Bacterial sludge: Stromatolites
A tale of two planet-destroying organisms [1700 words 7 mins
Aagh, but it took a long time to get here. The scatter of black pine trees in the fawn-coloured early light. The roar of the water in its sandstone-sided gorge. The long drag up the peaty path, getting steeper and stonier even as the views open out behind. Claggy sweat inside your waterproofed jacket, but in your nose the air is Christmas crisp. And at last, the ridgeline. Underfoot now is bare sandy rock slabs, reddish brown glowing almost warm in the sideways light. Wind roars like a river among the rocks, and there’s the edge of a frightful drop for that wind to blow you over. And the sandstone rises in lumps, layer on layer, to ridges that stand jagged against the sky, and distant silver-grey sea the colour of mermaids.
But you're an ecology conscious kind of person – or maybe you just wore some waterproofs you hadn't bothered to do the re-waterproofing of and you've been coming up the long relentless slope of Ben Alligin in a nasty little rain shower while gradually realising that the occasional day walk in the Lakes doesn't mean you're fit enough for Beinn Alligin.
Anyway, so what if the sparkling sea stretching towards the how-pretty-is-that the Hebrides, and the majestic walls of red brown rock, not just across the other side of the valley but under your feet and dropping almost vertically alongside you in both directions... But in the bad mood you're in, or your advanced level of ecological knowledge, you do have to be aware that this what you're looking at is damaged landscape. The birch trees, pines and rowan that should have been blocking out your view all the way up the lower slopes – all gone centuries past, nibbled away by sheep and by deer. The blackened patches on those slopes of Sgurr Dubh down there, a testament to us-induced global warming. Not to mention the giant elk and the dire wolf and the freshwater mussel and the pine hoverfly and all the other creatures that ought to be here but aren't.
What you might not spot is: you are looking at not one but two episodes of planetary damage. And us humans, we are very much the small-timers in this connection. You can be forgiven for not noticing the effects – truly dreadful effects though they are – of the small near-invisible bacterial slime called stromatolites. Turned the atmosphere inside out, it did: destroyed almost every living form apart from itself.
But it did all happen around one and a half thousand million years ago...
The human damage is down on the hillsides, or the tree-free valley floor. But the stromatolite has been doing its stuff right below your boot soles. Where you see it (or in your case fail to see it) is in the cheerful red brown colour of the Torridonian rock.
Don’t complain about the weather of Wester Ross: these rocks underneath you were formed in a torrid, unbearable desert. And not one of your cosy little continent-sized deserts: in Torridonian times, the desert was everywhere. There was no grass or trees to slow down the wind; pebbles 2cm across are rounded in the style of sand-grains, showing that they were blown across the rocky surface. When rain fell, there was no soil to hold it, and flash floods piled the gravel and sand in slanty drifts.
And yet this bleak reddish-black world did contain life. Life – but not as we know it. Across the bottom of shallow seas and lakes, single-celled blue-green algae formed what’s been called a lawn or carpet, but was more like a layer of slime, stiffened with a little calcium carbonate (the limescale that forms in your kettle). As the sea drops silt, the slime climbs up through it to form reef-like mounds: these beings, half slime half stone, are the stromatolites.
The green slime didn’t have sex so it didn't evolve. It stayed much the same, ruling the earth in a soda-water ocean for three thousand million years. This is ten times longer than the dinosaurs, and roughly 1500 times as long as mankind has managed so far.
Stromatolites made their living by photosynthesis; using energy from sunlight to break up carbon dioxide, then using the carbon to build their bodies. The polluting by-product is oxygen. Oxygen: on the Periodic Table of the chemical elements, it's next companion to chlorine - and chlorine is such lethal stuff it's banned by the Geneva convention as a weapon of mass destruction.
As anyone who’s attempted to maintain rusty motor cars will know, the chemical element which enthusiastically scavenges oxygen is iron. Iron dissolved in seawater seized hold of the oxygen and formed the black, powdery oxide of iron called magnetite. This dropped safely out of the way onto the ocean bottom, where it formed stripy-layered banded ironstone.
So far, so bad: we’ve removed the protective blanket of CO2, and dropped the ocean temperature from its cosy 70°C towards where we could even have some ugly icefields forming. But you can see what’s coming. After 1.5 billion years of this, the stromatolites have converted all the ocean’s iron to ironstone, and are making oxygen faster than fresh iron was being washed into the sea.
