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At least 721 records · Page 40Linked to original sources

USGS2014

Coupled hydrological and biogeochemical processes controlling variability of nitrogen species in streamflow during autumn in an upland forest

Autumn is a season of dynamic change in forest streams of the northeastern United States due to effects of leaf fall on both hydrology and biogeochemistry. Few studies have explored how interactions of biogeochemical transformations, various nitrogen sources, and catchment flow paths affect stream nitrogen variation during autumn. To provide more information on this critical period, we studied (1) the timing, duration, and magnitude of changes to stream nitrate, dissolved organic nitrogen (DON), and ammonium concentrations; (2) changes in nitrate sources and cycling; and (3) source areas of the landscape that most influence stream nitrogen. We collected samples at higher temporal resolution for a longer duration than typical studies of stream nitrogen during autumn. This sampling scheme encompassed the patterns and extremes that occurred during base flow and stormflow events of autumn. Base flow nitrate concentrations decreased by an order of magnitude from 5.4 to 0.7 µmol L −1 during the week when most leaves fell from deciduous trees. Changes to rates of biogeochemical transformations during autumn base flow explained the low nitrate concentrations; in-stream transformations retained up to 72% of the nitrate that entered a stream reach. A decrease of in-stream nitrification coupled with heterotrophic nitrate cycling were primary factors in the seasonal nitrate decline. The period of low nitrate concentrations ended with a storm event in which stream nitrate concentrations increased by 25-fold. In the ensuing weeks, peak stormflow nitrate concentrations progressively decreased over closely spaced, yet similarly sized events. Most stormflow nitrate originated from nitrification in near-stream areas with occasional, large inputs of unprocessed atmospheric nitrate, which has rarely been reported for nonsnowmelt events. A maximum input of 33% unprocessed atmospheric nitrate to the stream occurred during one event. Large inputs of unprocessed atmospheric nitrate show direct and rapid effects on forest streams that may be widespread, although undocumented, throughout nitrogen-polluted temperate forests. In contrast to a week-long nitrate decline during peak autumn litterfall, base flow DON concentrations increased after leaf fall and remained high for 2 months. Dissolved organic nitrogen was hydrologically flushed to the stream from riparian soils during stormflow. In contrast to distinct seasonal changes in base flow nitrate and DON concentrations, ammonium concentrations were typically at or below the detection limit, similar to the rest of the year. Our findings reveal couplings among catchment flow paths, nutrient sources, and transformations that control seasonal extremes of stream nitrogen in forested landscapes.

Vermont↗

USGS2014

Precipitation isotopes link regional climate patterns to water supply in a tropical mountain forest, eastern Puerto Rico

Like many mountainous areas in the tropics, watersheds in the Luquillo Mountains of eastern Puerto Rico have abundant rainfall and stream discharge and provide much of the water supply for the densely populated metropolitan areas nearby. Projected changes in regional temperature and atmospheric dynamics as a result of global warming suggest that water availability will be affected by changes in rainfall patterns. It is essential to understand the relative importance of different weather systems to water supply to determine how changes in rainfall patterns, interacting with geology and vegetation, will affect the water balance. To help determine the links between climate and water availability, stable isotope signatures of precipitation from different weather systems were established to identify those that are most important in maintaining streamflow and groundwater recharge. Precipitation stable isotope values in the Luquillo Mountains had a large range, from fog/cloud water with δ 2 H, δ 18 O values as high as +12 ‰, −0.73 ‰ to tropical storm rain with values as low as −127 ‰, −16.8 ‰. Temporal isotope values exhibit a reverse seasonality from those observed in higher latitude continental watersheds, with higher isotopic values in the winter and lower values in the summer. Despite the higher volume of convective and low-pressure system rainfall, stable isotope analyses indicated that under the current rainfall regime, frequent trade -wind orographic showers contribute much of the groundwater recharge and stream base flow. Analysis of rain events using 20 years of 15 -minute resolution data at a mountain station (643 m) showed an increasing trend in rainfall amount, in agreement with increased precipitable water in the atmosphere, but differing from climate model projections of drying in the region. The mean intensity of rain events also showed an increasing trend. The determination of recharge sources from stable isotope tracers indicates that water supply will be affected if regional atmospheric dynamics change trade- wind orographic rainfall patterns in the Caribbean.

