Fe Isotope Compositions Of The Upper Cretaceous Black Shales And Oceanic Red Beds And Its Implications To Paleoceanography And Paleoclimate

The Cretaceous period is one of the typical examples of “greenhouse states” in earth history, thus unraveling the mechanisms of paleoceanographic and paleoclimate change during this period will help us make accurate projections of future climate change on the warming earth. Upper Cretaceous oceanic sequence witnessed a significant sedimentation change from organic-rich black shales to organic-poor Cretaceous oceanic red beds(CORBs), which were associated with the Cretaceous oceanic anoxic event 2 happened near the Cenomanian-Turonian boundary and the subsequent oceanic oxic event. This transition represents a major global carbon cycle perturbation and the paleoceanographic and paleocliamte fluctuation during the “greenhouse” climate, and therefore has become a new research focus in Cretaceous research.In this thesis, we use sedimentary, element geochemistry, Fe speciation, and Fe isotope proxies in three sections(Chuangde section, Tibet, Contessa Quarry section, Italy, and ODP Leg 103 Site 641A) that recorded the Upper Cretaceous “black-red” sedimentation change from eastern Tethys, western Tethys and North Atlantic, to reconstruct the paleoceanographic and paleocliamte changes in Late Cretaceous. The Fe isotope compositions and other geochemical proxies exhibited systematic variations from the black shales to CORBs in all three sections. The Fe isotope compositions of the CORBs, which have never been reported before, are relatively consistent with lighologic background but also showed certain variation, with δ56Fe ranging from-0.400‰ to 0.505‰. The Fe isotope perturbations in CORBs are associated with rock type and local Fe geochemical cycle. In contrast, the black shales showed variable δ56Fe values, and the three sections exhibit different excursions. Composite analysis of Fe isotope, Fe speciation and element geochemistry, during OAE 2 the Chuangde section was deposited under oxygen-depleted environments, and the Contessa Quary section of western Tethys was within anoxic/euxinic environment. The geochemical proxies of black shales from ODP Leg 103 Site 641 A in North Atlantic implied restricted exunic basin during OAE 2. The transition interval from black shales to CORBs showed variable Fe isotope values, which may suggest unstable paleoceanographic conditions. The difference in redox conditions and Fe geochemical cycles of the three sections during their deposition implied the effect of local paleogeography and local process on paleoceanographic environment and geochemical cycle under overall cliamte change background. The geochemical results from Site 641 A in North Atlantic suggested a likely early breach of the Equatorial Atlantic Gateway(EAG) in(late) Turonian, which may have facilitated the formation of CORBs in this site, and it took at least about 0.60 Ma to have stable deep-water connection between North and South Atlantic after the initial open of EAG. In western Tethys(Italy), it took about 1.1 Myr to change from euxinic conditions during OAE 2 to well oxygenated, ventilated conditions, but required a long time(>9 Myr) in Eastern Tethys(south Tibet). Based on the geochemical results from this thesis and the global paleoceanographic and paleoclimate proxies, the sedimentation transition of organic-rich black shale to the CORBs was likely driven by the combination of global cooling and the opening and widening of the ocean gateways.