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USGS2024

Microplastic and associated black particles from road-tire wear: Implications for radiative effects across the cryosphere and in the atmosphere

The environmental effects of airborne micro- and nano-size plastic particles are poorly understood. Microscopy and chemical analyses of atmospherically deposited particles on snow surfaces at high elevation (2,865–3,690 m) in the Upper Colorado River basin (UCRB; Colorado Rocky Mountains) revealed the presence of black substances intimately associated with microplastic fibers, particles interpreted to have originated as tire matter. Identical and similar particles occur in shredded tires and road-surface samples. The substance responsible for the black color of all tires is carbon black, a graphitic light-absorbing tire additive produced by hydrocarbon combustion that homogeneously permeates the mixture of tire polymers and other additives. Such black tire matter may thus exert radiative effects closely similar to those of black carbon. The presence in snow of many organic compound types common to tires, measured by two-dimensional gas chromatography, suggests that atmospherically deposited black road-tire-wear matter is among the light-absorbing particulates that advance the onset and rate of snow melt in the UCRB. The mass of road-tire-wear particles shed from vehicles may be estimated by multiplying measured amounts of eroded tire-per-distance traveled by vehicular distances. Under a combination of measurements and assumptions about the amounts and radiative properties of atmospheric tire-wear particles, the radiative effects of these particles might add about 10%–30% to those effects from black carbon, an estimate ripe for revision. On regional and global scales, the amounts and effects of emitted and deposited tire-wear matter likely vary by factors of geographic source, transport pathway, and depositional setting.

JGR - Atmospheres↗

USGS2007

Relations between winter precipitation and atmospheric circulation simulated by the Geophysical Fluid Dynamics Laboratory general circulation model

General circulation model (GCM) simulations of atmospheric circulation are more reliable than GCM simulations of temperature and precipitation. In this study, temporal correlations between 700 hPa height anomalies simulated winter precipitation at eight locations in the conterminous United States are compared with corresponding correlations in observations. The objectives are to 1) characterize the relations between atmospheric circulation and winter precipitation simulated by the GFDL, GCM for selected locations in the conterminous USA, ii) determine whether these relations are similar to those found in observations of the actual climate system, and iii) determine if GFDL-simulated precipitation is forced by the same circulation patterns as in the real atmosphere. -from Authors

International Journal of Climatology↗

USGS1991

Atmospheric deposition and solute export in giant sequoia: Mixed conifer watersheds in the Sierra Nevada, California

Atmospheric depostion and stream discharge and solutes were measured for three years (September 1984 — August 1987) in two mixed conifer watersheds in Sequoia National Park, in the southern Sierra Nevada of California. The Log Creek watershed (50 ha, 2067–2397 m elev.) is drained by a perennial stream, while Tharp's Creek watershed (13 ha, 2067–2255 m elev.) contains an intermittent stream. Dominant trees in the area include Abies concolor (white fir), Sequoiadendron giganteum (giant sequoia), A. magnifica (red fir), and Pinus lambertiana (sugar pine). Bedrock is predominantly granite and granodiorite, and the soils are mostly Pachic Xerumbrepts. Over the three year period, sulfate (SO 4 2− ), nitrate (NO 3 − ), and chloride (Cl −1 ) were the major anions in bulk precipitation with volume-weighted average concentrations of 12.6, 12.3 and 10.0 μeq/1, respectively. Annual inputs of NO 3 -N, NH 4 -N and SO 4 -S from wet deposition were about 60 to 75% of those reported from bulk deposition collectors. Discharge from the two watersheds occurs primarily during spring snowmelt. Solute exports from Log and Tharp's Creeks were dominated by HCO 3 − , Ca 2+ and Na + , while H + , NO 3 − , NH 4 + and PO 4 3− outputs were relatively small. Solute concentrations were weakly correlated with instantaneous stream flow for all solutes (r 2 <0.2) except HCO 3 − (Log Cr. r 2 = 0.72; Tharp's Cr. r 2 = 0.38), Na + (Log Cr. r 2 = 0.56; Tharp's Cr. r 2 = 0.47), and silicate (Log Cr. r 2 = 0.71; Tharp's Cr. r 2 = 0.49). Mean annual atmospheric contributions of NO 3 -N (1.6 kg ha −1 ), NH 4 -N (1.7 kg ha −1 ), and SO 4 -S (1.8 kg ha −1 ), which are associated with acidic deposition, greatly exceed hydrologic losses. Annual watershed yields (expressed as eq ha −1 ) of HCO 3 − exceeded by factors of 2.5 to 37 the annual atmospheric deposition of H + .

