Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO2 consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has delta S-34 Values of 16 to 20 parts per thousand(CDT), indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS2. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from similar to 1.1 to similar to 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite. Estimated long-term average CO2 fluxes from silicate weathering range from similar to 0.012 x 10(6) to 0.039 x 10(6) mol/km(2)/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the and climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water-rock contact times dominate CO2 consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO2 consumption rates in particular, based on major ion abundances, should be treated with skepticism. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.