Archean Seawater Research Articles

Oxygen and silicon isotopic compositions of Archean silicified lava and cherts of the Onverwacht Group: Implication for seafloor hydrothermalism and the nature of recycled components in the source of granitoids

Cherts are commonly used as proxies for Archean environmental conditions such as oceanic temperature. However, because cherts encompass a wide variety of silica-rich rocks, including post-depositional silicified of clastic sediments or chemical precipitates from seawater, reconstructions of the Archean environment based on their O and Si isotopic compositions remain controversial. In this study, we present triple O isotope analyses associated with SIMS O and Si isotope measurements of Palaeoarchean pervasively silicified lavas and clastic sediments, which have been less studied than the associated cherts within the Onverwacht Group (Barberton Greenstone Belt, South Africa). We also provide triple O isotopic compositions of seawater-precipitated cherts. The O and Si isotope composition of silicified clastic sediments and lavas is not considerably variable through the Onverwacht succession. These lithologies display relatively low δ18O (11.3–14.9 ‰) and high Δ’17O values (−0.05 to - 0.07 ‰) attributed to precipitation of silica from hydrothermal fluids at lower temperatures than 200 °C. Using these results, we reconstruct the approximate O isotope composition of the hydrothermal fluids. Water-rock interaction models suggest that this fluid composition is consistent with the existence of low δ18O oceans, but the exact δ18O value of the Palaeoarchean ocean remains elusive. Seawater-precipitated cherts show a mixture of hydrothermal and seawater-like triple O isotope signatures, confirming that cherts are not a straightforward proxy for constraining Archean seawater composition or temperature. The average δ30Si value of the pervasively silicified lavas and clastic sediments is positive (> +0.19 ‰) and compositionally similar to published values of the seawater-precipitated cherts. This suggests comparable δ30Si values between Archean hydrothermal fluids and seawater. A gradual increase of δ18O values of silicified lavas and cherts from 10 to 13 ‰ to 15–20 ‰ observed along the Onverwacht succession is ascribed to local lithospheric cooling occurring between 3.47 Ga and 3.3 Ga, while Si isotope compositions remained mostly unchanged in the silica-rich rocks. As silicified lavas are likely part of materials that melted to generate Palaeoarchean granitoids, the evolutional difference between O and Si isotopes may have characterised the silicified rocks that were recycled in different generations of granitoid sources. This may explain the variation in O isotopic compositions measured in different granitoid generations in the Barberton area.

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Using a novel approach to characterize the surface reactivities of silica-rich ferrihydrite and biogenic cyanobacteria-ferrihydrite aggregates and the implications for Archean ocean geochemistry

Precambrian banded iron formations (BIF) are iron- and silica-rich chemical sedimentary rocks that are commonly used as paleo-redox proxies for Archean and Paleoproterozoic seawater geochemistry. At the onset of the Great Oxidation Event (herein GOE) around 2.4 Ga, cyanobacteria flourished with increasing nutrient fluxes due to oxidative weathering on land. In turn, this led to increased primary productivity that facilitated the permanent shift from a reducing Earth atmosphere to an oxidizing one. Interestingly, the duration of GOE also overlapped with one of the most prolific periods of BIF deposition.It is widely accepted that cyanobacteria were likely responsible for BIF formation during the GOE. Oxidation of dissolved Fe(II) by oxygen produced from cyanobacteria forms a metastable and amorphous mineral phase ferrihydrite, Fe(OH)3. As an essential component in both ancient BIF deposits and various modern ecosystems, the surface reactivity of ferrihydrite has been extensively studied under different conditions (i.e., pH and ionic strengths). Not only are the highly reactive surfaces of ferrihydrite particles important shuttles for trace element transport from the water column to the sediment pile, but previous studies have also demonstrated that cyanobacterial cells and ferrihydrite tend to aggregate at seawater pH. This means that ferrihydrite was also a vector for the transport of organic carbon to the seafloor. However, a complicating issue is how co-ions affect the surface reactivity of ferrihydrite, specifically dissolved silica which was abundant in ancient seawater. Although previous studies have demonstrated that silica can passivate the surface reactivity of ferrihydrite, what remains unclear is how silica impacts ferrihydrite-biomass aggregation. To fill this knowledge gap, we formed both silica-spiked ferrihydrite and cyanobacteria-ferrihydrite aggregates in situ and subsequently conducted empirical potentiometric acid-base titrations and Cd adsorption experiments on the fresh aggregate samples at three different ionic strengths (0.56 M, 0.1 M and 0.01 M). We minimized sample processing (i.e., drying and powdering) to a simple washing step, in which the aggregate pellets remained hydrated to avoid any mineral transformation thus altering their true surface reactivity in seawater. Experimental results were then fitted with non-electrostatic model to predict both surface charges and metal-adsorption behavior of ferrihydrite aggregates. Different from previous surface-complexation modelling studies, here we used a novel and more powerful modelling program called Phreefit. It utilizes the global optimization algorithms instead of more commonly used Newton-Raphson method in FITEQL program, which is often too limited for precisely modelling complex systems such as the two samples in this study. Furthermore, we also measured the surface charges of both samples over the pH range from 3 to 9 on a Malvern Zetasizer and characterized the surface functional groups through Fourier-Transform Infrared Spectroscopy to help with our interpretation of the experimental data.Preliminary results show that cyanobacteria-ferrihydrite aggregates formed primarily due to ionic bridging. Cyanobacterial cells likely facilitated the precipitation of dissolved silica. Findings from titration and Cd adsorption experiments indicate that the surface reactivity and capacity of both silica-rich ferrihydrite cyanobacteria-ferrihydrite aggregates to adsorb trace elements differ from their individual components, likely due to site blockage. This distinction is particularly prominent when considering the expected Archean seawater pH from 6 to 8. This disparity implies that the biogenic ferrihydrite aggregates do not exhibit an additive surface reactivity, which is in agreement with similar previous studies. Our combined results are crucial to accurately predict the adsorption of trace elements onto the aggregate surface and, ultimately, comprehend the archive of trace elements in sedimentary rocks used to reconstruct Precambrian ocean chemistry.

