Archaean sea-floor hydrothermal fluids

The circulation of ocean water through new oceanic crust not only cools oceanic lithosphere sufficiently for it to droop and help drive sea-floor spreading.  It also re-emerges as hot submarine springs that today host curious ecosystems, which depend entirely on energy and chemicals that spew out of these “smokers”.  The chemistry of life molecules, particularly the metals in them, reveals a blend that is surprisingly similar to that of hydrothermal fluids.  This, along with other matters, such as the highly primitive genetics of thermophilic bacteria, make sea-floor hydrothermal vents or the crust beneath them excellent candidates for the cradle of life’s origin.  So getting samples of the very earliest such fluids has to be among the most exciting discoveries relevant to palaeobiology.  Jacques Touret of the Free University of Amsterdam, one of the pioneers of fluid inclusion studies, believes that he has found some (Touret , J.L.R. 2003. Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland.  Precambrian Research, v. 126, p. 219-233).  The host rock is an undeformed, but metamorphosed breccia made of basaltic pillows from the famous Isua greenstone belt of West Greenland, which formed about 2.8 billion years ago.  Quartz crystals in amygdales and veins that cement the breccia together contain minute fluid inclusions.  There is little of interest in that fact alone, for most igneous or metamorphic minerals trap samples of the fluids involved in the origin of the host rocks.  What is intriguing abut the Isua fluids is their high content of methane and brine; just as expected from low temperature hydrothermal fluids.  Their chemistry compares well with that of inclusions in altered basalts from modern oceanic crust, in which bacterial activity is implicated.  Metamorphism generally results in carbon dioxide as the main carbon-containing gas in fluid inclusions.  Formation of methane in sea-floor environments can be biologically controlled, but the hydration of deeper ultramafic rocks to serpentine can also generate enough hydrogen to reduce CO2 to methane abiogenically.  The full association at Isua suggests carbon-dominated hydrothermal activity, which today precipitates carbonates at vents, forming so-called “white smokers”.  [“Black smokers” are sulphur dominated, and take their name from the massive precipitation of metal sulphides when the fluids emerge at the seabed.]  These create alkaline conditions that are well suited to bacterial growth.  Touret does not claim that the inclusions indicate living processes, merely that the right conditions were around in the earliest Archaean for life to thrive.  It would be an immense feat if he subsequently discovers bacterial fossils in the inclusions, but that is highly unlikely.  However, the brines might provide proxy evidence, because living cells uniquely accumulate bromine from sea water.  Anomalous ratios of chlorine to bromine might point strongly towards life having been around during Isua times.

See also:  Hecht, J.  2003.  Droplets may reveal life’s oceanic beginnings.  New Scientist, 13 September 2003, p. 25.


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