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Sea Science: Ice governs aquatic life on Earth

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Sea Science: by Scott McDowell

You need not be a sober scientist to realize that ice is lighter than water. We see cubes floating in our cocktails, and we’re aware that tips of icebergs rise above the sea. Also, we remember that bottles of water explode when frozen. This ice-density condition is apparent in our everyday lives.

Chemical principles indicate that when water freezes, the resulting ice is 8 percent less dense than its liquid state. Equally fascinating (maybe to a physics geek) is that liquid water at 39 degrees F (4 degrees C) is more dense than liquid water at any temperature, either hotter or colder. This is counter intuitive but it’s true: as 39-degree water cools, it expands. The same occurs when ice is formed; it expands beyond its volume as a liquid.

Water is the only naturally occurring liquid on our planet that decreases its density (expands) upon freezing. Ocean life is totally dependent upon this physical characteristic. If ice were more dense than water, ice in the sea or a lake would descend immediately upon formation at the surface.

At high-latitude regions, our lakes and oceans would fill with ice from the bottom up and we would never again have water at these locations because the deep ice would never melt (except near undersea volcanoes, which are uncommon on the vast sea floor).

This doomsday scenario might remind baby boomers of Kurt Vonnegut’s novel “Cat’s Cradle”, which identified Ice-9 as a molecule that would freeze all water upon contact. Fortunately, this was pure fiction as there is no ice form with this dangerous property or behavior.

Although researchers have identified 15 crystalline forms of ice in laboratory studies, only one (Ice-1) exists under normal temperatures and pressures encountered on our planet.

Jupiter’s bright moon Europa is totally covered with ice, with thickness of 60 miles. It is possible that this ice is a rare crystalline form because of that moon’s low surface temperature (-260 degrees F).

We need not worry that our planet’s water will transition to a different, rare form of ice. Even at the bottom of the seven-mile deep Mariana Trench in the Pacific Ocean, the pressure is not sufficient to create any forms of ice other than Ice-1.

Let’s get back to the important issue of ice being less dense than water. When high-latitude lakes experience the onset of winter, their surface waters cool until they reach 39 degrees F, the maximum density of liquid water. This chilled water immediately sinks to the bottom and the lake gradually fills from bottom to top with  39-degree water as more descends from the surface during the winter months. It is fortunate for aquatic life that liquid water, albeit cold, fills the bottom of lakes rather than descending ice (a drastic scenario if ice were more dense than water).

When the lake is full of 39-degree water, it continues to cool at the surface but the water with cold-air contact does not descend because, you’ll recall, further cooling reduces water density. Therefore, surface-water temperatures approach 32 degrees F and ice formation begins. Water beneath and in contact with the surface ice layer continues to decrease in temperature, eventually forming additional ice and adding to the thickness of the layer from beneath.

Ice growth is a slow process because the trapped waters have no further contact with the frigid, winter atmosphere. Cooling occurs only from contact with the overlying ice sheet.

This same scenario of ice formation occurs in the sea but the dissolved salts in seawater make the process of sea-ice formation occur at a lower temperature and with secondary, impeding effects of salt brine formation.

The take-home lesson is that, if ice were more dense than water, all bodies of water at high latitudes on our planet would be filled with ice and aquatic life as we know it in those regions would be non-existent. Certainly, that’s a worse result than having ice cubes sink in our cocktails.

Whomever or whatever is responsible for establishing the chemical Equation of State for water, with maximum density at 39 degrees F, I toast you as the ice floats atop my Dark ‘N Stormy.

Scott E. McDowell has a doctorate in ocean physics, a 100-ton Merchant Mariner license, and is author of Marinas: a Complete Guide, available at www.scottemcdowell.com. Comments are welcome below.

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