The Influence of Physical Breakdown and Solute Transport on Chemical Properties of Permafrost Soils: Insights From Radiogenic (Sr and Nd) and Elemental Composition.

This study in a continuous permafrost region investigated the influence of cold temperatures on soil formation, weathering, and solute transport by determination of radiogenic Sr and Nd isotopes in solid, labile, and dissolved reservoirs in the soil-vegetation system. The site is located in Caledonian belt of northeast Greenland with soils developed on glacial and colluvial deposits of metamorphic rocks of the Paleoproterozoic basement complex. Soils are well-drained Arctic Browns (Typic Haploturbels) characterized by an overall higher content of fines in upper soil horizons and accumulation of clay minerals in the B-horizon. The Sr and Nd isotope ratios and element composition of bulk soil and grain size separates indicate enrichment of biotite, amphibole, and the accessory minerals apatite, garnet, and zircon in fine fraction of upper soil horizons. The higher abundance of these minerals in dark striae of gneiss suggests a higher susceptibility of mafic layers to physical break down that is enhanced in upper soil layers. Accumulation of biotite and its weathering products in the clay fraction of B-horizons is documented in high Sr isotope ratios. Lower Nd isotope ratios in B than A-horizons suggest that clay accumulation is related to mechanical rather than in situ weathering. The overall change in mineralogy decouples the chemical evolution of upper soil layers from lower soil layers. To determine sources of dissolved materials and solute transport we measured Sr isotope ratios in the exchangeable complex (ammonium-acetate) and soil water collected with suction cups, and calculated the intercept of regressing Sr isotopes ratios versus Al/Sr ratios of bulk upper soil horizons. Exchangeable Sr isotope ratios are uniform for each soil profile, higher than in bulk soil, and match isotope ratios in vegetation. They document that solutes extracted with this method give the composition of the plant available nutrient reservoir and indicates the strong influence of biotite. In contrast, Sr isotope ratios calculated from regression analyses are rather uniform and lower than observed in soil/exchange solution. The agreement with isotope composition of soil water indicates that this approach yields the longer-term Sr isotope signature of solutes that are exported by percolating water. The difference in the isotope composition in these reservoirs point to different sources and limited exchange between both reservoirs. The accumulation of clay enriched in biotite and its weathering products in B-horizons may provide a preferential source for vegetation. A good correlation between organic carbon content and cation exchange capacity indicates that biomass cycling stabilizes the high Sr isotope signature. Lower Sr isotope ratios are observed in Na-rich feldspar, amphibole, and apatite that are abundant in sand and silt fraction of soils. Seeking these minerals as primary sources for Sr isotope ratio in percolating water suggest more rapid release of solutes to water. The difference between Sr isotope ratios of both solute reservoirs diminishes when soils are less well drained providing longer time for isotope equilibration.