George and the Blue Moon
The Oceans of Earth by Professor Ros E. M. Rickaby
Earth—our blue planet—is exceptional in our solar system as it is nearly three-quarters covered by the oceans. But why are our oceans here? Intriguingly, Earth’s oceans arrived from outer space. When the Earth was forming, it was too hot for water to condense on the planet. Just as tall mountains have snowy white tops above the “snow-line,” where the cooling of the atmosphere with height allows snow to persist, so too was there a gradient of cooling to a snow-line away from the ferociously hot early Sun.
Temperatures cold enough for ice grains to form were only reached farther out in the solar system, in the asteroid belt somewhere between Mars and Jupiter. Earth’s oceans, therefore, had to be imported: many think this happened with a shower of water-rich meteorites or comets from the asteroid belt bombarding the early Earth.
Since then these extraterrestrial water molecules have been neither created nor destroyed! For the subsequent 3.8 billion years (the first evidence for liquid water comes from sediments of this age on southwest Greenland), our oceans have been trapped on the Earth’s surface, where they go around in two cycles.
First, the warmth of the Sun in the tropics turns some of the ocean to vapor (just as you see coming from a boiling kettle or steam engine) and clouds. Rising clouds cool and create rain, which trickles across the land and into streams and rivers before gushing back into the oceans.
Second, small amounts of water pop down into Earth’s interior, through deep-sea trenches in the ocean crust. This water rapidly returns to the surface through volcanoes or hydrothermal vents.
So the very same water molecules coming out of your taps at home have witnessed every second of Earth’s history, from before the start of self-reproducing life itself to the emergence of multi-celled organisms. Most probably, these water molecules passed through a dinosaur at some point. You could be making a cup of tea out of water that was once slurped down by a thirsty T. rex!
What makes water so extraordinary and the oceans so key to life is its ability to dissolve things. Put some salt in a glass of water, or sugar in your tea, and those crystals will disappear or dissolve. This is because
of the slight charge or “polarity” of water molecules, which attracts elements into solution.
Water is even better at dissolving things if it is made a little acidic, by reacting with something like carbon dioxide to make carbonic acid. Take a sip of sparkling water (those bubbles are carbon dioxide) and see if you can taste the acidity; both my sons wrinkle their noses on doing so. Now, when water cycles from the oceans to clouds, then to rain and down rivers, it becomes a bit acidic by reacting with carbon dioxide in our atmosphere. As a result, this carbonated rainwater dissolves elements out of the land (this is called weathering), takes them into the rivers, and the elements end up going into the oceans. Have you ever seen reddish-brown rivers? These are full of iron that has been leached out of the rocks.
The oceans accumulate all the elements dissolved from the land (and from reaction with the deep ocean floor at hydrothermal vents, such as spectacular black smokers). But only the water molecules themselves keep on cycling back to clouds—the elements are left behind. Some elements get so concentrated in the ocean that they turn back into minerals and fall out as sediments, notably limestone (calcium carbonate) and cherts (silica), a process which limits their concentration in the sea.
Unlike most elements, however, the elements sodium or chlorine—the two ingredients of salt—only fall out from the ocean episodically and in exceptional circumstances. For example, the entire Mediterranean dried up to a puddle about 6 million years ago, leaving huge salt deposits. The lack of a natural continuous “sink” for sodium and chlorine means that the sea is always salty.
The weathering of land by water is the very reason why life could appear and remain on Earth: It acts as a thermostat for Earth’s temperature. The speed of weathering depends on Earth’s temperature. So if, for any reason, the temperature rises—due to the increase in solar luminosity over Earth’s history, for example—or if there is an increase of carbon dioxide (a greenhouse gas that warms the Earth) in the planet’s atmosphere, then the rocks on land dissolve more quickly. This leads to a
rush of elements (and carbon) into the oceans—which in turn speeds up the formation of sediment. This locks additional carbon dioxide into limestones, thus resetting the planet to its previous conditions and stopping everything from overheating. How do you think weathering works to stop the Earth completely freezing over?
While weathering maintained temperatures favorable for life to appear, we do not know, and perhaps might never know, where life did begin on our Earth (now there’s a challenge for you!). Was it in some “warm little pond,” as the great naturalist Darwin suggested, or at the depths of the ocean? Whichever it was, one thing we do know is that life’s origins and evolution depended on water. Elements are bound rigidly in rocks in the Earth’s crust, but the ocean is a watery cocktail of all those rocky elements (and organic molecules) highly available, all free to diffuse and react. This is the key to initiating life.
It is believed that the deeper oceans likely provided a safe haven for life’s very first stirrings—the surface of the early Earth would have been a much harsher environment. Down in the oceans, harmful radiation was filtered out, and the seas provided buffering against extreme temperatures and protected the development of life against bombardments of meteorites and intense volcanic outpourings.
From uncertain origins perhaps 2.7 billion years ago, scientists believe that the first 2 billion years of life’s history almost certainly then played out in the ocean. But an inescapable feedback spurred life to become more and more complex. The increasing success of microbes created more chemical byproducts (notably oxygen in the atmosphere), most of which were initially toxic. So to afford more and better control of internal chemistry, simple cells became compartmentalized (these kinds of cells are called eukaryotes) and ultimately differentiated.
This appearance of multi-cellularity coincided with the most spectacular of life’s inventions—that of the skeleton. During this “Cambrian explosion,” 0.54 billion years ago, the rock record of life shows a change from faint ambiguous imprints to a diversity of robust yet intricate shell fossils, undoubtedly sculpted by organisms of complexity (indeed Darwin misread this explosion as the dawn of life).
The solution of Earth’s minerals concentrated in the ocean—
as explained before—made making hard parts like shells relatively easy. Just as the horned dinosaurs developed ever more elaborate ornamentation against the increasing ferocity of the Tyrannosaurs, these first “biominerals” afforded armored protection against forces, poisons, and, importantly, predators.
Skeletons—shells and bones—gave rigidity to support animal life in the first steps onto land!
Over Earth’s history, the weathering thermostat has maintained a balance between the amount of acidity (the carbon dioxide) and the amount of alkalinity (the dissolved ions in the ocean). You might think of the continents as an indigestion remedy or “antacid” for the ocean. As long as the oceans have been present, they have always been slightly alkaline—perfect for making skeletons.
But we—and future generations on Earth—face a growing problem.
The booming of mankind and our thirst for fossil fuels is adding carbon dioxide—hence acidity—to the ocean at an unprecedented rate. In a million years or so, the dissolution of the land masses of our continents will accelerate sufficiently to start to neutralize this great burp of carbon dioxide into our waters. But this weathering is inherently slow, so in the meantime, the oceans are becoming a bit less alkaline and a bit less saturated. This process is often termed ocean acidification. “Ocean slightly less alkalization” would be a more accurate description, though less headline-grabbing!
Vulnerable organisms such as coral reefs will find skeleton generation increasingly challenging. This could have enormous ramifications across the marine ecosystem. Unless organisms can adapt—and fast!
Some scientists believe we should intervene to redress global warming and acidification by “geoengineering” carbon dioxide removal. This could include manipulating the weathering of the land, to release more alkaline elements into the seas.
But should we really embark on yet another global-scale experiment with our Earth?
What do you think?