Luquillo Mountains↗

USGS2015

Source limitation of carbon gas emissions in high-elevation mountain streams and lakes

Inland waters are an important component of the global carbon cycle through transport, storage, and direct emissions of CO 2 and CH 4 to the atmosphere. Despite predictions of high physical gas exchange rates due to turbulent flows and ubiquitous supersaturation of CO 2 —and perhaps also CH 4 —patterns of gas emissions are essentially undocumented for high mountain ecosystems. Much like other headwater networks around the globe, we found that high-elevation streams in Rocky Mountain National Park, USA, were supersaturated with CO 2 during the growing season and were net sources to the atmosphere. CO 2 concentrations in lakes, on the other hand, tended to be less than atmospheric equilibrium during the open water season. CO 2 and CH 4 emissions from the aquatic conduit were relatively small compared to many parts of the globe. Irrespective of the physical template for high gas exchange (high k ), we found evidence of CO 2 source limitation to mountain streams during the growing season, which limits overall CO 2 emissions. Our results suggest a reduced importance of aquatic ecosystems for carbon cycling in high-elevation landscapes having limited soil development and high CO 2 consumption via mineral weathering.

Journal of Geophysical Research G: Biogeosciences↗

USGS2016

The role of crystallization-driven exsolution on the sulfur mass balance in volcanic arc magmas

The release of large amounts of sulfur to the stratosphere during explosive eruptions affects the radiative balance in the atmosphere and consequentially impacts climate for up to several years after the event. Quantitative estimations of the processes that control the mass balance of sulfur between melt, crystals, and vapor bubbles is needed to better understand the potential sulfur yield of individual eruption events and the conditions that favor large sulfur outputs to the atmosphere. The processes that control sulfur partitioning in magmas are (1) exsolution of volatiles (dominantly H 2 O) during decompression (first boiling) and during isobaric crystallization (second boiling), (2) the crystallization and breakdown of sulfide or sulfate phases in the magma, and (3) the transport of sulfur-rich vapor (gas influx) from deeper unerupted regions of the magma reservoir. Vapor exsolution and the formation/breakdown of sulfur-rich phases can all be considered as closed-system processes where mass balance arguments are generally easier to constrain, whereas the contribution of sulfur by vapor transport (open system process) is more difficult to quantify. The ubiquitous “excess sulfur” problem, which refers to the much higher sulfur mass released during eruptions than what can be accounted for by amount of sulfur originally dissolved in erupted melt, as estimated from melt inclusion sulfur concentrations (the “petrologic estimate”), reflects the challenges in closing the sulfur mass balance between crystals, melt, and vapor before and during a volcanic eruption. In this work, we try to quantify the relative importance of closed- and open-system processes for silicic arc volcanoes using kinetic models of sulfur partitioning during exsolution. Our calculations show that crystallization-induced exsolution (second boiling) can generate a significant fraction of the excess sulfur observed in crystal-rich arc magmas. This result does not negate the important role of vapor migration in sulfur mass balance but rather points out that second boiling (in situ exsolution) can provide the necessary yield to drive the excess sulfur to the levels observed for crystal-rich systems. In contrast, in crystal-poor systems, magma recharge that releases sulfur-rich bubbles is necessary and most likely the primary contributor to sulfur mass balance. Finally, we apply our model to account for the effect of sulfur partitioning during second boiling and its impact on sulfur released during the Cerro Galan supereruption in Argentina (2.08 Ma) and show the potential importance of second boiling in releasing a large amount of sulfur to the atmosphere during the eruption of large crystal-rich ignimbrites.

Journal of Geophysical Research B: Solid Earth↗

USGS2017

What determines water temperature dynamics in the San Francisco Bay-Delta system?

