The Archaean is the oldest time division in Earth’s extant crustal record. Radioisotope dating gives it an age of 3.9-2.5 billion years, spanning just over a third of the total length of Earth history. The only remains older than 3.9 billion years are isolated crystals of the super-hard mineral zircon (ZrSiO₄), which give ages of up to 4.4 billion years. In recolonisation theory the Archaean equates to the first few thousand years immediately after the Cataclysm, at the start of which the planet was under water and new land was forming in place of the old. How well does this idea match up with the evidence?

Water before there was land

Rocks from the earliest Archaean are rare, and they are predominantly igneous – that is, crust that solidified from rising magma. Apart from their rarity, which in itself suggests that most of the Earth’s crust was oceanic, later to be subducted, most of them do not tell us directly whether there was water or not. Pillow basalts – igneous rocks that contracted into pillow shapes as they came into contact with water – testify directly of submarine deposition, as do the rocks that consist of chemical precipitates. The rarity of clastic sediments – the products of weathering, river action and tectonic upheavals – also indicates that there was little land above sea-level. The general picture for the early Archaean is of a world that was under water, with here and there small volcanic islands protruding like ancient versions of Surtsey Island and rapidly shedding material from their sides.

Contrary to what one would expect if the Earth had a solar nebula origin, its geological record shows that after the cataclysm the world was dominated by oceans. There was no dry land. It was situation just like that described in the Hebrew tradition: ‘The waters prevailed so mightily upon the earth that all the high mountains under the whole heaven were covered.’ With the old creation destroyed, terrestrial crust had to form anew, by massive underwater extrusions.

Where did the water come from?

According to the nebula hypothesis for the origin of the solar system, Earth should not have any oceans, since it lies a long way inside of the ‘snow line’ (well beyond Mars) where water in the nebula could have begun to exist as a liquid. Whether there was water before 3.9 Ga is unclear (the zircon evidence is debatable), but there certainly was thereafter.

The continents were mostly flooded until the end of the Archaean. Only 2–3% of the Earth’s area consisted of emerged continental crust until about 2.5 Ga.

Nicholas Flament et al. (2008) Earth & Planetary Science Letters 275:326

Since this water had to have come from somewhere, the most common suggestion is that it came from comets hitting the Earth after the proto-crust had solidified (and after the supposed collision with another planet that gave birth to the Moon). However, the origin of cometary water is itself a mystery, so the suggestion only shifts the problem elsewhere. That Earth’s oceans did come from ‘elsewhere’ is nonetheless not unreasonable.

A further difficulty is that the heat energy released by the bombardment at the end of the Hadean was sufficient to boil all surface oceans away. Although it has been calculated that most of this water would have remained in the atmosphere and condensed back within 2,000 years, no marine life could have survived the onslaught. Yet, as is discussed below, marine life is certainly attested from 3.5 Ga and possibly from as early as 3.8 Ga.

By itself, then, the scientific evidence seems insufficient to solve the mystery. This is where the Hebrew tradition gives us important additional information. First, it indicates that the original Earth was two-layered, with a body of deep water lying underneath the land rather than encircling it – this much has in fact been scientifically inferred (see Kamber et al 2003). Hence, when asteroids struck the planet, marine life in this subterranean deep would have been protected. Second, the tradition indicates that the cataclysm destroyed the architecture of this two-layered system. The pillars holding up the land collapsed, so that the deep burst through the upper layer and submerged it; the ocean that was once underneath the land was now above. And thirdly, water also rained down from above the whole planet, from the waters that originally enclosed the whole solar system.

As we explain more fully elsewhere, the Kuiper Belt is the present-day remnant of this aqueous envelope. Over time, the Sun’s gravity caused much of this water to diffuse inwards and, as is evident from the ubiquity of water in all parts of the solar system, by 4.5 Ga ago interplanetary space must have hosted a substantial amount of the liquid. For a while, Earth’s upper atmosphere presumably absorbed the water that had been diffusing through space, preventing its precipitation on the land below. However, as asteroids ripped through the atmosphere, the stored water added to the deluge.

Thus, we can now explain the origin of Earth’s oceans: part of the water came from beneath the land and part from the aqueous cocoon that once enclosed the whole solar system. Since comets themselves originate from the Kuiper Belt, the idea that the oceans derived from these bodies may not be entirely wrong. However, they are unlikely to have been the form in which water reached the Earth during or at the end of the Hadean.

Could anything have survived?

