Initiation of modern-style plate tectonics recorded in … · 2017-05-11 · Initiation of...

Initiation of modern-style plate tectonics recordedin Mesoarchean marine chemical sediments

Aaron M. Satkoski a,b,!, Philip Fralick c, Brian L. Beard a,b, Clark M. Johnson a,b

aUniversity of Wisconsin-Madison, Department of Geoscience, 1215 West Dayton Street, Madison, WI 53706, United StatesbNASA Astrobiology Institute, United States

cDepartment 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

Abstract

The chemistry of the oceans in part reflects a balance between inputs from the continents and mantle. Traditionally, it hasbeen thought that Archean ocean chemistry was dominated by mantle sources, but recent work has suggested that continentalweathering during the Archean provided a much higher flux to the oceans than previously recognized. Here, we present newRb-Sr and Sm-Nd isotope compositions on carbonate (dolomite and limestone) from the 2.94 Ga Red Lake and 2.80 GaSteep Rock groups in the Superior Province, Canada to assess the potential impact continental weathering had on oceanchemistry 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 a!ect 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 at2.94 Ga and 0.70182 at 2.80 Ga. Strontium isotope compositions from both Red Lake and Steep Rock indicate that seawaterwas significantly more radiogenic than contemporaneous mantle, and suggests that weathering of evolved continental crustwas an important input to seawater. Continental weathering likely a!ected seawater chemistry through uplift of continentallithosphere during the initiation of modern-style plate tectonics at 3.2 Ga, a model that is contrary to those that suggest theArchean continents were small in extent and largely submerged. Initiation of modern-style plate tectonics and associated con-tinental weathering had an important e!ect 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.! 2017 Elsevier Ltd. All rights reserved.

Keywords: Plate tectonics; Archean; Carbonate; Strontium

1. INTRODUCTION

The interplay between continental crustal evolution andthe beginning of plate tectonics has been a long-standingquestion in Earth science (Condie and Pease, 2008, and ref-erences therein). Stabilization of significant quantities of

evolved continental crust, of substantial thickness such thatit was emergent, seems likely to have been tied to orogenesis(e.g., Kemp and Hawkesworth, 2003). Multiple lines of evi-dence suggest that modern-style plate tectonics probablybegan to operate by 3.2–3.0 Ga (Shirey and Richardson,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 underconditions of high CO2 levels in the Archean atmosphere(e.g., Kasting, 2010), there should have been a markedincrease in continental weathering fluxes to the oceans. Sup-port for this model is supplied by studies of clastic

http://dx.doi.org/10.1016/j.gca.2017.04.0240016-7037/! 2017 Elsevier Ltd. All rights reserved.

! Corresponding author at: University of Wisconsin-Madison,Department of Geoscience, 1215 West Dayton Street, Madison, WI53706, United States.

E-mail address: [email protected] (A.M. Satkoski).

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Geochimica et Cosmochimica Acta 209 (2017) 216–232

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sequences suggesting that weathering conditions through-out the Archean were extreme (Hessler and Lowe, 2006and 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 theoceans, relative to ocean mixing timescales (Broecker andPeng, 1982; Hodell et al., 1990), and hence is amenable totracing using marine chemical sediments. Historically, thereis a strongly held notion that Archean ocean chemistry wasalmost entirely bu!ered by oceanic hydrothermal fluid cir-culation, with little influence from continental input (e.g.,Shields, 2007; Pons et al., 2013). In part this reflects theinfluence 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 forArchean ocean chemistry have been influential in inferringa large mantle contribution, but these have been hinderedby the lack of pristine chemical sedimentary rocks in thegeologic record throughout much of the Archean (e.g.,Kamber, 2010). The wide scatter in 87Sr/86Sr ratios inArchean carbonates (Shields and Veizer, 2002) may reflectissues of siliciclastic contamination, diagenesis, and alter-ation by fluid-rock interaction. This scatter has led to thewidespread assumption that the lowest 87Sr/86Sr ratios mustbe closest to seawater, and yet it can be shown that thisassumption can lead to an under-estimate of seawater87Sr/86Sr ratios if a local hydrothermal component was pre-sent during mineral formation or during post-depositionalalteration. For example, using the relatively insoluble min-eral barite, which is likely resistant to post-depositionalalteration, Satkoski et al. (2016) showed that the globalocean at 3.2 Ga had a significantly more radiogenic Sr iso-tope composition than contemporaneous mantle, suggest-ing that continental weathering a!ected seawaterchemistry at that time. Although such an approach ispromising, barite is only sporadically preserved in theArchean rock record, and hence provides limited snapshotsof Archean ocean chemistry.

In this contribution, we take a new look at the Sr isotoperecord for Archean carbonates through study of two excep-tionally well-preserved Mesoarchean sequences of 2.94 and2.80 Ga age at Red Lake and Steep Rock in Superior Pro-vince, Canada. Combining Sr isotope data with REE andNd isotope data, as well as stable isotope data, we canassess issues of siliciclastic contamination and post-depositional alteration, allowing us to isolate the seawatersignals. The goal is to evaluate the potential impact of con-tinental weathering on ocean chemistry in a time periodthat 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

2.1. Red Lake carbonate – 2.94 Ga

The Red Lake carbonate platform is part of the RedLake greenstone belt in the Uchi Subprovince of the larger

Fig. 1. Location maps for Red Lake and Steep Rock samples. (C)The numbered dots (1–3) show sampling sites for Red Lakesamples. (D) The numbered dots (4–6) show sampling sites forSteep Rock samples. The numbered dots (sampling locations) arelinked to specific samples in Appendix 1.

