Characterization And Mechanism Of The Cretaceous Oceanic Anoxic Event 1d Carbon Cycle Perturbation In Southern Tibet

During the mid-Cretaceous,typical greenhouse climate period in the geological history,the Earth underwent numerous global and local carbon cycle perturbations to the ocean–atmosphere system,especially during periods of known widespread organic-carbon burial and marine carbonates carbon isotope excursions termed oceanic anoxic events（OAEs）.OAE 1d preceded the Albian-Cenomanian boundary（～100.5 Ma）.Most studies on OAE 1d are established in the western Tethys and North Atlantic Oceans Oceans,but not yet in the eastern Tethys Ocean.Former studies highlighted increases of upper ocean water temperature in the western Tethys and the Atlantic and elevated atmospheric p CO₂ prior to OAE 1d and speculated a causal link with volcanism.However,to date conclusive geochemical evidence of the link between OAE 1d and volcanism is absent.Moreover,as another light carbon source,what is the role of methane seeps during the OAEs is still indistinct.In this paper,we present biostratigraphic,chemostratigraphic,cyclostratigraphic,organic geochemical,mineralogical and petrographic characteristics of the Albian–Turonian intervals in two eastern Tethyan sections（Youxia and Qiangdong,southern Tibet）.Our study provides a high-resolution Albian-Cenomanian boundary interval（ACBI）and OAE 1d carbon-isotope reference curves for the eastern Tethys and establish a composite orbital chronology with estimated duration of ACBI（～311kyr）and OAE 1d（～233kyr）.Our study also provides the first mercury（Hg）concentration and isotope record of OAE 1d.Hg anomaly prior to the onset of OAE 1d is derived from a volcanic source.Based on the atmospheric p CO₂ and ocean water temperature evidence prior to OAE 1d and the absolute age of coeval Large Igneous Provinces（LIPs）,we conclude that the Central Kerguelen LIP volcanism may have triggered OAE 1d global climate change.Different from other sites of OAE 1d records,ACBI and OAE 1d in the Qiangdong section show much larger fluctuations in the carbon isotope curve.We suggest that the nine layers of seep carbonates outcropped in the Qiangdong section serve as the main cause for the peculiar carbon isotope perturbation.Similar with other records of mid-Cretaceous methne seeps along the passive continental margins of Tethys,the main control factors of the methane seepage are enhanced organic matter accumulations and rapid sea-level changes.Furthermore,at low seawater sulfate concentrations,elevated organic matter accumulations would increase the flux of organic carbon into an expanded zone of methanogenesis and ultimately result in a greater flux of methane toward the sediment surface,possibly further contributing to the establishment of anoxia and black shale deposition.The increased methane seepages would alter the carbonate pool in the pore water of coeval sediments and eventually changed the primary carbon isotope signals which may recorded the global carbon cycle perturbations.This study firstly fills the research gap of OAE 1d record in the eastern Tethys,reveals the trigger mechanism of its carbon cycle perturbations and highlights the superposition of regional sedimentary processes（e.g.early diagenesis）on global carbon cycle signals,providing a paradigm for the integrated study of the regional and global characteristics of the carbon cycle perturbations of the OAEs.