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At least 613 records · Page 34Linked to original sources
USGS2014

Lacustrine responses to decreasing wet mercury deposition rates: results from a case study in northern Minnesota

We present a case study comparing metrics of methylmercury (MeHg) contamination for four undeveloped lakes in Voyageurs National Park to wet atmospheric deposition of mercury (Hg), sulfate (SO 4 –2 ), and hydrogen ion (H + ) in northern Minnesota. Annual wet Hg, SO 4 –2 , and H + deposition rates at two nearby precipitation monitoring sites indicate considerable decreases from 1998 to 2012 (mean decreases of 32, 48, and 66%, respectively). Consistent with decreases in the atmospheric pollutants, epilimnetic aqueous methylmercury (MeHg aq ) and mercury in small yellow perch (Hg fish ) decreased in two of four lakes (mean decreases of 46.5% and 34.5%, respectively, between 2001 and 2012). Counter to decreases in the atmospheric pollutants, MeHg aq increased by 85% in a third lake, whereas Hg fish increased by 80%. The fourth lake had two disturbances in its watershed during the study period (forest fire; changes in shoreline inundation due to beaver activity); this lake lacked overall trends in MeHg aq and Hg fish . The diverging responses among the study lakes exemplify the complexity of ecosystem responses to decreased loads of atmospheric pollutants.

Minnesota↗
USGS2009

Application of a rule-based model to estimate mercury exchange for three background biomes in the continental United States

Ecosystems that have low mercury (Hg) concentrations (i.e., not enriched or impactedbygeologic or anthropogenic processes) cover most of the terrestrial surface area of the earth yet their role as a net source or sink for atmospheric Hg is uncertain. Here we use empirical data to develop a rule-based model implemented within a geographic information system framework to estimate the spatial and temporal patterns of Hg flux for semiarid deserts, grasslands, and deciduous forests representing 45% of the continental United States. This exercise provides an indication of whether these ecosystems are a net source or sink for atmospheric Hg as well as a basis for recommendation of data to collect in future field sampling campaigns. Results indicated that soil alone was a small net source of atmospheric Hg and that emitted Hg could be accounted for based on Hg input by wet deposition. When foliar assimilation and wet deposition are added to the area estimate of soil Hg flux these biomes are a sink for atmospheric Hg. ?? 2009 American Chemical Society.

Environmental Science & Technology↗
USGS2010

Isotopic composition and origin of indigenous natural perchlorate and co-occurring nitrate in the southwestern United States

Perchlorate (ClO 4 − ) has been detected widely in groundwater and soils of the southwestern United States. Much of this ClO 4 − appears to be natural, and it may have accumulated largely through wet and dry atmospheric deposition. This study evaluates the isotopic composition of natural ClO 4 − indigenous to the southwestern U.S. Stable isotope ratios were measured in ClO 4 − (δ 18 O, Δ 17 O, δ 37 Cl) and associated NO 3 − (δ 18 O, Δ 17 O, δ 15 N) in groundwater from the southern High Plains (SHP) of Texas and New Mexico and the Middle Rio Grande Basin (MRGB) in New Mexico, from unsaturated subsoil in the SHP, and from NO 3 − -rich surface caliche deposits near Death Valley, California. The data indicate natural ClO 4 − in the southwestern U.S. has a wide range of isotopic compositions that are distinct from those reported previously for natural ClO 4 − from the Atacama Desert of Chile as well as all known synthetic ClO 4 − . ClO 4 − in Death Valley caliche has a range of high Δ 17 O values (+8.6 to +18.4 ‰), overlapping and extending the Atacama range, indicating at least partial atmospheric formation via reaction with ozone (O 3 ). However, the Death Valley δ 37 Cl values (−3.1 to −0.8 ‰) and δ 18 O values (+2.9 to +26.1‰) are higher than those of Atacama ClO 4 − . In contrast, ClO 4 − from western Texas and New Mexico has much lower Δ 17 O (+0.3 to +1.3‰), with relatively high δ 37 Cl (+3.4 to +5.1 ‰) and δ 18 O (+0.5 to +4.8 ‰), indicating either that this material was not primarily generated with O 3 as a reactant or that the ClO 4 − was affected by postdepositional O isotope exchange. High Δ 17 O values in ClO 4 − (Atacama and Death Valley) are associated with high Δ 17 O values in NO 3 − , indicating that both compounds preserve characteristics of O 3 -related atmospheric production in hyper-arid settings, whereas both compounds have low Δ 17 O values in less arid settings. Although Δ 17 O variations in terrestrial NO 3 − can be attributed to mixing of atmospheric (high Δ 17 O) and biogenic (low Δ 17 O) NO 3 − , variations in Δ 17 O of terrestrial ClO 4 − are not readily explained in the same way. This study provides important new constraints for identifying natural sources of ClO 4 − in different environments by multicomponent isotopic characteristics, while presenting the possibilities of divergent ClO 4 − formation mechanisms and(or) ClO 4 − isotopic exchange in biologically active environments.

