One of the fundamental discoveries about climate change during the Plio-Pleistocene ice ages is how many climate fluctuations with periods too short to be ascribed to astronomical forcing link to shifts in deep-ocean circulation. In the case of the North Atlantic Ocean, if high-latitude seas become diluted by fresh water cold dense brines are less able to form. It is their sinking as a residue from the formation of sea ice that helps drive the “ocean conveyor” and draws warmer water into the Arctic from the tropics. If they do not form, then the conveyor shuts down and high-latitudes cool. The most spectacular of these ocean-driven events was the Younger Dryas cooling from about 12.9 to 11.6 ka, and it may well have occurred because of the sudden drainage of a giant lake of glacial meltwater down the St Lawrence Seaway to dilute the North Atlantic. The waning of every major ice sheet covering North America would have generated vast amounts of freshwater, and because repeated glaciation created basins by erosion and sagging of the low-relief surface, drainage of such lakes would have been characteristic of every transition to interglacial warmth. Steven Colman of the US Geological Survey reviews recent attempts to model how flooding may have escaped from the ice-sheet margins (Colman, S.M. 2002. A fresh look at glacial floods. Science, v. 296, p. 1251-1252).
The Hadean was cool
James Hutton’s observation that the geological history of Scotland had “no vestige of a beginning” applies everywhere, for no-one has dated rocks that are older than about 4.0 billion years (Ga) old, despite a great deal of effort. It seems that continental crust only became capable of remaining at the surface in large volumes almost 600 Ma after the Earth formed from the Solar nebula. Indirect isotopic evidence and dating of meteorites do indicate that the Earth accreted from dust and planetesimals about 4.56 Ga ago. There are terrestrial materials that break the 4 Ga barrier, but they are so few and so tiny that they could be lost with one powerful sneeze. These are crystals of the highly resistant mineral zircon, found as detrital grains in mid-Archaean sandstones in Western Australia. The oldest of these is a single grain dated at 4.404 Ga. All of them formed in igneous rocks produced by partial melting of the mantle, which concentrates zirconium in magma. Following their liberation to sedimentary processes by weathering, the zircons have probably been through several sedimentary cycles since the formed. So the pre-Archaean history of our world has left relics, but they are minuscule. Because of the absence of pre-4Ga crust, that period was probably turbulent, partly through rapid convective turnover of the mantle and higher degrees of melting because of higher heat production, and partly due to far more large impacts that the lunar surface shows during those times. Dating of lunar cratering and impact glasses suggests that bombardment reached a crescendo around 4.0 to 3.9 Ga. It is now fairly certain that the Moon formed from incandescent material ejected from the Earth when it collided with a Mars-sized planet around 4.45 Ga. Earth and its companion would, in that likely scenario, have begun their geological evolution completely molten in the case of the Moon and with a deep magma ocean on Earth. “Hellish” is a barely adequate adjective for such conditions, and the period before 4 Ga has been termed the Hadean. A vital question concerns when such extreme conditions waned to become potentially supportive of biochemistry and the origin of life.
Minute as they are, the pre-4.0 Ga zircons provide useful oxygen-isotope data, and their d18O is no different from that of more common zircons throughout the Archaean Aeon. The explanation for this is that the mantle and the magmas produced from it contained an H2O phase. Either the mantle has always had a water content – no surprise as it still does – or the magmas from which the zircons crystallized encountered near-surface water vapour, possibly as a result of hydrothermal exchange with a hydrosphere. Reviewing these data, John Valley and colleagues from the University of Wisconsin USA and Curtin University Australia pursue the second conjecture (Valley, J.W. et al. 2002. A cool early Earth. Geology, v. 30, p. 351-354), and argue for a surface temperature below the boiling point of water since 4.4 Ga, only 50 Ma years after geochemical “year zero”. The crux of their argument is that the high d18O values of four Hadean zircons indicate their equilibration with water vapour at temperatures below water’s critical point (374°C). If crystallization at depth was below that temperature, then the Earth would have had surface oceans. But is this such a surprising conclusion? Loss of heat by radiation being proportional to the fourth power of absolute temperature, an incandescent Earth’s surface at the time of Moon formation would have cooled below 100°C well within 50 Ma, unless it was blanketed by an opaque atmosphere. Impacts of the size of those which produced the lunar maria around 4.0-3.9 Ga could have boiled away any surface water from time to time, only for the surface to cool quickly once again. Conditions for bio-geochemistry could well have been present throughout the Hadean. The significance of that for the origin of life is hard to judge, because large impacts and ocean boiling would have extinguished any progress, so that the process may have had to restart again and again.