Variability in sea ice carbonate chemistry: a case study comparing the importance of ikaite precipitation, bottom-ice algae, and currents across an invisible polynya
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 Archipe...
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Veröffentlicht in: | The cryosphere 2022-09, Vol.16 (9), p.3685-3701 |
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Zusammenfassung: | 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 |
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ISSN: | 1994-0424 1994-0416 1994-0424 1994-0416 |
DOI: | 10.5194/tc-16-3685-2022 |