In a contribution to the journal Nature entitled ‘Microbial habitability of the Hadean Earth during the late heavy bombardment’ Oleg Abramov and Stephen Mojzsis recently addressed this question. They based their calculations on the assumption that the bombardment continued, intermittently, for 100 million years and that the total mass of the asteroids was 2 x 10¹⁷ tons. Although the mass estimate may have been excessive, they concluded that within the crust there would still have been zones where microbial life could have survived. They also found that:

• Earth would not have been sterilised even if all the impacts had occurred simultaneously.
• The surface would have cooled within days, at which point the rate of heat loss would have been limited by heat conduction in the subsurface.
• The upper 350 metres of the crust could have cooled down to habitable temperatures in under 1,000 years.
• If water had saturated the subsurface, heat would have been lost up to ten times faster and habitable conditions re-established up to an order of magnitude more rapidly.

The impact energy would have been colossal – the equivalent of a trillion hydrogen bombs – but most of it would have been absorbed by the thick layer of land, not the water beneath. In contrast to today’s continents, which are already 1300° C at their base, this primeval layer would have been cold from top to bottom. Undoubtedly the upper few kilometres would have totally melted, but as water engulfed the land, rapid quenching of the melt would have formed an insulating crust, just as, on a much smaller scale, an insulating crust forms around pillow basalts, and further cooling would have taken place much more slowly.

Thus, although the oceans would certainly have been hotter than now, marine life borne up in the erupting water might well have survived – animal life as well as microbes.

The first signs of life

Just how warm the oceans were is difficult to say. One recent study used oxygen and silicon isotope data to infer extremely high temperatures during the Archaean (above 60° C). Silicon is the main constituent of chert, and in Archaean greenstone sequences chert deposits are both ubiquitous and voluminous (see next page for what greenstones are). If the cherts had simply been precipitated from silica-saturated seawater, temperatures at this level might have been a valid deduction. However, it has become apparent that many chert deposits formed around hydrothermal vents that were bathed in hot silica-rich fluids. They cannot therefore be taken as normal seawater precipitates.

Perhaps a more reliable indicator is the oxygen isotope composition of phosphates. Phos- phates are also potential evidence of life, since organic phosphates have higher proportions of the heavier isotope, ¹⁸O. The oldest such evidence comes from the 3.5-3.2 Ga-old Barberton greenstone belt of South Africa. Near-surface ocean temperatures were in the range 26–35° C, similar to temperatures now around the equator. On the other hand, some of the ocean-floor cherts indicate that the water further down was warmer. Since warm water will rise, the ocean cannot have been in thermal equilibrium. Being the remains of a terrestrial surface melted and broken up by asteroid impacts, the now submerged crust was still hot and hydrothermally active.

Other evidence of life from this time includes the diverse stromatolites preserved in the Strelley Pool Chert of the Pilbara craton, Australia. Stromatolites are sedimentary mats or mounds formed by microbes, and these remarkable examples occur in a setting that provides a snapshot of environments extending from the rocky shore all the way to the volcaniclastic deposits of deep water.

The stromatolites formed during ‘a rare pause in igneous and hydrothermal activity’, in areas where the water was shallow. They decreased in abundance towards the deeper parts of the basin and also higher up in the succession, as evaporation increased salinity and eventually made conditions intolerable. Here the hydrothermal activity was not that of fluids escaping from the ocean floor but of a molten body of granite ascending through the craton margin and causing the shelf to dome up and fracture.

The microbial structures of the Strelley Pool Chert represent the oldest compelling evidence of an ecosystem anywhere. Microbially formed tubes in the rims of pillow lava from the Barberton are about the same age. Claims of still earlier evidence of life from Akilia, SW Greenland, where some of the world’s oldest rocks are found, are controversial. According to one recent assessment,

The rocks of Akilia provide no evidence that life existed at or before c. 3.82 Ga, or indeed before 3.67 Ga, or even that traces of life occur at all. The complexity of the rocks (mafic, ultramafic and tonalitic gneisses), multiple episodes of intrusion, intense deformation and high-grade metamorphism, and the absence of any unequivocal evidence that any of the rocks were deposited on the Earth’s surface, suggest that these rocks are wholly unsuitable for the discovery of primordial traces of life.

M J Whitehouse et al, Journal of the Geological Society 166:335-48 (2009)

Though bluntly delivered, the assessment may not be unfair. Indeed, immediately after the bombardment, and immediately next to the site of an impact, one would hardly expect conditions to have been suitable, either for the establishment of life or for its preservation. Conditions would remain extreme for some time.