Biological Fractionation of Ge/Si by Antarctic Diatoms - JGOFS/AESOPS Trap Results

Germanium (Ge) and silicon (Si) behave similarly both geochemically and biologically. As a result, today's oceanic Ge versus Si relationship is linear with a Ge/Si ratio of 0.7 x 10^-6 (atom/atom). This same ratio is also observed in diatomaceous opal from core tops and from earlier interglacial periods. However, ratios in glacial sediments are ~30% lower: (Ge/Si)_opal ~0.5 x 10^-6. Two plausible mechanisms have been proposed for these glacial-interglacial shifts: 1) whole ocean changes in Ge and Si sources and/or sinks (changes in continental weathering sources or reverse weathering sinks inducing differential mass balance variations of Ge/Si); and 2) biological fractionation during diatom shell formation in surface waters coupled with incomplete utilization of Si (Rayleigh distillation and variable preformed Si). A statistically-significant positive Ge-intercept in the oceanic Ge vs. Si relationship can be interpreted as evidence of biological fractionation. On the other hand, the absence of significant fractionation in diatom cultures and the perfect linearity of the seawater Ge vs. Si relationship both argue against significant Ge-discrimination during opal formation. Most diatom production and opal preservation occur in the Southern Ocean. To detect biological fractionation by diatoms, the Ge/Si ratio of intrinsic opal was measured in time-series trap samples from the southwest Pacific sector of the Southern Ocean - the JGOFS/AESOPS moorings across the Polar Front Zone along 170°W. Trap fluxes were collected for ~one year (1997), during which a doublet spring diatom bloom was captured in the 1000 m and 2000 m traps. Preliminary results show fractionation in the (Ge/Si)_opal ratio of the trapped diatom shells. The first bloom coincides with a decrease in the opal ratio from (0.70 +/- 0.03) x 10^-6 to 0.55 x 10^-6 and then a rapid return to the background oceanic ratio. The second bloom several weeks later displayed no fractionation - opal ratios do not differ from seawater. There are at least two explanations for the differences between the two blooms: 1) species' dependent Ge/Si diatom fractionation - perhaps the dominant diatom species changed between the two blooms; or 2) iron-dependent fractionation - perhaps the first bloom initiated under iron-replete conditions while the second bloom occurred in iron-depleted waters. Consequently, glacial-interglacial changes in (Ge/Si)_opal may record changes in the dominant diatom species or changes in surface water iron concentrations, rather than changes in local Si bio-limitation or whole ocean changes in the Ge/Si ratio of seawater. The plot thickens.