Brief Communication: Ikaite (CaCO<sub>3</sub>·6H<sub>2</sub>O) discovered in Arctic sea ice

Abstract

Abstract. We report for the first time on the discovery of calcium carbonate crystals as ikaite (CaCO 3 ·6H 2 O) in sea ice from the Arctic (Kongsfjorden, Svalbard) as confirmed by morphology and indirectly by X-ray diffraction as well as XANES spectroscopy of its amorophous decomposition product. This finding demonstrates that the precipitation of calcium carbonate during the freezing of sea ice is not restricted to the Antarctic, where it was observed for the first time in 2008. This observation is an important step in the quest to quantify its impact on the sea ice driven carbon cycle.

Ikaite crystals in melting sea ice – implications for <i>p</i>CO<sub>2</sub> and pH levels in Arctic surface waters

“…x-ray diffraction as ikaite had a morphology similar to the ones reported from Antarctica (Dieckmann et al, 2008) and from Arctic coastal fast ice locations in Svalbard (Dieckmann et al, 2010) and off Station Nord NE Greenland (Rysgaard et al, unpublished results). Furthermore, we observed ikaite crystals throughout the sea ice column with the largest crystals in the upper layers of the sea ice, in agreement with the observations from the other locations.…”

“…The observation of TA : T CO 2 ratios as high as 2 indicate that calcium carbonate is formed in natural sea ice (Rysgaard et al, 2007(Rysgaard et al, , 2009. Ikaite crystals in sea ice have recently been discovered in both hemispheres (Dieckmann et al, 2008(Dieckmann et al, , 2010. The specific conditions promoting ikaite precipitation in sea ice are currently poorly understood, but if precipitation occurs in the porous bottom sea-ice layer, where the brine volume is larger than 5 % (Weeks and Ackley, 1986;Golden et al, 1998Golden et al, , 2007, the CO 2 -enriched brine will exchange with seawater via gravity drainage (Notz and Worster, 2009).…”

Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008

“…This approach reconciles the results of Sander et al (2006) and Morin et al (2008b). Also, observations are now available showing that ikaite precipitation occurs in the Arctic (Dieckmann et al, 2010) as well as in the Antarctic (Dieckmann et al, 2008). It should be noted though, that all results and discussions presented here assume thermodynamic equilibrium.…”

Laboratory study on coprecipitation of phosphate with ikaite in sea ice

“…Ikaite (CaCO 3 ·6H 2 O) in sea ice was only recently discovered [ Dieckmann et al ., ] and for a long time PO 4 has been considered to be crucial for ikaite formation as ikaite is very often found in PO 4 ‐rich environments [ Bischoff et al ., ; Buchardt et al ., ; Council and Bennett , ; Selleck et al ., ]. A recent study by Hu et al .…”