Antarctica 2019

There were so many exciting things happening on the ship that I almost forgot to write about the main work that I have been doing during this cruise: CTDs! The fact that the sea ice conditions didn’t allow us to deploy the mooring that I brought, doesn’t mean I could lean back and have holidays. CTDs are almost taken all of the time and our team was split up in 2 shifts. I always worked from 12:00 – 16:00h and from 20:00 – 04:00h, which means that we basically only got a few hours of daylight (it was dark from about 18:00 – 04:00 UTC). This can be tiring, but from the CTDs we can learn a lot about the physics, chemistry and biogeochemistry in the ocean, which again is important when studying oceanography and the ecosystem in the ocean.

But now a deeper explanation of the work:

CTD means Conductivity (Salinity) -Temperature-Depth. You can imagine a big yo-yo that goes all the way to the ocean floor and up again, measuring those three values throughout the whole water column. Since we already send down those sensors (which can takes about 100 min for 3000 m water depth), we can equally do more measurements on the way down. We had also an LADCP (Lowered Acoustic Doppler Current Profiler to measure velocity), a flourometer, transmissometer, Biospherical/Licor and an oxigen sensor. In addition, there are Niskin bottles attached to the CTD frame (the grey bottles on the picture), which take water samples at different depths. The water samples are used for calibration of the sensors, but also to measure biogeochemistry and chemistry in the water.

In total, we did 65 CTDs and took about 3800 water samples from the Niskin bottles to analyse them afterwards in the lab. Whenever the CTD came up from it’s yo-yo dive, we had to take the water samples, filter them and analyse. It was my task to analyse the concentration of dissolved oxygen in the water samples, which tells about the age of the water and the biological activity. Super exciting! Sometimes we were just done with the tasks, when the CTD came on deck again to do it all over again at the next location.

I am sure, we will have interesting results, once we have looked at the data more closely!

While most Norwegians associate Ægir with something else, did you know that Ægir is the jötunn (some kind of god) of the oceans? Here on board of RV Kronprins Haakon, we have an ROV (remotely operated vehicle) called Ægir  https://oceanlab-no.weebly.com/rov.html which is some kind of robot that dives along the ocean floor filming and picking up whatever could be interesting to study. It is attached to a winch on the ship and can go down to 6000 m depth! While it films the ocean floor, we can watch the movies live from the ship. Like that, the biologists on board can tell the ROV-pilots which animals, corals, sponges or other plants should be picked up or sucked up with the ROV and brought on board for later studies.

It was very unclear to us, how the ocean floor would look like in Dronning Maud Land and what to expect, so it was very exciting to watch! Here is a collection of pictures that the ROV took of corals, fish, anemones, sponges, squid, sea stars and jellyfish.

In the last blog post, Elin wrote that I called her with bad news from Dronning Maud Land, because the ice didn’t allow us to deploy the mooring. But what was actually our aim with that mooring? What did we want to learn? For that I need to tell you about the Southern Ocean and ice shelf ocean interaction…

The Southern Ocean is very important for the global system for different reasons:

• Cold temperatures and high salinities close to the Antarctic coast form very dense water masses. They sink down to the bottom of the ocean and drive the whole global ocean circulation.
• The ocean can melt ice shelves – the floating extensions of the ice sheet. Consequently, more of the Antarctic Ice Sheet flows into the ocean and causes sea level rise.

Regarding the ice shelves, they are mainly melting because “warm” water reaches the ice. “Warm” in this sense is not really warm… It only means warmer than the freezing point, which can go Down below -1.9^oC in salt water and under higher pressure at depth. It is very difficult to measure temperatures and currents under an ice shelf. But 10 years ago, one of our oceanographers on board, Tore Hattermann from NPI, went to the ice shelf Fimbulisen here in Dronning Maud Land to drill holes in the ice shelf and install moorings underneath. Since then, those mooring have been measuring temperature, salinity, oxygen and the current speed underneath the ice, where we otherwise can’t measure anything Those are really impressive data!

So what did Tore find out?
There are three different ways how warm water can reach an ice shelf (Hattermann et al., 2012):

• Water that is a the freezing point at the surface cannot melt ice at the surface. But ones it sinks further down, the freezing point decreases because of the higher pressure, so the water is warmer than the local freezing point and can actually melt the ice.
• Warm Deep Water (T < 0.6^oC), which is off the continental shelf break and in greater depths can spill over the continental shelf break in short pulses. It flows then into the ice shelf cavity on the bottom as Modified Warm Deep Water.
• In summer, the surface heats up by the sun. The Winds can then press the surface heated water Down to Depths where it can enter the ice shelf cavity.

However, the heat entering the cavity through those processes is relatively small in the case of Fimbulisen and the melt rate beneath  the ice shelf is low. But what does it have to do with our cruise?
We want to know, how the water masses in front of the ice shelf, at the continental shelf break change with time. Therefore we wanted to install more moorings that could measure the depth of the Warm Deep Water etc. Maybe the winds change with time and consequently also the currents? Also changes in the sea ice can change the density of the water and cause a change in water masses.

It would have been very interesting to study those processes in an area, where so good data from the ice shelf cavity and ice shelf melt exist. We were therefore quite sad, that we couldn’t reach far enough south due to the sea ice restrictions. Luckily, we managed to install two other mooring that the oceanographers from NPI brought, although we had to place them further north than intended. We’ll find out in two years if the data are interesting! And keep the fingers crossed that the instruments won’t get overrun by icebergs or break for other reasons!

 

On the way to our intensive study area in Dronning Maud Land, the work package leaders on RV Kronprins Haakon constantly have to keep the cruise plan updated with the time schedule and with sea ice conditions, weather and whatever interesting is coming up on the way.

In one location, the observers on board spotted more than 100 whales, compared to about 5 on other days. The whales are there for one reason: food! And the food chain starts all the way down with primary producers that are the food for krill. The Antarctic krill has an estimated biomass of around 389 000 000 tonnes, which is more than the global population of humans! More than 50% of this mass fuels the ecosystem: the whales, seals, penguins, squid and fish.

Primary production in the Southern Ocean is generally low despite high nutrient concentrations. This is due to low iron concentrations. It increases close to islands, coasts, sea ice edge and icebergs. In Dronning Maud Land, many processes that increase primary production come together and green sea ice filaments can even be seen on satellite images.