The emergence of subaerial crust and onset of weathering 3.7 billion years ago

Reconstructing the emergence and weathering of continental crust in the Archean is crucial for our understanding of early ocean chemistry, biosphere evolution and the onset of plate tectonics. However, considerable disagreement exists between the elemental and isotopic proxies that have been used to trace crustal input into marine sediments and data are scarce prior to 3 billion years ago. Here we show that chemical weathering modied the Sr isotopic composition of seawater as recorded in 3.52-3.20 Ga stratiform barite deposits from three different cratons. Using a combination of Sr, S and O isotope data, barite petrography and a hydrothermal mixing model, we calculate a novel Sr isotope evolution trend for Paleoarchean seawater that is much more radiogenic than the curve previously determined from carbonate rocks. Our ndings require the presence and weathering of subaerial and evolved (high Rb/Sr) crust from 3.7 ± 0.1 Ga onwards. This Eoarchean onset of crustal weathering affected the chemistry of the oceans and supplied nutrients to the marine biosphere 500 million years earlier than previously thought.


Background
One of the most accurate and complete records for tracing the weathering of continental crust over geological time are strontium isotopes ( 87 Sr/ 86 Sr) in marine chemical sediments. Strontium in seawater is derived from two sources with distinct radiogenic isotopic compositions: hydrothermal alteration of oceanic crust with low and mantle-like 87 Sr/ 86 Sr, and subaerial weathering of continental crust with higher 87 Sr/ 86 Sr 1 . Because Sr has a long residence time relative to the ocean mixing rate, 87 Sr/ 86 Sr in seawater is globally homogenized and the balance between oceanic and continental inputs can be recorded in the Sr isotope ratios of authigenic marine minerals. Typically the lowest 87 Sr/ 86 Sr (least radiogenic) are taken as the best estimate of seawater at any time since post-depositional alteration is most likely to increase Sr isotope ratios 2 . Throughout the Phanerozoic, carbonate shells and limestones show many secular variations in seawater 87 Sr/ 86 Sr that can be linked to changes in sea oor spreading rates, as well as shifting tectonic, geographic and climatic controls on the Sr isotopic composition of riverine runoff 3 . Further back in time, the seawater Sr isotope record becomes compromised by the scarcity of unaltered sedimentary carbonate rocks. Sparse Archean carbonates with highly unradiogenic 87 Sr/ 86 Sr have therefore been interpreted to represent seawater, suggesting a mantle-dominated ocean chemistry at this time 4,5,6 . This curve and the concomitant rise in seawater 87 Sr/ 86 Sr at the Archean-Proterozoic boundary supports models and proxies that argue for the late emergence of continental crust and onset of weathering around 2.5 Ga 7,8,9,10,11,12,13 . However, recent work on two barite deposits suggested much higher seawater 87 Sr/ 86 Sr than the mantle-like value assumed from the Archean carbonate record at 3.2 Ga 14,15 . This nding questions the assumption that the unradiogenic carbonates truly re ect seawater, and challenges the validity of a mantle-like Sr isotope seawater evolution curve for the Paleoarchean. The earlier onset of weathering inferred from these barite data would be consistent with Ti isotopic evidence for emerged felsic crust at 3.5 Ga (Table S1). All barite deposits occur in volcanicsedimentary successions consistent with shallow to deep marine environments 19,20,21,22,23,24,25 .
Multiple sulfur isotope data provide evidence for atmospherically-derived sulfate in the barites from a well-mixed global seawater reservoir 26,27,28 or local felsic volcanic eruptions 29,30 . In contrast, eld observations suggest a hydrothermal origin for the barium 25 . In all localities, barite is strongly associated with chert 21 , silica dykes feed into barite horizons at Barite Valley and North Pole 25,31 , and barium-rich hydrothermal alteration zones with Ba-feldspar underlie barite at Londozi and Sargur 19,29 . However, all deposits lack polymetallic sul de deposits, indicating that hydrothermal uid temperatures were relatively low and likely below 150°C 32 . Low-temperature hydrothermal activity is also consistent with the lack of an underlying magmatic system, absence of sintering and the small vertical extent of chert dykes at Barite Valley 25 .
