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

Abstract

During the mid-Cretaceous extreme greenhouse (∼ 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 δ⁶⁶Zn, δ¹⁵N, δ¹³C and ¹⁸⁷Os/¹⁸⁸Os 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 [ε_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 ε_Nd(t) of residues indicate that the HH-extracted Nd in this study is mainly sourced from authigenic FeMn oxides. To evaluate the robustness of our ε_Nd(t) records, we examined the potential influences of the detrital fraction. Our ɛ_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 ε_Nd(t) records shed new lights on our understanding of OAE2 during the mid-Cretaceous.