A first look at the gallium-aluminium systematics of Early Earth’s seawater: Evidence from Neoarchean banded iron formation

Geochemical proxies archived in banded iron formations (BIFs) can provide information on the physico-chemical conditions in the Precambrian atmosphere-hydrosphere system and the element sources to ambient Early Earth’s seawater. We here report results of the first investigation of the Ga-Al systematics of BIFs and use adjacent magnetite and metachert bands of the well-studied and very pure 2.7 Ga-old Temagami BIF in Ontario, Canada, in an attempt to test the suitability of the Ga/Al ratio as a geochemical proxy. Due to very low Ga concentrations in BIFs and multiple mass interferences on the stable isotopes 69Ga and 71Ga, we conducted comparative analyses of dissolved samples with sector-field (SF-)ICP-MS and tandem ICP-MS/MS as well as laser-ablation (LA-)SF-ICP-MS on nano-particulate pressed powder tablets (NP tablets). The unexpected results indicate uniform Ga/Al weight ratios between 2.0×10−4 and 7×10−4 for the Temagami BIF, i.e. Ga/Al ratios that are slightly above those of potential clastic detritus but well below those of modern seawater or marine hydrothermal fluids. Consistent results from solution SF- and ICP-MS/MS and a two-component mixing experiment with BIF reference material IF-G and synthetic pure quartz sand corroborate the high analytical quality of these Ga and Al data. The Ga/Al ratios of the metachert bands show the same range of Ga/Al ratios as the magnetite bands and appear to be dominated by small amounts of finely dispersed Fe oxide particles that were deposited together with the chert. The Ga/Al ratios of the magnetite bands either suggest that (in contrast to other trace elements such as the rare earth elements and Y) Ga and Al were fractionated from each other during scavenging from Archean seawater by Fe (oxyhydr)oxides or that ambient Temagami seawater had significantly lower Ga/Al ratios than modern seawater, possibly due to different weathering, riverine, and estuarine processes in the Archean. While these first results demonstrate the potential of Ga-Al systematics as an additional geochemical tool for studies of Early Earth, they also reveal the knowledge gaps that still need to be addressed in future investigations.

Interstitial carbonates in pillowed metabasaltic rocks from the Pilbara Craton, Western Australia: A vestige of Archean seawater chemistry and seawater-rock interactions

