Hydrochemical and Isotopic Constraints on the Temporal and Spatial Variability of Chemical Weathering and CO2 Fluxes: An Example From the Australian Victorian Alps

Water from a network of 11 pristine rivers draining the Australian Victorian Alps was collected at different locations during (i) high discharge (June 2006) and (ii) low discharge (February 2007) conditions and was analyzed for dissolved major ions, δ2H and δ18O, and δ34S of dissolved sulphate. River water chemistry implies that solutes are largely derived from precipitation and chemical weathering of silicate lithologies. Cl/Br ratios as low as 30 molar suggest that rivers have not dissolved halite, however, higher salinity (≥100 mmol/L) winter samples have intermediate Cl/Br ratios (600 to 2000 molar) that are attributed to minor halite dissolution at the onset of the rainy season. Subsequent mixing of river water homogenizes ratios and evaporation is the dominant process that increases downstream salinities. Oxygen and Hydrogen isotopes also indicate that mixing and evaporation have occurred. Despite the lack of carbonate outcrops in the study area and uniform negative calcite saturation indices, the dissolution of hydrothermal calcite may account for up to 67% of the total dissolved cations, generating up to 92% of all dissolved Ca and Mg. The sulphur isotope data (16 to 20°CDT) indicates that the dissolved SO4 is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS. This militates against sulphuric acid weathering in Victorian rivers. Si/(Na* + K*) ratios suggest that silicate weathering is dominated by the transformation of plagioclase (An40) to smectite and, to a lesser extend, the production of kaolinite. In total, chemical weathering consumes 17.6 x 106 (summer) to 71.59 x 106(winter) mol/km2/yr CO2, with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This range is at the upper end of the global scale and shows that the predominance of fresh silicate lithologies exerts the main control on higher CO2 fluxes; temperature and runoff, in turn, are crucial variables for the inter- seasonal variability in this region. Data on discharge and major ion chemistry, measured in regular intervals between 1977 and 1990, support this; however, the timing of absolute maxima of Si/(Na* + K*) and CO2 flux peaks do not coincide. We suggest that the combination of dissolution of diatoms that precipitated under low flow- and high temperature conditions in the tributaries and Na-adsorption by suspended clay particles, that were probably redistributed locally after bushfires and/or duststorms during drought periods in the early 1980's, as mechanisms to spontaneously elevate Si/(Na* + K*) ratios and, when coupled with irregular discharge fluctuations, explain deviations from seasonal CO2 fluxes.