HR: 17:30h
AN: OS24B-03    [Abstracts]
TI: The Silicon Isotope Composition of Silicic Acid in the Equatorial Pacific
AU: * Brzezinski, M A
EM: brzezins@lifesci.ucsb.edu
AF: Marine Science Institute, University of California, Santa Barbara, CA 93106, United States
AU: Beucher, C P
EM: Beucher@lifesci.ucsb.edu
AF: Marine Science Institute, University of California, Santa Barbara, CA 93106, United States
AB: Natural variations in silicon isotopes within silicic acid were investigated between 4° N and 3° S latitude along 110° W longitude in the equatorial Pacific. Variations in silicon isotopes map onto discrete water masses between 0 and 300 m. Silicic acid in equatorial surface waters is isotopically light compared to the subtropical waters to the north and south. The Equatorial Undercurrent (EUC) is revealed as a local maximum in δ ³⁰Si of +1.8 to +2.4 ‰ compared to values of +1.6 to +1.8 ‰ in waters with similar silicic acid concentrations to the north and to the south. The anomalously positive isotopic composition of silicic acid within the undercurrent may reflect the biological consumption of silicic acid in the Southern Ocean and North Pacific water masses that supply silicic acid to the EUC. Estimates of the fractionation factor ε calculated using an open system isotope model range between -0.6 and -1.0 ‰ with most of the variability in ε being due to uncertainties in the characteristics of source waters supplying silicic acid to surface waters. Deep waters beneath the equator show relatively higher δ ³⁰Si values, +1.4 ‰, compared to previous measurements from the north Pacific of ca. +1.0 ‰. Deep waters flow from the north to the south at 110°W suggesting that the deep waters beneath the equator at 110°W may be influenced by the North Pacific silicic acid plume that lies to the north of our study site. Samples of silicic acid from the core of the Si plume reveal δ ³⁰Si values all >+1.4 ‰ consistent with this hypothesis.
DE: 4231 Equatorial oceanography
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4808 Chemical tracers
DE: 4845 Nutrients and nutrient cycling (0470, 1050)
DE: 4870 Stable isotopes (0454, 1041)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting