The
Origin of Life at a submarine alkaline seepage
Michael
J. Russell, Allan J. Hall, Laiq Rahman &
Dugald Turner
Heat in the Hadean Earth was generated at more than five times the present rate (Turcotte 1980; Abbott and Hoffman 1984).
The large temperature gradient between core and surface of the Earth generated by gravitational and nuclear energy was (and is) resolved largely by convection cells.
Thus convective updrafts within the dense but liquid core focused cellular convection in the mantle (Fig. 3). Because of the very high heat flow in the Hadean the annual rates of generation, advection and obduction (stacking of overthrusts) of ocean floor were decimetric.
Tectonic patterns would have been somewhat chaotic. Many short spreading ridges "connnected by unstable triple junctions" would have generated new ocean floor, much as occurs today over the hot upper mantle in the Southwest Pacific (Lagabrielle et al. 1997).
Although comprised mainly of iron-rich basalts, gabbros and peridotites; komatiitic (Mg-rich silicate) intrusives and volcanics would also have been well represented in this early oceanic crust.
The rain of meteorites and comets, some of them tens of kilometres across, was unremitting (Melosh 1989). Iron-nickel impactors, though quickly gravitating to the core, buffered the oxidation state of Hadean crust and mantle at iron-wustite (Fe-FeO) (see Saxena 1989; Righter and Drake 1997) (Fig. 5,8), though the QFM (quartz - fayalite - magnetite or SiO2 - FeSiO4 - Fe3O4) buffer was attained by 3900 Ma (Canil 1997; Delano 2001).
The energy released at impact melted the surrounding mantle causing substantial areas of the advecting oceanic crust to be quickly resurfaced with flood basalts or komatiites (cf. Price 2001). Notwithstanding a one to ten bar atmosphere of CO2, radiative heat loss is likely to have lowered temperatures on Earth to near the freezing point, because of atmospheric smog as well as the stratospheric dust raised by meteorites, both barriers to photons.
Sulfur-bearing gases, pyrophosphate and hydrogen chloride were exsolved during depressurization and crystallization of rising magma and exhaled through volcanoes and fractures.
The convection of ocean water in fractures would have generated and transferred a quantity of heat from the fresh hot crust, in a myriad of open convection cells, to the cold ocean where it was lost by radiation to space.
We argue that the emergence of Life itself is a corollary of coupled convective heat loss on our sunlit planet (e.g., Russell and Hall 1997).
Solar energy input in those times was around 70 watts per square metre (cf. the average, 300 milliwatts, provided then by the high geothermal flux).
Convection onsets whenever the Rayleigh number for a particular system is exceeded (e.g., Solomon 1976). Hydrothermal convection cells operating in oceanic crust organize into two distinct classes (Fehn and Cathles 1986). The first of these are the very high temperature cells, derived from cooled ocean water and driven directly (forced) by magmatic heat, and physically buffered to exhale at ~400°C at shallow depth by the sudden drop in density of water at its critical point even at pressures of about 250 bars (Bischoff and Rosenbauer 1984).
These high temperature solutions are rendered acidic by addition of volatiles from the mantle and loss of magnesium from the 'metamorphosed' seawater.
Mg2+ + 2H2O -> Mg(OH)2 (silicate) +2H+
Even in the relatively oxidizing conditions of the present day they are able to convey up to 25 millimoles of ferrous iron, as well as hydrogen sulfide, minor manganese and traces of nickel, cobalt and zinc, back to the ocean (Von Damm 1990; pers. comm. 2001).
Yet, unlike modern times, very little mineral precipitation would have taken place around these 400°C springs as their pH did not contrast greatly with that of the acidulous Hadean ocean.
That the ocean
was acidulous was a consequence of the high CO2 partial pressure
on the early Earth (1 to 10 bars; Walker 1985; Kasting 1993). Even though
CO emanations from the highly reduced mantle via volcanic exhalation may,
initially, have outweighed CO2 by 3 to 1, carbon dioxide is
generated from water and carbon monoxide photolytically in the atmosphere.
In these conditions hydrothermal iron remained in solution (pH ~ 5.5,
25 millimoles Fe, (Fig. 25) cf. Lake Nyos
(Kling et al. 1989; Evans et al. 1994 and pers comm. 1999).
( http://volcano.und.nodak.edu/vwdocs/volc_images/africa/nyos.html )
Meteoritic dust also may have
contributed additional iron, nickel and some small concentratations of
the platinum group elements to the Hadean ocean (Kasting and Brown 1998).
The second class of hydrothermal system operates, in the absence of magmatic intrusion, up to about 260°C in tectonically stressed, and therefore permeable, crust (Russell 1973,1978,1986). Aqueous fluids are normally buffered at the lower end of this 25°-260°C range in oceanic crust by hydration and pressure solution of minerals comprising the walls of initially permeable fracture systems. Although mafic and ultramafic crust is particularly prone to hydration, carbonation and oxidation, fractures retained their permeability as the crust was continually flexed by active tectonics and the tidal forces exerted by the close and rapidly orbiting moon.
We therefore surmise that waters,
derived from this same ocean, exothermically serpentinised the mafic/ultra
mafic crust to become the warm alkaline and hydrogen-bearing convecting
fluids (e.g., hydration of pyroxene; equ. 2) with up to 20 millimole of
hydrosulfide (HS-) (Fyfe 1974; McLeod et al. 1994; Snow and
Dick 1995; and see Barnes et al. 1973) which ultimately seeped into the
still carbonic, somewhat oxidised, deep ocean (Fig.
1) (Russell 1978; Fehn and Cathles 1986; Cathles 1990; Mottl et al
1998):
Kelley and her coworkers (2001)
have now discovered just such a warm (=<75°C) alkaline (=<9.8)
spring emanating from 15 million year old crust in the North Atlantic.
The physical process of convection can be said to leave chemical energy 'in the lurch'. The Earth at 4.4 to 4.2 Ga can be considered as a giant photoelectrochemical cell with an output of about a volt, commensurate with the requirements of chemosynthetic life (Russell and Hall 2001a) (Figs. 6,7a,22)).
Constantly replenishable hydrogen (Fig. 1,5) focused at submarine warm seepages maintains the negative electrodes, while the ferric oxyhydroxides generated photolytically from exhalative ferrous iron form the dispersed positive electrodes or terminals (Cairns-Smith et al. 1992; Russell et al. 1994). The ocean, and the iron sulfides/hydroxides which comprise the barriers, together constitute the electrolyte. Ferric oxyhydroxides were eddy-pumped to the vicinities of the mounds by the hundred metre or so, four-hourly tides generated by a proximal Moon (Goldreich 1966; Gaffey 1997).
Thus the potential chemiosmotic energy available at the deep supmarine seepages comprised a delta pH (acidulous ocean/alkaline spring) and a delta Eh (Fe3+/H2 couple), (Figs. 13, 24).
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18th December 2001