Stardust

What is a pebble? It is a wave-smoothed piece of rock, and a complex mineral framework, and a tiny part of a beach, and a capsule of history too. All these guises have their own stories, and these we shall come to. But from yet another viewpoint the pebble is a collection of atoms of different kinds—of many, many atoms—and that might be the best way to start. Considering it at this level, it is a little like taking the equivalent of a large sack of mixed sweets and separating them out into their different types. How big a sack, though? Or, to put it another way, how many atoms in our pebble? There is a simple formula for estimating the number of atoms in a piece of anything. The basic idea was first glimpsed by Amadeo Avogadro, Count of Quereta and Cerreto in Piedmont, now Italy: scholar, savant and teacher (though his teaching was briefly interrupted because of his revolutionary and republican leanings—a little impolitic when the king lives nearby). Avogadro was interested in how the particles (atoms, molecules) in matter are related to the volume and mass of that matter. Years later, his early studies were refined by other scientists and the upshot, a century or so later, came to be called Avogadro’s constant. Thus, in what is called the mole of any element there are a little over 600,000 million million million—or, to put it more briefly, 623—atoms. A mole here is not a small furry burrowing quadruped, or a minor skin blemish, but the atomic weight of any element expressed in grams. For oxygen a mole would therefore be 16 grams, as 16 is its atomic weight, an oxygen atom having a total of 16 protons and neutrons in its nucleus. The kitchen scales tell us that our pebble weighs some 50 grams. About half of it is made up of oxygen, and much of the rest is silicon (atomic weight 28) and aluminium (atomic weight 27) with a scattering of other elements, most somewhat heavier. A judiciously averaged atomic weight might therefore reasonably be something like 25.

Futures

The pebble is on the beach, once more, unmarked by its brief contact with human sentience. Almost unmarked. The fingerprints that it lightly bears will, however, be washed away by the next tide. It has a long future, still, but probably not as a pebble—though quite how long it remains as a pebble may well depend on human action. Not on immediate, direct human action—whether it is scooped up by a digger and converted into concrete for a sea-front esplanade, for instance, or even collected as a souvenir by some passing tourist. Either of these fates should cause only a brief deflection from its long-term future (the esplanade is, after all, only a cliff to be attacked by the elements, while beach souvenirs are soon discarded). A larger perturbation of its trajectory more probably hinges on wider human effects—but more of that anon. We might assume, first, that nature runs its course. A pebble on a beach, its natural environment, is changing all the time. Not long ago, it was part of a slab of slate in a cliff, then it briefly became an angular chunk of rock, before the waves and water smoothed it down. They are still smoothing it, wearing away at it, making it smaller. Even the contact with human hands probably removed a grain or two. A pebble has the appearance of permanence, but it is not permanent. How long does it take to wear down a pebble? This can happen astonishingly quickly. Even over a single tide, being washed backwards and forwards by every incoming wave, a pebble can become detectably lighter—by less than one tenth of one per cent, admittedly, but that weight difference can easily be measured using modern electronic scales. Over a season, on an exposed part of the coast, a pebble can lose between a third and a half of its mass. The rates will vary—on a stormy day the banging of pebbles against each other can produce distinct percussion marks on their surfaces, while on a calm day the attrition rate will drop markedly. Night and day, though, the pebble is disintegrating.

