Subglacial meltwater supported aerobic marine habitats during Snowball Earth

Abstract

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.

Keywords: Fe isotopes; Snowball Earth; glaciation; iron formation; oxygenation.

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) Representative stratigraphic columns of the IF-bearing Sturtian glacial intervals of the Chuos Formation, Yudnamutana Subgroup, and Kingston Peak Formation (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. . Copyright (2017), with permission from Elsevier.

Fig. 2.

Box plots of the Sturtian IF redox proxy data. (A) Fe/Al ratios, (B) Mn/Al ratios, (C) Ce anomalies (Ce_n/Ce_n*), and (D) Fe isotope ratios (δ⁵⁶Fe) from the ice-contact, ice-proximal, and ice-distal facies. Note that multiple facies are present in each study area (SI Appendix). (E) Box plots showing the range of Fe isotopic compositions of the Sturtian IFs (this study; n = 81) relative to marine sediments (IFs, shales, carbonates, hydrothermal deposits, Fe sulfides, and Fe oxides; literature data updated from ref. ; SI Appendix) from 3 time bins: prior to and during the Great Oxidation Event (>2,200 Ma; n = 1118), between the Great Oxidation Event and the Phanerozoic (2,200 to 541 Ma; n = 321), and the Phanerozoic Eon (<541 Ma; n = 252). Boxes represent 50% of the data (the interquartile range); the mean is represented by a black circle and the median by a black line. Outliers (outside the interquartile range) are represented by a gray circle; extreme outliers (greater than double the interquartile range from the median; n = 6) are not shown. Whiskers represent extreme values that are not outliers.

Fig. 3.

Schematic model for the outwash of oxygenated basal meltwater to anoxic glaciomarine environments (not to scale). IFs deposited in ice-contact settings (within ∼2 km of the grounding line) are characterized by negative δ⁵⁶Fe values and may feature negative Ce anomalies and Mn-oxide enrichment, indicative of oxidizing conditions. Ice-proximal IFs (∼2 to 10 km from the grounding line) display predominantly positive δ⁵⁶Fe values and negligible to slight positive Ce anomalies. Ice-distal IFs (greater than ∼10 km from the grounding line) feature slight positive Ce anomalies and highly positive δ⁵⁶Fe values, with negligible Mn oxides. These geochemical trends are indicative of oxygenated glaciomarine environments which could have supported aerobic eukaryotes despite widespread anoxia during extreme glaciation. Microaerophilic Fe-oxidizing bacteria may have proliferated where O₂-bearing meltwater mixed with ferruginous seawater, potentially supporting heterotrophic food webs.