Cu isotopes in marine black shales record the Great Oxidation Event.
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 (δ(65)CuERM-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 (65)Cu, along with the preferential removal of (65)Cu by iron oxides, left seawater and marine biomass depleted in (65)Cu but enriched in (63)Cu. 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 δ(65)Cu values coincides with a shift to negative sedimentary δ(56)Fe 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 δ(65)Cu 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.
Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, SE-10691 Stockholm, Sweden; ernest.chifru@geo.su.se.
Ernest
0000-0003-2673-0565
E
Chi Fru
Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, SE-10691 Stockholm, Sweden;
Nathalie P
NP
Rodríguez
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2;
Camille A
CA
Partin
CNRS-UMR 6538 Laboratoire Domaines Océaniques, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Technopôle Brest-Iroise, Place Nicolas Copernic, Plouzané 29280, France;
Stefan V
0000-0003-1318-2280
SV
Lalonde
Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden;
Per
P
Andersson
Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2BP, United Kingdom;
Dominik J
DJ
Weiss
Institut de Chimie des Milieux et Matériaux de Poitiers, Université de Poitiers UMR 7285-CNRS, 86073 Poitiers, France;
Abderrazak
A
El Albani
ALS Scandinavia AB, ALS Laboratory Group, S-977 75 Lulea, Sweden;
Ilia
I
Rodushkin
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2EG.
Kurt O
KO
Konhauser
Precambrian; Proterozoic; copper cycling; paleoceanography; trace metals