On this first day of the New Year, it seems appropriate to celebrate with cake. I don’t mean real cake – I mean ice cake. Ice cakes really do look like round sugar-dusted cakes sitting along the shore. As I looked at them one snowy Monday afternoon when the seascape faded from white to gray, I noticed just what perfect polka dots the ice had formed. Salt ice is one of those strange materials caught between the liquid and solid worlds just as much as the intertidal zone is caught between sea and land.
Most people know that it takes much colder temperatures to freeze salt water than it does to freeze fresh water. And the saltier the water, the colder it has to be to freeze. Ocean water has about 35 grams of salt for every 1000 units of water, which means it freezes around 29 F. But if you’ve ever made home-made ice cream, or tried to chill the beer in your cooler quickly, you may have learned that adding salt essentially super-cools the water by allowing it to stay liquid at a lower temperature, thus more quickly chilling your cream or beer than you could with freshwater ice. This is important to ocean creatures for two reasons. First, they can live in a liquid environment that is quite chilly. Lots of ocean animals have amazing adaptations to allow them to do this like super slow metabolism and thick layers of fatty blubber. And second, when the surface ice does freeze, it helps to insulate the waters underneath, keeping them warmer than the air up above. Sea ice also allows animals to travel places they otherwise can’t in order to access winter’s scarce sources of food. In fact, as I’m writing this, my neighbor just sent a photo of a seal floating across the water on a chunk of sea ice.
So, sea ice is important for many reasons. But, the funny thing is that there is no salt in it. When water freezes, its molecules bond together to form the crystalline structure of ice. But, those pesky molecules of salt get in the way and keep the crystals from forming. So, the water molecules simply push the salt aside and form freshwater ice on top of the ocean water. I first discovered this when my girls, thirsty from ice exploration on the seashore, decided to try a sample and noted that it didn’t taste “ocean-y”. This is of interest for another reason. If the ice that forms on the ocean is composed only of fresh water, where does the extra salt go?
One of the other properties of salt water, aside from freezing at a lower temperature, is that it is denser or heavier than fresh water. That means that salt water sinks. On top of that, cold water is denser than warm water. So, cold salty water really sinks. It is the movement of this heavy water along the sea floor that drives much of the circulation of water in the world’s oceans. One place where this is particularly important is in the Arctic, where the cold, briny slug of water is needed to drive our major ocean currents, bringing nutrients to places like the Gulf of Maine and directing the flow of the Gulf Stream.
But I still haven’t gotten to why sea ice forms into cakes? There are these neat things called frazils that are tiny needle-like crystals that form in water as it begins to freeze. When they float to the surface and stick together, they form different shapes. They form sheets when the water is particularly calm – as you might see on a pond or even a puddle. Frazils form into cakes when the crystals are swirled together in areas of more water movement. You can see this in fresh water such as along the edge of the Androscoggin River as well. These cakes, also known by some as pancakes, often have raised edges that are formed when they bump up against each other and become more and more round. In super cold places like the Arctic, pancakes can pile up on top of each other and form into sheet ice up to 60 feet thick. Or, closer to home, they might pile up into odd-shaped chunks tossed up along the shore by the tide. So, there you have it – why the ice cakes along the shore. And now you know that you can actually eat them if you’re in need of a bit of fresh water.
Here’s to a New Year of celebration and appreciation for our coast and all the wonder that it holds.
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