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Geochimica et Cosmochimica Acta 209 (2017) 216–232 www.elsevier.com/locate/gca
a,b, c a,b a,b Aaron M. Satkoski ⇑, Philip Fralick , Brian L. Beard , Clark M. Johnson
a University of Wisconsin-Madison, Department of Geoscience, 1215 West Dayton Street, Madison, WI 53706, United States b NASA Astrobiology Institute, United States c Department of Geology, Lakehead University, Thunder Bay, ON PB7 5E1, Canada
Received 16 January 2017; accepted in revised form 14 April 2017; Available online 24 April 2017
The chemistry of the oceans in part reﬂects a balance between inputs from the continents and mantle. Traditionally, it has been thought that Archean ocean chemistry was dominated by mantle sources, but recent work has suggested that continental weathering during the Archean provided a much higher ﬂux to the oceans than previously recognized. Here, we present new Rb-Sr and Sm-Nd isotope compositions on carbonate (dolomite and limestone) from the 2.94 Ga Red Lake and 2.80 Ga Steep Rock groups in the Superior Province, Canada to assess the potential impact continental weathering had on ocean chemistry during the Mesoarchean, a time when initiation of modern-style plate tectonics has been proposed to have occurred. The low Rb contents of all carbonate samples suggest that clastic contamination does not aﬀect the Sr isotope compositions. Using O and Sr isotope modeling, we identiﬁed unaltered samples and estimate a 87Sr/86Sr ratio of 0.70173 for seawater at 2.94 Ga and 0.70182 at 2.80 Ga. Strontium isotope compositions from both Red Lake and Steep Rock indicate that seawater was signiﬁcantly more radiogenic than contemporaneous mantle, and suggests that weathering of evolved continental crust was an important input to seawater. Continental weathering likely aﬀected seawater chemistry through uplift of continental lithosphere during the initiation of modern-style plate tectonics at 3.2 Ga, a model that is contrary to those that suggest the Archean continents were small in extent and largely submerged. Initiation of modern-style plate tectonics and associated con- tinental weathering had an important eﬀect on the biosphere, including increased nutrient delivery, as well as creation of eco- logical niches that allowed development of the ﬁrst biologically produced shallow marine redox gradients. Ó 2017 Elsevier Ltd. All rights reserved.
Keywords: Plate tectonics; Archean; Carbonate; Strontium
1. INTRODUCTION evolved continental crust, of substantial thickness such that it was emergent, seems likely to have been tied to orogenesis The interplay between continental crustal evolution and (e.g., Kemp and Hawkesworth, 2003). Multiple lines of evi- the beginning of plate tectonics has been a long-standing dence suggest that modern-style plate tectonics probably question in Earth science (Condie and Pease, 2008, and ref- began to operate by 3.2–3.0 Ga (Shirey and Richardson, erences therein). Stabilization of signiﬁcant quantities of 2011; Næraa et al., 2012; Dhuime et al., 2012, 2015; Satkoski et al., 2013), and if this coincided with stabiliza- tion of evolved, thick continental crust, particularly under conditions of high CO2 levels in the Archean atmosphere ⇑ Corresponding author at: University of Wisconsin-Madison, Department of Geoscience, 1215 West Dayton Street, Madison, WI (e.g., Kasting, 2010), there should have been a marked 53706, United States. increase in continental weathering ﬂuxes to the oceans. Sup- E-mail address: [email protected] (A.M. Satkoski). port for this model is supplied by studies of clastic http://dx.doi.org/10.1016/j.gca.2017.04.024 0016-7037/Ó 2017 Elsevier Ltd. All rights reserved. A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 217 sequences suggesting that weathering conditions through- out the Archean were extreme (Hessler and Lowe, 2006 and references therein). Of the radiogenic isotope systems that respond to conti- nental crustal evolution, only the 87Rb-87Sr system is asso- ciated with an element that has a long residence time in the oceans, relative to ocean mixing timescales (Broecker and Peng, 1982; Hodell et al., 1990), and hence is amenable to tracing using marine chemical sediments. Historically, there is a strongly held notion that Archean ocean chemistry was almost entirely buﬀered by oceanic hydrothermal ﬂuid cir- culation, with little inﬂuence from continental input (e.g., Shields, 2007; Pons et al., 2013). In part this reﬂects the inﬂuence of models that suggest continental crust was lar- gely submerged in the early to middle Archean (e.g., Shields and Veizer, 2002; Flament et al., 2013). In addition, however, Sr isotope studies of carbonates as a proxy for Archean ocean chemistry have been inﬂuential in inferring a large mantle contribution, but these have been hindered by the lack of pristine chemical sedimentary rocks in the geologic record throughout much of the Archean (e.g., Kamber, 2010). The wide scatter in 87Sr/86Sr ratios in Archean carbonates (Shields and Veizer, 2002) may reﬂect issues of siliciclastic contamination, diagenesis, and alter- ation by ﬂuid-rock interaction. This scatter has led to the widespread assumption that the lowest 87Sr/86Sr ratios must be closest to seawater, and yet it can be shown that this assumption can lead to an under-estimate of seawater 87Sr/86Sr ratios if a local hydrothermal component was pre- sent during mineral formation or during post-depositional alteration. For example, using the relatively insoluble min- eral barite, which is likely resistant to post-depositional alteration, Satkoski et al. (2016) showed that the global ocean at 3.2 Ga had a signiﬁcantly more radiogenic Sr iso- tope composition than contemporaneous mantle, suggest- ing that continental weathering aﬀected seawater chemistry at that time. Although such an approach is promising, barite is only sporadically preserved in the Archean rock record, and hence provides limited snapshots of Archean ocean chemistry. In this contribution, we take a new look at the Sr isotope record for Archean carbonates through study of two excep- tionally well-preserved Mesoarchean sequences of 2.94 and 2.80 Ga age at Red Lake and Steep Rock in Superior Pro- vince, Canada. Combining Sr isotope data with REE and Nd isotope data, as well as stable isotope data, we can assess issues of siliciclastic contamination and post- depositional alteration, allowing us to isolate the seawater signals. The goal is to evaluate the potential impact of con- tinental weathering on ocean chemistry in a time period that immediately follows the proposed onset of modern- style plate tectonics at 3.2–3.0 Ga (e.g., Dhuime et al., 2015).
