1
Title: Two-billion-year-old evaporites capture Earth’s great oxidation
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Authors: C.L. Blättler1*, M.W. Claire2,3,4, A.R. Prave2, K. Kirsimäe5, J.A. Higgins1, P.V.
2
Medvedev6, A.E. Romashkin6, D.V. Rychanchik6, A.L. Zerkle2,3, K. Paiste7, T. Kreitsmann5, I.L.
3
Millar8, J.A. Hayles9, H. Bao10, A.V. Turchyn11, M.R. Warke2, A. Lepland12,7,5,13
4
Affiliations:
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1Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
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2School of Earth and Environmental Sciences, University of St Andrews, St Andrews KY16
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9AL, Scotland/UK.
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3Centre for Exoplanet Science, University of St Andrews, St Andrews KY16 9AL, Scotland/UK.
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4Blue Marble Space Institute of Science, 1001 4th Ave., Suite 3201 Seattle, WA 98154, USA.
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5University of Tartu, Department of Geology, 50411 Tartu, Estonia.
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6Institute of Geology, Karelian Science Centre, Pushkinskaya 11, 185610 Petrozavodsk, Russia.
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7Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geosciences, UiT The
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Arctic University of Norway, 9037 Tromsø, Norway.
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8NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham
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NG12 5GG, UK.
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9Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005, USA.
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10Department of Geology and Geophysics, E235 Howe-Russell Geoscience Complex, Louisiana
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State University, Baton Rouge, Louisiana 70803, USA.
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11Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2
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3EQ, UK.
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12Geological Survey of Norway, 7491 Trondheim, Norway.
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13Tallinn University of Technology, Institute of Geology, 19086 Tallinn, Estonia.
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*Correspondence to: blattler@princeton.edu.
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This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science , (), doi:
10.1126/science.aar2687"
2 Abstract:
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Major changes in atmospheric and ocean chemistry occurred in the Paleoproterozoic Era
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(2.5–1.6 billion years ago). Increasing oxidation dramatically changed Earth’s surface, but few
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quantitative constraints exist on this important transition. This study describes the
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sedimentology, mineralogy, and geochemistry of a two-billion-year-old and ~800 meter-thick
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evaporite succession from the Onega Basin in Russian Karelia. The deposit consists of a basal
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unit dominated by halite (~100 meters) followed by anhydrite-magnesite (~500 meters) and
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dolomite-magnesite (~200 meters) dominated units. The evaporite minerals provide a robust
32
constraint that marine sulfate concentrations were at least 10 mmol/kg, representing an oxidant
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reservoir equivalent to over 20% of the modern ocean-atmosphere oxidizing capacity. These
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results show that substantial amounts of surface oxidant accumulated during this critical
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transition in Earth’s oxygenation.
36
37
One Sentence Summary:
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A well preserved, extensive evaporite deposit reveals that a large oxidant reservoir in the form of
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marine sulfate accumulated two billion years ago.
40
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Main Text:
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The geological record preserves evidence of Earth’s dynamic surface oxygenation
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(reviews by 1, 2), but quantifying this history remains a challenge. The presence/absence of
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redbeds, banded iron formations, and detrital grains of pyrite and uraninite (1, 3) qualitatively
45
indicate increasing oxidation during the Paleoproterozoic, and the disappearance of large-
46
3
magnitude mass-independent fractionation (MIF) of sulfur isotopes at 2.4–2.3 Ga (4, 5) shows
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that the atmosphere exceeded a redox threshold of ~1 ppm pO2 (6, 7). However, this limit
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reflects only a tiny fraction of the potential surface oxidant budget and does not capture
49
subsequent redox changes in the Earth system. Today, marine sulfate ([SO42-
(aq)] = 28 mmol/kg)
50
constitutes one of the largest surface oxidant reservoirs, equivalent to almost twice the modern
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atmospheric O2 inventory. Therefore, quantitative bounds on marine sulfate concentrations are
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essential for constraining the net electron balance and accumulation of oxidants on Earth’s
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surface.
