Multiple impacts set back oxygen build-up in the Archaean

Earth’s present atmosphere contains oxygen because of one form of photosynthesis that processes water and carbon dioxide to make plant carbohydrates, leaving oxygen at a waste product. The photochemical trick that underpins oxygenic photosynthesis seems only to have evolved once. It was incorporated in a simple, single-celled organism or prokaryote, which lacks a cell nucleus but contains the necessary catalyst chlorophyll. Such an organism gave rise to cyanobacteria or blue-green bacteria, which still make a major contribution to replenishing atmospheric oxygen. Chloroplasts that perform the same function in plant cells are so like cyanobacteria that they were almost certainly co-opted during the evolution of a section of nucleus-bearing eukaryotes that became the ancestors of plants. A range of evidence suggests that oxygenic photosynthesis appeared during the Archaean Eon, the most tangible being the presence of stromatolites, which cyanobacteria mats or biofilms form today. These knobbly structures in carbonate sediments extend as far back as 3.5 billion years ago (see: Signs of life in some of the oldest rocks; September 2016). Yet it took a billion years before the first inklings of biogenic oxygen production culminated in the Great Oxygenation Event or GOE (see: Massive event in the Precambrian carbon cycle; January, 2012) at around 2400 Ma. Then, for the first time, oxidised iron in ancient soils turned them red. If oxygen was being produced, albeit in small amounts, in shallow, sunlit Archaean seas, why didn’t it build up in the atmosphere of those times? Geochemical analyses of Archaean sediments do point to trace amounts, with a few ‘whiffs’ of more substantial amounts. But they fall well below those of Meso- and Neoproterozoic and Phanerozoic times. One hypothesis is that Archaean oceans contained dissolved, ferrous iron (Fe2+) – a powerful reducing agent – with which available oxygen reacted to form insoluble ferric iron (Fe3+) oxides and hydroxides that formed banded iron formations (BIFS). The Fe2+ in this hypothesis is attributed to hydrothermal activity in basaltic oceanic crust. There is, however, another possibility for suppression of atmospheric oxygen accumulation in the Archaean and early-Palaeoproterozoic.

Summary of the evolution of atmospheric oxygen and related geological features. The percentage scale is logarithmic with the modern level being100%. Credit Alex Glass, Duke University

Simone Marchi of the Southwest Research Institute of Boulder, CO, USA and colleagues from the US, Austria and Germany suggest that planetary bombardment offers a plausible explanation (Marchi, S. et al 2021. Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth. Nature Geoscience, v. 14 advance publication; DOI: 10.1038/s41561-021-00835-9). Over the last 20 years evidence of extraterrestrial impacts has emerged, in the form of thin spherule-bearing layers in Archaean sedimentary strata, probably formed by impacts of objects around 10 km across. So far 35 such layers have been identified from several locations in South Africa and Western Australia. They span the last billion years of the Archaean and the earliest Palaeoproterozoic, although they are not evenly spaced in time. The spherules represent droplets of mainly crustal but some meteoritic rocks that were vaporised by impacts and then condensed as liquid. Meteorites in particular contain reduced elements and compounds, including iron, whose oxidation by would remove free oxygen.

The evidence from spherule beds is supplemented by the team’s new calculations of the likely flux of impactors during the Archaean. These stem from re-evaluation of the lunar cratering record that is used to estimate the number and size of impacts on Earth up to 2.5 Ga ago. This flux amounts to the ‘leftovers’ of the catastrophic period around 4.1 Ga when the giant planets Jupiter and Saturn ran amok before they settled into their present orbits. Their perturbation of gravitational fields in the solar system injected a long-lived supply of potential impactors into the inner solar system, which is recorded by craters on the post-4.1 Ga lunar maria. The calculations suggest that the known spherule layers underestimate the true number of such collisions on Earth. Modelling by Marchi et al., based on the meteorite flux and the oxidation of vaporised materials produced by impacts, plausibly accounts for the delay in atmospheric oxygen build-up.

It is worth bearing in mind, however, that large impacts and their geochemical aftermath are, in a geological sense, instantaneous events widely spaced in time. They may have chemically ‘sucked’ oxygen out of the Archaean and early-Palaeoproterozoic atmosphere. Yet photosynthesising bacteria would have been generating oxygen continuously between such sudden events. The same goes for the supply of reduced ferrous iron and its circulation in the oceans of those times, capable of scavenging available oxygen through simple chemical reactions. In fact we can still observe that in action around ocean-floor hydrothermal vents where a host of reduced elements and compounds are oxidised by dissolved oxygen. The difference is that oxygen is now produced more efficiently on land and in the upper oceans and a less vigorous mantle is adding less iron-rich basalt magma to the crust: the balance has changed. Another issue is that the Great Oxygenation Event terminated the oxygen-starved conditions of the Archaean and Palaeoproterozoic in about 200 million years, despite the vast production of BIFs before and after it happened. The Wikipedia entry for the GOE provides a number of hypotheses for how that termination came about. Interestingly, one idea looks to a shortage of dissolved nickel that is vital for methane generating bacteria: a nickel ‘famine’. A geochemical setback for methanogens would have been a boost for oxygenic photosynthesisers and especially their waste product oxygen: methane quickly reacts with oxygen in the atmosphere to produce CO2 and water. Anomalously high nickel is a ‘signature element’ for meteorite bombardment, though it can be released by hydrothermal alteration of basalt. Had meteoritic nickel been fertilising methane-generating bacteria in the oceans prior to the GOE?

See also: A new Earth bombardment model. Science Daily, 21 October 2021.

