Supercontinent: Ten Billion Years in the Life of Our Planet (27 page)

It is interesting that cratonization did not occur at the same pace all over the Earth’s surface. At the time that the supercontinent of Ur was forming (it connected areas of South Africa, Madagascar, Sri Lanka, India, Western Australia and parts of East Antarctica),
elsewhere on the Earth small convection cells were still producing ‘greenstone belts’, the typical product of the Earth’s earliest tectonics: small kernels of continental rocks surrounded by highly deformed, ocean-floor-type rocks.

Yet on Ur, processes of erosion and deposition were taking place in modern style. Between about 3.1 and 2.7 billion years ago, for example, on what is now the western side of southern Africa, large thicknesses of sediments were laid down in the Witwatersrand and Pongola basins. Here, geologists have been able to decipher distinctive environments, such as intertidal flats and braided streams (some containing gold), sand ripples, mud cracks (and the very first glacial deposits to form on Earth).

Ur was also, it seems, the first supercontinent to experience glaciation, which in turn supports the idea that even then the global climate was not too dissimilar from that of today. That said, the atmosphere contained very low concentrations of oxygen, perhaps less than 1 per cent of modern levels. How glaciations could happen at all when the atmosphere was essentially full of greenhouse gases is one of those questions that further research into the Earth system must answer. As the Supercontinent Cycle that would continue to our own day was just beginning, emergent life unleashed the first of its many great climatic catastrophes on Earth, as oxygen – this new and toxic gas, hitherto just the rare product of chemical dissociation – began to appear in rising quantities as life escaped from the upper deep.

Life’s ancient sleep had ended. It came to the surface as bugs living in the pore spaces of rocks that, instead of being subducted beneath them, were scraped off on to continental masses. Under those
shelf-seawaters,
lit by the sickly light of a weak Sun perhaps 20 per cent less luminous than today, beneath the suffocating twilight of a greenhouse atmosphere full of carbon dioxide, methane and nitrogen, life learnt to photosynthesize.

Oxygen is a highly reactive gaseous element that now makes up about 16 per cent of the atmosphere. The growth of oxygen has not been steady: there have been times when much more oxygen was present in the air than now; for example during the Carboniferous Period, just as Pangaea was forming. Coal forests then covered much of the planet, pumping out oxygen and sequestering carbon. These forests were therefore even more inflammable than they are now, and the higher oxygen levels also made it possible for dragonflies of the time to have wingspans of a metre.

But oxygen was almost entirely absent from the (to us) toxic atmosphere of the early Earth, composed as it was mainly of volcanic exhalations. That this was so is proved by the fact that Archean sediments often contain detrital grains of the mineral pyrite (iron sulphide), which means that this unstable chemical could then survive intact at surface. You may have found Jurassic fossils preserved in this mineral (also known as fool’s gold because of its brassy colour) on the beach at, for example, the world-famous fossil site of Lyme Regis in Dorset, England. It formed there under anoxic conditions below the Jurassic seabed. But if you have ever taken these fossils home you may also know how short a time they last, all shiny and lovely, unless protected from air. For oxygen corrodes the pyrite and one day you find that your fossil has disintegrated into an ash-like powder. Such is the fate of all pyrite at the Earth’s surface today – but not in the Archean. Yet as soon as life had escaped from its submarine lair and gone green, this was to change.

Free oxygen was pouring into a world where elements such as iron and sulphur had always been able to exist quite happily – on land or in the ocean – in their unoxidized (or ‘reduced’) form. Today unoxidized iron cannot exist on the Earth’s surface for long, and we see the evidence all around us in the rusting hulks of old motor vehicles. But before ‘free’ oxygen could build up in the air to levels that could
support oxygen-breathing animals, the products of the first photosynthesizing organisms had to oxidize all the iron and sulphur. These elements constituted a vast oxygen ‘sink’; one that geological evidence suggests was not completely filled for as long as 1.6 billion years.

