Preprints
https://doi.org/10.5194/tc-2021-320
https://doi.org/10.5194/tc-2021-320

  27 Oct 2021

27 Oct 2021

Review status: this preprint is currently under review for the journal TC.

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

Brent G. T. Else1, Araleigh Cranch1, Richard P. Sims1,a, Samantha Jones1, Laura A. Dalman2,b, Christopher J. Mundy2, Rebecca A. Segal3,4, Randall K. Scharien5, and Tania Guha1 Brent G. T. Else et al.
  • 1Department of Geography, University of Calgary, Calgary, Alberta, Canada
  • 2Centre for Earth Observation Science, University of Manitoba, Winnipeg, Manitoba, Canada
  • 3Arctic Eider Society, Sanikiluaq, Canada
  • 4SmartIce Sea Ice Monitoring & Information Inc., St. John’s, Canada
  • 5Department of Geography, University of Victoria, Victoria, British Columbia, Canada
  • anow at: College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom
  • bnow at: Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia

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 offgassing, 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 be explained by the salinity distribution, with the strong currents at the polynya site potentially playing a role in desalinisation; biology did not have a noticeable impact. 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 modeling 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.

Brent G. T. Else et al.

Status: open (until 25 Dec 2021)

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Brent G. T. Else et al.

Brent G. T. Else et al.

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Short summary
Sea ice helps control how much carbon dioxide polar oceans absorb. We compared ice cores from two sites to look for differences in carbon chemistry: one site had thin ice due to fast ocean currents and thick snow; the other site had thick ice, thin snow, and slow currents. We did find some differences in small layers near the top and the bottom of the cores, but for most of the ice volume the chemistry was the same. This result will help build better models of the carbon sink in polar oceans.