Two types of barite are observed in the six deposits: bladed barite consisting of course blades up to several centimeters long, and ne-grained granular barite (Fig. S1). Crystal morphology alone does not reveal the origin of the barite. However, a combination of sedimentological evidence and morphology supports a primary origin for bladed barite at Barite Valley, except for some isolated blades cutting through barite sands that appear to have grown diagenetically 23,25 . A primary origin for bladed barite is consistent with the well-formed, tabular to bladed crystal morphology that is predicted to grow in settings with low to moderate degrees of barite oversaturation 33 and is found in modern hydrothermal settings 34 . In contrast, granular barite is often found in association with heavy minerals and reworked quartz, indicating a detrital origin for this morphological type 23,25 . Equigranular textures can also form by recrystallization of barite during diagenesis or metamorphism that has affected all deposits 35 38,39 . In order to determine which barite can be used to constrain Paleoarchean seawater 87 Sr/ 86 Sr, eld data and mineral morphology must therefore be integrated with geochemical proxies.
Within individual deposits, bladed barite samples are Sr isotopically distinct from granular barite.
Unlike the marine carbonate record, the lowest 87 Sr/ 86 Sr values in bladed barite cannot be unambiguously interpreted to re ect seawater as ratios may have been lowered by hydrothermal input of unradiogenic Sr 15 . We therefore combine 87 Sr/ 86 Sr data with oxygen and sulfur isotopic compositions to select which barite is most representative of seawater ( Fig. 1b-d) 27 Importantly, we observe the highest δ 18 O values for each deposit in bladed barite and in association with the most negative, and therefore most seawater-like 46 , anomalous sulfur isotope signatures (D 33 S, see Methods for calculation, Table S2). These samples also display a strong positive correlation (R 2 = 0.95) between 87 Sr/ 86 Sr and Δ 33 S (Fig. 1d), in contrast to a weaker correlation for granular barite (R 2 = 0.64, not shown in Fig. 1d). Previous work has demonstrated that the magnitude of seawater sulfate D 33 S decreases throughout the Paleoarchean, as shown in Fig. 1d 26,27,42 . The observed correlation between Sr and S isotopes in bladed barite is therefore best explained by co-evolution of D 33 S and 87 Sr/ 86 Sr in seawater due to progressive decay of 87 Rb. In contrast, this correlation may have been blurred in the granular barite as a result of alteration or contamination, which is consistent with granular textures resulting from recrystallization processes. At water-rock ratios of 1 to 10, metamorphic uids with 50-1000 ppm Sr and 87 Sr/ 86 Sr ~ 0.703-0.706 can shift Sr isotopic compositions from those measured in the bladed barite towards the higher values in granular barite samples (Fig. S2).
Based on the O, S and Sr isotope systematics outlined above, we conclude that 87 Sr/ 86 Sr of bladed barite is as close to Paleoarchean seawater as possible for a hydrothermal deposit. The 87 Sr/ 86 Sr values in the bladed barite samples de ne a strong regression line (Fig. 2, R 2 = 0.98), and are more radiogenic than the Paleoarchean primitive mantle (Fig. 1a) (Table S5). Calculated saturation indices are low (<0.2), which is consistent with our interpretation above that bladed barite formed as primary crystals 33 .
We use this result and a two-component mixing model 34 (Fig. 3). Our results empirically constrain the seawater Sr isotope evolution trend signi cantly further back in time compared to the curve predicted from the extrapolation of 3.2 Ga barite 87 Sr/ 86 Sr 15, 56 (Fig. 3).

