Abstract
Abstract It has been suggested that geochemical processes related to sea-ice growth and melt might be important for the polar carbon cycle via the so called sea-ice carbon pump (SICP). The SICP affects the air-sea CO2 exchange by influencing the composition of dissolved inorganic carbon (DIC) and total alkalinity (TA) in the surface ocean. Here we quantify the strength of the SICP-induced air-sea CO2 flux using the global three-dimensional ocean-sea-ice-biogeochemical model MPIOM/HAMOCC. Simulations prescribing the range of observed DIC and TA concentrations in the sea ice were performed under two idealized climate scenarios for the present-day and the future oceanic and sea-ice state, both forced with a fixed atmospheric CO2 concentration. Model results indicate that the SICP-induced air-sea CO2 uptake increases with higher ratios of TA:DIC prescribed in the sea ice relative to the basic oceanic TA:DIC ratios. Independent of the modeled scenario, the simulated strength of the SICP is larger in the Antarctic than in the Arctic, because of more efficient export of brine-associated DIC from the Antarctic mixed layer. On an annual basis, we generally find an enhanced SICP-induced oceanic CO2 uptake in regions with net sea-ice melt, and enhanced SICP-induced oceanic CO2 out-gassing in regions with net sea-ice growth. These general regional patterns are modified further by the blockage of air-sea gas exchange through sea-ice coverage. Integrated over the sea-ice zones of both hemispheres, the SICP-induced oceanic CO2 uptake ranges from 2 to 14 Tg C yr−1, which is up to 7% of the simulated net CO2 uptake in polar regions, but far less than 1% of the current global oceanic CO2 uptake. Hence, while we find that the SICP plays a minor role in the modern global carbon cycle, it is of importance for the regional carbon cycle at high latitudes.
Highlights
The magnitude of CO2 exchange between the atmosphere and the ocean is determined by the difference in partial pressure of CO2 between the two reservoirs
In the Arctic, the simulated sea-ice extent in the EQ scenario is in good agreement with the observational data of the National Oceanic and Atmospheric Administration (NOAA)/National Snow and Ice Data Center (NSIDC) (Meier et al, 2011)
Surface ocean areas that are characterized by a melt-water footprint show an enhanced sea-ice carbon pump (SICP)-induced oceanic CO2 uptake. Both the concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) and the TA:DIC ratio prescribed in the sea ice relative to the basic oceanic TA and DIC setting further determine the strength of the SICP-induced CO2 uptake, which is higher for high ratios of TA:DIC prescribed in the sea ice (R2l, R2h) than for low ratios (R1.1l, R1.1h)
Summary
The magnitude of CO2 exchange between the atmosphere and the ocean is determined by the difference in partial pressure of CO2 (pCO2) between the two reservoirs. The release of melt-water to the ocean dilutes the oceanic TA and DIC concentrations, and the dissolution of ikaite potentially increases the oceanic TA:DIC ratio which is usually 1.1
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