Inorganic carbon dynamics of melt-pond-covered first-year sea ice in the Canadian Arctic

Abstract

Melt pond formation is a common feature of spring and summer Arctic sea ice, but the role and impact of sea ice melt and pond formation on both the direction and size of CO₂ fluxes between air and sea is still unknown. Here we report on the CO₂–carbonate chemistry of melting sea ice, melt ponds and the underlying seawater as well as CO₂ fluxes at the surface of first-year landfast sea ice in the Resolute Passage, Nunavut, in June 2012. <br><br> Early in the melt season, the increase in ice temperature and the subsequent decrease in bulk ice salinity promote a strong decrease of the total alkalinity (TA), total dissolved inorganic carbon (<i>T</i>CO₂) and partial pressure of CO₂ (<i>p</i>CO₂) within the bulk sea ice and the brine. As sea ice melt progresses, melt ponds form, mainly from melted snow, leading to a low in situ melt pond <i>p</i>CO₂ (36 &mu;atm). The percolation of this low salinity and low <i>p</i>CO₂ meltwater into the sea ice matrix decreased the brine salinity, TA and <i>T</i>CO₂, and lowered the in situ brine <i>p</i>CO₂ (to 20 &mu;atm). This initial low in situ <i>p</i>CO₂ observed in brine and melt ponds results in air–ice CO₂ fluxes ranging between −0.04 and −5.4 mmol m^&minus;2 day^&minus;1 (negative sign for fluxes from the atmosphere into the ocean). As melt ponds strive to reach <i>p</i>CO₂ equilibrium with the atmosphere, their in situ <i>p</i>CO₂ increases (up to 380 &mu;atm) with time and the percolation of this relatively high concentration <i>p</i>CO₂ meltwater increases the in situ brine <i>p</i>CO₂ within the sea ice matrix as the melt season progresses. As the melt pond <i>p</i>CO₂ increases, the uptake of atmospheric CO₂ becomes less significant. However, since melt ponds are continuously supplied by meltwater, their in situ <i>p</i>CO₂ remains undersaturated with respect to the atmosphere, promoting a continuous but moderate uptake of CO₂ (~ −1 mmol m^&minus;2 day^&minus;1) into the ocean. Considering the Arctic seasonal sea ice extent during the melt period (90 days), we estimate an uptake of atmospheric CO₂ of −10.4 Tg of C yr^&minus;1. This represents an additional uptake of CO₂ associated with Arctic sea ice that needs to be further explored and considered in the estimation of the Arctic Ocean's overall CO₂ budget.