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At least 955 records · Page 53Linked to original sources

USGS2018

Herbivory and eutrophication mediate grassland plant nutrient responses across a global climatic gradient

Plant stoichiometry, the relative concentration of elements, is a key regulator of ecosystem functioning and is also being altered by human activities. In this paper we sought to understand the global drivers of plant stoichiometry and compare the relative contribution of climatic vs. anthropogenic effects. We addressed this goal by measuring plant elemental (C, N, P and K) responses to eutrophication and vertebrate herbivore exclusion at eighteen sites on six continents. Across sites, climate and atmospheric N deposition emerged as strong predictors of plot‐level tissue nutrients, mediated by biomass and plant chemistry. Within sites, fertilization increased total plant nutrient pools, but results were contingent on soil fertility and the proportion of grass biomass relative to other functional types. Total plant nutrient pools diverged strongly in response to herbivore exclusion when fertilized; responses were largest in ungrazed plots at low rainfall, whereas herbivore grazing dampened the plant community nutrient responses to fertilization. Our study highlights (1) the importance of climate in determining plant nutrient concentrations mediated through effects on plant biomass, (2) that eutrophication affects grassland nutrient pools via both soil and atmospheric pathways and (3) that interactions among soils, herbivores and eutrophication drive plant nutrient responses at small scales, especially at water‐limited sites.

Ecology↗

USGS2011

Deposition and accumulation of airborne organic contaminants in Yosemite National Park, Calfornia

Deposition and accumulation of airborne organic contaminants in Yosemite National Park were examined by sampling atmospheric deposition, lichen, zooplankton, and lake sediment at different elevations. Passive samplers were deployed in high‐elevation lakes to estimate surface‐water concentrations. Detected compounds included current‐use pesticides chlorpyrifos, dacthal, and endosulfans and legacy compounds chlordane, dichlorodiphenyltrichloroethane‐related compounds, dieldrin, hexachlorobenzene, and polychlorinated biphenyls. Concentrations in snow were similar among sites and showed little variation with elevation. Endosulfan concentrations in summer rain appeared to coincide with application rates in the San Joaquin Valley. More than 70% of annual pesticide inputs from atmospheric deposition occurred during the winter, largely because most precipitation falls as snow. Endosulfan and chlordane concentrations in lichen increased with elevation, indicating that mountain cold‐trapping might be an important control on accumulation of these compounds. By contrast, chlorpyrifos concentrations were inversely correlated with elevation, indicating that distance from source areas was the dominant control. Sediment concentrations were inversely correlated with elevation, possibly because of the organic carbon content of sediments but also perhaps the greater mobility of organic contaminants at lower elevations. Surface‐water concentrations inferred from passive samplers were at sub–parts‐per‐trillion concentrations, indicating minimal exposure to aquatic organisms from the water column. Concentrations in sediment generally were low, except for dichlorodiphenyldichloroethane in Tenaya Lake, which exceeded sediment guidelines for protection of benthic organisms.

California↗

USGS2015

An evaluation of the residual toxicity and chemistry of a sodium hydroxide-based ballast water treatment system for freshwater ships

Nonnative organisms in the ballast water of freshwater ships must be killed to prevent the spread of invasive species. The ideal ballast water treatment system (BWTS) would kill 100% of ballast water organisms with minimal residual toxicity to organisms in receiving waters. In the present study, the residual toxicity and chemistry of a BWTS was evaluated. Sodium hydroxide was added to elevate pH to >11.5 to kill ballast water organisms, then reduced to pH <9 by sparging with wet-scrubbed diesel exhaust (the source of CO 2 ). Cladocerans ( Ceriodaphnia dubia ), amphipods ( Hyalella azteca ), and fathead minnows ( Pimephales promelas ) were exposed for 2 d to BWTS water under an air atmosphere (pH drifted to ≥9) or a 2.5% CO 2 atmosphere (pH 7.5–8.2), then transferred to control water for 5 d to assess potential delayed toxicity. Chemical concentrations in the BWTS water met vessel discharge guidelines with the exception of concentrations of copper. There was little to no residual toxicity to cladocerans or fish, but the BWTS water was toxic to amphipods. Maintaining a neutral pH and diluting BWTS water by 50% eliminated toxicity to the amphipods. The toxicity of BWTS water would likely be minimal because of rapid dilution in the receiving water, with subsurface release likely preventing pH rise. This BWTS has the potential to become a viable method for treating ballast water released into freshwater systems.

Environmental Toxicology and Chemistry↗

USGS2013

Inorganic carbon loading as a primary driver of dissolved carbon dioxide concentrations in the lakes and reservoirs of the contiguous United States

Accurate quantification of CO 2 flux across the air-water interface and identification of the mechanisms driving CO 2 concentrations in lakes and reservoirs is critical to integrating aquatic systems into large-scale carbon budgets, and to predicting the response of these systems to changes in climate or terrestrial carbon cycling. Large-scale estimates of the role of lakes and reservoirs in the carbon cycle, however, typically must rely on aggregation of spatially and temporally inconsistent data from disparate sources. We performed a spatially comprehensive analysis of CO 2 concentration and air-water fluxes in lakes and reservoirs of the contiguous United States using large, consistent data sets, and modeled the relative contribution of inorganic and organic carbon loading to vertical CO 2 fluxes. Approximately 70% of lakes and reservoirs are supersaturated with respect to the atmosphere during the summer (June–September). Although there is considerable interregional and intraregional variability, lakes and reservoirs represent a net source of CO 2 to the atmosphere of approximately 40 Gg C d –1 during the summer. While in-lake CO 2 concentrations correlate with indicators of in-lake net ecosystem productivity, virtually no relationship exists between dissolved organic carbon and p CO 2,aq . Modeling suggests that hydrologic dissolved inorganic carbon supports p CO 2,aq in most supersaturated systems (to the extent that 12% of supersaturated systems simultaneously exhibit positive net ecosystem productivity), and also supports primary production in most CO 2 -undersaturated systems. Dissolved inorganic carbon loading appears to be an important determinant of CO 2 concentrations and fluxes across the air-water interface in the majority of lakes and reservoirs in the contiguous United States.

Global Biogeochemical Cycles↗

USGS2013

Characterizing and estimating noise in InSAR and InSAR time series with MODIS

InSAR time series analysis is increasingly used to image subcentimeter displacement rates of the ground surface. The precision of InSAR observations is often affected by several noise sources, including spatially correlated noise from the turbulent atmosphere. Under ideal scenarios, InSAR time series techniques can substantially mitigate these effects; however, in practice the temporal distribution of InSAR acquisitions over much of the world exhibit seasonal biases, long temporal gaps, and insufficient acquisitions to confidently obtain the precisions desired for tectonic research. Here, we introduce a technique for constraining the magnitude of errors expected from atmospheric phase delays on the ground displacement rates inferred from an InSAR time series using independent observations of precipitable water vapor from MODIS. We implement a Monte Carlo error estimation technique based on multiple (100+) MODIS-based time series that sample date ranges close to the acquisitions times of the available SAR imagery. This stochastic approach allows evaluation of the significance of signals present in the final time series product, in particular their correlation with topography and seasonality. We find that topographically correlated noise in individual interferograms is not spatially stationary, even over short-spatial scales (<10 km). Overall, MODIS-inferred displacements and velocities exhibit errors of similar magnitude to the variability within an InSAR time series. We examine the MODIS-based confidence bounds in regions with a range of inferred displacement rates, and find we are capable of resolving velocities as low as 1.5 mm/yr with uncertainties increasing to ∼6 mm/yr in regions with higher topographic relief.

California↗

USGS2015

Climate-water quality relationships in Texas reservoirs

Water temperature, dissolved oxygen, and concentrations of salts in surface water bodies can be affected by the natural environment, local human activities such as surface and ground water withdrawals, land use, and energy extraction, and variability and long-term trends in atmospheric conditions including temperature and precipitation. Here, we quantify the relationship between 121 indicators of mean and extreme temperature and precipitation and 24 water quality parameters in 57 Texas reservoirs using observational data records covering the period 1960 to 2010. We find that water temperature, dissolved oxygen, pH, specific conductance, chloride, sulfate, and phosphorus all show consistent correlations with atmospheric predictors, including high and low temperature extremes, dry days, heavy precipitation events, and mean temperature and precipitation over time scales ranging from one week to two years. Based on this analysis and published future projections for this region, we expect climate change to increase water temperatures, decrease dissolved oxygen levels, decrease pH, increase specific conductance, and increase levels of sulfate, chloride in Texas reservoirs. Over decadal time scales, this may affect aquatic ecosystems in the reservoirs, including altering the risk of conditions conducive to algae occurrence, as well as affecting the quality of water available for human consumption and recreation.

