HR: 15:10h
AN: PP33D-07    [Abstracts]
TI: A Germanium Isotope Record From Diatom Opal
AU: * Mantoura, S
EM: sman02@esc.cam.ac.uk
AF: Department of Earth Sciences, Univeristy of Cambridge, Downing Street, Cambridge, CB23EQ United Kingdom
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: Department of Earth Sciences, Univeristy of Cambridge, Downing Street, Cambridge, CB23EQ United Kingdom
AU: De La Rocha, C L
EM: crocha@awi-bremerhaven.de
AF: Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, Bremerhaven, 27568 Germany
AU: Elderfield, H
EM: he101@esc.cam.ac.uk
AF: Department of Earth Sciences, Univeristy of Cambridge, Downing Street, Cambridge, CB23EQ United Kingdom
AU: Shemesh, A
EM: Aldo.Shemesh@weizmann.ac.il
AF: Department of Environmental Sciences and Energy Research (ESER), The Weizmann Institute of Science, Rehovot, 76100 Israel
AB: Here we present the first known record of the germanium isotopic composition of sedimentary diatom opal and the results of diatom culturing experiments. Analyses were performed using a hydride generation system coupled to a MC-ICP-MS and variations in the ⁷⁴Ge/⁷⁰Ge ratio are reported as δ⁷⁴Ge. Diatom cultures grown in the laboratory showed no notable fractionation; For all experiments, the δ⁷⁴Ge of the cultured opal fell within analytical error of the culture media. This lack of fractionation suggests that the δ⁷⁴Ge of opal may be used as a paleorecorder of seawater δ⁷⁴Ge. We tested procedures for the cleaning and dissolution of sedimentary samples and the purification of Ge. Comparison of the δ⁷⁴Ge and trace metal content of the opal and fine fractions of sediments from 9 locations in the Southern Ocean indicate that the < 0.2 μm fraction has a lighter isotopic composition than pure opal and that care must be taken to prevent this ultra fine fraction interfering with opal based measurements. Fe, Al, and Ti levels in measured samples may be used to indicate contamination. There is an insignificant difference between the average glacial and average interglacial δ⁷⁴Ge of opal from a down core record spanning the penultimate deglaciation (MIS 6-5e) in the Southern Ocean. However, there appears to be a small (0.6‰) but notable perturbation at the glacial termination. The average δ⁷⁴Ge of sedimentary opal indicates a seawater δ⁷⁴Ge of +4.0 ± 0.4 ‰. This value is almost 3 ‰ heavier than a bulk silicate earth average of +1.1 ± 0.3‰ [1], requiring the existence of a further sink for isotopically light Ge. The lack of variation in δ⁷⁴Ge opal in this record stands in contrast to the systematic changes in Ge/Si opal. A one box model can be used to compare competing explanations for changes in Ge/Si opal by simulating their effect on δ⁷⁴Ge, although this work is limited by the large uncertainties which exist in the the Ge isotope budget. [1] Rouxel,O. Galy,A. and Elderfield,H. Ge/Si and Ge isotope systematic in marine sediments, GCA 68, S1, A365.
DE: 4825 Geochemistry
DE: 4870 Stable isotopes (0454, 1041)
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005