Tag: Phosphorus

Life’s origins: a new variant on Darwin’s “warm little pond”

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In 1871 Charles Darwin wrote to his friend Joseph Hooker, a botanist:

“It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts, light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.”

There have been several attempts over the last 150 years, starting with Miller and Urey in 1952, to create physical analogues for this famous insight (See:  The origin of life on Earth: new developments). What such a physico-chemical environment on the early Earth could have been like has also been a fertile topic for discussion: literally warm pools at the surface; hot springs; seawater around deep-ocean hydrothermal vents; even droplets in clouds in the early atmosphere. Attention has recently moved to Darwin’s original surface pools through examination of modern ones. The most important content would be dissolved phosphorus compounds, because that element helps form the ‘backbone’ of the helix structure of RNA and DNA. But almost all natural waters today have concentrations of phosphorus that are far too low for such linkages to form by chemical processes, and also to produce lipids that form cell membranes and the ATP (adenosine triphosphate) so essential in all living metabolism. Phosphorus availability has been too low for most of geological time simply because living organisms are so efficient at removing what they need in order to thrive.

Mono Lake in semi-arid eastern California – a ‘soda lake’- is so concentrated by evaporation that pillars of carbonate grow above its surface

For the first life to form, phosphorus would somehow have had to be concentrated in watery solution as phosphate ions – [PO ₄]³⁻. The element’s source, like that of all others in the surface environment, is in magmas and the volcanic rocks that they form. Perhaps early chemical weathering or reactions between lavas and hydrothermal fluids could have released phosphate ions to solution from a trace mineral present in all lavas: the complex phosphate apatite (Ca10(PO4)6(OH,F,Cl)2). But that would still require extreme concentration for it to be easily available to the life-forming process. In January 2024 scientists at the University of Washington in Seattle, USA (Haas, S. et al. 2024. Biogeochemical explanations for the world’s most phosphate-rich lake, an origin-of-life analog. Nature Communications, v. 5, article 28; DOI: 10.1038/s43247-023-01192-8) showed that the highest known concentrations of dissolved phosphorus occur in the so called “soda lakes” that are found in a variety of modern environments, from volcanically active continental rifts to swampy land. They contain dissolved sodium carbonate (washing soda) at very high concentrations so that they are extremely alkaline and often highly salty. Usually, they are shallow and have no outlet so that dry weather and high winds evaporate the water. Interestingly, the streams that flow into them are quite fresh, so soda lakes form where evaporation exceeds annual resupply of rainwater.

The high evaporation increases the dissolved content of many ions in such lakes to levels high enough for them for them to combine and precipitate calcium, sodium and magnesium as carbonates. In some, but not all soda lakes, such evaporative concentration also increases their levels of dissolved phosphate ions higher than in any other bodies of water. That is odd, since it might seem that phosphate ions should combine with dissolved calcium to form solid calcium phosphate making the water less P-rich.  Haas et al. found that lakes which precipitate calcium and magnesium together in the form of dolomite (Ca,Mg)CO3 have high dissolved phosphate. Removal of Ca and other metal ions through bonding to carbonate (CO3) deprives dissolved phosphate ions in solution of metal ions with which they can bond. But why has dissolved phosphate not been taken up by organisms growing in the lakes: after all, it is an essential nutrient. The researchers found that some soda lakes that contain algal mats have much lower dissolved phosphate – it has been removed by the algae. But such lakes are not as salty as those rich in dissolved phosphate. They in turn contain far less algae whose metabolism is suppressed by high levels of dissolved NaCl (salt). Hass et al.’s hypothesis has now been supported by more research on soda lakes.

In an early, lifeless world phosphate concentrations in alkaline, salty lakes would be controlled by purely inorganic reactions. This strongly suggests that ‘warm little soda lakes’ enriched in dissolved sodium carbonate by evaporation, and which precipitated dolomite could have enabled phosphorus compounds to accumulate to levels needed for life to start. They might have been present on any watery world in the cosmos that sustained volcanism.