Water temperature is an important factor determining estuarine species habitat conditions. Water temperature is mainly governed by advection (e.g., from rivers) and atmospheric exchange processes varying strongly over time (day-night, seasonally) and the spatial domain. On a long time scale, climate change will impact water temperature in estuarine systems due to changes in river flow regimes, air temperature, and sea level rise. To determine which factors govern estuarine water temperature and its sensitivity to changes in its forcing, we developed a process-based numerical model (Delft3D Flexible Mesh) and applied it to a well-monitored estuarine system (the San Francisco Estuary) for validation. The process-based approach allows for detailed process description and a physics-based analysis of governing processes. The model was calibrated for water year 2011 and incorporated 3-D hydrodynamics, salinity intrusion, water temperature dynamics, and atmospheric coupling. Results show significant skill in reproducing temperature observations on daily, seasonal, and yearly time scales. In North San Francisco Bay, thermal stratification is present, enhanced by salinity stratification. The temperature of the upstream, fresh water Delta area is captured well in 2-D mode, although locally—on a small scale—vertical processes (e.g., stratification) may be important. The impact of upstream river temperature and discharge and atmospheric forcing on water temperatures differs throughout the Delta, possibly depending on dispersion and residence times. Our modeling effort provides a sound basis for future modeling studies including climate change impact on water temperature and associated ecological modeling, e.g., clam and fish habitat and phytoplankton dynamics.

California↗

USGS2017

Controls of multi-modal wave conditions in a complex coastal setting

Coastal hazards emerge from the combined effect of wave conditions and sea level anomalies associated with storms or low-frequency atmosphere-ocean oscillations. Rigorous characterization of wave climate is limited by the availability of spectral wave observations, the computational cost of dynamical simulations, and the ability to link wave-generating atmospheric patterns with coastal conditions. We present a hybrid statistical-dynamical approach to simulating nearshore wave climate in complex coastal settings, demonstrated in the Southern California Bight, where waves arriving from distant, disparate locations are refracted over complex bathymetry and shadowed by offshore islands. Contributions of wave families and large-scale atmospheric drivers to nearshore wave energy flux are analyzed. Results highlight the variability of influences controlling wave conditions along neighboring coastlines. The universal method demonstrated here can be applied to complex coastal settings worldwide, facilitating analysis of the effects of climate change on nearshore wave climate.

California↗

USGS2018

The role of environmental driving factors in historical and projected carbon dynamics of wetland ecosystems in Alaska

Wetlands are critical terrestrial ecosystems in Alaska, covering ~177,000 km 2 , an area greater than all the wetlands in the remainder of the United States. To assess the relative influence of changing climate, atmospheric carbon dioxide (CO 2 ) concentration, and fire regime on carbon balance in wetland ecosystems of Alaska, a modeling framework that incorporates a fire disturbance model and two biogeochemical models was used. Spatially explicit simulations were conducted at 1‐km resolution for the historical period (1950–2009) and future projection period (2010–2099). Simulations estimated that wetland ecosystems of Alaska lost 175 Tg carbon (C) in the historical period. Ecosystem C storage in 2009 was 5,556 Tg, with 89% of the C stored in soils. The estimated loss of C as CO 2 and biogenic methane (CH 4 ) emissions resulted in wetlands of Alaska increasing the greenhouse gas forcing of climate warming. Simulations for the projection period were conducted for six climate change scenarios constructed from two climate models forced under three CO 2 emission scenarios. Ecosystem C storage averaged among climate scenarios increased 3.94 Tg C/yr by 2099, with variability among the simulations ranging from 2.02 to 4.42 Tg C/yr. These increases were driven primarily by increases in net primary production (NPP) that were greater than losses from increased decomposition and fire. The NPP increase was driven by CO 2 fertilization (~5% per 100 parts per million by volume increase) and by increases in air temperature (~1% per °C increase). Increases in air temperature were estimated to be the primary cause for a projected 47.7% mean increase in biogenic CH 4 emissions among the simulations (~15% per °C increase). Ecosystem CO 2 sequestration offset the increase in CH 4 emissions during the 21st century to decrease the greenhouse gas forcing of climate warming. However, beyond 2100, we expect that this forcing will ultimately increase as wetland ecosystems transition from being a sink to a source of atmospheric CO 2 because of (1) decreasing sensitivity of NPP to increasing atmospheric CO 2 , (2) increasing availability of soil C for decomposition as permafrost thaws, and (3) continued positive sensitivity of biogenic CH 4 emissions to increases in soil temperature.

