Zn isotope ratios vary on small scales throughout the terrestrial biosphere. Zn is released into soils during mineral weathering, and isotopes of Zn fractionate upon interaction with mineral and organic components in the soil. In 5 soil profiles collected from Iceland (all derived from the same parent basalt), soil δZn values varied from +0.10 to +0.35‰, and the organic-rich layers were Zn-depleted relative to the mineral-rich layers, likely due to contribution by isotopically light organic matter and Zn loss by leaching.
Isotopic discrimination of Zn varies in different components of higher plants, likely due to the various processes involved in Zn uptake, binding, transport, diffusion, speciation and compartmentalization. For example, Weiss et al. observed heavier δZn values in the roots of several plants (rice, lettuce and tomato) relative to the bulk solution in which the plants were grown, and the shoots of those plants were Zn-depleted relative to both their roots and bulk solution. Furthermore, Zn isotopes partition differently between different Zn-ligand complexes, so the form of Zn incorporated by organisms in the terrestrial biosphere plays a role in driving Zn isotope compositions of the organisms. In particular, based on ab initio calculations, Zn-phosphate complexes are expected to be isotopically heavier than Zn-citrates, Zn-malates and Zn-histidine complexes by 0.6 to 1‰.
The discharge- and [Zn]-weighted average δZn value of rivers throughout the world is +0.33‰. In particular, the average δZn values of the Kalix and Chang Jiang rivers are +0.64 and +0.56‰, respectively. The Amazon, Missouri and Brahmaputra rivers have average δZn values near +0.30‰, and the average δZn value of the Nile river is +0.21‰.