Glacial Lake Agassiz Peatlands Research Articles

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 Δ14C 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.

Stable isotopes of water show deep seasonal recharge in northern bogs and fens

Ground water recharge is assumed to occur primarily at raised bog crests in northern peatlands, which are globally significant terrestrial carbon reservoirs. We synoptically surveyed vertical profiles of peat pore water δ18O and δ2H from a range of bog and fen landforms across the Glacial Lake Agassiz Peatlands, northern Minnesota. Contrary to our expectations, we find that local-scale recharge penetrates to not only the basal peat at topographically high bog crests but also transitional Sphagnum lawns and low-lying fen water tracks. Surface landscape characteristics appear to control the isotopic composition of the deeper pore waters (depths ≥0.5 m), which are partitioned into discrete ranges of δ18O on the basis of landform type (mean ± standard deviation for bog crests = −11.9 ± 0.4‰, lawns = −10.6 ± 0.1‰, fen water tracks = −8.8 ± 1.0‰). Fen water tracks have a shallow free-water surface that is seasonally enriched by isotope fractionating evaporation, fingerprinting recharge to underlying pore waters at depths ≥3 m. Isotope mass balance calculations indicate on average 12% of the waters we sampled from the basal peat of the fen water tracks was lost to surface evaporation, which occurred prior to advection and dispersion into the underlying formation. These new data provide direct support for the hypothesis that methane production in deeper peat strata is fuelled by the downward transport of labile carbon substrates from the surface of northern peat basins. Copyright © 2013 John Wiley & Sons, Ltd.

Investigating dissolved organic matter decomposition in northern peatlands using complimentary analytical techniques

The chemical transformations that govern storage, degradation, and loss of organic matter in northern peatlands are poorly characterized, despite the significance of these peat deposits as pivotal reservoirs in the global carbon cycle. One of the most challenging problems concerns the character of dissolved organic matter (DOM) in peat porewaters, particularly higher-molecular weight compounds that may function either as non-reactive sinks or reactive intermediates for organic byproducts of microbial decay. The complexity of these large molecules has defied attempts to characterize their molecular structure in bulk samples with a high degree of precision. We therefore determined the composition and reactivity of DOM from representative bog and fen sites in the Glacial Lake Agassiz Peatlands (GLAP) in northern Minnesota, USA. We applied four complementary techniques: electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS), proton nuclear magnetic resonance spectroscopy (1H NMR), specific UV absorbance (SUVA) and excitation–emission matrix (EEM) fluorescence spectroscopy. We observed that the vast majority (>80%) of molecular formulas that appear in the surface bog DOM are also present at 2.9m depth, indicating that much of DOM in the bog is resistant to microbial degradation. In contrast to bog samples, a considerable number of new compounds with low O/C and high H/C elemental ratios were observed in the 3m fen horizon relative to surface samples. These results indicate a more pronounced difference in the composition of surface and deep DOM in the fen.SUVA, determined at 254nm, indicated significantly lower aromaticity in deep fen samples relative to deep bog samples. This trend was verified by 1H NMR. Aromatic and carbohydrate components represented up to 70% of deep bog DOM but comprised a much smaller proportion of deep fen DOM, which was dominated by functionalized and non-functionalized aliphatics. Molecular formula data determined by FT-ICR mass spectrometry were consistent with results from optical and NMR spectroscopy measurements and showed that compounds with low O/C and high H/C were generated with depth in the fen. Such compounds were absent in both surface fen and in surface and deep bog samples respectively, providing further evidence of qualitative and quantitative differences in the evolution of DOM in fens and bogs. These differences, attributed to either variations in source vegetation or environmental factors that render DOM more reactive in fen sites or less reactive in bog sites, have important implications for the response of peatlands to climate change, since climatic change leading to moister conditions may enhance the abundance of sedge-dominated fens and increase the pool of more labile soil carbon.

Microbial community structure and activity linked to contrasting biogeochemical gradients in bog and fen environments of the Glacial Lake Agassiz Peatland.