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Sedimentary evaporite minerals are one of the best archives of ancient seawater
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chemistry (e.g. 8, 9) and specific isotopic signals (e.g. 10, 11). Unfortunately, most Precambrian
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evaporite deposits consist of pseudomorphic replacements (12), and until recently, the oldest
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known preserved evaporitic halite and bedded sulfates dated from ~830 Ma (13, 14) and ~1.2 Ga
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(15), respectively. This study presents analyses from a remarkably preserved ~2.0 Ga marine
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evaporite succession bearing carbonates, sulfates, halites, and bittern salts. This succession was
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discovered during the 2007–2009 drilling of the Onega Parametric Hole (OPH), which
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intersected 2.9 km of Paleoproterozoic sedimentary and volcanic rocks and 600 m of Archean
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gneiss in the Onega Basin, Karelia, Russia (16, 17). By extending the record of extensive marine
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evaporites by almost a billion years, core samples from the OPH provide a unique window into
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surface conditions and redox balance in the aftermath of the initial rise of oxygen on Earth.
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The interval of the OPH studied here lies between 2940 and 2115 m depth and
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corresponds to the ~2.0 Ga Tulomozero Formation (age discussed in supplementary materials).
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In other cores and outcrop exposures, this formation contains abundant pseudomorphic
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replacements of evaporite minerals (18, 19), but original evaporite minerals are only preserved in
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4
the OPH where they define three major units. Unit 1 (2940–2833 m; 40% average core recovery
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in cored intervals) comprises dark red-pink, recrystallized halite with intraclasts of anhydrite,
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magnesite, and mudstone (Fig. 1A,B) and ~10% various magnesium- and potassium-sulfate salts
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(Fig. 2). Unit 2 (2833–2330 m; 56% average core recovery in cored intervals) consists of
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decimeter- to meter-scale interlayered anhydrite, magnesite, and laminated dark-gray to red
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mudstone (Fig. 1C) with minor glauberite, gypsum, and halite in its lower part. Unit 3 (2330–
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2115 m; 44% average core recovery in cored intervals) is typified by pink-tan, commonly
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microbially laminated dolostone (Fig. 1D), laminated red-brown-gray mudstone, and variable
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amounts of magnesite; quartz and dolomite pseudomorphs of calcium-sulfate minerals occur
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throughout the lower half of this unit, forming laths, nodules, discs, swallow-tail crystals, and
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chicken-wire fabric (Fig. S2).
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Considering the Tulomozero Formation in the OPH and its development across the
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18,000 km2 of the Onega Basin (18, 19), the interpreted depositional setting is a restricted marine
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embayment with sabkha/coastal plain and shallow-marine environments (Fig. S3). The OPH
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succession captures a decreasing degree of evaporation from a state of halite and magnesium-
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/potassium-sulfate saturation (Unit 1), through calcium-sulfate deposition (Unit 2), and then to
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more open marine dolostone precipitation (Unit 3). The extent, thickness (>800 m in the OPH
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core), and mineral sequence of the evaporite succession are comparable to Phanerozoic evaporite
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basins.
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Isotope geochemistry of the OPH evaporites presents an opportunity to investigate
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Earth’s ocean-atmosphere system at ~2.0 Ga (Fig. 2). Quadruple sulfur isotope analyses of
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samples from Units 1 and 2 reveal ∆33S and ∆36S values indistinguishable from zero, confirming
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an oxic atmosphere (6, 7) and production of the Tulomozero Formation sulfate well after that
92
5
atmospheric transition occurred. Triple oxygen isotope measurements of sulfates yield resolvably
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negative ∆17O values. Oxygen MIF derived from atmospheric O3/O2 photochemistry (20) cannot
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be ruled out, but the small magnitude ∆17O signals preclude a unique interpretation (see
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supplementary materials) and a quantification of pO2 is not possible.
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The mass-dependent behavior of sulfur and calcium isotopes provides compositional
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constraints on ancient seawater. Sulfate δ34S values lie between 5 and 7‰, except for the
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uppermost sample in Unit 2 (further discussion in supplementary materials). Given the small 34S
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enrichment during sulfate evaporite formation (10), the seawater sulfate δ34S composition is
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estimated to have been 4–6‰ during deposition of Units 1 and 2. The homogeneous sulfur
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isotopic composition across ~400 m of OPH stratigraphy and the composition and sheer volume
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of evaporite minerals suggest that the OPH evaporites must have derived from seawater and
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preserve robust, primary isotopic signals. Additionally, the presence of halite and highly soluble
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bittern salts in Unit 1 argues against interaction with large volumes of fluid and supports the
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interpretation of primary isotopic ratios for the major mineral-forming elements. Calcium isotope
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ratios show a clear stratigraphic relationship following mineralogical trends, with the highest
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δ44/40Ca values in Unit 1, decreasing values in Unit 2, and even lower values in Unit 3. Three
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samples in Unit 3 with the lowest δ44/40Ca values (–1.6 to –1.4‰ relative to modern seawater)
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also exhibit relatively high strontium content, with one sample containing minor relict aragonite;
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these observations indicate that the bulk sediment likely contained primary aragonite which has
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now largely been converted to other carbonate minerals (see supplementary materials).