A.M. Satkoski et al. /Geochimica et Cosmochimica Acta 209 (2017) 216–232 217

Superior Province, Canada (Tomlinson et al., 1998)(Fig. 1). The Ball assemblage of the Red Lake greenstonebelt is composed of mafic (pillowed) and felsic extrusiveigneous rocks, iron formation, dolomite-chert beds, andstromatolitic carbonate (Pirie, 1981; Hofmann et al.,1985). The depositional environment for carbonate rangedfrom shallow marine, as highlighted by mounded stromato-lites associated with flat-pebble breccias, to deeper marinefor massive, slide brecciated and slumped carbonate layersassociated with iron formation and chert (McIntyre, 2014).The age of the stromatolites is constrained by a rhyolitictu! (2940 + 2.4/!1.7 Ma) below and a 2925+ 3.4/!2.9 Ma rhyolitic flow above (Corfu and Wallace,1986). We use 2940 Ma to represent the age of the carbon-ate samples analyzed as part of this study as the sedimen-tary rocks appear to lie conformably on the volcanicrocks. The regional metamorphic grade ranges from green-schist to lower amphibolite facies. Stromatolites in the RedLake area include columnar, stratiform, and mounded mor-phologies (Hofmann et al., 1985; McIntyre, 2014), whichsuggests a great diversity in energy levels of the environ-ments and microbial life at this time. Crystal fans occuras layers interbedded with the stromatolites and as mounds,which probably occupied areas further o!shore (McIntyre,2014). The crystals grew directly on the seafloor, reaching20 cm in height, and by analogy with Archean crystal fansat other locations, were probably originally aragonite(Sumner and Grotzinger, 2000). Detailed sample informa-tion can be found in Appendix 1.

2.2. Steep Rock carbonate – 2.80 Ga

The Steep Rock carbonate platform sampled here is partof the larger 2.8 Ga Steep Rock Group of the central Wabi-goon Subprovince in the Canadian Shield (Riding et al.,2014; Fralick and Riding, 2015) (Fig 1). The Steep RockGroup, at Steep Rock, contains a basal conglomeratedeposited in paleo-valleys incised into "3.0 Ga tonalite,as well as a 500 m thick carbonate unit (Mosher Carbonate;focus of this study) that rests on the basal conglomerate ordirectly on tonalitic basement (Fralick et al., 2008). Theentire package represents a transgressive sequence thatwas deposited primarily as Ca-carbonate, with lesseramounts of dolomite, ankerite, and siderite (Riding et al.,2014). Laterally, the carbonate transitions into deep-wateriron formation, and therefore the Steep Rock carbonaterepresents a platform that was constructed next to a deeperportion of the basin.

The tonalitic basement underlying the Steep Rock car-bonate has an age of 3001.6 ± 1.7 Ma (Tomlinson et al.,2003) and pyroclastics of the Dismal Ashrock overlying itare 2780.4 ± 1.4 Ma (Tomlinson et al., 2003), which repre-sents the minimum age of the Steep Rock Group, at SteepRock Lake. The youngest detrital zircon found in the basalconglomerate is 2779 ± 22 Ma, requiring the limestone toprobably be younger than 2801 Ma. A correlative unit tothe northeast (Lumby Lake belt) overlies volcanic rockswith an age of 2828 ± 1 Ma. Using the age of the overlyingvolcanic rocks and the youngest detrital zircon in the basalconglomerate, Fralick and Riding (2015) suggest that the

Steep Rock carbonate has an age between 2801 and2780 Ma. We use 2800 Ma to represent the age of the car-bonate samples analyzed as part of this study. The regionalmetamorphic grade is lower greenschist facies (Wilks andMisbet, 1985), however, work by Veizer et al. (1982),Riding et al. (2014) and Fralick and Riding (2015) suggeststhat Steep Rock carbonate largely preserves its primarychemistry. Detailed sample information can be found inAppendix 1. Based on texture, carbonate samples at SteepRock are divided into the following categories, (1) crystalfan, (2) void filling cement and (3) stromatolite. Samplesanalyzed as part of this study were also studied by Ridinget al. (2014) and Fralick and Riding (2015).

3. METHODS

3.1. Rb-Sr isotopes

Approximately 10–40 mg of powder was spiked with anenriched 87Rb-84Sr tracer solution and dissolved with 4 MHNO3 in clean Teflon on a hotplate for a minimum of24 h. Strontium was separated from matrix elements(including Rb) using Sr Spec" resin in 4 M HNO3 (Beardet al., 2013). Strontium was stripped from the resin using2% HNO3. The resin was cleaned prior to use followingusing the method of Charlier et al. (2006). Rubidium wasfurther separated from matrix elements using cation-exchange chromatography (BioRad AG-MP-50 100–200mesh) in 2.5 M HCl (Beard et al., 2013). Strontium sampleswere loaded onto Ta filaments with H3PO4 and isotopicanalyses were made using a dynamic multi-collector routineon a VG Instruments Sector 54 thermal ionization massspectrometer (TIMS) at the University of Wisconsin-Madison. Typical 88Sr ion intensities were 3 # 10!11 ampsand the Sr isotope ratios were exponentially normalizedusing 86Sr/88Sr = 0.1194; this procedure removes anymass-dependent Sr isotope variations that might exist inthe samples. Measurements of NIST SRM-987, E&A Srand EN-1 Sr yielded an average 87Sr/86Sr ratio of0.710270 ± 0.000016 (n = 35, 2-SD), 0.708055 ± 0.000024(n = 8, 2-SD) and 0.709194 ± 0.000013 (n = 8, 2-SD)respectively. These uncertainties are comparable to thoseestimated for initial 87Sr/86Sr ratios of ± 0.00002 based onSr and Rb (see below) isotope dilution analyses. MeasuredSr isotope ratios are not normalized to any external stan-dard. Total procedural Sr blanks of 107, 15, and 16 pg weremeasured, which are negligible relative to samples.

Rubidium samples were loaded onto Ta filaments withH3PO4. Samples were analyzed in static-mode on Faradaycups or using single Daly detector peak-hopping mode ona TIMS. Standard NIST SRM-984 (87Rb/85Rb = 0.386± 0.002) was analyzed by both methods and produced anaverage 87Rb/85Rb = 0.3863 ± 0.0042 (n = 8, 2-SD) in staticmode and an average 87Rb/85Rb = 0.3804 ± 0.0020 (n = 9,2-SD) for single Daly detector peak-hopping. Relative toNIST SRM-984, these measured ratios produce exponentialb factors of -0.0304 and 0.6247, respectively, which wereapplied to the measured 87Rb/85Rb ratios obtained on sam-ples. Total procedural Rb blanks of 58, 0.61 and 1.49 pgwere measured, which are negligible relative to samples.