Onset of subaerial crustal weathering
The radiogenic Sr isotope values for 3.52-3.20 Ga Paleoarchean seawater imply detectable weathering of an emerged and felsic crustal source at least 300 million years further back in time than what has previously been reported at 3.2 Ga 15 . We further advance this by constraining the start of subaerial weathering from the intersection of our seawater evolution trend and mantle curves, which represents the time at which the input of crustal Sr started to modify the 87 Sr/ 86 Sr of seawater away from mantledominated values. Our calculated trend for Paleoarchean seawater 87 Sr/ 86 Sr indicates an onset at approximately 3.7 ± 0.1 Ga based on the intersection with the primitive mantle curve, 3.6 ± 0.1 Ga from depleted mantle curve DM1 and 3.8 ± 0.1 Ga from depleted mantle curve DM2 (Fig. 2). Improved constraints on the Paleoarchean Sr isotope mantle curve are required to further reduce the uncertainty on this estimate. However, the late Eoarchean onset of crustal weathering observed from chemical signatures is consistent with the siliciclastic rock record which shows that physical weathering products appear for the rst time in greenstone belts around 3.4 Ga 57 and possibly as early as 3.7 Ga 58 (Fig. 3), as well as examples of exposed land surfaces by 3.5 Ga 59 and evidence for the existence of felsic crust at this time 60 . Our ndings indicate that weathering subs tantially modi ed the Sr isotope budget of Paleoarchean seawater, in contrast to its oxygen isotope composition that was recently shown to be unaffected by this 12 .
Comparison of our Sr isotope data and revised seawater trend with other records of weathering (e.g. 6,11,61 ) highlights the scarcity of data in the Paleoarchean and the importance of the barite record for this period in Earth's history. Paleoarchean carbonate from the Pilbara and Kaapvaal craton 1, 5 displays signi cantly more radiogenic 87 Sr/ 86 Sr than barite (Fig. 3), re ecting the higher preservation potential of insoluble barite during diagenesis and metamorphism 35 . The combined updated barite (blue lines in Fig.   3) and carbonate 56 (green line in Fig. 3 If the extrapolation of our seawater Sr isotope trend is correct, it implies that the late Eoarchean geodynamic regime generated granitic magmas and su cient continental freeboard to support weathering of emerged felsic crust from 3.7 Ga. The globally signi cant changes in seawater 87 Sr/ 86 Sr de ned by our samples from three different cratons suggest that subaerial weathering was a relatively widespread phenomenon, likely requiring a larger relative area of emerged crust than the 4% previously reported for the Neoarchean (see Supplementary Information) 63 . Following the same reasoning as above, the absence of radiogenic Sr in seawater before 3.7 Ga suggests that prior to this time the extent of felsic landmass and subaerial weathering was very limited. This in turn would have hampered the colonization of land, inhibited the supply of nutrients to the oceans 64 and restricted the extent of epicontinental seas in the early Eoarchean, possibly limiting the evolution of photosynthetically fueled ecosystems. The lack of undisputed evidence for microbial activity in supracrustal rocks older than 3.5 Ga may re ect such an Eoarchean planet with fewer favorable environments for life to ourish than in the Paleoarchean, when crustal emergence and weathering facilitated life in shallow marine settings.

Analytical
Barite powders were drilled from the same spots as used for multiple sulfur isotope analysis 27

Model
Barite solubility products at 0.6M NaCl (modern seawater salinity) were approximated by linear extrapolation of experimental results at 0.2M NaCl and 1M NaCl and 25-250°C 53   Measured and modelled Paleoarchean seawater curves. Sr isotope evolution trend for Paleoarchean seawater calculated from the hydrothermal mixing model and bladed barite 87Sr/86Sr (dark blue lines). Also shown is the regression line for the bladed barite (light blue line with 95% con dence intervals).
Primitive and depleted mantle curves are the same as those shown in Fig. 1a (see text for calculations).
The onset of large scale subaerial crustal weathering is de ned by the intersection of the seawater and mantle curves at 3.7 ± 0.1 Ga.

Figure 2
Measured and modelled Paleoarchean seawater curves. Sr isotope evolution trend for Paleoarchean seawater calculated from the hydrothermal mixing model and bladed barite 87Sr/86Sr (dark blue lines).
Also shown is the regression line for the bladed barite (light blue line with 95% con dence intervals).
Primitive and depleted mantle curves are the same as those shown in Fig. 1a (see text for calculations). The onset of large scale subaerial crustal weathering is de ned by the intersection of the seawater and mantle curves at 3.7 ± 0.1 Ga.