Greg Michalski

Neogene climate change and uplift in the Atacama Desert, Chile

The relationship between Andean uplift and extreme desiccation of the west coast of South America is important for understanding the interplay between climate and tectonics in the Central Andes, yet it is poorly understood. Here we use soil morphological char- acteristics, salt chemistry, and mass independent fractionation anomalies ( 17 O values) in dated paleosols to reconstruct a middle Miocene climatic transition from semiaridity to extreme hyperaridity in the Atacama Desert. Paleosols along the southeastern margin of the Calama Basin change from calcic Vertisols with root traces, slickensides, and gleyed horizons to an extremely mature salic Gypsisol with pedogenic nitrate. We interpret this transition, which occurred between 19 and 13 Ma, to represent a change in precipitation from 200 mm/yr to 20 mm/yr. This drastic reduction in precipitation likely resulted from uplift of the Central Andes to elevations 2 km; the uplift blocked moisture from the South American summer monsoon from entering the Atacama. The mid-Miocene Gyp- sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the Cal- ama Basin, significantly higher than modern nitrate soils, which occur below 2500 m. Modern and Quaternary soils in this elevation zone contain soil carbonate and lack ped- ogenic gypsum and nitrate. We infer that 900 m of local surface uplift over the past 10 m.y. displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the Calama Basin by decreasing local air temperatures and creating an orographic barrier to Pacific air masses.

Sulfate oxygen-17 anomalies in desert varnishes

Rock varnishes are ubiquitous in arid regions on Earth, and are believed to be commonly present on Mars. Here we report high water-soluble sulfate and nitrate contents in desert varnishes from the Death Valley region of southwestern U.S.A., and that sulfate in varnishes possess δ17O/δ18O ratios that do not fall on terrestrial mass-dependent fractionation line. Sulfate from wet and dry atmospheric deposition is probably the source of the δ17O anomalies. The anomalies are only moderately lower than that of aerosol sulfates collected from the greater Los Angeles area, indicating probably more than half of the sulfate in desert varnish is supplied by atmospheric deposition. This finding suggests that Earth surface environments are constantly accumulating δ17O-anomalous sulfate from the atmosphere; arid and stable conditions facilitate the preservation of these atmospheric signatures. This finding also indicates that different δ17O/δ18O ratios found in different components in the Martian meteorites may result from atmospheric chemical processes.

Reply to Comment on “Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from 15N-Stable Isotope in Size-Resolved Aerosol Ammonium”

Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from N‐Stable Isotope in Size-Resolved Aerosol Ammonium” W appreciate the opportunity to respond to the comments of Chang and Ma regarding our article, and we also hope to further clarify the findings of our work. Their comments on our work focus on the source apportionment of ammonia (NH3) during haze episodes in Beijing. We do not think that their objections are well founded, and their speculations do not change our conclusions.

Nitrogen Stable Isotope Composition (δ15N) of Vehicle-Emitted NOx

The nitrogen stable isotope ratio of NOx (δ(15)N-NOx) has been proposed as a regional indicator for NOx source partitioning; however, knowledge of δ(15)N values from various NOx emission sources is limited. This study presents a detailed analysis of δ(15)N-NOx emitted from vehicle exhaust, the largest source of anthropogenic NOx. To accomplish this, NOx was collected from 26 different vehicles, including gasoline and diesel-powered engines, using a modification of a NOx collection method used by the United States Environmental Protection Agency, and δ(15)N-NOx was analyzed. The vehicles sampled in this study emitted δ(15)N-NOx values ranging from -19.1 to 9.8‰ that negatively correlated with the emitted NOx concentrations (8.5 to 286 ppm) and vehicle run time because of kinetic isotope fractionation effects associated with the catalytic reduction of NOx. A model for determining the mass-weighted δ(15)N-NOx from vehicle exhaust was constructed on the basis of average commute times, and the model estimates an average value of -2.5 ± 1.5‰, with slight regional variations. As technology improvements in catalytic converters reduce cold-start emissions in the future, it is likely to increase current δ(15)N-NOx values emitted from vehicles.

Using 15N, 17O, and 18O to determine nitrate sources in the Yellow River, China.

Many previous studies have used δ(15)N and δ(18)O of nitrate (δ(15)NNO3 and δ(18)ONO3) to determine the nitrate sources in rivers but were subject to substantial uncertainties and limitations, especially associated with evaluating the atmospheric contribution. The Δ(17)O of nitrate (Δ(17)ONO3) has been suggested as an unambiguous tracer of atmospheric NO3(-) and may serve as an additional nitrate source constraint. In the present study, triple nitrate isotopes (δ(15)NNO3, Δ(17)ONO3, and δ(18)ONO3) were used for the first time to assess the sources and sinks of nitrate in the Yellow River (YR) basin, which is the second longest river in China. Results showed that the Δ(17)ONO3 of the water from the YR ranged from 0‰ to 1.6‰ during two normal-water seasons. This suggested that unprocessed atmospheric nitrate accounted for 0-7% of the total nitrate in the YR. The corrected δ(15)NNO3 and δ(18)ONO3 values with atmospheric imprints being removed indicated that the main terrestrial sources of nitrate were sewage/manure effluents in the upstream of the YR and manure/sewage effluents and ammonium/urea-containing fertilizer in the middle and lower reaches which made comparable contributions to the nitrate. In addition, there was a significant positive relationship between δ(15)NNO3 and δ(18)ONO3 values of river water (p < 0.01) which may signal the presence of denitrification. This study indicates that the triple nitrate isotope method is useful for assessing the nitrate sources in rivers, especially for the measurements of atmospheric nitrate contribution.