218 A.M. Satkoski et al. /Geochimica et Cosmochimica Acta 209 (2017) 216–232

3.2. Sm-Nd isotopes

Approximately 500 mg of powdered carbonate wasspiked with an enriched 149Sm-150Nd tracer solution anddissolved with 4 M HNO3 in clean Teflon on a hotplatefor a minimum of 24 h. The bulk Rare Earth Elements(REEs) were separated from other matrix elements usingcation-exchange chromatography (BioRad AG 50W X8200–400 mesh resin) and 2.5 M HCl. The REEs are strippedfrom the resin using 6 M HCl. The REEs were then sepa-rated from themselves using a 2-methylactic acid procedure(Johnson and Thompson, 1991). Neodymium was loadedonto single Re filaments with Si-gel and H3PO4 and ana-lyzed as NdO+ using a dynamic multi-collector routine ona VG Instruments Sector 54 TIMS at the University ofWisconsin-Madison. To enhance formation of NdO+ thepressure in the source was raised using an O2 gas bleed.Analyses were run with a 144Nd16O+ ion signal of between0.74 and 1 # 10!11 amps. Measured data were exponen-tially corrected using 146Nd/144Nd = 0.7219. Measurementsof UW AMES I, II, LaJolla and JNdi-1 standards yielded143Nd/144Nd ratios of 0.512135 ± 0.000014 (n = 9, 2-SD),0.511968 ± 0.000012 (n = 4, 2-SD), 0.511849 ± 0.000022(n = 10, 2-SD), and 0.512106 ± 0.000021 (n = 6, 2-SD).The correlation between our measurements of LaJollaand JNdi-1 is 1.000507, which lies within the range(1.000503 ± 15 1-SD) reported by Tanaka et al. (2000).

These uncertainties are comparable to those estimated forinitial eNd(T) values at ±0.5 e-units based on Nd and Smisotope dilution analyses. Measured Nd isotope ratios arenot normalized to any external standard. A total procedu-ral Nd blank of 20 pg was measured, which is negligible rel-ative to samples.

Samarium was loaded onto single Re filaments with Si-gel and H3PO4 and analyzed by TIMS in static mode as ametal (Sm+) using Faraday cups. Typical ion beams were5 # 10!12 amps. The measured 149Sm/147Sm ratio wasexponentially corrected using 147Sm/152Sm = 0.5608. Atotal procedural Sm blank of 6 pg was measured, which isnegligible relative to samples.

4. RESULTS

4.1. Oxygen isotopes

Oxygen isotope compositions were determined on thesame powders as used for Rb-Sr and Sm-Nd analyses,and are reported in McIntyre (2014) and Fralick andRiding (2015). The Red Lake samples have a range ofd18O values from 14.7‰ to 20.3‰ (Fig. 2c). Dolomite sam-ples have d18O values that range from 14.7‰ to 15.8‰. TheCa-carbonate samples range from 18.0‰ to 20.3‰ and aresystematically higher than the dolomite samples. There isno di!erence in d18O values between carbonate morpholo-

Fig. 2. MgO/CaO (A, D), Sr content in ppm (B, E) and d18Ocarb (C, F) versus initial 87Sr/86Sr ratios for the Red Lake and Steep Rocksamples, respectively.


gies. The Steep Rock samples have d18O values that rangefrom 19.7‰ to 22.4‰ (Fig. 2f), and there is no systematicdi!erence in d18O values between carbonate morphologies.

4.2. Rb-Sr isotopes

Measured 87Rb/86Sr ratios for all samples are very low,less than 0.017, which results in very small corrections forin situ decay, <0.00071 for 87Sr/86Sr ratios. Although thereis a broad positive relation between measured 87Rb/86Srand 87Sr/86Sr ratios (Appendix 2), the slope of this correla-tion is far greater than that of the age of formation, indicat-ing that 87Rb/86Sr-87Sr/86Sr relations do not recordincorporation of a high Rb/Sr component (e.g., clays) atthe time of formation. Data are discussed below in termsof initial 87Sr/86Sr ratios, as calculated using the measured87Rb/86Sr and 87Sr/86Sr ratios and age of formation.

Initial 87Sr/86Sr ratios for Red Lake samples fall intotwo groups based on mineralogy. Carbonate samples thathave high MgO/CaO ratios (>0.1; dolomite), low Sr con-tents (<50 ppm), and low d18O values (<16‰) have highinitial 87Sr/86Sr ratios that range from 0.70273 to 0.70337,whereas samples that have low MgO/CaO ratios (<0.1; cal-cite), high Sr contents (>200 ppm), and high d18O values(>18‰) have significantly lower initial 87Sr/86Sr ratios thatrange from 0.70173 to 0.70198 (Fig. 2a–c).

Similarly, initial 87Sr/86Sr ratios of Steep Rock samplesalso vary based on mineralogy and composition (Fig. 2d–f). Samples that have MgO/CaO ratios <0.1 have initial87Sr/86Sr ratios that vary from 0.70305 to 0.70182. Withinthe low MgO/CaO samples, initial 87Sr/86Sr ratios varyby carbonate morphology. On average, carbonates thathave a crystal fan morphology have higher Sr contents,consistent with the inferred aragonite precursor of the fans(Edwards et al., 2015 and references therein), and lessradiogenic initial 87Sr/86Sr ratios than carbonate associatedwith microbial mats.

4.3. Sm-Nd isotopes

Red Lake carbonates broadly define a147Sm/144Nd-143Nd/144Nd errorchron with an apparentage of 2596 ± 450 Ma and an eNd(2596) of -1.9 (initial143Nd/144Nd = 0.50918 ± 0.00037; MSWD = 15; Appendix2), which overlaps the accepted depositional age of thesesamples (Section 2.1). No significant correlation exists onthe same diagram if the samples are separated by MgO/CaO ratios (calcite versus dolomite). The samples showno correlation (R2 = 0.1) when plotted on a 1/Nd versus143Nd/144Nd diagram (Appendix 2), which suggests theSm-Nd relations are not controlled by binary mixing, butrecord small di!erences in Sm/Nd ratios followed byin situ growth since formation.