Breaking the surface

The pebble is a small but perfectly integrated part of a metal factory. This factory has produced copper, silver, zinc, lead and gold (real gold, not its iron sulphide facsimile, pyrite). It is about 100 kilometres long and 60 kilometres across, by about 6 kilometres deep. It is called Wales. The metals have sustained, puzzled, frustrated, and finally abandoned many generations of Welsh miners. Many hundreds of generations, indeed, for these metals have been sought, avidly, since at least the Bronze Age, more than 3000 years ago, when shafts were dug through solid rock with little more than hand-held antler bone and rounded cobble. It is no small feat to chase the metal underground, for its path is tortuous, its presence capricious and its surroundings dangerous. The Welsh miners have been celebrated at home in literature and songs, and also in more surprising quarters, as in the Japanese filmmaker Hayao Miyazaki’s portrayal of them in Castle in the Sky (a children’s animé film, perhaps, but deeply serious at core, like everything that Miyazaki has done). So how is a country-sized metal factory created? Tiny fragments of the answer reside within the pebble. A streak of white crosses the pebble, cutting across both the strata and the tectonic cleavage surfaces. Cutting both these fabrics, it must then be younger. Such evidence of what-came-first and what-came-next is at the heart of geology, and has been so since the very beginnings of the science, since before geological time was pinned and measured by the application of atomic clocks and of fossil time-zonations. And for all today’s shiny atom-counting machines and well-stocked libraries and museums, this kind of logic is still the first thing the geologist applies when any new and unfamiliar problem comes into view. But what is it in the pebble that is younger? Peer with the hand lens, and the white streak is resolved as a mineral vein: that is, as a mass of tiny crystals that have grown within a fracture in the rock.

Ghosts in absentia

One of the books that changed my perception of the world is The Open Sea, Part 1, by the marine biologist Sir Alister Hardy. He had set out to write one book about the sea, but found that there was so much to say about the world of the plankton that it took up a whole book (he then had to write another book about everything else). It’s now more than half a century old, and yet this hidden world remains marvellously evoked by his words, and by the antique black and white photographs and line drawings. Coming to this as a palaeontologist, it was eye-opening. I was aware that in the strata, one normally only finds the remains of those forms of life that had some hard parts to fossilize. Bones, teeth, shells—and in the case of the acritarchs, chitinozoa and graptolites, their tough organic casings and homes. I knew that there had been other soft-bodied things out there of course, but alas these don’t register often enough on the radar of the geologically programmed. So the sheer variety and exuberance of this world, revealed in those pages, took me by surprise. The remains of some of this life, within the pebble, lie somewhere within the amorphous black carbon that gives this object its dark colour, and in some of the subtle chemical signals of the rock itself. Parts of the hidden Silurian sea are beginning to be decoded from this unpromising material, and the stories emerging—fragmentary, ambiguous, tantalizing— sometimes have surprising uses. Tow a fine-mesh net behind a ship for a few minutes, as Hardy did as a working scientist, and then examine its contents with a microscope, and a small fraction of this world is revealed—enough to reveal its almost boundless diversity. There are microscopic plants, the base of the food chain: the diatoms, for instance, single-celled algae with a silica skeleton that looks like a tiny ornate hatbox; the coccolithophores, even smaller algae with a bizarre calcium carbonate skeleton made of overlapping shield-like discs, and the dinoflagellates, too.

The sea

Some things are just infuriatingly difficult to pin down in geology. For instance, just how deep was our pebble sea, the Silurian sea of the Welsh Basin at the spot that became, some 400 million years later, the beach beneath our feet? Well, one can estimate some kind of minimum depth. It was deeper than the depth to which waves and tides can leave a trace on a sea floor, because no traces of these phenomena have been found in the pebble stuff or—rather more convincingly as evidence—in any of the strata of those Welsh cliffs from which the pebble could have been derived. As a rule of thumb, that means that the sea was more than a couple of hundred metres deep, that being the depth to which the very biggest waves of the very biggest storms on a wide open sea can stir the sea floor. Now, if strata have been deposited above that level, then one can make some reasonable estimates of ancient water depth. Thus, if one finds fossilized beach-strata, that is an obvious signal that those rocks were formed virtually at sea level. And below that, we can make a distinction between those shallow sea floors that are stirred pretty well all the time, even by the small waves of a fair-weather day (on this kind of sea floor, mud is winnowed away, and only sand and pebbles can settle); and those deeper sea floors only affected by the biggest storms (where thick muddy layers can settle in between major storms that may have been a decade—or a century—apart). But below even that? It is, in practical terms, hard to tell from the rock strata whether the ancient sea floor on which they were laid down was 300m or 3000m deep, or perhaps even more. So it is with the pebble rock. This Welsh sea floor was deep in general terms, but its precise depth remains a mystery—working out even a reasonably imprecise depth remains as a puzzle for future generations of geologists to solve.