Alaska↗

USGS2018

Assessing historical and projected carbon balance of Alaska: A synthesis of results and policy/management implications

We summarize the results of a recent interagency assessment of land carbon dynamics in Alaska, in which carbon dynamics were estimated for all major terrestrial and aquatic ecosystems for the historical period (1950–2009) and a projection period (2010–2099). Between 1950 and 2009, upland and wetland (i.e., terrestrial) ecosystems of the state gained 0.4 Tg C/yr (0.1% of net primary production, NPP), resulting in a cumulative greenhouse gas radiative forcing of 1.68 × 10 −3 W/m 2 . The change in carbon storage is spatially variable with the region of the Northwest Boreal Landscape Conservation Cooperative (LCC) losing carbon because of fire disturbance. The combined carbon transport via various pathways through inland aquatic ecosystems of Alaska was estimated to be 41.3 Tg C/yr (17% of terrestrial NPP). During the projection period (2010–2099), carbon storage of terrestrial ecosystems of Alaska was projected to increase (22.5–70.0 Tg C/yr), primarily because of NPP increases of 10–30% associated with responses to rising atmospheric CO 2 , increased nitrogen cycling, and longer growing seasons. Although carbon emissions to the atmosphere from wildfire and wetland CH 4 were projected to increase for all of the climate projections, the increases in NPP more than compensated for those losses at the statewide level. Carbon dynamics of terrestrial ecosystems continue to warm the climate for four of the six future projections and cool the climate for only one of the projections. The attribution analyses we conducted indicated that the response of NPP in terrestrial ecosystems to rising atmospheric CO 2 (~5% per 100 ppmv CO 2 ) saturates as CO 2 increases (between approximately +150 and +450 ppmv among projections). This response, along with the expectation that permafrost thaw would be much greater and release large quantities of permafrost carbon after 2100, suggests that projected carbon gains in terrestrial ecosystems of Alaska may not be sustained. From a national perspective, inclusion of all of Alaska in greenhouse gas inventory reports would ensure better accounting of the overall greenhouse gas balance of the nation and provide a foundation for considering mitigation activities in areas that are accessible enough to support substantive deployment.

Alaska↗

USGS2011

Mars Global Digital Dune Database (MGD3): North polar region (MC-1) distribution, applications, and volume estimates

The Mars Global Digital Dune Database (MGD 3 ) now extends from 90°N to 65°S. The recently released north polar portion (MC‐1) of MGD 3 adds ~844 000 km 2 of moderate‐ to large‐size dark dunes to the previously released equatorial portion (MC‐2 to MC‐29) of the database. The database, available in GIS‐ and tabular‐format in USGS Open‐File Reports, makes it possible to examine global dune distribution patterns and to compare dunes with other global data sets (e.g. atmospheric models). MGD 3 can also be used by researchers to identify areas suitable for more focused studies. The utility of MGD 3 is demonstrated through three example applications. First, the uneven geographic distribution of the dunes is discussed and described. Second, dune‐derived wind direction and its role as ground truth for atmospheric models is reviewed. Comparisons between dune‐derived winds and global and mesoscale atmospheric models suggest that local topography may have an important influence on dune‐forming winds. Third, the methods used here to estimate north polar dune volume are presented and these methods and estimates (1130 km 3 to 3250 km 3 ) are compared with those of previous researchers (1158 km 3 to 15 000 km 3 ). In the near future, MGD 3 will be extended to include the south polar region.

Earth Surface Processes and Landforms↗

USGS2007

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

The stable isotopic composition of dissolved inorganic carbon (??13C-DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed ?? 13C-DIC increased between 3-5??? from the stream source to the outlet weir approximately 0??5 km downstream, concomitant with increasing pH and decreasing PCO2. An increase in ??13C-DIC of 2.4 ?? 0??1??? per log unit decrease of excess PCO2 (stream PCO2 normalized to atmospheric PCO2) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO2 outgassing rather than exchange with atmospheric CO2 may be the primary cause of increased ?? 13C-DIC values downstream when PCO2 of surface freshwater exceeds twice the atmospheric CO2 concentration. Although CO2 outgassing caused a general increase in stream ??13C-DIC values, points of localized groundwater seepage into the stream were identified by decreases in ??13C-DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO2 during the early spring growing season. Thus, in spite of effects from CO2 outgassing, ??13C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments. Copyright ?? 2007 John Wiley & Sons, Ltd.