The ‘rusting’ of the Earth began about 3.5 billion years ago, which is how we can date the approximate emergence of photosynthesis. The first of a new and distinctive rock type began to be deposited in the oceans: rocks that are today the world’s dominant source of iron ore. They occur on every continent and contain about 100,000,000,000,000 (one hundred trillion) tonnes of iron, nearly all of it deposited between 3.2 and 1.5 billion years ago. These spectacular rocks, marking the second-biggest biochemical event since the creation of life, are the Banded Iron Formations, or BIFs.

BIFs consist of interbedded iron ore and chert, a flinty rock that began as a gelatinous deposit formed by the chemical precipitation of volcanic silica from seawater. As fast as the first photosynthesizers pumped their waste into the air, reduced iron dissolved in the acidic seas combined with the reactive new element and precipitated out as insoluble oxides in unimaginable quantities: not only in the shallow shelf seas, but presumably all over the oceans too. This means that the 100 trillion tonnes of iron that has survived to the present day is but a tiny fraction of the total iron deposited at this time, partly because the ironstones that formed on the deep ocean floor must have been destroyed by subduction. David Dobson and John Brodholt of University College London, in research first published in 2005, think they know exactly where this ancient iron now is: deep inside the planet, sitting at the boundary between the core and the mantle.

Seismologists have long puzzled over the nature of what they call Ultra Low Velocity Zones (ULVZs) between one and ten kilometres in thickness, where seismic waves, generated by earthquakes far
above, suddenly and anomalously slow down as they travel close to the core-mantle boundary. If BIFs did form everywhere in the Archean ocean, including over ocean floor, because a BIF is 25 per cent denser than the mantle, once subducted it would tend to go on sinking until it hit – literally – rock bottom, where the silicate mantle touches the roiling, molten iron and nickel of the outer core. If Dobson and Brodholt are right, the processes of life, far from being a superficial veneer, have affected this planet to its deepest interior.

For this reason, it may also be that life changed the speed of the Earth’s rotation. The sinking of such a huge mass of iron to the coremantle boundary would have had an effect similar to the gradual drawing in of a pirouetting skater’s arms, causing the days to get shorter. Scientists remain undecided about this idea, however, since we know (from the growth lines on fossil corals) that since the Devonian period (a mere 360 million years ago) the drag of the ocean tides has tended to
slow
the Earth’s rotation. However, the period over which the BIFs were sinking occurred so long before the evolution of complex life that its opposite effect on day-length remains a plausible but untested idea.

Oxygen’s chemical sinks began to be near ‘full’ about 2.3 billion years ago. By 1.9 billion years ago, BIFs had ceased to form (though they did make a brief comeback, as we shall see) and levels of oxygen in the atmosphere began climbing towards modern levels, with further disastrous consequences.

During this time more large continental groupings were beginning to coalesce. At about 2.5 billion years a second more northerly
supercontinent
emerged, called Arctica because the Arctic Ocean opened through its middle, incorporating much of northern and central Canada, Greenland and Siberia. At about two billion years a second northerly supercontinent, called Baltica (consisting of much of
north-west
Europe as far as the modern Urals), assembled and eventually
collided with Arctica to form (1.6 billion years ago) another larger continent, known variously as Nena, Nuna or Columbia.

Also at about two billion years ago, a southerly megacontinent dubbed ‘Atlantica’ (because eventually the Atlantic Ocean would open through it when Gondwanaland broke up) united much of north-eastern South America with West Africa and the Congo, and forming the heart of what would one day be West Gondwana. These three components of different ages, ancient Ur, Nena/Nuna/Columbia and Atlantica, finally came together about one billion years ago to form the first true supercontinent, motherland of complex life, Rodinia.

But this is to race ahead. After BIFs ceased to be deposited and oxygen began to be more freely available, sediments exposed on land became oxidized too, and ‘red beds’ became common. At about 1.7 billion years ago there was also a marked increase in weathering, because thick deposits of quartzite, a rock type made from pure silica sand, suddenly became widespread all over the continents. Such an outbreak of silicate weathering would have had a drastic cooling effect on climate, as carbon dioxide in the air became combined with rock materials to form bicarbonates, which were washed into the seas. What happened then, effectively sequestering the carbon and keeping it from returning to the atmosphere, relied on another development happening in the oceans.