Texas↗

USGS2016

Spatial and temporal patterns of dissolved organic matter quantity and quality in the Mississippi River Basin, 1997–2013

Recent studies have found insignificant or decreasing trends in time-series dissolved organic carbon (DOC) datasets, questioning the assumption that long-term DOC concentrations in surface waters are increasing in response to anthropogenic forcing, including climate change, land use, and atmospheric acid deposition. We used the weighted regressions on time, discharge, and season (WRTDS) model to estimate annual flow-normalized concentrations and fluxes to determine if changes in DOC quantity and quality signal anthropogenic forcing at 10 locations in the Mississippi River Basin. Despite increases in agriculture and urban development throughout the basin, net increases in DOC concentration and flux were significant at only 3 of 10 sites from 1997 to 2013 and ranged between −3.5% to +18% and −0.1 to 19%, respectively. Positive shifts in DOC quality, characterized by increasing specific ultraviolet absorbance at 254 nm, ranged between +8% and +45%, but only occurred at one of the sites with significant DOC quantity increases. Basinwide reductions in atmospheric sulfate deposition did not result in large increases in DOC either, likely because of the high buffering capacity of the soil. Hydroclimatic factors including annual discharge, precipitation, and temperature did not significantly change during the 17-year timespan of this study, which contrasts with results from previous studies showing significant increases in precipitation and discharge over a century time scale. Our study also contrasts with those from smaller catchments, which have shown stronger DOC responses to climate, land use, and acidic deposition. This temporal and spatial analysis indicated that there was a potential change in DOC sources in the Mississippi River Basin between 1997 and 2013. However, the overall magnitude of DOC trends was not large, and the pattern in quantity and quality increases for the 10 study sites was not consistent throughout the basin.

Hydrological Processes↗

USGS2020

Chronic and episodic acidification of streams along the Appalachian Trail corridor, eastern United States

Acidic atmospheric deposition has adversely affected aquatic ecosystems globally. As emissions and deposition of sulfur (S) and nitrogen (N) have declined in recent decades across North America and Europe, ecosystem recovery is evident in many surface waters. However, persistent chronic and episodic acidification remain important concerns in vulnerable regions. We evaluated acidification in 269 headwater streams during 2010–2012 along the Appalachian Trail (AT) that transits several ecoregions and is located downwind of high levels of S and N emission sources. Discharge was estimated by matching sampled streams to those of a nearby gaged stream and assuming equivalent daily mean flow percentiles. Charge balance acid‐neutralizing capacity (ANC) values were adjusted to the 15th (Q15) and 85th flow percentiles (Q85) by applying the ANC/discharge slope among sample pairs collected at each stream. A site‐based approach was applied to streams sampled twice or more and a second regression‐based approach to streams sampled once to estimate episodic acidification magnitudes as the ANC difference from Q15 to Q85. Streams with ANC <0 μeq/L doubled from 16% to 32% as discharge increased from Q15 to Q85 according to the site‐based approach. The proportion of streams with ANC <0 μeq/L at low flow and high flow decreased from north to south. Base cation dilution explained the greatest amount of episodic acidification among streams and variation in sulfate (SO 4 2− ) concentrations was a secondary explanatory variable. Episodic SO 4 2− patterns varied geographically with dilution dominant in northern streams underlain by soils developed in glacial sediment and increased concentrations dominant in southern streams with older, highly weathered soils. Episodic acidification increased as low‐flow ANC increased, exceeding 90 μeq/L in 25% of streams. Episodic increases in ANC were the dominant pattern in streams with low‐flow ANC values <30 μeq/L. Chronic and episodic acidification remain an ecological concern among AT streams. The approach developed here could be applied to estimate the magnitude and extent of chronic and episodic acidification in other regions recovering from decreasing levels of atmospheric S and N deposition.