Hydrological Processes↗

USGS1995

Hydrological and thermal response of lakes to climate: Description and modeling

Lake systems continually respond to climatic conditions that vary over broad scales of space and time. The spatial distribution of lakes on the Earth’s surface is indicative of long-term patterns of atmospheric circulation, and the annual cycle of climate over lake basins is reflected in seasonal change in the size and temperature of lakes. Lake size is determined by the balance of water inputs and outputs, and lake temperature is governed by the balance of heat inputs and outputs. The lake hydrological and energy balances are coupled to the atmosphere. In response to the inputs of mass, energy, and momentum (precipitation, radiation, and wind stress), lakes return heat and moisture to the atmosphere through conduction and evaporation. Global, regional, or local change in the hydrological or thermal states of lakes thus represent interactive responses to climatic variation in the supply of water and energy.

Book chapter↗

USGS2024

Microbial communities in standing dead trees in ghost forests are largely aerobic, saprophytic, and methanotrophic

Standing dead trees (snags) are recognized for their influence on methane (CH 4 ) cycling in coastal wetlands, yet the biogeochemical processes that control the magnitude and direction of fluxes across the snag-atmosphere interface are not fully elucidated. Herein, we analyzed microbial communities and fluxes at one height from ten snags in a ghost forest wetland. Snag-atmosphere CH 4 fluxes were highly variable (− 0.11–0.51 mg CH 4 m −2 h −1 ). CH 4 production was measured in three out of ten snags; whereas, CH 4 consumption was measured in two out of ten snags. Potential CH 4 production and oxidation in one core from each snag was assayed in vitro. A single core produced CH 4 under anoxic and oxic conditions, at measured rates of 0.7 and 0.6 ng CH 4 g −1 h −1 , respectively. Four cores oxidized CH 4 under oxic conditions, with an average rate of − 1.13 ± 0.31 ng CH 4 g −1 h −1 . Illumina sequencing of the V3/V4 region of the 16S rRNA gene sequence revealed diverse microbial communities and indicated oxidative decomposition of deadwood. Methanogens were present in 20% of the snags, with a mean relative abundance of < 0.0001%. Methanotrophs were identified in all snags, with a mean relative abundance of 2% and represented the sole CH 4 -cycling communities in 80% of the snags. These data indicate potential for microbial attenuation of CH 4 emissions across the snag-atmosphere interface in ghost forests. A better understanding of the environmental drivers of snag-associated microbial communities is necessary to forecast the response of CH 4 cycling in coastal ghost forest wetlands to a shifting coastal landscape.

Current Microbiology↗

USGS2025

Avian navigation: Comparing the olfactory navigational “map” and the infrasound direction-finding hypotheses to aeronautics

Animal navigation has long been a fascinating but bewildering subject. Humans and animals might well share similar navigational strategies because they developed within the same physical environments. A “map-and-compass” model has been proposed to explain the two-step avian navigational process, but the “map” step has remained elusive. Although scalar values from bicoordinate geomagnetic or atmospheric olfactory gradients have been considered foundational to the avian map, neither has proved convincing engendering decades of controversy. The olfactory map, and an alternative infrasound direction-finding (IDF) hypothesis, are discussed in this review. The olfactory map hypothesis currently requires extensive stable gradients of trace-odor ratios, but such gradients are highly unlikely within a turbulent and rapidly mixed lower atmosphere. The IDF hypothesis, on the other hand, postulates a two-step navigational model analogous to the maritime and aeronautical radio direction-finding technique. This review was also written to encourage further investigation, and direct testing, of the acoustic navigational process. The IDF hypothesis, at present, appears the better explanation of observed avian navigational behavior and accuracy within the atmosphere’s physical environment.

