Zinc isotope composition of settling particles as a proxy for biogeochemical processes in lakes: Insights from the eutrophic Lake Greifen, Switzerland

The isotope composition of zinc (Zn) was determined in settling particles collected monthly over a year in the hypolimnion of the eutrophic Lake Greifen (Switzerland) in order to explore the potential of this novel isotope system as a proxy for biogeochemical processes in aqueous environments. Significant variation in delta(66)Zn(JMC3-0749L) of 0.9 parts per thousand (where delta(66)Zn(JMC3-0749L) = [{(66)Zn : (64)Zn}(sample)/{(66)Zn : (64)Zn}(JMC3-0749L) - 1] x 1000) was found throughout the seasonal cycle of productivity. The most negative delta(66)Zn(JMC3-0749L) values (down to -0.66 parts per thousand +/- 0.08 parts per thousand) were observed during the productive summer period from June to September, when the Zn in the settling particles was predominantly associated with organic material, and when Zn concentrations in the epilimnion were lowest. This observation is likely explained with preferential uptake of the light Zn isotopes during algal growth. During the other months, the Zn in the sediment traps showed higher delta(66)Zn values, reflecting increasing contributions of anthropogenic or lithogenic components. The average isotopic signature of the settling particles during the autumn-winter-spring period (similar to-0.2 parts per thousand for delta(66)Zn(JMC3-0749L)) was somewhat lower than typical for anthropogenic and natural sources, and this possibly reflects the effect of the preferential adsorption of lighter dissolved Zn isotopes onto the settling particles. We suggest that Rayleigh fractionation plays a minor role in controlling the isotopic composition of the settling particles in the lake. Our work demonstrates the presence of an obvious seasonal cycle in delta(66)Zn, and this suggests that Zn isotopes can be employed to study nutrient utilization and other biogeochemical processes in lakes and other aquatic environments.