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Rapid Recovery of Ediacaran Oceans in the Aftermath of the Marinoan

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Rapid Recovery of Ediacaran Oceans in the Aftermath of the Marinoan Clim. Past Discuss., https://doi.org/10.5194/cp-2018-119 Manuscript under review for journal Clim. Past Discussion started: 20 September 2018 c Author(s) 2018. CC BY 4.0 License. 1 Rapid recovery of Ediacaran oceans in the aftermath of the 2 Marinoan glaciation 3 4 Anthony Dosseto1,*, Holly L. Taylor1, Juraj Farkaš2,3, Grant M. Cox2, Andrew 5 Kingston4, Andrew Lorrey5, Alexander J. Corrick2 and Bing Shen6 6 7 1 Wollongong Isotope Geochronology Laboratory, School of Earth & Environmental 8 Sciences. University of Wollongong. Wollongong, NSW, Australia 9 2 Department of Earth Sciences. University of Adelaide. Adelaide, SA, Australia 10 3 Department of Environmental Geosciences, Czech University of Life Sciences, Prague, 11 Czech Republic 12 4 National Institute of Water and Atmospheric Research. Wellington, New Zealand 13 5 National Institute of Water and Atmospheric Research. Auckland, New Zealand 14 6 Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, School of 15 Earth and Space Sciences, Peking University, Beijing 100871, People’s Republic of China 16 17 18 19 20 21 22 23 24 25 * corresponding author: [email protected] 1 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-119 Manuscript under review for journal Clim. Past Discussion started: 20 September 2018 c Author(s) 2018. CC BY 4.0 License. 26 ABSTRACT 27 The termination of Cryogenian glaciations would have undoubtedly impacted the 28 chemistry of Neoproterozoic oceans, with possible consequences for life; but the extent and 29 duration of this impact are poorly constrained. In this study, we use the lithium (Li) isotope 30 composition of Ediacaran cap dolostones from South Australia (Nuccaleena Formation) and 31 China (Doushantuo Fm) to investigate changes in ocean chemistry that followed the Marinoan 32 deglaciation. The effect of diagenesis was evaluated and while the Nuccaleena Fm is likely to 33 have preserved the primary composition of cap dolostone deposition, the offset in Li isotope 34 ratios observed for the Doushantuo Fm could possibly reflect partial overprinting by diagenetic 35 fluids. The Li isotope composition of Ediacaran seawater was estimated and we suggest it was 36 similar to that of late Cenozoic oceans for most of the cap dolostone deposition. Using a box 37 model for the oceanic Li cycle, we show that at the onset of deglaciation, the supply of riverine 38 Li to the oceans was up to 50 times the modern flux. The modelled riverine Li isotope 39 composition suggests that continents resembled modern high-latitude regions during this time. 40 This episode was short-lived (up to 1 Myr) and the subsequent supply of riverine Li was similar 41 to modern conditions, both in flux and isotope composition, for the whole duration of cap 42 dolostone deposition. These results suggest that Ediacaran oceans and continents rapidly 43 recovered from the Marinoan glaciation to reach environmental conditions similar to the late 44 Cenozoic. From the standpoint of the Li oceanic budget, the Ediacaran oceans in which 45 complex lifeforms emerged may have not been that different from our modern oceans. 46 2 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-119 Manuscript under review for journal Clim. Past Discussion started: 20 September 2018 c Author(s) 2018. CC BY 4.0 License. 47 1. Introduction 48 The Neoproterozoic era (1,000-542 Myr ago) is characterised by major environmental 49 changes including global glaciations (Hoffman et al., 2017), a second major increase in 50 atmospheric oxygen (Och and Shields-Zhou, 2012;Sahoo et al., 2016;Scott et al., 2008) and 51 the radiation of complex lifeforms (Knoll and Carroll, 1999). Biomarkers suggest that the first 52 metazoans emerged sometime in between the Sturtian and Marinoan glaciations (Love et al., 53 2009), with algae replacing phototropic bacteria as the top marine primary producer between 54 659 and 650 Ma (Brocks et al., 2017). This shift has been suggested as resulting from a massive 55 supply of nutrients to the oceans at the end of the Sturtian glaciation (Brocks et al., 2017). The 56 end of the Marinoan glaciation could have triggered another major leap in evolution, since it is 57 followed by the appearance of centimetre-scale macroalgae and a significant increase in 58 metazoan complexity and diversity (Yuan et al., 2011;Yin et al., 2007;Yin et al., 2015). 