Waiting out the weather


A blustery day at McMurdo Station.

Well, we were hoping for another chance at the sea ice yesterday but a storm came in and kept us confined to McMurdo Station.  The winds continued through the night, and have really picked up in intensity today after a brief lull this morning.  We are getting frequent gusts to 50 knots (93 kph/58 mph) and from the feel of things occasionally higher than that.  If it was just windy this wouldn’t be so bad, but the dry icy snow around McMurdo feels like sand.  Walking from building to building is not so much like being in a blizzard as being in a sandstorm.  Conditions are even worse out on the flat sea ice.  Visibility out there would be just a few feet.  So it’s a good day to stay inside and catch up on other work.

We took the opportunity to have a meeting with some of the safety personnel and the station manager about how we will operate when conditions improve.  No one has worked as far from the station this early in the season as we hope to do, and we have to alleviate a lot of their concerns to be allowed to do this.  After going over plans for safe travel to the ice edge, possible locations for a shelter between the ice edge and McMurdo, and our procedures for managing incidents at the ice edge we are closer to getting a stamp of approval but we aren’t there yet.  Once the weather clears we will spend a few more days on the sea ice around the Dellbridge Islands and working with Dan and Jen to set a safe route to the ice edge near Cape Royds.  Once that’s done we should be able to do some science!

In the meantime we have one major task that we are slowly making progress on; making and sterilizing around two thousand liters of very salty water.  This is definitely not a trivial process, but without it we simply couldn’t process our samples.  This is a result of the way sea ice forms and the way bacteria and ice algae (and viruses and anything else biological) lives within sea ice.  Seawater starts to turn into ice at about -1.8 C, but although it might look solid below this temperature it actually consists of two portions, relatively fresh ice crystals and salty liquid water.  The crystals give the ice its structure, and the colder it gets the larger they get (and the smaller the liquid fraction gets).  The crystals however, don’t hold onto the salt in seawater when they form.  They reject it into the remaining liquid fraction.  So the colder the sea ice gets, the smaller the pockets of liquid water get, and the saltier the water in those pockets becomes.

Microscope images of the pore spaces within sea ice (left), and a sea ice diatom living within one of these pore spaces (right). The liquid around the diatom is very saline relative to seawater, while the crystals have a very low salinity. Image from Krembs et al., 2002.

Bacteria, algae, and everything else that isn’t pure water are forced into those pockets along with the salt.  Because of this bacteria in cold sea ice are living in a very salty environment.  Bacteria (like any cell) must reach an osmotic balance with this salty environment.  There are a variety of mechanisms for doing this that I’ll talk about in a later post.  The end result however, is that if you take these bacteria from their salty environment and place them in fresh water they will suddenly take in a lot of water and pop!  Directly melting a chunk of sea ice does exactly this.  The bacteria might be living at a salinity of 150 ppt (parts per thousand), about five times the salinity of the ocean.  The melted ice might have a salinity of only 10 ppt.  So to keep the cells in sea ice from lysing (a fancy word for bursting) we have to melt the ice into water that is very, very salty.  That way the final melt is a mix of relatively fresh seawater and very salty brine and approximates the salinity of the liquid in which the bacteria live.  So Shelly and I have around 420 kgs (925 lbs) of salt that we need to transform into saltwater, filter to make clean, ultrafilter to make sterile, and store somewhere…


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2 Responses to Waiting out the weather

  1. rangotti says:

    As you wait on the weather, I am hoping you can help me find some data. I am looking for a data set to make a visualization of Antarctic Ice (or any ice for that matter). I have lots of individual data sets that have variables such as depth of ice, year and then one other variable, mercury for example. I am wondering if there are any that have multiple other variables and even some categorical variables as well as quantitative ones. I don’t know anything about ice, but working through and trying to create a graphical visualization helps me think mathematically through all the interacting variables involved and I would like to have more than just one variable to see the interactions. Can you point me toward some data?

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