Making mountains

It has been a quiet 20 million years for the pebble: an interlude, at somewhere around 3–4 kilometres under the sea floor. The rock has still been crystallizing, but only very slowly. The water has by now mostly been squeezed out, so little fluid has flowed through that rock. At this depth it is hot, well above 1008°C. The pebble-form is sterile, lifeless. The time is now a little under 400 million years ago. We are in the Devonian Period. Above, at the Earth’s surface, changes have been taking place, but as far as they affected the pebble they could be on another planet. In the sea, the graptolites have been going through an evolutionary rollercoaster, with explosions of diversity separated by bad times, when they only just survive. Soon, one of those bad times will be terminal, and they will disappear from the open seas, never to return. By contrast, the fish are beginning to thrive both in the sea and in rivers and lakes. The land is greening, almost explosively, as plants evolve furiously. None of this affects the future pebble. But something soon will. The sea above has been gradually shallowing, filled in with sediment from the encroaching land. Eventually, it changed, some few million years ago, into a vast coastal plain, traversed by rivers. We are about at the time, now, when that lowland is about to rear up to form a range of mountains that—although much reduced from their early glory—can still be climbed today. What took them so long? For the Iapetus Ocean to the north, which, 50 million years ago, was more than 1000 kilometres across, had effectively disappeared 20 million years ago, the ocean plate sliding beneath the northern continent of Scotland and north America. But on Avalonia, the effect was as if these continents had just slid neatly into place, with only minor distortion of the Avalonian crust (and, in truth, these landmasses did approach each other partly from the side, rather than headon). Did the mountain-building force still come from the north, perhaps as some mysteriously delayed intensification of the vice-like grip that held these landmasses together?

Gold!

This is the beginning of the long goodbye to the surface realm. The flakes and grains of the pebble material are now in utter darkness (except perhaps for occasional flickers of phosphorescence from some of that microbial life), at the bottom of that deep, stagnant sea. The strata that we see in the pebble are a few centimetres thick. But now, of course, they are made of good, hard, respectable, tightly compressed rock. Back then, they made a layer of mud—waterlogged, sticky, slimy, and very likely evil-smelling mud—a quarter of a metre thick or more, that formed part of a layer on the sea floor that extended for tens of kilometres in every direction. Let us catch it at just this point in time, before it became buried by further influxes of sediment from those endless turbidity currents. The mud was full of life, particularly at the surface, most of which will have been occupied by those infinitely complex microscopic city-states that are microbial mats. But even below that, in the buried mud itself, there will have been considerable activity. In fact, as microbes are extremely good at clinging to life in all kinds of conditions, that activity was to carry on for quite some time yet. Those indefatigable microbes, though, still had to earn their keep. One way of doing that was by making use of the soft tissues of the fallen plankton, that were dismantled and recycled in the process that we call decay. Even in these anoxic conditions, where decay was slow, the magnificent, complex molecular architecture of body tissues was beginning to degrade, to transform into smaller, simpler molecules, leaving just the considerable inedible remnants that are the cases of the acritarchs and the chitinozoa, and the living quarters of the graptolites, upon which the microbes did not seem to manage to get much of a foothold (so to speak), even though they had decades and centuries in which to make the attempt. It is one thing to be occupied in this microscopic breaker’s yard, amid the wreckage of proteins, fats, and carbohydrates.

Where on Earth?

