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 δ18O values compared to pristine silicate crust (∼5.5 to 12‰). pH ~ 8 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 – Δ’17 0.1 (24 27, 31). Most of the Ochert data do not fall on the black (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 p CO [bar] 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). By combining data 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 from the two techniques, Lowe et al. (31) conclude that Time [Ga ago] “...Archean surface temperatures were well above those of the Fig. 1. Constraints on the R size over time with implications for pCO . present day, perhaps as high as 66 to 76 °C.” A+O+OC 2 δ18 A illustrates the size of the RA+O+OC carbon reservoir and its distribution This interpretation hinges on the assumption that Osw was between the oceanic crust (ROC in gray), ocean water (RO in dark blue), and always close to its present-day value. The classic and apparently the atmosphere (R in light blue). Transition of this carbon to the long-term 18 A strong argument for a constant δ Osw of the oceans is derived RCC and the mantle (green) decreases the size of the RA+O+OC reservoir, which from the roughly constant δ18O composition of oceanic crust was still large at 3.2 Ga (see SI Appendix) and minimal at the onset of the δ18 global glaciations at 2.4 Ga. B shows two recent pH curves over Earth’s his- profiles through time (29). It has been proposed that low Osw ocean water would drive altered oceanic crust to low values (29) tory to illustrate how the pH-dependent distribution of carbon between RA and RO may translate into pCO2 at a given time. (C) The panel summarizes as observed for rocks altered by isotopically light meteoric water pCO2 estimates [adopted from Catling and Zahnle (4)] and proposed pCO2 (32–34). However, while local crust alteration with meteoric evolution curves illustrated as dashed lines: a (2), b (18), and c (proposed water constitutes the case of an open system, alteration of the here). Qualitative evidence to construct curve c comes from rare evidence for oceanic crust by the ocean-water reservoir as a whole should be glaciers (44) and lower degrees of carbonatization of oceanic crust at 3.5 Ga regarded as a closed system. In order to extract 18O from seawater, compared to 3.2 Ga (13), suggesting a transient interval of somewhat low 18 pCO in the Paleoarchean. Low pCO is indicated prior to the onset of cold a complementary reservoir must incorporate O. Interestingly, 2 2 18 climates later in the rock record. The black bar at 4.5 Ga is derived from Archean greenstones tend to be slightly enriched in δ O (29) and carbon-flux arguments and the primordial carbon reservoir (1, 2). The black thus provided a sink for 18O. A typical feature of Archean ter- bar at 3.2 Ga applies the same carbon-flux arguments to translate the high ranes >3 Ga is their high degree of silicification, which may well carbonate content observed in the oceanic crust in Pilbara into tentative explain the overall elevated δ18O observed in Archean green- pCO2 estimates (SI Appendix), which are most consistent with curve a (2). stones (11, 12, 29), which act as the complementary reservoir for depleted ocean water δ18O in the scenario suggested herein. 18 only small effects of carbonatization on the δ Osw value are We propose that the significance of CO2 sequestration through expected for post-Archean seawater (16). Here, we focus on the carbonatization along with intense silicification, both associated very high carbonatization (8–10, 13) and silicification (11, 12) with high Archean pCO2, have been overlooked and that they fluxes before the Mesoarchean pCO2 drop, and we model the offer an explanation for the long-standing oxygen isotope conun- 18 respective effects on ancient δ Osw. drum of chemical sediments deep in time.