2. GEOLOGICAL BACKGROUND AND SAMPLES Fig. 1. Location maps for Red Lake and Steep Rock samples. (C) The numbered dots (1–3) show sampling sites for Red Lake 2.1. Red Lake carbonate – 2.94 Ga samples. (D) The numbered dots (4–6) show sampling sites for Steep Rock samples. The numbered dots (sampling locations) are The Red Lake carbonate platform is part of the Red linked to speciﬁc samples in Appendix 1. Lake greenstone belt in the Uchi Subprovince of the larger 218 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232
Superior Province, Canada (Tomlinson et al., 1998) Steep Rock carbonate has an age between 2801 and (Fig. 1). The Ball assemblage of the Red Lake greenstone 2780 Ma. We use 2800 Ma to represent the age of the car- belt is composed of maﬁc (pillowed) and felsic extrusive bonate samples analyzed as part of this study. The regional igneous rocks, iron formation, dolomite-chert beds, and metamorphic grade is lower greenschist facies (Wilks and stromatolitic carbonate (Pirie, 1981; Hofmann et al., Misbet, 1985), however, work by Veizer et al. (1982), 1985). The depositional environment for carbonate ranged Riding et al. (2014) and Fralick and Riding (2015) suggests from shallow marine, as highlighted by mounded stromato- that Steep Rock carbonate largely preserves its primary lites associated with ﬂat-pebble breccias, to deeper marine chemistry. Detailed sample information can be found in for massive, slide brecciated and slumped carbonate layers Appendix 1. Based on texture, carbonate samples at Steep associated with iron formation and chert (McIntyre, 2014). Rock are divided into the following categories, (1) crystal The age of the stromatolites is constrained by a rhyolitic fan, (2) void ﬁlling cement and (3) stromatolite. Samples tuﬀ(2940 + 2.4/ 1.7 Ma) below and a 2925 analyzed as part of this study were also studied by Riding À + 3.4/ 2.9 Ma rhyolitic ﬂow above (Corfu and Wallace, et al. (2014) and Fralick and Riding (2015). À 1986). We use 2940 Ma to represent the age of the carbon- ate samples analyzed as part of this study as the sedimen- 3. METHODS tary rocks appear to lie conformably on the volcanic rocks. The regional metamorphic grade ranges from green- 3.1. Rb-Sr isotopes schist to lower amphibolite facies. Stromatolites in the Red Lake area include columnar, stratiform, and mounded mor- Approximately 10–40 mg of powder was spiked with an phologies (Hofmann et al., 1985; McIntyre, 2014), which enriched 87Rb-84Sr tracer solution and dissolved with 4 M suggests a great diversity in energy levels of the environ- HNO3 in clean Teﬂon on a hotplate for a minimum of ments and microbial life at this time. Crystal fans occur 24 h. Strontium was separated from matrix elements Ò as layers interbedded with the stromatolites and as mounds, (including Rb) using Sr Spec resin in 4 M HNO3 (Beard which probably occupied areas further oﬀshore (McIntyre, et al., 2013). Strontium was stripped from the resin using 2014). The crystals grew directly on the seaﬂoor, reaching 2% HNO3. The resin was cleaned prior to use following 20 cm in height, and by analogy with Archean crystal fans using the method of Charlier et al. (2006). Rubidium was at other locations, were probably originally aragonite further separated from matrix elements using cation- (Sumner and Grotzinger, 2000). Detailed sample informa- exchange chromatography (BioRad AG-MP-50 100–200 tion can be found in Appendix 1. mesh) in 2.5 M HCl (Beard et al., 2013). Strontium samples were loaded onto Ta ﬁlaments with H3PO4 and isotopic 2.2. Steep Rock carbonate – 2.80 Ga analyses were made using a dynamic multi-collector routine on a VG Instruments Sector 54 thermal ionization mass The Steep Rock carbonate platform sampled here is part spectrometer (TIMS) at the University of Wisconsin- 88 11 of the larger 2.8 Ga Steep Rock Group of the central Wabi- Madison. Typical Sr ion intensities were 3 10À amps Â goon Subprovince in the Canadian Shield (Riding et al., and the Sr isotope ratios were exponentially normalized 2014; Fralick and Riding, 2015)(Fig 1). The Steep Rock using 86Sr/88Sr = 0.1194; this procedure removes any Group, at Steep Rock, contains a basal conglomerate mass-dependent Sr isotope variations that might exist in deposited in paleo-valleys incised into 3.0 Ga tonalite, the samples. Measurements of NIST SRM-987, E&A Sr 87 86 as well as a 500 m thick carbonate unit (Mosher Carbonate; and EN-1 Sr yielded an average Sr/ Sr ratio of focus of this study) that rests on the basal conglomerate or 0.710270 ± 0.000016 (n = 35, 2-SD), 0.708055 ± 0.000024 directly on tonalitic basement (Fralick et al., 2008). The (n = 8, 2-SD) and 0.709194 ± 0.000013 (n = 8, 2-SD) entire package represents a transgressive sequence that respectively. These uncertainties are comparable to those was deposited primarily as Ca-carbonate, with lesser estimated for initial 87Sr/86Sr ratios of ± 0.00002 based on amounts of dolomite, ankerite, and siderite (Riding et al., Sr and Rb (see below) isotope dilution analyses. Measured 2014). Laterally, the carbonate transitions into deep-water Sr isotope ratios are not normalized to any external stan- iron formation, and therefore the Steep Rock carbonate dard. Total procedural Sr blanks of 107, 15, and 16 pg were represents a platform that was constructed next to a deeper measured, which are negligible relative to samples. portion of the basin. Rubidium samples were loaded onto Ta ﬁlaments with The tonalitic basement underlying the Steep Rock car- H3PO4. Samples were analyzed in static-mode on Faraday bonate has an age of 3001.6 ± 1.7 Ma (Tomlinson et al., cups or using single Daly detector peak-hopping mode on 2003) and pyroclastics of the Dismal Ashrock overlying it a TIMS. Standard NIST SRM-984 (87Rb/85Rb = 0.386 are 2780.4 ± 1.4 Ma (Tomlinson et al., 2003), which repre- ± 0.002) was analyzed by both methods and produced an sents the minimum age of the Steep Rock Group, at Steep average 87Rb/85Rb = 0.3863 ± 0.0042 (n = 8, 2-SD) in static Rock Lake. The youngest detrital zircon found in the basal mode and an average 87Rb/85Rb = 0.3804 ± 0.0020 (n = 9, conglomerate is 2779 ± 22 Ma, requiring the limestone to 2-SD) for single Daly detector peak-hopping. Relative to probably be younger than 2801 Ma. A correlative unit to NIST SRM-984, these measured ratios produce exponential the northeast (Lumby Lake belt) overlies volcanic rocks b factors of -0.0304 and 0.6247, respectively, which were with an age of 2828 ± 1 Ma. Using the age of the overlying applied to the measured 87Rb/85Rb ratios obtained on sam- volcanic rocks and the youngest detrital zircon in the basal ples. Total procedural Rb blanks of 58, 0.61 and 1.49 pg conglomerate, Fralick and Riding (2015) suggest that the were measured, which are negligible relative to samples. A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 219