California↗

USGS1989

Recent increases in atmospheric deposition of mercury to North-Central Wisconsin lakes inferred from sediment analyses

Profiles of total mercury (Hg) concentrations in sediments were examined in 11 lakes in north-central Wisconsin having a broad range of pH (5.1 to 7.8) and alkalinity (-12 to 769 μeq/L). Mercury concentrations were greatest in the top 15 cm of the cores and were much lower in the deeper strata. The Hg content in the most enriched stratum of individual cores ranged from 0.09 to 0.24 μg/g dry weight, whereas concentrations in deep, precolonial strata ranged from 0.04 to 0.07 μg/g. Sediment enrichment factors varied from 0.8 to 2.8 and were not correlated with lake pH. The increase in the Hg content of recent sediments was attributed to increased atmospheric deposition of the metal. Eight of the 11 systems studied were low-alkalinity lakes that presumably received most (≥90%) of their hydrologic input from precipitation falling directly onto the lake surface. Thus, the sedimentary Hg in these lakes seems more likely linked to direct atmospheric deposition onto the lake surfaces than to influxes from the watershed. The data imply that a potentially significant fraction of the high Hg burdens measured in game fish in certain lakes in north-central Wisconsin originated from atmospheric sources.

Wisconsin↗

USGS2016

Climate-change signals in national atmospheric deposition program precipitation data

National Atmospheric Deposition Program (NADP)/National Trends Network precipitation type, snow-season duration, and annual timing of selected chemical wet-deposition maxima vary with latitude and longitude within a 35-year (1979–2013) data record for the contiguous United States and Alaska. From the NADP data collected within the region bounded by 35.6645°–48.782° north latitude and 124°–68° west longitude, similarities in latitudinal and longitudinal patterns of changing snow-season duration, fraction of annual precipitation recorded as snow, and the timing of chemical wet-deposition maxima, suggest that the chemical climate of the atmosphere is linked to physical changes in climate. Total annual precipitation depth has increased 4–6 % while snow season duration has decreased from approximately 7 to 21 days across most of the USA, except in higher elevation regions where it has increased by as much as 21 days. Snow-season precipitation is increasingly comprised of snow, but annually total precipitation is increasingly comprised of liquid precipitation. Meanwhile, maximum ammonium deposition occurs as much as 27 days earlier, and the maximum nitrate: sulfate concentration ratio in wet-deposition occurs approximately 10–21 days earlier in the year. The maximum crustal (calcium + magnesium + potassium) cation deposition occurs 2–35 days earlier in the year. The data suggest that these shifts in the timing of atmospheric wet deposition are linked to a warming climate, but the ecological consequences are uncertain.

Climate Dynamics↗

USGS2020

Evolving infrasound detections from Bogoslof volcano, Alaska: Insights from atmospheric propagation modeling