See also: Service, R.F. 2025. Early life’s phosphorus problem solved? Science, v. 387, p. 917; DOI: 10.1126/science.z78227f; Soda Lakes: The Missing Link in the Origin of Life? SciTechDaily, 26 January 2024. .

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Multiple Archaean gigantic impacts, perhaps beneficial to some early life

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In March 1989 an asteroid half a kilometre across passed within 500 km of the Earth at a speed of 20 km s-1. Making some assumptions about its density, the kinetic energy of this near miss would have been around 4 x 1019 J: a million times more than Earth’s annual heat production and humanity’s annual energy use; and about half the power of detonating every thermonuclear device ever assembled. Had that small asteroid struck the Earth all this energy would have been delivered in a variety of forms to the Earth System in little more than a second – the time it would take to pass through the atmosphere. The founder of “astrogeology” and NASA’s principal geological advisor for the Apollo programme, the late Eugene Shoemaker, likened the scenario to a ‘small hill falling out of the sky’. (Read a summary of what would happen during such an asteroid strike).  But that would have been dwarfed by the 10 to 15 km impactor that resulted in the ~200 km wide Chicxulub crater and the K-Pg mass extinction 66 Ma ago. Evidence has been assembled for Earth having been struck during the Archaean around 3.6 billion years (Ga) ago by an asteroid 200 to 500 times larger: more like four Mount Everests ‘falling out of the sky’ (Drabon, N. et al. 2024. Effect of a giant meteorite impact on Paleoarchean surface environments and life. Proceedings of the National Academy of Sciences, v. 121, article e2408721121; DOI: 10.1073/pnas.2408721121

Impact debris layer in the Palaeoarchaean Barberton greenstone belt of South Africa, which contains altered glass spherules and fragments of older carbonaceous cherts. (Credit: Credit: Drabon, N. et al., Appendix Fig S2B)

In fact the Palaeoarchaean Era (3600 to 3200 Ma) was a time of multiple large impacts. Yet their recognition stems not from tangible craters but strata that contain once glassy spherules, condensed from vaporised rock, interbedded with sediments of Palaeoarchaean ‘greenstone belts’ in Australia and South Africa (see: Evidence builds for major impacts in Early Archaean; August 2002, and Impacts in the early Archaean; April 2014), some of which contain unearthly proportions of different chromium isotopes (see: Chromium isotopes and Archaean impacts; March 2003). Compared with the global few millimetres of spherules at the K-Pg boundary, the Barberton greenstone belt contains eight such beds up to 1.3 m thick in its 3.6 to 3.3 Ga stratigraphy. The thickest of these beds (S2) formed by an impact at around 3.26 Ga by an asteroid estimated to have had a mass 50 to 200 times that of the K-Pg impactor.

Above the S2 bed are carbonaceous cherts that contain carbon-isotope evidence of a boom in single-celled organisms with a metabolism that depended on iron and phosphorus rather than sunlight. The authors suggest that the tsunami triggered by impact would have stirred up soluble iron-2 from the deep ocean and washed in phosphorus from the exposed land surface, perhaps some having been delivered by the asteroid itself. No doubt such a huge impact would have veiled the Palaeoarchaean Earth with dust that reduced sunlight for years: inimical for photosynthesising bacteria but unlikely to pose a threat to chemo-autotrophs. An unusual feature of the S2 spherule bed is that it is capped by a layer of altered crystals whose shapes suggest they were originally sodium bicarbonate and calcium carbonate. They may represent flash-evaporation of up to tens of metres of ocean water as a result of the impact. Carbonates are less soluble than salt and more likely to crystallise during rapid evaporation of the ocean surface than would NaCl.   

Time line of possible events following a huge asteroid impact during the Palaeoarchaean. (Credit: Drabon, N. et al. Fig 8)

So it appears that early extraterrestrial bombardment in the early Archaean had the opposite effect to the Chicxulub impactor that devastated the highly evolved life of the late Mesozoic. Many repeats of such chaos during the Palaeoarchaean could well have given a major boost to some forms of early, chemo-autotrophic life, while destroying or setting back evolutionary attempts at photo-autotrophy.

