HR: 14:40h
AN: V53B-05    [Abstracts]
TI: Controls on the Transition Metal Isotopic Composition of Seawater: Diatom Culture Experiments
AU: * Vance, D
EM: d.vance@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ United Kingdom
AU: Archer, C
EM: c.archer@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ United Kingdom
AU: Kennaway, G
EM: gabrielle@kennaway.net
AF: Department of Botany, The Natural History Museum, Cromwell Road, London, SW7 5BD United Kingdom
AU: Cox, E
EM: E.J.Cox@nhm.ac.uk
AF: Department of Botany, The Natural History Museum, Cromwell Road, London, SW7 5BD United Kingdom
AU: Statham, P J
EM: pjs1@soc.soton.ac.uk
AF: School of Ocean and Earth Sciences, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AB: Many transition metals are essential micronutrients for marine phytoplankton. As a result the expectation is that biological processes play an important, perhaps a dominant, role in their marine isotope geochemistry. These observations raise the prospect of using isotope records to trace transition metal micronutrient usage in the past oceans, an issue that is of importance to the efficiency of the biological pump and atmospheric carbon dioxide. As such, the characterisation of trace metal isotopic fractionations associated with marine primary productivity are an important scientific goal. Here we report fractionations associated with Fe, Cu and Zn sequestration by diatoms, one of the main primary producers in the oceans. Axenic unialgal cultures of {\it Thalassiosira weissflogii} and {\it Thalassiosira pseudonana} were established in artificial seawater + F/2 medium at 18$\deg$C on a 16:8 light:dark cycle. The cultures were filtered to separate diatom material from residual media and analysed for Zn, Cu and Fe concentrations and isotope composition using techniques described elsewhere$^{1,2}$. Aliquots of the starting medium were also measured for each batch of cultures. The diatom organic material shows small, but consistent and resolvable, positive fractionations (0.1-0.3 per mil) for Fe, Cu and Zn relative to the starting medium. In the case of all three metals, but particularly for Zn (70-95% depending on experiment size), the diatoms had sequestered a large proportion of the available metal, suggesting that the fractionation factor for metal usage by the diatoms is much greater than 1.0001 to 1.0003. Time-series experiments are under way to determine the exact magnitude of the fractionation factor. The mass-balance is supported by the fact that the residual medium is around -0.4 per mil for Zn. The fact that diatoms incorporate trace metals that are isotopically heavier than the nutrient pool is a surprising result, the expectation having been that, as with carbon, the biological usage of trace metals would result in kinetic fractionations$^{3}$. The positive fractionations necessitate an equilibrium process and, perhaps, active extra-cellular sequestration of trace metals. The second broader implication is that given the proposed role of diatoms in controlling the extreme depletion of Zn in open ocean surface waters, particularly in the Pacific where surface waters are have up to a factor of 250 less Zn than deep waters$^{4}$, the depletion of the light isotope in surface waters and its enrichment in deep waters are predicted to be extreme. Zn, and other trace metal, isotopes may have an important role in recording this process in the past oceans. $^{1}$ C. Archer and D. Vance, 2004, J. Anal. Atom. Spectr. 19, 656-665. $^{2}$ J. Bermin, et al., 2004, this volume. $^{3}$ Pichat, S et al., 2003, Earth Planet. Sci. Lett. 210, 167-178. $^{4}$ Lohan, M.C. et al., 2002, Deep-Sea Res. II 49, 5793-5808.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 0400 Biogeosciences
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting