by Ted Nield
These two elements are very similar in many ways. Both sit close together in Group 3 of the Periodic Table, which occupies two long lines at the bottom of the chart, and which are collectively known as the ‘rare Earth elements’. On the top row (the ‘Lanthanide series’) you will find neodymium (Nd) at number 60 and samarium (Sm) at number 62. The atoms of these two elements are about the same size, and they react similarly in chemical processes because in their cloud of electrons both elements have the same number available for forming bonds – two. However, something different happens to neodymium and samarium atoms when rocks in the Earth’s mantle begin to melt and form the magma that will eventually build the ocean floor.
The Periodic Table of the Elements. The Lanthanide Series is one of the so-called ‘transition element’ groups (horizontal lines) arranged in a double line below the main diagram.
Lanthanide-series elements display a very strange property. As their atomic number (and hence their atomic weight) rises, the atoms actually shrink rather than expand. This is known as the Lanthanide contraction. When melting begins, rock becomes a mixture of liquid and solid. Lanthanide contraction makes the heavier and denser Lanthanides (such as samarium) more likely to stay where they are in the solid phase, while the lighter, bigger atoms such as neodymium will tend to prefer to melt. This means that neodymium atoms have a higher tendency to leave the mantle, and samarium to stay put. Mantle rocks have therefore become progressively depleted in neodymium since the planet formed 4700 million years ago.
If you could count all the atoms of samarium in the Earth and divide that number by the number of atoms of neodymium, you would get a figure that expressed their relative global abundance. If the two elements were present in equal amounts, the number would, of course, be one. If you found more neodymium than samarium, then the resulting number would go down. For example, if there were twice as much neodymium as samarium, you would be dividing two into one, and the result would be 0.5. These numbers are called ratios, and they are useful in expressing the relative abundance of two things.
If you do the calculation for real, taking the Earth as a whole and dividing the number of Nd atoms into the number of Sm atoms, it works out at about 0.32: the ‘bulk Earth’ ratio. Or, putting it another way, there are about a third as many samarium atoms as neodymium atoms inside planet Earth.
But because, through geological time, Nd has been continually leaving the mantle in volcanic melts that have gone to form oceanic crust, Nd is more concentrated in the crust than in the Earth as a whole, so the samarium-to-neodymium (Sm/Nd) ratio of crustal rocks is lower, at 0.2. Conversely, Nd has been depleted from the mantle, so the mantle now contains less than average amounts of that element, making the Sm/Nd ratio of ‘depleted mantle’ higher (about 0.5, in fact).
So far we have not considered the matter of different isotopes of samarium and neodymium, but have thought of all the isotopes of each element collectively. However, both samarium and neodymium have isotopes aplenty. Samarium has seven natural ones, one of which, samarium-147, is unstable and undergoes radioactive decay to neodymium-143. This radioactive transformation is very slow indeed, with a half-life of 106 billion years – or over seven times the age of the universe. Neodymium has nine isotopes, seven of which are stable. One of them, neodymium-144, is not a product of any radioactive decay series, so it does not change in concentration in a given rock with time and therefore can be used as a benchmark.
The gradual decay of samarium-147 (147Sm) to neodymium-143 (143Nd) therefore has the effect of making 143Nd more common through geological time in all rocks, gradually increasing the ‘bulk Earth’ ratio between its daughter element 143Nd and the unchanging 144Nd. However, remember that there is more samarium in the depleted mantle than in the crust because of neodymium’s tendency to fractionate into melts that head upwards. Therefore the increase through time in the ratio of the two isotopes of neodymium will be faster in the depleted mantle (where the parent element samarium is relatively abundant) and slower in the crust, where Sm is rarer.
This means that the isotopic signature provided by the ratio of 143Nd to 144Nd gives the rock a fingerprint for its place of origin. Because radioactive decay processes are known, unchanging and predictable, you can then, by extrapolating backward, determine when the rock from which you obtained the sample left the mantle. It is as though the rock has never lost its accent: you can take the melt out of the mantle, but you can’t take the mantle out of the melt. Moreover, because samarium and neodymium are almost identical chemically, this fingerprint is almost indelible: it is almost unaffected by most subsequent changes that a rock might undergo.
This allows geologists to take more or less any rock that formed by the crystallization of magma – even if it formed by the remobilization of previous crustal rocks – and work out when its chemistry began to go its own way and depart from the isotope chemistry of the depleted mantle. This works because, in the end, nearly all rocks were originally derived from the upper mantle. As long as there has not been contamination from other melts with different histories (and this is usually evident from the field geology) the method is a sound way of telling when the rocks were first born.
This crucial date, the rock’s first birthday, is called the Depleted Mantle Model Age, abbreviated as TDM. This is the tool we have been looking for: a way of telling when these pieces of ocean floor, now preserved in mountain belts, first left the mantle at a mid-ocean ridge.
Back now to the rocks. By taking as many samples as possible from ophiolite suite rocks that were emplaced during the elimination of the ocean we are studying, and then comparing their TDM ages with the date that the mountain range formed (which we know independently from straightforward radiometric dating of that event), we should at last be able to determine whether that process was one of extroversion or introversion.
Using this technique, Murphy and Nance found that the rocks from Brazil that were caught up in the formation of Pannotia after what would become West Gondwana rifted off the previous supercontinent Rodinia, provide TDM ages of about 1200 million years, and that similar rocks from south-west Algeria and southern Morocco come out at between 1200 and 950 million years. In the Mozambique Belt the TDM ages come out at between 800 and 900 million years. From this they concluded that the vanished ocean, whose tombstone is those ancient mountain belts, formed part of Rodinia’s exterior ocean, because the TDM ages are all older than the date of the break-up of Rodinia. As predicted by the Paul Hoffman model, Pannotia formed by the extroversion of Rodinia. Rodinia turned inside out.
In the case of Pangaea, the supercontinent after Pannotia, the results have been, as expected, very different, but also consistent with predictions. The main mountain ranges that formed as Pangaea reached its maximum packing – the great suture scars that mark the healing up of oceans – are the Appalachians in the USA, the Caledonian mountains of the UK and Norway, the Variscan mountains of southern Europe and the Urals of Russia. We know that Pangaea’s predecessor Pannotia began to fragment at about 550 million years, when oceans like Iapetus began to form. So, as oceanic rocks caught up in the formation of Pangaea by the destruction of those ‘interior’ oceans, they should all have TDM ages of less than 550 million years. No data are available as yet from the Urals, but data from the other mountain belts all indicate that they were derived from the Earth’s mantle less than 550 million years ago. Pangaea formed by the accordion tectonics of introversion.
Quadrille
So, even from times 1000 million years in the past, geologists now are finding ways of determining how continental fragments moved about the face of the planet, consuming the ocean before them as they went. But this stately dance of the continents, which, like partners in a quadrille, move apart, twist around one another and come together again in new combinations, has not gone on for ever. Geologists can recognize the probable existence of even older supercontinents than Rodinia, though they will remain even more dubious and controversial until new techniques ca
n be found to recover more information from the geological record.
But what makes ancient secrets more difficult still to unlock is the probability that in Earth’s deepest past the tectonic processes that operated were quite different from those we see around us today. For these distant pasts, the present no longer provides the key. The Earth may then have worked in ways perhaps as different from the tectonics of today as the Ediacara garden’s inhabitants may have been from modern life forms. And when it comes to life on Earth, geologists tracing the evolution of the planet from its Hadean origins are increasingly wondering how biology itself may owe both its beginning, and its drive to complexity, to the workings of our planet’s inner life – as told in the greatest palimpsest of all.
10
BIRTH
We must be humble. We are so easily baffled by appearances And do not realise that these stones are one with the stars.
HUGH MACDIARMID, ON A RAISED BEACH
When geologists hit upon the notion of constraining their dreams of the past in terms of processes observed operating today, it made geology a science. Although debate has raged ever since about whether those processes always operated with the same intensity at all times in the long history of the Earth, the method still held together. But it only held for those youngest, lightly scraped and overwritten pages of the great palimpsest that was open to geologists of the nineteenth century and much of the twentieth: namely, the 542 million years since the beginning of the Cambrian Period, when the age of animals (and eventually plants) with easily fossilizable bodies dawned, heralding complex life’s long march – or random walk – through increasing complexity.
But with radiometric dating came the shocking realization that this segment only represented about the last 12 per cent of Earth history; and that, in the conventional stratigraphic tables of the time, the tiny unregarded plinth of complex rocks labelled ‘Precambrian’ on which the geological column rested, actually contained nearly all the time that had elapsed since the planet formed. It was like digging a well, only to find what you had thought to be solid bedrock giving way into a black, bottomless, unsuspected cavern, loud with a vast and terrifying silence.
An old-fashioned stratigraphic table, dating from 1898. The basal part labelled ‘Precambrian’ actually contains 88% of Earth history. Taken from Charles Lapworth’s Intermediate Text-Book of Geology (Blackwood & Sons). From the collection of the author.
As geologists now looked for ways to decipher the rare and often badly damaged pages of the Precambrian chapter, they began to realize something else deeply shocking. They began to see that there were things in the deepest places of Earth history for the unlocking of whose secrets the present no longer provided the key. True, up to a point the old tools still worked; after all, a poorly sorted conglomerate full of mud and cobbles and boulders the size of a man were still probably dumped by glaciers. (However, the same difficulties and controversies would attend their interpretation in the late twentieth century as in the mid-nineteenth, the only difference being that now the arguments were more sophisticated.)
But the problem went deeper than just interpreting the meaning of particular rock types. The Precambrian world that the old tools and other new tools such as isotope analysis revealed was not the familiar, endlessly cycling Huttonian or Lyellian world, ringing to what Thomas Hardy described as ‘the full-fugued song of the universe unending’: a world with no beginning, offering no prospect of an end. By contrast the vast spans of Precambrian time were dominated by progressive, secular processes that had wrought permanent change upon the evolving Earth system.