See also: King, A. 2024. Meteorite 200 times larger than one that killed dinosaurs reset early life. Chemistry World 23 October 2024.

Geochemistry and the Ediacaran animals

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Hopefully, readers will be fairly familiar with the sudden appearance of the Ediacaran fauna – the earliest abundant, large animals – at the start of the eponymous Period of the Neoproterozoic around 635 Ma. If not, use the Search Earth-logs box in the side bar to find extensive coverage since the start of the 21st century. A June 2019 Earth-logs review of the general geochemical background to the Ediacaran Period can be found here. Ten years ago I covered the possible role of the element phosphorus (P) – the main topic here – in the appearance of metazoans (see: Phosphorus, Snowball Earth and origin of metazoans – November 2010).

One of the major changes in marine sedimentation seen during the Ediacaran was a rapid increase in the deposition on the ocean floor of large bodies of P-rich rock (phosphorite), on which a recent paper focuses (Laakso, T.A. et al. 2020. Ediacaran reorganization of the marine phosphorus cycle. Proceedings of the National Academy of Sciences, v. 117, p. 11961-11967; DOI: 10.1073/pnas.1916738117). It has been estimated that on million-year time scales phosphorites remove only a tiny amount of the phosphorus carried into the oceans by rivers. So, conversely, an increase in deposition of marine P-rich sediment would have little effect on the overall availability of this essential nutrient from the oceans. The Ediacaran boost in phosphorites suggests a connection between them and the arrival of totally new ecosystems: the global P-cycle must somehow have changed. This isn’t the only change in Neoproterozoic biogeochemistry. Thomas Laakso and colleagues note signs of slightly increased ocean oxygenation from changes in sediment trace-element concentrations, a major increase in shallow-water evaporites dominated by calcium sulfate (gypsum) and changes in the relative proportions of different isotopes of sulfur.

Because all marine cycles, both geochemical and those involving life, are interwoven, the authors suggest that changes in the fate of dead organic matter may have created the phosphorus paradox. Phosphorus is the fifth most abundant element in all organisms after carbon, hydrogen, nitrogen and oxygen, followed by sulfur (CHNOPS), P being a major nutrient that limits the sheer bulk of marine life. Perhaps changes to dead organic matter beneath the ocean floor released its phosphorus content, roughly in the manner that composting garden waste releases nutrients back to the soil. Two chemical mechanisms can do this in the deep ocean: a greater supply of sinking organic matter – essentially electron donors – and of oxidants that are electron acceptors. In ocean-floor sediments organic matter can be altered to release phosphorus bonded in organic molecules into pore water and then to the body of the oceans to rise in upwellings to the near surface where photosynthesis operates to create the base of the ecological food chain.

Caption The Gondwana supercontinent that accumulated during the Neoproterozoic to dominate the Earth at the time of the Ediacaran (credit: Fama Clamosa, at Wikimedia Commons)

There is little sign of much increase in deep-ocean oxygen until hundreds of million years after the Ediacaran. It is likely, therefore, that increased availability of oxidant sulfate ions (SO42-) in ocean water and their reduction to sulfides in deep sediment chemically reconstituted the accumulating dead organic matter to release P far more rapidly than before. This is supported by the increase in CaSO4 evaporites in the Ediacaran shallows. So, where did the sulfate come from? Compressional tectonics during the Neoproterozoic Era were at a maximum, particularly in Africa, South America, Australia and Antarctica, as drifting continental fragments derived from the break-up of the earlier Rodinia supercontinent began to collide. This culminated during the Ediacaran around 550 Ma ago with assembly of the Gondwana supercontinent. Huge tracts of it were new mountain belts whose rapid erosion and chemical weathering would have released plenty of sulfate from the breakdown of common sulfide minerals.

So the biological revolution and a more productive biosphere that are reflected in the Ediacaran fauna ultimately may have stemmed from inorganic tectonic changes on a global scale