Bogoslof volcano, a back-arc volcano in Alaska’s Aleutian arc, began an eruptive sequence in mid-December 2016 that ended in late August 2017, with 70 individual eruptive episodes. Because there were no local seismic or infrasound stations on the island, the Alaska Volcano Observatory (AVO) relied on distant geophysical networks and remote sensing techniques to assess activity during the eruption. AVO maintains six infrasound arrays to monitor activity along the Aleutian arc: Adak, the Island of Four Mountains, Okmok, Akutan, Sand Point, and Dillingham. Eruption detection at infrasound arrays is subject to local as well as mesoscale meteorological conditions that vary greatly over both short and long timescales. Infrasound detections from the array nearest to Bogoslof (Okmok), with a latency of about 3 min, played a crucial role in monitoring activity during the eruption. Despite the relative proximity of the Okmok array to Bogoslof (60 km), infrasound detections were not uniformly observed with only about two-thirds of the events successfully detected. The farthest array at Dillingham (816 km) detected approximately half of the explosive events, with all other arrays detecting less than half of the events. We compare observations with infrasound propagation model predictions, using both normal mode and parabolic equation forward models, to interpret the variation in detections of the 70 explosive events across the AVO infrasound network. The forward models utilize the newly created, publicly available AVO-G2S atmospheric reconstruction using numerical weather predictions data for the lower atmosphere, coupled with upper atmosphere empirical models of wind speeds and temperature. We find that long-range detections (> 100 km) of Bogoslof events are largely aligned with seasonal variability in favorable propagation conditions, while regional detections (< 100 km) are less consistent with propagation modeling. Understanding the output of numerical models in comparison to past observations will facilitate their use in future operational settings for AVO and other observatories.

Alaska↗

USGS2013

Climate, not atmospheric deposition, drives the biogeochemical mass-balance of a mountain watershed

Watershed mass-balance methods are valuable tools for demonstrating impacts to water quality from atmospheric deposition and chemical weathering. Owen Bricker, a pioneer of the mass-balance method, began applying mass-balance modeling to small watersheds in the late 1960s and dedicated his career to expanding the literature and knowledge of complex watershed processes. We evaluated long-term trends in surface-water chemistry in the Loch Vale watershed, a 660-ha. alpine/subalpine catchment located in Rocky Mountain National Park, CO, USA. Many changes in surface-water chemistry correlated with multiple drivers, including summer or monthly temperature, snow water equivalent, and the runoff-to-precipitation ratio. Atmospheric deposition was not a significant causal agent for surface-water chemistry trends. We observed statistically significant increases in both concentrations and fluxes of weathering products including cations, SiO 2 , SO 4 2− , and ANC, and in inorganic N, with inorganic N being primarily of atmospheric origin. These changes are evident in the individual months June, July, and August, and also in the combined June, July, and August summer season. Increasingly warm summer temperatures are melting what was once permanent ice and this may release elements entrained in the ice, stimulate chemical weathering with enhanced moisture availability, and stimulate microbial nitrification. Weathering rates may also be enhanced by sustained water availability in high snowpack years. Rapid change in the flux of weathering products and inorganic N is the direct and indirect result of a changing climate from warming temperatures and thawing cryosphere.

Loch Vale Watershed, Rock Mountain National Park↗

USGS2010

Response of lake chemistry to changes in atmospheric deposition and climate in three high-elevation wilderness areas of Colorado

Trends in precipitation chemistry and hydrologic and climatic data were examined as drivers of long-term changes in the chemical composition of high-elevation lakes in three wilderness areas in Colorado during 1985–2008. Sulfate concentrations in precipitation decreased at a rate of −0.15 to −0.55 μeq/l/year at 10 high-elevation National Atmospheric Deposition Program stations in the state during 1987–2008 reflecting regional reductions in SO 2 emissions. In lakes where sulfate is primarily derived from atmospheric inputs, sulfate concentrations also decreased although the rates generally were less, ranging from −0.12 to −0.27 μeq/l/year. The similarity in timing and sulfur isotopic data support the hypothesis that decreases in atmospheric deposition are driving the response of high-elevation lakes in some areas of the state. By contrast, in lakes where sulfate is derived primarily from watershed weathering sources, sulfate concentrations showed sharp increases during 1985–2008. Analysis of long-term climate records indicates that annual air temperatures have increased between 0.45 and 0.93°C per decade throughout most mountainous areas of Colorado, suggesting climate as a factor. Isotopic data reveal that sulfate in these lakes is largely derived from pyrite, which may indicate climate warming is preferentially affecting the rate of pyrite weathering.

