Cruise Day 12
Speed 1 knot (kts) (on station)
Course 331° (NNW)
Location SW Flank of Mendelev Rise, approx. 310 nm NNE of Wrangel Island, Russia
Depth 2303 m
GO DEEPER DISCUSSION: (see previous journal for the questions.)
Although July is the warmest month in the Arctic, the average temperatures in August and the first half of September are still warm enough to have a melting effect on sea ice. For this reason, measurements are taken every year in mid-September to document the annual Arctic sea ice minimum. We have had freezing temperatures, occasional snow flurries, and ice accumulation on railings and on the deck, but overall the sea ice we are beginning to see is melting. After mid-September, the days start getting short enough and average temperatures drop enough to start re-forming Arctic sea ice.
TODAY’S JOURNAL:
I want to start today’s journal by welcoming my earth science students to the beginning of 8th grade at Centennial Middle School in Boulder, Colorado! One of the hardest things about embarking on this expedition is missing my family, friends, colleagues, and getting to know all of my new students. I’ll catch up fast when I get back in mid-October, and I know you all will learn tons and be off to a great start by the time I return. Have fun, work hard, and get the most out of every day!
I’m proud to fly the Centennial Cyclone flag in the Arctic Ocean!
We’re on a long station today, which began around noon and should wrap up after lunch tomorrow. As you can see in the summary information in the header, we are in the deepest water so far of the cruise, and so we are doubling up on the casts with most of the teams doing a shallow cast early in the station and a deep cast later on. For example, we did a GEOTRACES cast down to 750 meters at the beginning of the station and will do another to nearly the bottom some time around 10:30 tonight. Casts also take longer to do in deep water, simply because it takes time to let out and reel in so much cable. A rule of thumb is to plan on an hour of cable time for every 1000 meters of water.
Yesterday I latched on to one of our pumping teams to get a little more detail about their process. They are interested in beryllium-7 (7Be), an isotope that is produced when cosmic rays collide with nitrogen atoms in the atmosphere. 7Be is unstable, with a half-life of approximately 53 days. (This means that half of the 7Be atoms in any sample will decay every 53 days. In 53 more days, half of the remaining 7Be will decay, and so on.) So 7Be makes a good surface water tracer, as a supply of 7Be is constantly being deposited into surface waters from the atmosphere at predictable rates while deep water has no significant source of the isotope. As surface water mixes with deeper water it brings 7Be down with it, and by measuring the amount of the isotope remaining a calculation can be made for the rate that surface water is mixing into deeper water or to track water masses that were recently at the surface.
A challenge with measuring 7Be is that it is present in very small amounts. So to get enough Beryllium to measure, large amounts of sea water are needed. The 7Be team employs a high-volume pump that can be lowered into the layer they are interested in sampling. It pumps water back up into large tanks strapped to the rail on the starboard side of the fantail via a large hose. Many hundred liters of water are then slowly pumped back through a filter packed with iron oxide-laden acrylic fiber. The Beryllium binds to the iron oxide (essentially, rust) in the filter. After drying the filter media out, samples are brought back to the lab. There the acrylic fiber is incinerated, leaving behind the metals. These are pressed into pellets that can be analyzed for 7Be by looking at its nuclear decay signature.
The team’s pump is connected to a small CTD unit to record the salinity & temperature profile at any layer it pumps water from. It is lowered to the desired depth, powered on, and sends water back on deck via the hose. I’ll note here that CTDs can be used alone or with other devices such as our sampling bottle rosettes, but in general parlance when planning a station etc. a CTD cast means that a sampling rosette (with its CTD and other instruments along with the sampling bottles) will be going overboard.
The 7Be sampling pump ready to lower off the fantail A-Frame crane. There are lead weights hung from the pump to help sink it, a black power cable leading to the pump, and a blue hose that will return water to holding tanks on deck. The square cage holds a small CTD that records a salinity and temperature profile for the cast.
Down goes the 7Be pump assembly. Note the CTD in the square housing next to the round pump body.
Large tanks are used to store water that has been pumped up for 7Be filtering. Two of the tanks, totaling several hundred liters, are used for each sampling level.
A smaller pump is used to slowly filter the sample. Water comes from the large tank through the gray hose, into the pump, and out through the red hose to the filter assembly, before finally draining back into the ocean.
The filter housing consists of a pipe to hold the filter media, hose fittings, and a flow meter to record the volume of water filtered.
The filter is packed with acrylic fiber impregnated with iron oxide (note the rusty color.) Beryllium bonds with the iron oxide.
After pumping all of the sample water through the filter, the media is dried in a desiccating oven. This is the final shipboard step for the 7Be team, with the remaining analytical steps done at their home lab.
GO DEEPER!
It will be important for the 7Be team to complete their analysis quickly after they get back to their home lab. Why are their samples very time sensitive?
Aloft Con web cam updated every hour
Healy Track
That's all for now. Best- Bill