Steep Rock carbonates similarly define a147Sm/144Nd-143Nd/144Nd errorchron with an apparentage of 2918 ± 410 Ma and an eNd(2918) of +2.6 (initial143Nd/144Nd = 0.50898 ± 0.00029; MSWD = 2.9; Appen-dix 2), which overlaps the accepted depositional age of thesesamples (Section 2.2). Samples show no correlation whenplotted on a 1/Nd versus 143Nd/144Nd diagram (Appendix

2), which suggests the Sm-Nd relationship is not controlledby binary mixing (R2 = 0.0), and instead records small vari-ations in initial Sm/Nd ratios and in situ growth.

4.4. REE + Y contents

REE and Y contents were measured on the same pow-ders as used for Rb-Sr and Sm-Nd analysis, and arereported in McIntyre (2014) and Fralick and Riding(2015). Total REE contents for Red Lake samples rangefrom 1.8–7.6 ppm and 1.6–19.7 ppm for Steep Rock sam-ples. Elements with ‘‘SN” refer to those that have been nor-malized to PAAS (Post Archean Average Shale) valuesfrom Taylor and McLennan (1985). All samples have pos-itive La anomalies, with Ce/Ce* = CeSN/(0.5 LaSN + 0.5PrSN) less than unity; positive Eu anomalies, with Eu/EuSN* = EuSN/(2/3 SmSN + 1/3 TbSN) greater than unity;and Y/Ho ratios greater than crustal rocks.

5. DISCUSSION

5.1. Assessing primary chemical signatures of carbonate

The goal of the study is to constrain Archean seawatercompositions as an indicator of continental weathering,but before this may be done, the e!ects of clastic contami-nation and post-formation alteration must be determined.

5.1.1. Clastic contaminationClastic contamination potentially a!ects Sr and Nd iso-

tope compositions of carbonates, as well as the REE + Ycontents. Here we used high-precision Rb contents, deter-mined by isotope dilution, to quantify the extent of clasticcontamination, as Rb contents in carbonate are exclusivelyderived from clastic erosion products. Carbonate that isfree of clastic material will typically have positive La andY anomalies in PAAS-normalized REE + Y patterns(Kamber and Webb, 2001), which may be quantified usingCe/Ce* (Ce/Ce* = CeSN/[0.5 LaSN + 0.5 PrSN]) and Y/Horatios, respectively. Relative enrichments in La and Y aredue to the increased solution stability of these elements,similar to the HREE (Nozaki et al., 1997; Alibo andNozaki, 1999). The chondritic ratio for Y/Ho ranges from26 to 28 (Kamber and Webb, 2001), which is the rangeexpected for bulk clastic contamination and crustal Ce/Ce* is expected to be unity. Siliciclastic contaminationwould therefore be expected to produce a negative correla-tion between Y/Ho and Rb and a positive correlationbetween Ce/Ce* and Rb, which is not observed (Fig. 3).At the very low Rb contents measured in the samples, sili-ciclastic contamination is negligible, and this cannotexplain the elevated initial 87Sr/86Sr ratios for some samples(Fig. 3). Based on the trends in REEs and Sr isotopes, weinfer that siliciclastic contamination has not a!ected theSr and Nd isotope compositions.

5.1.2. Fluid-rock alterationThe high 87Sr/86Sr ratios for a number of carbonate

samples, if not due to clastic contamination (Fig. 3), mayreflect mixing with a high-87Sr/86Sr component after depo-


sition, as supported by the 87Rb/86Sr-87Sr/86Sr relations,which show data that plot far above an appropriate iso-chron relation (Appendix 2). Post-depositional mixing witha high-87Sr/86Sr component would most likely occurthrough fluid-rock interaction, which may be tested usingO isotope compositions. Using the fluid-rock interactionmodel of McCulloch et al. (1981), samples that have highinitial 87Sr/86Sr ratios, which also tend to have low d18Ovalues, can be explained by fluid-rock interaction over tem-peratures ranging from 100 to 300 #C (Fig. 4; Appendix 4).These results demonstrate that none of the dolomites fromRed Lake (Fig. 4a) likely represent seawater Sr isotopecompositions; this is consistent with other studies that notedolomite may not retain primary compositions (e.g.,Kamber and Webb, 2001; Sena et al., 2014). Based on themodeling shown in Fig 4a, the crystal fan samples fromRed Lake are closest to seawater. For comparison of thecrystal fans and microbialite from Steep Rock, the broadlylower d18O values and higher initial 87Sr/86Sr ratios of themicrobialite samples also suggests that the crystal fans arecloser to seawater compositions (Fig. 4b). This is consistent

with the interpretation that the crystal fans were originallyaragonite, which has substantially higher Sr contents thancalcite (Edwards et al., 2015 and references therein), andhence more likely to retain its original Sr isotope composi-tion during fluid-rock interaction. Variations in Sr/Caratios and d18O values can also identify primary carbonatecompositions (e.g., Edgar et al., 2015), and we interpret theSteep Rock samples with the highest Sr/Ca ratios and d18Ovalues and lowest 87Sr/86Sr ratios to be closest to seawatercompositions (Appendix 5) and are shown as the leastaltered samples on Fig 4b.

Although O and Sr isotope compositions of the RedLake and Steep Rock carbonates have been a!ected byfluid-rock interaction, including dolomitization, it is unli-kely these processes changed the REE + Y contents orNd isotope compositions (e.g., Banner et al., 1988). Thehigh stability of REEs in carbonate reflects their substitu-tion for the Ca2+ ion in the calcite lattice (Zhong andMucci, 1995) and, short of complete dissolution; the REEswill not be a!ected by later dolomitization (Webb andKamber, 2000). Recrystallization in the Red Lake and

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 pristinecarbonate. Full details of the modeling are in Appendix 3.


Steep Rock carbonates appears to be neomorphic(McIntyre, 2014; Fralick and Riding, 2015), and thus theREE chemistry should be primary. Modeling fluid-rockinteraction between seawater with a Nd content of2.8 # 10!6 ppm and carbonate with a Nd content 0.1 ppmshows that the initial eNd(T) values only change by 0.01eNd units over the range of d

18Ocarb values observed as partof this study (Appendix 6). We therefore interpret all car-bonates studied, including dolomite samples, to record pri-mary REE compositions.