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Excluding those samples associated with aragonite, where mineralogy rather than evaporitic
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processes is the first-order control on calcium isotope ratios, the increase in δ44/40Ca values in the
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more highly evaporated facies is consistent with evaporite precipitation driving isotopic
115
6
distillation of calcium, the magnitude of which is sensitive to the initial composition of seawater
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(21).
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An estimate for seawater sulfate concentrations at ~2.0 Ga can be derived from the
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observed sequence, mineralogy, and calcium isotope ratios of the OPH evaporites (Fig. 3).
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Constraints were assessed by comparison to batch evaporation simulations with varying initial
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ion concentrations. The relative concentrations of calcium and sulfate are the principal controls
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governing the precipitation sequence that is expressed in the OPH, as in modern evaporites, of
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carbonates (Unit 3), followed by calcium sulfates (Unit 2), halite, and eventually magnesium
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sulfates (Unit 1). As such, the OPH preserves a reversed evaporite sequence where the degree of
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evaporation decreases stratigraphically upwards, progressing from the most evolved brine at the
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base of the Tulomozero Formation towards more open marine conditions. During a forward
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evaporite sequence, calcium precipitates as sulfate minerals and minor carbonate with an isotopic
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fractionation, such that calcium in the remaining brine becomes enriched in the heavier isotopes
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through Rayleigh distillation. If sufficient sulfate is present to remove a large fraction of the
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original calcium content of seawater, later calcium-bearing phases can record large δ44/40Ca
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enrichments (21). The ~1‰ δ44/40Ca range captured in the OPH between shallow-marine
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carbonates in Unit 3 and halite-hosted anhydrite in Unit 1 therefore places a lower limit on
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sulfate concentrations. Together with mineralogical constraints, and assuming modern
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concentrations of other major ions and a conservative interpretation of δ44/40Ca values (but see
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supplementary materials for further discussion), the minimum sulfate concentration consistent
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with these observations is ~10 mmol/kg (Fig. 3).
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The OPH core provides quantitative evidence that marine sulfate concentrations at ~2.0
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Ga were at least a third those of modern seawater. This constraint validates assertions that a large
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7
Paleoproterozoic sulfate reservoir existed (18, 22) and increases fourfold the previous lower
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bound of 2.5 mmol/kg sulfate, derived from observing that gypsum evaporites precipitated
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before halite (22, 23). Although the ancient ocean volume is unknown, 10 mmol/kg sulfate in a
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modern-sized ocean represents an oxidant reservoir equivalent to 23% of the present ocean-
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atmosphere oxidizing capacity (or 62% of the present atmospheric O2 inventory). The growth of
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such a reservoir from < 200 µmol/kg sulfate in the Neoarchean (24) would account for a redox
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imbalance of at least 8–24×1010 mol/yr in equivalent moles of O2 produced or organic carbon
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buried over 100–300 Myr. Compared to estimates of modern organic carbon burial (5×1012
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mol/yr (25)), the accumulation of such a sizable fluid oxidant reservoir within the given time
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constraints can be explained by either a large and rapid decline in reductant sinks (i.e. sulfide) or
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a prolonged 2–5% imbalance over 108-year timescales. In either case, the observations suggest
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that a sustained increase in net O2 production occurred in the Paleoproterozoic.
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The geologically rapid growth of a massive sulfate reservoir, with or without a
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commensurate increase in atmospheric O2, also has implications for feedbacks between the
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global biogeochemical cycles of O2 and CO2 and Earth’s climate. In particular, the oxidation of
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large amounts of reduced sulfur requires additional sources of carbon to offset the inferred
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organic carbon burial (the initial source of oxygen) and prevent catastrophic cooling (26).