The abundances, compositions, and activities of microbial communities were investigated at bog and fen sites in the Glacial Lake Agassiz Peatland of northwestern Minnesota. These sites contrast in the reactivity of dissolved organic matter (DOM) and the presence or absence of groundwater inputs. Microbial community composition was characterized using pyrosequencing and clone library construction of phylogenetic marker genes. Microbial distribution patterns were linked to pH, concentrations of dissolved organic carbon and nitrogen, C/N ratios, optical properties of DOM, and activities of laccase and peroxidase enzymes. Both bacterial and archaeal richness and rRNA gene abundance were >2 times higher on average in the fen than in the bog, in agreement with a higher pH, labile DOM content, and enhanced enzyme activities in the fen. Fungi were equivalent to an average of 1.4% of total prokaryotes in gene abundance assayed by quantitative PCR. Results revealed statistically distinct spatial patterns between bacterial and fungal communities. Fungal distribution did not covary with pH and DOM optical properties and was vertically stratified, with a prevalence of Ascomycota and Basidiomycota near the surface and much higher representation of Zygomycota in the subsurface. In contrast, bacterial community composition largely varied between environments, with the bog dominated by Acidobacteria (61% of total sequences), while the Firmicutes (52%) dominated in the fen. Acetoclastic Methanosarcinales showed a much higher relative abundance in the bog, in contrast to the dominance of diverse hydrogenotrophic methanogens in the fen. This is the first quantitative and compositional analysis of three microbial domains in peatlands and demonstrates that the microbial abundance, diversity, and activity parallel with the pronounced differences in environmental variables between bog and fen sites.

Influence of acidification on the optical properties and molecular composition of dissolved organic matter

Acidification is a common method for preserving dissolved organic matter (DOM) in natural water samples until sophisticated laboratory analyses can be performed. However, little is known about the effects of this practice on the composition and optical properties of DOM. In this study, the effects of acidification on DOM in porewater samples collected from the RL IV bog system of the Glacial Lake Agassiz Peatlands in northern Minnesota were characterized. Molecular composition was determined by ultrahigh resolution mass spectrometry and optical properties by UV absorption and three-dimensional fluorescence spectroscopy. Excitation–emission matrix fluorescence spectroscopy results indicate that the fluorescence properties of the peatland porewater DOM were sensitive to pH and that the observed changes were fluorophore dependent. Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry revealed the appearance of newly formed, oxygen-rich compounds upon acidification. The extent to which these oxygen-rich compounds were formed was also dependent on the composition of the DOM.

Characterization of dissolved organic matter in northern peatland soil porewaters by ultra high resolution mass spectrometry

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) was used to identify the qualitative differences between dissolved organic matter (DOM) in fen and bog porewaters from the Red Lake II system in the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota. Approximately 80% of the molecular composition in surface porewater was maintained throughout the upper portion of the bog profile (0.17–2.50 m). The qualitative stability of the composition of the DOM was accompanied by a quantitative increase in dissolved organic carbon (DOC) with depth. The composition of DOM in the fen was significantly different at depth, with slightly varying DOC levels. Aromaticity index (AI) values were used to identify condensed aromatic and phenol-type compounds in the porewater of both peatlands. Surface bog and deep fen DOM had surprisingly similar molecular composition. Differences in enzymatic degradation rates via phenol oxidase in the bog and surface fen horizons, slower transport down the bog vertical profile and the presence of a stratum of Sphagnum-woody peat at depth in the fen are suggested as being responsible for the observed variations in DOM composition.

Variations in free‐phase gases in peat landforms determined by ground‐penetrating radar

The spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified. This study uses ground‐penetrating radar to noninvasively estimate the vertical distribution of biogenic free‐phase gas (FPG) in two distinct peat landform types in the Glacial Lake Agassiz Peatland, Minnesota: a near‐crest bog and a midslope lawn (i.e., both as defined by surface vegetation communities). Ground‐penetrating radar velocities retrieved from common midpoint surveys were modeled using the Complex Refractive Index Model to estimate free‐phase gas volumes along one‐dimensional vertical profiles. Near‐crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24% by volume of FPG localized within the deep peat (e.g., 2–4 m depth). Midslope lawn sites show lower total volumes (up to 12% free‐phase gas) and less variability that result in a more even free‐phase gas distribution throughout the vertical profile. High‐resolution data also suggests the presence of thinner layers (<1 m) containing up to 40% free‐phase gas at a near‐crest site. Our results suggest that spatial distribution of free‐phase gas along the peat column may vary in the Glacial Lake Agassiz Peatlands depending on peat landform. Statistical analysis supports vertical variability of ground‐penetrating radar velocity data with a 95% confidence limit. Our results could potentially be upscaled using surface vegetation communities to estimate landscape‐scale gas storage potential in peat landforms.

