Robina Shaheen

Detection of oxygen isotopic anomaly in terrestrial atmospheric carbonates and its implications to Mars

The debate of life on Mars centers around the source of the globular, micrometer-sized mineral carbonates in the ALH84001 meteorite; consequently, the identification of Martian processes that form carbonates is critical. This paper reports a previously undescribed carbonate formation process that occurs on Earth and, likely, on Mars. We identified micrometer-sized carbonates in terrestrial aerosols that possess excess 17O (0.4–3.9‰). The unique O-isotopic composition mechanistically describes the atmospheric heterogeneous chemical reaction on aerosol surfaces. Concomitant laboratory experiments define the transfer of ozone isotopic anomaly to carbonates via hydrogen peroxide formation when O3 reacts with surface adsorbed water. This previously unidentified chemical reaction scenario provides an explanation for production of the isotopically anomalous carbonates found in the SNC (shergottites, nakhlaites, chassignites) Martian meteorites and terrestrial atmospheric carbonates. The anomalous hydrogen peroxide formed on the aerosol surfaces may transfer its O-isotopic signature to the water reservoir, thus producing mass independently fractionated secondary mineral evaporites. The formation of peroxide via heterogeneous chemistry on aerosol surfaces also reveals a previously undescribed oxidative process of utility in understanding ozone and oxygen chemistry, both on Mars and Earth.

Tales of volcanoes and El-Niño southern oscillations with the oxygen isotope anomaly of sulfate aerosol

The ability of sulfate aerosols to reflect solar radiation and simultaneously act as cloud condensation nuclei renders them central players in the global climate system. The oxidation of S(IV) compounds and their transport as stable S(VI) in the Earth’s system are intricately linked to planetary scale processes, and precise characterization of the overall process requires a detailed understanding of the linkage between climate dynamics and the chemistry leading to the product sulfate. This paper reports a high-resolution, 22-y (1980–2002) record of the oxygen-triple isotopic composition of sulfate (SO4) aerosols retrieved from a snow pit at the South Pole. Observed variation in the O-isotopic anomaly of SO4 aerosol is linked to the ozone variation in the tropical upper troposphere/lower stratosphere via the Ozone El-Niño Southern Oscillations (ENSO) Index (OEI). Higher ∆17O values (3.3‰, 4.5‰, and 4.2‰) were observed during the three largest ENSO events of the past 2 decades. Volcanic events inject significant quantities of SO4 aerosol into the stratosphere, which are known to affect ENSO strength by modulating stratospheric ozone levels (OEI = 6 and ∆17O = 3.3‰, OEI = 11 and ∆17O = 4.5‰) and normal oxidative pathways. Our high-resolution data indicated that ∆17O of sulfate aerosols can record extreme phases of naturally occurring climate cycles, such as ENSOs, which couple variations in the ozone levels in the atmosphere and the hydrosphere via temperature driven changes in relative humidity levels. A longer term, higher resolution oxygen-triple isotope analysis of sulfate aerosols from ice cores, encompassing more ENSO periods, is required to reconstruct paleo-ENSO events and paleotropical ozone variations.

Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate

RATIONALE Triple oxygen isotopes of sulfate and nitrate are useful metrics for the chemistry of their formation. Existing measurement methods, however, do not account for oxygen atom exchange with quartz during the thermal decomposition of sulfate. We present evidence for oxygen atom exchange, a simple modification to prevent exchange, and a correction for previous measurements. METHODS Silver sulfates and silver nitrates with excess (17)O were thermally decomposed in quartz and gold (for sulfate) and quartz and silver (for nitrate) sample containers to O(2) and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Helium carries O(2) through purification for isotope-ratio analysis of the three isotopes of oxygen in a Finnigan MAT253 isotope ratio mass spectrometer. RESULTS The Δ(17)O results show clear oxygen atom exchange from non-zero (17)O-excess reference materials to zero (17)O-excess quartz cup sample containers. Quartz sample containers lower the Δ(17)O values of designer sulfate reference materials and USGS35 nitrate by 15% relative to gold or silver sample containers for quantities of 2-10 µmol O(2). CONCLUSIONS Previous Δ(17)O measurements of sulfate that rely on pyrolysis in a quartz cup have been affected by oxygen exchange. These previous results can be corrected using a simple linear equation (Δ(17)O(gold) = Δ(17)O(quartz) * 1.14 + 0.06). Future pyrolysis of silver sulfate should be conducted in gold capsules or corrected to data obtained from gold capsules to avoid obtaining oxygen isotope exchange-affected data.

