Microbial alteration of oceanic crust

The transformation of ocean-floor lavas from pristine assemblages of anhydrous minerals to cold, wet masses of hydrated silicates is of central importance to subduction processes that pull oceanic lithosphere apart and generate the hydrous arc magmas that can eventually become parts of the continents. This geochemical heat engine is usually ascribed to hydrothermal circulation of seawater through hot new oceanic crust. When these fluids emerge as hydrothermal vents they sustain seething colonies of prokaryote and eukaryote life from the most minute Archaea to substantial metazoans. That this long-hidden part of the biosphere might play a role in plate-tectonic systems is beginning to seem possible. Evidence is emerging from the study of altered basaltic glass that the biosphere does extend deep into the ocean floor (Staudigel, H. et al. 2006. Microbes and volcanoes: A tale from the oceans, ophiolites and greenstone belts. GSA Today, v. 16, October 2006 issue, p. 4-10). The US, Canadian and Norwegian team reviews observations of modern unicellular organisms in the cracks that permeate volcanic glass when it forms by rapid cooling of lava erupted into seawater. They seem rapidly to colonise tiny cracks and to act as a medium through which water is more easily able to transform the sterile glass into complex clay assemblages known as palagonite. The bugs are everywhere, down to at least 300 m in modern ocean floor. High-powered microscopy of ancient ophiolites, such as those of the Cretaceous Troodos Complex on Cyprus, reveals structures that appear exactly the same, including convincing evidence of the organisms themselves. Similar structures, but no irrefutable cell-like structures, occur in Archaean greenstone belt lavas too, as far back as 3.4 Ga: possibly the oldest tangible signs of living processes.

From a cell-biology standpoint, hydration reactions in mafic to ultramafic lavas are potentially highly fertile, the formation of serpentine minerals by hydration being a well-known generator of hydrogen. Modern methanogens use the reaction of hydrogen with carbon dioxide as an energy source, with methane as a by-product. Other organisms exploit the oxidation of sulfides or the reduction of sulfates in a similar way. All these processes can go on inorganically, and the possibility that tiny cracks in volcanic glasses may have harboured the origin of life, as well as thriving ‘ecosystems’, is a possibility worth further exploration. If there is one process that has undoubtedly occurred since the Earth cooled sufficiently for liquid water to exist, it is the alteration of mantle-derived lavas.

Oxygen in the atmosphere: why the delay?

Several lines of evidence suggest that the Earth’s atmosphere only accumulated sufficient oxygen for it to be significantly oxidising around 2.4 Ga ago. Yet the much earlier emergence of blue-green bacteria, assumed to be the organisms that secreted the intricate biofilms that make up stromatolites, suggest that it was being generated by photosynthesis as a much as a billion years beforehand. Many geochemists now suggest that oxygen was readily mopped up in the oceans by the conversion of soluble iron(II) ions derived from sea-floor lavas to insoluble compounds of iron(III), through oxidation reactions. As the rate of production of oceanic lithosphere gradually slowed, there would come a point when all available iron(II) was precipitated leaving excess photosynthetic oxygen to accumulate and enter the atmosphere. But other factors would have been at work: burial of organic carbon produced by photosynthesisers also works to increase the rate at which oxygen remains uncombined (otherwise it combines with oxygen to reproduce carbon dioxide). Complicating the geochemistry of atmospheric oxygen is the way in which it may combine with biogenic methane by reactions catalysed by ultraviolet radiation. Since UV penetration also falls as oxygen levels rise, because of the formation of ozone. That makes possible extremely complex systems of positive and negative feedback. Assessing such mechanisms, three British environmental scientists suggest a kind of potential ‘flipping’ from two possible states for the Archaean to Palaeoproterozoic atmosphere; one rich in oxygen the other forced to have low levels (Goldblatt, C. et al. 2006. Bistability of atmospheric oxygen and the Great Oxidation. Nature, v. 443, p. 683-686). Various permutations of the rates of carbon burial, methane and oxygen production might have locked the pre-2.4 Ga atmosphere in a low-oxygen state. The authors estimate that just a 3% increase in organic carbon burial could have flipped the dynamics towards a state of rapid oxygen accumulation that by generating ozone would be destined to persist. Their model helps resolve a number of awkward geochemical observations that an iron buffering model cannot explain.

See also: Kasting, J.F. 2006. Ups and downs of ancient oxygen. Nature, v. 443, p. 643-645.

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