Calcites hosted in the interpillow void spaces of extremely well preserved, 3.47–3.12 Ga pillow lavas of the Archean Pilbara Craton, Australia, provide new geochemical insights into the composition of Archean seawater and its interaction with basaltic crust. We present a comprehensive dataset of major and trace elements, radiogenic 147Sm-143Nd, 87Rb-87Sr, and stable C-O isotopes for these calcites.Based on their elemental composition and Post-Archean Australian Shale-normalized rare earth element and yttrium (REYPAAS) patterns, two types of calcites can be distinguished. Type I (n = 9) are coarse-grained calcites with elevated Mn concentrations (478–14,790 ppm) and high REY concentrations that match the basaltic host rock compositions. Petrographic textures in combination with geochemical data indicate precipitation from boiling seawater trapped between pillows during basalt eruption. Small light-REYPAAS depletions, only slightly super-chondritic Y/Ho (34.5–39.1), and low δ18O(VSMOW2) (9.25–16.66 ‰) suggest relatively high fluid-rock interactions of boiling seawater with the basaltic host rock. These characteristics are similar to those of interstitial carbonates found in modern mid-ocean ridge basalts. Type II carbonates (n = 6) are fine-grained, low-Mn (13.7–420 ppm) calcites that exhibit comparably high Sr concentrations (825–2,516 ppm) and anoxic modern seawater-like REYPAAS patterns, strong super-chondritic Y/Ho (39.1–55.8) and δ18O(VSMOW2) values of 9.10–13.18 ‰. These features hint to direct calcite precipitation from seawater within the degassing space with little fluid-rock interaction.Almost all (Type I and Type II) of the calcites investigated here, combined with their respective host rocks, yield well-defined Sm-Nd isochron ages for the Apex (3,466 ± 23 Ma), Euro (3,365 ± 43 Ma), Honeyeater (3,187 ± 33 Ma), and Bradley (3,131 ± 24 Ma) formations. Only calcites from the Mt. Ada Basalt and their associated host rocks show a ∼ 100 Ma younger Sm-Nd isochron age (3,361 ± 22 Ma) compared to the U-Pb zircon age of formation.Highly radiogenic initial Sr isotope compositions of the high-Mn calcites (Type I) and low Sr concentrations, suggest Sr remobilization for these calcites. In contrast, the low-Mn calcite (Type II) exhibit higher Sr concentrations and lower 87Sr/86Sr(i) of 0.7010–0.7011 at 3.34 Ga and 0.70137 at 3.12 Ga, suggesting increasing influence of crustal weathering on the composition of Paleoarchean seawater through time, and progressive decoupling from the Archean mantle (87Sr/86Sr(i) = 0.7003–0.7007).

Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe- and Si-rich chemical sedimentary rocks formed have remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated.In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral.The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume-driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.

Fe-Si-C isotope constraints on the genesis of iron ores in Gongchangling iron deposit in northeastern China

The Gongchangling iron deposit is an oxide-facies Algoma-type banded iron formation (BIF) and one of the most representative iron deposits in northeastern China. We present new data from petrological, whole-rock geochemical, and Fe-Si-O isotope compositional data from the Gongchangling BIFs to constrain the origin of ore-forming materials and mineralization processes. The low Al2O3 and TiO2 contents in iron ores indicate that the sources of the BIFs were not contaminated by terrigenous clastic sediments. The Gongchangling BIFs are characterized by depletions in LREEs and 30Si and enrichments in HREEs, with positive Eu and Y anomalies. Given these and the Fe isotope results, we conclude that both high-temperature hydrothermal fluid and seawater contributed to the formation of the BIFs and that a mixture of seawater and ~ 0.1 % hydrothermal fluid, which percolated and dissolved the submarine volcanic rocks, provided the ore-forming materials. Combining the Ce anomalies, enrichments in heavy Fe isotopes and low δ13C compositions, we conclude that the fO2 of Archean seawater was limited. Integrated petrographic and geochemical evidence indicates that the high-grade iron ores from the Gongchangling BIFs experienced incomplete oxidation and low amounts of Fe precipitation in a marine environment and that dissimilatory iron reduction and metamorphic hydrothermal fluid played important roles.

Triple silicon isotope insights into the formation of Precambrian cherts

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.

Low-phosphorus concentrations and important ferric hydroxide scavenging in Archean seawater.

The availability of nutrients in seawater, such as dissolved phosphorus (P), is thought to have regulated the evolution and activity of microbial life in Earth's early oceans. Marine concentrations of bioavailable phosphorus spanning the Archean Eon remain a topic of debate, with variable estimates indicating either low (0.04 to 0.13 μM P) or high (10 to 100 μM P) dissolved P in seawater. The large uncertainty on these estimates reflects in part a lack of clear proxy signals recorded in sedimentary rocks. Contrary to some recent views, we show here that iron formations (IFs) are reliable recorders of past phosphorus concentrations and preserved a primary seawater signature. Using measured P and iron (Fe) contents in Neoarchean IF from Carajás (Brazil), we demonstrate for the first time a clear partitioning coefficient relationship in the P-Fe systematics of this IF, which, in combination with experimental and Archean literature data, permits us to constrain Archean seawater to a mean value of 0.063 ± 0.05 μM dissolved phosphorus. Our data set suggests that low-phosphorus conditions prevailed throughout the first half of Earth's history, likely as the result of limited continental emergence and marine P removal by iron oxyhydroxide precipitation, supporting prior suggestions that changes in ancient marine P availability at the end of the Archean modulated marine productivity, and ultimately, the redox state of Earth's early oceans and atmosphere. Classification: Physical Sciences, Earth, Atmospheric and Planetary Sciences.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Exploring the secondary mineral products generated by microbial iron respiration in Archean ocean simulations.