5.2. The REE + Y characteristics of Archean seawater

The primary REE input into the modern oceans isthrough continental weathering (Bau and Dulski, 1996).Before entering the oceans, however, the LREE are scav-enged in estuaries due to adsorption onto the surfaces ofsuspended or sinking particles (Webb and Kamber, 2000).This scavenging results in a REE + Y pattern (PAAS nor-malized) that is typically LREE depleted relative to the

HREE and has a positive La and Y anomaly (Ce/Ce* < 1,Y/Ho > 27). This LREE depletion has been observed inthe 3.45 Ga Strelley Pool carbonates, which suggests thatthe scavenging process has been operating since early inthe Archean (van Kranendonk et al., 2003). In modern sea-water, REEs are also input into the oceans through oceanichydrothermal fluids, but these are immediately scavengedby Fe-Mn oxyhydroxides and therefore make a negligiblecontribution to the overall seawater REE budget. In theArchean, however, when the heat flux from the interior ofthe Earth was greater and seawater contained low oxygencontents, REE budgets would have been dominated byhydrothermal REE, which would include a characteristicpositive Eu anomaly and high Sm/Nd ratios (Danielsonet al., 1992; Bau and Dulski, 1996). Alternatively, for car-bonates formed in shallow-water, proximal settings wherecontinental input would be greater, we would expect highY/Ho ratios due to estuarine scavenging, decreased or nopositive Eu anomalies, and low Sm/Nd ratios (Bau andDulski, 1996).

Fig. 4. Plots of d18O versus initial 87Sr/86Sr for (A) Red Lake and (B) Steep Rock samples. The dashed lines are modeled e!ects of water/rockinteraction 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.


The two suites studied here appear to be distinct in termsof water mass mixing for the REE + Y. The range in REE+ Y contents for the Red Lake carbonates suggests possiblemixtures between components (Fig. 5). The positive trendobserved between Sm/YbSN and Eu/EuSN* (Eu/EuSN* = -EuSN/[2/3 SmSN + 1/3 TbSN]), and negative trend betweenSm/YbSN and Y/Ho, suggest that seawater in the Red Lakedepositional environment represented a mixture of deep(hydrothermal) and shallow water masses (Fig. 5c and d).The REE + Y patterns for Steep Rock carbonates, how-ever, suggest a di!erent interpretation (Fig. 6). When com-pared to a Holocene microbialite and carbonate from the3.45 Ga Strelley Pool carbonate occurrence, all Steep Rocksamples show similar features (Fig. 6a and b), whichinclude depleted LREE and positive La (Ce/Ce* < 1) andY anomalies very similar to those observed for Red Lakesamples. No clear correlations exist, however, betweenSm/YbSN versus Eu/EuSN* (Fig. 6c) and Y/Ho (Fig. 6d),suggesting they do not preserve mixing of deep (hydrother-mal) and shallow water masses.

5.3. Evolution of seawater Sr and Nd

In modern seawater, Sr has a residence time of 2.5 m.y.,which is much longer than the ocean circulation time of1500 yrs (Broecker and Peng, 1982), and hence at any time

the Sr isotope composition of the oceans is homogenous,even for restricted regions such as the Arctic Ocean(Winter et al., 1997). It has been proposed, however, thatvery low Sr contents measured in the 2.5 Ga Campbellrandcarbonates of South Africa reflect a significantly lower sea-water Sr residence time in the Neoarchean (Kamber andWebb, 2001). In support of a short residence time for Sr,Kamber and Webb (2001) show a correlation (R2 = 0.72)between Sr and Nd isotopes in the 2.5 Ga Campbellrandcarbonates of South Africa. In modern seawater, the resi-dence time of Nd is very short ("500 yrs; Tachikawaet al., 2003), and assuming a similar residence time in thelate Archean, a reasonable assumption because Nd is nota redox-sensitive element, a correlation between Sr andNd isotopes could be taken as support for a short residencetime, and hence isotopic provinciality for Sr isotopes.

The eNd(T) values for the carbonates studied here varysignificantly, particularly those of the Red Lake suite(Appendix 7), and variations in Eu/EuSN* , Y/Ho, and Sm/YbSN with Nd isotope compositions for Red Lake carbon-ates suggest mixing relations between a hydrothermal com-ponent and a shallow seawater component (Fig. 7). Nosuch relations are seen in the data from Steep Rock(Appendix 7), consistent with the lack of clear mixing rela-tions among the REEs (Fig. 6), and the limited range in Ndisotope compositions. It is di"cult, 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 averageshale 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). Apositive correlation between Sm/YbSN and Eu/EuSN* could suggest mixing between shallow seawater and oceanic hydrothermal fluids. (D) Aplot of Y/Ho versus Sm/YbSN. The observed negative trend can be explained by mixing of oceanic hydrothermal fluids and seawater.


clear relation between Sr and Nd isotope compositions forthe Red Lake suite, despite the large range in initial Nd iso-tope compositions because, as argued above, none of thedolomites are considered to have preserved their primarySr isotope compositions. This leaves three samples of Ca-carbonate, and although there is a hint of a correlation ofdecreasing eNd(T) values and initial 87Sr/86Sr for these sam-ples (Fig. 7a), the range in initial 87Sr/86Sr is quite small.

It is important to note that the Sr contents of the 2.5 GaCampbellrand carbonates studied by Kamber and Webb(2001) are exceptionally low, 10s of ppm Sr. The averageSr content for Archean Ca-carbonate is 297 ppm, which isclose to the average Sr content of the samples from thisstudy (328 ppm; Appendix 8). The high Sr contents fromthe majority of Archean carbonates imply that the2.52 Ga carbonate studied by Kamber and Webb (2001)may not generally represent the chemistry of the Archeanoceans. Whether the Sr residence time during the Archeanwas truly similar to the REEs is still unknown, but thehigher Sr contents of the samples measured in this study,which is comparable to those of average Archean carbon-ate, suggests that there is no clear evidence for Sr isotope

heterogeneity in seawater based on the Red Lake and SteepRock suites. We therefore take the inferred initial 87Sr/86Srratios for Red Lake and Steep Rock carbonates, once thee!ects of post-depositional changes are accounted for (iden-tified as the least altered samples of Fig 4 and listed inTable 1), to reflect ambient seawater in the Mesoarchean.