Connecticut, Georgia, Maine, Massachusetts, Maryla↗

USGS2001

Effect of basin physical characteristics on solute fluxes in nine alpine/subalpine basins, Colorado, USA

Alpine/subalpine basins may exhibit substantial variability in solute fluxes despite many apparent similarities in basin characteristics. An evaluation of controls on spatial patterns in solute fluxes may allow development of predictive tools for assessing basin sensitivity to outside perturbations such as climate change or deposition of atmospheric pollutants. Relationships between basin physical characteristics, determined from geographical information system (GIS) tools, and solute fluxes and mineral weathering rates were explored for nine alpine/subalpine basins in Rocky Mountain National Park, Colorado, using correlation analyses for 1993 and 1994 data. Stream-water nitrate fluxes were correlated positively with basin characteristics associated with the talus environment; i.e., the fractional amounts of steep slopes (??? 30??), unvegetated terrain and young debris (primarily Holocene till) in the basins, and were correlated negatively with fractional amounts of subalpine meadow terrain. Correlations with nitrate indicate the importance of the talus environment in promoting nitrate flux and the mitigating effect of areas with established vegetation, such as subalpine meadows. Total mineral weathering rates for the basins ranged from about 300 to 600 mol ha-1 year -1. Oligoclase weathering accounted for 30 to 73% of the total mineral weathering flux, and was positively correlated with the amount of old debris (primarily Pleistocene glacial till) in the basins. Although calcite is found in trace amounts in bedrock, calcite weathering accounted for up to 44% of the total mineral weathering flux. Calcite was strongly correlated with steep slope, unvegetated terrain, and young debris-probably because physical weathering in steep-gradient areas exposes fresh mineral surfaces that contain calcite for chemical weathering. Oligoclase and calcite weathering are the dominant sources of alkalinity in the basins. However, atmospherically deposited acids consume much of the alkalinity generated by weathering of calcite and other minerals in the talus environment. Published in 2001 by John Wiley and Sons, Ltd.

Hydrological Processes↗

USGS2004

The application of an integrated biogeochemical model (PnET-BGC) to five forested watersheds in the Adirondack and Catskill regions of New York

PnET-BGC is an integrated biogeochemical model formulated to simulate the response of soil and surface waters in northern forest ecosystems to changes in atmospheric deposition and land disturbances. In this study, the model was applied to five intensive study sites in the Adirondack and Catskill regions of New York. Four were in the Adirondacks: Constable Pond, an acid-sensitive watershed; Arbutus Pond, a relatively insensitive watershed; West Pond, an acid-sensitive watershed with extensive wetland coverage; and Willy's Pond, an acid-sensitive watershed with a mature forest. The fifth was Catskills: Biscuit Brook, an acid-sensitive watershed. Results indicated model-simulated surface water chemistry generally agreed with the measured data at all five sites. Model-simulated internal fluxes of major elements at the Arbutus watershed compared well with previously published measured values. In addition, based on the simulated fluxes, element and acid neutralizing capacity (ANC) budgets were developed for each site. Sulphur budgets at each site indicated little retention of inputs of sulphur. The sites also showed considerable variability in retention of NO3-. Land-disturbance history and in-lake processes were found to be important in regulating the output of NO3- via surface waters. Deposition inputs of base cations were generally similar at these sites. Various rates of base cation outputs reflected differences in rates of base cation supply at these sites. Atmospheric deposition was found to be the largest source of acidity, and cation exchange, mineral weathering and in-lake processes served as sources of ANC. ?? 2004 John Wiley and Sons, Ltd.

