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

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 δ 13C excursion; whereas "Db" and "Dd" probably correspond to the two intervals with positive δ 13C values. • Four segments of the Cryogenian nonglaciatial epoch in South China are identified and the time of each segment is constrained. • Environmental magnetism analyses show a climate cooling ∼4 Myr before the Nantuo glaciation (or Marinoan snowball Earth). • Siliciclastic depositions in South China are linked to carbonate depositions in low-latitude continents. [ABSTRACT FROM AUTHOR]