2of5 | PNAS Herwartz et al. 18 16 https://doi.org/10.1073/pnas.2023617118 A CO2 greenhouse efficiently warmed the early Earth and decreased seawater O/ O before the onset of plate tectonics Downloaded by guest on October 2, 2021 volatilized, and the respective CO2 is reintroduced into the atmosphere with 150 18 A Continental weathering ( δ a base scenario + FCO2+SiO2 mantle-like O. Because of the practically infinite mantle oxygen reservoir, 0.01 High T 18 b like `a´ but 10x Fsfw and 0.2x Fcw CO2 sequestration and recycling (defined here as FCO2) extracts O from the c like `b´ but Tprecip = 100°C and 0.1x Fsp hydrothermal (F ) hydrosphere. 100 50°C 100 Heavily silicified greenstones are common in the Archean (11, 12) simply c 0.1 2 because CO2 weathering of silicates produces both carbonate (CaCO3)and seawater (empirical at 2.4 Ga silica (SiO ). For each mole of consumed CO and precipitated CaCO , 1 mol 50 F 10 2 2 3 1 cw without F Z&B 2019) ) sp CO2+SiO2 SiO forms (CaSiO +CO → CaCO + SiO , Urey reaction) so that F equals 100°C seawater (ice free) 2 3 2 3 2 SiO2 5 δ18 0.5 0.5 seawater (base scenario) FCO2. As for carbonates, quartz comprises high O even at elevated pre- VSMOW 18 b cipitation temperatures and removes heavy O from the oceans (SI Ap- 0 a seawater (modern) 150°C pendix). Therefore, enhanced CO -driven carbonatization and silicification 2 2 ∆ ‘¹ 7 O [ppm] 18 carbonatization and 10 of oceanic crust provides a means of lowering the δ O value of Archean 200°C 0.1 100 siliziﬁcation (FCO2+SIO2) MORB seawater (Fig. 2A). These CO2-related fluxes, FSiO2 and FCO2, are not considered -50 – 300°C mantle in existing models (19 21, 24, 29) and could dramatically differ from the cont. crust present day owing to the potentially very different atmospheric composition.

-100 The CO2-sequestration flux FCO2 by carbonatization of oceanic crust at 3.2 Ga -15 -10 -5 0 5 10 is estimated as 1.5 × 1014 mol · yr−1 (17), which is two orders of magnitude larger 18 − δ O [‰] than the modern carbonatization rate of only 0.45 to 2.4 × 1012 mol · yr 1 150 (14, 15). This high estimate is based on the assumption that spreading rates at

B ]

‰ typical error the time were three times as high as those at present (36, 37), which is incon- (8 ppm)

23 [ 23 sistent with suggestions that plate tectonics only started around 3 Ga (38, 39) but 100 200°C increased 2 is consistent with proposals that the mode of plate tectonics simply evolved over F 50 1 cw or reduced time (40). The various competing tectonic models proposed for the Archean 0.5 impose large uncertainties not only on newly introduced FCO2+SiO2 but for all F fluxes. Respective modifications, especially for the classic F and F fluxes, have sp cw sp 0 18 200°C previously been invoked for the Archean (21, 30) to argue for very low δ Osw, VSMOW down to −13.3‰. These particular flux modifications can be ruled out, as they -50 100°C Δ’17 Δ’17 75°C induce high Osw, which is inconsistent with the observed low Ochert ∆ ‘¹ 7 O [ppm] 0°C – 50°C (24 27). By adding the CO2-sequestration flux of Shibuya et al. (17) to our base -100 L 2019 model, we can explore how it affects the triple oxygen isotopic composition of Z 2021 Archean 25°C seawater. Moderate modification of all fluxes then allows identification of fea- -150 S 2020 cherts > 3 Ga Proterozoic Δ’17