Banded iron formation, 3000 million years old (Chambers Street Museum, Edinburgh)
At this point, dangerous and unstable oxygen starts to pollute the ocean itself, ripping into any existing organisms and breaking them back into the carbon-dioxide them have so laboriously photosynthesised themselves out of. At the same time it starts to attack iron compounds lying about on land, oxidising them to the cheerful orange colour we refer to as rust.
Life’s first work was the conversion of Earth’s atmosphere. The challenge of coping with, and then exploiting, an air poisoned with oxygen is was what led to the complex eukaryotic cell, the first multi-celled organism, and eventually to us. We think of green as being the colour of living things. But nearly a billion years before the first ferns emerged on land, the life colour marking new atmosphere and newly challenged life is oxygen plus iron: the rusty-red of the Torridonian sandstone.
Enduring slime
And yet, in this holocaust of death and adaptation, the polluting slime survived. Along the shoreline of Stromness, on a cold autumn morning (though all mornings are cold on Orkney: cold and clear, with that sharp white light that comes from having the sea in every direction, and a wind that’s blown across that clean sea from half way round the world and smells of the sea and the seaweed and nothing else at all….)
Across the pale grey water, the dark grey island of Hoy is humped against the sky. Out there, someone’s just spotted a killer whale. But inshore, at the top of the scruffy beach, the low cliffs are layered pinkish and beige. It’s the Old Red Sandstone, so called because of being less than half as old as the Torridonian (okay, 400 million years is still quite a lot). And yes, the oxygen is still poisoning Earth’s atmosphere, and the sandstone is as the name has it, iron-oxide red. But look, here it is! A squlggy pattern disturbing the layers of pinkish-brown and beige, the slime-stone still lives on a billion years later, at the bottom of the Orcadian Lake, with strange Devonian fishes swimming overhead, their four fins preparing to drag them out onto the muddy land towards eventually becoming us people.
Fast forward, and also fast upward, through the rock layers to the top edge of the Jurassic, a mere 140 million years ago. Down on the Dorset coast, the slime-rock is still at it, this time killing off some trees. Tall pine trees, they were, waving in the warm breeze while the occasional Upper Jurassic dinosaur wandered by. But the sea came in, and the bacterial slime smothered their roots. The trees fell, and the slime grew over them and covered them up. The fossil forest, they call it. But really it’s fossil slime, encasing the holes where the forest used to be.
But now, in a ‘living fossil’ moment that makes the coelacanth seem like a Johnny-gone-lately, in 1956 stromatolites were found still alive and smelling like cow-pats in the brackish waters of Shark Bay in Western Australia. The tetrapod fishes are gone: the dinosaurs are away: now stick-nest rats, brush-tailed mulgara and the occasional human in swimming trunks look down from the shoreline. UNESCO has written a stiff letter to Western Australia: the tourists might tread on the precious ancient slime.
So, when you’re tempted to complain about the damage caused by man, just remember: we’re only the second worst. And in another thousand million years or so, will some five centimetre long degenerate remnant of the big brained bald ape creature, now finding ourselves a sordid little ecological niche In the corner of the decomposition pits maybe, or the trapped silt among the air tendrils of the pseudo baobabs or - and this would be a neat irony given the fate of that hoverfly, in the rot holes of some fallen ancient pine trees. And whatever still-to-come creature, composed of quantum field variations or tangles of string theory or possibly even material substance, who knows? will look down sadly at these strange arcane entities. “Remember that microstratum I showed you? The thing we call the anthropopause, the little layer of carbonized forests, non-biodegradable plastics, fossil typewriters and radioactive waste. Believe it we must, these beings once transformed the planet into something simultaneously worse and better. Just fancy! They don’t smell very nice, do they?”
But now 1,000m below us, the lights are coming on in Alligin village. The grassy ridge-end above Coire Mhic Nobaill makes for a quick descent; there will be a nice cup of tea waiting, and a jolly big slice of fruit cake.
For now, the future will just have to wait.
Thank for making it interesting, vivid and simple enough to understand.
Gawd, Nature, huh? Fascinating.