A persistently low level of atmospheric oxygen in Earth\u2019s middle age

Xiao-Ming Liu,Chao Wang,Andrey Bekker,Robert M Hazen,Andrew H Knoll,Linda C Kah,Huan Cui

doi:10.1038/s41467-020-20484-7

Abstract

Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth’s history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 – 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/Ce* data to estimate the partial pressure of atmospheric oxygen (pO2) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO2 of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).

Highlights

Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time
Our compilation of Ce/Ce* in marine carbonate rocks is displayed in Fig. 2, with a curve fit to the lowest 10% of Ce/Ce*
Ce anomalies towards the GOE and a broad, though modest, increase in the latter part of the mid-Proterozoic interval that accelerated into a marked Ediacaran to mid-Paleozoic change

Summary

Introduction

Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. Additional geochemical proxy data from marine carbonate phases suggest atmospheric O2 levels intermediate between these estimates[10,11]. Most of these studies were based on individual stratigraphic units and did not provide long term estimates of pO2 through time. Under suboxic or anoxic conditions of the modern ocean, Ce anomalies are weak to absent, reflecting the reductive dissolution of Mn-rich and Fe-rich particles below the redoxcline[18,19]; Ce/Ce* shifts closer to 1 For this reason, Ce anomalies are useful for distinguishing oxic depositional environments from suboxic and anoxic settings[20]

Methods

Results

Conclusion