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Evolution of continental inputs and watermass during Cretaceous Oceanic Anoxic Event 2 in the Neo-Tethyan region: Evidence from Nd-isotopes in southern Tibet

Neo-Tethyan,HH-extraction,Shales,Weathering inputs,Oceanic circulation,OAE2

Theme 3: Earth-Life Co-evolution Through Time > Session 3h: Transient Ocean Deoxygenation in Earth History: Biotic and Abiotic Controls

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 (Jenkyns, 2010). Investigation of these changes is critical for an understanding of contemporaneous global-ocean evolution. Previous studies on δ⁶⁶Zn, δ¹⁵N, δ¹³C and ¹⁸⁷O_S/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) (Zhang et al., 2019; Chen et al., 2021; Li et al., 2022). 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 (Martin et al., 2010). 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 Fe-Mn 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.



References

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Jenkyns, H. C. (2010). Geochemistry of oceanic anoxic events. Geochemistry, Geophysics, Geosystems, 11(3).

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Martin, E. E., Blair, S. W., Kamenov, G. D., Scher, H. D., Bourbon, E., Basak, C., & Newkirk, D. N. (2010). Extraction of Nd isotopes from bulk deep sea sediments for paleoceanographic studies on Cenozoic time scales. Chemical Geology, 269(3-4), 414-431. https://doi.org/10.1016/j.chemgeo.2009.10.016

Zhang, X., Gao, Y., Chen, X., Hu, D., Li, M., Wang, C., & Shen, Y. (2019). Nitrogen isotopic composition of sediments from the eastern Tethys during Oceanic Anoxic Event 2. Palaeogeography, Palaeoclimatology, Palaeoecology, 515, 123-133. https://doi.org/10.1016/j.palaeo.2018.03.013