sible flux combinations that most closely resemble the measured O data. EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES L 2020 cherts Phanerozoic chert cherts Accounting for the respective F yields δ18O of seawater be- 0°C CO2+SiO2 sw-ss -200 − − ‰ -15 -10 -5 0 5 15 25 30 35 40 45 50 tween 7.5 and 2 (Fig. 2) and mainly depends on the average temperature 10 20 18 for carbonation and silicification within oceanic crust (Tprecip). Heavy Ois δ18O [‰] extracted more efficiently from the hydrosphere at low Tprecip. Most notably, consideration of carbonatization and silicification fluxes F and precipita- Fig. 2. Plots of Δ’17O versus δ18O for the various models and published CO2+SiO2 sw tion of the silica and carbonate at 100 to 200 °C lowers the ocean’s δ18O but chert compositions. A shows how seawater composition changes when in- sw-ss does not lead to the increase in Δ’17O as seen in (24) (Model S1 and Fig. 2A). dividual fluxes are manipulated. The δ18O of seawater decreases for sub- sw-ss sw Accounting for CO sequestration and silicification provides a much better stantially lower F (green) or higher F (gray) but comes along with a 2 sp cw fit to observed Δ’17O of Precambrian cherts compared to previous sugges- Δ’17O increase. Tick marks refer to multiples of the fluxes used in the base sw tions to lower δ18O without F (Fig. 2B). Simply using the modern- scenario (SI Appendix). Accounting for Archean carbonatization and silicifi- sw CO2+SiO2 day fluxes of Muehlenbachs (20) in combination with the estimated CO flux cation (F ) also decreases the δ18O of seawater (brown) but leaves 2 CO2+SiO2 sw at 3.2 Ga (17) and T of 150 °C yields δ18O of −3‰ (scenario “a” in Δ’17O largely unchanged (a). Temperatures in A indicate average precipi- precip sw-ss sw Fig. 2A). The δ18O can be forced to very low values, but the Δ’17O in- tation temperatures (T ) for carbonates and silicates. B shows how sw-ss sw precip creases for such end-members (Fig. 2), making respective scenarios incom- measured triple oxygen isotope compositions of cherts compare to the patible with chert data. A range of more realistic flux combinations cluster model. The data are derived from refs. 25–27 and 31. Silica precipitated in around a resultant δ18O of seawater of −5‰ with near zero Δ’17O (e.g., equilibrium with modern ocean water must fall on the black equilibrium sw-ss scenario “b” in Fig. 2A; see Model S1 and SI Appendix for a sensitivity curve at the respective precipitation temperature. The greenish field illus- Δ’17 analysis). Such a scenario is best compatible with triple oxygen isotope ratios trates O compositions expected for cherts that precipitated from a low – δ18 of Archean cherts (25 27, 41) and shales (42). Osw due to increasing Fcw or decreasing Fsp as modeled by Sengupta and 18 18 A direct seawater estimate [δ Osw = −1.7 ± 1.1‰ (43)] at 2.4 Ga corre- Pack (24). When accounting for FCO2+SiO2, however, the δ Osw of seawater Δ’17 sponds to a time when the degree of carbonatization of oceanic crust was decreases, and Osw remains broadly similar, consistent with Precambrian 18 17 even lower than today (13), suggesting that slightly lower δ O persisted in the chert Δ’ O (brown field). The solid and stippled gray curves indicate pre- sw Paleoproterozoic even without significant CO sequestration and silicification. cipitation from scenarios a and b, respectively. The light brown field en- 2 δ18 − ± ‰ δ18 = ‰ We suggest that Osw further decreased to 5 2 when pCO2 was high at compasses all possible solutions. The stippled brown line at O 23 18 18 3.2 Ga. We propose a slightly higher δ O of ∼−3 ± 2‰ for the Paleoarchean indicates the highest Archean δ O analyzed in situ by microanalytical sw (at 3.5 Ga) to account for apparently lower degrees of carbonatization (13), techniques (27, 31). The red arrow depicts an expected slope for alteration. 18 18 observations of cold climate (44), and high δ Ocarbonate and δ Ophosphate (45) at the time (Fig. 3). Both approximations are used in combination with direct 18 δ Osw estimates (43, 46, 47) to construct a seawater curve over time (Fig. 3). Methods and Results 18 18 Shifts in δ Osw on these timescales are viable because δ Osw can adapt to a new

To explore how variable CO2 sequestration through carbonatization and steady state within a few tens of millions of years (SI Appendix). silicification affects the oxygen isotope composition of the oceans at variable temperatures, we use Muehlenbachs’ model (20), which was recently Discussion extended for δ17O (24) (Fig. 2A). After slight modifications (SI Appendix), 18 sw Even with the moderately low δ Osw of seawater and warm pre- the base scenario for modern-day fluxes gives a seawater steady state 18 cipitation temperatures proposed here, alteration is still required to δ Osw-ss = −0.53‰, intermediate between the modern ocean and the 18 explain the triple oxygen isotopic compositions of most Precambrian presumed value for an ice-free world (i.e., δ Osw ∼−1‰). The respective 17 cherts (Fig. 2). Bedded cherts are frequently interpreted to reflect Δ’ Osw-ss = −9 ppm (i.e., −0.009‰) matches the measured value of −5 ppm for modern seawater within uncertainty (35) (Fig. 2A). precipitation from warm/hot bottom water brines or hydrothermal plumes (25–27). Cherts of superior-type banded iron formations, Volcanic CO2 equilibrates with silicates in the magma chamber before it is outgassed with near-mantle–like δ18O ∼5.5‰ (SI Appendix). It is subject to however, certainly originate from primary precipitates from ambient various fractionation processes but is eventually immobilized as carbonate seawater temperatures on continental shelves. Surface waters within 18 with higher δ18O. Such carbonates, if they become subducted, are mostly epicontinental seaways may have been low in δ O due to a partly