5.4. Comparison to previously published Sr isotope data

The initial 87Sr/86Sr ratios (0.70173 and 0.70182)inferred here for seawater at 2.94 and 2.80 Ga are withinthe range reported by Veizer et al. (1989) for 2.8 ± 0.2 Gaseawater (0.7025 ± 0.0015). Specifically, Veizer et al.(1989) report 87Sr/86Sr ratios for Steep Rock with a rangefrom 0.70224 to 0.70178, with one sample at 0.70130. Com-pared to the rest of the Archean carbonates (n = 39) pub-lished by Veizer et al. (1989), however, the leastradiogenic Sr sample (87Sr/86Sr = 0.70130) has a d18O valueof 17.6‰, which is much lower than the average d18O valuemeasured in the current study. Therefore, the87Sr/86Sr = 0.70130 carbonate likely represents a samplethat underwent high-temperature water-rock interaction

Fig. 6. Normalized REE patterns for samples from Steep Rock. For comparison a Holocene carbonate (Webb and Kamber, 2000) andcarbonate (crystal fan and stromatolite) from the 3.45 Ga Strelley Pool (van Kranendonk et al., 2003). PAAS is post Archean average shalefrom Taylor and McLennan (1985). (C) A plot of Sm/YbSN versus Eu/EuSN* . Traditionally, the Eu anomaly is defined using Gd, howeverwhen samples have a Gd anomaly the Eu anomaly should be expressed as (Eu/[2/3Sm + 1/3 Tb] (Kamber and Webb, 2001). A positivecorrelation between Sm/YbSN and Eu/EuSN* could suggest mixing between shallow seawater and oceanic hydrothermal fluids, however, nodefinitive correlation exists. (D) A plot of Y/Ho versus Sm/YbSN. No definitive trend exists, suggesting that these samples do not preservemixing between shallow seawater and deep oceanic hydrothermal fluids.


Fig. 7. Initial Nd isotope composition for Red Lake samples versus (A) initial 87Sr/86Sr (B) Sm/YbSN (C) Y/Ho and (D) Eu/EuSN* . The arrowsrepresent expected correlations to be observed if shallow seawater mixed with oceanic hydrothermal fluids.


and hence does not reflect equilibrium with seawater. Theremaining samples from Veizer et al. (1989) have a rangein Sr and O isotope compositions that are similar to thosereported here (Fig. 2). Of note, the large compilations ofShields and Veizer (2002) and Prokoph et al. (2008) dis-count the majority of Sr isotope compositions from SteepRock of Veizer et al. (1989) in that they assume the lowest87Sr/86Sr ratio most closely matches that of seawater, whichis not always true (e.g., Satkoski et al., 2016). Importantly,the seawater Sr isotope curve produced by Shields andVeizer (2002) is not defined by any actual data between3.2 and 2.8 Ga, and essentially assumes mantle-like compo-sitions are correct through this time interval.

6. ARCHEAN CRUSTAL EVOLUTION –IMPLICATIONS FOR SEAWATER CHEMISTRY

The 87Sr/86Sr ratios inferred for seawater from thisstudy, and that of Satkoski et al. (2016), indicate that sea-water was significantly more radiogenic than the mantlebetween 3.2 and 2.8 Ga (Table 1). The 87Sr/86Sr ratios ofthe continental crust during this time depends upon theaverage 87Rb/86Sr of the crust and average crustal age,but assuming crustal ages between 100 and 300 my duringthis time, the Sr isotope compositions of seawater couldhave ranged from "25% to 75% of crustal compositions rel-ative to the mantle (Table 1). These observations require are-assessment of an assumed mantle dominance on seawa-ter chemistry in the Archean (e.g., Shields and Veizer,2002; Kamber, 2010; Flament et al., 2013), which has foundsupport from arguments that increased ridge formation andhydrothermal circulation was high in the Archean due to anelevated mantle temperature (Kamber, 2015), features thatmay provide e"cient modes of heat loss on a silicate plan-etary body (Stern, 2008). What is less understood is the pos-sibility that continental weathering had a significant impacton ocean chemistry during the Archean. The question weaddress in this section is that, given our current understand-ing of Archean crustal dynamics, how reasonable is it toexpect that Archean continental crust contributed substan-tially to Archean seawater signals, and if this is reasonable,how does this conclusion require re-consideration of thenature of the Archean crust in terms of its thickness, rigid-ity, and composition.

6.1. Architecture of Archean crust

Preserved Archean continental crust is primarily tonalite-trondhjemite-granodiorite (TTG) in composition (e.g.,Moyen, 2011). Experimental results suggest that tonalitecan be formed by directly melting a basaltic source(Foley, 2008), which is consistent with geologic studiesusing trace element and Hf isotopes (e.g., Satkoski et al.,2013). Archean TTGs are unique in that they are typicallydepleted in HREE, producing high La/YbCN ratios (‘‘CN”is chondrite normalized), and show no EuCN anomaly (Eu/EuCN* " 1; Eu/EuCN* = EuCN/[

pSmCN

* GdCN]) when com-pared to Archean granites (e.g., Kleinhanns et al., 2003)and Phanerozoic and Proterozoic felsic igneous rocks(Moyen, 2011). These geochemical characteristics areascribed to formation of TTG in the lower crust, at depths>"30–40 km, below the plagioclase stability field and inequilibrium with residual garnet (Kemp andHawkesworth, 2003) and possibly amphibole (Rapp et al.,2003), although amphibole does not by itself explain thevery low HREE abundances. Extensive experimental stud-ies show that for mafic lithologies, dehydration melting inthe lower crust breaks down plagioclase and produces gar-net at depths of 35–45 km (e.g., Wolf and Wyllie, 1993; Senand Dunn, 1994; Rapp and Watson, 1995), processes thatcan explain high La/YbCN ratios and Eu/EuCN* " 1. For-mation of TTG at the base of thick basaltic crust is consis-tent with the extensive mantle melting that would beexpected from the higher mantle potential temperatures inthe Archean, relative to today (Davies, 1992). Using meta-morphic petrology, a study of ten undisturbed Archeangranite-greenstone belts indicates they formed in a crustthat was, on average, 47 km thick, consistent with theabove arguments (Galer and Mezger, 1998).