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

Significance The highest S-isotope anomaly is observed in a nonvolcanic period, and the magnitude of anomaly is similar to the largest volcanic eruptions of the 20th century. S-quadruple isotope data provided the first evidence of how super El Niño Southern Oscillation (ENSO) events (1997–1998) have affected the transport and transformation of aerosols to the stratosphere; thus, record of paleo-ENSO events of this magnitude can be traced with the S-isotopic anomaly. High-resolution and high-precision S-isotopic fingerprinting also revealed that the tropospheric sulfate produced during fossil-fuel and biomass burning contributes to the stratospheric sulfate aerosol layer, a contribution previously unrecognized. The distribution of sulfur anomalies mimics the Archean isotope record, which is used to track the origin and evolution of oxygen on earth. Sulfur-isotopic anomalies have been used to trace the evolution of oxygen in the Precambrian atmosphere and to document past volcanic eruptions. High-precision sulfur quadruple isotope measurements of sulfate aerosols extracted from a snow pit at the South Pole (1984–2001) showed the highest S-isotopic anomalies (Δ33S = +1.66‰ and Δ36S = +2‰) in a nonvolcanic (1998–1999) period, similar in magnitude to Pinatubo and Agung, the largest volcanic eruptions of the 20th century. The highest isotopic anomaly may be produced from a combination of different stratospheric sources (sulfur dioxide and carbonyl sulfide) via SOx photochemistry, including photoexcitation and photodissociation. The source of anomaly is linked to super El Niño Southern Oscillation (ENSO) (1997–1998)-induced changes in troposphere–stratosphere chemistry and dynamics. The data possess recurring negative S-isotope anomalies (Δ36S = −0.6 ± 0.2‰) in nonvolcanic and non-ENSO years, thus requiring a second source that may be tropospheric. The generation of nonvolcanic S-isotopic anomalies in an oxidizing atmosphere has implications for interpreting Archean sulfur deposits used to determine the redox state of the paleoatmosphere.

Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere

Significance Martian meteorite ALH 84001 serves as a witness plate to the history of the Martian climate ∼4 Ga ago. This study describes ion microprobe δ18O analyses coupled with δ13C, δ18O, and Δ17O analyses from stepped acid dissolution of the meteorite that identifies a new carbonate phase with distinct isotope compositions. These new measurements of the oxygen isotope composition of carbonates within this meteorite reveal several episodes of aqueous activity that were strongly influenced by atmospheric chemistry. When paired with carbon isotope measurements, these data suggest that the ancient atmosphere of Mars was significantly depleted in 13C compared to the present day. This implies substantial enrichment in the δ13C of the atmosphere since the Noachian which may have occurred through extensive atmospheric loss. Carbonate minerals provide critical information for defining atmosphere–hydrosphere interactions. Carbonate minerals in the Martian meteorite ALH 84001 have been dated to ∼3.9 Ga, and both C and O-triple isotopes can be used to decipher the planet’s climate history. Here we report Δ17O, δ18O, and δ13C data of ALH 84001 of at least two varieties of carbonates, using a stepped acid dissolution technique paired with ion microprobe analyses to specifically target carbonates from distinct formation events and constrain the Martian atmosphere–hydrosphere–geosphere interactions and surficial aqueous alterations. These results indicate the presence of a Ca-rich carbonate phase enriched in 18O that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess 17O (0.7‰) that captured the atmosphere–regolith chemical reservoir transfer, as well as CO2, O3, and H2O isotopic interactions at the time of formation of each specific carbonate. The carbon isotopes preserved in the Ca-rich carbonate phase indicate that the Noachian atmosphere of Mars was substantially depleted in 13C compared with the modern atmosphere.

Ammonium deficiency caused by heterogeneous reactions during a super Asian dust episode

Mineral dust particles exert profound impacts on air quality, visibility, and ocean biogeochemistry. Interactions between dust particles and other anthropogenic pollutants modify not only the size spectrum and morphology but also physicochemical properties of dust particles, thereby affecting their radiative properties and ability to act as cloud condensation nuclei and in turn their impact on climate. Here we report field observations on the surface chemical transformations in a super Asian dust plume captured in coastal areas of China and the adjacent marginal seas. The dust plume showed enhanced concentrations of sulfate, nitrate, and calcium along with a decrease in ammonium. The percentages of total Ca in water-soluble form increased from an intrinsic value of ~5% to 25–40% at four stations along the path of the dust plume. From these increases, we estimated the extent to which carbonate was modified by heterogeneous reactions and calculated that the enhanced sulfate and nitrate could account for 40–60% of the observed concentrations. Our observation suggests that the formation of ammonium sulfate via the H2SO4-NH3-H2O ternary system was impeded by heterogeneous reactions in the marine boundary layer when dust loads exceeded a certain threshold. A conceptual model is proposed to elucidate the heterogeneous reactions during the super Asian dust event and their impacts on atmospheric chemistry.

