Stepwise oxygenation of the Paleozoic atmosphere

Abstract

Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O2 currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O2 sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O2 that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.

Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling

“…However, there was considerable variability in the temporal and spatial extent of deep ocean oxygenation at this time, including the possibility of pulsed 'Oceanic Oxic Events' (6). Evidence for periodic deep water anoxia remains frequent up until the mid-Paleozoic, when a final major rise in atmospheric oxygen concentration occurred around 450 to 400 Ma (7). This 'Paleozoic Oxygenation Event' (POE) appears to have elevated atmospheric O2 to present day levels, and established a dominantly oxygenated deep ocean, which persisted throughout the Mesozoic and Cenozoic Eras.…”

“…where animals began to dominate marine ecosystems. The POE is accompanied by a major increase in animal body size, more diverse and specialized predators, and the evolution of vascular land plants (7). However, determining causality between rises in marine and atmospheric O2 levels and the evolution of the biosphere is complex, and there is considerable debate over the role of oxygen in driving biological evolution versus the role of life in 'bioengineering' oxygen to higher levels (10,11).…”

Copper and its Isotopes in Organic-Rich Sediments: From the Modern Peru Margin to Archean Shales

“…Geosciences 2019, 9, x FOR PEER REVIEW 11 of 27 Oxidation Event (NOE; [61]), while the excursion in 65 Cu at 444-446 Ma coincides with the recently proposed Paleozoic Oxygenation Event (POE; [62]).…”

“…Again, data are scarce at this early stage, but the exception to these generally high δ 65 Cu values occurs in the interval 570-520 Ma, after which there are more very high values in samples associated with the Hirnantian Glaciation at about 444-446 Ma. The dataset is much too small to do more than hint at this stage, but it is intriguing that this step occurs close in time to the Neoproterozoic Oxidation Event (NOE; [61]), while the excursion in δ 65 Cu at 444-446 Ma coincides with the recently proposed Paleozoic Oxygenation Event (POE; [62]).…”

Response to anonymous referee #2 - corrected version

“…As an integral part in the research of the Oxygen cycle, the oceanography community issued the Kiel Declaration (https://www.ocean-oxygen.org/declaration), addressing that the ongoing loss of oxygen is a rapidly increasing threat to marine ecosystems and improved understanding of its causes and consequences is immediately required. In addition, in the field of geology, scientists are also making their effort to investigate the mechanism of The Great Oxidation Event (GOE) (Bekker et al, 2004;Canfield, 2014;Kump, 2008;Lyons et al, 2014) and rebuild the evolution of atmospheric oxygen during geologic period (Berner, 2009;Krause et al, 2018;Royer, 2013). The Oxygen cycle deserves the attention of the science community and the research on global Oxygen cycle should be valued.…”