HR: 17:15h
AN: B42E-06    [PDF]
TI: Kinetic Fractionation of Carbon Isotopes During Carbonate Weathering in Glaciated Catchments: Implications for the Detection of Subglacial Microbial Activity
AU: * Skidmore, M
EM: skidmore@citrus.ucr.edu
AF: Department of Environmental Sciences, 2217 Geology Building University of California at Riverside, Riverside, CA 92521 United States
AU: Sharp, M
EM: martin.sharp@ualberta.ca
AF: Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Tranter, M
EM: m.tranter@bristol.ac.uk
AF: Bristol Glaciology Centre, School of Geographical Sciences University of Bristol, Bristol, BS8 1SS United Kingdom
AU: Bottrell, S
EM: s.bottrell@earth.leeds.ac.uk
AF: Dept. of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Microbes are abundant at the water-rock-ice interface beneath valley glaciers at Haut Glacier d'Arolla, Switzerland (HGA) and at John Evans Glacier, Ellesmere Island Nunavut, Canada (JEG). However, the importance of in-situ microbial activity in driving subglacial weathering reactions remains unknown. This is a key question when considering the potential role of microbes in mediating subglacial weathering and carbon cycling on a continental scale beneath the Pleistocene mid-latitude ice sheets. This study measured the chemical composition of meltwaters, including $\delta$${^13}$C-DIC at the two glaciers to quantify microbial CO$_{2}$ inputs to the DIC budget using isotope mass balance techniques. However, PCO$_{2}$ data indicates that most of the glacial meltwaters are far from equilibrium with respect to atmospheric CO$_{2}$ and thus kinetic processes are important in determining the water chemistry. Consequently, conventional equilibrium isotope mass balance techniques were inappropriate in this case. Hence, laboratory experiments were conducted with calcium carbonate and carbonate rich glacial sediments from JEG under simulated subglacial conditions ($<$ 63 micron size fraction, sediment concentrations 0.01 to 5 g/l, 5$\deg$C) to investigate potential kinetic isotopic effects and aid in interpretation of the field data ($\delta$${^13}$C-DIC values ranging from -2.4 to -15.7 $\permil$). The laboratory experiments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis (closed system conditions) and early stages of carbonate dissolution driven by atmospheric CO$_{2}$ (open system conditions). Preferential dissolution of Ca$^{12}$CO$_{3}$, results in $\delta$${^13}$C-DIC values that are significantly isotopically lighter than the bulk carbonate. This kinetic isotopic effect (KIE) is more pronounced at higher sediment concentrations and can be up to -17.4 $\permil$ for glacial sediments under closed system conditions and sediment concentrations of 5g/l. The KIE is also significant during the first 6 hrs of carbonate dissolution driven by atmospheric CO$_{2}$. Incorporating KIE into the geochemical weathering models and isotopic mass balance calculations allows identification of significant ($>$ 10 %) microbial CO$_{2}$ contributions to the DIC budget in glacial meltwaters. However, where weathering processes producing KIE dominate the DIC budget, small microbial CO$_{2}$ contributions may be masked. Examples of subglacial environments from JEG and HGA exhibiting these contrasting weathering processes and differing levels of microbial CO$_{2}$ input will be presented.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1827 Glaciology (1863)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting