Sigurður R. Gislason

Dissolution of diopside and basaltic glass: the effect of carbonate coating

Abstract Far-from-equilibrium dissolution experiments with diopside and basaltic glass in mixed-flow reactors at 70°C and pH 8.2 show that solute concentrations do not reach steady state over the experimental duration of 45-60 days. Chemical modelling indicates that during the dissolution experiments, solutions have become supersaturated with respect to carbonates in the case of diopside, and carbonates, clay minerals and zeolites in the case of the basaltic glass. Decreasing dissolution is therefore interpreted as a result of secondary surface precipitates blocking the reactive surface area. Calcite formation was supported in both experiments by a significant increase in Ca (and Sr) concentrations as pH was abruptly lowered from 8.2 to 7 because this change increased carbonate solubility and caused all potential carbonate precipitates to re-dissolve. The reduction in pH also led to an increase in Si concentration for diopside and a decrease in Si concentration for basaltic glass. This observation is in accordance with previous experiments on the pH-dependent dissolution rates of pyroxenes and basaltic glass.

Dissolution rates of crystalline basalt at pH 4 and 10 and 25–75°C

Abstract Far-from-equilibrium dissolution rates of crystalline basalt were measured in a mixed-flow reactor at pH 4 and 10, and at temperatures from 25 to 75°C. The material used was obtained from a dyke on Stapafell Mountain on Reykjanes peninsula in Iceland because of its similarity with previous experiments on dissolution rates on basaltic glass by Oelkers and Gislason (2001) and Gislason and Oelkers (2003). Comparison of dissolution rates of basaltic glass and of crystalline basalt of similar chemical composition (from Gislason and Oelkers, 2003) indicates lower rates for crystalline material.

12. Chemical weathering, chemical denudation and the CO2 budget for Iceland

The average relative mobility of elements in SW Iceland during chemical weathering and denudation is: S>F>Na>K≫Ca>Si>Mg>P>Sr≫Mn>Al>Ti>Fe. The sequence of mobility of the elements in the youngest rocks is different from that of the oldest rocks and it changes with increased vegetation cover. The secondary minerals that are left behind in soil are predominantly allophane, poorly crystalline ferrihydrite, and some imogolite. The total chemical denudation rate of the rock derived elements (TCDR) increases with increasing runoff, but decreases with increasing age of the rocks. Where the variation in the primary variables, runoff and age of rocks, is small the denudation rate for Ca, Mg and Si increases with increased vegetative cover. The TCDR for the major Icelandic catchments range from 19 to 146 t km −2 yr 1 . The calculated average for Iceland is 37 compared to the 26 t km −2 yr −1 for the continents. The high rate in Iceland is attributed to high runoff and high reactivity of glassy and crystalline basalt. The chemical denudation rate of relatively mobile elements like S,F,Na and Ca increased by a factor of 2 to 5 at constant runoff and vegetative cover but 0% to 100% glass content of the rocks. The chemical denudation rates of 5 m yr old crystalline basalt, including data on cation uptake by growing trees, indicate that the rate of release of Ca and HCO 3 to streams is about 3 times higher in tree-covered areas than in unvegetated areas. The variation in the chemical denudation rate in Iceland is dictated by the composition of the soil solutions and the saturation state of primary and secondary minerals as reflected in the dependence on runoff. More atmospheric CO 2 is fixed during transient chemical weathering in Iceland than is released to the atmosphere from Icelandic volcanoes and geothermal systems. However, on a geological time scale, more CO 2 is released to the atmosphere than will eventually precipitate in the ocean as Ca and Mg carbonates, as the result of Ca and Mg released during chemical weathering. The overall net transient CO 2 budget for Iceland in 1990, including CO 2 fixation by chemical weathering and vegetation, and the anthropogenic release of CO 2 and the release from volcanoes, was ‘positive’. That is there was a net CO 2 release to the atmosphere of the order 0.3–1.3 m t CO 2 yr −1 .

Hekla cold springs (Iceland): groundwater mixing with magmatic gases

We have analysed the chemical and stable isotope compositions of four spring waters situated just northwest of the Hekla volcano, where cold water emerges from the base of the lava flows. The stable isotope ratios of water (H, O), dissolved inorganic carbon (C) and sulphate (S) were used to determine whether magmatic gases are mixing with the groundwater. The waters can be characterised as Na-HCO3 type. The results show that deep-seated gases mix with groundwater, substantially affecting the concentration of solutes and the isotopic composition of dissolved carbon and sulphate.

High reactivity of deep biota under anthropogenic CO2 injection into basalt

Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO2 emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO2 injections based on a microbiological survey of a basaltic CCS site. Acidic CO2-charged groundwater results in a marked decrease (by ~ 2.5–4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage.The impacts of carbon capture and storage (CCS) on subsurface microorganisms are poorly understood. Here, the authors show that deep ecosystems respond quickly to CO2 injections and that the environmental consequences of their metabolic activities need to be properly assessed for sustainable CCS in basalt.