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.
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 …
Having an interior that is dominated by reducing conditions and oxidising surface environments since free oxygen gradually permeated from its initial build up in the atmosphere to the ocean depths, the Earth has been likened to a massive self-charging battery. Electrons flow continually as a consequence of the nature of the linked oxidation-reduction: in terms of electrons, oxidation involves loss while reduction involves gain (the OILRIG mnemonic). Although there are natural electrical currents, most of the electron flow is in the form of reduced compounds rich in electrons that make their way through the flow of fluids from the deep Earth – effectively an anode – towards the surface where the reduced compounds lose electrons to create the equivalent of a cathode. Reduction-oxidation (redox) is therefore a power source. Inorganic reactions, such as the precipitation on the sea floor of sulfides from hydrothermal fluids at ‘black smokers’ dissipate energy. Yet the power has considerable potential for organic life. Some bacteria oxidise hydrogen sulfide carried by hydrothermal fluids and others do the same to upwelling methane. In 1977 a teeming biome of worms, molluscs and higher animals was discovered in a totally dark environment around ocean-floor vents. It soon became clear that it could only subsist on chemical energy of this kind, rather than any form of photosynthesis. The key to some metazoans’ success had to be symbiosis with bacteria that could perform the chemical tricks possible in the cathode region of the Earth’s electron flow. There are several candidate compounds: H2S, CH4, NH4, metal ions and even hydrogen gas.
As hydrothermal fluids cycle ocean water into the basaltic crust and underlying peridotite mantle, they not only hydrate the olivines and pyroxenes that dominate the oceanic lithosphere but trigger other reactions one of whose products is hydrogen. As well as a reaction being eyed by those keen on a cheap source of clean fuel, it generates more energy potential for biological metabolism in the guise of hydrogen than those which form other common compound in the returning fluids. Although the nature of hydrogen’s organic use has been elusive, it has now come to light in a surprising guise (Petersen, J.M. and 14 others 2011. Hydrogen is an energy source for hydrothermal vent symbioses. Nature, v. 476, p. 176-180).
One highly successful animal in ocean-floor hot spring systems is a mussel called Bathymodiolus. Genetic experiments by the German-French-US team revealed that a gene known as hupL is present in the mussels’ gill tissue; a gene found in bacteria that use either carbon monoxide or hydrogen as an electron donor. The hupL gene encodes for enzymes known as hydrogenases that are needed to set off the reaction H2 = 2H+ + 2e– that provides electrons needed in bacterial metabolism; a sort of living fuel cell. Hydrogen-using bacteria interact symbiotically with the mussels, which would otherwise be unable to live in the pitch black environment. Genomic sequencing of tube worms and shrimps that occur in the vent communities also contain the bacterial hupL gene. Hydrogenase enzymes are proteins with an iron-nickel core, and probably evolved far back in bacterial evolution around metal-rich hot springs. Interesting as the specific detail of hydrogen-based symbiosis is, the general concept of Earth’s redox systems’ having battery-like behaviour is very useful. On land groundwater sometimes comes into contact with sulfide ore bodies that are oxidised to yield hydrogen and sulfate ions ,while the groundwater is reduced: a battery comes into being with a cathode in the aerated groundwater and electrons flow from the unaltered orebody towards it. Such currents are useful in revealing hidden orebodies using the ‘self-potential’ or SP method. Indeed the downward change from oxidising to reducing groundwater, caused by the redox reactions involved in weathering and soil formation also result in weak negative and positive ‘electrodes’ with a sluggish flow of compounds that bacteria can exploit and thereby encourage metazoan life through symbiosis. In doing so, changes in redox conditions affect the inorganic load of the slowly moving groundwater so that reduced metal ions can be precipitated once they rise into the oxidising horizon. The general enrichment of the upper horizons of soils in iron oxides and hydroxides, and metal depletion in lower horizons probably stem from the ‘Earth battery’ produced by an interplay between inorganic and organic redox reactions. Be on the look-out for more on this topic as the quest for hydrogen fuels becomes more urgent. A former colleague, Gordon Stanger, investigating groundwater in the Semail ophiolite of the Oman for his PhD in the 1970s discovered to his surprise that in outcrops of the mantle sequence there were springs from which hydrogen bubbled freely: fortunately he was not a smoker…
Orphan, V.J. & Hoehler, T.M. 2011. Hydrogen for dinner. Nature, v. 476, p. 154-155.