Colorado↗

USGS1982

Phase relations in the system NaCl-KCl-H2O. Part I: Differential thermal analysis of the NaCl-KCl liquidas at 1 atmosphere and 500, 1000, 1500, and 2000 bars

A simple differential thermal analysis (DTA) technique has been developed to study phase relations of various chemical systems at elevated pressures and temperatures. The DTA system has been calibrated against known melting temperatures in the system NaCl-KCl. Isobaric sections of the liquidus in the system NaCl-KCl have been determined at pressures of 1 atmosphere and 500, 1000, 1500, and 2000 bars. Using the least-squares method, the following equation was used to fit the experimental data: T(°C)= ∑ i=0 6 a i X i KCl "> T(°C)=∑i=06a i X i KCl where T is the liquidus temperature, X KCl is mole fraction of KCl, and a i (listed below) are the derived empirical constants. P (bars) a o "> ao a 1 "> a1 a 2 "> a2 a 3 "> a3 a 4 "> a4 a 5 "> a5 a 6 "> a6 1 atm. 800.1 −334.2 781.6 −6490.3 17553.1 −17638.4 6098.3 500 813.5 −354.9 743.3 −6011.7 16406.4 −16516.3 5702.8 1000 824.5 −406.7 1446.8 −8818.4 21253.5 −20343.7 6839.4 1500 838.6 −418.7 1434.7 −8819.0 21557.9 −20908.4 7123.1 2000 848.5 −381.5 1246.9 −8605.0 21785.8 −21449.1 7375.8 The liquidus temperatures estimated from these equations are within ±3°C of experimental values. The measured liquidus temperatures at 1 atmosphere agree with the best available data to within 5°C. The melting temperatures for pure end members at higher pressures agree with the values calculated from the Simon equation (Clark, 1959) to within 3°C. No previous melting data are available for the intermediate compositions at elevated pressures. Using the data in both heating and cooling scans, the minimum melting temperature at 1 atmosphere in the system was located at 658° ± 3° C where the sample has an equimolar composition.

Geochimica et Cosmochimica Acta↗

USGS1972

The martian atmosphere: Mariner 9 television experiment progress report

Atmospheric phenomena appearing in the Mariner 9 television pictures are discussed in detail. The surface of the planet was heavily obscured by a global dust storm during the first month in orbit. Brightness data during this period can be fitted by a semi-infinite scattering and absorbing atmosphere model with a single-scattering albedo in the range 0.70–0.85. This low value suggests that the mean radius of the particles responsible for the obscuration was at least 10 μm. By the end of the second month, this dust storm had largely dissipated, leaving a residual optical depth ∼0.1. Much of the region north of 45°N was covered by variable clouds comprising the north polar hood. The cloud structures revealed extensive systems of lee waves generated by west-to-east flow over irregular terrain. Extensive cloud systems in this region resembled baroclinic wave cyclones. Clouds were also observed over several of the large calderas; these clouds are believed to contain water ice. Several localized dust storms were seen after the global dust storm cleared. These dust clouds appeared to be intensely convective. The convective nature of these storms and the stirring of large dust particles to great heights can be explained by vertical velocities generated by the absorption of solar radiation by the dusty atmosphere.

Icarus↗

USGS1991

40Ar/39Ar systematics and argon diffusion in amber: Implications for ancient earth atmospheres

