A case for low atmospheric oxygen levels during Earth's middle history

Abstract

The oxygenation of the atmosphere — one of the most fundamental transformations in Earth's history — dramatically altered the chemical composition of the oceans and provides a compelling example of how life can reshape planetary surface environments. Furthermore, it is commonly proposed that surface oxygen levels played a key role in controlling the timing and tempo of the origin and early diversification of animals. Although oxygen levels were likely more dynamic than previously imagined, we make a case here that emerging records provide evidence for low atmospheric oxygen levels for the majority of Earth's history. Specifically, we review records and present a conceptual framework that suggest that background oxygen levels were below 1% of the present atmospheric level during the billon years leading up to the diversification of early animals. Evidence for low background oxygen levels through much of the Proterozoic bolsters the case that environmental conditions were a critical factor in controlling the structure of ecosystems through Earth's history.

On the co‐evolution of surface oxygen levels and animals

“…For example, the “Canfield ocean” featured anoxic and euxinic (sulfide‐rich) deep‐ocean conditions through the Proterozoic, relieved only by a relatively unidirectional rise in oxygen during the late Ediacaran (Canfield, ). Broader stratigraphic and geographic coverage, including onshore–offshore transects, as well as new redox proxies have provided a more nuanced view of Proterozoic redox with much less stability (Diamond & Lyons, ; Doyle, Poulton, Newton, Podkovyrov, & Bekker, ; He et al, ; Li, Cheng, et al, ; Li, Zhang, et al, ; Planavsky, Cole, et al, ; Planavsky, Slack, et al, ; Sperling et al, ; Tang, Shi, Wang, & Jiang, ). Any attempts to model Proterozoic redox conditions are similarly left with the conclusion that the marine redox landscape was patchy and complicated (Reinhard, Planavsky, Olson, Lyons, & Erwin, ).…”

“…The defining characteristic of the Proterozoic ocean rather appears to be the lack of a strong redox buffer (Planavsky, Cole, et al, ; Planavsky, Slack, et al, ), in stark contrast to the strongly ferruginous (anoxic and iron‐rich) Archean oceans and the largely oxic modern oceans. Although Proterozoic oceans were dominantly reducing, conditions varied spatially and temporally—there were almost certainly regions that were ferruginous, euxinic, and oxic throughout the Eon.…”

A paleosol record of the evolution of Cr redox cycling and evidence for an increase in atmospheric oxygen during the Neoproterozoic

“…Traditional estimates for Proterozoic atmospheric oxygen levels following the GOE range between 1% and 40% PAL, based on assumed retention of iron in paleosols, and the assumed presence of an anoxic deep ocean (Kump, ). However, this interpretation was based heavily on the 1.1 Ga Sturgeon Falls paleosol (Zbinden, Holland, Feakes, & Dobos, ), which has subsequently been found to exhibit iron and manganese loss instead of retention (Mitchell & Sheldon, , , ), an observation now repeated for at least two other penecontemporaneous paleosols (Planavsky et al, ).…”

Terrestrial microbialites provide constraints on the mesoproterozoic atmosphere

“…For example, give an estimate of 10-200 PAL pCO 2 at 1.4 Ga based on the carbon isotopic composition of large microfossils, Kah and Riding (2007) give an estimate of ≤10 PAL at 1.2 Ga based on microbialite sheath calcification and a number of palaeosols from the Midcontinent Rift (Mitchell and Sheldon, 2010;Sheldon, 2013) have consistently indicated 4-6 PAL at 1.1 Ga. A fundamental question is whether or not pCO 2 of this range could maintain temperate, non-glacial conditions without an additional greenhouse gas present. Given that a variety of proxies are consistent with low atmospheric pO 2 in the Mesoproterozoic (reviewed in Planavsky et al, 2018), CH 4 would likely have been stable in the atmosphere, and given that it has a warming potential about 25 times that of CO 2 , it has often been proposed as a candidate Proterozoic greenhouse gas (Sheldon, 2013).…”

“…The Mesoproterozoic has long been considered to be part of the 'boring billion' years between the Palaeoproterozoic Huronian and Neoproterozoic 'snowball Earth' events, when there was comparatively little change in Earth surface, greenhouse gas or oceanic conditions (Shields and Veizer, 2002;Lyons et al, 2014; accompanied by relatively steady atmospheric pO 2 . More recently, however, new data indicate complex patterns of oceanic oxygenation and euxinia through time (Planavsky et al, , 2018 and suggest that by the mid-Proterozoic, atmospheric pCO 2 likely dropped to less than one quarter of the Palaeoproterozoic levels Sheldon, 2006Sheldon, , 2013. For example, Kanzaki and Murakami (2015) estimate 23-210 times present atmospheric levels (PAL) at 1.85 Ga based on palaeosol geochemistry, but a number of studies have estimated the pCO 2 of the Mesoproterozoic Bartley and Kah, 2004;Kah and Riding, 2007;Mitchell and Sheldon, 2010) and report widely varying, but generally much lower estimates.…”