Hydrological Processes↗

USGS2004

Evaluating the source of streamwater nitrate using δ 15 N and δ 18 O in nitrate in two watersheds in New Hampshire, USA

The natural abundance of nitrogen and oxygen isotopes in nitrate can be a powerful tool for identifying the source of nitrate in streamwater in forested watersheds, because the two main sources of nitrate, atmospheric deposition and microbial nitrification, have distinct δ 18 O values. Using a simple mixing model, we estimated the relative fractions in streamwater derived from these sources for two forested watersheds with markedly different streamwater nitrate outputs. In this study, we monitored δ 15 N and δ 18 O of nitrate biweekly in atmospheric deposition and in streamwater for 20 months at the Hubbard Brook Experimental Forest, New Hampshire, USA (moderate nitrogen export), and monthly in streamwater at the Bowl Research Natural Area, New Hampshire, USA (high nitrogen export). For rain, δ 18 O values ranged from +47 to +77&permil; (mean: +58&permil;) and δ 15 N from −5 to +1&permil; (mean: −3&permil;); for snow, δ 18 O values ranged from +52 to +75&permil; (mean: +67&permil;) and δ 15 N from −3 to +2&permil; (mean: −1&permil;). Streamwater nitrate, in contrast to deposition, had δ 18 O values between +12 and +33&permil; (mean: +18&permil;) and δ 15 N between −3 and +6&permil; (mean: 0&permil;). Since nitrate produced by nitrification typically has δ 18 O values ranging from −5 to +15&permil;, our field data suggest that most of the nitrate lost from the watersheds in streamflow was nitrified within the catchment. Our results confirm the importance of microbial nitrogen transformations in regulating nitrogen losses from forested ecosystems and suggest that hydrologic storage may be a factor in controlling catchment nitrate losses.

Hydrological Processes↗

USGS2006

The hydrology of northern peatlands as affected by biogenic gas: Current developments and research needs

Recent research indicates that accumulation and release of biogenic gas from northern peatlands may substantially affect future climate. Sudden release of free-phase gas bubbles into the atmosphere may preclude the conversion of methane to carbon dioxide in the uppermost oxic layer of the peat, resulting in greater contribution of methane to the atmosphere than is currently estimated. The hydrology of these peatlands also affects and is affected by this process, especially when gas is released suddenly and episodically. Indirect hydrological evidence indicates that ebullitive gas releases are relatively frequent in some peatlands and time-averaged rates may be significantly greater than diffusive releases. Estimates of free-phase gas contained in peat have ranged from 0 to nearly 20% of the peat volume. Abrupt changes in the volume of gas may alter hydraulic gradients and movement of water and solutes in peat, which in turn could alter composition and fluxes of the gas. Peat surfaces also move vertically and horizontally in response to accumulation and release of free-phase gas. Future research should address the distribution, temporal variability, and relative significance of ebullition in peatlands and the consequent hydrological responses to these gas-emission events. Copyright ?? 2006 John Wiley & Sons, Ltd.

Hydrological Processes↗

USGS2013

Representing the effects of alpine grassland vegetation cover on the simulation of soil thermal dynamics by ecosystem models applied to the Qinghai-Tibetan Plateau

Soil surface temperature is a critical boundary condition for the simulation of soil temperature by environmental models. It is influenced by atmospheric and soil conditions and by vegetation cover. In sophisticated land surface models, it is simulated iteratively by solving surface energy budget equations. In ecosystem, permafrost, and hydrology models, the consideration of soil surface temperature is generally simple. In this study, we developed a methodology for representing the effects of vegetation cover and atmospheric factors on the estimation of soil surface temperature for alpine grassland ecosystems on the Qinghai-Tibetan Plateau. Our approach integrated measurements from meteorological stations with simulations from a sophisticated land surface model to develop an equation set for estimating soil surface temperature. After implementing this equation set into an ecosystem model and evaluating the performance of the ecosystem model in simulating soil temperature at different depths in the soil profile, we applied the model to simulate interactions among vegetation cover, freeze-thaw cycles, and soil erosion to demonstrate potential applications made possible through the implementation of the methodology developed in this study. Results showed that (1) to properly estimate daily soil surface temperature, algorithms should use air temperature, downward solar radiation, and vegetation cover as independent variables; (2) the equation set developed in this study performed better than soil surface temperature algorithms used in other models; and (3) the ecosystem model performed well in simulating soil temperature throughout the soil profile using the equation set developed in this study. Our application of the model indicates that the representation in ecosystem models of the effects of vegetation cover on the simulation of soil thermal dynamics has the potential to substantially improve our understanding of the vulnerability of alpine grassland ecosystems to changes in climate and grazing regimes.

