High K/Rb ratios in stream waters — Exploring plant litter decay, ground water and lithology as potential controlling mechanisms

The mechanisms causing the K/Rb ratio in stream waters to attain considerably higher ratios (c. 1000) than in the major source rocks such as granites and basalts (c. 275 and 250) and in the upper continental crust (c. 260) are explored. Because K is a nutrient and Rb is not, the K/Rb ratio in biological materials is typically higher than in soils and rocks. It is therefore possible, as recently suggested in the scientific literature, that a considerable portion of the K carried by rivers has circulated via biota, and been leached into water bodies during litter decay. In this study we can provide detailed information on the development of the K/Rb ratio during long-term decay of litter from typical plants in the boreal zone. This data show that it is indeed possible for decaying litter to deliver degradation products strongly enriched in K over Rb. However, after revisiting data collected in regional hydrogeochemical studies over Europe and ground-water data collected at two contrasting sites in the boreal region, we suggest that lithology could also be an important controlling factor. Over Phanerozoic sedimentary (and/or limestone-rich) rocks, in particular, the K/Rb ratio is generally high, perhaps mostly due to extensive retention of Rb released by the weathering of source minerals. We thus argue, that geochemistry, together with biological and climatological factors, is responsible for the high K/Rb ratios in streams and rivers worldwide.