The Seven Sisters

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Some operating systems for personal computers come with spectacular photographs for monitor backgrounds. The Seven Sisters scene in the Canadian Rocky Mountains is one of them (Fig. 1). Alberta’s Moraine Lake is in the foreground and the Seven Sisters mountain range in the background. You can see it all personally in Canada’s Banff National Park but the photograph will be familiar to many readers.

If you look closely at the picture (Fig. 1), you’ll notice distinct layers on the mountain sides, indicating various layers of stone. These layers are deposits of limestone that were once deep in the oceans and were uplifted by the tectonic forces pushing the American continents towards the west.

The Seven Sisters

Fig. 1. Moraine Lake with the Seven Sisters mountain range, Banff National Park, Canada.

Sediment Layers

The sediment layers in this mountain are visible due the limestone deposits with different resistance to erosion. The harder layers erode slower and give rise to ledges upon which the snow accumulates. You may have seen similar mountain ranges elsewhere; for example in the European Alps. What do these mountains have in common?

They are Limestone Deposits

More specifically, they are all deposits of limestone and formed in the oceans many millions of years ago. Limestone and dolomite are carbonate-type rocks, consisting of calcium and magnesium carbonate. The crucial term here is carbonate, meaning a salt of carbonic acid (H2CO3). You may wonder where all that carbonic acid for those rocks came from? Well, that’s the real surprise: from the carbon dioxide in the air.

Carbonic Acid

Carbonic acid is nothing but carbon dioxide (CO2) dissolved in water. Of course, CO2 has been a constituent of the Earth’s atmosphere from the beginning. In fact, early on, it was a major component. However, once photosynthesizing algae came about and the photosynthesis process started doing its thing in earnest, the CO2 levels in the atmosphere began to decline substantially. This process also elevated the pH (a measure of acidity) of the oceans to alkaline (the opposite of acidic) conditions. Under such alkaline conditions, dissolved CO2 and calcium and magnesium ions form insoluble carbonate-type salts which precipitate and slowly sink to the bottom. This process of calcium carbonate precipitation is still continuing to this day, both in oceans and freshwater lakes. It can actually be observed on occasion as a “whiting” event in some lakes. The term describes the visible precipitation of white carbonate minerals in the water column.

The next question you may wonder about then is: how much of the (formerly atmospheric) carbon dioxide is locked-up in those mountain rocks?

CO2 Locked-up in Carbonate Rocks

The amount of carbon dioxide locked up in carbonate rocks around the world is staggering. Together with residues from biological origin, it is in the order of 10^20 tons, more than a thousand times the amount of all carbon in the entire world’s known plants, and all coal, oil and natural gas deposits COMBINED!

But what is really mind-boggling is the fact that all that limestone carbon was once in the atmosphere. All that carbon dioxide precipitated in the oceans and freshwater which is in the mountain rocks was once in the atmosphere before it became locked-up in these rocks. Numerous remnants of aquatic organisms prove that.

So, what then is the take-home lesson here?

Take-Home Lesson

The take-home lesson here is: Carbon dioxide is an important constituent of the Earth’s atmosphere. The world’s ecosystems need it to function. The CO2 levels in the earth’s atmosphere used to be much higher than now and, finally,

CO2 in the atmosphere is vital for survival and growth of nearly every organism on earth.