3.2. Sm-Nd isotopes These uncertainties are comparable to those estimated for initial eNd(T) values at ±0.5 e-units based on Nd and Sm Approximately 500 mg of powdered carbonate was isotope dilution analyses. Measured Nd isotope ratios are spiked with an enriched 149Sm-150Nd tracer solution and not normalized to any external standard. A total procedu- dissolved with 4 M HNO3 in clean Teﬂon on a hotplate ral Nd blank of 20 pg was measured, which is negligible rel- for a minimum of 24 h. The bulk Rare Earth Elements ative to samples. (REEs) were separated from other matrix elements using Samarium was loaded onto single Re ﬁlaments with Si- cation-exchange chromatography (BioRad AG 50W X8 gel and H3PO4 and analyzed by TIMS in static mode as a 200–400 mesh resin) and 2.5 M HCl. The REEs are stripped metal (Sm+) using Faraday cups. Typical ion beams were 12 149 147 from the resin using 6 M HCl. The REEs were then sepa- 5 10À amps. The measured Sm/ Sm ratio was Â 147 152 rated from themselves using a 2-methylactic acid procedure exponentially corrected using Sm/ Sm = 0.5608. A (Johnson and Thompson, 1991). Neodymium was loaded total procedural Sm blank of 6 pg was measured, which is onto single Re ﬁlaments with Si-gel and H3PO4 and ana- negligible relative to samples. lyzed as NdO+ using a dynamic multi-collector routine on a VG Instruments Sector 54 TIMS at the University of 4. RESULTS Wisconsin-Madison. To enhance formation of NdO+ the pressure in the source was raised using an O2 gas bleed. 4.1. Oxygen isotopes Analyses were run with a 144Nd16O+ ion signal of between 11 0.74 and 1 10À amps. Measured data were exponen- Oxygen isotope compositions were determined on the Â 146 144 tially corrected using Nd/ Nd = 0.7219. Measurements same powders as used for Rb-Sr and Sm-Nd analyses, of UW AMES I, II, LaJolla and JNdi-1 standards yielded and are reported in McIntyre (2014) and Fralick and 143Nd/144Nd ratios of 0.512135 ± 0.000014 (n = 9, 2-SD), Riding (2015). The Red Lake samples have a range of 0.511968 ± 0.000012 (n = 4, 2-SD), 0.511849 ± 0.000022 d18O values from 14.7‰ to 20.3‰ (Fig. 2c). Dolomite sam- (n = 10, 2-SD), and 0.512106 ± 0.000021 (n = 6, 2-SD). ples have d18O values that range from 14.7‰ to 15.8‰. The The correlation between our measurements of LaJolla Ca-carbonate samples range from 18.0‰ to 20.3‰ and are and JNdi-1 is 1.000507, which lies within the range systematically higher than the dolomite samples. There is (1.000503 ± 15 1-SD) reported by Tanaka et al. (2000). no diﬀerence in d18O values between carbonate morpholo-
18 87 86 Fig. 2. MgO/CaO (A, D), Sr content in ppm (B, E) and d Ocarb (C, F) versus initial Sr/ Sr ratios for the Red Lake and Steep Rock samples, respectively. 220 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 gies. The Steep Rock samples have d18O values that range 2), which suggests the Sm-Nd relationship is not controlled from 19.7‰ to 22.4‰ (Fig. 2f), and there is no systematic by binary mixing (R2 = 0.0), and instead records small vari- diﬀerence in d18O values between carbonate morphologies. ations in initial Sm/Nd ratios and in situ growth.
4.2. Rb-Sr isotopes 4.4. REE + Y contents
Measured 87Rb/86Sr ratios for all samples are very low, REE and Y contents were measured on the same pow- less than 0.017, which results in very small corrections for ders as used for Rb-Sr and Sm-Nd analysis, and are in situ decay, <0.00071 for 87Sr/86Sr ratios. Although there reported in McIntyre (2014) and Fralick and Riding is a broad positive relation between measured 87Rb/86Sr (2015). Total REE contents for Red Lake samples range and 87Sr/86Sr ratios (Appendix 2), the slope of this correla- from 1.8–7.6 ppm and 1.6–19.7 ppm for Steep Rock sam- tion is far greater than that of the age of formation, indicat- ples. Elements with ‘‘SN” refer to those that have been nor- ing that 87Rb/86Sr-87Sr/86Sr relations do not record malized to PAAS (Post Archean Average Shale) values incorporation of a high Rb/Sr component (e.g., clays) at from Taylor and McLennan (1985). All samples have pos- * the time of formation. Data are discussed below in terms itive La anomalies, with Ce/Ce = CeSN/(0.5 LaSN + 0.5 87 86 of initial Sr/ Sr ratios, as calculated using the measured PrSN) less than unity; positive Eu anomalies, with Eu/ 87 86 87 86 * Rb/ Sr and Sr/ Sr ratios and age of formation. EuSN = EuSN/(2/3 SmSN + 1/3 TbSN) greater than unity; Initial 87Sr/86Sr ratios for Red Lake samples fall into and Y/Ho ratios greater than crustal rocks. two groups based on mineralogy. Carbonate samples that have high MgO/CaO ratios (>0.1; dolomite), low Sr con- 5. DISCUSSION tents (<50 ppm), and low d18O values (<16‰) have high initial 87Sr/86Sr ratios that range from 0.70273 to 0.70337, 5.1. Assessing primary chemical signatures of carbonate whereas samples that have low MgO/CaO ratios (<0.1; cal- cite), high Sr contents (>200 ppm), and high d18O values The goal of the study is to constrain Archean seawater (>18‰) have signiﬁcantly lower initial 87Sr/86Sr ratios that compositions as an indicator of continental weathering, range from 0.70173 to 0.70198 (Fig. 2a–c). but before this may be done, the eﬀects of clastic contami- Similarly, initial 87Sr/86Sr ratios of Steep Rock samples nation and post-formation alteration must be determined. also vary based on mineralogy and composition (Fig. 2d– f). Samples that have MgO/CaO ratios <0.1 have initial 5.1.1. Clastic contamination 87Sr/86Sr ratios that vary from 0.70305 to 0.70182. Within Clastic contamination