Climate changes in the Cryogenian nonglacial epoch: a global synthesis with new findings from the Datangpo Formation in South China

Bao, Xiujuan, Zhang, Shihong, Jiang, Ganqing, Chan, Duo, Hu, Yongyun, Wu, Huaichun, Li, Haiyan, Wang, Xinqiang and Yang, Tianshui (2023) Climate changes in the Cryogenian nonglacial epoch: a global synthesis with new findings from the Datangpo Formation in South China. Global and Planetary Change, 229, [104234]. (doi:10.1016/j.gloplacha.2023.104234).

Abstract

The Cryogenian nonglacial epoch (CNE) spans from the termination of the Sturtian snowball Earth (ca. 659 Ma) to the initiation of the Marinoan snowball Earth (ca. 649.2 Ma). The carbonate sequences formed in low latitudes record paleoclimatic and paleoceanographic changes during the CNE. Three negative carbon isotope (δ¹³C) excursions, named Rasthof, Tayshir and Trezona, are present at the bottom, middle, and top of the carbonate sequences, bracketing two intervals with highly positive δ¹³C values. Uranium isotope (δ²³⁸U) data from Mongolia reveal ocean redox changes linked to δ¹³C variations during the CNE, marked by the development of anoxic deep ocean to extensive oxygenation and subsequent deoxygenation. However, the timing and duration of these paleoceanographic events and δ¹³C excursions have not been adequately investigated. Here, we report new environmental magnetic results of the higher paleo-latitudinal, siliciclastic-dominated Datangpo Formation (Fm) in South China that may help constrain the age and correlation of the CNE geochemical events. We identified four segments of the Datangpo Fm based on the magnetic susceptibility (MS) profile and other environmental magnetic parameters, which are, in ascending order, the ascent segment (Da), lower stable-segment (Db), transition segment (Dc), and upper stable-segment (Dd). These segments are dated by the astronomical time scale as 659–655.6 Ma (Da), 655.6–653.3 Ma (Db), 653.3–652.3 Ma (Dc), and 652.3–649.2 Ma (Dd). Environmental magnetism analyses show that the highest magnetite concentration but lowest fine-grained magnetite contents are present in “Da”, followed by “Dd” and “Db”. These results suggest a sea-level fall and a potential decrease in oxygenation after the deposition of the transition segment (Dc), implying that a cooling climate started ∼4 Myr before the Nantuo glaciation (Marinoan snowball Earth). Ocean redox variations revealed by environmental magnetism in the Datangpo Fm are consistent with those reported from the low-latitudinal carbonate sequences. “Da” likely corresponds to the Rasthof δ¹³C excursion and “Dc” corresponds to the Tayshir δ¹³C excursion; whereas “Db” and “Dd” probably correspond to the two intervals with positive δ¹³C values.

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Contributors

Author: Duo Chan

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