Rapid physically driven inversion of the air-sea ice CO_2 flux in the seasonal landfast ice off Barrow, Alaska after onset of surface melt

摘要

The air-sea ice CO_2 flux was measured over landfast sea ice in the Chukchi Sea, off Barrow, Alaska in late May 2008 with a chamber technique. The ice cover transitioned from a cold early spring to a warm late spring state, with an increase in air temperature and incipient surface melt. During melt, brine salinity and brine dissolved inorganic carbon concentration (DIC) decreased from 67.3 to 18.7 and 3977.6 to 1163.5γmolkg~(-1), respectively. In contrast, the salinity and DIC of under-ice water at depths of 3 and 5m below the ice surface remained almost constant with average values of 32.4±0.3 (standard deviation) and 2163.1±16.8γmolkg~(-1), respectively. The air-sea ice CO_2 flux decreased from +0.7 to -1.0mmolm~(-2)day~(-1) (where a positive value indicates CO_2 being released to the atmosphere from the ice surface). During this early to late spring transition, brought on by surface melt, sea ice shifted from a source to a sink for atmospheric CO_2, with a rapid decrease of brine DIC likely associated with a decrease in the partial pressure of CO_2 of brine from a supersaturated to an undersaturated state compared to the atmosphere. Formation of superimposed ice coincident with melt was not sufficient to shut down ice-air gas exchange.