Inorganic Carbon and pCO₂ Variability During Ice Formation in the Beaufort Gyre of the Canada Basin

Abstract

Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (AT). The brine sinks, potentially exporting DIC and AT to deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 year pCO2 mooring time series where a ~35‐μatm increase in pCO2 was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resulting pCO2 is sensitive to the ratio of AT and DIC retained in the ice and the mixed layer depth, which controls dilution of the ice‐derived AT and DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing the pCO2 at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC and pCO2 in the surface of the Arctic Ocean as seasonal ice production and wind‐driven mixing increase with diminishing ice cover.

Introduction to Special Collection on Arctic Ocean Modeling and Observational Synthesis (FAMOS) 2: Beaufort Gyre Phenomenon

“…Ecosystem and biogeochemical analyses targeting estimation of biological production rates in both water and sea ice, and characteristics of dissolved organic and inorganic matter, are another focus, with papers employing syntheses of multimodel experiments in combination with collective data analysis (e.g. Dainard et al, 2019;DeGrandpre et al, 2019;Ji et al, 2019;Watanabe et al, 2019).…”

“…The data set analyzed in the paper serves as a valuable baseline for future estimates of biological production rates. Journal of Geophysical Research: Oceans DeGrandpre et al (2019) show how inorganic carbon export from the surface layers is related to sea ice formation. Modeling of the ecosystem is performed by Watanabe et al (2019), who conducted an intercomparison of ice algal productivity in the Chukchi and Barents seas and in the Eurasian and Canada basins at seasonal to decadal scales employing five different models.…”

Changes in the Arctic Ocean Carbon Cycle With Diminishing Ice Cover

“…Steele and Dickinson (2016) also found that SST is typically <0°C at DOR. Ice melt and formation could change pCO 2 by diluting and concentrating DIC, respectively, and by altering the ratio of DIC and A T in the ice or brine (Cai et al, 2010;DeGrandpre et al, 2019;Rysgaard et al, 2007Rysgaard et al, , 2009. Ice melt occurs before DOR and could decrease pCO 2 levels (Cai et al, 2010).…”

Changes in the Arctic Ocean carbon cycle with diminishing ice cover

“…The pCO 2 was then recalculated using the new DIC, SST, and salinity and a constant total alkalinity (A T ) in the CO 2 equilibrium program CO2sys (Pierrot et al, 2006). The A T was estimated using an A T -salinity relationship derived from bottle samples during the BGOS cruises (DeGrandpre et al, 2019;Yamamoto-Kawai et al, 2005). A T is a conservative property of seawater that does not change with temperature or air-sea CO 2 exchange (Millero, 2007).…”