Zinc isotopes as a tool to investigate zinc biogeochemical cycling in the SW Pacific Ocean

Abstract

Marine phytoplankton account for more than 40% of global primary production and hence play an important role in moderating global climate through ocean-atmosphere CO2 exchange. Zinc (Zn) is an essential micronutrient and plays an important role in the carbon acquisition system within marine phytoplankton, thus, it has the potential to influence marine primary production. In seawater, Zn bioavailability is dependent on its concentration and its chemical speciation. The free Zn2+ ion concentration of the surface ocean is usually in the low picomolar range with the potential to be growth limiting for some phytoplankton species. The aim of this research is to better understand Zn cycling in the ocean and the relationship between primary production and Zn bioavailability by exploiting variations in the Zn isotope composition of phytoplankton and seawater samples. To realise this aim, a procedure was developed to measure the Zn isotope composition of marine samples. This procedure utilises the double spike (DS) technique in conjunction with the pre-concentration of Zn from seawater to determine processes that influence the Zn biogeochemical cycling in the ocean. Seawater samples, collected as part of a 2010 GEOTRACES process study, contrasting oligotrophic waters of north Tasman Sea and the mesotrophic waters of south Tasman Sea, were analysed using this technique. In this study, variability in δ66Zn of dissolved Zn is observed in the upper ocean (0-200 m) for the mesotrophic waters and is attributed to biological activity of eukaryotic phytoplankton. At stations where eukaryotic phytoplankton dominated, heavier δ66Zn values coincided with the chlorophyll maxima suggesting preferential uptake of lighter Zn isotopes by phytoplankton. To complement the field work, a Tasman Sea isolate of the coccolithophore Emiliania huxleyi was cultured across a range of free Zn2+ ion concentrations to determine the extent of Zn isotope fractionation during Zn uptake. The laboratory results support the field observation that the resident phytoplankton community controls δ66Zn composition of the upper water column in the south Tasman Sea. The intermediate and deep waters of the south Tasman Sea have Southern Ocean origin. The Southern Ocean plays a significant role in the global carbon cycle influencing both ocean circulation and biogeochemistry. Much of the surface waters of the Southern Ocean have low iron (Fe) bioavailability. I investigated the role Fe limitation plays in Zn accumulation and Zn isotope fractionation by the Southern Ocean haptophyte Phaeocystis antarctica, a major component of the Southern Ocean phytoplankton assemblage. Under Fe-limiting condition an increased cellular quota for Zn (expressed as Zn:P) and heavier δ66Zn values within the cells is observed; Whereas, cells grown under Fe-replete conditions have a lower Zn quota and a lighter δ66Zn composition. Thus, Fe bioavailability could regulate the dissolved Zn isotope composition of the Southern Ocean, which in turn would be reflected in other parts of the world ocean. This study highlights the importance of carefully analysing phytoplankton community structure and the trace metal composition while interpreting δ66Zn composition of the biologically active upper water column of the ocean.