The CO2 System at Below Zero Temperatures
The polar oceans are a rather under-explored territory for marine geochemists, because sampling, analytical protocols and techniques are not all reliable yet at below-zero temperatures. In the last decade a persistent limitation for the chemical oceanographic data set for the reaction rates and equilibrium constants that control biogeochemical reactions at the below-zero temperatures of polar oceanic environments has become apparent. Sea ice with all its features (permeable and impermeable brine channels and gas pockets, fractures, ponds, gaps, leads) changes the mode of contact of the sea-ice-covered ocean with the atmosphere in high latitudes. This multi-phase setting harbours a dynamic carbon cycle that is driven by biological activity and inorganic equilibrium reactions amongst the dissolved CO2 species, gaseous CO2, and the carbonate mineral ikaite.
Our research has led to the discovery of the hydrated carbonate mineral, ikaite, in artificial and field sea ice conditions, in collaboration with the Alfred Wegener Institution in Germany. For the last 6 years we have investigated the occurrence and distribution of discrete minerals in sea ice brine channels as a result of frigid conditions. We have determined the reaction rates and equilibrium constant of ikaite, which is part of the suite of
hydrated sea salts that are emergent features of sea ice.
We found that the presence of in the cryosphere is sensitive to the dissolved CO2 in the brine of sea ice and has fast kinetics of precipitation and dissolution that drive a fast internal cycle driven by temperature and CO2 fluctuations and established that the sea ice cover of the high latitude oceans is a potent carbon reactor (Papadimitriou et al., 2013, 2014).
It is currently not possible to determine the dissolved CO2 system directly, fully, and reliably, certainly not in the desired spatial and temporal detail afforded in more approachable, single-phase natural environments. Of the 4 measurable parameters of the aqueous CO2 system (total alkalinity, total dissolved inorganic carbon, pH, fCO2), the measurement of pH at below-zero temperature was a particularly intractable problem. In collaboration with NOC and Southampton University, we have made this measurement possible by spectrophotometry and the indicator dye meta-Cresol Purple. In collaboration with Prof Dickson at Scripps Institution of Oceanography, in the USA we have undertaken a rigorous characterization of Tris pH buffers at sub-zero temperatures and high salinities, conditions typical of sea ice brines. Thus adding to our capability to determine routinely and rigorously to international standards total alkalinity, total dissolved inorganic carbon, and fCO2 in our laboratories. Most recently we have rigorously determined the equilibrium dissociation constants of carbonic acid at below-zero temperatures. Previously only extrapolated equations have been used to provide indicative values whereas we have provided reliable trends for these parameter down to -6°C. Our research in the sub-zero temperature domain will make the Arctic and the Antarctic regions more amenable to oceanic studies for the monitoring of the carbon dioxide exchange across the boundary between the ocean and the atmosphere.