HR: 0800h
AN: B21B-0882    [Abstracts]
TI: Oxygen Isotopic Disequilibrium in Bacteriogenic Soil Calcite Precipitated Near Leaking Oil and Gas Wells in Western Canada
AU: * Arkadakskiy, S V
EM: serguey@gpu.srv.ualberta.ca
AF: University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Canada
AU: Muehlenbachs, K
EM: Karlis.Muehlenbachs@ualberta.ca
AF: University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Canada
AU: Mendoza, C
EM: Carl.Mendoza@ualberta.ca
AF: University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Canada
AU: Szatkowski, B
EM: gchem@telusplanet.net
AF: GCHEM Ltd., 101 Airport Road, Lloydminster, AB T9V 2S1 Canada
AB: About one third of all oil and gas wells drilled in Western Canada leak natural gas to surface. Leaking gas is oxidized, sometimes completely, by metanotrophic bacteria in soil near the wells. Depending on soil permeability and rate of gas leakage, zones of bacterial oxidation extend from 20 to $>$500 cm away from the well bores of leaking wells. Thin coatings and aggregates $<$300 micrometers across of authigenic calcite of both abiotic and bacteriogenic origin are found in the oxidation zones. Abiotic calcite forms crusts comprised of submicron to micron size sparry, euhedral crystals typical of inorganic soil calcite precipitates. Bacteriogenic calcite is closely associated with bacteria and microbial film and forms massive to porous aggregates of subhedral rounded crystals coating or cementing mineral grains. Growth of nanometer size calcite crystals on top of microbial surfaces indicates active involvement of bacteria with calcite precipitation. To constrain the conditions of calcite precipitation \delta$^{13}$C and \delta$^{18}$O of soil CO$_{2}$ and soil temperature at two leaking well sites in Saskatchewan were monitored for ca. two years. The \delta$^{18}$O of local soil moisture and groundwater were also measured. Results show that \delta$^{18}$O (PDB) of bacteriogenic calcite is from 3 to 6 permil lower than this that would precipitate in isotopic equilibrium with local soil moisture at any time during the year. Although \delta$^{13}$C of bacteriogenic calcite may be in isotopic equilibrium with local soil CO$_{2}$ at temperatures close to freezing, decrease of bacterial metabolic rates at such low temperatures would not favor bacteriogenic calcite precipitation. Therefore, \delta$^{13}$C may also reflect disequilibrium. In contrast, both \delta$^{18}$O and \delta$^{13}$C of abiotic calcite are in isotopic equilibrium with soil moisture and soil CO$_{2}$ at temperatures close to or higher than the average soil temperatures in the area. The small sizes of bacteriogenic calcite aggregates indicate that these precipitated in confined pools of water where local chemistry and pH are easily modified by the microorganisms. Therefore, oxygen and/or carbon isotopic disequilibrium of bacteriogenic calcite is likely the result of bacterial metabolism. The \delta$^{18}$O of soil calcite in paleosols has been widely used as a paleothermometer. Bacteriogenic calcite precipitation however is common in soils and not confined to zones of leaking gas oxidation. Likewise oxygen isotopic disequilibrium of bacteriogenic soil calcite may also be a common phenomenon. Therefore, paleosols should be examined for evidence of bacterial involvement in calcite precipitation prior to the use of \delta$^{18}$O of soil calcite as a paleothermometer.
DE: 4870 Stable isotopes
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting