Cracking the code: Sea Ice Edition

Sea Ice forms seasonally across the Ross Sea during the Winter months in Antarctica. Every year brings a unique landscape, and with it new formations and cracks. As the sea surface starts to freeze, we can observe frazil ice begin to form. Depending on the wind stress over them, they can take the texture of an icy drink, called Nilas ice, or a form of pancakes called, wait for it, pancake ice!

Left: McMurdo Sound (red pin) during Summer season 2025; Right: McMurdo Sound during Winter Sound 2025. Both images from NASA WorldView.

As the ice accumulates, it can thicken over 5 feet easily! Now called columnar ice, it is usually paired with stalactites-looking ice underneath called platelet ice (although this fragile ice below is not considered part of the ice true thickness).

Below the sea ice, platelet ice formation in the McMurdo Sound.

As ice sheets collide into each other, they form pressure ridges between them - which leads to cracks. Due to the cool geometry of ice molecules, working cracks usually expand in a stair like pattern. The start of the stair like feature marks the actual crack width, while the crack width that it is unreliable to cross due to little sea ice thickness is considered the effective crack width. To safely cross them, either by foot or by our vehicles like the Pisten Bully, we dig them up and profile them!

A cool trick to spot sea ice cracks: look for the seals! Seals love the cracks as a way to get in and out of the water - where there is a seal, there must be a crack.

To profile a sea ice crack, we dig into the snow until we hit ice. Starting from the crack itself, we dig outward until we find the first “stair” step. Once we found ice, our goal is to get:

1. Actual and Effective crack width

2. Ice Thickness

On the crack itself, we drill (see the picture below!) until we hit water. The drill itself is 100cm, so if no water is touched, we are good to pass it! If it does, we proceed to measure the sea ice thickness with a measuring tape through the hole.

How do we know what is the minimum thickness? We have a handy little table! Depending of the vehicle you are driving, different minimum values for thickness and width will exist to ensure a safe passage of the vehicles.

For our Pisten Bully vehicle we require at least 12in of ice thickness with an effective crack width of no more than 36in. Because we found less than 12in of thickness, we moved outward the crack in 1ft increments and drilled again. With each drill, we were expanding our effective crack width. If the drilling went past 36in without getting a thickness of more than the 12in, it would not have been safe to cross. Lucky for us, we were able to find a thickness of more than 12in within a length of 36in, safe to cross!

No year or day is the same. Cracking the code on the cracks is a daily effort from anyone that passes its way!

A wall of rocks... and life

One of the sites that we sometimes get to work at is called Arrival Heights. This is one of the spots that the Antarctic Explorers (Robort Falcon Scott, notably) landed to setup the first, and still present permanent hut on the Continent. Also why our location is known as the Hut Point Peninsula. It is on the north side of town, just outside and is a relatively steep rocky face with lots of areas that have undergone some landslides over time.

It is also an area that can pick up some current, and so we tend to be very aware of the current here before we get in, but it also means there are TONS of filter feeding organisms, like anemones and sponges.

In particular the large anemones here are really everywhere and conspicuous members of the benthic (seafloor community).

It was my first dive with Dexter and Paola under the ice, and it was fun to put the team back together again.

A Warm Welcome to McMurdo

Welcome to McMurdo Station! After a quick ride in the Cress vehicles, which took us from the Phoenix Airfield and past the New Zealand Antarctica Base “Scott Base”, we were brought into the polar town we would be calling home for this season. We are planning to be here until the first week in December.

This humble mining town supports scientific operations around Antarctica. Most stay around the station, but this is also a hub that leads work out in the McMurdo Dry Valleys, the South Pole and the West Antarctic Ice Sheet (WAIS).

Once we arrived, we made our way to Building 155, the big blue galley building where meals and stores are held, kind of the central building of the station. Inside waiting were fellow Antarcticans with signs welcoming new and returning faces. We met more members of our dive team – Alex Brett and Steve Rupp, legends we had heard Andrew talk about during our preparation. Many introductions and reunions compounded in the space.

We had a brief introduction to the station, received our room assignments, and grabbed some linens to set up our quarters. We all ended up in the same dorm building, 211. That’s 2-1-1, not 2-11. I’ve learned to enunciate each number to avoid confusion here.

Afterwards we grabbed our bags from building 140, which were palletized on the plane, and took them to our dorms, the Crary lab building, or the dive locker depending on their contents. We then went on a guided tour of the station by Andrew as we had no idea where anything was, and many buildings are just referred to as their building numbers. We'll give a full tour later on in the blog.

Now that we've made it to McMurdo, we have a plethora of trainings to do including Field Safety, Sea Ice, GPS, Light Vehicle, Waste Management, Fire Safety, Medical, Science Brief, Lab Chemical, all before we can go out diving in the ice.

It seems poetic that in our first week here was also the last sunset of the season. At 1am, the sun set, illuminating the southern most chapel in the world. For the rest of our fieldwork, we will have 24 hours of daylight. See you next time!