Date of Award

June 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Donald I. Siegel

Keywords

Climate, Geochemistry, Peatlands, Porewater, Prairie Wetlands, Wetlands

Subject Categories

Physical Sciences and Mathematics

Abstract

Wetlands develop where climate and physiography conspire to maintain saturated soils at the land surface, support diverse plant and animal communities, and serve as globally important sinks for atmospheric carbon. The chemistry of wetland porewaters impacts near-surface biological communities and subsurface biogeochemical processes that influence carbon cycling in the environment. Wetland porewater chemistry is a dynamic byproduct of complex hydrogeological processes that cause meteoric waters to enter groundwater systems (recharge) or groundwater to flow to the land surface (discharge). Changes in climate can alter subsurface hydraulic gradients that determine the recharge and discharge functions of wetlands, which in turn control the hydrogeochemical evolution of wetland porewaters.

The climate of mid-continental North America is influenced by competing air masses with vastly different temperature and moisture contents originating from the Pacific Coast, the Gulf of Mexico, and the Arctic. The interactions of these air masses result in large dynamic shifts of climate regimes characterized by decadal-scale oscillations between periods of drought and heavy rain. Over the course of the 20th century, a shift occurred towards wetter climate in the mid-continental region. This dissertation examines the impact of this climate shift on the porewater chemistry of two very different wetland systems, located only 350 km apart: the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota and the Cottonwood Lake Study Area (CLSA) of North Dakota. The former study site consists of a large (7,600 km2), circumboreal peatland that developed an extensive blanket of peat over the last ~5000 years on a relatively flat glacial lake bed within a sub-humid to semi-arid climate gradient characterized by small annual atmospheric moisture surpluses and frequent droughts. The latter study site consists of a 0.92 km2 complex of small (meter-scale) “prairie pothole” wetlands located on a hummocky glacial stagnation moraine under semi-arid climate where wetlands frequently fill and dry with surface ponds over low-permeability glacial till in response to snowmelt runoff and evapotranspiration. Both sites have been the subject of long-term hydrological study since c. 1980 and are well-established examples of the sensitivity of wetland functions to changes in climate.

The first chapter of this dissertation utilizes a semi-conservative tracer suite (pH, Ca, Mg, Sr, 87Sr/86Sr) to fingerprint discharge of calcareous groundwater to GLAP peat along a ~6 km transect from a bog crest downslope to an internal fen water track and bog islands. However, stable isotopes of the peat porewaters (δ18O and δ2H) show that the subsurface throughout the entire study area is currently flushed with recharge from the near surface peat. I hypothesize that back-diffusion of groundwater-derived solutes from the peat matrix to active pore-spaces has allowed the geochemical signal from paleo-hydrogeologic discharge to persist into the current regime of dilute recharge. This effect promotes methane generation in the peatland subsurface by allowing transport of labile carbon compounds from the land surface to depth while maintaining geochemical conditions (i.e. pH) in the deep peat favorable to biogenic methane production. The results of this study show that autogenic hydrogeochemical feedback mechanisms contribute to the resilience of peatlands systems and associated ecological functions against climate change.

The second chapter of this dissertation consists of a detailed geoelectrical survey of a well-studied, closed-basin prairie wetland (P1) in the CLSA that has experienced record drought and heavy rains (i.e. deluge) during the late 20th century. Subsurface storage of sulfate (SO4) salts allows many such closed-basin prairie wetlands to maintain moderate surface water salinities (TDS from 1 to 10 g L-1) that influence communities of aquatic biota. I imaged saline lenses of sulfate-rich porewater (TDS > 10 g L-1) in wetland sediments beneath the bathymetric low of the wetland and within the currently ponded area along the shoreline of a prior pond stand. Analyses of long-term (1979 – 2014) groundwater and surface water levels in the wetland suggest that the saline lenses formed during paleo-droughts when the groundwater levels dropped below the wetland bed and are stable in the subsurface on at least centennial timescales. I hypothesize a “drought-induced recharge” mechanism by which wetlands maintain moderate surface water salinity by subsurface storage during droughts when the wetlands dry and intermittent runoff events flush surface salts down secondary porosity created by desiccation fractures and terrestrial plant roots. Drought-derived saline groundwater has the potential to increase wetland salinity during record wet climate conditions currently prevalent in the Prairie Pothole Region.

The third chapter of this dissertation extends the findings of the second chapter by a detailed geochemical survey of wetland porewater, pond water, and upland groundwater in the P1 basin. I use a natural geochemical tracer suite of halogens (Cl, Br, and I) to better understand the hydrogeochemical evolution of saline groundwater in the wetland subsurface during prior droughts. I found that saline porewater lenses contain SO4, Cl, and Br that are ~6x more concentrated from levels measured in the surface pond, due to the effects of evapo-concentration and recharge during paleo-droughts. However, I found the highest concentrations of porewater iodine (up to ~4 µM; the highest dissolved iodine concentration ever reported for a terrestrial aquatic system) occurred in vertical profiles above the saline lenses. I hypothesize that chromatographic separation of iodine from SO4, Cl, and Br occurs during droughts when sedimentary iodine oxidizes from its reduced forms (I- and organically bound I) to the less-mobile iodate compound (IO3-). Understanding the hydrogeochemical evolution and composition of drought-derived, saline groundwater can be used to fingerprint sources of salinity to wetland ponds during the record wet climate conditions currently prevalent in the Prairie Pothole Region.

The three studies that comprise this dissertation illustrate diverse and complicated ways by which different wetland systems store and release salinity to their porewaters under dry and wet climate conditions. In the GLAP, geochemical buffering from the peat matrix stabilizes subsurface biogeochemical processing against the effects of climate change. At the CLSA, subsurface salinity storage during droughts allows closed-basin prairie wetlands to maintain moderate surface water salinities under semi-arid climate, which can be re-released back to wetland surface ponds during wetter conditions. These studies both have practical applications for forecasting the response of mid-continental wetlands to changes in climate, and highlight autogenic hydrogeochemical feedback mechanisms that help wetlands stabilize their ecological and biogeochemical functions under a changing climate.

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