Salt, Sea Ice and Science

Science-wise, our team has been busy getting settled in to the lab and taking all the trainings required for us to be allowed to do things. We won’t be able to get out on the ice until after our Sea Ice Training on Friday, so I thought I’d take this opportunity to talk a little about the actual science we’ll be doing.

You may recall from earlier posts how increased albedo can lead to a Snowball and CO2 can end one. As the planet cooled off and the climate system change, new types of ice could form that are rarely, if ever, found on our modern Earth. That’s why we’re down here in Antarctica in the winter–to find modern analogs of this very, very cold ice.

During our first couple of weeks here we’ll be looking for cold sea ice. “Sea ice” in science terms means ice that forms when seawater freezes–ice that formed from fresh water just happens to be floating on the sea, like icebergs, doesn’t count. When sea water begins to freeze, it forms a large number of small crystals of ice, called frazil. This stage is also called “grease ice” because it looks a bit like an oil slick on the water. Eventually the crystals begin freezing together into a solid sheet. As they freeze they trap small pockets of salt water, and these are what we’re really interested in.

As you may know, salt lowers the melting point of ice so that it becomes liquid at lower temperatures, a property that is useful when melting ice off of driveways or making ice cream by hand. Sea water averages about 3.5% salt (that’s six or seven teaspoons in a quart of salt for you American types, or 35g in a liter for the rest.) This particular salinity level means it freezes at 28F or -2C. As it freezes, the ice pushes out the salt, and the water in the pockets of salt water–the brine pores–gets saltier. If the ice gets colder, this saltier water will also start to freeze, pushing out more salt and making the brine pockets smaller and saltier still.

At -9F/-23C, some of the salt in the water starts to form crystals, called hydrohalite. Like the many small crystals in snow or table salt, these crystals of hydrohalite are good at scattering light. By sending light back out the way it came, the crystals in the brine pockets can increase the amount of light reflected from the sea ice. Our colleague Bonnie Light demonstrated this effect in the lab, and we are hoping to find it out in the field. These albedo measurements will help improve models of Snowball Earth. Because so much area in the sunlit tropics is covered in sea ice on Snowball Earth, small changes in albedo can have large effects, so it’s helpful to have measurements that are as accurate as possible.

Not all of our equipment is here or unpacked yet, but I’ll post photos when it is.

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One Response to “Salt, Sea Ice and Science”

  1. Psychroteuthis Says:

    […] as frost flowers are, they are making our life difficult. As you may recall from my previous post Salt, Sea Ice and Science we are looking at the way the albedo of sea ice changes when the salt in brine pockets forms […]

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