It’s been an interesting month for me – I’ve been studying a few fossil specimens which I’ve been meaning to get round to for most of the last two years, and starting to dabble in making some 3D reconstructions from them (on which more later, if it works). But summer is finally here, and that means it’s also been time for fieldwork.
This time, the adventure was to Islay, an island off the west coast of Scotland famous mostly for its whisky, but also (among a much smaller, more geologically inclined group of people), for its rocks. Islay has some globally important Precambrian rock successions with some amazing stories to tell, so when my friend Breandán asked me if I wanted to come on fieldwork with him there for a couple of days, I of course said yes.
Between 850 and 635 million years ago, in the Cryogenian period, Earth went through a series of massive glaciations, much more extreme than anything since. The last ice age, which we associate with beleaguered cave men hunting mammoths through the snow, was a summer picnic by comparison with the Cryogenian, and has lasted for less than 3 million years with ice sheets restricted to areas close to the poles. The longest of the Cryogenian glaciations, by contrast, locked most of the planet into an icy shell for around 60 million years, longer than the whole of the Jurassic Period.
Ice sheets reached down to very near the equator, leaving beds of glacial sediments and geochemical clues that the whole Earth was in the deep freeze. Some geologists suspect that the ice sheets might actually have covered the entire planet, creating what has been called a ‘Snowball Earth’ scenario. Islay has some of the best Cryogenian rocks in the UK, and is a key locality for studying this somewhat crazy time in Earth’s climatic history.
Like many other deposits from Cryogenian glaciations, the rock succession we looked at on Islay started with a thick glacial deposit called the Port Askaig Tillite, which is a thick, dark layer of mostly fairly fine grained sedimentary rocks, with a few much larger rock clasts embedded in it. Most of these were scraped and cracked off a continent by ice sheets which carried them out to sea before dropping them along with the rest of their much finer sediment load. Over that, in a succession of rocks called the Bonahaven Formation, is limestone – what is known as a ‘cap carbonate’, because it ‘caps’ the glacial rocks below like icing on a Christmas cake.
All over the world, Cryogenian glacial deposits are topped with cap carbonate, and this poses puzzles of its own. We usually associate this kind of limestone formation with warm water conditions, not necessarily what we would expect in the periods of respite between massive glaciations. It’s been suggested that the Cryogenian glaciations only ended when something (volcanic eruptions, the release of methane from permafrost or methane hydrate from the deep seas, for example) pumped carbon dioxide into the atmosphere, triggering a strong greenhouse effect and effectively defrosting the Earth. If this is true, then the Cryogenian Period was a time of climatic extremes – not just intense and lasting cold, but intense heat too.
Understandably, given the science fiction flavoured subject matter and its vast scope, the Cryogenian is a buzzing area of research with a mass of unanswered questions. We are still wondering what caused the glaciations and how they ended. Work on Jupiter’s moon Europa, which is entirely covered with a thick shell of ice but has an ocean of liquid water underneath, shows that ‘Snowball’ scenarios are a real possibility, and poses interesting questions about life on other planets. Life got through the Cryogenian on Earth, so why shouldn’t we find it on a frozen moon of Jupiter, too?
We know that some microbial life must have survived the cold, since we have good evidence of fossil cells dating back to at least 1800 million years ago, well before the Cryogenian. This shouldn’t surprise us too much; microbes can live in Antarctic permafrost, so the idea of life surviving more or less permanent ice cover shouldn’t seem too implausible. On the face of it, life on a Snowball Earth sounds impossible, yet somewhere under all that ice the ancestors of all life on Earth today made a living.
Thanks to Breandán MacGabhann, of Edge Hill University, for allowing me to tag along on his fieldwork with my notebook and camera, and for arranging the route of said fieldwork to include five different distilleries.