title: Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation.
abstract: Cryogenian (∼720-635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth's history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)-indicative of an episode of intense chemical weathering-occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.
authors.affiliation: Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China.|Shaanxi Key Laboratory of Early Life and Environments, Department of Geology, Northwest University, Xi'an 710069, People's Republic of China.|State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, People's Republic of China.
authors.forename: Kang-Jun
authors.initials: KJ
authors.lastname: Huang
authors.affiliation: Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195.
authors.forename: Fang-Zhen
authors.initials: FZ
authors.lastname: Teng
authors.affiliation: Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China; bingshen@pku.edu.cn.
authors.forename: Bing
authors.initials: B
authors.lastname: Shen
authors.affiliation: Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
authors.forename: Shuhai
authors.initials: S
authors.lastname: Xiao
authors.affiliation: Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Xianguo
authors.initials: X
authors.lastname: Lang
authors.affiliation: Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Hao-Ran
authors.initials: HR
authors.lastname: Ma
authors.affiliation: College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, People's Republic of China.
authors.forename: Yong
authors.initials: Y
authors.lastname: Fu
authors.affiliation: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803.
authors.forename: Yongbo
authors.initials: Y
authors.lastname: Peng
keywords: South China; cap carbonate; chemical weathering; magnesium isotopes; snowball Earth