Qinghai;Tibet↗

USGS2012

Spring onset variations and trends in the continental United States: past and regional assessment using temperature-based indices

Phenological data are simple yet sensitive indicators of climate change impacts on ecosystems, but observations have not been made routinely or extensively enough to evaluate spatial and temporal patterns across most continents, including North America. As an alternative, many studies use weather-based algorithms to simulate speciﬁc phenological responses. Spring Indices (SI) are a set of complex phenological models that have been successfully applied to evaluate variations and trends in the onset of spring across the Northern Hemisphere’s temperate regions. To date, SI models have been limited by only producing output in locations where both the plants’ chilling and warmth requirements are met. Here, we develop an extended form of the SI (abbreviated SI-x) that expands their application into the subtropics by ignoring chilling requirements while still retaining the utility and accuracy of the original SI (now abbreviated SI-o). The validity of the new indices is tested, and regional SI anomalies are explored across the data-rich continental United States. SI-x variations from 1900 to 2010 show an abrupt and sustained delay in spring onset of about 4–8 d (around 1958) in parts of the Southeast and southern Great Plains, and a comparable advance of 4–8 d (around 1984) in parts of the northern Great Plains and the West. Atmospheric circulation anomalies, linked to large-scale modes of variability, exert modest but signiﬁcant roles in the timing of spring onset across the United States on interannual and longer timescales. The SI-x are promising metrics for tracking spring onset variations and trends in mid-latitudes, relating them to relevant ecological, hydrological, and socioeconomic phenomena, and exploring connections between atmospheric drivers and seasonal timing.

International Journal of Climatology↗

USGS2026

Aquatic reflectance derived from Sentinel-2 Multispectral Imager data for inland waters in the conterminous United States

Satellite-based earth observation is a robust tool for tracking change in ecosystems. While terrestrially focused applications of remote sensing have empowered wide adoption for research and management, remote sensing of inland aquatic ecosystems remains comparably nascent. This divergence, in part, stems from the lack of standardized, accessible, and near real-time remotely sensed surface reflectance, atmospherically corrected for aquatic environments. To date, surface reflectance products at national scales and with minimal latency are typically designed exclusively for terrestrial environments. Rectifying this situation can be accomplished by applying aquatic-focused atmospheric correction algorithms independent of those used for terrestrial ecosystems. As a first step to filling this data gap, we present the first national scale, dynamically updated, analysis-ready, aquatic reflectance dataset for inland water derived from Sentinel-2 for the conterminous United States.

conterminous United States↗

USGS2018

Examining the role of unusually warm Indo‐Pacific sea‐surface temperatures in recent African droughts

Southern Africa (SA) and eastern Africa (EA) experienced a sequence of severe droughts in December–February (SA DJF) 2015–2016, October–December (EA OND) 2016 and March–April–May 2017 (EA MAM). This sequence contributed to severe food insecurity. While climate variability in these regions is very complex, the goal of this study is to analyse the role played by unusually warm Indo–Pacific SSTs, where unusual is defined as a 1‐in‐6 year event. We use observed sea‐surface temperatures (SST) and satellite–gauge rainfall observations, a 20‐member ensemble of Community Atmospheric Model version 5.1 simulations (CAM5), and a 40‐member ensemble of climate change simulations from the Community Earth Systems Model version 1 (CESM1) Large Ensemble Community Project (LENS) to explore climate conditions associated with warm events identified based on eastern and western Pacific SSTs. Our analysis suggests that strong El Niño's may be followed by warm western Pacific SST conditions, which can lead to conditions conducive to successive and potentially predictable droughts in SA DJF, EA OND and EA MAM. We show that different regions of warm SST are related to recent droughts—SA DJF: Niño 3.4; EA OND: western equatorial Pacific (WEP); and EA MAM: western North Pacific (WNP). For DJF and MAM, respectively, the CAM5 model driven with observed SST and the same model driven within a climate change experiment indicate that warmer El Niño's and WNP events produce more intense atmospheric responses, potentially associated with more severe droughts. OND climate seems to be strongly influenced by the Indian Ocean Dipole, which corresponds with some WEP events. Given global warming, we suggest that the extreme Niño 3.4 and west Pacific SST events responsible for 2015–2017 droughts are likely to reoccur, thus humanitarian agencies should prepare to predict and respond to multi‐year drought and substantial food insecurity in SA and EA.