Analysis of atmospheric inputs of nitrate to a temperate forest ecosystem from Δ17O isotope ratio measurements

(dry deposited HNO3 and wet deposited NO3 )i n northern Michigan is derived from atmospheric deposition. To test this idea, soil, rainfall, and cloud water were sampled in a temperate forest in northern Lower Michigan. The fraction of the soil solution NO3 pool directly from atmospheric deposition was quantified using the natural isotopic tracer, D 17 O. Our results show that on average 9% of the soil solution NO3 is unprocessed (no microbial turnover) N derived directly from the atmosphere. This points to the potential importance of anthropogenic N deposition and contributes to the long‐standing need to improve our understanding of the impacts of atmospheric nitrogen processing and deposition on forest ecosystems and forest productivity. Citation: Costa, A. W., G. Michalski, A. J. Schauer, B. Alexander, E. J. Steig, and P. B. Shepson (2011), Analysis of atmospheric inputs of nitrate to a temperate forest ecosystem from D 17 O isotope ratio measurements, Geophys. Res. Lett., 38,

Sources and Transport of Nitrogen in Arid Urban Watersheds

Urban watersheds are often sources of nitrogen (N) to downstream systems, contributing to poor water quality. However, it is unknown which components (e.g., land cover and stormwater infrastructure type) of urban watersheds contribute to N export and which may be sites of retention. In this study we investigated which watershed characteristics control N sourcing, biogeochemical processing of nitrate (NO3-) during storms, and the amount of rainfall N that is retained within urban watersheds. We used triple isotopes of NO3- (δ15N, δ18O, and Δ17O) to identify sources and transformations of NO3- during storms from 10 nested arid urban watersheds that varied in stormwater infrastructure type and drainage area. Stormwater infrastructure and land cover--retention basins, pipes, and grass cover--dictated the sourcing of NO3- in runoff. Urban watersheds were strong sinks or sources of N to stormwater depending on runoff, which in turn was inversely related to retention basin density and positively related to imperviousness and precipitation. Our results suggest that watershed characteristics control the sources and transport of inorganic N in urban stormwater but that retention of inorganic N at the time scale of individual runoff events is controlled by hydrologic, rather than biogeochemical, mechanisms.

The role of symmetry in the mass independent isotope effect in ozone

Understanding the internal distribution of “anomalous” isotope enrichments has important implications for validating theoretical postulates on the origin of these enrichments in molecules such as ozone and for understanding the transfer of these enrichments to other compounds in the atmosphere via mass transfer. Here, we present an approach, using the reaction NO2− + O3, for assessing the internal distribution of the Δ17O anomaly and the δ18O enrichment in ozone produced by electric discharge. The Δ17O results strongly support the symmetry mechanism for generating mass independent fractionations, and the δ18O results are consistent with published data. Positional Δ17O and δ18O enrichments in ozone can now be more effectively used in photochemical models that use mass balance oxygen atom transfer mechanisms to infer atmospheric oxidation chemistry.

Nitrogen Isotope Composition of Thermally Produced NOx from Various Fossil-Fuel Combustion Sources.

The nitrogen stable isotope composition of NOx (δ(15)N-NOx) may be a useful indicator for NOx source partitioning, which would help constrain NOx source contributions in nitrogen deposition studies. However, there is large uncertainty in the δ(15)N-NOx values for anthropogenic sources other than on-road vehicles and coal-fired energy generating units. To this end, this study presents a broad analysis of δ(15)N-NOx from several fossil-fuel combustion sources that includes: airplanes, gasoline-powered vehicles not equipped with a three-way catalytic converter, lawn equipment, utility vehicles, urban buses, semitrucks, residential gas furnaces, and natural-gas-fired power plants. A relatively large range of δ(15)N-NOx values was measured from -28.1‰ to 8.5‰ for individual exhaust/flue samples that generally tended to be negative due to the kinetic isotope effect associated with thermal NOx production. A negative correlation between NOx concentrations and δ(15)N-NOx for fossil-fuel combustion sources equipped with selective catalytic reducers was observed, suggesting that the catalytic reduction of NOx increases δ(15)N-NOx values relative to the NOx produced through fossil-fuel combustion processes. Combining the δ(15)N-NOx measured in this study with previous published values, a δ(15)N-NOx regional and seasonal isoscape was constructed for the contiguous U.S., which demonstrates seasonal and regional importance of various NOx sources.

Using 17O to Investigate Nitrate Sources and Sinks in a Semi-Arid Groundwater System

We apply a triple isotope approach for nitrate that utilizes Δ(17)O as a conservative tracer, in combination with δ(18)O and δ(15)N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ(18)O and δ(15)N to determine nitrate sources and cycling, but the atmospheric δ(18)O signature can be overprinted by biogeochemical processes. In this study, δ(18)O and δ(15)N values of nitrate were coupled with δ(17)O values of nitrate to quantify atmospheric nitrate inputs and denitrification amounts. Results show generally low groundwater nitrate concentrations (<0.2 mmol/L) throughout the basin; high nitrate concentrations (up to 1 mmol/L) with evidence for some denitrification were detected in areas where effluent was the predominant source of recharge to groundwater. Furthermore, the denitrification was inferred from elevated δ(18)O and δ(15)N values which were reinforced by increases in observed δ(17)O values. Finally, relatively low, but significant atmospheric nitrate concentrations were measured in groundwater (up to 6% of total nitrate). This study concludes that the triple isotope approach improves determination of the proportion of atmospheric nitrate and the significance of denitrification in natural waters, allowing us to develop a conceptual model of the biogeochemical processes controlling nitrogen in an urban setting.