HR: 1340h
AN: OS33A-0984    [Abstracts]
TI: Continent – ocean fluxes and isotopic compositions of dissolved silicon contributed by groundwater
AU: * Georg, R B
EM: Bastian.Georg@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Basu, A R
EM: abasu@earth.rochester.edu
AF: University of Rochester, Earth and Environmental Sciences, 227 Hutchison Hall, Rochester, NY 14627, United States
AU: West, A J
EM: Joshua.West@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Halliday, A N
EM: Alex.Halliday@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: The global cycle of silicon (Si) is closely linked to that of carbon (C). The breakdown of silicate rocks provides essential nutrients, such as Si, to the marine system and is thought to regulate global climate on geological time scales. It is thought that nearly 85% of the oceanic Si budget is directly maintained via riverine pathways, with the remainder being derived from atmospheric deposition and low-temperature alteration of sea-floor basalt (ELDERFIELD and SCHULTZ, 1996; TRÉGUER et al., 1995). Although groundwater-derived fluxes add to the global Si balance their magnitude and effect on the Si isotopic composition of the oceans have not been elucidated. We have investigated the Si fluxes and associated isotopic compositions of subsurface flow within the Bengal Basin. We find that the groundwater Si fluxes are high and comparable to riverine Si fluxes. For instance, the groundwater derived Si fluxes, directly provided by submarine groundwater discharge into the Bay of Bengal, reach ~70% of the combined Ganges-Brahmaputra Si flux. Moreover the Si isotope composition evolves to lighter and even negative δ³⁰Si values with age and depth. The deep groundwater carries δ³⁰Si as negative as -0.1‰, clearly distinct from the positive δ³⁰Si values typically found for river water. The Si isotope composition of the Brahmaputra appears to be a mixture of isotopically heavier Si from the upper tributaries and lighter Si derived from shallow groundwater. Our data indicate that groundwater derived Si fluxes are significant for the global Si cycle. Silicon isotopes might represent a new tool to trace the relative proportions of groundwater versus riverine Si fluxes. We propose that the continental Si flux into the ocean is enhanced by groundwater, and that the net isotopic composition of continental Si delivered to the oceans will reflect the relative proportion of surface waters versus groundwater. References: Elderfield H. and Schultz A. (1996) An. Rev. Earth Planet. Sci. 24, 191-224. Tréguer P., Nelson D. M., et al. (1995) Science 268, 375-379.
DE: 1030 Geochemical cycles (0330)
DE: 4808 Chemical tracers
DE: 4825 Geochemistry
DE: 4924 Geochemical tracers
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting