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A CO2 Greenhouse Efficiently Warmed the Early Earth and Decreased ACO2 greenhouse efficiently warmed the early Earth and decreased seawater 18O/16O before the onset of plate tectonics Daniel Herwartza,1, Andreas Packb, and Thorsten J. Nagelc aInstitut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany; bGeowissenschaftliches Zentrum, Georg-August-Universität Göttingen, 37073 Göttingen, Germany; and cDepartment of Geoscience, Aarhus University, 8000 Aarhus C, Denmark Edited by Donald E. Canfield, University of Southern Denmark, Odense, Denmark, and approved April 15, 2021 (received for review November 13, 2020) The low 18O/16O stable isotope ratios (δ18O) of ancient chemical Today, carbonatization, which is the formation of carbonates sediments imply ∼70 °C Archean oceans if the oxygen isotopic during alteration of the oceanic crust, mainly occurs in relatively composition of seawater (sw) was similar to modern values. Mod- cool, off-axis hydrothermal systems over the first 20 Mya after δ18 els suggesting lower Osw of Archean seawater due to intense crust formation at midocean ridges (14–16). Elevated degrees of continental weathering and/or low degrees of hydrothermal alter- carbonatization in oceanic crust from the Cretaceous and Jurassic ation are inconsistent with the triple oxygen isotope composition are assigned to higher dissolved inorganic carbon at the time (14). (Δ’17O) of Precambrian cherts. We show that high CO sequestration 2 Hence, higher pCO2 (i.e., a larger RA+O+OC) directly translates fluxes into the oceanic crust, associated with extensive silicification, into higher degrees of carbonatization. While carbonate is mainly δ18 lowered the Osw of seawater on the early Earth without affect- observed as vein fillings in the upper 300 m of oceanic crust today Δ’17 ing the O. Hence, the controversial long-term trend of increasing (14, 15), it extensively replaces glass and igneous minerals in δ18 ’ O in chemical sediments over Earth s history partly reflects in- Archean greenstones down to depths of 2 km below the ancient δ18 p creasing Osw due to decreasing atmospheric CO2. We suggest seafloor (8–10, 17). The amount of CO fixed in 3.2-Ga-old δ18 − ‰ 2 that Osw increased from about 5 at3.2Gatoanewsteady- oceanic crust from Pilbara, Australia is estimated at 1.2 × 107 − ‰ − state value close to 2 at 2.6 Ga, coinciding with a profound drop mol · m 2 (±10%) (17)—a remarkable figure that is about two in pCO2 that has been suggested for this time interval. Using the orders of magnitude more compared to today (SI Appendix). EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES moderately low δ18O values, a warm but not hot climate can be sw Much lower degrees of carbonatization in oceanic crust are al- inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the “faint young ready observed at 2.6 Ga, suggesting a drastic Mesoarchean drop in pCO2 (13) (Fig. 1A). Sun” was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga. The qualitative evidence from the sediment record (5) and from the degree of carbonatization and silicification of oceanic p triple oxygen isotopes | Archean | climate | faint young Sun | crust (13) has not been included in proposed CO2 curves (4, 18) p plate tectonics because the quantitative conversions into absolute CO2 require some assumptions (SI Appendix). Nevertheless, pCO2 during the early Archean may have been as high as initially predicted by he amount of carbon that degassed from a solidifying magma Kasting (2), followed by a pronounced Mesoarchean drop (13) to Tocean on the infant Earth 4.5 Ga ago was probably similar to levels consistent with available paleo-pCO2 estimates toward the the amount of CO2 that is now present in the atmosphere of p ∼ Neoarchean (4) (stippled line “c” in Fig. 1C). Further decreasing Venus ( CO2 90 bar) (1). Subsequently, dynamic carbon cycling p between the early Earth’s atmosphere (atmospheric reservoir CO2 toward modern levels partly reflects increasing ocean pH (Fig. 1) rather than a shrinking RA+O+OC (SI Appendix). Hence, [RA]), the ocean (RO), and the oceanic crust (ROC) reservoirs stabilized a primordial Earth–atmospheric pCO2 to about 1.5 bar (2). Decreasing pCO2 over the Earth’s history reflects the net transfer of Significance large masses of carbon out of the atmosphere-ocean-oceanic crust (RA+O+OC) system into the mantle and the emerging continental Due to the lower luminosity of the young Sun, climate modelers crust reservoir (RCC), the latter providing a long-term sink for struggle to explain why the climate on early Earth was not carbon in the form of inorganic carbon (carbonate rocks) and or- freezing cold. This “faint young Sun paradox” is in conflict with ganic material (organic matter–rich shales, coal, oil, and gas) (1, 2). apparently hot