δ26Mg, δ44Ca and 87Sr/86Sr isotope differences among bedrock minerals constrain runoff generation in headwater catchments: An acidified granitic site in Central Europe as an example

Magnesium, calcium and strontium isotope systematics were studied in a Central European headwater catchment underlain by granite. The Uhlirska catchment (Czech Republic) is recovering from acidification following 40 years of acid rain. A combination of isotope and non-isotope data was used to constrain the origin of base cations in runoff. Whole-rock delta 26Mg, delta 44Ca values and 87Sr/86Sr ratios were complemented by isotope analysis of apatite, biotite, plagioclase, orthoclase and titanite. Isotope composition of Mg, Ca and Sr in precipitation, throughfall and runoff was monitored for 12 months. Soil, soil solutions and Norway spruce tissues were also analyzed. Non -isotope data included a 24-year time-series of input/output Mg and Ca fluxes. Biotite was the likely main source of geogenic Mg. Apatite and plagioclase were the likely main sources of geogenic Ca, and plagioclase was the likely main source of geogenic Sr. Magnesium in biotite was isotopically too heavy to dominate runoff. Calcium in apatite and plagioclase was isotopically indistinguishable from whole-rock Ca and could play a major role in runoff generation. Plagioclase had a significantly lower 87Sr/86Sr ratio than bulk bedrock, close to the low 87Sr/86Sr ratio of runoff. Plagioclase weathering was consistent with a sizeable geogenic Sr contribution to runoff but if only bulk-rock 87Sr/86Sr was considered predominance of geogenic Sr in runoff would be unlikely. Higher Mg, Ca and Sr runoff fluxes, compared to deposition, suggested geogenic control of runoff. A decrease in runoff fluxes of Mg and Ca coincided with a decrease in deposition fluxes but there may not be a causal relationship pointing to a large atmospheric contribution of base cations to runoff. Decreasing fluxes of base cations via runoff were mostly related to decreasing sulfate export accompanying retreat of acidification. Mg/Ca/Sr isotope sys-tematics at Uhlirska are discussed in light of analogous data from four other headwater catchments.