Journal of the Royal Meteorological Society↗

USGS2008

Effects of nitrate and water on the oxygen isotopic analysis of barium sulfate precipitated from water samples

BaSO 4 precipitated from mixed salt solutions by common techniques for SO isotopic analysis may contain quantities of H 2 O and NO that introduce errors in O isotope measurements. Experiments with synthetic solutions indicate that δ 18 O values of CO produced by decomposition of precipitated BaSO 4 in a carbon reactor may be either too low or too high, depending on the relative concentrations of SO and NO and the δ 18 O values of the H 2 O, NO , and SO . Typical δ 18 O errors are of the order of 0.5 to 1‰ in many sample types, and can be larger in samples containing atmospheric NO , which can cause similar errors in δ 17 O and Δ 17 O. These errors can be reduced by (1) ion chromatographic separation of SO from NO , (2) increasing the salinity of the solutions before precipitating BaSO 4 to minimize incorporation of H 2 O, (3) heating BaSO 4 under vacuum to remove H 2 O, (4) preparing isotopic reference materials as aqueous samples to mimic the conditions of the samples, and (5) adjusting measured δ 18 O values based on amounts and isotopic compositions of coexisting H 2 O and NO . These procedures are demonstrated for SO isotopic reference materials, synthetic solutions with isotopically known reagents, atmospheric deposition from Shenandoah National Park, Virginia, USA, and sulfate salt deposits from the Atacama Desert, Chile, and Mojave Desert, California, USA. These results have implications for the calibration and use of O isotope data in studies of SO sources and reaction mechanisms.

Rapid Communications in Mass Spectrometry↗

USGS2011

Investigation of preparation techniques for δ2H analysis of keratin materials and a proposed analytical protocol

Accurate hydrogen isotopic measurements of keratin materials have been a challenge due to exchangeable hydrogen in the sample matrix and the paucity of appropriate isotopic reference materials for calibration. We found that the most reproducible δ 2 H VSMOW‐SLAP and mole fraction of exchangeable hydrogen, x (H) ex , of keratin materials were measured with equilibration at ambient temperature using two desiccators and two different equilibration waters with two sets of the keratin materials for 6 days. Following equilibration, drying the keratin materials in a vacuum oven for 4 days at 60 °C was most critical. The δ 2 H analysis protocol also includes interspersing isotopic reference waters in silver tubes among samples in the carousel of a thermal conversion elemental analyzer (TC/EA) reduction unit. Using this analytical protocol, δ 2 H VSMOW‐SLAP values of the non‐exchangeable fractions of USGS42 and USGS43 human‐hair isotopic reference materials were determined to be –78.5 ± 2.3 ‰ and –50.3 ± 2.8 ‰, respectively. The measured x (H) ex values of keratin materials analyzed with steam equilibration and N 2 drying were substantially higher than those previously published, and dry N 2 purging was unable to remove absorbed moisture completely, even with overnight purging. The δ 2 H values of keratin materials measured with steam equilibration were about 10 ‰ lower than values determined with equilibration in desiccators at ambient temperatures when on‐line evacuation was used to dry samples. With steam equilibrations the x (H) ex of commercial keratin powder was as high as 28 %. Using human‐hair isotopic reference materials to calibrate other keratin materials, such as hoof or horn, can introduce bias in δ 2 H measurements because the amount of absorbed water and the x (H) ex values may differ from those of unknown samples. Correct δ 2 H VSMOW‐SLAP values of the non‐exchangeable fractions of unknown human‐hair samples can be determined with atmospheric moisture equilibration by normalizing with USGS42 and USGS43 human‐hair reference materials when all materials have the same powder size.

Rapid Communications in Mass Spectrometry↗