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Subglacial Meltwater Supported Aerobic Marine Habitats During Snowball Earth

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Subglacial Meltwater Supported Aerobic Marine Habitats During Snowball Earth Subglacial meltwater supported aerobic marine habitats during Snowball Earth Maxwell A. Lechtea,b,1, Malcolm W. Wallacea, Ashleigh van Smeerdijk Hooda, Weiqiang Lic, Ganqing Jiangd, Galen P. Halversonb, Dan Asaele, Stephanie L. McColla, and Noah J. Planavskye aSchool of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia; bDepartment of Earth and Planetary Science, McGill University, Montréal, QC, Canada H3A 0E8; cState Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, 210093 Nanjing, China; dDepartment of Geoscience, University of Nevada, Las Vegas, NV 89154; and eDepartment of Geology and Geophysics, Yale University, New Haven, CT 06511 Edited by Paul F. Hoffman, University of Victoria, Victoria, BC, Canada, and approved November 3, 2019 (received for review May 28, 2019) The Earth’s most severe ice ages interrupted a crucial interval in Cryogenian ice age. These marine chemical sediments are unique eukaryotic evolution with widespread ice coverage during the geochemical archives of synglacial ocean chemistry. To develop Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have sur- a global picture of seawater redox state during extreme glaci- vived the “Snowball Earth” glaciations, requiring the persistence of ation, we studied 9 IF-bearing Sturtian glacial successions across 3 oxygenated marine habitats, yet evidence for these environments paleocontinents (Fig. 1): Congo (Chuos Formation, Namibia), is lacking. We examine iron formations within globally distributed Australia (Yudnamutana Subgroup), and Laurentia (Kingston Cryogenian glacial successions to reconstruct the redox state of the Peak Formation, United States). These IFs were selected for synglacial oceans. Iron isotope ratios and cerium anomalies from a analysis because they are well-preserved, and their depositional range of glaciomarine environments reveal pervasive anoxia in the environment can be reliably constrained. ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial melt- Cryogenian Glacial Environments water supplied oxygen to the synglacial oceans, creating glaciomarine All 9 of the studied IF-bearing successions show sedimentolog- oxygen oases. The confluence of oxygen-rich meltwater and iron-rich ical evidence for deposition in glaciomarine environments, in- seawater may have provided sufficient energy to sustain chemosyn- cluding a stratigraphic association with ice-rafted debris-bearing thetic communities. These processes could have supplied the requisite marine strata and diamictites containing glacially striated clasts. oxygen and organic carbon source for the survival of early animals and Sedimentation in glaciomarine environments is controlled by dis- other eukaryotic heterotrophs through these extreme glaciations. tance from the grounding line where the ice shelf begins to float (23–25), and glaciomarine deposits can be broadly categorized oxygenation | glaciation | Snowball Earth | iron formation | Fe isotopes into ice-contact, ice-proximal, and ice-distal facies (e.g., refs. 23 and 26). The ice-contact environment is a narrow setting within he Cryogenian ice ages, known as the Sturtian (717 to 660 Ma) ∼2 km of the grounding line where coarse-grained, massive Tand Marinoan (ca. 640 to 635 Ma) glacial episodes (1), fea- diamictites are abundant due to basal ice melting and subice tured low-latitude ice sheets at sea level (2–6) and widespread shelf rain-out processes (23, 26). The ice-distal environment (greater marine ice cover that drastically reduced the habitable ecospace than ∼10 km from the grounding line) is predominantly characterized for eukaryotes for millions of years. The oceans prior to these by fine-grained, laminated sediments deposited from the settling “Snowball Earth” events were characterized by oxygenated sur- face waters overlying a low-O2 water column (7), and global ice Significance coverage would have exacerbated oceanic anoxia. This has been suggested to have led to the mass mortality of marine aerobes (4, The “Snowball Earth” ice ages featured global ice sheets for 5), possibly restricting aerobic eukaryotes to supraglacial envi- millions of years, leading to oceanic anoxia and threatening the ronments such as low-latitude areas with partial melt on glacial perseverance of aerobic life on Earth. Crucially, aerobic eukary- surfaces or cracks in ice shelves (3, 8–10). Yet, the Cryogenian otes survived, possibly including early animals. Iron-rich chemical fossil record (although sparse) does not appear to have a signifi- sediments preserved in glacial deposits offer insights into the cantly lower