Germanium cycling in the waters across a frontal zone: the Chatham Rise, New Zealand

Abstract

Transect and profile data reveal that Si and Ge concentrations are depleted in the surface waters across a frontal zone to the east of New Zealand. These results are consistent with Si and Ge uptake and regeneration from siliceous organisms. The Ge/Si ratio along three transects is not constant with Si utilisation indicating that there is fractionation between Si and Ge during uptake by phytoplankton. A fractionation factor ( K D) of 0.36 is obtained from the transect data for Si concentrations below about 6 μM, assuming a Rayleigh distillation-like process. Although Si utilisation is slight faster than that of Ge, assuming that the Si(OH) 4 and Ge(OH) 4 are the chemical species utilised, such chemical fractionation is unlikely to contribute to the variations seen in the Ge/Si transect data, rather biological fractionation appears to dominate. Profiles for Ge/Si versus depth reveals a subsurface maximum in the Ge/Si data suggesting that Ge is being recycled faster than Si from phytoplankton. Such Ge/Si fractionation during Si and Ge uptake and regeneration is the most likely explanation for the positive Ge intercept seen for the global Ge versus Si relationship. Biological fractionation of Ge is contrary to the results of Bareille et al. [Geology 26 (1998) 82], who observed little variation in Ge/Si opal values for diatom frustules isolated from sediments along a transect where a strong Si concentration gradient exists.