Juillot Farid, Noel V., Louvat P., Gelabert A., Jouvin D., Gottlicher J., Belin S., Muller B., Morin G., Voegelin A. (2023). Can Zn isotopes in sediments record past eutrophication of freshwater lakes ? A pilot study at Lake Baldegg (Switzerland). Chemical Geology, 620, p. 121321 [12 p.]. ISSN 0009-2541.
Titre du document
Can Zn isotopes in sediments record past eutrophication of freshwater lakes ? A pilot study at Lake Baldegg (Switzerland)
Juillot Farid, Noel V., Louvat P., Gelabert A., Jouvin D., Gottlicher J., Belin S., Muller B., Morin G., Voegelin A.
Source
Chemical Geology, 2023,
620, p. 121321 [12 p.] ISSN 0009-2541
In this study, the speciation and isotopic composition of Zn were traced across the sediments of a freshwater lake that experienced one hundred years of strong eutrophication, in order to assess the potential of sedimentary Zn isotopes to record such an environmental disturbance. The results indicate that the sedimentary Zn isotope signal varied with the change from pre-eutrophic to eutrophic conditions in the investigated lake. The average delta 66ZnJMC value of the dominantly allochthonous lithogenic sediments deposited during the pre-eutrophic period was +0.27%o +/- 0.05%o (i.e. similar to the delta 66ZnJMC value of +0.28%o +/- 0.05%o proposed for Bulk Silicate Earth), while enhanced autochthonous biochemical sedimentation during the eutrophic period resulted in significantly lower delta 66ZnJMC values down to +0.04%o +/- 0.06%o. Synchrotron-based X-ray absorption spectroscopy data revealed a concomitant change in Zn speciation from a dominant fraction of Zn in clay minerals during the pre-eutrophic period to a major fraction of Zn in ZnS during the eutrophic period. A linear regression relating the sedimen-tary Zn isotope signal to the fraction of Zn in ZnS indicated delta 66ZnJMC values of +0.27%o +/- 0.06%o and 0.00%o +/- 0.08%o for Zn in clay minerals and in ZnS, respectively. The enrichment of light Zn in ZnS in the eutrophic sediments is tentatively attributed to enhanced biological uptake of light Zn in the water column, which resulted in an enhanced flux of organic-bound Zn towards the sediments and further transformation of organic Zn into ZnS upon biomass mineralization during early diagenesis. This hypothesis is in agreement with the fractionation towards lighter Zn reported for both biological uptake of Zn and ZnS precipitation. The results of this study emphasize the potential of sedimentary Zn isotopes to register past eutrophic periods in freshwater lakes, and thus to serve as a probe of paleo-environmental conditions and/or past land use at the catchment scale.
