Study Of 200,000-year-old History Of Cold Seeps In The NE South China Sea

Cold seeps are methane seepage at seafloor,which are widespread at continental margins worldwide and are commonly associated with the dissociation of gas hydrates and greenhouse effect.They have significant importance in resources and environment.However,the history of past seepages in the South China Sea（SCS）and the relationship between methane seepage and glacial-interglacial transitions remain unclear.Here,this study present high-resolution mineralogical,geochemical（carbon and oxygen isotopes,REE and trace elements）,and chronological(¹⁴C and U/Th)data of seep carbonate(¹⁴C from bivalve shells)from a drill core recovered during China’s second gas hydrate drilling expedition（GMGS-2）in the SCS.（1）Three stages of methane seepage linked to the dissociation of biogenic methane hydrate:～130.3 ka BP before,MIS 5（～130.3 to 111.4 ka BP）and MIS 1（～11.1 to 10.0 ka BP）.Carbonates formed in stage 2 are mainly occurred as aragonite and show extremely high molybdenum contents,and seawater-like REE pattern.These suggest the presence of intermittent or even permanent euxinic conditions generated by sulfidic bottom water that result from intensive methane seepages.In contrast,mineral compositions and trace element patterns indicate that the carbonates in stages 1 and 3 form within sulfidic conditions restricted to pore water under relatively lower methane fluxes.（2）Compared to regular indicator of intense of methane seepage,Mo isotope has stronger stability and show various degrees of fractionation due to different seepage intensities.This represents that Mo isotopic compositions can be used as a tool in reconstructing methane intense.Meanwhile,the Mo isotopic composition of euxinic sediments（aragonite）with[H₂S]>11μM associated with strong methane seepage,reflects the coeval seawater record.（3）The strong methane seepage will affect the ecosystems on the seabed.Three genera of fossil Cold-water corals（CWCs,Crispatotrochus sp.,Solenosmilia sp.and Enallopsammia sp.）were identified.It appears that seep carbonates provide a substratum for coral colonization.Bathymetric high points,appropriate water temperature and stronger bottom-water currents might be crucial to keep conditions favorable for the growth of CWCs in the studied area.（4）Cold seeps occurred due to an increase in bottom water temperature during periods of sea-level highstands,which are totally at variance with the conclusion that past hydrate dissociation events mostly coincided with sea-level lowstands.Glacial-interglacial transitions usually witnessed rapid increase of temperature and subsequently significant decrease of hydrostatic pressure.These prominent changes should be responsible for the dissociation of gas hydrates in the sediment column and associated methane seepages on the seafloor.We suspect that there are possible links between methane seepages and glacial-interglacial transitions in the South China Sea and this is probably a widespread phenomenon,possibly accelerating global warming.We systematically reveal,for the first time,links between methane seepages and global climate change.