Supercontinent: Ten Billion Years in the Life of Our Planet

by Ted Nield

  So, as you watch the advancing tide from your vantage point on the slopes of the Pennine mountains, and see the dune tops slowly vanish beneath a scummy, turbid tide of thick, slimy, bitter water, you will be rewarded from time to time by the sight and sound of sudden bubbling.

  Zechstein

  Because at least part of this transgression of the sea was caused by the global rise of sea levels, this story was repeated all over the edges of the fragmenting supercontinent. But this particular example, which geologists call the Zechstein Sea, was (like many of the others) not stable. Like the modern Mediterranean, it could not exist for long without being connected to the global ocean. But because the Zechstein was a shelf sea underlain by continental crust, it was much shallower than the Mediterranean, which is a true ocean floored by dense ocean crust that sits low on the Earth’s surface. This made the Zechstein especially vulnerable to drying.

  This cannot have been a simple ‘on-off’ process. Zechstein sediments, now buried deep below the bed of the North Sea, are hundreds of metres thick. To make just thirty centimetres of evaporite (as minerals produced this way, including anhydrite and gypsum, are known), you need to drive off five hundred metres of seawater: twice the depth of the Zechstein Sea at its deepest. Clearly, fresh supplies of ocean water had to be entering the sea continuously, over long periods, and evaporating under the intense heat of the Permian desert.

  And this, of course, is where Ripon’s troublesome soluble gypsum comes from. The Zechstein Sea may have dried up almost completely as many as five times in its relatively short lifespan of barely ten million years. In doing so, it left behind regular cycles of chemical deposits, each series beginning with the most insoluble minerals (which precipitate first) and ending with those that crystallize only when there is hardly any water left to be dissolved in. So the first minerals to appear are limestone (calcium carbonate, which precipitates readily in warm, saturated water and may do so in your kettle) and dolomite, an impure limestone made of a chemical mixture of calcium and magnesium carbonate.

  The first such rock to be deposited, known generally as the Magnesian Limestone, was the very rock chosen to build the grand new Palace of Westminster, home of the British Parliament, which was rising as the Blanford brothers were leaving for India in 1856. Called Anstone, it came from quarries near Worksop, and proved a disaster in the metropolis’s acid rain. Alas, despite its workability and lovely biscuit colour, the Mother of Parliaments’ new home soon began dissolving before its builders’ eyes, giving rise to a lot of amusing but chemically suspect jokes about why the geologists advising the Parliamentary commission had suggested building the Palace of Westminster out of laxative (Epsom Salt is magnesium sulphate).

  The new sea brought some relief to the barren heart of northern Pangaea, and it is likely that around its edges the land grew green, or at least greener than it had been. But the supercontinent that enclosed it turned the Earth into a very different world from ours.

  Immortal cells

  The Earth’s climate is largely controlled by a set of fairly simple physical constants, but as scientists are increasingly finding, the combination of simple things can have results of almost unpredictable complexity.

  As it orbits the Sun, at a distance of ninety-four million miles, the Earth receives a certain amount of radiation from it, known as insolation. The Sun’s output has been increasing with time, and over hundreds of millions of years this small increase – brought about by the gradual exhaustion of its primary fuel, hydrogen – is significant enough to have to be taken into account. It is one of those secular changes of which Sir Charles Lyell would not have approved.

  This radiation hits the Earth and warms it up, and the atmosphere of the Earth keeps the heat in by the well-known ‘greenhouse effect’, and moves it around. By and large, the average energy received at the top of Earth’s atmosphere is a fairly constant 343-watts per square metre: a bit more than three lightbulbs’worth. But the complex interaction of axial tilt and other superimposed cycles made the distribution of heat over the surface of the planet a very complex thing to model.

