基于年代学的南海东北部50万年来古冷泉活动演化历史的重建

The study of cold seeps is becoming the science frontier because of its closely relationship with gas hydrate resouces , global environmental change, extreme enviroment creature resources and the origin of life. Authigenic carbonates and seep biota are direct archives of seep history and record paleo-environmental condition.A variety of proxies has been used to unravel the history of past fluid activity recorded in methane –derived authigenic carbonates(MDAC)...Foremost among cold seep reseach is the age determination of MDAC and seep biota.Most cold seeps have their specific lifetimes and cyclicities, which might be controlled by the exhaustion of hydrocarbon soureces and shift of pressure gradients between pore water and hydraulic pressure of seawater. The recent studies indicate that the physical,chemical and biological processes at cold seeps can change significantly with time, and such temporal variability can be expressed by difference in geochemical and mineralogical signatures of associated MDAC and seep biota. Therefore, defining the geochronology of MDAC formation and seep biota is critical to understand the timescales of past methane seepage and reveal their relationship to the global sea-level change. Direct information on the timing of past methane release events can be obtained through 14C dating and U-Th dating of MDAC and seep biota. To date, U-Th dating of MDAC has mostly focused on samples from mid-latitudes and was used to assess the periodicity of fluid migration along fault system and the effects of long-term changes in sea-level and bottom water temperature on methane efflux or gas hydrate dissociation...The northern SCS is a passive continental margin controlled by complex interactions between the Eurasian, Pacific, and Indian-Australian plates. Massive authigenic carbonates and seep biota on the seafloor were discovered in SCS，especially Dongsha Area of the northeastern SCS. Seep carbonates and seep biota were discovered during the joint Chinese-German RV SONNE Cruise 177 in 2004 at the northeastern slope of SCS. An active cold seep with a methane gas plume, living mussels and authigenic carbonates was reported at Site F in the Dongsha Area. To date, The stable C and O isotopes and petrography/mineralogy and type of MDAC in the Dongsha Area have been deep studies, and the research of U-Th dating of MDAC was started in recent years. Two sets of U/Th-ages have been published of which one coincides with high sea-level stand and another sample coincides with rising sea level, whereas all others (16 samples)coincide with low sea-level stands. Other mechanisms have been discussed such as warming of bottom water that destabilizes methane hydrate, increase in the sediment loading, salt diapirism or erosion of the seafloor or tectonism in SCS. There are limited age data on seep carbonares and seep biota so far. Meanwhile, U-Th dating of MDAC has mostly focused on samples from seafloor, not the drilling core samples, and the dating of seep biota is less in SCS...13 layers authigenic carbonates and bivalve shells were obtained from GMGS08 site ,which is one of 5 drilled sites with cored and recovered gas hydrate and has a ~95m long during the second China’ s major gas hydrate expedition GMGS2 in 2013 in Dongsha Area of the northeastern SCS. It can provides important information on past seepage activity history. In our project, petrography, mineralogy, stable carbon and oxgen isotopic compositions, element geochemistry and radiocarbon dating and U/Th dating of seep carbonates and bivalve shells will be analyzed from GMGS08 site and other sites(GMGS16, GMGS09, GMGS07), in order to provide information on fluid source and environmental conditions, seepage fluid intensities, reconstruct the absolute timing and tempo of methane seepage , reveal their relationship to gas hydrate destabilisation and sea-level variations since 500ka of the northeastern SCS.

冷泉生物和碳酸盐岩是冷泉渗漏的直接标志，也是水合物可能存在的重要证据。开展冷泉生物和碳酸盐岩的研究对了解冷泉活动的形成与演化、与海平面变化关系等具有重要意义。目前的研究主要局限于出露海底的冷泉碳酸盐岩，而对钻孔岩心中的冷泉生物和碳酸盐岩还没有系统深入的研究，南海东北部水合物钻探获取的钻孔沉积物中的多层冷泉生物和冷泉碳酸盐岩为研究冷泉系统流体渗漏活动演化历史提供了绝佳的机会。本项目拟通过14C 测年和U 系定年法确定冷泉生物和碳酸盐岩的形成年龄，建立南海东北部完整的水合物分解事件年龄-海平面变化图，探索古冷泉活动演化历史，分析冷泉活动与冰期间冰期旋回的耦合性，确定冷泉渗漏活动的强度是否受海平面变化的控制；并综合冷泉生物和碳酸盐岩同位素、元素地球化学及矿物学、岩石学等特征，揭示冷泉活动与天然气水合物分解释放事件的关系，探讨流体性质与来源，恢复不同时期冷泉活动时的环境条件。

天然气水合物是个动态变化的系统，对温度和压力及其敏感，在低海平面时期更容易分解释放，该结论主要基于出露海底的冷泉碳酸盐岩。本项目首次对南海东北部水合物钻孔冷泉碳酸盐岩开展水合物释放活动演化历史及其触发机制、水合物分解的环境响应研究。.研究钻孔自下而上发育3层产状、年龄和组分均不同的冷泉碳酸盐岩，第1层和第3层的呈结核状、结壳状，其δ13C值绝大多数介于-40.52～-56.80‰，均低于-40‰，δ18 O 值介于1.31～5.7‰；其Mo含量介于0.17 ～36.6 μg/g ，平均含量7.12 μg/g；由文石、高镁方解石、镁方解石、石英、长石和白云石等组成。第2层的呈成岩的灰岩，其δ13C值介于-43.61～-54.02‰，均低于-40‰，δ18 O 值介于2.90～5.37‰；其Mo含量变化范围大，介于0.6～118μg/g ，平均含量17.8μg/g，14个样品Mo含量超过25μg/g；以文石为主，文石含量介于74 ～96%。碳酸盐岩的形成源自水合物分解，水合物分解释放分别发生在MIS1、MIS5和前MIS5等3个时期，不同时期甲烷渗漏强度和氧化还原条件不同，第1和第期甲烷渗漏较弱，MIS5e第2期甲烷渗漏最强。.创新性提出新的水合物分解释放触发机制—海水升温。与学界普遍认为的低海平面弱静水压力为水合物的主要触发机制不同，促使重新审视水合物与全球变化的关系。MIS5e阶段的最强烈甲烷渗漏发生在112,700-133,300yearsBP（U-Th定年结果），该时间与末次间冰期（MIS5e）高海平面温暖期相吻合。本研究首次提供了与MIS5e相关的高海平面期强烈甲烷活动的证据，阐明了MIS5e期间流体活动与气候变化的关系，有助于更好的理解气候变化与天然气水合物稳定性的相互依存关系。.首次提出天然气水合物分解可能导致海底局部硫化环境。MIS5e第2时期甲烷渗漏强度大，在海底附近钼（Mo）含量达到最高值(118ug/g)，底层水高度硫化。