The dissolution of olivine added to soil: Implications for enhanced weathering

Abstract

Chemical weathering of silicate minerals consumes atmospheric CO2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10−16.4 and 10−15.5 moles(Mg)cm−2s−1. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.