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Episode of Intense Chemical Weathering During the Termination of the 635 Ma Marinoan Glaciation

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Episode of Intense Chemical Weathering During the Termination of the 635 Ma Marinoan Glaciation Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation Kang-Jun Huanga,b,c, Fang-Zhen Tengd, Bing Shena,1, Shuhai Xiaoe, Xianguo Langa, Hao-Ran Maa, Yong Fuf, and Yongbo Pengg aMinistry 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; bShaanxi Key Laboratory of Early Life and Environments, Department of Geology, Northwest University, Xi’an 710069, People’s Republic of China; cState Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, People’s Republic of China; dIsotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195; eDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; fCollege of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, People’s Republic of China; and gDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved November 3, 2016 (received for review May 14, 2016) Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) relationship between intense chemical weathering and cap carbonate represent the most extreme ice ages in Earth’s history. The termi- precipitation remains equivocal. In this study, we use Mg isotopes as nation of these snowball Earth glaciations is marked by the global a geochemical proxy to constrain the intensity of chemical weath- precipitation of cap carbonates, which are interpreted to have ering during the deposition of the terminal Cryogenian Nantuo been driven by intense chemical weathering on continents. How- Formation and the overlying cap carbonate of the basal Ediacaran ever, direct geochemical evidence for the intense chemical weath- Doushantuo Formation in the Yangtze Platform of South China. ering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Mar- Tracer for the Intensity of Chemical Weathering inoan-age Nantuo Formation and the overlying cap carbonate of Traditional proxies for the intensity of chemical weathering in- the basal Doushantuo Formation in South China. A positive excur- clude chemical index of alteration (CIA) and strontium isotopes sion of extremely high δ26Mg values (+0.56 to +0.95)—indicative (87Sr/86Sr). However, CIA is sensitive to the compositions of source of an episode of intense chemical weathering—occurs in the top rocks (13), and Sr isotopes are susceptible to diagenetic alterations Nantuo Formation, whereas the siliciclastic component of the over- (14). Magnesium (Mg) isotopes have several advantages to sur- EARTH, ATMOSPHERIC, 26 AND PLANETARY SCIENCES lying Doushantuo cap carbonate has significantly lower δ Mg mount these obstacles. First, owing to the limited fractionation values (<+0.40), suggesting moderate to low intensity of chemical during magmatic processes (15), the primary components of the weathering during cap carbonate deposition. These observations upper continental crust (UCC, e.g., granitoids and basalts) display a suggest that cap carbonate deposition postdates the climax of narrow range of Mg isotopic compositions [δ26Mg = –0.3 to –0.1, chemical weathering, probably because of the suppression of car- Fig. 1A; δ26Mg is the per mil deviation of 26Mg/24Mg with respect to bonate precipitation in an acidified ocean when atmospheric CO2 the standard Dead Sea Magnesium (DSM3) (16)], minimizing the concentration was high. Cap carbonate deposition did not occur source effect encountered in CIA study. In addition, unlike Sr iso- until chemical weathering had consumed substantial amounts of topes, the Mg isotopic system in dolomite and siliciclastic rocks is atmospheric CO2 and accumulated high levels of oceanic alkalinity. immune to secondary alteration related to low-grade metamorphism Our finding confirms intense chemical weathering at the onset of and diagenesis (17, 18). More importantly, Mg isotopes significantly deglaciation but indicates that the maximum weathering predated fractionate (up to 2‰) during chemical weathering, with the pref- cap carbonate deposition. erential retention of 26Mg in weathering residues (Fig. 1A)(19–21), and, as such, δ26Mg values of weathering residues become progres- snowball Earth | magnesium isotopes | cap carbonate | sively higher with increasing intensity of chemical weathering (22). As chemical weathering | South China Significance he worldwide distribution of Cryogenian glacial deposits (∼720–635 Ma) implies the occurrence of global glaciations or T At least two extreme glaciations (dubbed as snowball Earth snowball Earth events, with ice sheet extending to paleo-equatorial events), each lasting millions of years and affecting the entire regions (1, 2). The termination of the end-Cryogenian Marinoan globe, occurred during the Cryogenian Period (∼720–635 Ma). glaciation was followed by ocean oxygenation and diversification of These glaciations are recorded by globally distributed glacial complex eukaryotes (3–5), suggesting the possible