Herwartz et al. PNAS | 3of5 18 16 ACO2 greenhouse efficiently warmed the early Earth and decreased seawater O/ O https://doi.org/10.1073/pnas.2023617118 before the onset of plate tectonics Downloaded by guest on October 2, 2021 Precambrian Phan- atmosphere. The exact level of pCO2 in the Archean most Hadean Archean Proterozoic erozoic Eo Paleo Meso Neo Paleo Meso Neo probably fluctuated quite a bit as it has done for the past billion 40 glaciations years. Large fluctuations, such as the proposed Mesoarchean p δ18 cherts 35 drop in CO2 (5, 13), should induce a significant shift in Osw. cherts carbonates phosphates carbonates 0 Indeed, a respective shift is broadly recorded in the oxygen iso- 30 tope record (Fig. 3), implying that carbonatization and silicifi- 0 cation processes had indeed been large enough to significantly 25 25 18 decrease the δ Osw. 25 0 Even with the moderately low δ18O proposed herein, the kinetics/alteration sw 50 20

50 [‰] temperatures estimated from chemical sediments imply a very 75 25 75 15 warm climate on the early Earth, which is consistent with the rare VSMOW T [°C] T evidence for ice before 3 Ga ago (44). With respect to the vigor- O

50 18 10

δ ously debated faint young Sun paradox (3, 49, 50), this implies generally high concentrations of greenhouse gases for most of the 75 5 Archean. Our study supports the original idea that CO2 and not CH4 counterbalanced the lower luminosity of the faint young Sun 0 (2, 3), at least prior to the stark Mesoarchean drop in pCO2 (5, 13).

-5 The apparently warmer temperatures compared to the Phanero- seawater zoic leads to the question of how pCO2 can be maintained at high -10 levels in a constantly warm climate, where it accelerates CO2 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 weathering, which leads to a decrease in pCO2 and keeps Earth’s Age [Ga] surface in a temperature window that is “just right” (51). R R Fig. 3. Compiled oxygen-isotope record over Earth’s history with suggested Carbon transfer from the A+O+OC reservoirs to the CC and 18 δ Osw evolution. The black curve from Jaffres et al. (21) is constructed the mantle (i.e., to the long-term storage reservoirs) was probably δ18 = through the database for Ocarbonate (calcite gray points; dolomite and inefficient on the early Earth. Continental crust was less abundant, other = gray diamonds) and slightly extended (stippled). Chert data com- and thus the continental reservoir was small. The residence times piled by Knauth (28) (green circles) is extended by data from refs. 22, 25–27, for carbon on early continents was probably shorter than today δ18 and 55 (green dots). Ranges for modern and ancient Ophosphate (45) are due to efficient recycling of thin continental-crust margins and indicated by brownish boxes. The proposed seawater curve is constructed p 18 18 acidic rain (due to high CO2). Sagduction tectonics, proposed for from published δ Osw (43, 46, 47) and Archean δ Osw proposed herein (see 18 the early Earth (39), is associated with high thermal gradients Methods and Results). High δ Osw estimates (56, 57) are not included (SI Appendix). The curve and its blue margin of 2‰ are not based on statistics. leading to efficient CO2 recycling and thus inefficient carbon Arrows indicate warming (red) and cooling (blue) trends expected from the storage in the mantle.