species richness than the preglacial interval (11), and synglacial marine redox state. We provide evidence for oxy- multiple eukaryotic clades evidently endured through the ice ages genated waters near ice sheet grounding lines, where oxic sub- (12, 13). Molecular clock and biomarker evidence may also sup- glacial meltwater would be supplied to the anoxic oceans. The port the persistence and diversification of key eukaryotic lineages, energy released from this mixing of oxic and anoxic waters could including metazoans, across the Cryogenian ice ages (14–17). The have fueled microbial communities and sustained localized aero- survival of aerobic eukaryotes through the Snowball Earth glaciations bic ecosystems. This meltwater oxygen pump may therefore have seemingly requires continually oxygenated refugia with an available been critical in the survival and evolution of aerobic eukaryotes organic carbon supply (12). during episodes of extreme glaciation. Geochemical and sedimentological investigation of Cryogenian synglacial sedimentary successions is essential for testing hypoth- Author contributions: M.A.L. and M.W.W. designed research; M.A.L., M.W.W., A.v.S.H., W.L., D.A., S.L.M., and N.J.P. performed research; M.A.L., M.W.W., A.v.S.H., W.L., D.A., and eses regarding the potential habitats for aerobic eukaryotes during N.J.P. contributed analytic tools; M.A.L., M.W.W., A.v.S.H., W.L., G.J., G.P.H., and N.J.P. Snowball Earth. Although there is evidence for an oxic atmo- analyzed data; and M.A.L., M.W.W., and N.J.P. wrote the paper with contributions from sphere (18, 19), the few geochemical studies of Cryogenian syn- all authors. glacial sediments are indicative of an anoxic water column during The authors declare no competing interest. glaciation (7, 18, 20, 21). Sedimentological evidence has been used This article is a PNAS Direct Submission. to argue for ice-free conditions during Cryogenian glaciation (e.g., ref. Published under the PNAS license. 22), but this evidence continues to be contentious (3) and the nature 1To whom correspondence may be addressed. Email: [email protected]. and location of potential oxic marine environments remain elusive. This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Iron formations (IFs) are found in many successions deposited doi:10.1073/pnas.1909165116/-/DCSupplemental. globally during the Sturtian glaciation (20), the first and longest First published December 2, 2019. 25478–25483 | PNAS | December 17, 2019 | vol. 116 | no. 51 www.pnas.org/cgi/doi/10.1073/pnas.1909165116 Downloaded by guest on September 25, 2021 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Fig. 1. Geological setting and sedimentology of the studied Sturtian IFs. (A) Geological maps of the study areas of the Chuos Formation, Kunene, Namibia (1, Okavare; 2, Landeck; 3, Lowenfontein; 4, Orusewa); the Yudnamutana Subgroup, Adelaide Fold Belt, South Australia (5, Oraparinna; 6, Willippa; 7, Holowilena South); and the Kingston Peak Formation, southern Death Valley, California (8, Sperry Wash; 9; Kingston Range). (B)Repre- sentative stratigraphic columns of the IF-bearing Sturtian glacial intervals of the Chuos Formation, Yudnamutana Subgroup, and Kingston Peak For- mation (see SI Appendix,Figs.S1–S8 for stratigraphic columns from all study areas). (C) Interbedded diamictite and IF of the Chuos Formation, interpreted as evidence of IF deposition in an ice-contact glaciomarine environment. (D) Laminated IF clast within ferruginous diamictite, Yudnamutana Subgroup. (E) Laminated IF interbedded with siltstone and sandstone turbidites, interpreted to represent an ice-distal glaciomarine environment, Kingston Peak Formation. (F) Simplified global continental reconstruction at ca. 680 Ma highlighting the paleogeographic distribution of Sturtian IFs (red squares). The dark gray areas correspond to cratonic regions and the light gray areas represent the inferred distribution of the fragments of the supercontinent Rodinia; inferred plate boundaries and subduction zones are represented by black and red lines, respectively. Reprinted from ref. 67. Copyright (2017), with permission from Elsevier. of suspended sediment from meltwater plumes and turbidite The Sturtian glacial successions of the Chuos Formation (northern deposits (23, 26). The ice-proximal environment is transitional Namibia) are dominated by massive diamictites containing abundant between the ice-contact and ice-distal facies. This setting features faceted and striated clasts and in places show evidence for subglacial diamictites at a lower relative abundance than the ice-contact deformation diagnostic of the ice-contact glaciomarine environ- environment; these diamictites lack evidence for glaciotectonism ment (27, 28). The Chuos IFs are typically interbedded
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