One by one, the panes of the greenhouse that had kept Earth warm for billions of years were being smashed.

It is one of the most remarkable discoveries in deep time that for most of the history of the biosphere, a period four times longer than all the time since the first complex fossils began to be preserved, the
most advanced life-form on the face of the Earth, and the absolute pinnacle of evolution, was
slime
.

When a slimy surface, coated in some kind of alga or bacterium, is exposed to sand and mud on the floor of the sea, the sediment tends to stick. The organisms then grow through the sediment to expose a fresh surface of slime to the sea, and in this way great thicknesses of thinly layered rock consisting of interleaved slime and lime can be deposited. These lasagne-like sedimentary structures are called stromatolites.

The very oldest stromatolites come from 3.5-billion-year-old rocks in Western Australia, a time when we still find no firm fossil evidence for the existence of photosynthetic organisms. But ‘stromatolite’ just means ‘layered stone’, irrespective of what creatures built it. It is entirely possible for many different sorts of bacteria to produce layered structures, and what exactly made these very ancient examples is still a mystery. However, at around 2.7 or 2.8 billion years ago something changed dramatically. Isotopes of carbon obtained from rocks of this age indicate the activity of a group of primitive organisms called Archaebacteria, and highly resistant organic molecules called steranes also suggest the presence of another bacterial grouping, the cyanobacteria: photosynthetic bugs formerly known as ‘blue-green algae’.

Before levels of atmospheric oxygen rose very far, these cyanobacteria had the new trick of photosynthesis more or less to themselves and began to coat the shallow shelf seas surrounding the early supercontinents of Ur and Arctica. Their heyday was brief, though, and came to an end about 2.2 billion years ago, when oxygen in the environment built up enough for oxides of nitrogen (the nitrates) to form. Nitrates are a powerful plant fertilizer which cyanobacteria can do without; but from that point stromatolites built by invigorated true algae (primitive plants whose cells’ genetic material is organized into a nucleus) took over. Lasagne World was born just as Rust World was dying.

Algal stromatolites were soon coating every available square kilometre of shelf sea, and building the first massive limestones in the geological record. In fact, by about one billion years ago (at just about the time that all the continental fragments of the Earth were coming together in the supercontinent Rodinia) limestones were forming a greater percentage of sedimentary rocks than ever before or since. Some of these limestones are of truly awesome dimensions, kilometres thick, and the effect of trapping all that calcium carbonate – coming on the heels of increased continental weathering – was to reduce the amount of carbon dioxide in the atmosphere even further.

Earth’s atmosphere has two main ‘greenhouse’ gases that cause it to trap the heat of the Sun: methane and carbon dioxide. Methane, also known as natural gas, is by far the more powerful; but it is very susceptible to oxidation (burning, by another name). With global oxygen building up, free methane in the atmosphere went into steep decline. Now, as carbon dioxide started to be drawn down by weathering on the emergent continents and then locked away in limestones at the bottom of shelf seas, the planet began finally to lose the last threads of its insulating atmospheric blanket.

Stromatolites reached the high point of their distribution just as Rodinia was forming: a major turning point in Earth history. For it seems from geological evidence that at about this time, one billion years ago, a fundamental and irreversible change occurred to the way our planet functioned.

The fact that the modern ocean basins are more or less the right size to accommodate all the water currently available to fill them is one of the great apparent coincidences of geology. True, from time to time, notably when supercontinents break up and volcanic mid-ocean ridge
systems become more active, the ocean basins become a little less voluminous as the ridges all over the world swell up. (Imagine yourself lying in a full bath. You breathe in. Your body expands, and the water of the bath spills on to the floor. In the Earth’s case, the oceans spill on to the continents, creating vast areas of shelf sea that can persist for millions of years.)

But even this phenomenon, which has been responsible for most of the great marine transgressions of geological history, is mere tinkering. If studies of fossil ocean floor from before one billion years ago are correct, at that time the ocean crust was over twice as thick as it was after that crucial moment in Earth history. This switch may have changed for ever the way the world looked and functioned.