changing size of the RA+O+OC reservoir. Temperature calibrations are an- Modern-style plate tectonics provides a mechanism to accrete δ18 = − ‰ δ18 = − ‰ chored at Osw 5 for cherts and calcite and at Osw 3 for land masses and to thicken continental crust (52), which generates δ18 phosphates. Apparently, too-high Ocarbonate are dolomites that generally topography and subaerial land surfaces. The two negative feed- comprise ∼2‰ higher δ18O compared to calcite. backs for pCO2 are the following: 1) the establishment of stable, long-term storage reservoirs for carbon and 2) an increased supply Δ’17 of weatherable rocks, enhancing continental CO2-weathering meteoric origin (29), and respective O would have been only fluxes. Subaerial land mass is assumed to be small before the slightly elevated. Alternatively, isotopic exchange between iron δ18 δ18 Neoarchean, with the large-scale emergence of continents above phases (with low OFeOx) and silica bands (with high OopalA) R δ18 Δ’17 sea level proposed to occur at 2.5 Ga (53). The A+O+OC carbon decreased Ochert and generated Ochert below the equilib- reservoir drastically decreased in size in the preceding several rium curve (41). Alteration with (meteoric) water resembles a hundred million years, suggesting that carbon was already relo- similar mixing mechanism that has been suggested to lower cated to the long-term storage reservoir on evolving continental Δ’17 – Ochert (25 27). The variable combinations of these processes platforms (38, 40, 52) and that carbon transfer to the mantle be- induce the considerable scatter in the oxygen-isotopic composi- came more efficient within proto-Phanerozoic–type subduction tion of chemical sediments through time (Fig. 3). Collectively, zones. Hence, sharply decreasing pCO could be a consequence of δ18 2 the Archean O record can be rationalized by a combination of a rapid or gradual switch from vertical sagduction–type tectonics δ18 the classic explanations: 1) a moderately low Osw, 2) warm to a horizontal subduction regime (39), generating thick, silica-rich ocean temperatures, and 3) alteration. continental crust from about 3 Ga onwards (52). Although even the most pristine Archean cherts are altered to some extent, their oxygen isotope composition reflects lower δ18Oof Conclusions Archean ocean water as a result of high degrees of carbonatization In this contribution, we propose that the intense carbonatization (8–10, 13) and silicification at >3 Ga ago (11, 12). This hypothesis and silica precipitation due to high CO2 fluxes in the Archean led needs further verification from high-precision triple oxygen isotope 18 17 to lower δ Osw values of seawater without affecting the Δ’ O. analyses of other types of chemical sediments. Eventually, concepts The shift in δ18O is mirrored in the lower δ18O of early Archean “ ” sw to see through diagenesis (48) can reveal the most viable triple chemical sediments. The decreasing intensity of carbonatization oxygen isotope composition of Archean seawater. Analyzing the and silica precipitation around ∼3 Ga shifted the isotope composition triple oxygen isotopic composition of apparently unaltered phos- of the ocean to similar values as seen in the oceans today. On a phates (45) (but see ref. 31) may even put direct constraints both on smaller scale, however, variable CO fluxes in the Proterozoic and Δ’17 2 Archean seawater Osw and precipitation temperatures. Knowl- Phanerozoic also induce variations in FCO2+SiO2 and thus in seawater Δ’17 18 18 edge of the exact Osw would further restrict feasible Archean δ Osw. Therefore, long-term fluctuations in the δ Oofchemical 18 oxygen fluxes including the relative proportion of the CO2-seques- sediments do not only reflect temperature but also changing δ Osw. tration flux. Respective constraints may also provide information on The apparent temperature change estimated for long-term climate the tectonic regime (e.g., sagduction versus subduction, high versus fluctuations (e.g., ref. 54) are thus systematically overestimated. low spreading rates, etc.) that is active at a given time (36–40). From the model presented here, lowering the seawater δ18O Data Availability. All data are included in the article and/or requires high CO2 fluxes, which are most viable for a high-pCO2 supporting information.

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Herwartz et al. PNAS | 5of5 18 16 ACO2 greenhouse efficiently warmed the early Earth and decreased seawater O/ O https://doi.org/10.1073/pnas.2023617118 before the onset of plate tectonics Downloaded by guest on October 2, 2021