,其δ34S值分别高达32.49‰（VCDT）和20.80‰（VCDT）.认为现代海洋沉积物中AOM成因的自生矿物能有效地记录海洋的甲烷异常渗漏事件,其矿物组合和稳定同位素组成可用于探索地史时期古海洋的甲烷渗漏事件.新元古代＂雪球地球＂结束后陡山沱组＂盖帽＂碳酸盐岩中极低碳稳定同位素的特征和扇状重晶石等自生矿物特征,指示了新元古代冰后期古海洋中曾经出现异常的甲烷渗漏事件,可能造成了古海洋环境和吉气候的突变.根据＂将今论古＂原理,利用AOM成因的自生矿物探索深时地球重大转折期古海洋的甲烷异常渗漏事件具有广阔的应用前景.
