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Revealing the Climate of Snowball Earth from Δ17 O Systematics Of Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks Daniel Herwartza,1,2, Andreas Packa, Dmitri Krylovb, Yilin Xiaoc, Karlis Muehlenbachsd, Sukanya Senguptaa, and Tommaso Di Roccoa aAbteilung Isotopengeologie, Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, 37073 Göttingen, Germany; bInstitute of Precambrian Geology and Geochronology, Russian Academy of Sciences, 199034 St. Petersburg, Russia; cCAS Key Laboratory of Crustal-Mantle Materials and Environments, School of Earth and Space Science, University of Science and Technology of China, Hefei, 230026, China; and dDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved March 13, 2015 (received for review December 1, 2014) The oxygen isotopic composition of hydrothermally altered rocks water compositions of δ18O ≈−34 ± 10‰, probably repre- partly originates from the interacting fluid. We use the triple senting Marinoan meltwaters (7). Apart from chemical sedi- oxygen isotope composition (17O/16O, 18O/16O) of Proterozoic rocks ments (5, 6) or ancient weathering products (7), it has also been 18 16 18 to reconstruct the O/ O ratio of ancient meteoric waters. Some of suggested to estimate δ Omw from hydrothermally altered rocks these waters have originated from snowball Earth glaciers and thus that have interacted with meltwater of meteoric origin (8). give insight into the climate and hydrology of these critical intervals Interaction of rocks with meteoric water at hydrothermal ∼ – 18 18 in Earth history. For a Paleoproterozoic [ 2.3 2.4 gigayears ago (Ga)] conditions (∼350 °C) shifts δ O of the rocks (δ Or) toward snowball Earth, δ18O = −43 ± 3‰ is estimated for pristine meteoric lower values. Modern examples for such shifts are known from waters that precipitated at low paleo-latitudes (≤35°N). Today, such volcanically active regions such as Iceland (9) or Yellowstone. low 18O/16O values are only observed in central Antarctica, where Fossil Phanerozoic and Precambrian hydrothermal systems like long distillation trajectories in combination with low condensation the Dabie−Sulu ultra-high-pressure terrain (China) (10) or the temperatures promote extreme 18O depletion. For a Neoproterozoic Belomorian Belt (Russia) (11) are suggested to have formed ∼ – δ18 18 ( 0.6 0.7 Ga) snowball Earth, higher meltwater O estimates of similarly to their modern analogs. The lowest δ Or values −21 ± 3‰ imply less extreme climate conditions at similar paleo- 18 provide an upper limit for the δ Omw (8). At full equilibration, ≤ 18 EARTH, ATMOSPHERIC, latitudes ( 35°N). Both estimates are single snapshots of ancient wa- AND PLANETARY SCIENCES rocks have a δ Or that is only 2–3‰ higher than the water they ter samples and may not represent peak snowball Earth conditions. interacted with (12), but the degree of equilibration between a 17 16 We demonstrate how O/ O measurements provide information rock and a given water is generally unknown, compromising beyond traditional 18O/16O measurements, even though all fraction- 18 absolute δ Omw estimates(8).Herewepresentanewap- ation processes are purely mass dependent. 18 proach to reconstruct the absolute δ Omw from measuring not only 18O/16Obutalsothe17O/16O ratios of hydrothermal low triple oxygen isotopes | hydrothermal alteration | snowball Earth | 18 δ Or Precambrian rocks. climate | paleo-temperatures Concept of Using Triple Oxygen Isotopes ≤ 17 18 lacial successions deposited near the paleo-equator ( 15°) The δ Omw is closely coupled with the δ Omw (Fig. S1); a re- Gsuggest that the Earth was entirely covered by ice several lation hereafter called meteoric water line (MWL). The slope of times during the Precambrian. Such episodes were termed “snowball the MWL is a result of equilibrium-dominated fractionation, Earth” climates. Presumably, the concentration of continents at low latitudes enhanced chemical weathering rates and thus removal of Significance CO2 from the atmosphere (1). Low pCO2 led to global cooling and formation of polar and continental ice sheets. Once ice caps ex- The snowball Earth hypothesis predicts that the entire Earth ∼ tended to latitudes below 50°, a runaway ice albedo cooling effect was covered with ice. Snowball Earth events were suggested occurs (2), global temperatures drop far below zero, and the entire to have occurred several times during the Precambrian. Classic Earth becomes covered with ice (a snowball Earth) (2, 3). At least paleo-thermometers (e.g., 18O/16O in marine carbonates) are one “total glaciation” occurred in the Paleoproterozoic era not available from snowball Earth episodes, and only a few [Makganyene at ∼2.4 gigayears ago (Ga)] (1), and at least two more reconstructions of 18O/16O in ancient meteoric water exist. arose in the Cryogenian (Sturtian at 720 Ma; Marinoan at 635 Ma) (3). Here we present a novel approach to reconstruct the 18O/16O The climatic and hydrologic conditions of these critical epi- composition of ancient meteoric waters using the triple oxygen sodes are poorly understood because classic paleo-thermometers isotopic composition (17O/16O and 18O/16O) of hydrothermally (e.g., marine carbonates) are not viable for snowball Earth states altered rocks. The inferred 18O/16O for waters that precipitated and ancient water samples are missing. The 18O/16Oratioofme- at (sub)tropical paleo-latitudes on a Paleoproterozoic (∼2.4 18 teoric water (expressed as δ Omw) can serve as a proxy for paleo- gigayears ago) snowball Earth are extremely low. Today, sim- temperature if the hydrogeological context is known (4). A few ilar compositions are observed only in central Antarctica. 