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1 Contents 1. Data Tables 2. Sample Descriptions 2.1. Geology and Stratigraphy 2.2 Samples 2.3 Mineralogy 3. δ 18 O w of Archaean seawater 4. Supplementary References 1

2 1. Data Tables Table S1. Geographic coordinates, lithology, selected geochemical characteristics, phosphate extraction results and isotopic composition of phosphate (δ 18 O P ) of studied samples. Sample Name Latitude Longitude Lithology SiO 2 Al 2 O 3 P 2 O 5 Fe 2 O 3 TC TOC Total PO 4 PO 4 (%) a (%) a (%) a (%) a (%) b (%) c extracted by HCl (μmole/g) extracti on (%) d AL03-1A Massive chert < < AL03-2A Silicified tuff < < 0.07 < n/a AL03-3D Black and white banded chert < n/a 1-2 AL03-5B Black and white banded chert < < AL03-11A Carbonaceous chert AL03-12A Carbonaceous chert < AL03-13B Banded chert < 0.01 < < 0.07 < AL03-13D Banded ferruginous chert, < AL03-20A Silicified sandstone with bands of jaspilite < < AL03-25A Banded ferruginous chert < < AL03-26A Hematite BIF < n/a 1-2 AL03-28A Black and white banded chert < < AL03-28C Carbonaceous-ferruginous chert AL03-28F Banded chert < 0.01 < < 0.07 < AL03-28G Black and white banded chert < < AL03-28I Banded ferruginous chert < AL03-29B Silicified tuff, 10 m above gigant lapilli < 0.07 < n/a 1-1 AL03-29C Silicified carbonaceous sandstone n/a 1-1 AL03-30B Silicified tuff, laminated < < n/a 1-2 AL03-88A Jaspilite-hematite BIF < < 0.07 < n/a 1-2 Standard g - - Internal Standard KH 2 PO a XRF data. b TC=Total carbon, analytical uncertainty: ± 0.07 %. c TOC=Total organic carbon, analytical uncertainty: ± 15 % rel. d Phosphate was extracted sequentially from samples using 10 M HNO 3, followed by 6 M HCl. The percentage shown is the fraction of phosphate, extracted by HCl e, f δ 18 O P =oxygen isotope composition of phosphate in per mil ( ) relative to VSMOW (Vienna Standard Mean Ocean Water). Two different 18 O-labeled waters (δ 18 O H2O : -6 and ) were used for the preparation of 10 M HNO 3 and 6 M HCl. No difference in δ 18 O P values (within error) was found between the two 18 O-labeled acid extractant solutions, indicating that incorporation of oxygen from water into PO 4 due to hydrolysis of any condensed or organic phosphate that may have been present 38 was negligible. g δ 18 O P value of internal laboratory standard (KH 2 PO 4 ): 14.2 ± 0.3 (standard deviation, n=5). Abbreviations: s.d.=standard deviation, n/a=not applicable. δ 18 O P ( ) (δ 18 O H2O : -6 ) e s.d. n δ 18 O P ( ) (δ 18 O H2O : ) f Group marked in Fig. 1 2

3 Table S2. δ 18 O P values of inorganic phosphate bound to abiogenic, high-temperature, plume particle iron-oxides from 9 N EPR. Sample name Vent type P (wt %) δ 18 O P (, VSMOW) s.d. n HOBO Unknown 9N Active L-vent marker plate Active Q-vent HOBO tip area #1 (deep) Active Q-vent HOBO tip area #2 (middle) Active Vent chimney wall Inactive n/a 1 a Internal standard KH 2 PO a δ 18 O P value of KH 2 PO 4 internal laboratory standard: 14.2 ± 0.3 (s.d., n=5). Abbreviations: VSMOW=Vienna Standard Mean Ocean Water, s.d.=standard deviation, n/a= not applicable 3

4 Table S3. Oxygen isotope compositions of dissolved phosphate (δ 18 O P ) in 10 M HNO 3 at 21 and 70 ºC. δ 18 O Sample ID Temp. (ºC) a Time δ 18 O H2O (, VSMOW) b P (, VSMOW) c LL days LH days LL days LH days TL-1 d 70 7 hours TH-1 d 70 7 hours TL-2 e hours TH-2 e hours TL-3 e days TH-3 e days TL-4 e days TH-4 e days a Precision of temperature: < ± 1 ºC for 21 ºC, < ± 2 ºC for 70 ºC. b Different 18 O-labeled water was used for the preparation of 10 M HNO 3. c Precision of δ 18 O P based on replicate mass spectrometric analyses of single samples: < ±0.3 (1σ). The starting PO 4 δ 18 O value: 14.2 ± 0.3 (standard deviation, n=5) d Sample volume was decreased from 47 ml to ~10 ml during its evaporation for 7 hours. e After the evaporation (TL-1 and TH-1), remaining samples (~10 ml) were stored in sealed borosilicate glass bottles thus, there was no change in sample volume. Abbreviations: VSMOW=Vienna Standard Mean Ocean Water 4

