The ASGARD cruise is all about rates. ASGARD, in fact, is an acronym that stands for Arctic Shelf Growth, Advection, Respiration, and Deposition Rate Experiments. I participated on this cruise last year and I’m back for another 3 weeks doing the second leg. Last fall I realized there was one rate that we didn’t measure last summer: the total amount of organic carbon that is sinking through the water column, called flux. We deployed two stationary sediment traps that collect organic matter as it sinks to the seafloor over the course of the year, but they don't provide high-resolution flux measurements like a drifting sediment trap will. A drifting sediment trap will drift with a water mass as it moves in the current. When you recover the traps, you can measure how much organic carbon sank into the trap over the amount of time that it was in the water, which is called carbon flux. This flux from the surface ocean to the seafloor either feeds benthic critters or is sequestered and buried in deeper sediments. When experiencing an atmosphere with rising carbon dioxide levels, understanding the mechanisms of carbon burial feels like an urgent need.
Successfully operating a drifting sediment trap won’t be easy. We’re studying the continental shelf in the Bering and Chukchi Seas, which means that it’s shallow. If the sediment trap drifts into water that’s too shallow, it will hit the bottom and stop effectively collecting particles and potentially get damaged or be impossible to recover.
Photo credit: Brendan Smith
Additionally, there is an international border that we have to mind. The shelf is split almost in the middle, with the U.S. owning the eastern half, and Russia owning the western half. If the trap floated into Russian waters we would not be allowed to recover it. I would have to bid farewell to my flux data and any chance of recovery and redeployment of the trap at another station.
Finally, they say time is money, but that is really exemplified when paying for research cruises. Ship time is really expensive and steaming extra hours tracking down a drifting trap isn’t a good use of time or money. If the trap floats away too fast or in the opposite direction of our next station we might have to grab it early before my ideal 12-hour deployment, possibly compromising the quality of the data I am hoping to collect.
With a shallow coast on the east, Russian waters on the west and our cruise plan heading north, there is a focused set of parameters that have to be met for successful sample collection. The night before our first deployment I couldn’t sleep.
At 3:30 AM my alarm goes off. I get dressed and drink coffee. I go down to prep the trap by adding filtered sea water to the tubes. At 4 AM my advisor, Andrew McDonnell, and I head out to the deck of the ship to start shackling buoys together and hose clamping the tracking beacons to the 20-foot-tall aluminum pole that sticks out of the main buoy. As we’re out on deck we see a picturesque snow-dusted island off the starboard side. The bottom depth at the study station is around 40 meters, if the trap floats much closer to that island it’ll definitely run amuck in too-shallow water. Once everything is prepped that we can do ahead of time, we go inside to check the ADCP (Acoustic Doppler Current Profiler) data, which tells us the speed and direction of the currents. They’re heading southeast away from the island fortunately, but unfortunately, in the opposite direction of the cruise plan. Looks like we can rule out that beautiful notion that the trap would float in the direction of the next station. It’ll come down to speed; hopefully the trap won’t move too fast and therefore we won’t have to travel too far to recover it.
At 6 AM we reach the station, the deck crew comes out, and we start lifting buoys with the ship’s A-frame to make the last attachments. Finally, I clip the sediment trap to the line and it’s ready to go over. It is such an odd feeling watching something that you’ve thrown into the ocean drift away, hoping you’ll be able to find it and get it back on board after an unknown amount of time. The trap will be following the water mass wherever it moves, while the ship works to stay in the same spot so the rest of the measurements taken that day will be collected at the same geographic location.
Deployment went on without a hitch. Andrew and I headed back inside and start checking the pings from the tracking beacon. It’s flowing southeast just like the currents.
The day proceeds. Nets are cast, trawls are trawled, water is filtered. After 4 hours the trap had only moved two nautical miles. This is good. Even with the trap floating in the opposite direction of travel, we can catch it pretty quick if it’s only moving half a mile per hour. Finally, after 11 hours, the time comes to recover. We reach the location of the trap pretty quickly and we are able to find it using the tracking beacon and radar reflection off of the physical buoy at the surface of the water. Paul the bosun hooks the lift point with a detachable hook at the end of a 5-meter-long pole. We pull the line, bringing the rig to the back of the ship, where we can use the A-frame to lift it out of the water. As it comes out, a huge wave of relief floods my body. Even from meters away, I can see the particles.
Photo credit: Andrew McDonnell
So many particles! The stars had aligned and I got what I came for.
I’m feeling much better about our next three deployments, though there is still an irrational fear of starting international conflict by accidentally floating a sediment trap over to Russia, but these are the things you have to deal with when doing research in the Bering and Chukchi Seas.
Photo credit: Andrew McDonnell