  Seasons, the most obvious climate changes of which we are aware, are caused by the tilt of the Earth’s axis relative to the Sun, which currently stands at about 23.4 degrees from the ‘vertical’ (defined as the right angle to the plane of the Earth’s orbit around the Sun, called the ecliptic). Thus, as the Earth revolves around the Sun, for half the year the Northern Hemisphere is tilted towards it, while for the other six months it’s the turn of the Southern Hemisphere. This tilting effectively concentrates the Sun’s heat first in one hemisphere and then in another, just like leaning towards the fire to warm your face (Northern Hemisphere Summer) and then walking around to the other side, so that you face away from the fire and the heat warms your bottom instead (Southern Hemisphere Summer). At the Equator, of course, this axial tilt has little effect and seasonality is less noticeable.

  But there’s much more to it than that. If you have ever watched a spinning top that isn’t moving perfectly, or the behaviour of a double pendulum, you will have a feeling for the complex way in which harmonic systems behave. There are also eccentricities in the system to consider, and cycles that affect the degrees of eccentricity. Several such long-period cycles affect the orbit of the Earth around the Sun, and these in turn change the climate because they affect the amount of insolation: how much heat hits a unit area of Earth in any one place. How all these cycles interact, sometimes reinforcing one another, sometimes cancelling one another out, creates a highly complex system that means that the Earth’s climate is never constant.

  Take the Earth’s elliptical orbit. The Sun does not sit at the centre of the ellipse, so the Earth–Sun distance that every schoolboy thinks he knows is actually only an average. However, this ellipticity varies (over a period of 98,500 years) from very elliptical to almost circular. At its most elliptical, the extra distance from the Sun can cut the amount of insolation by as much as 30 per cent from when the Earth is closest. This cycle has almost no effect at all on the total amount of heat received by the Earth per year, because it all averages out. However, it does increase ‘seasonality’ (the contrast between the seasons) in one hemisphere, while reducing it in the other.

  The inclination of the Earth’s axis to the ecliptic varies (over a timespan of 41,000 years), between extreme values of 21.39 (nearest to ‘vertical’) and 24.36 degrees (most inclined). This cycle also affects the length of the dark polar winters, and has a marked effect on climate in high latitudes. In addition, the Earth’s axis of spin describes a circle (over a period of 21,700 years). You can make a spinning top do this very easily by giving it a nudge. This is called precession.

  If you combine two of these factors – the 98,500-year cycle in orbital ellipticity with the 21,700-year cycle in the Earth’s axial tilt (precession) – you generate a harmonic interference between the two cycles: they produce another cycle. At one extreme the Earth will come closest to the Sun during the Southern Hemisphere Summer; and at the other it will come closest during the Northern Hemisphere Summer. At these extreme points in the cycle, the additive effect of axial tilt and proximity to the Sun makes the summer more intense (and, six months later, the winters deeper, occurring as they will, when the Earth is at its farthest from the Sun). Intense summer conditions will increase the heating of land areas (land heats up and cools down much more quickly than the more even-tempered ocean), with striking effects on rainfall, as we shall see. The overall effect creates cycles of seasonality. But by the same token, when the seasons are at their most contrasted in one hemisphere, they will be at their least contrasted in the other.

  Orbital climate-forcing effects were first described by Scots geologist James Croll (1821–90) and later developed by Serb mathematician Milutin Milankovich (1879–1958), and for this reason they are known collectively as Croll–Milankovich cycles. But the climate is not all about angles of tilt and rays per square metre. The Earth’s fluid shells – the air
and water – are what make it completely different from any other space rock struck by starlight. Earth’s atmosphere and the hydrosphere absorb and transport the Sun’s heat around, creating an equable average temperature at surface (currently about 25 degrees Celsius). In the oceans this circulation is achieved by a set of interlocking convection-driven cells called gyres; there’s one in the North Atlantic and one in the South Atlantic, for example. But they are not discrete: they mesh like cogs in a gearbox, shunting water (and heat) from one gyre to another. In fact, the ocean basins are connected by a three-dimensional ‘global conveyor’, as it has become known, refreshing and warming bottom waters, creating fertile upwellings of cold, mineral-rich waters elsewhere, and preventing stratification: the tendency of warm water to float on cold, light on dense. This keeps the whole ocean system oxygenated and healthy.

