Joel Savarino

Observation of wavelength‐sensitive mass‐independent sulfur isotope effects during SO2 photolysis: Implications for the early atmosphere

Mass-independent isotopic signatures for δ 33 S, δ 34 S, and δ 36 S produced in the photolysis of sulfur dioxide exhibit a strong wavelength dependence. Photolysis experiments with three light sources (ArF excimer laser (193 nm), mercury resonance lamp (184.9 and 253.7 nm), and KrF excimer laser (248 nm)) are presented. Products of sulfur dioxide photolysis undertaken with 193-nm radiation exhibit characteristics that are similar to sulfur multiple-isotope data for terrestrial sedimentary rock samples older than 2450 Ma (reported by Farquhar et al. [2000a]), while photolysis experiments undertaken with radiation at other wavelengths (longer than 220 nm and at 184.9 nm) exhibit different characteristics. The spectral window between 190 and 220 nm falls between the Schumann-Runge bands of oxygen and the Hartley bands of ozone, and its absorption is therefore more sensitive to changes in altitude and atmospheric oxygen content than neighboring wavelengths. These two observations are used to suggest a link between sulfur dioxide photolysis at 193 nm and sulfur isotope anomalies in Archean rocks. This hypothesis includes the suggestion that UV wavelengths shorter than 200 nm penetrated deep in the Earths atmosphere during the Archean. Potential implications of this hypothesis for the chemistry, composition, and UV absorption of the atmosphere are explored. We also explore the implications of these observations for documentation of bacterial sulfur metabolisms early in Earths history.

Sulfate Formation in Sea-Salt Aerosols: Constraints from Oxygen Isotopes

imparts a large D 17 O signature to the resulting sulfate (8.8%) relative to oxidation by H2O2 (0.9% )o r by OH or O 2 (0%). Ship data from two Indian Ocean Experiment (INDOEX) cruises in the Indian Ocean indicate D 17 O values usually 70%) and increases MBL sulfate concentrations by typically >10% (up to 30%). Globally, this mechanism contributes 9% of atmospheric sulfate production and 1% of the sulfate burden. The impact on H2SO4 (g) formation and implications for the potential formation of new particles in the MBL warrants inclusion in models examining the radiative effects of sulfate aerosols.

Evidence of atmospheric sulphur in the martian regolith from sulphur isotopes in meteorites.

Sulphur is abundant at the martian surface, yet its origin and evolution over time remain poorly constrained. This sulphur is likely to have originated in atmospheric chemical reactions, and so should provide records of the evolution of the martian atmosphere, the cycling of sulphur between the atmosphere and crust, and the mobility of sulphur in the martian regolith. Moreover, the atmospheric deposition of oxidized sulphur species could establish chemical potential gradients in the martian near-surface environment, and so provide a potential energy source for chemolithoautotrophic organisms. Here we present measurements of sulphur isotopes in oxidized and reduced phases from the SNC meteorites—the group of related achondrite meteorites believed to have originated on Mars—together with the results of laboratory photolysis studies of two important martian atmospheric sulphur species (SO2 and H2S). The photolysis experiments can account for the observed sulphur-isotope compositions in the SNC meteorites, and so identify a mechanism for producing large abiogenic 34S fractionations in the surface sulphur reservoirs. We conclude that the sulphur data from the SNC meteorites reflects deposition of oxidized sulphur species produced by atmospheric chemical reactions, followed by incorporation, reaction and mobilization of the sulphur within the regolith.

Tracing the Origin and Fate of NOx in the Arctic Atmosphere Using Stable Isotopes in Nitrate

Atmospheric nitrogen oxides (NOx =NO+ NO2) play a pivotal role in the cycling of reactive nitrogen (ultimately deposited as nitrate) and the oxidative capacity of the atmosphere. Combined measurements of nitrogen and oxygen stable isotope ratios of nitrate collected in the Arctic atmosphere were used to infer the origin and fate of NOx and nitrate on a seasonal basis. In spring, photochemically driven emissions of reactive nitrogen from the snowpack into the atmosphere make local oxidation of NOx by bromine oxide the major contributor to the nitrate budget. The comprehensive isotopic composition of nitrate provides strong constraints on the relative importance of the key atmospheric oxidants in the present atmosphere, with the potential for extension into the past using ice cores.

Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass‐independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth

The oxygen isotopic composition (16O, 17O, and 18O) of sulfate formed from different oxidative reactions has been investigated. In the aqueous phase, sulfur oxidation by H2O2, O3, and O2, catalyzed by Fe(III) and Mn(II) were studied. In the gas phase we have investigated the only relevant reaction for the atmosphere: SO2+OH and its chain termination reaction SO3+H2O. The results show that none of these reactions, gas or aqueous phase, produce a mass-independent oxygen isotopic composition in sulfate. Since H2O2 and O3 are known to possess a mass-independent isotopic signature, we have investigated the possible transfer of this anomaly to sulfate. It appears that both these oxidant species transfer their anomaly. Isotopic analysis shows that two oxygen atoms from H2O2 are found in the product H2SO4. This result is in accord with previous work. For O3 we found that only one of the original ozone oxygen transfers to the product sulfate. These isotopic results contradict the free radical reaction mechanism proposed by Penkett et al. [1979] but agree with the nonfree radical mechanism suggested by Erickson et al. [1977]. Therefore, it appears that only aqueous phase oxidation produces a mass-independent oxygen isotopic composition in sulfate. This finding is a response of the origin of the mass-independent oxygen isotopic composition of atmospheric and mineral deposits of sulfate on Earth [Bao et al., 2000; Lee, 1997]. Furthermore, this finding allows us to quantify the relative proportion of sulfate production by OH (gas phase formation) and by H2O2 and O3 (aqueous phase formation). The results can be used to test atmospheric chemical/transport models.

Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65°S to 79°N

The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the δ 15N of nitrate mostly deriving from anthropogenically emitted NO x is found to be significantly different (from 0 to 6‰) from nitrate sampled in locations influenced by natural NO x sources (−4 ± 2)‰. The effects on δ 15N(NO3 −) of different NO x sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Δ17O = δ 17O − 0.52 × δ 18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NO x sinks. Different Δ17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low δ 15N values (down to −40‰) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NO x from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.

Sulfur‐containing species (methanesulfonate and SO4) over the last climatic cycle in the Greenland Ice Core Project (central Greenland) ice core

A high-resolution profile covering the last two centuries and a discontinuous study spanning the complete last glacial-interglacial cycle of methanesulfonate (MSA) (CH3SO3−) and sulfate were obtained along Summit (central Greenland) ice cores. MSA concentrations were close to 4±1.4 ng g−1 from 1770 to 1870 A.D. and 3 ng g−1 in 1900, and exhibited a well-marked decreasing trend from 1945 to the present. These changes of Summit snow MSA concentrations between 1770 and 1945 are discussed in terms of possible modulation of dimethylsulfide (DMS) marine emissions influencing the Greenland Ice Sheet by past climatic fluctuations in these regions. The decrease of MSA levels in Summit snow layers deposited since 1945 suggests either a decline in marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in response to increasing anthropogenic NOx, and hydrocarbon emissions. While interglacial ice concentrations of MSA and sulfate are close to 2.9±1.9 ng g−1 and 27±10 ng g−1, respectively, reduced MSA (1.2±0.7 ng g−1) and enhanced sulfate (55±19 ng g−1) levels characterized the early Holocene stage (9000 to 11,000 years B.P.). MSA concentrations in glacial ice remain similar to the ones observed during interglacial stages. In contrast, sulfate levels are strongly enhanced (243±84 ng g−1) during the last glacial maximum (14,400 to 15,700 B.P.) compared with the interglacial ones. These variations of sulfur-containing species in response to past climatic conditions are similar to those found in other Greenland cores. In contrast, they are different from those revealed in the Antarctic Vostok ice core, where colder climates were associated with an increase by a factor of 5 and 2 in MSA and sulfate concentrations, respectively. These glacial-interglacial changes are discussed in terms of present and past contributions of marine DMS emissions versus other sulfate sources such as volcanic emissions and continental dust to the Greenland precipitation.

Cold decade (AD 1810-1819) caused by Tambora (1815) and another (1809) stratospheric volcanic eruption.

Climate records indicate that the decade of AD 1810–1819 including “the year without a summer” (1816) is probably the coldest during the past 500 years or longer, and the cause of the climatic extreme has been attributed primarily to the 1815 cataclysmic Tambora eruption in Indonesia. But the cold temperatures in the early part of the decade and the timing of the Tambora eruption call into question the real climatic impact of volcanic eruptions. Here we present new evidence, based on sulfur isotope anomaly (Δ33S), a unique indicator of volcanic sulfuric acid produced in the stratosphere and preserved in polar snow, and on the precise timing of the volcanic deposition in both polar regions, that another large eruption in 1809 of a volcano is also stratospheric and occurred in the tropics. The Tambora eruption and the undocumented 1809 eruption are together responsible for the unusually cold decade.

High northern latitude forest fires and vegetation emissions over the last millennium inferred from the chemistry of a central Greenland ice core

We have analyzed the soluble portion of impurities trapped in solid precipitation that accumulated at Summit (central Greenland) from 1193 A.D. to the present. Seventy-three ice layers show elevated concentrations of ammonium and formate, caused by high-latitude biomass burning debris reaching Greenland. While a mixture of ammonium and formate close to the molar ratio is generally observed in these ice layers, a large depletion of formate relative to ammonium is found in a few cases. The chemical composition of such layers indicates the presence of a mixture of ammonium, formate, and nitrate with a NH4+/(HCOO− +NO3−) molar ratio close to 1. These differences may be related to the fire type (flaming versus smoldering) or to meteorological conditions encountered by plumes during their transport toward Greenland. The high-resolution ammonium and formate profiles are used to reconstruct the frequency and the intensity of high-latitude biomass burning input having reached central Greenland since 1193 A.D. Three periods of enhanced biomass burning input over central Greenland are identified: 1200–1350 A.D., 1830–1930 A.D., and to a lesser extent 1500–1600 A.D. The 1200–1350 A.D. time period coincides with warm and dry conditions which characterized the Medieval Warm Period. After a period of infrequent biomass burning input during the coldest period of the Little Ice Age (1600–1850 A.D.), the frequency was enhanced at the turn of the last century and then decreased throughout this last century. Aside from high-latitude biomass burning, the background levels of formate show a slight and persistent decreasing trend over the last 800 years probably reflecting the deterioration of the boreal vegetation from North America.

A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: Potential Implications for atmospheres of Mars and Earth

Photodissociation of CO2 by ultraviolet light (λ = 185 nm) generates CO and O2, which are unusually enriched (more than 100‰) in 17O. The dissociation takes place through a spin forbidden process during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The 17O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin-orbit coupling of the low energy vibrational levels of the 16O12C17O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (λ < 160 nm) involves no spin violation and produces CO and O2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13C enriched CO2; in this case the products are enriched in both heavy isotopes but about 100‰ more in 18O. The 17O enrichment in CO and O2 generated by CO2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.