Sea ice helps carbon absorption

By Dave Armstrong - 22 Sep 2014 16:59:0 GMT
Sea ice helps carbon absorption

What a vista the buckled sea ice gives us in the winter. Come the thaw, we now have precious little remaining and the result of that loss will be?; Sea-ice image; Credit: © Shutterstock

Ice is an impenetrable barrier to a gas diffusing from the atmosphere into the ocean? The logical answer is yes, it is, but Danish scientists have proved otherwise. Dorte Haubjerg Søgaard and her colleagues at the Greenland Institute of Natural Resources, the Universities of Aarhus and of Southern Denmark in Odense, Bangor University in Wales, the University of Manitoba and the Scottish Marine Institute wrote this paper as as - The relative contributions of biological and abiotic processes to carbon dynamics in subarctic sea ice.

In 2012, we saw the least sea ice that the Arctic ever recorded. It was reported in 2012 Record for low sea-ice levels. Most important now is to gauge just how important this huge area is to world climate. Subarctic sea ice almost disappears completely in the summer, but fixes carbon at a rate of 56mg per metre2. This figure is based on bacterial absorption and the formation of calcium carbonate. The large surprise is in the interaction between dissolved organic matter within the ice, phosphates in the ice, the brine outflow from the ice carrying calcium carbonate (and therefore carbon) and the precipitation of more carbonate to replace it.

The dense brine flowing downward into deeper layers carries carbon dioxide and other solutes with it and it dissolves in the ocean. The special crystals of calcium carbonate (ikaite) that form in the ice below -2.2oC. create alkaline conditions between the crystals of ice. The result is a loss of surface carbon dioxide and an increase in the oceanic levels of the dissolved gas.

During spring, the dilution processes cause levels of CO2 to drop in both the depths and the surface. The result will be flow of the gas from the atmosphere to the sea. Obviously, our thinking about the global air/sea dynamics of carbon dioxide need rethinking.

The extracellular polymeric substances (EPS) that are produced by bacteria and algae on the ice may feed bacterial activity if they can be proved to be a high quality substrate. That is debateable, it seems. In the bottom parts of the ice in March, great activity is observed, with a net loss of carbon dioxide but the figures don’t add up, meaning the major processes involved could be inorganic. There are few nutrients left for heterotrophs by March. The calcium carbonate levels in the bottom of the sea ice in March are the highest, associated with low salinity and nutrient levels there. Concluding that abiotic and probably chemical processes were responsible for 96% of the sea-ice driven carbon dioxide uptake, the fluxes involved seemed quite complex. They certainly involved 2 phases.

In winter the sea ice forms calcium carbonate crystals (ikaite) that cause CO2 to dissolve in heavy cold brine that sinks. In summer the carbonate is still in the ice, but dissolves with it and uses up CO2. Dorte even explains the formation of the famed frost flowers on the ice: they too contain high concentrations of the ikaite. Perhaps in future they will act as indicators of where these processes are more frequent or of how the chemistry of the ice may change.