Title

Carbon dynamics, nutrient cycling, and the material properties of peat in the Glacial Lake Agassiz Peatlands, Northern Minnesota

Date of Award

1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Donald I. Siegel

Keywords

Carbon dynamics, Nutrient cycling, Glacial Lake Agassiz, Peatlands, Minnesota

Subject Categories

Biogeochemistry | Earth Sciences | Environmental Sciences | Physical Sciences and Mathematics

Abstract

The phenomenon of global warming is a problem that is likely to have lasting repercussions on global circulation, temperatures, and hydrologic functions in temperate regions. Large wetlands are capable of exchanging vast quantities of greenhouse gases between the atmosphere and terrestrial reservoirs. The three research studies in this dissertation investigate this exchange. Results of the first study, a stochastic model of carbon cycling, suggest the Glacial Lake Agassiz peatland complex (GLAP) in northern Minnesota now sequesters approximately 12.74 g C m -2 yr -1 . The current accumulation of carbon may shift to a net carbon loss as atmospheric temperatures increase and water tables lower in this region, which would liberate dissolved and free CO 2 and CH 4 currently stored in the peatlands.

Nutrient balances (especially nitrogen) in wetlands may also impact greenhouse gas emission because the end product of several pathways of the nitrogen cycle is N 2 O. The release of this greenhouse gas increases with rising temperatures and lowering of water tables. The results of the second, study on the spatial and temporal nutrient dynamics in the GLAP show that ammonia (NH 4 ) changes to nitrate (NO 3 ) in the lower anoxic depths of both bog and fen peat in the Glacial Lake Agassiz Peatlands. Other oxidation-reduction reactions documented in this study include NO 3 to NH 4 , SO 4 to H 2 S and CH 2 O to CH 4 .

A large source of uncertainty associated with carbon and nitrogen dynamics in wetlands stems from difficulty in constraining material properties of peat. The dual porosity nature of peat can produce large spatial heterogeneity in concentrations of solutes. The third study uses computed tomography (CT) to non-invasively produce high-resolution, three-dimensional images of the porosity structure of peat. In a series of solute transport experiments, CT imagery was able to visually document that: (1) increasing salinity of porewater causes effective porosity of humified peat to increase up to 5%; (2) solutes moving through active pore spaces diffuse into large segments of dead pore space; and (3) biogenic gas accumulation and transport may occur first along horizontal planes until bubbling out in large macropores.

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