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Models for Earth’s oxidation must balance these considerations as well as the new evidence for a
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substantial oxidant reservoir in the form of marine sulfate. Finally, although sulfate likely
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represented the largest oxidant reservoir during deposition of the Tulomozero Formation, its
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concentration subsequently decreased such that evaporites at ~1.9 Ga no longer precipitated
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gypsum before halite (12, 27) and a fundamental change in the sedimentary sulfur isotopic
160
composition occurred (28). The apparent transient accumulation of surface oxidants is not yet
161
8
well understood (2, 29) but implies a protracted re-organization of the global redox budget on the
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timescales of sedimentary recycling of pyrite and organic carbon (i.e. hundreds of millions of
163
years). Regardless of the mechanisms involved, the observations presented here from the OPH
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core document a remarkably large oxidant pool at ~2.0 Ga – a pivotal new constraint on the
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history of Earth’s oxidation.
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350
Acknowledgements:
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Data presented in this study are available in the Supplementary Materials. Funding sources:
352
Simons Foundation (SCOL 339006 to C.L.B.), European Research Council (ERC Horizon 2020
353
grant 678812 to M.C.), Research Council of Norway (RCN Centres of Excellence funding
354
scheme project 223259 to K.P. and A.L.), Estonian Science Agency (PUT696 to K.K., A.L.,
355
K.P., T.K.). Thanks to T.H. Bui for assistance with sulfur isotope analyses and B.A. Wing for
356
helpful discussions. Core material from the Onega Parametric Hole is maintained by the Institute
357
of Geology, Karelian Science Centre, Petrozavodsk.
358
359
Fig. 1. Representative Tulomozero Formation evaporite rocks in the OPH. A. Halite cored
360
intervals (box length is 1 m; 2900 m depth); note rounded gray intraclasts consisting of
361
mudstone, anhydrite and magnesite. B. Halite with felted anhydrite grains and anhydrite-
362
magnesite intraclasts (2854 m depth). C. Magnesite-anhydrite (white) and halite overlain sharply
363
by laminated red-gray mudstone with desiccation cracks; magnesite-anhydrite bed infilling
364
compacted desiccation crack at top of image (2528 m depth). D. Laminated fine-grained
365
dolostone (2304 m depth). White bars are 1 cm in length.
366
367
15
Fig. 2. Interpretive stratigraphy of the Tulomozero Formation in the OPH and associated
368
mineralogical and geochemical data. Calcium isotope data for samples influenced by former
369
aragonite shown by open symbols with crosses. Additional δ34S from (30); cc–calcite, arag–
370
aragonite, dol–dolomite. Methods described in supplementary materials.
371
372
Fig. 3. Constraints on seawater chemistry during deposition of the Tulomozero Formation.
373
Circles show batch evaporation simulations with variable calcium and sulfate concentrations; all
374
other ions as in modern seawater. Small, empty circles indicate failure to precipitate gypsum
375
prior to halite. Color of filled circles indicates fraction of initial calcium (fCa) removed at halite
376
saturation. Conversion of fCa to δ44/40Ca range is based on a Rayleigh distillation model with α =
377
0.99905 (see supplementary materials). M and S identify composition of modern and estimated
378
Silurian seawater (8), respectively. Lines indicate constraints from OPH observations (arrows
379
give directionality), and the blue region shows the range of seawater compositions consistent
380
with these constraints.
381
Tulomozero FormationArchean gneissZaonega and younger Fms
dark gray = cored intervals of Tulomozero Formation OPH
2 km
3 km Unit 1 halite-rich interval:
restricted marine embayment Unit 2 anhydrite, magnesite, mudstone with evaporite pseudomorphs:
sabkha-marine embayment Unit 3
dolostone, magnesite, mudstone with evaporite fabrics:
sabkha-coastal plain to shallow- marine
2500
2600 2200
2300
2700
2800
2900
0 50 100 50 100
depth (m)
evaporite mineralogy (%)
insoluble fraction (%)
analytical error:
1.5 1.0 0.5 0 44/40Ca (‰ SW)
0 5 10
34S (‰ VCDT)
0.2 0 0.2
33S (‰)
2200
2300
2400
2500
2600
2700
2800
2900 17O (‰)
depth (m)
carbonates cc+arag+dol magnesite Ca sulfates halite Mg-K sulfates
this study Krupenik et al., 2011
(2)
0.2 0.1 0
0 5 10 15 20 25 30 0
5 10 15 20 25 30 35
Gypsum
bef ore halit
e
Mg sulf
ates occur Ca
isot ope
const raint
SO4(mmol/kg)
Ca (mmol/kg)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.2 0.4 0.6 0.8 1.0 1.5 2.0
0.0 44/40Ca range (‰) fCa
M S
1
Supplementary Materials for
Two-billion-year-old Evaporites Capture Earth’s Great Oxidation
C.L. Blättler, M.W. Claire, A.R. Prave, K. Kirsimäe, J.A. Higgins, P.V. Medvedev, A.E.
