The PETM-CIE and shallow-water biotic changes in the eastern Neotethys Ocean (Tibet/China and Ladakh/India)
Final Report Abstract
The Paleocene-Eocene thermal maximum (PETM), characterized by >4–5 °C global temperature rise within ~200 thousand years, was a hyperthermal event happening ~56 million years (Ma) ago. This event was associated with a ~2–7‰ negative Carbon Isotope Excursion (CIE) and with dramatic changes in the atmosphere, hydrosphere, and biosphere. The PETM was generally thought to be related to a massive addition of 13C-depleted carbon into the atmosphere-ocean system and is considered a potential ancient analog for future climate change. A prerequisite to the PETM studies is the recognition of CIE records from sedimentary sections. Traditionally, the CIE was thought to consist of a one-step decrease in carbon isotope compositions (δ13C) and a subsequent exponential recovery in δ13C. In this project, two parallel shallow marine carbonate sections at Tingri (south Tibet) were studied. Data of carbon isotope compositions in bulk carbonate (δ13Ccarb) from two sections showed that ~10 m thick CIE records, with stepped δ13Ccarb decreases and large magnitude of ~7‰, was preserved at Tingri. The stepped CIE consists of three discrete intervals of decreasing δ13Ccarb values intervened by intervals of relatively stable δ13Ccarb, which is significantly different from most CIE records with a one-step decrease in δ13C. We demonstrate that the δ13Ccarb records at Tingri cannot be an artifact of diagenesis, mixed different components in bulk carbonate, or local processes (freshwater discharge, evaporation, or oxidation of organic matter) in shallow epicontinental seas. The capability of an inter-regional correlation of the stepped CIE from different depositional environments (lake, carbonate ramp, continental slope, and open ocean) further confirms the fidelity of the stepped CIE. Thus, we propose that the stepped CIE should represent a more complete PETM-CIE record, and infer that multiple carbon releases might have happened during the PETM. In addition, in-situ δ13C analyses on foraminiferal shells suggest that the full CIE magnitude in the surface ocean should reach to ~7‰, which is much larger than the generally accepted value of ~4‰. We measured carbon isotope compositions of total organic carbon (δ13CTOC), and used paired data of δ13Ccarb and δ13CTOC to explore possible carbon sources during the PETM. We suggest that volcanic CO2 emission prior to the CIE might have triggered the PETM, evidenced by an evident 13C-depletion of total organic carbon relative to bulk carbonate immediately below the CIE onset. Except for carbon released from volcanism, we tentatively infer that light carbon from methane hydrates had probably contributed to the initiation of the CIE. We investigated the response of the larger benthic foraminifera (LBF) to the PETM at Tingri, and found that there was no evident change in LBF assemblage at the CIE onset. Instead, a major compositional change in LBF, different from the so-called Larger Foraminiferal Turnover, occurred in the CIE recovery. We defined this LBF change as “larger foraminiferal extinction and origination”, and tentatively ascribed the larger foraminiferal extinction and origination event to a eutrophication event, resulting from intensified continental weathering during the CIE recovery of the PETM.
Publications
- (2016). Stratigraphic distribution of the larger benthic foraminifera Lockhartia in south Tibet (China). Journal of Foraminifera Research, 46(1): 34-47
Kahsnitz, M.M., Qinghai Zhang & Willems, H.
(See online at https://doi.org/10.2113/gsjfr.46.1.34) - (2017). Structure and magnitude of the carbon isotope excursion during the Paleocene-Eocene thermal maximum. Gondwana Research, 46: 114-123
Zhang Qinghai, Wendler, I., Xu Xiaoxia, Willems, H. & Ding Lin
(See online at https://doi.org/10.1016/j.gr.2017.02.016) - (2018). Paleocene and Lower Eocene shallow-water limestones of Tibet: Microfacies analysis and correlation of the eastern Neo-Tethyan Ocean. Palaeoworld, 27(2): 226-246
Kahsnitz, M.M., Willems, H., Luo Hui & Zhou Zhicheng
(See online at https://doi.org/10.1016/j.palwor.2017.12.002) - (2019). Genesis of Paleocene and Lower Eocene shallowwater nodular limestone of South Tibet (China). Carbonates and Evaporites, 34: 199-218
Kahsnitz, M.M. & Willems, H.
(See online at https://doi.org/10.1007/s13146-017-0360-7) - (2019). Response of larger benthic foraminifera to the Paleocene-Eocene thermal maximum and the position of the Paleocene/Eocene boundary in the Tethyan Shallow Benthic Zones: Evidence from south Tibet. GSA Bulletin, 131(1/2): 84-98
Zhang Qinghai, Willems, H., Ding Lin & Xu Xiaoxia
(See online at https://doi.org/10.1130/B31813.1) - (2020). Constraining the magnitude of the carbon isotope excursion during the Paleocene-Eocene thermal maximum using larger benthic foraminifera: Global and Planetary Change, 184, p.1-13
Zhang, Qinghai, Ding Lin, Kitajima, K., Valley, J.W., Zhang, Bo, Xu Xiaoxia, Willems, H. & Klügel, A.
(See online at https://doi.org/10.1016/j.gloplacha.2019.103049)