potentially aﬀects Sr and Nd iso- the low MgO/CaO samples, initial 87Sr/86Sr ratios vary tope compositions of carbonates, as well as the REE + Y by carbonate morphology. On average, carbonates that contents. Here we used high-precision Rb contents, deter- have a crystal fan morphology have higher Sr contents, mined by isotope dilution, to quantify the extent of clastic consistent with the inferred aragonite precursor of the fans contamination, as Rb contents in carbonate are exclusively (Edwards et al., 2015 and references therein), and less derived from clastic erosion products. Carbonate that is radiogenic initial 87Sr/86Sr ratios than carbonate associated free of clastic material will typically have positive La and with microbial mats. Y anomalies in PAAS-normalized REE + Y patterns (Kamber and Webb, 2001), which may be quantiﬁed using * * 4.3. Sm-Nd isotopes Ce/Ce (Ce/Ce = CeSN/[0.5 LaSN + 0.5 PrSN]) and Y/Ho ratios, respectively. Relative enrichments in La and Y are Red Lake carbonates broadly deﬁne a due to the increased solution stability of these elements, 147Sm/144Nd-143Nd/144Nd errorchron with an apparent similar to the HREE (Nozaki et al., 1997; Alibo and age of 2596 ± 450 Ma and an eNd(2596) of -1.9 (initial Nozaki, 1999). The chondritic ratio for Y/Ho ranges from 143Nd/144Nd = 0.50918 ± 0.00037; MSWD = 15; Appendix 26 to 28 (Kamber and Webb, 2001), which is the range 2), which overlaps the accepted depositional age of these expected for bulk clastic contamination and crustal Ce/ samples (Section 2.1). No signiﬁcant correlation exists on Ce* is expected to be unity. Siliciclastic contamination the same diagram if the samples are separated by MgO/ would therefore be expected to produce a negative correla- CaO ratios (calcite versus dolomite). The samples show tion between Y/Ho and Rb and a positive correlation no correlation (R2 = 0.1) when plotted on a 1/Nd versus between Ce/Ce* and Rb, which is not observed (Fig. 3). 143Nd/144Nd diagram (Appendix 2), which suggests the At the very low Rb contents measured in the samples, sili- Sm-Nd relations are not controlled by binary mixing, but ciclastic contamination is negligible, and this cannot record small diﬀerences in Sm/Nd ratios followed by explain the elevated initial 87Sr/86Sr ratios for some samples in situ growth since formation. (Fig. 3). Based on the trends in REEs and Sr isotopes, we Steep Rock carbonates similarly deﬁne a infer that siliciclastic contamination has not aﬀected the 147Sm/144Nd-143Nd/144Nd errorchron with an apparent Sr and Nd isotope compositions. age of 2918 ± 410 Ma and an eNd(2918) of +2.6 (initial 143Nd/144Nd = 0.50898 ± 0.00029; MSWD = 2.9; Appen- 5.1.2. Fluid-rock alteration dix 2), which overlaps the accepted depositional age of these The high 87Sr/86Sr ratios for a number of carbonate samples (Section 2.2). Samples show no correlation when samples, if not due to clastic contamination (Fig. 3), may plotted on a 1/Nd versus 143Nd/144Nd diagram (Appendix reﬂect mixing with a high-87Sr/86Sr component after depo- A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 221
Fig. 3. LogRb concentration versus (A, D) Y/Ho, (B, E) Ce/Ce* and (C, F) initial 87Sr/86Sr for Red Lake and Steep Rock samples, respectively The arrow highlights the point along the mixing curve that represents 0.25% mixing of a PAAS composition and pristine carbonate. Full details of the modeling are in Appendix 3.
sition, as supported by the 87Rb/86Sr-87Sr/86Sr relations, with the interpretation that the crystal fans were originally which show data that plot far above an appropriate iso- aragonite, which has substantially higher Sr contents than chron relation (Appendix 2). Post-depositional mixing with calcite (Edwards et al., 2015 and references therein), and a high-87Sr/86Sr component would most likely occur hence more likely to retain its original Sr isotope composi- through ﬂuid-rock interaction, which may be tested using tion during ﬂuid-rock interaction. Variations in Sr/Ca O isotope compositions. Using the ﬂuid-rock interaction ratios and d18O values can also identify primary carbonate model of McCulloch et al. (1981), samples that have high compositions (e.g., Edgar et al., 2015), and we interpret the initial 87Sr/86Sr ratios, which also tend to have low d18O Steep Rock samples with the highest Sr/Ca ratios and d18O values, can be explained by ﬂuid-rock interaction over tem- values and lowest 87Sr/86Sr ratios to be closest to seawater peratures ranging from 100 to 300 °C(Fig. 4; Appendix 4). compositions (Appendix 5) and are shown as the least These results demonstrate that none of the dolomites from altered samples on Fig 4b. Red Lake (Fig. 4a) likely represent seawater Sr isotope Although O and Sr isotope compositions of the Red compositions; this is consistent with other studies that note Lake and Steep Rock carbonates have been aﬀected by dolomite may not retain primary compositions (e.g., ﬂuid-rock interaction, including dolomitization, it is unli- Kamber and Webb, 2001; Sena et al., 2014). Based on the kely these processes changed the REE + Y contents or modeling shown in Fig 4a, the crystal fan samples from Nd isotope compositions (e.g., Banner et al., 1988). The Red Lake are closest to seawater. For comparison of the high stability of REEs in carbonate reﬂects their substitu- crystal fans and microbialite from Steep Rock, the broadly tion for the Ca2+ ion in the calcite lattice (Zhong and lower d18O values and higher initial 87Sr/86Sr ratios of the Mucci, 1995) and, short of complete dissolution; the REEs microbialite samples also suggests that the crystal fans are will not be aﬀected by later dolomitization (Webb and closer to seawater compositions (Fig. 4b). This is consistent Kamber, 2000). Recrystallization in the Red Lake and 222 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232
Fig. 4. Plots of d18O versus initial 87Sr/86Sr for (A) Red Lake and (B) Steep Rock samples. The dashed lines are modeled eﬀects of water/rock interaction between an assumed pristine carbonate sample (d18O = 23‰, 87Sr/86Sr = 0.70173 for Red Lake and d18O = 23‰, 87Sr/86Sr = 0.70182 for Steep Rock). Full details of the modeling are in Appendix 4.
