Climatic Drivers for Multi-Decadal Shifts in Solute Transport and Methane Production Zones within a Large Peat Basin

Northern peatlands are an important source for greenhouse gases but their capacity to produce methane remains uncertain under changing climatic conditions. We therefore analyzed a 43-year time series of pore-water chemistry to determine if long-term shifts in precipitation altered the vertical transport of solutes within a large peat basin in northern Minnesota. These data suggest that rates of methane production can be finely tuned to multi-decadal shifts in precipitation that drive the vertical penetration of labile carbon substrates within the Glacial Lake Agassiz Peatlands. Tritium and cation profiles demonstrate that only the upper meter of these peat deposits was flushed by downwardly moving recharge from 1965 through 1983 during a Transitional Dry-to-Moist Period. However, a shift to a moister climate after 1984 drove surface waters much deeper, largely flushing the pore waters of all bogs and fens to depths of 2 m. Labile carbon compounds were transported downward from the rhizosphere to the basal peat at this time producing a substantial enrichment of methane in Delta C-14 with respect to the solid-phase peat from 1991 to 2008. These data indicate that labile carbon substrates can fuel deep production zones of methanogenesis that more than doubled in thickness across this large peat basin after 1984. Moreover, the entire peat profile apparently has the capacity to produce methane from labile carbon substrates depending on climate-driven modes of solute transport. Future changes in precipitation may therefore play a central role in determining the source strength of peatlands in the global methane cycle

Geochemical characterization of groundwater flow processes in a large patterned peatland

For several decades, scientists have studied the hydrology and geochemistry of Northern peatlands with respect to their influence on potential climate change. The complex hydrology and geochemistry in peatlands lead to unique vegetation, which sequesters carbon in peat soils. Peatlands also creates biogeochemical conditions that induce release of greenhouse gases carbon dioxide and methane. Much of what we know about peatland hydrology and ecology evolved from over 30 years of research in the Glacial Lake Agassiz Peatlands (GLAP), MN. Here, gaseous methane, produced at depth, was first identified as a major potential contributor of methane flux to the atmosphere, along with diffusive transport to the atmosphere. The deep methane forms at the interface between downward moving groundwater under raised bogs and upwards moving groundwater from underlying mineral soils. It remains uncertain how groundwater upwells into these peatlands, keeps the peat saturated, and sustains vegetation growth, even under drought conditions. Moreover, seasonal variability in peatland hydrology controls dissolved organic carbon (DOC) mineralization, consequently releasing or sequestering greenhouse gases. In my dissertation, I have investigated three aspects of the peatland geochemical puzzle that bear on solute movement in large circumboreal peatlands and alter hydrology within peat due to methane formation. For this work, I sampled pore water from seven bog-fen complexes in the study area over a period of three years for geochemical and isotopic analyses. In chapter 1, I report the results of synnoptic studies of the pore water chemical and isotopic composition in multiple bog-fen complexes found in different hydrogeologic settings. This study was designed to better characterize the geochemical mixing and sources of pore water in peat. In chapter 2, I addressed the effect of decadal scale seasonal wet-dry cycles on the geochemical and isotopic composition of peat pore water and net changes in the rate of deep methanogenesis. In the 3 rd chapter, I heuristically assessed how recharging acidic water and upwelling minerotropic water affect the pore water composition. For this, I built a module in a geochemical computational program PHREEQC, incorporating organic acid dissociation constants to assess the acid-base equilibrium of pore waters in peat