HR: 0800h
AN: V51A-0510    [Abstracts]
TI: The Separation and Isotopic Analysis Seawater Cu and Zn
AU: * Bermin, J
EM: j.bermin@gl.rhul.ac.uk
AF: Department of Geology, Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX United Kingdom
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 Geology, Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX 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 key micronutrients and their concentration profiles in the oceans often show nutrient-like patterns, with strong surface depletions and deep enrichments$^{1}$. In addition, their biological usage has been shown to induce isotopic fractionations$^{2}$ so that the precise and accurate analysis of their isotope systems in seawater has potential applications in tracing metal micronutrient usage in the past ocean. The analytical challenges involved in realising this goal are, however, considerable, given the low concentrations of transition metals in seawater and the requirement to extract small amounts from large samples at low blank and with no artificial isotopic fractionation. Here we present a method for the separation an analysis of Cu and Zn isotopes that is applicable to 0.1-5 L samples of seawater. Trace metals were concentrated from seawater using a Chelex-100 ion-exchange column$^{3}$ and further purified and separated from each other using a small anion column$^{4,5}$. All isotopic analyses were performed on a ThermoFinnigan Neptune instrument at the University of Bristol. The main requirements for precise and accurate isotopic analyses are a low contribution from analytical blank and the robust correction for analytical mass discrimination. Our blanks allow the analysis of seawater samples of 50-250 mL for Cu, samples of about 100 mL for Zn in the deep oceans and for Zn-depleted open ocean surface water samples of around 5L. The correction for mass discrimination is most readily considered as two components - that occurring during the chemical separation procedure in response to non-100% yields and that occurring in the mass spectrometer. Correction of all mass discrimination throughout the procedure is most robustly done for Zn and Fe using a double-spike that is added prior to any chemical treatment. This approach has been tested using standard-doped seawater samples that had previously been stripped of their metal contents using the Chelex column. For Zn, the approach is highly successful and yields $\delta$$^{66}$Zn = -0.02$\pm$0.08 (n = 6) relative to the standard dopant. For Fe the precise analysis of low concentration samples is precluded by the propagation of large errors on the $^{57}$Fe/$^{54}$Fe through the double-spike calculation. Mass discrimination correction is simplified if the chemistry yield is 100% since then the mass spectrometric component can be corrected for using established techniques$^{4,5}$. The yield for Fe from the chemistry is not 100% but demonstrably is for Cu and Zn. Correction for instrumental mass discrimination using these conventional approaches is compromised by non-spectral matrix effects which cause changes in the behaviour of pure standards that have been (noted previously$^{5}$). This is overcome by comparing samples to a standard that has a matrix similar to seawater - e.g. a trace metal-stripped, standard-doped seawater sample. The two approaches yield identical results for the isotope composition of Zn in an English Channel sample relative to the Lyons JMC standard: $\delta$$^{66}$Zn = 0.38$\pm$0.06 (double spike, n = 12) and 0.46$\pm$0.08 per mil (standard-bracketing n = 6). $^{1}$ K.W. Bruland, 1980, Earth Planet. Sci. Lett. 47, 176. $^{2}$ B.L. Beard et al., 2003, Chem. Geol. 195, 87. $^{3}$ H.M. Kingston et al., 1978, Anal. Chem. 50, 2064. $^{4}$ C.N. Marechal et al., 1999, Chem. Geol. 156, 251. $^{5}$ C. Archer and D. Vance, 2004, J. Anal. Atom. Spectr. 19, 656.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting