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

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−2 s−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

Riverine silicon isotope variations in glaciated basaltic terrains: Implications for the Si delivery to the ocean over glacial–interglacial intervals

Marine primary production is dominated by diatoms and these are dependent upon the riverine delivery of silicon (Si) to the ocean. In paleoreconstruction of silicic acid utilisation by diatoms, it is assumed that the isotopic composition of the Si that is delivered from the continent to the oceans remains constant. In this study it is shown that glacier-fed Icelandic rivers differ from those directly draining basaltic catchments in their dissolved Si isotope compositions. Lighter values (δ30Si=+0.17±0.18‰) are associated with the high physical erosion rates in glacial rivers, and heavier values (δ30Si=+0.97±0.31‰) are associated with lower physical erosion rates and enhanced formation of secondary minerals in direct runoff rivers. The Si isotopic compositions correlate with those of Li and provide evidence of a climatic dependence that is likely to have led to glacial–interglacial differences. Based on existing δ30Si measurements from diatoms in a sediment record from the Southern Ocean, the interpretation of changes in Si utilisation between the Last Glacial Maximum (LGM) and the early Holocene is revisited taking into account changing isotopic compositions of the river water delivered to the ocean over glacial–interglacial intervals. During the LGM, Si utilisation values are higher when allowing for changing Si isotope input to the ocean (59±5%), than when a constant Si isotope input is assumed (42–47±5%). This reduces but does not eliminate the difference relative to the Holocene (88±5%). Therefore, changes in Si isotope delivery to the ocean need to be taken into account in the precise reconstruction of ocean Si utilisation and primary productivity over glacial–interglacial timescales

Continental weathering following a Cryogenian glaciation:evidence from calcium and magnesium isotopes

A marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger Cryogenian glaciation have been inferred from boron isotope measurements. Calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time. Data from Neoproterozoic sections in Namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate-dominated one. Combined with the effects of primary dolomite formation in the cap dolostones, this caused the ocean to depart from a state of acidification and return to higher pH after climatic amelioration. Differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern Congo craton shelf point to local influences modifying and amplifying the global signal, which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux

The effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes: Lake Myvatn, Iceland

Lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes, but little is known on their behaviour in groundwaters and hydrothermal springs, and how these sources might influence the weathering signal in surface waters. This study presents lithium isotope compositions (δ7Li) for cold groundwaters (3–7 °C) and hydrothermal springs that were at geothermal temperatures (200–300 °C) but have cooled during transport (17–44 °C). Both represent an important source of water and nutrients for Lake Myvatn, Iceland. We also present a time-series from the Laxa River, which is the single outflow from the lake. The δ7Li values in the input springs to Lake Myvatn are highly variable (5–27‰), and correlate inversely with temperature and total dissolved solids. These co-variations imply that even in such waters, the processes controlling δ7Li variations during weathering still operate: that is, the ratio of primary rock dissolution to secondary mineral formation, where the latter preferentially incorporates 6Li with a temperature-dependent fractionation factor. In high-temperature geothermal waters (> 300 °C) secondary mineral formation is inhibited, and has a low fractionation factor, leading to little δ7Li fractionation. Even in waters that have cooled considerably over several months from their geothermal temperatures, fractionation is still low, and δ7Li values are similar to those reported from waters measured at > 350 °C. In contrast, cooler groundwaters promote relatively high proportions of clay formation, which scavenge dissolved solids (including 6Li). The time series on the Laxa River, the single outflow from Lake Myvatn, shows little δ7Li variation with time over the 12 month sampling period (17–21‰), demonstrating that in contrast to tracers such as Si isotopes, Li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake. Thus, these results clearly illustrate that Li isotopes are ideally suited to constrain silicate weathering processes, because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant, and because Li isotopes are demonstrably unaffected by phytoplankton or plant growth