A CRITICAL ZONE PERSPECTIVE ON PRAIRIE POTHOLES ECOSYSTEM EVOLUTION

The critical zone (CZ) is the interval between the outer vegetation envelope and the lower limits of groundwater. This zone coincides spatially with the interval occupied by the majority of terrestrial ecosystems. Whereas ecosystem investigations tend to emphasize interactions among biota and bioactive elements, CZ research places greater emphasis on the operation of the geologic, hydrologic, geomorphologic, and geochemical underpinnings of ecosystems. In many environments, this more geocentric perspective is useful, even necessary, to fully understand ecosystem function. The Prairie Pothole Region (PPR) illustrates this point.

The PPR occupies 750,000 km² of the northern Great Plains of the US and Canada with millions of small (generally <1 ha) internally drained wetlands. These wetlands host over 7 million migratory shorebirds. The PPR wetlands arise from CZ processes operating on the underlying fine grained glacial till. The till is poorly permeable, causing precipitation to pond on the surface. Most hydrologic interactions between wetlands occur via groundwater flow within discrete km-scale basins. In the 92 ha Cottonwood Lake Study Area (CWLSA) ND, USA, upland wetlands typically have low salinity (TDS ~150 mg/kg- Ca-HCO3 dominant) and recharge the groundwater system. A lowland wetland only 200m distant (but 18m lower) discharges groundwater and has TDS>3000 mg/kg (Mg-SO4 dominant). This local geochemical variability produces equally marked variability in flora and fauna. Groundwater and wetland chemistry is controlled by CZ oxidation of pyrite (FeS₂) within a marine shale component of the glacial till, thus generating SO₄^2- dominant waters and a near surface brown iron oxide-bearing zone. Core logs from >30 drill holes within the CWLSA show that the oxidized zone occurs throughout and is up to 10m deep. Modeling of uranium-series isotope disequilibrium document that formation of this oxidized layer may have required nearly 10 Kyr. Regional data from both the US and Canadian portions of the PPR verify that both the surface brown oxidized layer and associated highly spatially variable wetland geochemistry are present throughout. Thus, long-term CZ oxidation processes, operating over very large areas dominate wetland water composition, which in turn is a primary control on ecology.