Oxidative elemental cycling under the low O2 Eoarchean atmosphere.

Robert Frei,Ali Polat,Michael Bau,Sean A Crowe,Lasse N Døssing,David A Fowle

doi:10.1038/srep21058

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.

Highlights

Reactions require at least traces of ROS14
We find positive Cr isotope values (average δ 53Cr = + 0.05 + /− 0.10 permil; δ 53Cr = (53Cr/52Cr)sample/ (53Cr/52Cr)SRM 979− 1) × 1000, where SRM 979 denotes Standard Reference Material 979; Fig. 1a) in both the Fe and Si-rich mesobands of 7 compositionally distinct quartz-magnetite and magnesian iron formation samples (Supplementary Table 1; petrographical description and classification in supplementary information; Supplementary Figs 2,3) collected from the eastern portion of the Isua banded iron formations (BIFs) (Western Greenland; supplementary information; Supplementary Fig. 1)
This is a region of low strain characterized by minimal metamorphic recrystallization where the BIFs preserve primary depositional features and geochemical compositions

Summary

Introduction

Reactions require at least traces of ROS14. The Cr isotopic composition of marine chemical sediments such as BIFs or carbonates can record chromium redox cycling, and when fractionated, appear to indicate the presence and action of molecular oxygen or hydrogen peroxide. High-temperature processes such as metamorphic reactions subsequent to deposition are unlikely to further fractionate Cr isotopes at the scale of whole rock hand samples which would require large-scale remobilization of poorly soluble Cr(III) species and the Cr redox proxy can be applied even to metamorphic rocks such as those from Isua

Methods

Results

Conclusion