Argon isotope data indicate retained argon in bulk amber (matrix gas) is radiogenic [ 40 Ar/ 39 Ar ≃32o] than the much more abundant surface absorbed argon [ 40 Ar/ 39 Ar ≃295.5]. Neutron-induced 39 Ar is retained in amber during heating experiments to 150° -250°C, with no evidence of recoiled 39 Ar found after irradiation. A maximum permissible volume diffusion coefficient of argon in amber (at ambient temperature) D≤1.5 x 10 −17 cm 2 S −1 is calculated from 39 Ar retention. 40 Ar/ 39 Ar age calculations indicate Dominican Republic amber is ≃ 45 Ma and North Dakota amber is ≃ 89 Ma, both at least reasonable ages for the amber based upon stratigraphic and paleontological constraints and upon the small amount of radiogenic 40 Ar. To date, over 300 gas analyses of ambers and resins of Cretaceous to Recent age that are geographically distributed among fifteen noted world locations identify mixtures of gases in different sites within amber (Berner and Landis, 1988). The presence of multiple mixing trends between compositionally distinct end-members gases within the same sample and evidence for retained radiogenic argon within the amber argue persuasivley against rapid exchange by diffusion of amber-contained gases with moder air. Only gas in primary bubbles entrapped between successive flows of tree resin has been interpreted as original “ancient air”, which is an O 2 -rich end-member gas with air-like N 2 /Ar ratios. Gas analyses of these primary bubbles indicate atmospheric O 2 levels in the Late Cretaceous of ≃ 35%, and that atmospheric O 2 dropped by early Tertiary time to near a present atmospheric level of 21% O 2 . A very low argon diffusion coefficient in amber persuasively argues for a gas in primary bubbles trapped in amber being ancient air (possibly modified only by O 2 reaction with amber).

North Dakota↗

USGS2008

Canopy structure and atmospheric flows in relation to the δ13C of respired CO 2 in a subalpine coniferous forest

Stable isotopes provide insight into ecosystem carbon cycling, plant physiological processes, atmospheric boundary-layer dynamics, and are useful for the integration of processes over multiple scales. Of particular interest is the carbon isotope content (δ 13 C) of nocturnal ecosystem-respired CO 2 ( δ R ). Recent advances in technology have made it possible to continuously examine the variation in δ R within a forest canopy over relatively long time-scales (months–years). We used tunable diode laser spectroscopy to examine δ R at within- and below-canopy spatial locations in a Colorado subalpine forest (the Niwot Ridge AmeriFlux site). We found a systematic pattern of increased δ R within the forest canopy ( δ R-c ) compared to that near the ground ( δ R-g ). Values of δ R-c were weakly correlated with the previous day's mean maximum daytime vapor pressure deficit (VPD). Conversely, there was a negative but still weak correlation between δ R-g and time-lagged (0–5 days) daily mean soil moisture. The topography and presence of sustained nightly drainage flows at the Niwot Ridge forest site suggests that, on nights with stable atmospheric conditions, there is little mixing of air near the ground with that in the canopy. Atmospheric stability was assessed using thresholds of friction velocity, stability above the canopy, and bulk Richardson number within the canopy. When we selectively calculated δ R-g and δ R-c by removing time periods when ground and canopy air were well mixed, we found stronger correlations between δ R-c and VPD, and δ R-g and soil moisture. This suggests that there may be fundamental differences in the environmental controls on δ R at sub-canopy spatial scales. These results may help explain the wide variance observed in the correlation of δ R with different environmental parameters in other studies.

Colorado↗

USGS2018

On the exchange of sensible and latent heat between the atmosphere and melting snow

The snow energy balance is difficult to measure during the snowmelt period, yet critical for predictions of water yield in regions characterized by snow cover. Robust simplifications of the snowmelt energy balance can aid our understanding of water resources in a changing climate. Research to date has demonstrated that the net turbulent flux ( F T ) between a melting snowpack and the atmosphere is negligible if the sum of atmospheric vapor pressure ( e a ) and temperature ( T a ) equals a constant, but it is unclear how frequently this situation holds across different sites. Here, we quantified the contribution of F T to the snowpack energy balance during 59 snowmelt periods across 11 sites in the FLUXNET2015 database with a detailed analysis of snowmelt in subarctic tundra near Abisko, Sweden. At the Abisko site we investigated the frequency of occurrences during which sensible heat flux ( H ) and latent heat flux ( λE ) are of (approximately) equal but opposite sign, and if the sum of these terms, F T , is therefore negligible during the snowmelt period. H approximately equaled - λE for less than 50% of the melt period and F T was infrequently a trivial term in the snowmelt energy balance at Abisko. The reason is that the relationship between observed e a and T a is roughly orthogonal to the “line of equality” at which H equals - λE as warmer T a during the melt period usually resulted in greater e a . This relationship holds both within melt periods at individual sites and across different sites in the FLUXNET2015 database, where F T comprised less than 20% of the energy available to melt snow, Q m , in 44% of the snowmelt periods studied here. F T / Q m was significantly related to the mean e a during the melt period, but not mean T a , and F T tended to be near 0 W m −2 when e a averaged ca . 0.5 kPa. F T may become an increasingly important term in the snowmelt energy balance across many global regions as warmer temperatures are projected to cause snow to melt more slowly and earlier in the year under conditions of lower net radiation ( R n ). Eddy covariance research networks such as Ameriflux must improve their ability to observe cold-season processes to enhance our understanding of water resources and surface-atmosphere exchange in a changing climate.