linkage between deposits overlain by cap carbonates. According to the snowball glaciation and evolution (6). As a marker for the end of the Mar- Earth hypothesis, cap carbonate deposition was driven by in- inoan glaciation, globally distributed cap carbonates immediately tense continental weathering during deglaciation, but geo- above the glacial deposits indicate a sharp transition from an ice- chemical evidence is lacking. Using Mg isotopes as a proxy, we house to a greenhouse climatic condition (7–9). The “snowball reconstruct the history of chemical weathering during and fol- Earth” hypothesis posits that deglaciation was abrupt and was lowing the terminal Cryogenian Marinoan glaciation in South triggered by extremely high atmospheric CO2 concentration ( pCO2) ∼ × –3.5 China. Our data confirm an episode of strong chemical weath- [ 350 present atmospheric level (PAL), about 10 bar] ac- ering during the termination of the 635-Ma Marinoan glaciation, cumulated during global freezing for tens of million years (8, 10). but its climax occurred before cap carbonate deposition. High atmospheric pCO2 levels are expected to cause intense chemical weathering on continents, which, in turn, elevates seawater alka- Author contributions: K.-J.H. and B.S. designed research; K.-J.H., F.-Z.T., B.S., S.X., X.L., linity and results in the rapid precipitation of cap carbonates all over H.-R.M., Y.F., and Y.P. performed research; K.-J.H., F.-Z.T., H.-R.M., and Y.P. analyzed the world (10, 11). This interpretation also explains the consistently data; K.-J.H., B.S., and S.X. wrote the paper; and X.L. and Y.F. conducted field work. negative carbon isotopic signatures of cap carbonates (10). Thus, The authors declare no conflict of interest. cap carbonate precipitation coupled with intense chemical weath- This article is a PNAS Direct Submission. ering is one of the key predictions of the snowball Earth hypothesis. 1To whom correspondence should be addressed. Email: [email protected]. However, direct geochemical evidence for intense chemical weath- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ering during deglaciation is not available (12), and the temporal 1073/pnas.1607712113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1607712113 PNAS Early Edition | 1of6 Downloaded by guest on September 26, 2021 A to the terminal Cryogenian or Marinoanglaciation(23).TheNantuo Peridotite & basalt (n=255) Formation is dated between 654 ± 3.8 Ma and 635.2 ± 0.6 Ma (24, 25), and is overlain by the 3- to 5-m-thick cap carbonate of the basal SW Granite & granitoid (n=78) Doushantuo Formation (25, 26). Although cap carbonate atop Marinoan glacial deposits may be time-transgressive, the magni- tude of this diachroneity is likely <103–104 y (9). In South China, Metamorphic rock (n=100) the contact between the Nantuo glacial deposits and the Doush- antuo cap carbonate is continuous, without a sedimentary break, Siliciclastic sedimentary rock (n=84) particularly in the sampled sections that were deposited in deep basinal environments beyond the reach of wave activities (26). Weathered residue (n=180) The upper part of the Nantuo Formation and the overlying Doushantuo cap carbonate were sampled from two localities: a Riverwater (n=159) drill core (14ZK) from the Songtao county and an outcrop section (Bahuang) in the Tongren city, both in northeastern Guizhou UCC Province, South China. These two sections are paleogeographi- Carbonate (n=343) cally located in the deep-water basinal facies in Neoproterozoic (23) (Fig. S1). The Nantuo Formation recovered from the 14ZK core is 225 m thick and consists of alternating siltstone, sandstone, -5.6 -4.8 -4.0 -3.2 -2.4 -1.6 -0.8 0.0 0.8 1.6 pebbly sandstone, and diamictite (Fig. 2). The focus of this study is 26 on the upper Nantuo Formation, which can be divided into six Mg lithological units (Fig. 2). Unit I is composed of 40-m-thick mas- B 2.0 sive diamictite (only the upper 20 m was sampled), followed by 2.0 Iceland (49) 15-m-thick pebbly sandstone/siltstone alternations (unit II). The South China (21) rest of the Nantuo Formation can be subdivided into, in strati- 1.5 1.0 South Carolina (22) graphic order, 28-m-thick sandstone and siltstone of unit III, 8-m-thick pebbly sandstone intercalated with siltstone (unit IV), 0.0 Columbia River (19) 10-m-thick siltstone (unit V), and 3-m-thick diamictite associated 1.0 Fresh Basalt (21-22) -1.0 with siltstone (unit VI). In the Bahuang section, only siltstone of 98 99 100 the topmost 1 m of the Nantuo Formation was sampled, and the sampled interval can be correlated with unit VI in the 14ZK core Mg 0.5 26 (Fig. S2). The Nantuo diamictites and pebbly sandstone/siltstone are generally considered as deposition in a glacial marine envi- 0.0 ronment, representing proximal glaciomarine and distal glacio- marine facies, respectively, whereas the pebble-free sandstone and siltstone may represent normal marine deposition without the -0.5 influence of glaciation (27, 28).
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