Steep Rock carbonates appears to be neomorphic HREE and has a positive La and Y anomaly (Ce/Ce* < 1, (McIntyre, 2014; Fralick and Riding, 2015), and thus the Y/Ho > 27). This LREE depletion has been observed in REE chemistry should be primary. Modeling ﬂuid-rock the 3.45 Ga Strelley Pool carbonates, which suggests that interaction between seawater with a Nd content of the scavenging process has been operating since early in 6 2.8 10À ppm and carbonate with a Nd content 0.1 ppm the Archean (van Kranendonk et al., 2003). In modern sea- Â shows that the initial eNd(T) values only change by 0.01 water, REEs are also input into the oceans through oceanic 18 eNd units over the range of d Ocarb values observed as part hydrothermal ﬂuids, but these are immediately scavenged of this study (Appendix 6). We therefore interpret all car- by Fe-Mn oxyhydroxides and therefore make a negligible bonates studied, including dolomite samples, to record pri- contribution to the overall seawater REE budget. In the mary REE compositions. Archean, however, when the heat ﬂux from the interior of the Earth was greater and seawater contained low oxygen 5.2. The REE + Y characteristics of Archean seawater contents, REE budgets would have been dominated by hydrothermal REE, which would include a characteristic The primary REE input into the modern oceans is positive Eu anomaly and high Sm/Nd ratios (Danielson through continental weathering (Bau and Dulski, 1996). et al., 1992; Bau and Dulski, 1996). Alternatively, for car- Before entering the oceans, however, the LREE are scav- bonates formed in shallow-water, proximal settings where enged in estuaries due to adsorption onto the surfaces of continental input would be greater, we would expect high suspended or sinking particles (Webb and Kamber, 2000). Y/Ho ratios due to estuarine scavenging, decreased or no This scavenging results in a REE + Y pattern (PAAS nor- positive Eu anomalies, and low Sm/Nd ratios (Bau and malized) that is typically LREE depleted relative to the Dulski, 1996). A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 223
The two suites studied here appear to be distinct in terms the Sr isotope composition of the oceans is homogenous, of water mass mixing for the REE + Y. The range in REE even for restricted regions such as the Arctic Ocean + Y contents for the Red Lake carbonates suggests possible (Winter et al., 1997). It has been proposed, however, that mixtures between components (Fig. 5). The positive trend very low Sr contents measured in the 2.5 Ga Campbellrand * * observed between Sm/YbSN and Eu/EuSN (Eu/EuSN =- carbonates of South Africa reﬂect a signiﬁcantly lower sea- EuSN/[2/3 SmSN + 1/3 TbSN]), and negative trend between water Sr residence time in the Neoarchean (Kamber and Sm/YbSN and Y/Ho, suggest that seawater in the Red Lake Webb, 2001). In support of a short residence time for Sr, depositional environment represented a mixture of deep Kamber and Webb (2001) show a correlation (R2 = 0.72) (hydrothermal) and shallow water masses (Fig. 5c and d). between Sr and Nd isotopes in the 2.5 Ga Campbellrand The REE + Y patterns for Steep Rock carbonates, how- carbonates of South Africa. In modern seawater, the resi- ever, suggest a diﬀerent interpretation (Fig. 6). When com- dence time of Nd is very short ( 500 yrs; Tachikawa pared to a Holocene microbialite and carbonate from the et al., 2003), and assuming a similar residence time in the 3.45 Ga Strelley Pool carbonate occurrence, all Steep Rock late Archean, a reasonable assumption because Nd is not samples show similar features (Fig. 6a and b), which a redox-sensitive element, a correlation between Sr and include depleted LREE and positive La (Ce/Ce* < 1) and Nd isotopes could be taken as support for a short residence Y anomalies very similar to those observed for Red Lake time, and hence isotopic provinciality for Sr isotopes. samples. No clear correlations exist, however, between The eNd(T) values for the carbonates studied here vary * Sm/YbSN versus Eu/EuSN (Fig. 6c) and Y/Ho (Fig. 6d), signiﬁcantly, particularly those of the Red Lake suite * suggesting they do not preserve mixing of deep (hydrother- (Appendix 7), and variations in Eu/EuSN, Y/Ho, and Sm/ mal) and shallow water masses. YbSN with Nd isotope compositions for Red Lake carbon- ates suggest mixing relations between a hydrothermal com- 5.3. Evolution of seawater Sr and Nd ponent and a shallow seawater component (Fig. 7). No such relations are seen in the data from Steep Rock In modern seawater, Sr has a residence time of 2.5 m.y., (Appendix 7), consistent with the lack of clear mixing rela- which is much longer than the ocean circulation time of tions among the REEs (Fig. 6), and the limited range in Nd 1500 yrs (Broecker and Peng, 1982), and hence at any time isotope compositions. It is diﬃcult, however, to determine a
Fig. 5. (A) and (B) Normalized REE patterns for samples from Red Lake. For comparison a Holocene carbonate (Webb and Kamber, 2000) and carbonate (crystal fan and stromatolite) from the 3.45 Ga Strelley Pool (van Kranendonk et al., 2003). PAAS is post Archean average * shale from Taylor and McLennan (1985). (C) A plot of Sm/YbSN versus Eu/EuSN. Traditionally, the Eu anomaly is deﬁned using Gd, however when samples have a Gd anomaly the Eu anomaly should be expressed as (Eu/[2/3Sm + 1/3 Tb] (Kamber and Webb, 2001). A * positive correlation between Sm/YbSN and Eu/EuSN could suggest mixing between shallow seawater and oceanic hydrothermal ﬂuids. (D) A plot of Y/Ho versus Sm/YbSN. The observed negative trend can be explained by mixing of oceanic hydrothermal ﬂuids and seawater. 224 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232