Agricultural and Forest Meteorology↗

USGS2015

Tectonic activity as a significant source of crustal tetrafluoromethane emissions to the atmosphere: observations in groundwaters along the San Andreas Fault

Tetrafluoromethane (CF4) concentrations were measured in 14 groundwater samples from the Cuyama Valley, Mil Potrero and Cuddy Valley aquifers along the Big Bend section of the San Andreas Fault System (SAFS) in California to assess whether tectonic activity in this region is a significant source of crustal CF4 to the atmosphere. Dissolved CF4 concentrations in all groundwater samples but one were elevated with respect to estimated recharge concentrations including entrainment of excess air during recharge (CreCre; &sim;30 fmol kg−1 H2O), indicating subsurface addition of CF4 to these groundwaters. Groundwaters in the Cuyama Valley contain small CF4 excesses (0.1–9 times CreCre), which may be attributed to an in situ release from weathering and a minor addition of deep crustal CF4 introduced to the shallow groundwater through nearby faults. CF4 excesses in groundwaters within 200 m of the SAFS are larger (10–980 times CreCre) and indicate the presence of a deep crustal flux of CF4 that is likely associated with the physical alteration of silicate minerals in the shear zone of the SAFS. Extrapolating CF4 flux rates observed in this study to the full extent of the SAFS (1300 km × 20–100 km) suggests that the SAFS potentially emits (0.3–1)×10−1 kg(0.3–1)×10−1 kg CF4 yr−1 to the Earth's surface. For comparison, the chemical weathering of &sim;7.5×104 km2&sim;7.5×104 km2 of granitic rock in California is estimated to release (0.019–3.2)×10−1 kg(0.019–3.2)×10−1 kg CF4 yr−1. Tectonic activity is likely an important, and potentially the dominant, driver of natural emissions of CF4 to the atmosphere. Variations in preindustrial atmospheric CF4 as observed in paleo-archives such as ice cores may therefore represent changes in both continental weathering and tectonic activity, including changes driven by variations in continental ice cover during glacial–interglacial transitions.

California↗

USGS2016

Sensitivity of Pliocene Arctic climate to orbital forcing, atmospheric CO 2 and sea ice albedo parameterisation

General circulation model (GCM) simulations of the mid-Pliocene Warm Period (mPWP, 3.264 to 3.025 Myr ago) do not reproduce the magnitude of Northern Hemisphere high latitude surface air and sea surface temperature (SAT and SST) warming that proxy data indicate. There is also large uncertainty regarding the state of sea ice cover in the mPWP. Evidence for both perennial and seasonal mPWP Arctic sea ice is found through analyses of marine sediments, whilst in a multi-model ensemble of mPWP climate simulations, half of the ensemble simulated ice-free summer Arctic conditions. Given the strong influence that sea ice exerts on high latitude temperatures, an understanding of the nature of mPWP Arctic sea ice would be highly beneficial. Using the HadCM3 GCM, this paper explores the impact of various combinations of potential mPWP orbital forcing, atmospheric CO 2 concentrations and minimum sea ice albedo on sea ice extent and high latitude warming. The focus is on the Northern Hemisphere, due to availability of proxy data, and the large data–model discrepancies in this region. Changes in orbital forcings are demonstrated to be sufficient to alter the Arctic sea ice simulated by HadCM3 from perennial to seasonal. However, this occurs only when atmospheric CO 2 concentrations exceed 300 ppm. Reduction of the minimum sea ice albedo from 0.5 to 0.2 is also sufficient to simulate seasonal sea ice, with any of the combinations of atmospheric CO 2 and orbital forcing. Compared to a mPWP control simulation, monthly mean increases north of 60°N of up to 4.2 °C (SST) and 9.8 °C (SAT) are simulated. With varying CO 2 , orbit and sea ice albedo values we are able to reproduce proxy temperature records that lean towards modest levels of high latitude warming, but other proxy data showing greater warming remain beyond the reach of our model. This highlights the importance of additional proxy records at high latitudes and ongoing efforts to compare proxy signals between sites.

