Are Glaciers and Ice Sheets Carbon Sources or Carbon Sinks?

Abstract:

Subglacial waters typically contain considerable quantities of HCO₃^-. Where this HCO₃^- is coupled with Ca²⁺ and Mg²⁺, it will ultimately precipitate as (Ca, Mg)CO₃ in the oceans. If the glacial HCO₃^- is derived from atmospheric CO₂, this pathway represents a long-term CO₂ sink. If the HCO₃^- is derived from carbonate minerals, precipitation is equal to dissolution and there is no net effect on the CO₂ balance of the atmosphere. Only the weathering of Ca or Mg-bearing silicates can potentially draw CO₂ out of the ocean/atmosphere system. Subglacial environments are potential habitats for a range of microbes that may generate CO₂ from organic C. If the production of CO₂ from organic sources exceeds the weathering of Ca and Mg from silicates, the subglacial environment is a long-term CO₂ source.

In order to determine whether ice bodies typically act as CO₂ sources or sinks, we modeled the evolution of pH and alkalinity through a range of typical subglacial weathering reactions, considering both the case in which CO₂ and O₂ can openly exchange with the atmosphere and the case in which the subglacial environment is closed from atmospheric interaction. We find that in the closed system scenario, subglacial waters cannot reach atmospheric P_CO2 levels under typical conditions. Initial open system weathering followed by closed system weathering can allow CO₂ supersaturation when sulfide oxidation is considered.

We use this result to analyze pH and alkalinity measurements from a geographically and geologically diverse selection of subglacial waters. The P_CO2 of most of the subglacial waters is near or above atmospheric values. This implies that exchange of gases between subglacial waters and the atmosphere is typical and widespread. This input of atmospheric CO₂ into the glacial weathering environment implies that about 5 mg of CO₂ are typically removed from the atmosphere per l of glacial discharge water. Similar P_CO2 values can be produced in an entirely closed system if organic C is broken down through anoxic fermentation. These closed system assumptions would require order of 10 mg of CO₂ production per l. As that rate of organic decomposition cannot be sustained over a glacial cycle, we favor the interpretation that glaciers and ice sheets are typically carbon sinks.