Journal of Comparative Physiology A↗

USGS2011

A westward extension of the warm pool leads to a westward extension of the Walker circulation, drying eastern Africa

Observations and simulations link anthropogenic greenhouse and aerosol emissions with rapidly increasing Indian Ocean sea surface temperatures (SSTs). Over the past 60 years, the Indian Ocean warmed two to three times faster than the central tropical Pacific, extending the tropical warm pool to the west by ~40° longitude (>4,000 km). This propensity toward rapid warming in the Indian Ocean has been the dominant mode of interannual variability among SSTs throughout the tropical Indian and Pacific Oceans (55°E–140°W) since at least 1948, explaining more variance than anomalies associated with the El Niño-Southern Oscillation (ENSO). In the atmosphere, the primary mode of variability has been a corresponding trend toward greatly increased convection and precipitation over the tropical Indian Ocean. The temperature and rainfall increases in this region have produced a westward extension of the western, ascending branch of the atmospheric Walker circulation. Diabatic heating due to increased mid-tropospheric water vapor condensation elicits a westward atmospheric response that sends an easterly flow of dry air aloft toward eastern Africa. In recent decades (1980–2009), this response has suppressed convection over tropical eastern Africa, decreasing precipitation during the ‘long-rains’ season of March–June. This trend toward drought contrasts with projections of increased rainfall in eastern Africa and more ‘El Niño-like’ conditions globally by the Intergovernmental Panel on Climate Change. Increased Indian Ocean SSTs appear likely to continue to strongly modulate the Warm Pool circulation, reducing precipitation in eastern Africa, regardless of whether the projected trend in ENSO is realized. These results have important food security implications, informing agricultural development, environmental conservation, and water resource planning.

Climate Dynamics↗

USGS1996

The initial cooling of pahoehoe flow lobes

In this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows. The accurate modeling of this initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation. This model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not an empirical fit to field data. We find that the only significant processes are (a) heat loss by thermal radiation, (b) heat loss by atmospheric convection, (c) heat transport within the flow by conduction with temperature and porosity-dependent thermal properties, and (d) the release of latent heat during crystallization. The numerical model is better able to reproduce field measurements made in Hawai'i between 1989 and 1993 than other published thermal models. By adjusting one parameter at a time, the effect of each of the input parameters on the cooling rate was determined. We show that: (a) the surfaces of porous flows cool more quickly than the surfaces of dense flows, (b) the surface cooling is very sensitive to the efficiency of atmospheric convective cooling, and (c) changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures. These model results provide a quantitative explanation for the recently observed relationship between the surface cooling rate of pahoehoe lobes and the porosity of those lobes (Jones 1992, 1993). The predicted sensitivity of cooling to atmospheric convection suggests a simple field experiment for verification, and the model provides a tool to begin studies of the dynamic crystallization of real lavas. Future versions of the model can also be made applicable to extraterrestrial, submarine, silicic, and pyroclastic flows.

Bulletin of Volcanology↗

USGS2013

On the twenty-first-century wet season projections over the Southeastern United States

This paper reconciles the difference in the projections of the wet season over the Southeastern United States (SEUS) from a global climate model (the Community Climate System Model Version 3 [CCSM3]) and from a regional climate model (the Regional Spectral Model [RSM]) nested in the CCSM3. The CCSM3 projects a dipole in the summer precipitation anomaly: peninsular Florida dries in the future climate, and the remainder of the SEUS region becomes wetter. The RSM forced with CCSM3 projects a universal drying of the SEUS in the late twenty-first century relative to the corresponding twentieth-century summer. The CCSM3 pattern is attributed to the “upped-ante” mechanism, whereby the atmospheric boundary layer moisture required for convection increases in a warm, statically stable global tropical environment. This criterion becomes harder to meet along convective margins, which include peninsular Florida, resulting in its drying. CCSM3 also projects a southwestward expansion of the North Atlantic subtropical high that leads to further stabilizing of the atmosphere above Florida, inhibiting convection. The RSM, because of its high (10-km grid) resolution, simulates diurnal variations in summer rainfall over SEUS reasonably well. The RSM improves upon CCSM3 through the RSM’s depiction of the diurnal variance of precipitation, which according to observations accounts for up to 40 % of total seasonal precipitation variance. In the future climate, the RSM projects a significant reduction in the diurnal variability of convection. The reduction is attributed to large-scale stabilization of the atmosphere in the CCSM3 projections.

