Identification of the processes leading to ikaite formation in polar sea ice
Dr. Gernot Nehrke
Helmholtz-Zentrum für Polar- und Meeresforschung
Recently we presented the first direct evidence of the systematic occurrence of ikaite, a metastable hydrated phase of calcium carbonate (CaCO3) in new and multi-year sea ice from the Weddell Sea, Antarctica (Dieckmann et al. 2008). The possibility of (CaCO3) formation during the freezing of sea-ice had been discussed since decades (based on thermodynamic calculations and laboratory experiments) but calcium carbonate was never observed to actually occur in the polar sea-ice. Since then we have also discovered the occurrence of ikaite in sea-ice from the Arctic (Kongsfjord, Svalbard) (Dieckmann et al. 2009). Calcium carbonate precipitation in sea ice is an emergent by-product of the physico-chemical changes attendant to seawater freezing and represents an additional potential carbon sink within the system along with internal primary production. The occurrence of authigenic CaCO3 has been considered central as a source of acidity to the processes on sea ice surfaces leading to tropospheric ozone depletion events in Polar Regions and as a source of CO2 potentially sequestered to the deep ocean with the dense brine expelled from sea ice, thus constituting an ice-driven carbon pump. However, the exact physico-chemical conditions leading to the formation of ikaite in sea-ice are still poorly understood. For example it has been suggested that the slightly higher phosphate concentration in Antarctic seawater would restrict the occurrence of ikaite to the southern Polar Regions, which we now have proven to be not the case. In 2009 we participated in an international large-scale sea-ice tank 4 experiment (INTERICE) at the HSVA in Hamburg (Germany). During these experiments a strong change in the carbonate system (CO2 and alkalinity) of the brine solution during ice formation was observed, which commonly is used as an “indirect evidence” of calcium carbonate formation. The fact that no calcium carbonate crystals formed during these experiments again proves how poorly this system is understood. As long as we do not understand the mechanisms leading to the calcium carbonate formation during the formation of sea-ice it is not possible to assess its impact on the global carbon cycle and other reactions such as the Bromide explosion. We therefore plan to perform ikaite precipitation experiments under controlled laboratory conditions, which will allow us to isolate the parameters leading to the formation of ikaite during the formation of sea-ice.
Beteiligte Person: Privatdozent Dr. Gerhard Dieckmann
Förderung von 2010 bis 2015