59 The termination of the Marinoan glaciation is capped by the occurrence of dolostones, 60 termed “cap carbonates” (Rose and Maloof, 2010;Hoffman et al., 2017). These unique 61 formations are useful archives of changes in ocean chemistry that took place following the 62 Marinoan glaciation. For instance, their boron isotopic composition suggests ocean 63 acidification (Ohnemueller et al., 2014;Kasemann et al., 2005) while triple oxygen isotopes 64 show that atmospheric carbon dioxide levels were at their highest levels in the past 750 million 65 years (Bao et al., 2008). Calcium (Ca) isotopes show that the Ca supply to the oceans would 66 have been 14 to 140 times greater than the modern flux, and this was interpreted as a large 67 pulse in continental weathering (Kasemann et al., 2005). Zinc and cadmium isotopes hint at a 68 rapid resumption of primary productivity (Kunzmann et al., 2013;John et al., 2017). Huang et 69 al. (2016) have also used magnesium isotopes (δ26Mg) to suggest that a rapid pulse of 70 continental weathering took place prior to the end of the Marinoan glaciation, and subsided 71 following cap carbonate deposition. While their results provide some important insights on 3 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-119 Manuscript under review for journal Clim. Past Discussion started: 20 September 2018 c Author(s) 2018. CC BY 4.0 License. 72 environmental changes associated with large Neoproterozoic glaciations, because they focus 73 on siliclastic sediments the changes observed could reflect changes in the provenance of 74 sediments mobilised by erosion. For instance, the pulse in weathering inferred from high δ26Mg 75 values at the end of the Marinoan glaciation could reflect preferential erosion of sediments 76 from continental regions characterised by higher weathering intensity instead of a globally 77 more intense continental weathering. Furthermore, while Ca isotopes suggest a large element 78 flux from the continents, there is the need for testing this hypothesis with an unequivocal proxy 79 for silicate weathering. In order to fill this gap, we use the lithium (Li) isotopic composition of 80 Ediacaran cap carbonates to investigate possible changes in riverine inputs to the ocean that 81 reflect continental weathering. 82 Lithium isotopes (7Li and 6Li) fractionate during the formation of secondary minerals, 83 with 6Li being preferentially incorporated into the solid, and thus have been used as a proxy 84 for silicate weathering (Burton and Vigier, 2011;Wimpenny et al., 2010;Pistiner and 85 Henderson, 2003;Verney-Carron et al., 2011;Decarreau et al., 2012). For instance, the Li 86 isotopic composition (noted δ7Li) of river waters records the extent of secondary mineral 87 formation at the catchment scale, where low δ7Li values are diagnostic of intense weathering 88 (Millot et al., 2010;Vigier et al., 2009). The Li isotopic composition of marine carbonates can 89 then be used to investigate how the riverine Li supply to the oceans has varied over time, 90 reflecting changes in continental weathering (Vigier and Goddéris, 2015;Li and West, 91 2014;Misra and Froelich, 2012;Wanner et al., 2014;Pogge von Strandmann et al., 2013;Lechler 92 et al., 2015;Pogge von Strandmann et al., 2017a). For instance, the increase in δ7Li values in 93 foraminifera throughout the Cenozoic has been interpreted as a consequence of the Himalayan 94 and Andean orogeneses, due to either a shift in continental weathering regime from transport- 95 limited, deeply weathered lowlands, to weathering-limited, poorly weathered steep terranes 96 (Misra and Froelich, 2012;Wanner et al., 2014;Vigier and Goddéris, 2015); or an increase in 4 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-119 Manuscript under review for journal Clim. Past Discussion started: 20 September 2018 c Author(s) 2018. CC BY 4.0 License. 97 submarine “reverse” weathering of authigenic clays (Li and West, 2014). For the former, it has 98 been proposed that the shift in continental weathering is represented either by a transition from 99 congruent to incongruent dissolution (Misra and Froelich, 2012), a decrease in weathering 100 intensity and increase in river suspended load (Wanner et al., 2014) or a decrease in Li storage 101 in secondary phases (Vigier and Goddéris, 2015). The δ7Li composition of calcite in chalk was 102 also used to evidence a large weathering pulse in the wake of the Ocean Anoxic Event 2 103 (OAE2), which could have contributed to a rapid recovery from the greenhouse state associated 104 with the OAE2 (Pogge von Strandmann et al., 2013). The δ7Li composition of Ordovician bulk 105 carbonates, brachiopods and shales showed that silicate weathering intensity was reduced 106 during the Hirnantian glaciation, resulting in inhibited CO2 drawdown which could have 107 facilitated deglaciation (Pogge von Strandmann et al., 2017a). 108 In this study, Li isotope compositions were measured on two Marinoan cap carbonate 109 formations from Australia and China: the Nuccaleena Formation (Fm) and Member I of the 110 Doushantuo Fm, respectively. In combination with the recent determination of Li isotope 111 fractionation during Ca-Mg carbonate precipitation (Taylor et al., 2018), results are used to 112 model changes in the Li budget of Ediacaran oceans. 113 114 2. Study area 115 The Doushantuo Fm is part of the Yantgze block in China (Figure 1a), and was 116 deposited in a passive-margin setting (Jiang et al., 2011).
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