  Oceanic convection cells are very much dependent on the shape of the ocean basins – and hence on the distribution of continents. But if you want stability, look to the atmosphere. Here three huge, sausage-like convection cells sit around each hemisphere like the folds of rubber flesh surrounding M. Michelin. They are invisible of course, though the cloud patterns give them away – if you know what to look for. They have existed for billions of years and continue their convection more or less irrespective of what the orbit is doing, or where the continents happen to lie on the shifting surface of the globe. Behind the fickle airs there is a dynamic stability that has easily outlasted the transient continents.

  These great convection tubes create the major climatic zones of the Earth, which like them lie in belts parallel to the Equator. The cells exist as the stable answer to the need to dissipate heat from where it is most plentiful – at the Equator – to the poles. At the Earth’s waistline, hot air rises, creating more or less permanent low pressure and rain as moisture condenses. The rising air hits the upper edge of the atmosphere and splits in two, some going south, some north. We shall follow the northern limb.

  This air travels north high up at the top of the atmosphere until it meets more – circulating in the next cell – coming in the opposite direction. The two currents collide and sink back to Earth again. This falling air is dry and creates permanent high pressure. Where it hits land it produces desert conditions everywhere on land except near coasts, where some moisture can blow a little way inland. Thus on either side of the wet equatorial region you find bands of deserts. They stand out well on those ‘where is the plane?’ simulations provided on long-haul flights.

  On hitting the Earth, the air splits again. Some goes back south, to pick up moisture and rise again at the Equator. The rest travels north along the Earth’s surface and does not rise again until it meets cold air travelling Equatorwards from the pole. The two then meet and rise, creating another line of low-pressure systems, and rain. Over the poles, in the final or Polar cell, cold dry air sinks, creating high pressure with (usually) relatively low evaporation – the dry arctic air of the tundra.

  Turning in the widening gyre

  A complication, introduced by the Earth’s rotation, is the Coriolis effect, named after French mathematician Gustave-Gaspard de Coriolis (1792–1843), who worked out the mathematics governing it. This is the apparent force, acting on all objects moving on the Earth’s rotating surface, that tends to deflect them to the right in the Northern Hemisphere and to the left in the Southern. This is why weather systems (and, allegedly, water disappearing down plugholes) rotate clockwise north of the Equator and anticlockwise south of it. On air moving in the cells, it acts to change the simple circular, ‘up-across-down’ convections I have just hinted at into helical ones.

  So in fact the winds within the cells spiral around inside them, like the rifling inside a gun barrel. And because these helical convection currents are wound in opposite directions either side of the Equator (coiling to the right in the north and left in the south thanks to the Coriolis effect), they give rise at surface to the famously reliable trade winds, beloved of sailors. The trade winds just north of the Equator blow from the north-east because the Coriolis effect deflects winds travelling south (completing their return leg to the Equator) to the right (i.e., the west). Contrariwise, below the Equator, the south-east trades blow from that quarter because these winds are deflected to the left (the west again) as they travel north.

  At higher latitudes than the tropics, the surface winds of the second great cell blow from the south-west in the Northern Hemisphere (bringing Britain its rain from the Atlantic) because those convection currents become deflected to the right, veering westerly. Above the southern tropics, winds that would be blowing back towards the South Pole (i.e., northerlies) are deflected to the left, backing westerly.

  What does all this mean for reconstructing vanished supercontinents? To some extent, no matter where the continents lie, the prevailing winds between the Equator and the tropics, and between the tropics and the pole, have always blown, and will always blow, in pretty much the same direction. These winds will be wet in the same places, and dry in the same places. Falling air will create high pressure; rising air will create lows. It’s simple – it’s physics.

  The way in which the atmosphere then interacts with them creates different environments, which the geologist can diagnose by looking at fossils, and the rocks that contain them, and by comparing this evidence with organisms and sedimentary environments around us today.

