Variability in sea ice carbonate chemistry: a case study comparing the importance of ikaite precipitation, bottom-ice algae, and currents across an invisible polynya

Abstract

Abstract. The carbonate chemistry of sea ice is known to play a role in global carbon cycles, but its importance is uncertain in part due to disparities in reported results. Variability in physical and biological drivers is usually invoked to explain differences between studies. In the Canadian Arctic Archipelago, “invisible polynyas” – areas of strong currents, thin ice, and potentially high biological productivity – are examples of extreme spatial variability. We used an invisible polynya as a natural laboratory to study the effects of inferred initial ice formation conditions, ice growth rate, and algal biomass on the distribution of carbonate species by collecting enough cores to perform a statistical comparison between sites located within, and just outside of, a polynya near Iqaluktuttiaq (Cambridge Bay, Nunavut, Canada). At both sites, the uppermost 10 cm ice horizon showed evidence of CO2 off-gassing, while carbonate distributions in the middle and bottommost 10 cm horizons largely followed the salinity distribution. In the polynya, the upper ice horizon had significantly higher bulk total inorganic carbon (TIC), total alkalinity (TA), and salinity potentially due to freeze-up conditions that favoured frazil ice production. The middle ice horizons were statistically indistinguishable between sites, suggesting that ice growth rate is not an important factor for the carbonate distribution under mid-winter conditions. The thicker (non-polynya) site experienced higher algal biomass, TIC, and TA in the bottom horizon. Carbonate chemistry in the bottom horizon could largely be explained by the salinity distribution, with the strong currents at the polynya site potentially playing a role in desalinization; biology appeared to exert only a minor control, with some evidence that the ice algae community was net heterotrophic. We did see evidence of calcium carbonate precipitation but with little impact on the TIC:TA ratio and little difference between sites. Because differences were constrained to relatively thin layers at the top and bottom, vertically averaged values of TIC, TA, and especially the TIC:TA ratio were not meaningfully different between sites. This provides some justification for using a single bulk value for each parameter when modelling sea ice effects on ocean chemistry at coarse resolution. Exactly what value to use (particularly for the TIC:TA ratio) likely varies by region but could potentially be approximated from knowledge of the source seawater and sea ice salinity. Further insights await a rigorous intercomparison of existing data.

Large‐Scale Summertime Variability of Carbonate Chemistry Across the East Siberian Sea: Primary Production Versus Ikaite Dissolution

“…(2012) observed a range of TA ice from 304 ± 63 to 533 ± 137 (σ) μmol kg −1 and DIC ice varying from 142 ± 24 to 292 ± 117 (σ) μmol kg −1 in sea ice cores taken in the Fram Strait, which was similar to TA ice of 380–800 μmol kg −1 and DIC ice of 250–600 μmol kg −1 measured from ice cores in the Young Sound area of northeastern Greenland (Rysgaard et al., 2013). These ranges also agreed well with a recent study in the Dease Strait in the Canadian Arctic Archipelago, ranging from 333 ± 30 to 552 ± 26 ( σ ) μmol kg −1 for TA ice and from 282 ± 22 to 538 ± 21 ( σ ) μmol kg −1 for DIC ice (Else et al., 2022). These variations in TA and DIC were strongly related to salinity of an ice core sample.…”

“…Previous studies have invoked the spatial heterogeneity of ikaite distribution in sea ice only using ice cores (e.g., Else et al., 2022; Miller et al., 2011; Rysgaard et al., 2007). It is not surprising to observe ikaite content differences between ice cores given their various ice growing patterns and formation of brine channels.…”

“…Primary production by sea-ice algae is also likely to play a role in heterogeneous distribution of TA and DIC in sea ice. Sea-ice algae bloom has been observed throughout the sea ice column in winter (Miller et al, 2011) and the bottom of sea ice in Arctic and Antarctic areas during spring and summer (Else et al, 2022;Leu et al, 2015;Meiners et al, 2012Meiners et al, , 2018.…”

High interannual surface pCO2 variability in the southern Canadian Arctic Archipelago's Kitikmeot Sea

“…For the seawater mixing endmember, surface TA (2034.43 µmo kg −1 ) and DIC (1958.82 µmol kg −1 ), SST (−1.38 • C), and salinity (28.64) are taken from seawater bottle data on 18 June 2018 (Duke et al, 2021) alongside surface silicate (4 µmol L −1 ) and phosphate (0.5 µmol L −1 ) from 2018 (Back et al, 2021). Average values from spring 2019 for TA (356.60 µmol kg −1 ), DIC (340.24 µmol kg −1 ) and salinity (4.56) in first-year sea ice are used for the sea ice mixing endmember (Else et al, 2022). Taking a sea ice thickness of 1.8 m and assuming water expands 10% when it freezes to form sea ice would suggest melting all the sea ice would add 1.64 m of water to reach the final salinity of 24.82 (the average recorded value from the RV Martin Bergmann measurements) and conservation of salinity would require this freshwater to be mixed with 8.68 m of seawater.…”