Temporal variability in the chemical weathering of Ca2 +-bearing phases in the Loch Vale watershed, Colorado, USA: A mass-balance approach

Abstract

The contributions of Ca2+-bearing minerals to the solute budgets of the subalpine to alpine Loch Vale watershed located in Rocky Mountain National Park, Colorado, USA, are investigated. Previously published mass-balance models of chemical weathering in Loch Vale used the Ca2+-phases oligoclase and calcite. However, hexagonal dissolution voids in detrital stream-sediment grains provide evidence that apatite is also dissolving, which is further supported by an absence of any apatite in stream sediments.The Ca2+ in stream waters attributable to the weathering of oligoclase, calcite, and apatite was studied using solute-based watershed geochemical mass-balance methods. Mineral weathering rates were calculated for eight triennia in the 24-year period from 1984 to 2008. For all mass-balance calculations, oligoclase must be weathered in order to balance the Na+, Ca2+, and HCO3−. Over the 24-year period of study, oligoclase contributes 25% of the Ca2+ in the stream waters. Depending on the proportion of chemical weathering attributable to sulfuric acid weathering, calcite contributes 40–65% of the Ca2+ in the stream waters, and apatite 10–35%. Apatite is the primary source of phosphorous to the Loch Vale ecosystem, with algal activity and lake-bottom sediments likely serving as intrawatershed phosphate sinks.The results of this study indicate that mineral weathering in the Loch Vale watershed is temporally variable. The weathering of calcite and apatite may be discontinuous through time, but ultimately experiencing complete dissolution within the watershed. The temporal variability of mineral weathering likely reflects changes in the mineral assemblages that are exposed to weathering agents as mechanical processes operate in the high elevation watershed. In addition, physical weathering is capable of influencing hydrologic flow paths, which in turn may determine the minerals interacting with weathering solutions. These observations underscore the importance of multi-decadal hydrogeochemical data sets for subalpine to alpine watersheds.