5 2. Sample Descriptions 2.1. Geology and Stratigraphy. The Swaziland Supergroup of the Barberton Greenstone Belt (BGB) consists of a succession of Archaean supracrustal rocks surrounded and intruded by granitoids (Fig. S1). The BGB comprises a sequence of ultramafic and mafic volcanic rocks (basalt, komatiite), inter-layered with volumetrically minor sedimentary units consisting of cherts, banded iron formations (BIFs) and variably silicified terrigenous and volcaniclastic sediments 16, 18,19,39,40 (Fig. S2). The succession of volcanic and sedimentary rocks provides a record of Earth evolution in the time interval from 3.55 to 3.22 Ga (early to mid-archaean) 16,41, 42. The BGB rocks have experienced low grade, greenschist facies metamorphism 17, Samples. Twenty outcrop samples, collected from main sedimentary units within Hooggenoeg, Kromberg and Mendon Formations of the Onverwacht Group and the Fig Tree Group have been analyzed (Fig. S2, Table S1). Care was taken to minimize the possibility of recent contamination (e.g., lichen P mining). Fractured parts of outcrops were avoided, and powders analyzed in this study were prepared from internal parts of hand specimens. 5

6 Fig. S1. General geologic map of the southwestern part of the Barberton Greenstone Belt (BGB), South Africa and Swaziland (after refs. 16 and 19). Lithostratigraphic criteria have allowed the subdivision of the BGB into Onverwacht, Fig Tree and Moodies Groups. Six formations (Sandspruit, Theespruit, Komati, Hooggenoeg, Kromberg and Mendon) have been recognized within the Onverwacht Group in the southern part of the BGB, south of the Inyoka fault 43. The Onverwacht rocks north of the Inyoka fault have been assigned to the Weltevreden Formation. The stratigraphy of the Onverwacht group is mostly coherent from the Komati Formation upwards. A major fault at the base of the Komati Formation marks a distinct tectonic boundary, and separates the lowermost formations, the Sandspruit and the Theespruit Formations form the rest of the succession. Yellow dots give the sampling localities for which geographic coordinates are given in Supplementary Table S1. 6

7 Fig. S2. Generalized stratigraphy and geochronology of rocks of the upper part of the Onverwacht Group and Fig Tree Group in the southern part of the Barberton Greenstone Belt (BGB) (after ref. 1). The BGB comprises a sequence of volcanic rocks, inter-layered with volumetrically minor sedimentary units consisting of cherts, banded iron formations (BIFs) and variably silicified terrigeneous and volcaniclastic sediments. Labeled rectangles give stratigraphic positions of analyzed samples. 7

8 2.3. Mineralogy. Quartz is the dominating phase in all samples (SiO 2 > 75%, Table S1) except hematite BIF AL03-26A. Hematite is the most common Fe-oxide, but magnetite and goethite are also present. Rare carbonates are typically represented by Fe-rich forms (siderite) that contain minor Mg, Mn and Ca, but impure dolomite, rhodochrosite and calcite are also present. Sericite, developed from feldspars, is a principal Al carrier phase in samples containing altered tuffs. Phosphate minerals are rare in analyzed samples (Table S1). Scarce crystals of apatite, monazite and xenotime were found in six samples, consistent with low P 2 O 5 contents. Relatively large (100 μm) apatite crystals were found in cross-cutting chlorite veins in samples AL03-29B and AL03-29C (Fig. 2a), a clear indication of secondary hydrothermal-metamorphic overprint in these samples. While the environmental characteristics of the Archean ocean stored in phosphates may be erased or modified in samples that are cut by veins, these samples can be used to trace the effect of alteration. Small (< 10 μm) disseminated apatite, monazite and xenotime crystals were typically observed in association with Fe-rich bands of four samples (AL03-3D, AL03-26A, AL03-28C, AL03-28I). This association suggests a genetic link between phosphate and Fe-phases, a relationship that has been documented in modern oceans 23,44 and has also been posited for Archean-Proterozoic oceans 30. It is plausible that dissolved phosphate, REE and Y were effectively scavenged by primary Fe-precipitates. Crystallization of apatite, monazite and xenotime would have occurred in such a case, during subsequent diagenesis or low-grade metamorphism. The isotope effect accompanying crystallization of monazite and xenotime from phosphate initially scavenged by Fe-precipitates has not been studied. The isotope effect accompanying precipitation of phosphate as apatite has been shown to be minor 45. Inclusions of monazite and xenotime in apatites observed in some samples (Fig. 2b) may have been formed during co-precipitation, or these inclusions may have exsolved from the REE- and 8