Heat transport in the Red Lake Bog, Glacial Lake Agassiz Peatlands

AbstractWe report the results of an investigation on the processes controlling heat transport in peat under a large bog in the Glacial Lake Agassiz Peatlands. For 2 years, starting in July 1998, we recorded temperature at 12 depth intervals from 0 to 400 cm within a vertical peat profile at the crest of the bog at sub‐daily intervals. We also recorded air temperature 1 m above the peat surface. We calculate a peat thermal conductivity of 0·5 W m−1 °C−1 and model vertical heat transport through the peat using the SUTRA model. The model was calibrated to the first year of data, and then evaluated against the second year of collected heat data. The model results suggest that advective pore‐water flow is not necessary to transport heat within the peat profile and most of the heat is transferred by thermal conduction alone in these waterlogged soils. In the spring season, a zero‐curtain effect controls the transport of heat through shallow depths of the peat. Changes in local climate and the resulting changes in thermal transport still may cause non‐linear feedbacks in methane emissions related to the generation of methane deeper within the peat profile as regional temperatures increase. Copyright © 2006 John Wiley & Sons, Ltd.

Flow path oscillations in transient ground-water simulations of large peatland systems

Transient numerical simulations of the Glacial Lake Agassiz Peatland near the Red Lakes in Northern Minnesota were constructed to evaluate observed reversals in vertical ground-water flow. Seasonal weather changes were introduced to a ground-water flow model by varying evapotranspiration and recharge over time. Vertical hydraulic reversals, driven by changes in recharge and evapotranspiration were produced in the simulated peat layer. These simulations indicate that the high specific storage associated with the peat is an important control on hydraulic reversals. Seasonally driven vertical flow is on the order of centimeters in the deep peat, suggesting that seasonal vertical advective fluxes are not significant and that ground-water flow into the deep peat likely occurs on decadal or longer time scales. Particles tracked within the ground-water flow model oscillate over time, suggesting that seasonal flow reversals will enhance vertical mixing in the peat column. The amplitude of flow path oscillations increased with increasing peat storativity, with amplitudes of about 5 cm occurring when peat specific storativity was set to about 0.05 m −1.

Use of hydraulic head to estimate volumetric gas content and ebullition flux in northern peatlands

Hydraulic head was overpressured at middepth in a 4.2‐m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free‐phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2‐m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous‐phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m−2 d−1. These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas‐flux chambers or tower‐mounted sensors.

Regional ground-water flow modeling of the Glacial Lake Agassiz Peatlands, Minnesota

Three-dimensional ground-water modeling experiments were done to test the hypothesis that regional ground-water flow is an important component of the water budget in the Glacial Lake Agassiz Peatlands of northern Minnesota. Previous data collected from the Glacial Lake Agassiz Peatlands suggest that regional ground-water flow discharges to these peatlands, maintaining saturation, controlling the peat pore-water chemistry, and driving ecological change. To test this hypothesis, steady-state MODFLOW models were constructed that encompassed an area of 10,160 km 2. Data used in this modeling project included surface-water and water-table elevations measured across the study area, digital elevation data, and well logs from scientific test wells and domestic water wells drilled in the study area. Numerical simulations indicate that the Itasca Moraine, located to the south of the peatland, acts as a recharge area for regional ground-water flow. Ground water recharged at the Itasca Moraine did not discharge to the Red Lake Peatlands, but rather was intercepted by the Red Lakes or adjacent rivers. Simulations suggest that ground-water flow within the peatlands consists of local-flow systems with streamlines that are less than 10 km long and that ground water from distant recharge areas does not play a prominent role in the hydrology of these peatlands. Ground-water flow reversals previously observed in the Red Lake Peatlands are either the result of interactions between local and intermediate-scale flow systems or the transient release of water stored in glacial sediments when the water-table is lowered.

87Sr/86Sr as a tracer of groundwater discharge and precipitation recharge in the Glacial Lake Agassiz Peatlands, northern Minnesota

A contrast in the 87Sr/86Sr ratio of atmospherically derived Sr (∼0.710) and Sr from regional bedrock (>0.730) allows quantification of the proportions of precipitation recharge and groundwater discharge into peat columns of bogs and fens in the Glacial Lake Agassiz Peatlands. Two contrasting bog‐fen pairs were studied: the Lost River peatland located over a regional groundwater discharge zone and the Red Lake peatland situated atop a hydrologic divide where recharge is expected to occur. Bog pore waters consist of 90–100% precipitation in the upper 2 m, below which they mix with groundwater. Adjacent fens consist mostly of groundwater, ranging from ∼30% near the surface to ∼100% with depth. At the Red Lake peatland, mixing relations suggest an additional Sr source that may be peat mineralization. Distinct 87Sr/86Sr and [Ca]/[Sr] ratios for groundwater discharging into bog and fen sites are indicative of distinct groundwater sources and are likely indicative of flow paths at two different length scales.