Marine chemical sedimentary deposits known as Banded Iron Formations (BIFs) archive Archean ocean chemistry and, potentially, signs of ancient microbial life. BIFs contain a diversity of iron- and silica-rich minerals in disequilibrium, and thus many interpretations of these phases suggest they formed secondarily during early diagenetic processes. One such hypothesis posits that the early diagenetic microbial respiration of primary iron(III) oxides in BIFs resulted in the formation of other iron phases, including the iron-rich silicates, carbonates, and magnetite common in BIF assemblages. Here, we simulated this proposed pathway in laboratory incubations combining a model dissimilatory iron-reducing (DIR) bacterium, Shewanella putrefaciens CN32, and the ferric oxyhydroxide mineral ferrihydrite under conditions mimicking the predicted Archean seawater geochemistry. We assessed the impact of dissolved silica, calcium, and magnesium on the bioreduced precipitates. After harvesting the solid products from these experiments, we analyzed the reduced mineral phases using Raman spectroscopy, electron microscopy, powder x-ray diffraction, and spectrophotometric techniques to identify mineral precipitates and track the bulk distributions of Fe(II) and Fe(III). These techniques detected a diverse range of calcium carbonate morphologies and polymorphism in incubations with calcium, as well as secondary ferric oxide phases like goethite in silica-free experiments. We also identified aggregates of curling, iron- and silica-rich amorphous precipitates in all incubations amended with silica. Although ferric oxides persist even in our electron acceptor-limited incubations, our observations indicate that microbial iron reduction of ferrihydrite is a viable pathway for the formation of early iron silicate phases. This finding allows us to draw parallels between our experimental proto-silicates and the recently characterized iron silicate nanoinclusions in BIF chert deposits, suggesting that early iron silicates could possibly be signatures of iron-reducing metabolisms on early Earth.

Questioning the Paradigm of a Phosphate‐Limited Archean Biosphere

AbstractThe nature of Archean life remains one of the most contested topics in the study of Earth history. The debate may be formulated as follows: When did present day metabolisms emerge to ecological significance? What limited the productivity of early biospheres? How did the existence and productivity of individual metabolisms affect the chemistry and oxidation state of the ocean‐atmosphere? In a new study, Ingalls et al. (2022, https://doi.org/10.1029/2022GL098100) apply a novel proxy, carbonate‐associated phosphate (CAP), to Neoarchean carbonate rocks and argue that seawater in the Neoarchean was more phosphate‐rich than through Phanerozoic time. Although the interpretation of CAP signals is currently burdened by uncertainties regarding Archean seawater chemistry, their results suggest Archean phosphate levels were comparable to modern seawater, if not higher. If true, then Earth's most successful metabolism, oxygenic photosynthesis, had either not achieved ecological prominence to exploit the relatively phosphate‐rich waters, or was curtailed by other under‐appreciated mechanisms.

Marine phosphate availability and the chemical origins of life on Earth

Prebiotic systems chemistry suggests that high phosphate concentrations were necessary to synthesise molecular building blocks and sustain primitive cellular systems. However, current understanding of mineral solubility predicts negligible phosphate concentrations for most natural waters, yet the role of Fe2+, ubiquitous on early Earth, is poorly quantified. Here we determine the solubility of Fe(II)-phosphate in synthetic seawater as a function of pH and ionic strength, integrate these observations into a thermodynamic model that predicts phosphate concentrations across a range of aquatic conditions, and validate these predictions against modern anoxic sediment pore waters. Experiments and models show that Fe2+ significantly increases the solubility of all phosphate minerals in anoxic systems, suggesting that Hadean and Archean seawater featured phosphate concentrations ~103–104 times higher than currently estimated. This suggests that seawater readily met the phosphorus requirements of emergent cellular systems and early microbial life, perhaps fueling primary production during the advent of oxygenic photosynthesis.

