Ca isotope fractionation in a permafrost-dominated boreal ecosystem (Kulingdakan watershed, Central Siberia)

Bagard, Marie-Laure; Schmitt, Anne-Desiree; Chabaux, Francois; Pokrovsky, Oleg S.; Viers, Jerome; Stille, Peter; Labolle, Francois and Prokushkin, Anatoly S. (2013). Ca isotope fractionation in a permafrost-dominated boreal ecosystem (Kulingdakan watershed, Central Siberia). In: Mineralogical Magazine, Mineralogical Society, 77(5) p. 641.




Ca isotope compositions were measured in different compartments (stream water, soil solutions, rocks, soils and soil leachates and vegetation) of a small permafrost-dominated forested watershed in the Central Siberian Plateau. Our results show that only the processes related to vegetation activity significantly fractionate Ca isotopes within the watershed. These fractionations occur during Ca uptake by roots and along the transpiration stream within the larch trees. Biomass degradation then significantly influences the Ca isotopic compositions of soil solutions and soil leachates via the release of light Ca. Furthermore, organic and organo-mineral colloids originated from organic matter degradation are thought to affect the Ca isotopic composition of soil solutions by preferential scavenging of 40Ca. This imprint of organic matter degradation on the δ44/40Ca of soil solutions is much more significant for the warmer south-facing slope of the watershed than for the shallow and cold soil active layer of the north-facing slope, indicating that the available stock of biomass and the decomposition rates are critical parameters that regulate the impact of vegetation on the soil-water system. Moreover, the obtained δ44/40Ca patterns contrast with those described for permafrost-free environments with a much lower δ44/40Ca fractionation factor between soils and plants, suggesting particular processes related either to the presence of permafrost or to the specific features of organic matter degradation in permafrost environments. Finally, biologically induced Ca fractionation observed at the soil profile scale is not visible in stream and river waters, whose isotopic variability in the course of the year is likely controlled by the lithological heterogeneity of the sources. As such, we suggest a negligible influence of biologically related fractionation on the long-term Ca isotopic signatures of riverine fluxes carried to the ocean.

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