VOLCANIC AND ANTHROPOGENIC CONTRIBUTIONS TO GLOBAL WEATHERING BUDGETS

We evaluate whether the global weathering budget is near steady state for the pre-anthropogenic modern environment by assessing the magnitud e of acidity-generating volcanic exhalations. The weathering rate indu ced by volcanic acid fluxes, of which the CO2 flux is the most importa nt, can be expressed as an average release rate of dissolved silica, b ased on a model feldspar-weathering scheme, and the ratio of carbonate -to-silicate rock weathering. The theoretically predicted flux of sili ca from chemical weathering is slightly smaller than the estimated glo bal riverine silica flux. After adjustment for carbonate weathering, t he riverine dissolved bicarbonate flux is larger than the volcanic car bon degassing rate by a factor of about three. There are substantial u ncertainties associated with the calculated and observed flux values, but the modern system may either not be in steady state, or additional , ''unknown'' carbon sources may exist. The closure errors in the pred icted budgets and observed riverine fluxes suggest that continental we athering rates might have had an impact on atmospheric CO2 levels at a time scale of 10(3)-10(4) years, and that enhanced weathering rates d uring glacial periods might have been a factor in the reduced glacial atmospheric CO2 levels. Recent anthropogenic emissions of carbon and s ulfur have a much larger acid-generating capacity than the natural flu xes. Estimated potential weathering budgets to neutralize these fluxes are far in excess of observed values. A theoretical scenario for a re turn to steady state at the current anthropogenic acidity emissions (d isregarding the temporary buffering action of the ocean reservoir) req uires either significantly lower pH values in continental surface wate rs as a result of storage of strong acids, and/or higher temperatures as a result of enhanced atmospheric CO2 levels in order to create weat hering rates that can neutralize the total flux of anthropogenic and n atural background acidity. (C) 1998 Elsevier Science B.V. All rights r eserved.