Date of Award

2005

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Daniel F. Belknap

Second Committee Member

Joseph T. Kelley

Third Committee Member

Emmanuel Boss

Abstract

Natural gas is observed in nearly every muddy estuary in Maine. In some locations, pockmarks provide evidence for escaping fluids. Methane, the largest component of the gas, is recognized as a potentially powerful greenhouse gas for which the marine realm may act as a sink and/or source. Inner shelf areas and estuaries are not included in most global models of methane's greenhouse effects. While the quantity of methane produced, released, and the potential for future production are of primary interest in global cycles, the resources and techniques available prevented addressing those questions. As an alternative, the source material for methanogenesis and its relationship to late Quaternary sea-level changes and potential implications were investigated in this study. Repeat surveys show that the Black Ledges area of Penobscot Bay actively produced new pockmarks at a rate of 137 per year and filled pockmarks at a rate of 45 per year between 2000 and 2002, thus reworking fine-grained sediments within the basin. The new features are smaller, thus the basin was a sediment sink, sequestering approximately 100,000 kg/day within a 17 km2 area. Comparisons of Black Ledges environments with those of Cranberry Basin, Muscongus Bay suggests that the geotechnical properties of sediments control pockmark development. Cranberry Basin sediments are slightly coarser and higher in water content. The gas front is generally within 1 m of the sediment-water interface. The sediments may allow gas to migrate vertically to the seafloor and escape without creating a pockmark. Direct sampling shows high (>15%) organic content in estuarine muds with identifiable plant fragments and a section of preserved salt marsh. These sediments are capable of supporting methanogenesis. Additionally, seismic data and reconstructions suggest the presence of drowned and filled lake basins. However, I was unable to directly sample paleo-lacustrine environments. The source material for the gas is stratigraphically higher than the glaciomarine sediments, which possess low (<5%) organic content. This boundary is typically the Pleistocene-Holocene unconformity that developed during the latest transgression. The source of the gas is related to the Holocene transgression, preservation potential, and present-day estuarine development.

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