Oxygen and Proterozoic Evolution: Interest has focused on events close to million years ago Ma and Ma, when increases in PO2 are thought to have stimulated the radiations of aerobically respiring eubacteria and via endosymbiosis protists, and macroscopic metazoans, respectively. Acceptance of these hypotheses requires 1 geochemical evidence of environmental change; 2 paleontological evidence of coeval evolutionary innovation; and 3 physiological, ecological, and phylogenetic reasons for linking the two records. New data from Paleoproterozoic weathering profiles are providing increasingly quantitative constraints on the timing and magnitude of an early Proterozoic PO 2 increase.
A simplified history of environmental oxygenation through Proterozoic and Phanerozoic time with data supporting a possible second event at ca.
The top panel shows relative oceanic—atmospheric O2 over the past 2. The bottom panel shows rhenium concentration data from organic-rich marine shales modified from ref. As outlined above, the Cr-isotope paleoredox proxy relies on the recognition of isotopically fractionated Cr in sedimentary rocks, reflecting redox transformations in the subaerial weathering environment and during subsequent transport e.
This tool has been most widely applied to iron-rich chemical precipitates, like iron formations, and organic-rich shales e. Oxidized Cr VI can substitute into the crystal lattice of carbonate minerals during precipitation. It has been shown, however, that Cr incorporated into both natural and experimentally grown carbonates can carry variable fractionations from surrounding waters, potentially resulting from partial Cr VI reduction through biological activity [ 7273 ].
Given this, it is unclear whether the presence of fractionated Cr in carbonates is 1 a ubiquitous feature in the geological record, caused by factors intrinsic to the incorporation of Cr into the carbonate lattice or those contributing to the precipitation of the carbonate itself i.
Few additional proxy data exist for the interval from ca. Interestingly, the most pronounced departure from the pattern of low mid-Proterozoic Re concentrations is in the ca. The Re data, however, return to near crustal values after ca. This relationship suggests that if a genuine oxygenation event did occur at ca.
While transient oxygenation at ca. Furthermore, the strong Re enrichments at ca. Furthermore, our intent is to stimulate an ongoing conversation e.
Multiple lines of geochemical evidence suggesting higher oceanic O2 at specific times within the mid-Proterozoic, however, point to the possibility of at least two instances during which environmental O2 concentrations transiently rose beyond this range.
At the same time, rift basins along the new continental margins become important loci for the burial of sediment and associated organic matter. Similarly, uplift of large mountain ranges during continent—continent collisions and associated delivery of nutrients and sediment to shelf systems can stimulate organic carbon burial and net O2 accumulation [ 8081 ].
The recognition of dynamic fluctuations in mid-Proterozoic oceanic redox conditions raises critical questions about possible relationships to biotic evolution. Specifically, if long-term average mid-Proterozoic conditions, dominated by low O2, acted mostly to inhibit the ultimate emergence of Metazoa, what consequences might transient oxygenation have had?
Despite indications for changing biospheric redox conditions throughout the Proterozoic, it is extremely challenging to extrapolate these data to quantitative estimates of surface—ocean O2, although iodine data from the ca.
In addition to the proxy data discussed above, another recent study of the Xiamaling Formation used a carbon export and oxidation model to estimate atmospheric pO2 based on the total organic carbon contents of the rocks, though several of the assumptions made in the model have been subsequently challenged in the literature [ 43 ]; c.
It is clear from the geochemical records summarized here that the purported oxygenation events at ca. Many of the same geochemical proxies have been used to propose redox fluctuations in the Neoproterozoic, and the magnitude of the chemical responses at that time was significantly larger, suggesting more intense expansions of oxygenated conditions.
This contrast, however, does not necessarily mean that the transient oxygenation events in the mid-Proterozoic had a negligible effect on the evolutionary trajectory of eukaryotes — on the contrary.
Perhaps, it is not a coincidence that both size and diversity of eukaryotic microfossils in the Proterozoic succession of the North China Craton reach a maximum in the Wumishan Formation, just below the Tieling Formation [ 83 ].
Furthermore, many authors have attributed the first occurrence of crown group eukaryotes to the fossil Bangiomorpha pubescens found in the Hunting and Angmaat formations of Arctic Canada [ 8485 ], the age of which has recently been updated to ca.
Schematic illustration of the evolution of atmospheric oxygen (dark line) with bounds provided by alternate Some of the tools used to study atmospheric evolution have only recently been developed. Environmental evolution of the Archean-early Proterozoic Earth. In Schopf, J.W. (ed.), Earth’s Earliest Biosphere: Its Origin and Evolution. A shale-hosted Cr isotope record of low atmospheric oxygen during the Proterozoic Devon B. Cole1*, Christopher T. Reinhard2, Xiangli Wang1, the early evolution and ecological expansion of animal life. study, as transition metal concentrations tend to. Early Proterozoic [~ million years ago (Ma)] (1, 3, 4), but the details remain elusive. Estimates for the timing of accumulation of appreciable concentrations of free oxygen in the Archean atmosphere range from as early as Ma (5, 6) to Ma (7).
All indications suggest a mid-Proterozoic world with low long-term average pO2, though dynamic fluctuations may have been prominent feature. The proxy data indicate that these fluctuations became larger and more frequent during the Neoproterozoic, potentially accompanied by incremental increases in baseline pO2, and eventually resulted in shelfal habitats that were stably oxygenated on longer timescales [ 21222526 ].
Nonetheless, specific questions about the relationship between these environmental changes and metazoan evolution remain open, and while the scope of the remaining challenges is great, the increasing resolution of geochemical and paleontological records coupled with the increasing sophistication of biogeochemical modeling is paving a path forward.
Summary The mid-Proterozoic was dominated by prolonged periods of intermediate but still very low levels of oxygen. Stable, long-term trend of low O2 levels could have been punctuated by transient oxygenation events. Competing Interests The Authors declare that there are no competing interests associated with the manuscript.
Acknowledgements This work benefited greatly from the constructive reviews of two anonymous reviewers.
Revision received May 30, Accepted June 2, Molecular oxygen (O2) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth’s biogeochemical cycles.
Although “whiffs” of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). Two possible scenarios in early terrestrial atmospheric evolution are examined using a one‐dimensional chemistry and flow model of the atmosphere.
This leads to ground‐level oxygen concentrations on the order of 10 Kenneth M.
Towe, Aerobic carbon cycling and cerium oxidation: significance for Archean oxygen levels and . The appeal of our preferred hypothesis is that it provides a causative link between remarkable features of the late Archean and early Paleoproterozoic: widespread plume activity, iron formation deposition, the rise of atmospheric oxygen, and global glaciation.
Sulfate Clues for the Early History of Atmospheric Oxygen Science 28 April , Volume , pp concentrations and are not diagnostic of Archean oxygen concentrations.
P. A. Trudinger, in Early Organic Evolution: Implications for Mineral and Energy Resources, M. Mass-independent fractionation (MIF) of sulfur isotopes has been reported in sediments of Archean and Early Proterozoic Age (> Ga) but not in younger rocks.
The only fractionation mechanism that is consistent with the data on all four sulfur isotopes involves atmospheric photochemical reactions such as SO2 photolysis.
We have used a one . Jun 29, · The simplest explanation for the appearance of O 2 in the atmosphere at concentrations in excess of ca 2 p.p.m. at ca Ga is that cyanobacteria using photosystem-II evolved at that time.
However, this is unlikely. The chemical evolution of the atmosphere and oceans. p. Early Proterozoic atmospheric change. In.