Alabama;Arkansas;Florida;Georgia;Kentucky;Louisian↗

USGS2013

A 17-year record of environmental tracers in spring discharge, Shenandoah National Park, Virginia, USA: use of climatic data and environmental conditions to interpret discharge, dissolved solutes, and tracer concentrations

A 17-year record (1995–2012) of a suite of environmental tracer concentrations in discharge from 34 springs located along the crest of the Blue Ridge Mountains in Shenandoah National Park (SNP), Virginia, USA, reveals patterns and trends that can be related to climatic and environmental conditions. These data include a 12-year time series of monthly sampling at five springs, with measurements of temperature, specific conductance, pH, and discharge recorded at 30-min intervals. The monthly measurements include age tracers (CFC-11, CFC-12, CFC-113, CFC-13, SF 6 , and SF 5 CF 3 ), dissolved gases (N 2 , O 2 , Ar, CO 2 , and CH 4 ), stable isotopes of water, and major and trace inorganic constituents. The chlorofluorocarbon (CFC) and sulfur hexafluoride (SF 6 ) concentrations (in pptv) in spring discharge closely follow the concurrent monthly measurements of their atmospheric mixing ratios measured at the Air Monitoring Station at Big Meadows, SNP, indicating waters 0–3 years in age. A 2-year (2001–2003) record of unsaturated zone air displayed seasonal deviations from North American Air of ±10 % for CFC-11 and CFC-113, with excess CFC-11 and CFC-113 in peak summer and depletion in peak winter. The pattern in unsaturated zone soil CFCs is a function of gas solubility in soil water and seasonal unsaturated zone temperatures. Using the increase in the SF 6 atmospheric mixing ratio, the apparent (piston flow) SF 6 age of the water varied seasonally between about 0 (modern) in January and up to 3 years in July–August. The SF 6 concentration and concentrations of dissolved solutes (SiO 2 , Ca 2+ , Mg 2+ , Na + , Cl − , and HCO 3 − ) in spring discharge demonstrate a fraction of recent recharge following large precipitation events. The output of solutes in the discharge of springs minus the input from atmospheric deposition per hectare of watershed area (mol ha −1 a −1 ) were approximately twofold greater in watersheds draining the regolith of Catoctin metabasalts than that of granitic gneisses and granitoid crystalline rocks. The stable isotopic composition of water in spring discharge broadly correlates with the Oceanic Niño Index. Below normal precipitation and enriched stable isotopic composition were observed during El Niño years.

Virginia↗

USGS2020

Decadal-scale decoupling of soil phosphorus and molybdenum cycles by temperate nitrogen-fixing trees

Symbiotic nitrogen- (N) fixing trees can influence multiple biogeochemical cycles by fixing atmospheric N, which drives net primary productivity and soil carbon (C) and N accumulation, as well as by mobilizing soil phosphorus (P) and other nutrients to support growth and metabolism. The soil micronutrient molybdenum (Mo) is essential to N-fixation, yet surprisingly little is known of whether N-fixing trees alter soil Mo cycling, and if changes to soil Mo are coupled to soil C, N, and P. We compared how symbiotic N-fixing red alder and non-N-fixing Douglas-fir trees modified surface soil C, N, P, and Mo across variation in climate and other site factors in the Pacific Northwest. We found that after two decades, N-fixing trees drove coupled increases in surface soil C, N, total P, and organic P. Consistent with contributions of N-fixing trees to soil organic matter, increased soil C and N were accompanied by lower δ13C in all sites, and lower δ15N in sites where non-fixer plots exhibited elevated soil δ15N. However, N-fixing trees did not affect surface soil Mo concentrations or fractions, suggesting that different factors control the cycling of P versus Mo over decadal timescales. Random forest analysis revealed that surface soil P was most strongly influenced by factors related to soil C accumulation, whereas surface soil Mo was related primarily to environmental factors, including potential differences in atmospheric Mo deposition across sites. Ratios of surface soil P:Mo were higher in extractable pools than in total soil digests, reinforcing the idea of stronger biotic cycling of P than Mo. Overall, our multi-site, multi-decadal field study found surprisingly small effects of N-fixing trees on soil Mo, despite rapid increases in soil organic C, N, and P. We hypothesize that, rather than direct effects of N-fixing vegetation, abiotic or indirect biotic factors such as soil sorption of atmospheric Mo inputs can link C–N–P–Mo cycles in terrestrial ecosystems on longer timescales.

British Columbia, Washington↗