Tag Archives: origin of life

Earliest hydrothermal vents and evidence for life

 

That seawater circulates through the axial regions of rifts associated with sea-floor spreading has been known since well before the acceptance of plate tectonics. The idea stems from the discovery in 1949 of brines with a temperature of 60°C on the central floor of the Red Sea, which in the early 60s turned out to be anomalously metal-rich as well. Advanced submersibles that can withstand the high pressures at great depth a decade later produced images of swirling clouds of sediment from large sea-floor springs, first on the Galapagos rift and subsequently on many others. The first shots were of dark, turbulent clouds, prompting the term ‘black smoker’ for such hydrothermal vents and it turns out that others produce light-coloured clouds – ‘white smokers’. Sampling revealed that the sediments in black smokers were in fact fine-grained precipitates of metallic sulfides, whereas those forming white smokers were sulfates, carbonates and oxides of barium calcium and silicon also precipitated from solute-rich brines produced by partial dissolution of ocean floor through which they had passes.

A black smoker known as "the brothers".

A black smoker with associated organism. (credit: Wikipedia)

Excitement grew when hydrothermal vents were shown to have complex animal ecosystems completely new to science. A variety of chemical evidence, most importantly the common presence of proteins and other cell chemicals built around metal sulfide groups in most living organisms, prompted the idea that hydrothermal vents may have hosted the origins of life on Earth. Many fossil vent systems also contain fossils; macrofossils in the Phanerozoic and microbial ones from the Precambrian. But tangible signs of life, in the form of mats ascribed to bacteria or archaea holding together fine-grained sediments, go back no further than 3830 Ma in the Isua area of SW Greenland. Purely geochemical evidence that carbonaceous compounds may have formed in living systems  are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen. Signs of deep sea hydrothermal activity are common in any geological terrain containing basalt lavas with the characteristic pillows indicating extrusion beneath water. So to trace life’s origins all that is needed to trigger the interest of palaeobiologists are the oldest known pillow lavas. Until quite recently, that meant the Isua volcano-sedimentary association, but heating, high pressures and  very strong deformation affected those rocks when they were metamorphosed half a billion years after they were formed; a cause for skepticism by some geoscientists.

The primacy of Isua metavolcanic rocks has been challenged by more extensive metamorphosed basalts in the Nuvvuagittuk area in Quebec on the east side of Hudson Bay, Canada. They contain hydrothermal ironstones associated with pillowed basalts that are cut by more silica-rich intrusive igneous rocks dated between 3750 and 3775 Ma. That might place the age of basalt volcanism and the hydrothermal systems in the same ball park as those of Isua, but intriguingly the basalts’ 146Sm-142Nd systematics suggest a possible age of magma separation from the mantle of 4280 Ma (this age is currently disputed as it clashes with  U-Pb dates for zircon grains extracted from the metabasalts around the same as the age at Isua). Nonetheless, some parts of the Nuvvuagittuk sequence are barely deformed and show only low-grade metamorphism, and they contain iron- and silicon-rich hot spring deposits (Dodd, M.S. et al. 2017. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, v. 543, p. 60-64; doi:10.1038/nature21377). As at Isua, the ironstones contain graphite whose carbon isotope proportions have an ambiguous sign of having formed by living or abiotic processes. It is the light deformation and low metamorphism of the rocks that gives them an edge as regards being hosts to tangible signs of life. Extremely delicate rosettes and blades of calcium carbonate and phosphate, likely formed during deposition, remain intact. These signs of stasis are in direct contact with features that are almost identical to minute tubes and filaments formed in modern vents by iron-oxidising bacteria. All that is missing are clear signs of bacterial cells. Ambiguities in the dating of the basalt host rocks do not allow the authors claims that their signs of life are significantly older than those at Isua, but their biotic origins are less open to question. Neither offer definitive proof of life, despite widespread claims by media science correspondents, some of whom tend  metaphorically to ‘run amok ‘ when the phrase ‘ancient life’ appears; in this case attempting to link the paper with life on Mars …

You can find more on early life here

 

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Signs of life in some of the oldest rocks

Vic McGregor (left) and Allen Nutman examine metasedimentary strata at Isua, West Greenland
For decades the record of tangible signs of life extended back to around 3.4 billion years ago, in the form of undulose, banded biofilms of calcite known as stromatolites preserved at North Pole in the Pilbara region of Western Australia. There have been attempts to use carbon-isotope data and those of other elements from older, unfossiliferous rocks to seek chemical signs of living processes that extracted carbon from the early seas. Repeatedly, claims have been made for such signatures being extracted from the 3.7 to 3.8 Ga Isua metasediments in West Greenland. But because this famous locality shows evidence of repeated metamorphism abiogenic formation of the chemical patterns cannot be ruled out. Isua has been literally crawled over since Vic McGregor of the Greenland Geological Survey became convinced in the 1960s that the metasediments could be the oldest rocks in the world, a view confirmed eventually by Stephen Moorbath and Noel Gale of Oxford University using Rb-Sr isotopic dating. There are slightly older rocks in Canada, which just break the 4 Ga barrier, but they were metamorphose at higher pressures and temperatures and are highly deformed. The Isua suprcrustals, despite deformation and metamorphism show far more diversity that geochemically can be linked to many kinds of sedimentary and volcanic rock types.

 

Two of the Isua addicts are Allen Nutman of the University of Wollongong, Australia and Clark Friend formerly of Oxford Brookes University, UK, who have worked together on many aspects of the Isua rocks for decades. Finally, thanks to melt-back of old snow pack, they and colleagues have found stromatolites that push the origin of life as far back as it seems possible for geoscientists to reach (Nutman, A.P. et al. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature). The trace fossils occur in a marble, formerly a limestone that retains intricate sedimentary structures, which show it to have been deposited in shallow water. The carbon and oxygen isotopes have probably been disturbed by metamorphism, and no signs of cell material remain for the same reason, but the shape is sufficiently distinct from those produced by purely sedimentary processes to suspect that they resulted from biofilm build-up. The fact that they are made of carbonates suggests that they may have been produced by cyanobacteria as modern stromatolites are.

isua strom

Stromatolite-like structures from a metasediment in the Isua area of West Greenland (credit Allen Nutman, University of Wollongong, Australia)

The age of the structures, about 3.7 Ga, is close to the end of the Late Heavy Bombardment (4. 1to 3.8 Ga) of the Solar System by errant asteroids and comets. So, if the physical evidence is what it seems to be, life emerged either very quickly after such an energetic episode or conditions at the end of the Hadean were not inimical to living processes or the prebiotic chemistry that led to them.

 You can find more on early life here

Allwood, A.C. 2016. Evidence of life in Earth’s oldest rocks. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature19429