Ikaite crystal distribution in Arctic winter sea ice and implications for CO<sub>2</sub> system dynamics

Abstract

The precipitation of ikaite (CaCO₃·6H₂O) in polar sea ice is critical to the efficiency of the sea ice-driven carbon pump and potentially important to the global carbon cycle, yet the spatial and temporal occurrence of ikaite within the ice is poorly known. We report unique observations of ikaite in unmelted ice and vertical profiles of ikaite abundance and concentration in sea ice for the crucial season of winter. Ice was examined from two locations: a 1 m thick land-fast ice site and a 0.3 m thick polynya site, both in the Young Sound area (74° N, 20° W) of NE Greenland. Ikaite crystals, ranging in size from a few &micro;m to 700 &micro;m were observed to concentrate in the interstices between the ice platelets in both granular and columnar sea ice. In vertical sea-ice profiles from both locations, ikaite concentration determined from image analysis, decreased with depth from surfaceice values of 700–900 &micro;mol kg⁻¹ ice (~ 25 &times; 10⁶ crystals kg⁻¹) to bottom-layer values of 100–200 &micro;mol kg⁻¹ ice (1–7 &times; 10⁶ kg⁻¹), all of which are much higher (4–10 times) than those reported in the few previous studies. Direct measurements of total alkalinity (TA) in surface layers fell within the same range as ikaite concentration whereas TA concentrations in bottom layers were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolved in bottom layers. From these findings and model calculations we relate sea ice formation and melt to observed <i>p</i>CO₂ conditions in polar surface waters, and hence, the air-sea CO₂ flux

Drivers of inorganic carbon dynamics in first‐year sea ice: A model study

“…Finally, the simulated ikaite concentrations, up to 210 mmol m 23 , are in good agreement with the literature [Dieckmann et al, 2008[Dieckmann et al, , 2010Fischer et al, 2013;Geilfus et al, 2013;Rysgaard et al, 2012Rysgaard et al, , 2013, although they underestimate the largest values reported [e.g., Rysgaard et al, 2013Rysgaard et al, , 2014. Despite this general agreement, there are other unaccounted contributors to CaCO 3 precipitation or dissolution (ionic strength, temperature, and ionic inhibitors such as orthophosphate), which were not considered here and could be important in different environments [Papadimitriou et al, 2014].…”

“…Field measurements of ikaite range from 15 to $900 mmol m 23 [Dieckmann et al, 2008[Dieckmann et al, , 2010Fischer et al, 2013;Geilfus et al, 2013;Rysgaard et al, 2012Rysgaard et al, , 2013]. In the model, the maximum ikaite concentration was limited, reaching an asymptotic maximum near 210 mmol m 23 when trying to increase calcification (i.e., when T CaCO3 is decreased, Figure 11b).…”

“…Low temperatures and high brine salinities further force the carbonate system toward higher CO 2 brine concentrations [Papadimitriou et al, 2003]. Low temperatures and high brine salinities also promote the precipitation of ikaite [Dieckmann et al, 2008[Dieckmann et al, , 2010Rysgaard et al, 2012Rysgaard et al, , 2013 which increases CO 2 brine concentration. In contrast, during the warming season, brine dilution, net carbon uptake by growing ice algae [Arrigo et al, 2010] and the dissolution of ikaite [Rysgaard et al, 2012[Rysgaard et al, , 2007 combine to decrease the brine pCO 2 and enhance the uptake of atmospheric CO 2 by sea ice Nomura et al, 2010aNomura et al, , 2013Semiletov et al, 2004].…”

“…Particulate tracers, such as ikaite do not move with brine motion and remain where they form [as indicated by microscope observations by Light et al, 2003;Rysgaard et al, 2013]. DIC and TA are treated as passive tracers for transport and diffusion.…”

Structural change induced by dehydration in ikaite (CaCO3·6H2O)

“…Ikaite is a metastable mineral in the sedimentary realm formed naturally from aqueous solutions in near-freezing anoxic marine sediments (Zabel and Schulz, 2001;Greinert and Derkachev, 2004;Selleck et al, 2007). In recent years, a variety of different occurrences have been reported, including in high salinity and alkaline lakes (Last et al, 2013;Oehlerich et al, 2013) and polar sea ice (Dieckmann et al, 2008;Rysgaard et al, 2013). Both in nature and in the laboratory, ikaite readily crystallizes from solution at temperatures near 0°C , but rapidly decomposes into a mush of anhydrous CaCO 3 and water at warmer temperatures (Bischoff et al, 1993;Council and Bennett, 1993;Mikkelsen et al, 1999;Tang et al, 2009 -168, 2014 served at the same time in the sedimentary record (Larsen, 1994).…”