The Archaean is the oldest time division of the extant crustal record. Radioisotope dating gives it an age of 3.9–2.5 billion years (Ga), spanning just over a third of the total length of Earth history. The only remains of the surface older than 3.9 Ga are isolated crystals of the super-hard mineral zircon (ZrSiO4), which give ages of up to 4.4 Ga. There is thus a mysterious 0.5 Ga period at the beginning which is missing from the primary rock record. It is missing because the original land – inhabited by people and animals – was totally destroyed. 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 began to form in place of the old.
Rocks from the earliest Archaean are predominantly igneous – crust solidified from rising magma. They are very rare, and that rarity in itself suggests that most of the Earth’s crust was oceanic and, as such, later to be subducted. But there are also other clues. Pillow basalts – lava that contracted into pillow shapes as they came into contact with water – testify directly of submarine deposition. The paucity of clastic sediments – products of weathering, erosion and tectonic upheavals – further indicates that there was little land above sea-level. The general picture is of a world that was submerged, with here and there small volcanic islands protruding like ancient versions of Surtsey Island, rapidly shedding material from their sides.
The geological record shows that after the cataclysm the world was dominated by oceans. For a very brief period there may have been no dry land at all, just like the situation 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.
According to the nebula hypothesis for the origin of the solar system, Earth should not have any oceans, since it is positioned 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, but thereafter surface ocean abounded.
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 – an idea that is now discredited). This is problematic, not least because the origin of cometary water is itself a mystery and the suggestion only displaces the problem. Nonetheless, that Earth’s oceans did come from ‘elsewhere’, just as the water that once existed on Venus and Mars must have done, does not seem 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 the water would have remained in the atmosphere and eventually back, no marine life could have survived the onslaught. Yet, as discussed below, marine life is attested from 3.5 Ga and possibly from as early as 3.8 Ga.
By itself, 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 indeed been scientifically inferred (Kamber et al 2003). Hence, when asteroids struck the planet, marine life in this subterranean deep would have been protected. Second, the cataclysm destroyed the architecture of this two-layered system. The pillars holding up the land collapsed, so that the great deep burst through the upper layer and submerged it; the ocean that was once underneath the land was now above. And third, water also rained down from above the planet, from the waters that originally enclosed the whole solar system.
As we explain more fully elsewhere, the now much depleted Kuiper Belt, another puzzling component of the solar system, is a remnant of this aqueous envelope. Over time, the Sun’s gravity caused most 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 the origin of Earth’s oceans can be explained: part of the water came from beneath the land and part from the aqueous cocoon that enclosed the whole solar system. Since comets themselves partly originate from the Kuiper Belt, the idea that the oceans derived from these bodies may not be entirely wrong, even if comets are unlikely to have been the form in which water reached the Earth during or at the end of the Hadean.
Oleg Abramov and Stephen Mojzsis addressed this question in a Nature article entitled “Microbial habitability of the Hadean Earth during the late heavy bombardment”. Basing their calculations on the assumption that the bombardment went on for 100 million years and that the total mass of the asteroids was 2 x 1017 tons (possibly an over-estimate), they concluded that within the crust there would have been zones where microbial life could have survived. They also found that:
- Earth would not have been sterilised even if the impacts had all occurred simultaneously.
- The surface would have cooled within days, at which point the rate of heat loss would have been limited by 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. However, 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 1300° C at their base, this primeval layer would have been relatively cool. 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 smaller scale, an insulating crust forms around pillow basalts. Further cooling would have taken place 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.
Just how warm the oceans were is difficult to say. One indicator is the oxygen isotope composition of phosphates. Phosphates 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–.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. Some ocean-floor cherts indicate that further down the water was warmer, in which case it would also have been denser, as a result of dissolved iron and other chemicals expelled from the mantle. Indeed the abundance of iron in the seawater of this time bears witness to the intensity of seafloor spreading. Stripped of oxygen, saturated in metals, hot and acidic, the deeper waters were uninhabitable. The submerged, formerly terrestrial crust, melted and broken up by asteroid impacts, was in hydrothermal ferment.
Other evidence of near-surface life from this time is preserved in the Strelley Pool Chert of Western Australia. Here we have a snapshot of environments all the way from the rocky shore to the volcaniclastic deposits of deep water. During “a rare pause in igneous and hydrothermal activity”, stromatolites, sedimentary mats and mounds formed by microbes, formed in the shallows, decreasing in abundance where the water deepened and also higher in the succession, where the increased salinity from evaporation made conditions intolerable. The hydrothermal activity was that of a molten blob of granite ascending through the craton margin and causing the shelf to dome up and fracture. Soon after the pause, lava up to 8 km thick flooded the region.
The microbial structures of the Strelley Pool Chert represent the oldest compelling evidence of an ecosystem anywhere. Microbial 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 blunt 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)
A recent report describes stromatolites from Greenland dating back to 3.7 Ga, much to everyone’s surprise. Soon after the bombardment, and close 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.