  But then the distribution of land and sea comes into play, and snarls up this simple convecting system. Think of how, joining Laurasia, the northern landmass to Gondwanaland cut off the equatorial currents and plunged Gondwana into a deep ice age. The distribution of continents clearly affects the way the oceans’ gyres work, and in much more unpredictable ways than the unchanging and imperturbable Polar, Ferrel and Hadley Cells of the atmosphere. Moreover, the monsoon is entirely dependent on the distribution of land and sea, the heat differential between them, and seasonal temperature differences across the Equator. These elements prevent the atmospheric circulations from perfectly overlaying an unchanging pattern of climatic stripes upon the shifting continents.

  Megamonsoon

  Children’s encyclopaedias never fail to include an explanation of onshore and offshore breezes; the former created during the day when the sun heats the land, and the latter at night when the land cools off quickly, leaving the sea warmer. Monsoons are in a way similar, but writ large, operating at continental scales over annual rather than daily cycles, and large enough to disrupt entire climate zones.

  The word ‘monsoon’ comes from the Arabic mausin, meaning ‘season’, and refers to winds that change from one part of the year to another. However, the common usage of ‘monsoon’ is for heavy rain associated with the summer monsoon winds of Asia, blowing ashore off the Indian Ocean.

  In India, for example, monsoon rains arrive in early summer. The winds blow onshore from the south-west, though these are actually the south-east trade winds of the subequatorial Hadley Cell, being pulled off course by hot air rising over the baking heart of India. The Himalayas and Tibetan Plateau intensify this process, by introducing hot air much higher in the atmosphere.

  The moisture-laden winds, which otherwise would have made landfall in the Horn of Africa, find themselves yanked back on themselves (the Horn receives its rains either side of the summer monsoon season, when the trades go back to normal). As the winds rise up over India’s great plateau, they are forced to drop their moisture; but as the rain condenses out, a runaway effect is created because condensation releases yet more heat, the ‘latent heat’, which pays back the extra energy needed to evaporate the water in the first place (which is why fountains feel cool: the evaporation they induce absorbs heat from the surroundings).

  To create a monsoon, therefore, all you need is a strong heat contrast between land and sea, and a source of moisture-laden air. At times in the Croll–Milankovich cycle when seasonality is most pronounced (say, a Southern Hemisphere Summer coinciding with the Earth’s closest approach to t
he Sun) any monsoon in that region will be enhanced. And that is the sort of cyclicity that geologists expect to find when looking at the rocks laid down through many such cycles over thousands of years.

  Computer climate models for a Pangaean Earth show that by far the greatest portion of global rainfall in that time was convective, and took place at the Equator – and hence almost entirely over the ocean, Panthalassa and its reef-fringed embayment, Tethys. As the great ‘C’ of Pangaea drifted slowly northwards, coming to straddle the Equator more symmetrically, the huge baking landmasses now sitting just north and south of the Equator exerted a gigantic effect upon the distribution of rainfall.

  Tethys, embraced by the supercontinent, was a warm ocean. An equatorial current flowed directly into its maw, concentrating heat and nutrients gathered from Panthalassa and introducing massive amounts of moisture into the atmosphere above it. However, the huge land areas north and south of the gulf would have set up Northern Hemisphere Summer monsoons on Tethys’s northern coast, and a Southern Hemisphere Summer monsoon on its southern flank. Climate modellers believe that this effect dwarfed even the biggest modern monsoon, and have dubbed it ‘megamonsoon’. Also, Pangaean mountain belts were probably among the mightiest ever seen on Earth. Those bordering Tethys’s northern coasts would have mirrored the enhancing effect of the modern Himalayas on the modern Asian summer monsoon, making the megamonsoons even more so.

  A new Lyell

  In conjuring these vanished worlds back into being in such great detail, geologists use two forms of uniformitarian reasoning. They project physical constants back into the past (adjusting for secular change, such as the Sun’s slowly increasing energy output) because physical laws do not change with time. And they interpret sediments in the light of what is known, by inspection, of sedimentary processes and environments around us today.