Romashkin, D.V. Rychanchik, A.L. Zerkle, K. Paiste, T. Kreitsmann, I.L. Millar, J.A.
Hayles, H. Bao, A.V. Turchyn, M.R. Warke, A. Lepland correspondence to: blattler@princeton.edu
This PDF file includes:
Materials and Methods Supplementary Text Figs. S1 to S14 Table S1
Other Supplementary Materials for this manuscript include the following:
Data tables S1 to S2
2 Materials and Methods
Geological context
The Paleoproterozoic sedimentary-volcanic succession in the Onega Basin (Fig. S1) is moderately deformed into open folds cut by high-angle faults and metamorphosed to lower greenschist facies during the 1.85 Ga Svecofennian orogeny (31). In the Onega Parametric Hole (OPH), the Tulomozero Formation sits above Archean gneiss and below the conformably overlying Zaonega Formation, composed of siliceous dolomarl-
rhythmite and sulfidic fine-grained siliciclastic and carbonaceous rocks. The lower contact with Archean rocks was not recovered, but downhole geophysical data were used to identify contacts in intervals without core recovery and indicate that the halite-
dominated unit sits directly on gneiss (32). Seismic reflection data (33) show that the Tulomozero Formation halite and associated salts form wedge-shaped bodies that thicken west-northwest (from ca. 150 to 400 m over 2 km) and that reflectors have uniformly low-angle dips, indicating that the evaporite units have not undergone significant mass flowage.
Although robust 3-D control for the units dominated by halite and anhydrite- magnesite are lacking (they are only known from the OPH core), the lithofacies
associations displayed in the Tulomozero Formation across the Onega Basin indicate that the major evaporite facies was fringed by sabkha-coastal plains and stromatolite reefs (observed in outcrop, e.g. (34); Fig. S2). Additional sedimentological features observed in the Tulomozero Formation in the OPH that support a shallow, tidally influenced marine sabkha environment include common desiccation cracks and quartz- and
dolomite-pseudomorphed sulfate minerals in the form of laths, swallow-tail-terminating crystals, cauliflower nodules and lenses, and chicken-wire fabric, as well as tidal bundles (cm-scale rhythmic thickening and thinning of layers) and combined-flow ripples with reactivation surfaces and mud drapes. The stromatolite reefs may have acted as barriers restricting exchange of water between the embayment and ocean. Garabogazköl Bay, Turkmenistan offers a partial modern analogue to envisage such a paleogeography (Fig.
S3). There, a slender bedrock ridge cut by a narrow inlet separates the Bay from the Caspian Sea; the inlet allows an influx from the Sea with salinities of 1–3% into the Bay where evaporation then raises concentrations to 30–35% in the salt-precipitating areas distal to the inlet (35, 36).
Age constraints
The age of the Tulomozero Formation is not precisely constrained. Carbonate carbon isotope ratios in the OPH from this study (see below) and previous work (30), as well as other cores in the Onega Basin (18, 19, 37), show that the Tulomozero Formation records heavy δ13C values, in line with the Paleoproterozoic Lomagundi-Jatuli positive carbonate δ13C excursion. The formation is older than 1956 ± 5 Ma, the age of igneous dykes (38) that intrude the conformably overlying Zaonega Formation, but a maximum age for deposition remains debated: a Pb-Pb age of 2090 ± 70 Ma has been obtained from Tulomozero Formation carbonates (39), but zircons from a putative lava underlying the Formation yielded U-Pb ages of 1976 ± 9 Ma (40) and 1975.3 ± 2.8 Ma (41). Thus, until resolved, the most judicious depositional age to consider for the Tulomozero Formation is ca. 2.0 Ga.
