INFLUENCE OF GROUNDWATER INPUT OF SULFATE ON METHANE EMISSIONS FROM PRAIRIE POTHOLE WETLANDS

The millions of wetland basins in the approximately 750,000 km² Prairie Pothole Region (PPR) of the North America have the potential to impact carbon budgets on a continental scale. Methane (CH₄) is an important greenhouse gas emitted by PPR wetlands, but controls on emissions are not well-defined. Many PPR wetland complexes are internally-drained and individual wetlands within a complex can range from fresh to hypersaline as a result of solute inputs via groundwater and evapoconcentration. The major ions SO₄^2-, HCO₃^-, Ca²⁺, and Mg²⁺ are elevated in local groundwater due to interaction with glacial till in which pyrite has been oxidized and the resulting acidity has been neutralized by carbonate dissolution. It is suspected that SO₄^2- exerts a primary control on CH₄emissions as observed for marine systems, but this has not been systematically investigated in the PPR.

We are combining long-term research on solute dynamics and CH₄ flux across the water-air interface at the Cottonwood Lake Study Area in North Dakota. This 92 ha site hosts a complex of wetlands having SO₄^2- concentrations in ponded water that currently range from <0.1 mM in groundwater recharge wetlands to almost 30 mM in some discharge wetlands. In addition to investigating the effects of evapoconcentration and dilution on SO₄^2- concentrations over time, we are investigating effects of remobilization of SO₄^2- via the dissolution of gypsum deposited during previous droughts. This process can result in gypsum saturated pore water with SO₄^2- concentrations >100 mM. Methane flux data have been collected from 12 wetland basins on a biweekly basis during growing seasons since 2009. Median CH₄ flux from central positions in ponded water from each wetland (0.1 to 34 mg CH₄ m^-2 hr^-1) negatively correlated with ponded water SO₄^2- concentrations. Methane flux increased exponentially with decreasing SO₄^2-. One wetland, which may have unique hydrology due to an underlying sand lens, was inconsistent with this trend. Its median CH₄ flux was relatively high (similar to freshwater wetlands) despite a SO₄^2- concentration of approximately 6 mM. In addition to the influence of SO₄^2- on CH₄ flux among wetlands, we are also considering the potential influence of spatial and temporal variability of SO₄^2- concentrations in ponded water and pore water within individual wetlands.