Archean ocean temperatures (∼70 °C) that can be 18 16 Early carbon-cycle models predicted high Archean pCO2 to estimated from the O/ O stable isotope ratio of chemical sedi- account for the faint young Sun paradox (2, 3), whereas direct ments. We show that the later temperatures had been over- 18 16 pCO2 estimates from the end of the Archean now imply much estimated because the O/ O of seawater also changed over time lower atmospheric pCO2 [∼0.01 to 0.1 bar (4)]. However, a due to intense carbonatization and silicification of the oceanic compilation of evidence from the sedimentary record implies crust, which consumes heavy 18O. Because these processes require much higher pCO2 between 3.2 to 3.0 Ga compared to the 2.9 to high fluxes of CO2, greenhouse warming by a CO2-rich atmosphere 2.7 Ga interval (5). These authors state that even several bars appears most feasible to explain all observations. pCO2 are feasible between 3.2 to 3.0 Ga, which is not in conflict Author contributions: D.H. designed research; D.H. performed research; and D.H., A.P., with much lower Neoarchean pCO2 estimates (Fig. 1) or paleo- atmospheric pressure estimates at 2.7 Ga [<2bar(6)and<0.5 bar and T.J.N. wrote the paper. The authors declare no competing interest. (7)]. A fundamental drop in atmospheric CO2 mixing ratio is also reflected in the observation that >3Ga,Archeanmaficcrust This article is a PNAS Direct Submission. (greenstones) is commonly characterized by very intense carbo- Published under the PNAS license. natization and silicification that is unparalleled in their modern 1To whom correspondence may be addressed. Email: [email protected]. analogs (8–12). Such observations provide evidence for deep-time This article contains supporting information online at https://www.pnas.org/lookup/suppl/ paleo-pCO2 fluctuations with a drastic pCO2 drop starting around doi:10.1073/pnas.2023617118/-/DCSupplemental. 3 Ga ago (5, 13). Published May 31, 2021. PNAS 2021 Vol. 118 No. 23 e2023617118 https://doi.org/10.1073/pnas.2023617118 | 1of5 Downloaded by guest on October 2, 2021 The Effect of an Increased CO Flux on the δ18O of A 100 atmosphere 2 sw ocean 11 % atmosphere-ocean Seawater oceanic crust 89 % -oceanic crust reservoir 18 75 δ Fi ~ 7 The Osw of seawater is controlled by the fluxes of oxygen pH 1 % 50 between rocks and the hydrosphere (10). Low-temperature (low- 99 % T) weathering products such as clays have ∼20 to 25‰ higher 25 ~ 8 δ18O values compared to pristine silicate crust (∼5.5 to 12‰). pH Hence, low-T weathering on the continents (expressed as flux 0 18 Fcw) and on the seafloor (Fsfw) lowers the δ Osw value of sea- 25 18 crust + mantle water by extracting heavy O from the hydrosphere. Alteration (long term storage of oceanic crust at hydrothermal temperatures (∼250 to 350 °C) 50 18 % of primordial reservoir reservoirs) produces rocks with ∼1to2‰ lower δ O(∼4.5‰) than the 75 pristine oceanic crust basalt (∼5.8‰), thereby adding 18O to the oceans (Fsp). Classic mass-balance models also account for minor 100 F 9 effects from water recycling through the mantle ( r) and the H&B 2017 B influence of continental growth (Fcg) (19, 20). δ18 ∼ ‰ 8 In order to explain the enigmatic shift in Oof 15 of chemical l&P 2018 sediments through time (21–28), extreme and difficult-to-reconcile changes of oxygen fluxes Fi are required (20, 29). As shown in 7 18 Ocean pH Fig. 2, even a moderate shift of ocean water δ Osw by −5‰ would necessitate very high continental weathering rates 6 (F >10 times present) or very low hydrothermal alteration fluxes 10 cw (Fsp <0.15 times present). Besides the substantial implications for C pH ~ 6 glaciations Precambrian paleoenvironments (21, 30), such flux manipulations pH ~ 7 Δ’17 SI Appendix Definitions (c) would induce much higher O( , )in 1 Precambrian cherts than documented (Fig. 2). This is presently 18 acknowledged as a strong argument against low δ Osw as an ex- pH ~ 8 planation for the low δ18O of Precambrian chemical sediments 2 (24–27, 31). Most of the Δ’17O data do not fall on the black CO [bar] 0.1 chert p (b) equilibrium curve in Fig. 2B (24–27), suggesting that these samples (a) are affected by postdepositional alteration processes. Alteration lowers both δ18OandΔ’17O, hence most triple oxygen isotope 0.01 data seem to suggest postdepositional alteration as the most viable Atmospheric hypothesis for the long-term shift in δ18O(24–27). Due to sample- 17 size requirements, the least altered cherts measured for Δ’ Ochert δ18 0.001 comprise somewhat lower O compared to the most pristine in situ δ18O analyses (27, 31). Nevertheless, triple oxygen isotope compositions of well-preserved cherts from the Barberton Greenstone Belt, South Africa fall close to the equilibrium line consistent with hot Archean seawater (31).
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