SO₂photoexcitation mechanism links mass-independent sulfur isotopic fractionation in cryospheric sulfate to climate impacting volcanism

Abstract

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO 2 , that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of 32 SO 2 , 33 SO 2 , 34 SO 2 , and 36 SO 2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO 2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.A ttribution of climate change relies on our understanding of natural climate variation. Volcanoes affect climate, but it is not easy to use proxy records to derive the climate impact of a given historical eruption, primarily because we lack knowledge about the volcanoes themselves. Some so-called Plinian eruptions penetrate the stratosphere, resulting in multiyear climate impacts (1). Volcanic sulfur dioxide (SO 2 ) in a plume is photooxidized in the atmosphere. In the troposphere, the sulfuric acid product is washed out as sulfate in acid rain in a matter of weeks. A Plinian eruption, in contrast, intensifies the stratospheric sulfate aerosol (SSA) layer (2), increasing the planet's albedo (3) and enhancing midlatitude O 3 depletion (4) for more than a year. Ice core records of sulfate provide an important record of volcanic activity (5), but the concentration of sulfate alone does not indicate the explosivity of the event and specifically, if the plume penetrated the stratosphere (6).A series of groundbreaking studies has shown that sulfur isotopes in sulfate from Plinian eruptions show mass-independent fractionation (MIF) (6-8). Carbonyl sulfide (OCS) is thought to be the main source of background SSA in volcanically quiescent periods (9), and the reactions breaking down OCS, mainly photolysis, show no evidence of sulfur MIF (10-13). MIF is not seen at the sour...

Large sulfur-isotope anomaly in nonvolcanic sulfate aerosol and its implications for the Archean atmosphere

“…A recent model that considers SO 2 photoexcitation rather than photolysis and volcanic plume chemistry (including heterogeneous stratospheric chemistry) suggests that UV photo excitation of SO 2 is another route to the observed S-MIF in volcanic sulfate. This new mechanism may also provide information about the ozone-depletion chemistry in the plume (30) and is relevant for the present data. Laboratory experiments indicate that SO 2 photodissociation is wavelength-dependent (Δ 36 S/Δ 33 S slopes vary from −1 at λ = 193 nm to −4 at λ = 248 nm as shown in Fig.…”

Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

“…This would also explain the very large variation in the isotopic fractionation with temperature (34), because relatively small differences in temperature lead to significant differences in the population of the rotational states, which can have a large effect on the fine structure of the cross-sections. High-resolution measurements of the isotopologue-specific cross-sections of CO 2 similar to studies performed for SO 2 (36,37) are needed to fully unravel isotopic fractionation fully on a subnanometer scale.…”

Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature

“…The photochemistry of SO 2 is complex, and modeling of the isotope effects, especially in the atmosphere, is challenging. In this issue, a unique photochemical photoexcitation mechanism that occurs in the stratosphere is presented (11). In Whitehill et al (18), a creative series of experiments and trapping the photoexcited sulfur dioxide a new mass independent isotopic signature pattern was observed.…”

“…Further supporting evidence is the disappearance of the isotopic anomaly at ∼2 billion years ago during the time associated with a global oxygenation event and development of the protective ozone layer. Various aspects of the physical chemistry of the process, biological interactions, and the preservation and new interpretations are provided in contributions to this special issue (8,9,11). The data are a collection of published data kindly provided by James Farquhar.…”

“…The applications have subsequently expanded, particularly to oxygen isotopes, and in this special issue, many unique uses of both oxygen and sulfur isotopes are presented (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18).…”