Fig. 6. Normalized REE patterns for samples from Steep Rock. For comparison a Holocene carbonate (Webb and Kamber, 2000) and carbonate (crystal fan and stromatolite) from the 3.45 Ga Strelley Pool (van Kranendonk et al., 2003). PAAS is post Archean average shale * from Taylor and McLennan (1985). (C) A plot of Sm/YbSN versus Eu/EuSN. Traditionally, the Eu anomaly is deﬁned using Gd, however when samples have a Gd anomaly the Eu anomaly should be expressed as (Eu/[2/3Sm + 1/3 Tb] (Kamber and Webb, 2001). A positive * correlation between Sm/YbSN and Eu/EuSN could suggest mixing between shallow seawater and oceanic hydrothermal ﬂuids, however, no deﬁnitive correlation exists. (D) A plot of Y/Ho versus Sm/YbSN. No deﬁnitive trend exists, suggesting that these samples do not preserve mixing between shallow seawater and deep oceanic hydrothermal ﬂuids. clear relation between Sr and Nd isotope compositions for heterogeneity in seawater based on the Red Lake and Steep the Red Lake suite, despite the large range in initial Nd iso- Rock suites. We therefore take the inferred initial 87Sr/86Sr tope compositions because, as argued above, none of the ratios for Red Lake and Steep Rock carbonates, once the dolomites are considered to have preserved their primary eﬀects of post-depositional changes are accounted for (iden- Sr isotope compositions. This leaves three samples of Ca- tiﬁed as the least altered samples of Fig 4 and listed in carbonate, and although there is a hint of a correlation of Table 1), to reﬂect ambient seawater in the Mesoarchean. 87 86 decreasing eNd(T) values and initial Sr/ Sr for these sam- ples (Fig. 7a), the range in initial 87Sr/86Sr is quite small. 5.4. Comparison to previously published Sr isotope data It is important to note that the Sr contents of the 2.5 Ga Campbellrand carbonates studied by Kamber and Webb The initial 87Sr/86Sr ratios (0.70173 and 0.70182) (2001) are exceptionally low, 10s of ppm Sr. The average inferred here for seawater at 2.94 and 2.80 Ga are within Sr content for Archean Ca-carbonate is 297 ppm, which is the range reported by Veizer et al. (1989) for 2.8 ± 0.2 Ga close to the average Sr content of the samples from this seawater (0.7025 ± 0.0015). Speciﬁcally, Veizer et al. study (328 ppm; Appendix 8). The high Sr contents from (1989) report 87Sr/86Sr ratios for Steep Rock with a range the majority of Archean carbonates imply that the from 0.70224 to 0.70178, with one sample at 0.70130. Com- 2.52 Ga carbonate studied by Kamber and Webb (2001) pared to the rest of the Archean carbonates (n = 39) pub- may not generally represent the chemistry of the Archean lished by Veizer et al. (1989), however, the least oceans. Whether the Sr residence time during the Archean radiogenic Sr sample (87Sr/86Sr = 0.70130) has a d18O value was truly similar to the REEs is still unknown, but the of 17.6‰, which is much lower than the average d18O value higher Sr contents of the samples measured in this study, measured in the current study. Therefore, the which is comparable to those of average Archean carbon- 87Sr/86Sr = 0.70130 carbonate likely represents a sample ate, suggests that there is no clear evidence for Sr isotope that underwent high-temperature water-rock interaction A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 225
87 86 * Fig. 7. Initial Nd isotope composition for Red Lake samples versus (A) initial Sr/ Sr (B) Sm/YbSN (C) Y/Ho and (D) Eu/EuSN. The arrows represent expected correlations to be observed if shallow seawater mixed with oceanic hydrothermal ﬂuids. 226 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232
Table 1 6.1. Architecture of Archean crust Sr isotope compositions of Archean Reservoirs. Seawater Mantle 100 my 200 my 300 my Preserved Archean continental crust is primarily tona Age (Ga) 87Sr/86Sr 87Sr/86Srb Crustc Crustc Crustc lite-trondhjemite-granodiorite (TTG) in composition (e.g., 3.20 0.70139a 0.70075 0.70168 0.70261 0.70355 Moyen, 2011). Experimental results suggest that tonalite 2.94 0.70173 0.70095 0.70188 0.70281 0.70374 can be formed by directly melting a basaltic source 2.80 0.70182 0.70115 0.70208 0.70301 0.70394 (Foley, 2008), which is consistent with geologic studies using trace element and Hf isotopes (e.g., Satkoski et al., a Seawater isotope composition is from Satkoski et al. (2016). b Mantle isotope composition is from McCulloch (1994). 2013). Archean TTGs are unique in that they are typically c The Sr isotope composition of juvenile upper continental crust depleted in HREE, producing high La/YbCN ratios (‘‘CN” that was extracted from the mantle 100, 200 or 300 million years is chondrite normalized), and show no EuCN anomaly (Eu/ prior to the age reported in the ﬁrst column. The values are Eu* 1; Eu/Eu* = Eu /[pSm * Gd ]) when com- CN CN CN CN CN calculated using mantle values from McCulloch (1994) and the Rb/ pared to Archean granites (e.g., Kleinhanns et al., 2003) Sr ratio of early Archean upper continental crust from Condie and Phanerozoic and Proterozoic felsic igneous rocks (1993). (Moyen, 2011). These geochemical characteristics are ascribed to formation of TTG in the lower crust, at depths and hence does not reﬂect equilibrium with seawater. The > 30–40 km, below the plagioclase stability ﬁeld and in remaining samples from Veizer et al. (1989) have a range equilibrium with residual garnet (Kemp and in Sr