Oxygen isotope insights into the Archean ocean and atmosphere

Accurately reconstructing the temperature of the ocean through time carries implications for Earth's climate and early habitability. Attempts to build these reconstructions using oxygen isotope records have led to three end-member interpretations. Namely, that the observed enrichment over time in 18O relative to 16O in ancient chemical sediments reflects a change in sea-surface temperatures (SST), a change in the 18O composition of the contemporaneous water, or that the primary signal has been subsequently overprinted. These questions become most salient in the Archean, where estimates of the isotopic composition of the ocean span ∼20‰, with a correspondingly wide range in estimated SSTs. Here, we introduce barite (BaSO4) as a robust new proxy for the oxygen isotope composition of the Archean ocean. We compile new and existing triple oxygen isotope and sulfur isotope data from the Fig Tree Group barite deposits in the Barberton Greenstone Belt, South Africa, with the goal of identifying the primary sources of sulfate to the Archean ocean. Using a simple Monte Carlo approach, we then constrain the possible isotopic composition of contemporaneous seawater. Our results suggest that microbial sulfur cycling played a limited role in setting the isotopic composition of Archean seawater sulfate. Additionally, it is likely that a significant flux of sulfate to the marine reservoir was atmospherically derived and carried a significant positive triple oxygen isotope signal. Importantly, our results support an Archean ocean with a somewhat enriched oxygen isotope composition (∼0-5‰), with the exact composition dependent on the relative contribution from each sulfate production pathway. This result points either to the decreased significance of low-temperature weathering and/or to elevated SSTs in the early Archean.

Earth’s geodynamic evolution constrained by 182W in Archean seawater

Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth. Applied to ancient marine chemical sediments such as banded iron formations, the short-lived 182Hf-182W isotope system can serve as key instrument to decipher Earth’s geodynamic evolution. Here we show high-precision 182W isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct 182W differences in alternating Si-rich (7.9 ppm enrichment) and Fe-rich (5.3 ppm enrichment) bands reflecting variable flux of W from continental and hydrothermal mantle sources into ambient seawater, respectively. Greater 182W excesses in Si-rich layers relative to associated shales (5.9 ppm enrichment), representing regional upper continental crust composition, suggest that the Si-rich bands record the global rather than the local seawater 182W signature. The distinct intra-band differences highlight the potential of 182W isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time.

A CO2 greenhouse efficiently warmed the early Earth and decreased seawater 18O/16O before the onset of plate tectonics

The low 18O/16O stable isotope ratios (δ18O) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower δ18Osw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ'17O) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ18Osw of seawater on the early Earth without affecting the Δ'17O. Hence, the controversial long-term trend of increasing δ18O in chemical sediments over Earth's history partly reflects increasing δ18Osw due to decreasing atmospheric pCO2 We suggest that δ18Osw increased from about -5‰ at 3.2 Ga to a new steady-state value close to -2‰ at 2.6 Ga, coinciding with a profound drop in pCO2 that has been suggested for this time interval. Using the moderately low δ18Osw values, a warm but not hot climate can be inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the "faint young Sun" was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga.

Ferric iron triggers greenalite formation in simulated Archean seawater

AbstractSedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

Triple oxygen isotopes of cherts through time

It has long been recognized that the stable oxygen isotope 18O/16O ratios of chemical sediments show a systematic decrease with increasing age. This continuous, 3.8 Ga long secular trend has been attributed to 80 °C hot early Earth oceans, a ~15‰ lower 18O/16O ratio of Archean seawater, or diagenetic obliteration of the original isotope signal. We demonstrate that high-precision triple oxygen isotope measurements including the rare third stable oxygen isotope 17O on silica rocks (cherts) can be used to shed light on this major problem in geochemistry. Our triple oxygen isotope data do not support 80 °C hot oceans in the Archean. The data also exclude that the 18O/16O of seawater was much lower in the Archean. Also, a simple freshwater-origin of cherts is not supported by the new data. We discuss the effects of diagenetic alteration on the triple oxygen isotope compositions of cherts and show that the Archean and Proterozoic cherts could have equilibrated with fluids with higher 18O/16O than today's oceans. For the Archean cherts, the high 18O/16O fluids required to explain the observations could reflect the composition of ambient seawater or of modified hydrothermal fluids. Oceans with higher 18O/16O ratio would be expected for an Archean “water world” without large continents. High 18O/16O fluids, however, could also have formed locally by reactions between seawater and the oceanic crust. Our study shows that high-precision triple oxygen isotope data provide new understandings of processes deep in time.