Quantification of polysaccharides in water using capillary electrophoresis

Two hydrolysis techniques were tested to optimize hydrolysis conditions for natural lake polysaccharides (PS). Modified sulphuric acid hydrolysis was found to be more efficient than 0.1 M HCl hydrolysis as assessed by a spectrophotometric method commonly known as TPTZ assay (2,4,6-tripyridyl-s-triazine). Capillary electrophoresis coupled with laser induced fluorescence (CE–LIF) was standardized to determine hydrolyzed saccharides in water. Two types of capillaries were tested for the CE technique. Fused silica capillary was found good for quantitative determination of different saccharides (r 2 = 0.998, P<0.001). Excellent resolution of monosaccharides was achieved using a gel capillary (CGE), but correlation coefficients (r 2<0.87, P<0.05) were not as strong as in the case of silica capillary. CE–LIF and TPTZ assay were applied to quantify the vertical polysaccharide profile of the Lake Geneva (0–309 m). Maximum concentration of PS (1.02 ± 0.03 mg L−1) was observed at surface. The concentration gradually decreased, with a minimum concentration of PS at 290 m (0.27 ± 0.04 mg L−1). A fairly good correlation (r 2 = 0.83, N = 9, P<0.001) between the total organic carbon (TOC) and APTS saccharide adduct measured by fused silica capillary was observed. The vertical profile of the lake as determined by CGE indicated glucose and fructose as dominant sugars with a minor contribution of xylose. The present study indicates the potential of CGE for qualitative as well as quantitative analysis of hydrolyzed saccharides in water samples.

Detection of deep stratospheric intrusions by cosmogenic 35S

Significance The recently revised stricter US National Ambient Air Quality Standard for ground-level ozone (O3) requires precise methods to screen “exceptional events,” such as naturally occurring deep stratospheric intrusions. However, existing approaches used in detecting stratospheric intrusions and evaluating their contributions to ground-level O3 enhancement are not satisfactory. Here, we introduce the use of cosmogenic 35S to assist in such quantifications. We measured the highest 35S concentration in natural sulfate aerosols ever reported in the literature during deep stratospheric intrusions. The downward transport of stratospheric O3 is confirmed by air quality data and meteorological analysis, showing the high sensitivity of 35S as a stratospheric tracer and its utility to understand atmospheric transport and chemistry processes. The extent to which stratospheric intrusions on synoptic scales influence the tropospheric ozone (O3) levels remains poorly understood, because quantitative detection of stratospheric air has been challenging. Cosmogenic 35S mainly produced in the stratosphere has the potential to identify stratospheric air masses at ground level, but this approach has not yet been unambiguously shown. Here, we report unusually high 35S concentrations (7,390 atoms m−3; ∼16 times greater than annual average) in fine sulfate aerosols (aerodynamic diameter less than 0.95 µm) collected at a coastal site in southern California on May 3, 2014, when ground-level O3 mixing ratios at air quality monitoring stations across southern California (43 of 85) exceeded the recently revised US National Ambient Air Quality Standard (daily maximum 8-h average: 70 parts per billion by volume). The stratospheric origin of the significantly enhanced 35S level is supported by in situ measurements of air pollutants and meteorological variables, satellite observations, meteorological analysis, and box model calculations. The deep stratospheric intrusion event was driven by the coupling between midlatitude cyclones and Santa Ana winds, and it was responsible for the regional O3 pollution episode. These results provide direct field-based evidence that 35S is an additional sensitive and unambiguous tracer in detecting stratospheric air in the boundary layer and offer the potential for resolving the stratospheric influences on the tropospheric O3 level.

Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene

Significance Signatures of sulfur isotopic anomalies (a proxy used in tracking the atmospheric oxygen/sulfur cycles in the past) preserved in the Himalayas (“Asian water towers”) reveal significant changes in the regional atmospheric sulfur cycle and glacial hydrological system during the second industrial revolution. The record extends our atmospheric sulfur isotopic anomaly observation to a unique region and different time and transitional period. Distinct from most existing aerosol measurements made in the twenty-first century, the 200-y record mimics the Archean (4–2.5 billion years ago) barite record and may provide a broader view of the mechanistic origin of sulfur isotopic anomalies in the modern atmosphere and another tool to deepen insights into the Earth’s sulfur cycle during the evolution of early life. Increased anthropogenic-induced aerosol concentrations over the Himalayas and Tibetan Plateau have affected regional climate, accelerated snow/glacier melting, and influenced water supply and quality in Asia. Although sulfate is a predominant chemical component in aerosols and the hydrosphere, the contributions from different sources remain contentious. Here, we report multiple sulfur isotope composition of sedimentary sulfates from a remote freshwater alpine lake near Mount Everest to reconstruct a two-century record of the atmospheric sulfur cycle. The sulfur isotopic anomaly is utilized as a probe for sulfur source apportionment and chemical transformation history. The nineteenth-century record displays a distinct sulfur isotopic signature compared with the twentieth-century record when sulfate concentrations increased. Along with other elemental measurements, the isotopic proxy suggests that the increased trend of sulfate is mainly attributed to enhancements of dust-associated sulfate aerosols and climate-induced weathering/erosion, which overprinted sulfur isotopic anomalies originating from other sources (e.g., sulfates produced in the stratosphere by photolytic oxidation processes and/or emitted from combustion) as observed in most modern tropospheric aerosols. The changes in sulfur cycling reported in this study have implications for better quantification of radiative forcing and snow/glacier melting at this climatically sensitive region and potentially other temperate glacial hydrological systems. Additionally, the unique Δ33S–δ34S pattern in the nineteenth century, a period with extensive global biomass burning, is similar to the Paleoarchean (3.6–3.2 Ga) barite record, potentially providing a deeper insight into sulfur photochemical/thermal reactions and possible volcanic influences on the Earth’s earliest sulfur cycle.