A compilation of REE data from Archean TTGs span-ning 3.9–2.5 Ga show no correlation between the La/YbCNnor EuCN anomaly and time (Fig. 8), suggesting that crustalthickness did not vary significantly over the whole of theArchean. Based on the stability fields of plagioclase andgarnet, we suggest an average continental crustal thicknessbetween 35 and 45 km throughout the Archean as the bestexplanation for the REE contents, in line with previousproposals noted above. Such a conclusion stands in con-trast, however, to recent proposals (Dhuime et al., 2015)that Archean continental crust was thin, between 15 and25 km, and that temporal changes occurred in crustal thick-ness (Fig. 8). The proposal of Dhuime et al. (2015) is basedon scaling changes in Rb/Sr ratios in igneous rocks to anassumed Rb/Sr-thickness relation in the modern crust.Although the increases in Rb/Sr ratios with decreasingage in the model of Dhuime et al. (2015) is attractive inthe sense of providing evidence for expected increasing87Sr/86Sr ratios with decreasing age, we feel that the scalingused is incorrect and inconsistent with the evidence forthick Archean continental crust noted above.

Thick Archean continental crust, by itself, does notrequire high-elevation, emergent crust. If, for example,Archean continental crust was relatively weak, high topog-raphy might not be supported (Rey and Coltice, 2008).Weaker crust would tend to reduce continental hypsometry,

Table 1Sr isotope compositions of Archean Reservoirs.

Seawater Mantle 100 my 200 my 300 myAge (Ga) 87Sr/86Sr 87Sr/86Srb Crustc Crustc Crustc

3.20 0.70139a 0.70075 0.70168 0.70261 0.703552.94 0.70173 0.70095 0.70188 0.70281 0.703742.80 0.70182 0.70115 0.70208 0.70301 0.70394

a Seawater isotope composition is from Satkoski et al. (2016).b Mantle isotope composition is from McCulloch (1994).c The Sr isotope composition of juvenile upper continental crust

that was extracted from the mantle 100, 200 or 300 million yearsprior to the age reported in the first column. The values arecalculated using mantle values from McCulloch (1994) and the Rb/Sr ratio of early Archean upper continental crust from Condie(1993).


which is also dependent on the nature of the lithosphericmantle (Gri"n et al., 1998). A higher continental geothermhas been invoked as a mechanism to reduce continentaltopography, and some studies have argued that early tomiddle Archean continental crust was largely submerged(Flament et al., 2013). There are, however, important argu-ments against higher continental geothermal gradients inthe Archean. Although it is true that overall heat flow fromthe Earth is expected to have been higher based on greaterradiogenic heat production and a larger heat flux from theheat of formation of the Earth, two-thirds of modern heatflow is lost through oceanic crust and oceanic crust genera-

tion (e.g., Sclater et al., 1980), and higher rates of heat lossthrough Archean ocean environments is expected (Burkeand Kidd, 1978). In fact, it has been estimated that Archeancontinental geothermal gradients were no more than "23 #C km!1, compared to a modern gradient of "17 #C km!1

(Burke and Kidd, 1978; Kramers et al., 2001, 2014), evenwhen considering higher concentrations of radionuclidesin 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 overtime, and (2) the preservation of coherent TTG crustal sec-tions that suggest the continental lithosphere was subject toless extreme geothermal gradients than where, for example,komatiite was generated, which likely only reflects localizedhigh 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 asubduction zone setting (e.g., Foley et al., 2002), or throughmelting 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, felsiccontinental crust can be generated from either of thesemodels, suggesting that felsic continental crust could haveexisted in the Hadean. The existence of continental-likecrust early in Earth history is supported by isotopic datafrom the Hadean Jack Hills zircon (e.g., Ushikubo et al.,2008). Most authors would agree that Archean tectonicsdid not operate in exactly the same manner as modern tec-tonics (see Hawkesworth et al., 2016 for a recent review).Questions have persisted on when the transition to a moremodern-style of plate tectonics began to operate on Earth(e.g., Condie and Kroner, 2008), but new research (utiliz-ing, in part, Hf isotopes in zircon) has suggested thatmodern-style plate tectonics began to operate by 3.2 Ga(Næraa et al., 2012; Satkoski et al., 2013; vanKranendonk and Kirkland, 2016) and definitely by 3.0 Ga(Shirey and Richardson, 2011; Dhuime et al., 2012). Thestart of modern-style plate tectonics would have manyimplications for Earth, including the beginning of large-scale orogenic activity (Shirey and Richardson, 2011; vanKranendonk and Kirkland, 2016). This would have beena time of relatively hot mantle and punctuated continentalgrowth (van Kranendonk and Kirkland, 2016), both ofwhich would increase the proportion of emergent continen-tal crust in response to higher mantle potential temperature(Flament et al., 2008).

