A Reef in Time
My nose was almost touching the shiny grey-white undulating slab of stone that was slick with moisture. My head was similarly close to the slab of stone above me, though I couldn’t see what it looked like. My elbows and forearms were inching me forward combined with whatever purchase I could get from my toes. In front of me my right hand held a carbide lamp with its silent elegant feather of acetylene flame guiding me forwards. This was me in my first year of doctoral research having decided I wanted something interesting and exciting to do. The Imperial College Caving Club provided this, heading out to the Brecon Beacons in an old ford car for a weekend of underground adrenaline. I can think of no other occasion when I was so closely surrounded by lime in different forms, though a caving trip to the Mendips as part of a management training exercise and trips to the disused chalk quarries above the Silent Pool in Surrey (coming home caked in chalk), come a close second.
The carbide lamp was an elegant testament to the many industrial uses that lime is a part of. The calcium carbide which was one part of the fuel which powered the light is a product of calcium oxide and coke heated to high temperatures in an electric arc furnace. The calcium oxide in turn is a product of limestone which has been heated in a lime kiln to over 800C, breaking down the calcium carbonate (which the limestone is composed of) into calcium oxide and CO2. The other part of the lamp’s fuel is water, which drips onto the powdered calcium carbide at the base of the lamp, reacting with it to produce acetylene. The acetylene burns with a small, pointed flame of bright light, perfect for caving. The lamp is literally in its element in a limestone cave and is still used by some cavers as their preferred source of light.
Carbide lamps have also been used by many generations of miners albeit the naked flame was a hazard in coal mines. This led to its replacement in this setting by the Davy lamp, with its gauze cover reducing the risk of igniting the coal gas.
The chemistry of calcium carbonate has not just been exploited by industrial man but also by a remarkable range of creatures throughout the history of life on earth. One of the earliest pieces of evidence for life are laminae of calcium carbonate in mounds about a metre across which resemble structures, called stromatolites. These are the product of microbial mats produced by cyanobacteria and growing in shallow water where they trap fine lime-rich sediment. The oldest of these are some 3.5 billion years old in the early Archaean eon and discovered in sandstone from western Australia.
Stromatolites reached the peak of their diversity in the late Proterozoic eon though they can still be seen today. These are not only the earliest life forms but also show the way that organisms started use lime as a way of protecting themselves and in doing so beginning to form reef like structures.
The journey from here to the astonishing diversity of life on a modern coral reef is a long one with periods of progressive ecological diversification and colonisation, but with some major setbacks too. By the Devonian period, some of the largest reefs that ever occurred on earth had developed.
These reefs were dominated by stromatoporid sponges, along with tabulate and rugose corals, a markedly different mix of animals from our modern reefs. Not only were there different ratios of reef-forming families, with sponges more common than corals, but also these Devonian sponge and coral orders are all now extinct.
Towards the close of the Devonian period, there was an extinction event, the fourth largest of the Phanerozoic (the current geological eon). This extinction event, thought to be caused by climatic cooling – a precursor of the glacial conditions which fluctuated throughout much of the Carboniferous period – wiped out the stromatoporoid corals and severely impacted both the tabulate and rugose coral families.
It wasn’t until the Triassic period, with the cold global climate of the Carboniferous long in the past that reef building on the scale of the Devonian returned. This also post-dated the “great dying” at the end of the Permian Period, the largest mass extinction the earth has seen, when life on earth was nearly wiped out. This extinction event finally finished off the tabulate and rugose corals and, in the Triassic, these old-order corals were replaced by the modern sclerectinian corals. These corals had a lighter structure than the rugose and tabulate corals and were made of aragonite (a polymorph of calcium carbonate) rather than calcite (the other polymorph of calcium carbonate). Sclerectinian corals are also know for their symbiosis with the photosynthesising algae, the Zooxanthellae. This partnership gives the corals an advantage in fixing lime from atmospheric CO2, and it may be that this collaboration gave these corals an edge in colonising and diversifying throughout the world.
