Quantifying temperature variation between Neoproterozoic cryochron – nonglacial interlude, Nanhua Basin, South China
Introduction
In the Neoproterozoic, the Earth experienced planetary wide, extreme climate change, swinging from warm to freezing conditions several times. The term “cryochrons” was proposed for the panglacial epochs, with onsets and terminations sharply defined in time (Hoffman et al., 2017). The snowball Earth hypothesis has emerged to explain these intense cryochrons, which drove paleosurface temperatures in the tropics to below −20 ℃ and the subsequent reversion to more temperate climes, with global climate models (Hoffman and Schrag, 2002; Kirschvink, 1992). However, precise values for the temperature differential between Neoproterozoic cryochrons and nonglacial interludes remains unknown. The presence of multiple glacial successions (Lan et al., 2014, Lan et al., 2015, Lang et al., 2018), petrographic evidence (Wang et al., 2017), and geochemical data (Feng et al., 2003, Huang et al., 2014, Xiao et al., 2014) from Neoproterozoic successions in the Nanhua Basin, South China, indicates the presence of three cryochrons. Published radiometric dates constrain their ages and enable their correlation with the worldwide Sturtian (717–660 Ma), Marinoan (650–635 Ma) and Gaskiers (ca. 579 Ma) cryochrons (Bao et al., 2018, Prave et al., 2016, Pu et al., 2016, Condon et al., 2005, Rooney et al., 2015).
Various chemical weathering indices have been developed to estimate the intensity of rock chemical weathering based on the geochemical compositions of sedimentary rocks, with variations related to changing climatic conditions (Fedo et al., 1995, Harnois, 1988, Nesbitt and Young, 1982, Parker, 1970, Rasmussen et al., 2011). This study complies geochemical data from six separate areas of the Nanhua Basin, to measure the late Neoproterozoic chemical weathering trends in South China. Compiled detrital zircon ages and geochemical analyses were combined to evaluate the potential effects from provenance, post-depositional alteration, sedimentary recycling and sorting, on the weathering indices. The corrected chemical index of alteration profiles in South China provide a reliable proxy for fluctuations in the intensity of chemical weathering and associated changing climate conditions during Neoproterozoic cryochrons and nonglacial interludes, and enable correlation with contemporaneous CIAcorr trends in Oman. Furthermore, we constrain the mean annual temperature (MAT) difference between cryochron and nonglacial interlude during the Neoproterozoic in South China and Oman based on a proxy for climate conditions using the relationship between τNa (sodium chemical depletion index) and MAT (Yang et al., 2016a, Yang et al., 2016b).
Section snippets
Geological setting and sampling sequence
Paleomagnetic data in combination with the presence of carbonate rocks, manganese, and/or iron-rich deposits resting directly on glacial successions indicate that the South China Craton occupied a mid-latitude northern hemisphere location from ca. 720–635 Ma (~30° N, Yu et al., 2016a, Zhang et al., 2013), subsequently migrating to an equatorial position from ca. 635–541 Ma, consistent with its inferred position in Gondwana (Jing et al., 2015, Macouin et al., 2004, Merdith et al., 2017). The
Methods
Only fine-grained samples were chosen for paleoclimate discussion in this study, avoiding medium to coarse-grained siliciclastic sedimentary rocks (cf., Yang et al., 2014). Whole-rock major and trace elements were analyzed by inductively coupled plasma-mass spectrometry and mineralogical compositions were determined by X-ray diffraction (XRD). Analytical data are provided in Supplementary Tables 1 and 2.
Fresh rock samples were cleaned with deionized water, and subsequently crushed and powdered
Results
XRD results show that all samples from the drill core have high quartz, feldspar and clay mineral contents, with minor pyrite. Total clay mineral content increases from 26 to 37 % (average 31.5%, n = 2) in the Tiesi’ao Formation, 30–38% (average 32.46%, n = 15) in the lower Datangpo Formation, to 33–51% (average 40.73%, n = 9) in the upper Datangpo Formation. Clay minerals are illite from the Tiesi’ao Formation, illite and chlorite are common in most samples from the Datangpo Formation, with a
Provenance interpretation and evaluating sedimentary recycling and sorting
In order to quantify the Neoproterozoic climate change of the South China Craton from calculated weathering indices, it is necessary to access the effects of source area, transportation and post-depositional diagenetic conditions.
Based on the following evidence, the variation of weathering indices is unaffected by source composition: A) Consistent sources for the sedimentary successions within the Nanhua Basin were maintained throughout the Cryogenian to Ediacaran (Fig. 5). B) Zr/Ti ratio,
Conclusion
Covariant change of chemical weathering trends in South China and Oman reflects several contemporaneous climate cooling events. Based on the correlation of τNa and MAT, the mean annual temperature differences reached ~20 ± 5.4 ℃ between the Marinoan cryochron and preceding/succeeding nonglacial interlude at around 30°latitude, and ~12 ± 5.4 ℃ between Gaskiers cryochron and preceding nonglacial interlude close to the equator. Concomitant fluctuation of CIAcorr values and MAT records during the
CRediT authorship contribution statement
Liang Qi: Writing - original draft, Investigation, Data curation. Peter A. Cawood: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Jianghai Yang: Writing - review & editing. Yajun Xu: Writing - review & editing, Supervision, Funding acquisition. Yuansheng Du: Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 41772106, 41472086 and U1812402), the Guizhou Science Innovation Team Project No. 2018-5618 and by the Australian Research Council (Grant FL160100168). The manuscript benefitted from the comments from the editor and anonymous reviewers.
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