Cu isotopes in marine black shales record the Great Oxidation Event

Abstract

The oxygenation of the atmosphere ∼2.45–2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth’s redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ65CuERM-AE633) in organic carbon-rich shales spanning the period 2.66–2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in 65Cu, along with the preferential removal of 65Cu by iron oxides, left seawater and marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ65Cu values coincides with a shift to negative sedimentary δ56Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ65Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Arsenic and high affinity phosphate uptake gene distribution in shallow submarine hydrothermal sediments

“…Crucially, most shallow submarine hydrothermal vent fluids are generally depleted in P, implying that unlike As, hydrothermal fluids are not a major P source but rather a strong seawater P sink (Wheat et al 1996; Edmonds and German 2004; Poulton and Canfield 2006; Hawkes et al 2014). Indeed, significant variation in As and P concentrations through Earth’s geological history have been linked to changing levels of submarine hydrothermal influence, Fe(III)(oxyhydr)oxide precipitation and sulfide content (Poulton and Canfield 2011; Chi Fru et al 2015a; Reinhard et al 2017; Chi Fru et al 2016a; Hemmingsson et al 2018). Importantly, shallow submarine hydrothermal activity in the modern oceans are suggested to account for up to 57% of global hydrothermal P removal from the ocean via Fe(III)(oxyhydr)oxide precipitation (Hawkes et al 2014).…”

Nitrous oxide from chemodenitrification: A possible missing link in the Proterozoic greenhouse and the evolution of aerobic respiration

“…Accumulation of N 2 O under Cu limitation occurs in denitrifying bacterial isolates (Granger & Ward, 2003), but not in natural waters (Twining, Mylon, & Benoit, 2007;Ward et al, 2008). Further, recent data suggest that marine Cu concentrations likely remained relatively stable throughout Earth history (Fru et al, 2016), making Cu limitation of Proterozoic N 2 O reduction less feasible as a N 2 O accumulation pathway. There is thus impetus for considering other potential pathways for producing N 2 O that do not rely on Cu-limited denitrification.…”

Bacterial resistance to arsenic protects against protist killing

“…Notably, Cu(I) and As(III) are far more toxic than Zn(II) and if available would be a more toxic mix. It is known that Great Oxidation Event increased the concentrations of both copper and arsenic but peaking at different time points (Chi Fru et al 2015, 2016). Therefore we suggest that arsenic and copper poisoning to be a potent weapon for protists, beginning after the onset of the Great Oxidation.…”

“…The Great Oxidation Event changed the bioavailability of many metals and metalloids and increased the concentrations of both copper and zinc but also arsenic but peaked at different time points (Chi Fru et al 2015, 2016). Recent data show that at least some protists forage on bacteria using toxic metals such as copper and maybe zinc in their phagosomes (Hao et al 2016).…”