Edge of the Arctic Shelf

Sunny Skies and CTDs
The Marine Science Technicians' forecast from yesterday was right on. The storm vented the rest of its fury through the night, but was spent by dawn. By the time I finished breakfast, the CTD was already back in the water taking measurements of our arctic eddy. Broken clouds scudded across the sky, and the sun peeked out from the behind the clouds throughout the day.

Last night we completed an extremely high resolution XCTD survey of the eddy. I asked Bob Pickart to share his thoughts on this accomplishment.

"During the previous SBI cruises we've crossed through a large number of eddies as we've carried out our predetermined track lines. The fact that we've seen so many of them suggests that the eddies play an important role in carrying water from the shelf to the basin. However, our serendipitous views up to this point have given us only limited information on their characteristics and how they actually accomplish this offshore transport. Hence one of the goals of our final cruise this year was to find such a feature and sample it in detail. This is easier said than done, however. It involves patience as well as some luck, and it was by no means clear that we could accomplish our objective.

After our "large scale" search the other night we found only one area that showed any promise of eddy activity. We were somewhat disappointed, but since this was our only lead we had no choice but to take our one shot and commence a detailed "search pattern". Much to our delight, our XCTD survey soon revealed the presence of a strong cold core eddy, just the type of feature we were hoping to find! It was extremely impressive that the Healy could carry out this survey through such rough seas and nasty conditions---the Marine Science Technicians deserve much credit for working so diligently throughout the night. By early morning the grid was complete and we had obtained a detailed three-dimensional view of a newly-spawned western Arctic eddy. Tonight we are taking one last cut through the eddy, collecting valuable chemical and biological information on these features. Among other things this will help us figure out how old this particular eddy is, and how large numbers of such eddies together might impact the Arctic food chain."

One of the important chemicals we measure in our water samples is the quantity of dissolved nutrients. Nutrients--silicate, phosphate, nitrite, and nitrate--can be thought of as the fertilizer that nourishes tiny marine plants called phytoplankton. With nutrients and sunlight, phytoplankton grow, and in turn form the base of the Arctic food web. Erik Quiroz is analyzing the nutrient content of our water samples on this cruise. I ran into Erik today while he was running the nutrient Auto-Analyzer machine in the lab.

This is quite a complicated piece of machinery. How does it work?
Erik: "It's definitely complicated, so let's start at the beginning. We have collected samples of water from different locations and depths. Each tiny sample has different concentrations of dissolved nutrients like silicate, etc. The challenge is finding out how much of each nutrient is present in each sample. That's where the Auto-Analyzer comes in. It extracts a bit of the water sample for each nutrient and then adds chemical reagents. The ensuing chemical reaction produces a color. The machine then measures how much of this color is produced, and using a known relation and an equation, we can compute how much of that particular nutrient is present in the sample."

What else can you share about the auto-analyzer?
Erik: "Although most of the harware for this machine, the Auto-Analyzer Technicon II, was designed in the 1970s, it has been substantially modified to withstand the rigors of at-sea work and the software is being continuously updated. Only a handful of people in the United States use this machine for marine chemistry... maybe 20 or so. Of those, only half run samples at sea. On short cruises, samples can be preserved in bottles and run back at the lab. On a cruise of this duration, we don't have that luxury. The samples must be analyzed quickly because we only have so many sample bottles. One other benefit to running the chemistry out here on the Healy is that we can see the data immediately and share this information with all of the other scientists. Having that information at hand is really valuable. The scientists on this cruise are interested in nutrients because it helps them better understand how Pacific water (which has a high nutrient content) travels across the shallow Chukchi Sea and into the central Arctic Ocean."

By tomorrow, our detailed section through the heart of the eddy will be complete, and our bow will be pointed southward. Destination: the Chukchi Sea central channel... Mission: recover the last mooring.