Evolution of continental inputs and watermass during cretaceous oceanic anoxic event 2 in the Neo-Tethyan region: Evidence from Nd-isotopes in southern Tibet

During the mid-Cretaceous extreme greenhouse (similar to 90 to 125 Ma), changes in continental inputs and water-mass circulation influenced nutrient distribution and set the stage for ocean anoxia. Investigation of these changes is critical for an understanding of contemporaneous global-ocean evolution. Previous studies on delta Zn-66, delta N-15, delta C-13 and Os-187/Os-188 isotopes in the Neo-Tethyan Ocean inferred both of these changes might be responsible for the continuous nutrient supply required to cause expansion of carbon burial during Oceanic Anoxic Event 2 (OAE2). Here, we applied the hydroxylamine hydrochloride (HH) extraction procedure to the analysis of Nd isotopic compositions of sediments collected from the mid-Cretaceous Dongshan and Lengqingre formations in southern Tibet. Our Nd isotope [epsilon Nd(t)] records exhibit a large negative excursion (from -9.8 to -6.1) during the Cenomanian-Turonian period, but little variation (-6.5 to -5.2) during the Aptian period. Major and trace elements as well as epsilon Nd(t) of residues indicate that the HH-extracted Nd in this study is mainly sourced from authigenic Fe-Mn oxides. To evaluate the robustness of our epsilon Nd(t) records, we examined the potential influences of the detrital fraction. Our epsilon Nd(t) records reflect a mixing signal of surface currents from the Central Pacific and continental weathering inputs, and the corresponding negative excursion is triggered by the changes of continental weathering inputs in the Neo-Tethyan region, which was synchronous with the change of riverine systems during the drift of the Greater Indian Plate in the mid-Cretaceous greenhouse. Further, the enhanced continental weathering inputs, which supplied continuous nutrients to the Neo-Tethyan ocean, have likely caused OAE2 and extended the duration of OAE2. Our epsilon Nd(t) records shed new lights on our understanding of OAE2 during the midCretaceous.