Articles | Volume 6, issue 4
The Cryosphere, 6, 901–908, 2012
The Cryosphere, 6, 901–908, 2012

Research article 15 Aug 2012

Research article | 15 Aug 2012

Ikaite crystals in melting sea ice – implications for pCO2 and pH levels in Arctic surface waters

S. Rysgaard1,2,3,6, R. N. Glud3,4,5, K. Lennert3, M. Cooper2, N. Halden1,2, R. J. G. Leakey4, F. C. Hawthorne2, and D. Barber1 S. Rysgaard et al.
  • 1Centre for Earth Observation Science, CHR Faculty of Environment Earth and Resources, University of Manitoba,
    499 Wallace Building, Winnipeg, MB R3T 2N2, Canada
  • 2Department of Geological Sciences, CHR Faculty of Environment Earth and Resources, University of Manitoba,
    499 Wallace Building, Winnipeg, MB R3T 2N2, Canada
  • 3Greenland Climate Research Centre, Greenland Institute of Natural Resources, Kivioq 2, 3900 Nuuk, Greenland
  • 4Scottish Association for Marine Science, Scottish Marine Institute, Oban. PA37 1QA, UK
  • 5Southern Danish University and NordCEE, Odense M, Denmark
  • 6Arctic Research Centre, Aarhus University, 8000 Aarhus C, Denmark

Abstract. A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air–sea CO2 exchange. This has been complicated by the recent discoveries of ikaite (a polymorph of CaCO3·6H2O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO2 and pH conditions in surface waters. Here, we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from a melting 1.7 km2 (0.5–1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice floe thickness by 0.2 m per week and resulted in an estimated 3.8 ppm decrease of pCO2 in the ocean surface mixed layer. This corresponds to an air–sea CO2 uptake of 10.6 mmol m−2 sea ice d−1 or to 3.3 ton km−2 ice floe week−1. This is markedly higher than the estimated primary production within the ice floe of 0.3–1.3 mmol m−2 sea ice d−1. Finally, the presence of ikaite in sea ice and the dissolution of the mineral during melting of the sea ice and mixing of the melt water into the surface oceanic mixed layer accounted for half of the estimated pCO2 uptake.