It remains an article of debate as to whether the older rugose and tabulate families of corals had this ability. This advantage to growth had its downsides though. When it came to mass extinction events, the zooxanthellate corals were at a disadvantage, constrained as they were to shallow water and more vulnerable to changes in temperature and ocean acidification. This was borne out in the mass extinction at the end of the Cretaceous period where the asteroid impact at Chicxulub in Mexico, not only wiped out the dinosaurs but many other families. The corals that survived this event tended to be deeper water, non-zooxanthellate organisms. You can’t hold a good idea down though, and following this catastrophe, the symbiotic relationship was re-established independently in several coral families.
The diversification of species of reef building animals to fill a wider range of ecological niches and larger areas of the sea-bed is not just controlled by evolution and natural disaster. There is wider rhythm to fluctuations in diversity and range related to changes in climate. One of the consequences of the change from a glaciated world to a hotter global climate with no polar ice caps is that higher sea levels flood continental shelves to create more extended shallow marine environments. As a result, hotter worlds such as the Silurian to mid-Devonian or the Triassic to Cretaceous had much larger habitats in which corals and other reef forming organisms could thrive. It may be that during the Carboniferous period, much of which sported polar ice caps, the consequent low sea levels and restricted shallow sea environments contributed to the slower development of reef habitats.
The Carboniferous period, however, was not without its reefs and was categorically not without limestones which formed from the remains of calcifying animals. Without these limestones the Romans would not have had a source for making lime with which to help stick the Wall together. The remains of reef like structures made up of rugose corals can be seen, for example, on the Northumberland coast. It is during this period that the thick shelled productid brachiopods become common a well as the elegant plant-like crinoids. These three animals are the dominant calcifying creatures of the Carboniferous, and their fossils are the ones you are most likely to find if you happen to come across a limestone outcrop when exploring the Wall.
I am certain that there would have been some of these fossils under my nose in that cave in the Brecon Beacons, however my mind was otherwise occupied at that moment. The steady drip of the water into the calcium carbide of my lamp making the acetylene to light my way forward seemed an appropriate reminder of the slow work of water in the cave I was crawling though. Limestone is soluble in water, particularly when acidified by dissolving CO2. As the limestone dissolves it produces some beautiful landforms, including the limestone pavement which was Mystery Rock number 22 for the WallCAP newsletter last month. It also completes the cycle started by the corals and crinoids and brachiopods fixing the calcium and carbonate ions into their shells, and now being dissolved and returned via the river to the sea.
It is also a reminder, as levels of anthropogenic CO2 continue to rise, that rising global sea temperatures and an acidifying ocean make our extraordinarily diverse, complex and beautiful reefs vulnerable. What will be the role of reefs be in the future, as we move into what is now commonly referred to as the Anthropocene?
Attributions
Stromatolite: Stage : Paleoarchean from 3 600 to 3 200 Ma (million years ago). Locality: Strelley Pool Chert (SPC) (Pilbara Craton) – Western Australia. By Didier Descouens – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=15944367
Modern stromatolite: Stromatolites growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia. Paul Harrison, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Stromatoporid Sponge: Jstuby at English Wikipedia, Public domain, via Wikimedia Commons. Stromatoporoids in the Silurian-Devonian Keyser Formation, Old Eldorado Quarry, in Blair County, Pennsylvania.
Corals through time: https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/corals/
Modern corals: By Toby Hudson – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11137678. A variety of corals form an outcrop on Flynn Reef, part of the Great Barrier Reef near Cairns, Queensland, Australia.
Zooxanthellae: By Todd C. LaJeunesse – flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=79980176 Symbiodinium, colloquially called “zooxanthellae”. Corals contain dense populations of round micro-algae commonly referred to as zooxanthellae. A typical coral will have one to several million symbiont cells in an area of tissue the size of a thumbnail.
Healthy reef: Holobionics, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Lodestone Reef Valentines Day 2016, Green Chromis on Coral.
Bleached reef: Bleached coral reef (Acropora) (Island of Réunion). The original uploader was Elapied at French Wikipedia., CC BY-SA 2.0 FR <https://creativecommons.org/licenses/by-sa/2.0/fr/deed.en>, via Wikimedia Commons
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