The more geologists thought about it, the more reasonable this began to seem. Just like the life of a human being, Earth’s growing years left their indelible marks upon her; and yet despite her difficult early traumas, by middle age she was leading a much more stable, routine, almost (but not quite) predictable life. She had reached a time in her life in which it was almost impossible to conceive of things ever being that radically different. Indeed, if things were so radically different then from now, perhaps they were too different for geologists to build a scientific picture of the Precambrian world? If the rule of the present could no longer be used to measure out the ancient world, what price the scientific method? How could a geologist’s imaginings of these very different times be constrained?
But all was not lost. Geologists turned first to the immutable laws of physics and chemistry; and in addition they found something new: the emerging techniques of computer modelling. Using these new approaches, John Sutton and Janet Watson’s dream of opening up the Precambrian gradually came to be realized. It was the final confirmation that the uniformitarian visions of Lyell and Hutton did not, and could not, do full justice to Earth’s chequered past. Moreover, as the idea that human activity might be affecting the Earth System became familiar to followers of current affairs, the whole question of how the climate works (a question rooted in how it evolved into its current state since the birth-time of the Earth) lent Precambrian geology a sudden relevance, even urgency. The purest of pure science, this expedition to an alien planet whose curiosity-driven mission directive had been drawn up without the slightest idea of practical application, suddenly moved politically centre-stage. For the tale of the Precambrian has proved to be a litany of terrible climate disasters, all of them brought about – or at least hastened – by life itself, and the Supercontinent Cycle.
Genesis
Erasmus Darwin (1731–1802), Charles’s eccentric, versifying, visionary ancestor, in his epic poem The Temple of Nature wrote: ‘Organic Life began beneath the waves.’ In 1871 his grandson, on the other hand, would speculate, in a letter to the Director of Kew Gardens, the explorer-botanist Sir Joseph Hooker:
It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
So who was right? Did life start in the seas, or in a little lightning-struck dish of lukewarm primeval soup? Today’s leading theory about how life came to planet Earth suggests that the older Darwin, on this occasion, came nearer the mark. Life, it now seems likely, originated not in a superficial pond, but deep below the waves on the gloomy floor of the Earth’s early oceans. What is more, it did so long before even the continents were fully formed and set sail across the globe. It isn’t only Hugh MacDiarmid’s stones that are ‘one with the stars’: life is, too.
The Earth began to accrete from a disc of space debris around 4.7 billion years ago, in a hellish birth-time colourfully referred to by some geologists as the ‘Hadean’ eon, though this name is not recognized by the International Commission on Stratigraphy, the body that decides such things. It prefers instead the rather more prosaic name ‘Eoarchean’ for the earliest, bottomless section of the Earth’s life story: the true and final plinth of the modern stratigraphic column below which there was simply no Earth.
As it vacuumed space debris orbiting the young Sun, the Earth gradually heated up. Gravitational energy from incoming bolides was converted into thermal energy. Deep within, the iron and nickel in the mixture separated out from the molten rocky materials and sank into the planet’s core, where it still remains, an eternal but querulous dynamo driving (and occasionally flipping) the Earth’s magnetic field. Up above, and for perhaps as long as 500 million years, our planet was a cratered, volcanic spaceball, sporadically molten, dark, sterile; blasted by solar wind, flayed by ultraviolet light, too hot for oceans, too hostile for life.
Although Lord Kelvin was quite wrong about how old the Earth could be because he assumed it had just cooled by radiating heat into space from its origina
l molten state, it is true that our planet was a very much hotter place four billion years ago. This was partly due to bombardment and gravitational heating; but it was also due to the much greater abundance at that time of highly radioactive isotopes.
Remember that all radioactive decay series eventually end up in stable isotopes, or at least in longer-lived and much less radioactive ones. Shortly after the solar system formed and the rocky planets coalesced from space junk, Earth’s nuclear reactor burned much hotter than today, just as radioactive waste, which will eventually become harmless, is at its hottest when it is fresh.
Because the young planet needed to dissipate maybe five times as much internal heat, the mantle convection systems deep below its crust would have been smaller and more active than today’s. Therefore, with greater production of volcanic material at surface, and faster movements among smaller plates, the number of spreading and subduction zones would have been greater. Moreover, the crust that they formed (and consumed) would also have been much thicker: somewhere between twenty-five and fifty kilometres. This has led geologists like Eldridge Moores to ask when this non-uniformitarian change from thick to thin oceanic crust took place, what the environmental implications of that change would have been, and whether it happened gradually, or more suddenly.
Birth of the continents
The crust of the newly accreted Earth would have been everywhere of the same composition (roughly speaking) as modern ocean floor, simply because this is the basic stuff of the Earth. Lighter rocks, which float high as the continents of today, had to be distilled from that crude material by the fractionation of lighter elements. Thus the silica and aluminium compounds, identified as ‘SiAl’ by Eduard Suess, had to be separated from the heavy ones, made predominantly out of silica and magnesium (‘SiMa’). So the continents cannot always have been the same size as today: they had to grow. The oceans, too, may well have been more voluminous than today because the hotter mantle could contain less water, chemically locked up in minerals, than it can today. Some scientists think there may have been twice as much water at surface, making the early Earth a truly panthalassic water-world.