Earth and Planetary Science Letters↗

USGS2004

Long term atmospheric deposition as the source of nitrate and other salts in the Atacama Desert, Chile: New evidence from mass-independent oxygen isotopic compositions

Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous 17 O enrichments in both minerals. Δ 17 O values of 14–21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ 17 O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ 17 O and δ 18 O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies.

Geochimica et Cosmochimica Acta↗

USGS2024

Revisiting the physical processes controlling the tropical atmospheric circulation changes during the Mid-Piacenzian Warm Period

The Mid-Piacenzian Warm Period (MPWP; 3.0–3.3 Ma), a warm geological period about three million years ago, has been deemed as a good past analog for understanding the current and future climate change. Based on 12 climate model outputs from Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), we investigate tropical atmospheric circulation (TAC) changes under the warm MPWP and associated underlying mechanisms by diagnosing both atmospheric static stability and diabatic processes. Our findings underscore the advantage of analyzing atmospheric diabatic processes in elucidating seasonal variations of TAC compared to static stability assessments. Specifically, by diagnosing alterations in diabatic processes, we achieve a quantitative understanding and explanation the following TAC changes (incl. Strength and edge) during the MPWP: the weakened (annual, DJF, JJA) Northern Hemisphere and (DJF) Southern Hemisphere Hadley circulation (HC), reduced (annual, DJF) Pacific Walker circulation (PWC) and enhanced (annual, JJA) Southern Hemisphere HC and (JJA) PWC, and westward shifted (annual, DJF, JJA) PWC. We further addressed that the increasing bulk subtropical static stability and/or decreasing vertical shear of subtropical zonal wind - two crucial control factors for changes in subtropical baroclinicity - may promote HC widening, and vice versa. Consequently, our study of spatial diabatic heating and cooling, corresponding to upward and downward motions within the TAC, respectively, provides a new perspective for understanding the processes controlling seasonal TAC changes in response to surface warming.

Quaternary International↗

USGS2021

Persistent nitrate in alpine waters with changing atmospheric deposition and warming trends

Nitrate concentrations in high-elevation lakes of the Colorado Front Range remain elevated despite declining trends in atmospherically deposited nitrate since 2000. The current source of this elevated nitrate in surface waters remains elusive, given shifts in additional nitrogen sources via glacial inputs and atmospheric ammonium deposition. We present the complete isotopic composition of nitrate (δ 15 N, δ 18 O, and Δ 17 O) from a suite of nitrate-bearing source waters collected during the summers of 2017–2018 from two alpine ecosystems to constrain the provenance of elevated nitrate in surface waters during the summer open-water season. The results indicate a consistent contribution of uncycled atmospheric nitrate throughout the summer (13–23%) to alpine lakes, despite seasonal changes in source water inputs. The balance of nitrate (as high as 87% in late summer) is likely from nitrate production within the catchment via nitrification of reduced nitrogen sources (e.g., thawed soil organic matter and ammonium deposition) and released with rock glacier meltwater. The role of microbially produced nitrate has become increasingly important over time based on historical surface water samples from the mid-90s to present, a trend coincident with increasing ammonium deposition to alpine systems.

Colorado↗