and O isotope compositions that are similar to those Hawkesworth, 2003) and possibly amphibole (Rapp et al., reported here (Fig. 2). Of note, the large compilations of 2003), although amphibole does not by itself explain the Shields and Veizer (2002) and Prokoph et al. (2008) dis- very low HREE abundances. Extensive experimental stud- count the majority of Sr isotope compositions from Steep ies show that for maﬁc lithologies, dehydration melting in Rock of Veizer et al. (1989) in that they assume the lowest the lower crust breaks down plagioclase and produces gar- 87Sr/86Sr ratio most closely matches that of seawater, which net at depths of 35–45 km (e.g., Wolf and Wyllie, 1993; Sen is not always true (e.g., Satkoski et al., 2016). Importantly, and Dunn, 1994; Rapp and Watson, 1995), processes that * the seawater Sr isotope curve produced by Shields and can explain high La/YbCN ratios and Eu/EuCN 1. For- Veizer (2002) is not deﬁned by any actual data between mation of TTG at the base of thick basaltic crust is consis- 3.2 and 2.8 Ga, and essentially assumes mantle-like compo- tent with the extensive mantle melting that would be sitions are correct through this time interval. expected from the higher mantle potential temperatures in the Archean, relative to today (Davies, 1992). Using meta- 6. ARCHEAN CRUSTAL EVOLUTION – morphic petrology, a study of ten undisturbed Archean IMPLICATIONS FOR SEAWATER CHEMISTRY granite-greenstone belts indicates they formed in a crust that was, on average, 47 km thick, consistent with the The 87Sr/86Sr ratios inferred for seawater from this above arguments (Galer and Mezger, 1998). study, and that of Satkoski et al. (2016), indicate that sea- A compilation of REE data from Archean TTGs span- water was signiﬁcantly more radiogenic than the mantle ning 3.9–2.5 Ga show no correlation between the La/YbCN 87 86 between 3.2 and 2.8 Ga (Table 1). The Sr/ Sr ratios of nor EuCN anomaly and time (Fig. 8), suggesting that crustal the continental crust during this time depends upon the thickness did not vary signiﬁcantly over the whole of the average 87Rb/86Sr of the crust and average crustal age, Archean. Based on the stability ﬁelds of plagioclase and but assuming crustal ages between 100 and 300 my during garnet, we suggest an average continental crustal thickness this time, the Sr isotope compositions of seawater could between 35 and 45 km throughout the Archean as the best have ranged from 25% to 75% of crustal compositions rel- explanation for the REE contents, in line with previous ative to the mantle (Table 1). These observations require a proposals noted above. Such a conclusion stands in con- re-assessment of an assumed mantle dominance on seawa- trast, however, to recent proposals (Dhuime et al., 2015) ter chemistry in the Archean (e.g., Shields and Veizer, that Archean continental crust was thin, between 15 and 2002; Kamber, 2010; Flament et al., 2013), which has found 25 km, and that temporal changes occurred in crustal thick- support from arguments that increased ridge formation and ness (Fig. 8). The proposal of Dhuime et al. (2015) is based hydrothermal circulation was high in the Archean due to an on scaling changes in Rb/Sr ratios in igneous rocks to an elevated mantle temperature (Kamber, 2015), features that assumed Rb/Sr-thickness relation in the modern crust. may provide eﬃcient modes of heat loss on a silicate plan- Although the increases in Rb/Sr ratios with decreasing etary body (Stern, 2008). What is less understood is the pos- age in the model of Dhuime et al. (2015) is attractive in sibility that continental weathering had a signiﬁcant impact the sense of providing evidence for expected increasing 87 86 on ocean chemistry during the Archean. The question we Sr/ Sr ratios with decreasing age, we feel that the scaling address in this section is that, given our current understand- used is incorrect and inconsistent with the evidence for ing of Archean crustal dynamics, how reasonable is it to thick Archean continental crust noted above. expect that Archean continental crust contributed substan- Thick Archean continental crust, by itself, does not tially to Archean seawater signals, and if this is reasonable, require high-elevation, emergent crust. If, for example, how does this conclusion require re-consideration of the Archean continental crust was relatively weak, high topog- nature of the Archean crust in terms of its thickness, rigid- raphy might not be supported (Rey and Coltice, 2008). ity, and composition. Weaker crust would tend to reduce continental hypsometry, A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 227
tion (e.g., Sclater et al., 1980), and higher rates of heat loss through Archean ocean environments is expected (Burke and Kidd, 1978). In fact, it has been estimated that Archean continental geothermal gradients were no more than 23 ° 1 1 C kmÀ , compared to a modern gradient of 17 °C kmÀ (Burke and Kidd, 1978; Kramers et al., 2001, 2014), even when considering higher concentrations of radionuclides in the crust. Geologic evidence in support of a non- extreme continental geothermal gradient are (1) P-T condi- tions in metamorphic rocks (3.7 Ga-modern; Brown, 2007, 2008) show relatively constant continental geotherms over time, and (2) the preservation of coherent TTG crustal sec- tions that suggest the continental lithosphere was subject to less extreme geothermal gradients than where, for example, komatiite was generated, which likely only reﬂects localized high heat ﬂow (Ernst, 2007).
