Past and present weathering rates in northern Sweden

Past and present chemical weathering rates in granitic till have been estimated. The Kalix River watershed in northern Sweden was used as a study area in which 17 evenly distributed soil profiles were investigated. The two estimations are based on elemental depletion trends in soil profiles and input/output budgets for the elements in the watershed, respectively. In the calculations of the past weathering rate it was assumed that zircon is resistant, and thus Zr was considered to be immobile during weathering. The long-term average chemical erosion rate since the area was deglaciated 8700 a ago, expressed as the sum of major element oxides (SiO2, Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O), was estimated to be 5.8 g·m−2·a−1. In terms of base cation (Ca2+, Mg2+, Na+, K+) depletion this corresponds to 0.36 keq·ha−2·a−1. All elements analysed have been depleted from the E-horizon, and the most affected elements are P with an average mass loss of 86% (as P2O5), La 81%, Co 78%, Cu 77% and Ni 76%. The present-day weathering rate was calculated as the difference between outputs and inputs in the Kalix River watershed. The input was considered as the contribution from precipitation, while the output was calculated as the sum of (1) the river-transported dissolved fraction, (2) the river-transported suspended non-detrital fraction (chemically precipitated Fe- and Mn-oxy-hydroxides and matter sorbed on these particles), and (3) the biotic nutrient net uptake. River-transported outputs were measured for an annual cycle starting in September 1991 and ending in August 1992. The present-day chemical erosion rate of the till was estimated to be 6.3 g·m−2·a−1 (sum of major element oxides), or a base cation flux of 1.42 keq·ha−2·a−1. Part of this present-day rate is related to carbonate weathering in the Caledonian mountain range which makes it difficult to compare the present weathering rate with the historical weathering rate. After correction for carbonate weathering the resulting present-day weathering rate of granitic till in terms of base cation flux was estimated to be 0.65–0.75 keq·ha−2·a−1. This result indicates that the present cation flux has increased by a factor of 1.8–2.1 compared to the long-term average. However, given the uncertainties introduced by the carbonates in the Caledonian mountain range it is not possible to prove any significant difference between the mean post-glacial and the present-day weathering rate with the methods used in this study.