Photosynthesis, arsenic and a window on the Archaean world

At the very base of the biological pyramid life is far simpler than that which we can see.  It takes the form of single cells that lack a nucleus and propagate only by cloning: the prokaryotes as opposed to eukaryote life such as ourselves. It is almost certain that the first viable life on Earth was prokaryotic, though which of its two fundamental divisions – Archaea or Bacteria – came first is still debated. At present, most prokaryotes metabolise other organisms’ waste or dead remains: they are heterotrophs (from the Greek for ‘other nutrition’). But there are others that are primary producers getting their nutrition by themselves, exploiting the inorganic world in a variety of ways: the autotrophs. Biogeochemical evidence from the earliest sedimentary rocks suggests that, in the Archaean prokaryotic autotrophs were dominant, mainly exploiting chemical reactions to gain energy necessary for building carbohydrates. Some reduced sulfate ions to those of sulphide, others combined hydrogen with carbon dioxide to generate methane as a by-product. Sunlight being an abundant energy resource in near-surface water, a whole range of prokaryotes exploit its potential through photosynthesis. Under reducing conditions some photosynthesisers convert sulfur to sulfuric acid , yet others combine photosynthesis with chemo-autotrophy. Dissolved material capable of donating electrons – i.e. reducing agents – are exploited in photosynthesis: hydrogen, ferrous iron (Fe2+), reduced sulfur, nitrite, or some organic molecules. Without one group, which uses photosynthesis to convert CO2 and water to carbohydrates and oxygen, eukaryotes would never have arisen, for they depend on free oxygen. A transformation 2400 Ma ago marked a point in Earth history when oxygen first entered the atmosphere and shallow water (see: Massive event in the Precambrian carbon cycle; January, 2012), known as Great Oxygenation Event (GOE). It has been shown that the most likely sources of that excess oxygen were extensive bacterial mats in shallow water made of photosynthesising blue-green bacteria that produced the distinctive carbonate structures known as stromatolites. These had formed in Archaean sedimentary basins for 1.9 billion years. It has been generally assumed that blue-green bacteria had formed them too, before the oxygen that they produced overcame the reducing conditions that had generally prevailed before the GOE. But that may not have been the case …

Microbial mats made by purple sulfur bacteria in highly toxic spring water flowing into a salt-lake in northern Chile. (credit: Visscher et al. 2020; Fig 1c)

Prokaryotes are a versatile group and new types keep turning up as researchers explore all kinds of strange and extreme environments, for instance: hot springs; groundwater from kilometres below the surface and highly toxic waters. A recent surprise arose from the study of anoxic springs laden with dissolved salts, sulfide ions and arsenic that feed parts of hypersaline lakes in northern Chile (Visscher, P.T. and 14 others 2020. Modern arsenotrophic microbial mats provide an analogue for life in the anoxic ArcheanCommunications Earth & Environment, v. 1, article 24; DOI: 10.1038/s43247-020-00025-2). This is a decidedly extreme environment for life, as we know it, made more challenging by its high altitude exposure to high UV radiation. The springs’ beds are covered with bright-purple microbial mats. Interestingly the water’s arsenic concentration varies from high in winter to low in summer, suggesting that some process removes it, along with sulfur, according to light levels: almost certainly the growth and dormancy of mat-forming bacteria. Arsenic is an electron donor capable of participating in photosynthesis that doesn’t produce oxygen. The microbial mats do produce no oxygen whatever – uniquely for the modern Earth – but they do form carbonate crusts that look like stromatolites. The mats contain purple sulfur bacteria (PSBs) that are anaerobic photosynthesisers, which use sulfur, hydrogen and Fe2+ as electron donors. The seasonal changes in arsenic concentration match similar shifts in sulfur, suggesting that arsenic is also being used by the PSBs. Indeed they can, as the aio gene, which encodes for such an eventuality, is present in the genome of PSBs.

Pieter Visscher and his multinational co-authors argue for prokaryotes similar to modern PSBs having played a role in creating the stromatolites found in Archaean sedimentary rocks. Oxygen-poor, the Archaean atmosphere would have contained no ozone so that high-energy UV would have bathed the Earth’s surface and its oceans to a considerable depth. Moreover, arsenic is today removed from most surface water by adsorption on iron hydroxides, a product of modern oxidising conditions (see: Arsenic hazard on a global scale; May 2020): it would have been more abundant before the GOE. So the Atacama springs may be an appropriate micro-analogue for Archaean conditions, a hypothesis that the authors address with reference to the geochemistry of sedimentary rocks in Western Australia deposited in a late-Archaean evaporating lake. Stromatolites in the Tumbiana Formation show, according to the authors, definite evidence for sulfur and arsenic cycling similar to that in that Atacama springs. They also suggest that photosynthesising blue-green bacteria (cyanobacteria) may not have viable under such Archaean conditions while microbes with similar metabolism to PSBs probably were. The eventual appearance and rise of oxygen once cyanobacteria did evolve, perhaps in the late-Archaean, left PSBs and most other anaerobic microbes, to which oxygen spells death, as a minority faction trapped in what are became ‘extreme’ environments when long before they ‘ruled the roost’. It raises the question, ‘What if cyanobacteria had not evolved?’. A trite answer would be, ‘I would not be writing this and nor would you be reading it!’. But it is a question that can be properly applied to the issue of alien life beyond Earth, perhaps on Mars. Currently, attempts are being made to detect oxygen in the atmospheres of exoplanets orbiting other stars, as a ‘sure sign’ that life evolved and thrived there too. That may be a fruitless venture, because life happily thrived during Earth’s Archaean Eon until its closing episodes without producing a whiff of oxygen.

See also: Living in an anoxic world: Microbes using arsenic are a link to early life. (Science Daily, 22 September 2020)