6.2. Crust generation, weathering, and the start of modern- style plate tectonics
It has been suggested that Archean TTG can form in a subduction zone setting (e.g., Foley et al., 2002), or through melting of hydrous basaltic rocks deep in thickened por- tions of the crust (Smithies, 2000), possibly in oceanic pla- teaus (Zegers and van Keken, 2001). Of note, felsic continental crust can be generated from either of these models, suggesting that felsic continental crust could have existed in the Hadean. The existence of continental-like crust early in Earth history is supported by isotopic data from the Hadean Jack Hills zircon (e.g., Ushikubo et al., 2008). Most authors would agree that Archean tectonics Fig. 8. (A) The thickness of new crust versus age from Dhuime et al. (2015). The curve in (A) is calculated based on observation did not operate in exactly the same manner as modern tec- that the Rb/Sr ratio of igneous rocks in modern-day Central and tonics (see Hawkesworth et al., 2016 for a recent review). South America increases with crustal thickness. (B) Age versus Eu/ Questions have persisted on when the transition to a more * EuCN. On average, Archean TTG does not have a Eu anomaly modern-style of plate tectonics began to operate on Earth compared to post-Archean granitoids, which has been linked to (e.g., Condie and Kroner, 2008), but new research (utiliz- depth of melting (30–40 km). (C) Age versus La/Yb ratio. Many ing, in part, Hf isotopes in zircon) has suggested that Archean TTGs have low Yb contents and high La/Yb ratios, which modern-style plate tectonics began to operate by 3.2 Ga are attributed to melting deep in the crust (30–40 km) in equilib- (Næraa et al., 2012; Satkoski et al., 2013; van rium with garnet. Samples with a green square inside the white Kranendonk and Kirkland, 2016) and deﬁnitely by 3.0 Ga squares represent those TTG samples with normalized Yb values (Shirey and Richardson, 2011; Dhuime et al., 2012). The below 5. The consistently low Yb and high La/Yb ratios of TTGs suggests the presence of thick continental lithosphere throughout start of modern-style plate tectonics would have many the Archean. The blue line represents the highest La/Yb ratio implications for Earth, including the beginning of large- measured from all post-Archean granitoids. TTG data shown here scale orogenic activity (Shirey and Richardson, 2011; van are compiled from Moyen (2011). (For interpretation of the Kranendonk and Kirkland, 2016). This would have been references to color in this ﬁgure legend, the reader is referred to the a time of relatively hot mantle and punctuated continental web version of this article.) growth (van Kranendonk and Kirkland, 2016), both of which would increase the proportion of emergent continen- which is also dependent on the nature of the lithospheric tal crust in response to higher mantle potential temperature mantle (Griﬃn et al., 1998). A higher continental geotherm (Flament et al., 2008). has been invoked as a mechanism to reduce continental Given the evidence discussed above that Archean (4.0– topography, and some studies have argued that early to 2.5 Ga) continental crust was likely thick, and that conti- middle Archean continental crust was largely submerged nental crustal heat ﬂow was moderate, which allows for a (Flament et al., 2013). There are, however, important argu- strong crust, and the fact that felsic continental crust could ments against higher continental geothermal gradients in exist throughout the entire Archean, there must have been a the Archean. Although it is true that overall heat ﬂow from global-scale change in crustal dynamics that could cause a the Earth is expected to have been higher based on greater signiﬁcant amount of continental crust to become emergent radiogenic heat production and a larger heat ﬂux from the by 3.2 Ga as suggested by Satkoski et al. (2016), and con- heat of formation of the Earth, two-thirds of modern heat tinue to be emergent throughout the Archean (this study; ﬂow is lost through oceanic crust and oceanic crust genera- Kamber and Webb, 2001). We suggest this change in crus- 228 A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 tal dynamics was caused by initiation of modern-style plate seawater, which bear on continental weathering and its tectonics (Shirey and Richardson, 2011; Næraa et al., 2012; input to the oceans. Extremely high atmospheric CO2 con- Satkoski et al., 2013; van Kranendonk and Kirkland, 2016). centrations have been estimated for the early Archean to According to Dhuime et al. (2015) the beginning of large- compensate for lower solar luminosity (Kasting, 2010, scale orogenic activity due to the initiation of modern- 2014), which in turn would enhance high weathering inten- style plate tectonics would be marked by an increase of con- sities, as shown in high chemical alteration indices for early tinental detritus into the oceans. Archean clastic rocks (Hessler and Lowe, 2006 and refer- In Fig 9 we integrate temporal variations of crustal vol- ences therein), the presence of extensive ﬁrst-cycle quart- ume, atmospheric CO2, crustal recycling, and Sr isotopes in zites (e.g., 3.2 Ga Moodies Group, Simpson et al., 2012),
Fig. 9. (A) The red curve plots age versus volumes of continental crust as determined by Dhuime et al. (2012). The black curves show the partial pressure of atmospheric CO2 that would be required to maintain a surface temperature of 15 and 0 °C given the diﬀerences in solar luminosity in the Archean (Kasting, 2010). The increased atmospheric CO2 relative to today would likely create a very aggressive global 87 86 weathering regime (Hessler and Lowe, 2006). (B) Age versus eHf (T) from zircon (Belousova et al., 2010). Age versus Sr/ Sr. The green (Red Lake) and yellow (Steep Rock) colored data are the best estimates of the seawater 87Sr/86Sr values at 2.94 and 2.80 Ga. The teal colored datum is from the Campbellrand carbonate (Kamber and Webb, 2001). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) A.M. Satkoski et al. / Geochimica et Cosmochimica Acta 209 (2017) 216–232 229 and compositions of chemical sediments (BIF) that suggest photosynthesizers evolved early in Earth’s history, with evi- deep weathering of the Slave Craton prior to 2.85 Ga dence for the oldest know marine oxygen gradient (and (Haugaard et al., 2016). Crustal growth curves suggest an hence cyanobacteria) at 3.2 Ga (Satkoski et al., 2015), as increase in continental volume from 40–60% relative to well as evidence for transient oxygen increases at 3.0 Ga today during the Paleoarchean (Fig. 9), and this is accom- (Planavsky et al., 2014) and 2.8 Ga (Riding et al., 2014). panied by an increase in crustal recycling, as recorded in Because the evolved continental crust is high in P, enhanced the isotopic compositions of detrital zircons, including weathering of continental crust would produce a high ﬂux marked decreases in eHf values (Fig. 9), as well as increases of P to the oceans (e.g., Ozaki and Tajika, 2013), and one in d18O values (Dhuime et al., 2012). We suggest that the can envision this as a trigger for bio-diversiﬁcation by ﬁrst sign of increased continental weathering ﬂuxes to the 3.2 Ga based on the discussion above. While it may be oceans coincides with this combination of high CO2- speculative that oxygenic photosynthesizers evolved in induced weathering, crustal volume, and uplifted continen- response to the initiation of modern-style plate tectonics tal crust due to the initiation of modern style plate tectonics and deep weathering of evolved, high-relief continental (Fig. 9). Although initiation of modern-style plate tectonics crust, it seems likely that large environmental changes does not necessarily require all of these components, our would have occurred in response to the large changes in interpretation is that the orogenic-related uplifts of crust crustal dynamics that are indicated by the revised Sr isotope that would occur when plate tectonics began is an attractive seawater curve for the Archean. mechanism for producing high weathering ﬂuxes to the oceans, in addition to climate. Perhaps not coincidentally, ACKNOWLEDGMENTS this time period corresponds to the age of the ﬁrst known redox gradient in the shallow marine environments, which We thank Bruno Dhuime, Kent Condie, and an anonymous has been interpreted to mark the emergence of oxygenic reviewer for constructive comments on the manuscript and Mark photosynthesis (Satkoski et al., 2015), and the role of Rehka¨mper for editorial handling and comments. This study was enhanced continental weathering during this time may have funded by the NASA Astrobiology Institute and NSF grant been in the delivery of nutrients to seawater, such as 1523697. PWF is supported by the Natural Sciences and Engineer- phosphorus. ing Research Council of Canada.
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