18 attempts have been made to reconstruct ancient δ Omw (5–8). Calcite cements that precipitated in methane seeps in the Author contributions: D.H. and A.P. designed research; D.H. and S.S. performed research; Nuccaleena Formation, Australia, probably sample meltwaters A.P. contributed new reagents/analytic tools; D.H., S.S., and T.D.R. analyzed data; D.H., 18 A.P., D.K., Y.X., K.M., S.S., and T.D.R. wrote the paper; and D.K., Y.X., and K.M. performed from the ∼635 Ma Marinoan snowball Earth with δ O ranging field work and provided samples. ∼− ‰ around 29 (5). The upper carbonate unit of the Lantian The authors declare no conflict of interest. Formation in Anhui, South China, probably formed during the This article is a PNAS Direct Submission. ∼ younger, 580 Ma Gaskiers glaciation within a meltwater-dom- 1Present address: Institut für Geologie und Mineralogie, Universität zu Köln, 50674 inated basin. These carbonates appear to be unaltered, hence Cologne, Germany. 18 low precipitation temperatures imply water compositions of δ O ≈ 2To whom correspondence should be addressed. Email: [email protected]. − ‰ − ‰ 20 to 27 (6). Barite- and malachite-associated sulfate This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. from a diamictite in Kaiyang, Guizhou, China, reveals meteoric 1073/pnas.1422887112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1422887112 PNAS Early Edition | 1of5 Downloaded by guest on September 26, 2021 whereas the intercept is due to kinetic fractionation in the exo- at 500–900 °C (10), well after the Neoproterozoic hydrothermal genic water (see SI Text and Fig. S1). Rocks, in general, do not alteration (16). However, the metamorphic petrology, the age, the plot on the MWL (13). Rocks that have exchanged to variable mineral equilibration temperatures, and even the rock type are degrees with meteoric water will define a mixing trend in a Δ′17O unimportant for our approach. Despite the later metamorphic 18 18 vs. δ′ O diagram (Fig. 1). The intersect between the mixing overprint, the bulk samples are still low in δ Or; hence part of the trend and the MWL gives, along with a small offset due to samples’ oxygen still originates from the meteoric waters that had equilibrium hydrothermal water−rock fractionation, the com- interacted with the samples. All hydrothermally altered samples position of the interacting water (Fig. 1). This allows re- (whole-rock estimates; Table S1) fall on a mixing trend between 18 construction of the δ Omw from hydrothermally altered rocks the unaltered and the fully altered end-members. This mixing 18 and overcomes the limitation of unknown degree of equilibra- trend crosses the MWL and implies a δ Omw = –21 ± 3‰ (Fig. 18 tion, when using δ Or only. Details regarding mass-dependent 1B). If the protolith exchanged oxygen not at hydrothermal, but at effects on Δ′17O in silicates (Fig. S2) and definitions are given in lower temperatures, a water composition of −22 ± 3‰ is sug- SI Text. gested (see Supporting Information). The Dabie−Sulu is located on the northern margin of the Results and Discussion South China block that was drifting between ∼35°N (at 750 Ma) 18 18 We first test the new approach on hydrothermally altered low-δ O (17) and ∼15°N (at 600 Ma) (18) in the Neoproterozoic. A δ Omw rocks from Iceland, where the δ18O of the interacting water is of –21 ± 3‰ at such low latitude is probably related to either approximately known. Icelandic samples are altered basalts from the Sturtian (∼720 Ma) or the Marinoan (∼635 Ma) glaciation two boreholes (KG7 and KG10) at the Krafla volcano, taken (3), but an origin from one of the smaller events (e.g., the 18 from variable depths between 978 m and 2,100 m below the Gaskiers glaciation) is also plausible. Published lower δ Omw surface. The samples fall on a mixing array in the Δ′17O vs. δ′18O estimates of ∼−29‰ (5), ∼−20‰ to −27‰ (6), and –34 ± space between the unaltered Iceland basalt and the altered end- 10‰ (7) differ from ours in space and time. Some estimates are member (Fig. 1A and Fig. S3A). The samples stem from depths likely related to other Neoproterozoic glaciations; hence large where temperatures were mostly ≥300 °C (9) and equilibrium variations between individual studies are expected.
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