9 Y-rich apatite during late diagenesis/metamorphism. Petrographic work did not reveal the presence of early diagenetic aluminophosphate minerals 46,47 in analyzed samples, but micron size or smaller crystals would have gone unnoticed. 3. δ 18 O of Archaean Seawater The δ 18 O value of seawater constrains temperatures calculated from δ 18 O values of cherts/silicates, phosphates and carbonates through geologic time. It is a subject of an intense and ongoing debate particularly in the case of ancient Precambrian systems where fewer samples are available for study. Arguments for a constant seawater δ 18 O w value of ~0 throughout Earth s history are based on consideration of reactions and hydrothermal circulation of seawater through basaltic/volcanic crust 12,48,49. Arguments for δ 18 O w values that were initially as low as - 13 in the Archean and increased gradually over time to reach present-day values are made principally on the basis of the Phaenerozoic and Precambrian marine carbonate record and include consideration of modeling of Earth s hydrologic system and chemical weathering 25 as well as variations in ocean depth, pelagic sedimentation and crustal thickness/heat flow 50. In this paper we calculated ocean temperatures using a δ 18 O w value of 0, a value that reflects conclusions of a study made specifically on seafloor rocks from the BGB 24 and one that results in more plausible, above-freezing, ocean temperatures. 9

10 4. Supplementary References 38 Liang, Y. & Blake, R. E. Oxygen isotope composition of phosphate in organic compounds: Isotope effects of extraction methods. Org. Geochem. 37, (2006). 39 Lowe, D. R. & Nocita, B.W. Foreland basin sedimentation in the Mapepe Formation, southern-facies Fig Tree Group. in Geologic Evolution of the Barberton Greenstone Belt, South Africa (eds. Lowe, D. R. & Byerly, G. R.), Geol. Soc. Am. Spec. Paper 329, (1999). 40 Hofmann, A. The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-archaean times. Precambrian Res., 143, (2005). 41 KrÖner, A., Byerly, G. R. & Lowe, D. R. Chronology of early Archaean granitegreenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation. Earth Planet. Sci. Lett. 103, (1991). 42 Kamo, S. L. & Davis, D. W. Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U Pb dating. Tectonics 13, (1994). 43 Viljoen, M. J. & Viljoen, R. P. An introduction to the geology of the Barberton granitegreenstone terrain. Geol. Soc. S. Afr. Spec. Publ. 2, 9 28 (1969). 10

11 44 Poulton, S. W. & Canfield, D. E. Co-diagenesis of iron and phosphorus in hydrothermal sediments from the southern East Pacific Rise: Implications for the evaluation of paleoseawater phosphate concentration. Geochim. Cosmochim. Acta 70, (2006) 45 Liang, Y. & Blake, R. E. Oxygen isotope fractionation between apatite and aqueousphase phosphate: C. Chem. Geol. 238, (2007). 46 Rasmussen, B. Early-diagenetic REE-phosphate minerals (florencite, gorceixite, crandallite, and Xenotime) in marine sandstones: A major sink for oceanic phosphorus. Am. J. Sci. 296, (1996). 47 Rasmussen, B., Buick, R. & Taylor, W. R. Removal of oceanic REE by authigenic precipitation of phosphatic minerals. Earth Planet. Sci. Letters 164, (1998). 48 Muelenbachs, K. The oxygen isotopic composition of the oceans, sediments and the seafloor. Chemical Geology 145, (1998). 49 Gregory, R.T. & Taylor, H.P. An Oxygen Isotope Profile in a Section of Cretaceous Oceanic Crust, Samail Ophiolite,Oman: Evidence for δ 18 O Buffering of the Oceans by Deep (>5 km) Seawater-Hydrothermal Circulation at Mid-Ocean Ridges. J. Geophysical Res.--Solid Earth 86, (1981). 50 Kasting, J. F. et al. Paleoclimates, ocean depth, and the oxygen isotopic composition of seawater. Earth Planet. Sci. Lett. 252, (2006). 11