The Zn abundance and isotopic composition of diatom frustules, a proxy for Zn availability in ocean surface seawater

We have developed cleaning methods for extracting diatom(opal) from bulk marine sediment samples, for measurement of both zinc (Zn) abundance and isotope composition. This cleaning technique was then applied to a set of Holocene core-top samples from the Southern Ocean. The measured delta(66)Zn ( reported relative to the JMC(Lyon) standard) and Zn/Si ratios from the Southern Ocean diatom(opal) samples range from 0.7 to 1.5 parts per thousand, and from 14 to 0.9 mu mol/mol, respectively. The Zn abundance and isotope composition data show a clear correlation with opal burial rates and other oceanographic parameters. In common with previous work, we interpret the systematic changes in the Zn/Si ratio to be linked to the variability in the concentrations of bioavailable Zn in the ambient surface seawater where the diatom opal is formed. This variability is likely to be primarily controlled by the degree to which Zn is taken up into phytoplankton biomass. The observed systematic pattern in the delta(66)Zn compositions of the diatomopal core-top samples is, similarly, likely to reflect changes in the delta(66)Zn composition of the ambient Zn in the surface waters above the core-top sites, which is progressively driven towards isotopically heavier values by preferential incorporation of the lighter isotopes into phytoplankton organic material. Thus, the systematic relationship between Zn isotopes and abundance observed in the core-top diatom(opal) samples suggests a potential tool for investigating the biogeochemical cycling of Zn in the past surface ocean for down-core diatom(opal) material. In this respect, it may be possible to test hypotheses that attribute variations in atmospheric CO(2) on glacial-interglacial timescales to the degree to which trace metals limited primary productivity in HNLC zones. (C) 2010 Elsevier B.V. All rights reserved.