3 X-ray diffractometry
Fifty-nine samples were taken for X-ray diffractometry (XRD); these were pulverized by hand with an agate mortar and pestle, and unoriented preparations were made on low-background sample holders of Si single-crystal wafers with high-index surface orientation. Preparations were measured on a Bruker D8 Advance using a Johannson-type Vario 1 focusing primary monochromator providing pure CuKα1 radiation to minimize peak overlaps. Scans were recorded with a LynxEye detector in the range 2θ = 2–70°, with a step size of 0.012° and counting time of 1 second per step.
Mineralogical compositions (Data table S1) were modeled and interpreted using the Rietveld algorithm-based codes Topaz and Siroquant 3.0 (42); the relative error of quantification is better than 10% for major phases (>5 wt%) and better than 20% for minor phases (<5 wt%).
Triple oxygen isotope and sulfur isotope analysis
Triple oxygen isotope analyses of sulfate were performed at Louisiana State University. Raw samples were treated with 3 M HCl for the extraction of acid-soluble sulfate (gypsum, anhydrite, carbonate-associated sulfate). To the resulting solution, ~4 mL of BaCl2-saturated 3 M HCl was added, resulting in the quantitative precipitation of BaSO4 which was then washed in distilled and deionized water until the washing solution was pH-neutral. The resulting BaSO4 powders were purified by repeated diethylene- triaminepentaacetic acid (DTPA) dissolution and reprecipitation (43). This purification procedure is critical for the removal of nitrate. Purified BaSO4 samples were converted to O2 by heating with a CO2-laser under BrF5. The O2 from laser fluorination was
analyzed for its triple oxygen isotope composition on a Thermo Finnigan MAT 253 (44).
All reported Δ17O values are the average of triplicate measurements and were computed using the logarithmic delta-prime notation with λ = 0.52 to facilitate comparison with published data: Δ17O = δʹ17O – 0.52 · δʹ18O, where δʹxO = ln(δxO/1000 + 1) · 1000. The precision on Δ17O values is better than ± 0.05‰.
The triple oxygen isotope composition is additionally reported using λ = 0.5305 (Data table S2). This alternate reference frame (45) is considered more appropriate for small magnitude Δ17O anomalies. The correction is performed using the more accurate δʹ18O from TCEA results (high temperature conversion elemental analyzer): Δ17O0.5305 = (0.52 · δʹ18OTCEA + Δ17O0.52) – 0.5305 · δʹ18OTCEA. Following this correction, the OPH samples average Δ17O0.5305 = –0.19 ± 0.03‰ (1 s.d.). Such small magnitude Δ17O values can have several origins, including but not restricted to stratospheric photochemical effects involving O2 and O3, which are in turn functions of the photosynthetic oxygen flux, oxygen consumption via respiration and weathering, the total atmospheric oxygen reservoir size (pO2), and its relative size to pCO2 (20). Additional processes such as low- temperature equilibrium fractionation and microbial sulfate cycling are also possible sources of the Δ17O signal.
Multiple sulfur isotope analyses were conducted on these same purified sulfate samples at McGill University, following an established methodology (46). Sulfate samples were purified with a chromium reduction procedure and converted to H2S gas through reaction with a Thode reduction solution (47, 48). The sulfide gas was captured as ZnS, converted to AgS2, and finally converted to SF6 gas through reaction with excess
4 F2. The resulting SF6 was purified cryogenically and chromatographically, and its
isotopic composition was quantified via gas source mass spectrometry on a Thermo Finnigan MAT 253. Long-term reproducibility (2σ) for δ34S values is better than
± 0.2‰, and for Δ33S and Δ36S values is ± 0.02‰ and ± 0.4‰, respectively. Triple- isotope and Δ-Δ plots (Fig. S4) demonstrate the relationships among the multiple sulfur isotopes.
Additional sulfate δ34S measurements were performed at the University of Cambridge in the Godwin Laboratory, following previously described methods (49).
Samples were dissolved in deionized water over 4–5 hours and sulfate was precipitated from the supernatant as barite by adding a saturated barium chloride solution. The barite precipitate was rinsed with deionized water, then dried and combusted at 1030 °C in a Flash Element Analyzer (EA) with the resulting sulfur dioxide (SO2) analyzed by continuous flow gas source isotope ratio mass spectrometry (Thermo Delta V Plus) and δ34S values corrected to NBS 127 = 20.3‰. The error for these δ34Ssulfate analyses is 0.3‰ based on replicate analyses of standards.
Calcium isotope analysis
Calcium isotope ratios were measured at Princeton University on fractions of the powdered samples used for XRD. The bulk powders were dissolved in 0.1 M buffered acetic acid–ammonium hydroxide solution (pH of ~4.5) for four hours. The soluble phase was then diluted and introduced into an automated ion chromatograph (Thermo- Dionex ICS-5000+) with a fraction collection system to isolate calcium for isotopic analysis (50). The isotope ratios therefore reflect the sum of calcium from all soluble calcium-bearing phases in a given sample, predominantly anhydrite and dolomite with minor gypsum, calcite, and aragonite (Data table S1).
Mass spectrometry was performed on a Thermo Neptune Plus multi-collector inductively coupled plasma mass spectrometer (ICP-MS) with an ESI Apex-IR sample introduction system (50). Corrections for isobaric strontium interferences were based on the double-charged 87Sr peak; polyatomic interferences (predominantly argon hydride, carbon dioxide, and nitrous oxide) were avoided with sufficient mass resolution. Sample- standard bracketing was used to calculate δ44/42Ca values relative to modern seawater. A triple-isotope plot shows the expected mass dependence among 44Ca, 43Ca, and 42Ca (Fig.
S5). Reported δ44/40Ca values are calculated following kinetic fractionation laws (51) and assuming no radiogenic 40Ca excess, which is consistent with previous work showing a lack of resolvable 40Ca excess in modern seawater or ancient marine precipitates (52);
alternatively, these can be considered initial δ44/40Ca values. External precision on δ44/40Ca values is ± 0.14‰ (2σ), derived from the long-term reproducibility of the
carbonate standard SRM-915b (n=155). The δ44/40Ca value obtained for SRM-915b is –1.15‰, which is indistinguishable from published values (53, 54).
Evaporation simulations
Evaporation simulations were conducted with the PHREEQC program (55) by the U.S. Geological Survey (https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/).
Simulations were initialized with varying concentrations of carbonate alkalinity, calcium, chloride, potassium, magnesium, sodium, sulfate, and trace strontium and set in
equilibrium with pCO2 = 10–1.5 atm (100× pre-industrial, see below for further
5 explanation) at a temperature of 25 °C with no charge balance imposed (i.e. net positive or negative charges were permitted and remained constant throughout a single
simulation). The minerals in Table S1 were permitted to precipitate if the respective saturations were attained. Note that calcite was the only carbonate mineral included;
dolomite and magnesite precipitation were suppressed because of known kinetic barriers (see below for discussion of magnesite). The pitzer.dat database provided with
PHREEQC was used for thermodynamic speciation and equilibrium calculations during the removal of over 99% of the initial water, and no back-reactions were allowed between fluid and solid phases.
Simulated evaporation of modern seawater yields a mineral assemblage of calcite, celestine, gypsum, glauberite, halite, polyhalite, bloedite, epsomite, kainite, kieserite, carnallite, and bischofite (listed in order of appearance with progressive evaporation).
Simulated evaporation of estimated Silurian seawater (8) yields a distinctly different assemblage, notably lacking magnesium sulfate minerals, of calcite, gypsum, halite, anhydrite, sylvite, carnallite, bischofite, and MgCl2 tetrahydrate. The constraints in Fig. 3 (main text) were established using simulations to determine the degree of evaporation (= initial water content / final water content) for the initial appearance of gypsum and halite, the presence/absence of select evaporite minerals, and the fraction of calcium removed at the onset of halite precipitation (see below for discussion of interpreting calcium isotopes and sensitivity tests of simulation behavior). The results using PHREEQC differ notably from simple calculations relying on stoichiometric
relationships among the major ions because of variable ion activities in concentrated brines.
Supplementary Text
Supporting geochemical data: Methods
Additional geochemical data, including carbonate carbon isotope ratios (δ13C), magnesium isotope ratios (δ26Mg), strontium isotope ratios (87Sr/86Sr), and trace element concentrations, provide additional context for the OPH (Data table S2). Carbon isotope ratios in calcium carbonate, dolomite, and magnesite were measured on micro-drilled powdered samples on a Thermo Delta V Advantage continuous flow isotope ratio mass spectrometer at Tartu University. Magnesite and dolomite-magnesite samples were reacted for 96 h at 70 °C to allow for the complete reaction of magnesite (method modified from (56)). Dolomite samples with <1 wt% magnesite content, as determined by XRD analysis, were reacted for 2.5 h at 70 °C. Carbon isotope ratios are reported relative to Vienna Pee Dee Belemnite (VPDB) with reproducibility (2σ) better than
± 0.1‰.
Magnesium isotopes and trace elements were measured at Princeton University on the same dissolved fractions used for calcium isotope analysis (see above). For
magnesium isotopes, automated ion chromatography (Thermo-Dionex ICS-5000+) was used to isolate magnesium, and isotopic ratios were then obtained by multi-collector ICP- MS (Thermo Neptune Plus) with wet aerosol introduction through a cyclonic spray chamber (50). Sample-standard bracketing in low resolution was used to calculate δ26Mg values relative to the standard DSM3. A triple-isotope plot shows the expected mass dependence among 24Mg, 25Mg, and 26Mg (Fig. S6). External precision on δ26Mg values is ± 0.09‰ (2σ), derived from the long-term reproducibility of the standard Cambridge-1
6 (n=76). The δ26Mg value obtained for Cambridge-1 is –2.56‰, which is
indistinguishable from published values (57).
Trace elements were analyzed on a quadrupole ICP-MS at Princeton University (Thermo iCAP). Samples were diluted so that major cations were no greater than 10 ppm, and scandium was added as an internal standard. External standard calibration curves bracketed the concentrations of each element of interest in the sample solutions, yielding precision of approximately ± 5%. Reported trace element concentrations reflect those of the acetic-acid-soluble component only (sum of carbonate, sulfate, and chloride evaporite minerals), not whole rock concentrations.
Strontium isotopes were analyzed at the NERC Isotope Geosciences Laboratory (British Geological Survey). The powdered samples were first leached in dilute (10%) acetic acid at 50 °C for 30 minutes to remove secondary carbonate phases. Residual material was washed repeatedly in water, then dissolved in distilled 6 M HCl at 50 °C for one hour. The dissolved material was removed after centrifuging, dried down, and converted to nitrate form using 1 mL 16 M HNO3. Strontium was separated using Sr- SPEC ion exchange resin, loaded onto outgassed single Re-filaments, and analyzed in a Thermo Scientific Triton thermal ionization mass spectrometer operated in multi-
dynamic mode with 100 measurement cycles for each sample. Eleven measurements of the strontium standard NBS-987 across the interval of analysis yield 87Sr/86Sr = 0.710256 (± 0.000007). Five seawater standards analyzed in the same period yield 87Sr/86Sr = 0.709182 (± 0.000004). The sample 87Sr/86Sri ratios have been corrected for the age of the Tulomozero Formation (2.0 Ga), and precision (2σ) for all samples is better than
± 0.000011.
Supporting geochemical data: Results
The additional geochemical datasets (Fig. S7 and S8) are generally consistent with the observations and interpretations presented in the main text. The magnesium isotope data are discussed below. Carbonate δ13C values from dolomite, magnesite, and rarely calcium carbonate range from +8.1 to +13.9‰, except for one sample in Unit 2 showing extreme 13C enrichment at +18.8‰. These values are characteristic of carbonates precipitated during the Lomagundi-Jatuli positive carbonate δ13C excursion (58).
Elevated strontium concentrations are found in three carbonate samples that also have very low δ44/40Ca values, one of which contains 4% aragonite (detected by XRD).
The combined geochemical evidence suggests that aragonite was likely the precursor carbonate mineral for these three samples, and that they were partially/completely
converted to calcite, dolomite, and magnesite. The sample with detectable aragonite also contains the highest proportion of calcite (55%) and highest strontium content (690 ppm) of any OPH sample, indicating relatively closed system (sediment-buffered) conditions during early marine neomorphism and recrystallization.
Relatively high strontium is also found in association with the anhydrite-magnesite of Unit 2, and to a lesser degree in Unit 1, where it scales with the abundance of calcium sulfate minerals (Fig. S9). This relationship suggests that where strontium is not
associated with aragonite, it may either be substituted into calcium sulfate minerals or exist as strontium sulfate (celestine), a trace phase predicted to co-precipitate with gypsum and anhydrite in evaporation simulations. Manganese is enriched in the
