摘要
极低碳稳定同位素是识别地质历史时期古冷泉及其甲烷释放事件的关键证据,目前,世界范围内尚无有关二叠纪冷泉活动的直接证据报道。本研究首次报道了西藏地区拉萨地块早二叠世碳酸盐岩的极低碳同位素数据,δ13CV-PDB值最低可达-34.69‰,碳、氧稳定同位素呈现较好的负相关性,指示为甲烷厌氧氧化作用成因。该碳酸盐岩发育于西藏申扎地区下二叠统昂杰组上段(碎屑岩段),以透镜状碳酸盐岩夹层及碳酸盐质胶结结核产出,可见方解石结晶扇,其碳同位素普遍较结核中碳酸盐胶结物更贫13C。推测研究区存在早二叠世古甲烷冷泉渗漏活动,其成因可能与冈瓦纳冰期后温暖海水导致的海洋甲烷水合物藏的失稳释放有关。作为研讨地质历史时期水合物-冰期反馈模型的重要实例,本研究对进一步探讨冰期海平面变化、古海洋化学、古生态和古气候等特殊环境变化事件具有重要的指示意义。
Extremely 1 3 C-depleted isotope is the key evidence to identify ancient cold seepage and methane re-lease event in Earth history.There has been no report of direct evidence of cold seepage in Early Permian period so far.Here we unprecedentedly present the carbonate carbon and oxygen isotopes with extreme 1 3 C depletion in Early Permian of Lasha Block,Tibet.The negative correlation of carbon and oxygen isotopes is pronounced and the lowest carbon isotope is to -34.69‰(VPDB),exclusively indicating anaerobic oxidation of methane. These carbonates occur in the upper member of the Lanfjie Formation in Xainza,Tibet as carbonate lens and carbonate-cemented concretion,and calcite crystal fans are more depleted in 1 3 C than carbonate cements in the concretions.We infer the occurrence of ancient cold seepage in Early Permian in the study area and attribute this event to the postglacial destabilization of gas hydrate reservoir.This study is significant to investigate glac-ioeustatic change,oceanographic geochemistry,paleoecology and climate change.
出处
《现代地质》
CAS
CSCD
北大核心
2016年第2期420-425,共6页
Geoscience
基金
中国地质调查局项目(1212011121257)