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland

To elucidate the roles of hydrology and vegetation in below ground carbon cycling within peatlands, radiocarbon values were obtained for pore water dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), CH4, and peat from the Glacial Lake Agassiz peatland. The major implication of this work is that the rate of microbial respiration within a peat column is greater than the peat decomposition rate. The radiocarbon content of DOC at both bog and fen was enriched relative to solid‐phase peat by ∼150‐300‰ consistent with the advection of recently photosynthesized DOC downward into the peat column. Fen Δ14C values for DIC and CH4 closely track the Δ14C of pore water DOC at depth, indicating that this recent plant production was the predominant substrate for microbial respiration. Aceticlastic methanogenesis apparently dominated the upper third of the peat column (α = 1.05), shifting toward CO2 reduction with depth (1.05 < α < 1.08). Upwelling groundwater contributed as much as 15% of the DIC to the bulk DIC pool at depth in the fen. The similarity of Δ14C values for DIC and CH4 suggests that methanogens utilized DIC from this source as well as DIC produced in situ. Bog Δ14C values for pore water DIC and CH4 differ by ≤ 15‰ at all depths and are depleted in 14C relative to DOC by ∼100‰, suggesting microbial utilization of a mixture of older and modern substrates. CO2 reduction was the primary pathway for methanogenesis at all depths in the bog (α = 1.08), and groundwater influence on bulk DIC was negligible. For both sites, Δ14C−DIC and Δ14C−CH4 are approximately equal at depths where stable isotope data indicate a predominance of CO2 reduction and dissimilar when acetate fermentation is indicated.

Methane Concentration and Stable Isotope Distribution as Evidence of Rhizospheric Processes: Comparison of a Fen and Bog in the Glacial Lake Agassiz Peatland Complex

This study evaluates relationships between vegetation and stable isotope distribution within a large, northern peat-accumulating wetland. Concentration and δ13C for both porewater and emitted methane were obtained from June–September for two systems characterized by different plant assemblages and hydrologic regimes: a Carex -dominated fen and a Sphagnum -dominated, forested bog crest. Average methane emissions were higher in the fen than at the bog crest across the entire growing season. Fen porewater methane concentrations were maintained at consistently low levels in the upper one-third of the peat column, and emitted methane was substantially13C-depleted (by approx. 6‰) relative to shallow porewater methane, trends which are characteristic of passive plant-mediated transport of rhizospheric methane to the atmosphere. Fen porewater δ13C-CH4values in shallow peat (approx. −59‰) suggest that microbial respiration was primarily driven by acetate fermentation. CO2reduction became more important deeper in the peat column with δ13C-CH4values ranging from approx. −65 to −69‰ between 1 and 2.8m. In contrast to the fen, porewater methane concentrations in the bog were usually at near-maximum levels just below the water table. δ13C values for emitted CH4in the bog were enriched relative to those for shallow porewater CH4by approx. 10‰, indicating that methane was subject to oxidation as it exited from the peat via passive diffusion. Methanogenesis in the peat at the bog crest appears to have been substrate-limited, with porewater δ13C-CH4(approx. −67‰) suggestive of CO2reduction at all depths.

Simulating vertical flow in large peatlands

Ground-water flow simulations are used to evaluate the importance of three parameters on vertical flow in peatlands: regional slope, permeability of the mineral soil underlying the peat, and peatland topography. Our results indicate that the extent of vertical ground-water flow in peatlands is primarily controlled by mineral soil permeability. Local ground-water flow cells that form under small water-table mounds at bog domes can drive peat pore water into permeable mineral soil. However, when peat forms over low permeability mineral soil, vertical movement of peat pore water becomes negligible and lateral flow of water in the upper portion of the peat column dominates the peatland hydrology. This effect is not due to the low permeability of humified peat layers, as is commonly assumed in many peatland studies. Field data from the Hudson Bay Lowland (Canada) and the Glacial Lake Agassiz peatlands of northern Minnesota confirm the validity of these models. These results are relevant in settings other than peatlands where relief is low and small topographic mounds exist.

