Early Cretaceous Cold Seep Carbonates In Dingri,Southern Tibet:the Feature And Formation Model

Conditions such as climate change,tectonic activities,and sea level changes cause the instability of submarine natural gas hydrates,and the process by which the strong greenhouse gas methane is released into the ocean and atmosphere is called marine methane leakage(or cold seepage).It has a significant greenhouse effect on the marine environment and climate system,and the formed carbonate rocks are called cold seep carbonate rocks.The middle Cretaceous period which the atmosphere-ocean system experienced multiple periods of major carbon cycle disturbances,in which cold seep activities triggered carbon cycles.At present,the research focus of the Cretaceous cold seep carbonate rocks is mainly focused on the formation environment and carbon source of the cold seep carbonate rocks.and the research on the formation process of the cold seep carbonate itself is still not comprehensive enough,therefore,it affects the understanding of the sedimentary environment of the ocean bottom sediment-water interface during the Cretaceous period and the relationship between cold seepage and carbon cycle disturbance events.A set of carbonate nodules with a thickness of about 1.38 meters developed under the oceanic anoxic events OAE1 a of the Gucuocun Formation of the Lower Cretaceous in the Tingri area of Tibet.In this paper,the characteristics of this set of carbonate nodules are studied by means of field and microscope observation of carbonate nodules,carbon and oxygen isotope testing,major and trace element testing of nodules and surrounding rocks,and nitrogen isotope testing of surrounding rocks,and reveal its genetic model on this basis,and infer whether it has a genetic connection with the Cretaceous oceanic anoxic events.The carbonate nodule layer can be subdivided into three layers: the lower layer is small pipe-shaped nodules with a length-to-width ratio of 2.4cm/1.8cm,which are produced in parallel or obliquely across the layers.and the middle layer is composed of ellipsoidal and plate-shaped nodules.The length-to-width ratio of nodules is 17cm/11.5cm.The upper layer is breccia,and the breccia is composed of limestone.It is a sandy composition,and the nodules are mainly composed of authigenic carbonate minerals and detrital materials,with the development of banded and botryoidal cements(bbc),framboidal pyrite and other common sedimentary fabrics of modern cold spring carbonate rocks.The nodules contain trace fossils,foraminifera,cephalopods,bivalves and other biological fossils.The 13 C value of carbonate rocks is-23.26 ‰?-7.14 ‰(V-PDB).CAS sulfur isotope value is +17.3‰?+19.0‰.The distribution of the enrichment factors of rock trace elements is similar to that of cold seep in other regions.The morphological,mineralogical and geochemical analysis of the early Cretaceous carbonate nodules on the Gucuo section in the Tingri area all show that the nodules are cold seep carbonate rocks,indicating the occurrence of cold spring leakage under the sediment-water interface;at the same time;There are also many clastic rock veins caused by seismic activity developed under the cold seep carbonate nodules,and the interstitials of the nodules were found to contain quartz and other sandy clastic components,indicating that the clastic rock veins are fluid It provides a channel for upward seepage;Mo and U enrichment factor of the surrounding rock indicate that the horizon water formed by the cold seep changes from an oxidizing environment to a reducing environment,that is,the methane released by the leakage activity of the cold seep consumes the oxygen in the bottom water,making the bottom water anoxic,It is also an indirect evidence of the existence of cold seepage activity.However,since the seepage activity of the cold seep discovered this time is mainly affected by regional seismic activity,Therefore,whether its formation process can affect the carbon signal of global ocean anoxic events requires further research.