Erwan Martin

Volcanic sulfate aerosol formation in the troposphere

The isotopic composition of volcanic sulfate provides insights into the atmospheric chemical processing of volcanic plumes. First, mass-independent isotopic anomalies quantified by Δ17O and to a lesser extent Δ33S and Δ36S in sulfate depend on the relative importance of different oxidation mechanisms that generate sulfate aerosols. Second, the isotopic composition of sulfate (δ34S and δ18O) could be an indicator of fractionation (distillation/condensation) processes occurring in volcanic plumes. Here we present analyses of O- and S isotopic compositions of volcanic sulfate absorbed on very fresh volcanic ash from nine moderate historical eruptions in the Northern Hemisphere. Most of our volcanic sulfate samples, which are thought to have been generated in the troposphere or in the tropopause region, do not exhibit any significant mass-independent fractionation (MIF) isotopic anomalies, apart from those from an eruption of a Mexican volcano. Coupled to simple chemistry model calculations representative of the background atmosphere, our data set suggests that although H2O2 (a MIF-carrying oxidant) is thought to be by far the most efficient sulfur oxidant in the background atmosphere, it is probably quickly consumed in large dense tropospheric volcanic plumes. We estimate that in the troposphere, at least, more than 90% of volcanic secondary sulfate is not generated by MIF processes. Volcanic S-bearing gases, mostly SO2, appear to be oxidized through channels that do not generate significant isotopically mass-independent sulfate, possibly via OH in the gas phase and/or transition metal ion catalysis in the aqueous phase. It is also likely that some of the sulfates sampled were not entirely produced by atmospheric oxidation processes but came out directly from volcanoes without any MIF anomalies.

A simple and reliable anion exchange resin method for sulfate extraction and purification suitable for multiple O- and S- isotope measurements.

RATIONALE The O- and S-isotope compositions of sulfates can be used as key tracers of the fate and sink of sulfate in both terrestrial and extra-terrestrial environments. However, their application remains limited in those geological systems where sulfate occurs in low concentrations. Here we present a simple and reliable method to extract, purify and concentrate sulfate from natural samples. The method allows us to take into account the separation of nitrate, which is known to be an issue in O-isotope analysis. METHODS The separation and concentration of sulfate from other anions in any aqueous solution are performed within a few hours via anion-exchange resin. The possible O- (δ18 O and Δ17 O) and S- (δ34 S, Δ33 S and Δ36 S) isotope exchanges, fractionations and/or contaminations are for the first time monitored during the whole procedure using initial O- and S-mass-dependent and mass-independent sulfate solutions. RESULTS After elution in HCl, pure sulfate is fully retrieved and precipitated into BaSO4 , which is suitable for O- and S-isotopic measurements using established techniques. The analysis of retrieved barite presents no variation within 2σ uncertainties: ±0.5‰ and ±0.1‰ in O- (δ18 O, Δ17 O) and ±0.2‰, ±0.02‰ and ±0.09‰ in S- (δ34 S, Δ33 S and Δ36 S) isotope ratios, respectively. CONCLUSIONS This study shows that the resin method for sulfate extraction and purification, in addition to being cheap, simple and quick, is applicable for the measurements of all O- and S-isotopic ratios in sulfates (including the Δ17 O, Δ33 S and Δ36 S values). Therefore, this method can be easily used for a high range of natural samples in which sulfate occurs in low concentration including aerosols, ice cores, sediments, volcanic deposits, (paleo)soils and rainwater, and thus it can be a key to our understanding of the sulfur cycle on Earth. Copyright

Insight on gem opal formation in volcanic ash deposits from a supereruption: A case study through oxygen and hydrogen isotopic composition of opals from Lake Tecopa, California, U.S.A.

Abstract At Lake Tecopa, in California, white play-of-color opals are found in vesicles of a volcanic ash layer from the Huckleberry Ridge Tuff super-eruption (2.1 Ma). They show characteristic traits of opal-AG by X-ray diffraction and scanning electron microscopy (silica spheres of ~330 nm). These properties are not typical for volcanic opals, and are usually associated with opals formed in a sedimentary environment, such as opal-AG from Australia. The conditions under which opal was formed at Lake Tecopa were determined by oxygen and hydrogen isotopic analyses and give a better understanding of the formation of opal in general. Tecopa opal’s δ18O is ~30‰, which leads to a formation temperature between 5 and 10 °C from water composition similar to the present spring water composition (δ18O = –12‰), or between 15 and 30 °C (the present day spring water temperatures) in water having a δ18O between –9.5 and –5.5‰. As a result, opal experienced 25–50% evaporation at the Tecopa basin. Contrary to long-held views, the formation of opal-AG vs. opal-CT (or opal-C) is not determined by the type of deposits, i.e., respectively sedimentary vs. volcanic, but mostly by the temperature of formation, low (≤45 °C for opal-AG) vs. high (≥160 °C for opal-CT) as suggested in most recent papers. The isotopic composition of water contained in Tecopa opals is assessed and results show that water in opal records different stages of opal formation from groundwater. Opal seems to precipitate from groundwater that is undertaking isotopic distillation during its circulation, most likely due to 15% up to 80–95% evaporation. Hydrogen isotopes are poorly documented in opal and require more systematic work, but this study reveals that, in Tecopa opals, molecular water (H2Om) is isotopically heavier than structural water (OH), a phenomena already well known in amorphous volcanic glass. Overall, opal isotopic composition reflects the composition of the water from which it precipitated and for that reason could be (as established for amorphous silicic glass) a useful tool for paleoenvironments, and paleoclimatic reconstitutions on Earth and on other terrestrial planets.