Oxidative elemental cycling under the low O2 Eoarchean atmosphere

Abstract

The Great Oxidation Event signals the first large-scale oxygenation of the atmosphere roughly 2.4 Gyr ago. Geochemical signals diagnostic of oxidative weathering, however, extend as far back as 3.3-2.9 Gyr ago. 3.8-3.7 Gyr old rocks from Isua, Greenland stand as a deep time outpost, recording information on Earth's earliest surface chemistry and the low oxygen primordial biosphere. Here we find fractionated Cr isotopes, relative to the igneous silicate Earth reservoir, in metamorphosed banded iron formations (BIFs) from Isua that indicate oxidative Cr cycling 3.8-3.7 Gyr ago. Elevated U/Th ratios in these BIFs relative to the contemporary crust, also signal oxidative mobilization of U. We suggest that reactive oxygen species were present in the Eoarchean surface environment, under a very low oxygen atmosphere, inducing oxidative elemental cycling during the deposition of the Isua BIFs and possibly supporting early aerobic biology.

Figure 1. Cr isotope values and U/Th ratios for BIF samples from Isua.

Cr isotope values (a) and U/Th ratios (b) of individual Fe and Si-rich mesobands of BIF samples and of associated metabasalts and metamorphosed clastic sediments from the Isua Greenstone Belt (data in Table S1). The grey-filled area in A delineates the δ⁵³Cr as defined by magmatic rocks. The dashed blue line and the blue-filled range in A denote the average δ⁵³Cr value and the 1σ confidence interval, respectively, for all samples, excluding 97-Isua-1 668.74M. Error bars in A denote 2σ errors (Supplementary Table 1). Error bars in B reflect the 2σ reproducibility of the U/Th ratios. The dashed blue line and the blue-filled area in B denote the average U/Th value and the 1σ confidence interval (excluding 97-Isua-1 668.74N). Sample IF-G (grey-filled symbols) is an iron ore standard from the IGB. The light green-filled symbol band in B mark the average, and the 1σ confidence interval of 59 least altered metabasalts and clastic metasediments from the IGB reported by.

Figure 2. Selected major and trace element relationships in BIF samples from Isua.

Relationships between Fe₂O₃/SiO₂ ratios and Cr concentrations (a), U concentrations (b) and δ⁵³Cr values (c) in mesobands from 7 tectonically least affected magnesian and quartz-magnetite BIF samples from the IGB. The grey-filled symbol marks the average of the IF-G standard. Error bars denote the conservative 10% 2σ reproducibility of the U and Cr concentrations, and the Fe₂O₃/SiO₂ ratios, and the 2σ error of “n” repeated mass spectrometry runs of every sample in case of δ⁵³Cr values (Supplementary Table 1). While the correlations between Fe₂O₃/SiO₂, Cr and U are consistent with Cr and U sequestration involving Fe(oxyhydr)oxides through sorption and reduction processes, respectively, the heterogeneous Cr isotope signatures likely signify dynamics in the δ⁵³Cr values of seawater.

Figure 3. U versus Fe₂O₃ diagram with BIF samples from Isua.

Data from Isua are compared with U and Fe₂O₃ contents of the 3.46 Ga Marble Bar Chert (MBC) samples, and with Phanerozoic cherts from three different tectonic settings (grey filled area; data sources in Li et al.. U* depict authigenic U concentrations in the BIFs (measured U corrected for 3.7 Ga decay and for detrital share using Th concentrations and an average crustal U/Th of 0.25). Using a conservative U K_d value of 10⁴ between Fe(III) hydroxides and aqueous solution, very low seawater U (U_SW) concentrations of between 1.2 to 32 ppt (dashed red lines) are estimated for the 3.7 Ga seawater. The dotted black line is the reference MBC 3.42 Ga seawater U = 20 ppt line. In conjunction with the Cr isotope data presented herein (Fig. 1a) and the positive U-Cr correlation (Supplementary Fig. 7) defined by the Isua BIF samples, we hypothesize that very low atmospheric oxgen levels were sufficient to mobilize small amounts of redox sensitive U and Cr through oxidative weathering conditions prevailing on land at 3.7 Ga.