In some ways the pebble is like one of the newer computer chips, tightly packed with more information than one could ever surmise from gazing on its smooth surface. That stored information can relate to any episode in the history of the pebble, and could be derived from nearby—a microbial mat growing on the exact spot on the sea floor where the pebble sediment accumulated, perhaps. But it could come from afar, such as a micrometeorite landing in the ocean and drifting slowly down to land on that very same spot (there are likely a few of those in the pebble, too). Some information is as pristine as the day it was written, in its own particular code, into the pebble fabric; some, on the other hand, has been almost completely overwritten, when yet further information was imprinted at some later point in time. We might consider here some information that has most likely been all but erased by the pebble’s tumultuous subsequent history—not that that should stop us trying to recover what we can of it. Nevertheless, when it was written into the fabric of the pebble, it provided a clear signal that travelled easily through some 4000 miles of solid rock, straight from the centre of the Earth. This signal gently nudged and guided certain of the flakes of sediment falling on to that sea floor. It made them line up, with almost military precision, to point polewards. They form a memory of latitude. The Earth’s magnetic field is a mysterious thing. What is magnetism? As a child, I used to push together the north poles of two toy magnets, and remember even now how frustratingly difficult it was to make them touch—or how tricky it was to prevent the north and south poles from locking together when I tried to keep them just a tiny bit apart. A few years later, I looked on, impressed but with incomprehension, as a physics teacher sprinkled iron filings around a magnet, to show how they lined up along the invisible lines of force.

To the rendezvous

Before any great expedition, there is a gathering of all of the forces—of the clans, the troops, the mercenaries—from near and far, by various routes. Once met, they will then travel en masse, their fortunes from then to be bound together, for good or ill. Sediment particles of the future pebble were gathering, around the shores of Avalonia, in the Silurian Period, for a journey that would take them to a resting place, one where they would not see the light of day for something over 400 million years. The grains of sand and flakes of mud, with all their variety and histories, were being washed into some long-vanished shoreline by Avalonian rivers, rivers that have not yet been discovered, or charted, or named, by modern-day explorer–geologists. Likely these rivers never will be charted, for in flowing they eroded themselves away, washing away their own tracks, as Avalonia was being dismantled, grain by grain, by the eternal, tireless action of the weather. All that is left is the freight they carried, the baggage of sand, mud and pebbles. The ancient shoreline lay not much more than 50 miles away from what is now our pebble beach in west Wales. It lay to the south, around what is now Pembrokeshire in South Wales. What did it look like, that ancient coastline? Well, it may even have resembled the rugged Pembrokeshire coastline of today, though it faced north rather than south, looking across an area of open sea that was later transformed into the Welsh mountains. For the pebble stuff, the passage across that coastline marked the entrance into a new realm. As the river waters entered the sea, their onrush slowed. The sediment grains, no longer driven by river flow, would have piled up around river mouths as deltas, or within silting-up estuaries. They would not have been stilled for long though, for coastlines are places where energy is exchanged. New forces acted on these sediment particles: wind and tides and waves, the forces that nowadays mariners need to respect, and understand, and predict.

Ghosts observed

Life is ubiquitous on the Earth’s surface. Exuberant, fantastical, tough, and very, very persistent, it gets pretty well everywhere. Darwin marvelled at what could be found in a simple tangled thicket by a footpath, while a spadeful of soil can keep a zoologist occupied for weeks—all those mites and worms and springtails and leatherjackets—and a microbiologist busy for months. There is life in the hottest deserts and in Antarctic ice and nestling up against boiling volcanic vents. It flies high through the air too—not just birds and bees, but spores and pollen and aerial bacteria (so abundant that they can make rain fall more copiously by acting as nuclei for the raindrops). In death, too, the organisms can be tough. Not every corpse gets recycled back to form new generations of the living, and not all fossils are such scarcities that each becomes a museum piece or commands a handsome reserve price at an auction of ancient curiosities. The ghosts of the past are all around us, in solid form. Indeed, we owe to them the comfortable contemporary life (not enjoyed by all, admittedly), of centrally heated houses and easy travel and an abundance of food. The remains of dead plants and animals power contemporary human civilization, in the form of oil and gas and coal. At a price, of course, that is still to be paid. The pebble contains a little coaly stuff within it—tiny flecks of what is now essentially carbon, which gives the dark laminae their colour. It probably makes up, today, something over one per cent of the pebble; when the pebble stuff had been layers of mud and sand on that Silurian sea floor, it would have been nearer 10 per cent. That carbon was once living things—but how does one go about finding what kind of living things these once were? The easiest way to release the cornucopia of ancient life locked in the pebble might strike a disinterested bystander as a little harsh. Indeed, it would be quite terminal for the pebble, albeit highly revealing. The procedure is, by now, quite standard.