Hadean Ocean

The primal crust beneath the Hadean ocean may well have consisted of ultramafic rocks (e.g., komathte), with... 

At present, little can be said with any degree of confidence about the composition of the proposed Hadean ocean, but it was probably not very different from that of the Early Archean ocean, about which a good deal can be inferred. 

There are several favorite notions of the site of the origin of life (Nisbet, 1987). The best known is the Marxist hypothesis of the primaeval soup — that the early ocean was a soup of organic molecules that had fallen in from meteorites (which frequently contain complex carbon-chain compounds organic chemicals, but made by prebiotic inorganic processes). In this soup, lipid blobs somehow evolved into living cells. The discovery of hydrothermal systems led to the realization that early oceans would have pervasively reacted with basalt, both in hydro-thermal systems and also with basalt ejecta after impacts. Thus, the late Hadean ocean was most unlikely to be a festering broth, but more likely a cool clean ocean not greatly dissimilar to the modem ocean exit the primaeval soup. 

Morse, J. W. Mackenzie, F. T. 1998. Hadean ocean carbonate geochemistry. Aquatic Geochemistry, 4, 301-319. 

Morita, A. and B. C. Garrett (2008) Molecular theory of mass transfer kinetics and dynamics at gas-water interface. Fluid Dynamics Research 40, 459-473 Moriwaki, R., Kanda, M. and H. Nitta (2006) Carbon dioxide build-up within a suburban canopy layer in winter night. Atmospheric Environment 40, 1394-1407 Morkovnik, A. F. and O. Yu. Okhlobystin (1979) Inorganic radical-ions and their organic reactions. Russian Chemical Review 40, 1055—1075 Morse, J. W., and F. T. MacKenzie (1998) Hadean ocean carbonate chemistry. Aquatic Geochemistry 4, 301-319... 

Hofmeister A. M. (1983) Effect of a Hadean terrestrial magma ocean on crust and mantle evolution. J. Geophys. Res. 88, 4963-4983. 

The broad outlines of Earth history during the Hadean are starting to become visible. The solar system originated 4.57 Ga (Allegre et al., 1995). The accretion of small bodies in the solar nebula occurred within lOMyr of the birth of the solar system (Lugmaier and Shukolyukov, 1998). The Earth reached its present mass between 4.51 Ga and 4.45 Ga (Halliday, 2000 Sasaki and Nakazawa, 1986 Porcelli et ah, 1998). The core formed in <30 Ma (Yin et al., 2002 Kleine et aL, 2002). The early Earth was covered by a magma ocean, but this must have cooled quickly at the end... 

The Hadean is the first of the four aeons of Earth history (Nisbet, 1991). Aeons are the largest divisions of geological time Hadean, Archean, Proterozoic, Phanerozoic. The first and last aeons are short (relatively, if 560 Ma can be called short) the middle two are bilhons of years long. The Hadean was the period of the formation of the Earth, from the first accretion of planetesimals at the start of the Hadean, to the end of the aeon, when the Earth was an ordered, settled planet, with a cool surface under oceans and atmosphere, and with a hot active interior mantle and core. 

There is no evidence for the existence of life before 4 Ga ago. Even if a living organism had appeared, life would probably have been obliterated within a few million years, killed in the intense Hadean bombardment. This was a time when from time to time (say every few million to tens of millions of years, large meteorite impact events would have occurred that so heated the oceans and the atmosphere as to make the Earth briefly uninhabitable, sterilized at several hundred °C (Sleep et al, 2001). 

The map of the surface of the Archean planet remains largely blank, populated by imagined beasts and perhaps some features seen dimly but tmly (Macgregor, 1949). The main input from the mantle to the surface is via volcanism. Late Hadean and early Archean volcanism would have provided thermodynamic contrast, placing material that had equilibrated with the mantle in contact with the ocean-atmosphere system that was open at the top to space and light. In the latest Hadean and earliest Archean this contrast would have been most likely thermodynamic basis of life. 

Model 3 Preservation of remnants of the crust of a Hadean magma ocean... 

Fig. 13. Four important reservoirs of CO2 are shown as functions of time for the models in Figure 12. High heat flow is denoted by continuous lines, low heat flow by dashed lines. Here we have chosen models in which the crustal reservoirs are initially constant in time i.e. we have started from the equilibrium reservoirs. In particular, the equilibrium continental reservoirs are small and so these models begin with very little continental carbonate. The high heat-flow models chiun the reservoirs fast enough that if we do not start at equilibrium values, the model quickly evolves to them, but in the low heat-flow models circulation is slow enough that the arbitrary initial conditions are remembered well into Archaean time. In general, the effect of abundant Hadean impact ejecta is to remove CO2 from the continents and oceans and put it into the mantle.

Figures 14 and 15 show the fluxes of CO2 between reservoirs in the models. Figure 14 addresses the high heat-flow (p = 0.7) model. In these, the largest fluxes in Hadean time are associated with the oceanic cycle of crustal...

Fig. 15. Fluxes of CO2 are shown as functions of time for the low heat-flow models shown in Figures 12 and 13. Impact ejecta dominate the CO2 cycle in Hadean time. The oceanic crustal cycle controls CO2 in Archaean time. Continents are important throughout Proterozoic time, with the transition from mantle to continental control occurring at c. 2.1 Ga.

Figure 15 addresses the low heat-flow (p = 0.2) model. In this model impact ejecta completely dominate the Hadean CO2 cycle. When impacts cease, control is passed to the oceanic crustal cycle, but continents become important fairly early and by c. 2 Ga the continental cycle is the more important. The transition between oceanic and continental control is driven in part by mantle cooling and in part by decreasing heat flow. Overall, the transition between regimes is clear and its timing reasonable. 

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