Given the evidence discussed above that Archean (4.0–2.5 Ga) continental crust was likely thick, and that conti-nental crustal heat flow was moderate, which allows for astrong crust, and the fact that felsic continental crust couldexist throughout the entire Archean, there must have been aglobal-scale change in crustal dynamics that could cause asignificant amount of continental crust to become emergentby 3.2 Ga as suggested by Satkoski et al. (2016), and con-tinue to be emergent throughout the Archean (this study;Kamber and Webb, 2001). We suggest this change in crus-

Fig. 8. (A) The thickness of new crust versus age from Dhuimeet al. (2015). The curve in (A) is calculated based on observationthat the Rb/Sr ratio of igneous rocks in modern-day Central andSouth America increases with crustal thickness. (B) Age versus Eu/EuCN* . On average, Archean TTG does not have a Eu anomalycompared to post-Archean granitoids, which has been linked todepth of melting (30–40 km). (C) Age versus La/Yb ratio. ManyArchean TTGs have low Yb contents and high La/Yb ratios, whichare attributed to melting deep in the crust (30–40 km) in equilib-rium with garnet. Samples with a green square inside the whitesquares represent those TTG samples with normalized Yb valuesbelow 5. The consistently low Yb and high La/Yb ratios of TTGssuggests the presence of thick continental lithosphere throughoutthe Archean. The blue line represents the highest La/Yb ratiomeasured from all post-Archean granitoids. TTG data shown hereare compiled from Moyen (2011). (For interpretation of thereferences to color in this figure legend, the reader is referred to theweb version of this article.)


tal dynamics was caused by initiation of modern-style platetectonics (Shirey and Richardson, 2011; Næraa et al., 2012;Satkoski et al., 2013; van Kranendonk and Kirkland, 2016).According to Dhuime et al. (2015) the beginning of large-scale orogenic activity due to the initiation of modern-style plate tectonics would be marked by an increase of con-tinental detritus into the oceans.

In Fig 9 we integrate temporal variations of crustal vol-ume, atmospheric CO2, crustal recycling, and Sr isotopes in

seawater, which bear on continental weathering and itsinput to the oceans. Extremely high atmospheric CO2 con-centrations have been estimated for the early Archean tocompensate for lower solar luminosity (Kasting, 2010,2014), which in turn would enhance high weathering inten-sities, as shown in high chemical alteration indices for earlyArchean clastic rocks (Hessler and Lowe, 2006 and refer-ences therein), the presence of extensive first-cycle quart-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 thepartial pressure of atmospheric CO2 that would be required to maintain a surface temperature of 15 and 0 #C given the di!erences in solarluminosity in the Archean (Kasting, 2010). The increased atmospheric CO2 relative to today would likely create a very aggressive globalweathering regime (Hessler and Lowe, 2006). (B) Age versus eHf (T) from zircon (Belousova et al., 2010). Age versus 87Sr/86Sr. The green (RedLake) 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 datumis from the Campbellrand carbonate (Kamber and Webb, 2001). (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)


and compositions of chemical sediments (BIF) that suggestdeep weathering of the Slave Craton prior to 2.85 Ga(Haugaard et al., 2016). Crustal growth curves suggest anincrease in continental volume from "40–60% relative totoday during the Paleoarchean (Fig. 9), and this is accom-panied by an increase in crustal recycling, as recorded inthe isotopic compositions of detrital zircons, includingmarked decreases in eHf values (Fig. 9), as well as increasesin d18O values (Dhuime et al., 2012). We suggest that thefirst sign of increased continental weathering fluxes to theoceans coincides with this combination of high CO2-induced weathering, crustal volume, and uplifted continen-tal crust due to the initiation of modern style plate tectonics(Fig. 9). Although initiation of modern-style plate tectonicsdoes not necessarily require all of these components, ourinterpretation is that the orogenic-related uplifts of crustthat would occur when plate tectonics began is an attractivemechanism for producing high weathering fluxes to theoceans, in addition to climate. Perhaps not coincidentally,this time period corresponds to the age of the first knownredox gradient in the shallow marine environments, whichhas been interpreted to mark the emergence of oxygenicphotosynthesis (Satkoski et al., 2015), and the role ofenhanced continental weathering during this time may havebeen in the delivery of nutrients to seawater, such asphosphorus.

7. SUMMARY AND CONCLUSIONS

A re-evaluation of the Sr isotope curve for Archean sea-water places important constraints on crustal evolution,changing the commonly held views on the Archean in termsof crustal compositions, thickness, and strength. Althoughsome thermal models have characterized Archean crust asthin and weak, other geochemical data suggest thatMesoarchean continental thickness and strength was moresimilar to that of modern crust. This would indicate thatthe continental lithosphere could support significant topog-raphy; topographic expression, therefore, would be con-trolled by the amount of crust present and the globaluplift rate. During crustal convergence and orogenesis at3.2–3.0 Ga, crust production and uplift to subaerial levelswould be expected to be accelerated. To keep surface tem-perature above freezing during this time, atmospheric CO2

partial pressures needed to be between 0.01 and 0.1 bars(Fig. 9), significantly higher than modern (0.0005 bartoday; Kasting, 2010), which would create conditions fordeep continental weathering, provided the continental crustwas emergent. Considering only Archean continentaldynamics and atmospheric composition it may be expectedthat a significant change in seawater chemistry should beobserved due to large amounts of continental runo! duringcrustal uplift.

The new Sr isotope data presented here, combined withdata from Satkoski et al. (2016) and Kamber and Webb(2001), suggest that seawater chemistry was a!ected by con-tinental erosion starting at 3.2 Ga, which we interpret toreflect the initiation of modern-style plate tectonics, whichcontinued for the rest of the Archean (Fig 9). Perhaps notcoincidentally, evidence continues to emerge that oxygenic

photosynthesizers evolved early in Earth’s history, with evi-dence for the oldest know marine oxygen gradient (andhence cyanobacteria) at 3.2 Ga (Satkoski et al., 2015), aswell as evidence for transient oxygen increases at 3.0 Ga(Planavsky et al., 2014) and 2.8 Ga (Riding et al., 2014).Because the evolved continental crust is high in P, enhancedweathering of continental crust would produce a high fluxof P to the oceans (e.g., Ozaki and Tajika, 2013), and onecan envision this as a trigger for bio-diversification by"3.2 Ga based on the discussion above. While it may bespeculative that oxygenic photosynthesizers evolved inresponse to the initiation of modern-style plate tectonicsand deep weathering of evolved, high-relief continentalcrust, it seems likely that large environmental changeswould have occurred in response to the large changes incrustal dynamics that are indicated by the revised Sr isotopeseawater curve for the Archean.

ACKNOWLEDGMENTS

We thank Bruno Dhuime, Kent Condie, and an anonymousreviewer for constructive comments on the manuscript and MarkRehkamper for editorial handling and comments. This study wasfunded by the NASA Astrobiology Institute and NSF grant1523697. PWF is supported by the Natural Sciences and Engineer-ing Research Council of Canada.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2017.04.024.

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