Blog from the RV Polarstern: MOSAiC trace gas fluxes update 4.5

Blog from the RV Polarstern: MOSAiC trace gas fluxes update 4.5

By Byron Blomquist

 

We had a period of about 6 days of clean air sampling for the bow-tower instruments over the past week or so, but for the last couple days wind is again behind the ship so gas analyzers are shut down. The Met City measurements are still going strong and we are out with the CO2/CH4/DMS chamber several times a week. Hope to be able to do an ozone profile on the tethered balloon soon.

 

Turbulent CO2/CH4 flux measurements from Met City have mostly been below detection limit, but reviewing data for the past week I noticed several days where a significant CO2 flux signal coincides with a large change in CO2 concentration as different air masses advect through our area. On day 185 a large drop in CO2 from 412 ppm to ~400 ppm over a few hours coincides with an emission flux of a couple mmoles/m2/day. This situation reversed on day 186, and then again on day 187. These small positive and negative fluxes may result from melt ponds and pore water re-equilibrating to changes in atmospheric concentration. The interesting point is that these small fluxes related to physical re-equilibration are the only signal we see. There's no indication of a broader net deposition flux of CO2 from ice/ocean photosynthetic productivity.

 

The DMS observations we've made so far also lack much evidence of a strong local emission source. Bulk DMS concentrations correlate with wind direction, with very low concentrations for northerly winds off of the pack ice and higher concentrations when winds come up from the ocean/marginal ice zone.

 

The appearance of a fresh water lens beneath the ice and in open leads throughout initial stages of the melt has been a focus of our activities over the past two weeks, and this may partially explain the very low air/sea transfer fluxes we've observed. Water from the melt ponds and open leads was fresh enough to drink – pond water in fact tastes like a clear mountain stream. Measured surface salinities in some of the leads were only a couple ppth and under-ice video clearly showed a fresh water lens under the ice. This strong surface density gradient is an effective barrier to turbulent transport of gases and heat with the atmosphere. The fresh water lens seems to be dissipating in the leads over the past couple days as the ice pack is opening up, but a strong salinity gradient is still present in the upper mixed layer. It will be interesting to see how fluxes respond when and if the ocean surface layer completely mixes. Hopefully we will be able to continue measurements throughout this process.