A stochastic appraisal of the annual carbon budget of a large circumboreal peatland, Rapid River Watershed, northern Minnesota

The probable limits of the carbon budget of the Rapid River Watershed, within the greater Glacial Lake Agassiz Peatland in northern Minnesota, were evaluated using a Monte Carlo simulation approach. Carbon enters the peatlands in groundwater, precipitation, and primary productivity. Carbon leaves the peatlands by groundwater and surface water outflow and by the outgassing of methane. Results of the simulations of the carbon budget show that the peatland is now probably a sink for carbon, supported by field data showing peat is, in fact, accumulating at the rate of about 1 mm yr−1 [Glaser et al., 1997]. Excluding extreme values, Monte Carlo simulation results indicate that the Rapid River Peatland stores between −28.98 g C m−2 yr−1 (release) and 50.38 g C m−2 yr−1 (storage) with a mean accumulation of 12.74 g C m−2 yr−1 over the 1,506,200 m2 watershed. The peatland appears to be delicately poised with respect to net gain or loss of carbon.

Climate-driven flushing of pore water in peatlands

NORTHERN peatlands can act as either important sources or sinks for atmospheric carbon1,2. It is therefore important to understand how carbon cycling in these regions will respond to a changing climate. Existing carbon balance models for peatlands assume that fluid flow and advective mass transport are negligible at depth3,4, and that the effects of climate change should be essentially limited to the near-surface. Here we report the response of groundwater flow and porewater chemistry in the Glacial Lake Agassiz peat-lands of northern Minnesota to the regional drought cycle. Comparison of field observations and numerical simulations indicates that climate fluctuations of short duration may temporarily reverse the vertical direction of fluid flow through the peat, although this has little effect on water chemistry5. On the other hand, periods of drought persisting for at least 3–5 years produce striking changes in the chemistry of the pore water. These longer-term changes in hydrology influence the flux of nutrients and dissolved organic matter through the deeper peat, and therefore affect directly the rates of fermentation and methanogenesis, and the export of dissolved carbon compounds from the peatland.

Hydraulic reversals and episodic methane emissions during drought cycles in mires

Research Article| March 01, 1993 Hydraulic reversals and episodic methane emissions during drought cycles in mires Edwin A. Romanowicz; Edwin A. Romanowicz 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Search for other works by this author on: GSW Google Scholar Donald I. Siegel; Donald I. Siegel 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Search for other works by this author on: GSW Google Scholar Paul H. Glaser Paul H. Glaser 2Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455 Search for other works by this author on: GSW Google Scholar Author and Article Information Edwin A. Romanowicz 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Donald I. Siegel 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Paul H. Glaser 2Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (3): 231–234. https://doi.org/10.1130/0091-7613(1993)021<0231:HRAEME>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edwin A. Romanowicz, Donald I. Siegel, Paul H. Glaser; Hydraulic reversals and episodic methane emissions during drought cycles in mires. Geology 1993;; 21 (3): 231–234. doi: https://doi.org/10.1130/0091-7613(1993)021<0231:HRAEME>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic-head and dissolved-methane profiles in peat, Glacial Lake Agassiz Peatlands, northern Minnesota, United States. During the summer 1990 drought, regional ground water discharged from the mineral soil underlying the peat to raised bogs. Concentrations of dissolved CH4 in pore water showed that much of the peat column was supersaturated with respect to a reference standard of I atm partial pressure CH4. By the summer of 1991, the severity of the drought lessened, and the upper peat column was resaturated with precipitation-derived water. Ground-water flow was then controlled by local, precipitation-driven, recharge flow systems, and the regional ground water that was discharged affected only the peat immediately above the mineral soil-peat interface. In contrast, during the summer of 1991, concentrations of dissolved CH4 in pore water were all undersaturated with respect to 1 atm partial pressure CH4. The decrease in the amount of dissolved CH4, in the pore water from the summer of 1990 to the summer of 1991 probably was caused by changes in the CH4 flux and degassing to the unsaturated zone and to a lesser extent by changes in the partial pressure of gaseous CH4 in the peat as the peat volume increased with expansion as it resaturated. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ground Water and the Evolution of Patterned Mires, Glacial Lake Agassiz Peatlands, Northern Minnesota

(1) The hydrogeological setting of the Glacial Lake Agassiz Peatlands in Minnesota was investigated by measuring ground-water levels in observation wells, by studies of soil types and thicknesses, and by computer model experiments to simulate ground-water flow. (2) Most ground water circulates along flow paths several kilometers long that pass through the peat column and into the underlying mineral soil. (3) Most ground-water flow is probably caused by the development and persistence of large raised bogs, and occurs because of ground-water mounds (elevated water tables) under the bogs. (4) Lateral bog growth may be limited by the neutralizing of bog water acidity by ground-water discharge ('artesian' flow) at raised bog margins.