Environmental Science & Technology↗
USGS1998

Above-ground sulfur cycling in adjacent coniferous and deciduous forest and watershed sulfur retention in the Georgia Piedmont, U.S.A.

Atmospheric deposition and above-ground cycling of sulfur (S) were evaluated in adjacent deciduous and coniferous forests at the Panola Mountain Research Watershed (PMRW), Georgia U.S.A. Total atmospheric S deposition (wet plus dry) was 12.9 and 12.7 kg ha-1 yr-1 for the deciduous and coniferous forests, respectively, from October 1987 through November 1989. Dry deposition contributes more than 40% to the total atmospheric S deposition, and SO2 is the major source (~55%) of total dry S deposition. Dry deposition to these canopies is similar to regional estimates suggesting that 60-km proximity to emission sources does not noticeably impact dry deposition at PMRW. Below-canopy S fluxes (throughfall plus stemflow) in each forest are 37% higher annually in the deciduous forest than in the coniferous forest. An excess in below-canopy S flux in the deciduous forest is attributed to leaching and higher dry deposition than in the coniferous forest. Total S deposition to the forest floor by throughfall, stemflow and litterfall was 2.4 and 2.8 times higher in the deciduous and coniferous forests, respectively, than annual S growth requirement for foliage and wood. Although A deposition exceeds growth requirement, more than 95% of the total atmospheric S deposition was retained by the watershed in 1988 and 1989. The S retention at PMRW is primarily due to SO2+4 adsorption by iron oxides and hydroxides in watershed soils. The S content in while oak and loblolly pine boles have increased more than 200% in the last 20 yr, possibly reflecting increases in emissions.

Water, Air, & Soil Pollution↗
USGS2003

Controls of streamwater dissolved inorganic carbon dynamics in a forested watershed

I investigated controls of stream dissolved inorganic carbon (DIC) sources and cycling along a stream size and productivity gradient in a temperate forested watershed in northern California. Dissolved CO2 (CO2 (aq)) dynamics in heavily shaded streams contrasted strongly with those of larger, open canopied sites. In streams with canopy cover > 97%, CO2 (aq) was highest during baseflow periods (up to 540 ??M) and was negatively related to discharge. Effects of algal photosynthesis on CO2 (aq) were minimal and stream CO2 (aq) was primarily controlled by groundwater CO2 (aq) inputs and degassing losses to the atmosphere. In contrast to the small streams. CO2 (aq) in larger, open-canopied streams was often below atmospheric levels at midday during baseflow and was positively related to discharge. Here, stream CO2 (aq) was strongly influenced by the balance between autotrophic and heterotrophic processes. Dynamics of HCO3- were less complex. HCO3- and Ca2+ were positively correlated, negatively related to discharge, and showed no pattern with stream size. Stable carbon isotope ratios of DIC (i.e. ??13C DIC) increased with stream size and discharge, indicating contrasting sources of DIC to streams and rivers. During summer baseflows, ??13C DIC were 13C-depleted in the smallest streams (minimum of -17.7???) due to the influence of CO2 (aq) derived from microbial respiration and HCO3- derived from carbonate weathering. ??13C DIC were higher (up to -6.6???) in the larger streams and rivers due to invasion of atmospheric CO2 enhanced by algal CO2 (aq) uptake. While small streams were influenced by groundwater inputs, patterns in CO2 (aq) and evidence from stable isotopes demonstrate the strong influence of stream metabolism and CO2 exchange with the atmosphere on stream and river carbon cycles.

Biogeochemistry↗
USGS2003

Modeling carbon dynamics in vegetation and soil under the impact of soil erosion and deposition

Soil erosion and deposition may play important roles in balancing the global atmospheric carbon budget through their impacts on the net exchange of carbon between terrestrial ecosystem and the atmosphere. Few models and studies have been designed to assess these impacts. In this study, we developed a general ecosystem model, Erosion-Deposition-Carbon-Model (EDCM), to dynamically simulate the influences of rainfall-induced soil erosion and deposition on soil organic carbon (SOC) dynamics in soil profiles. EDCM was applied to several landscape positions in the Nelson Farm watershed in Mississippi, including ridge top (without erosion or deposition), eroding hillslopes, and depositional sites that had been converted from native forests to croplands in 1870. Erosion reduced the SOC storage at the eroding sites and deposition increased the SOC storage at the depositional areas compared with the site without erosion or deposition. Results indicated that soils were consistently carbon sources to the atmosphere at all landscape positions from 1870 to 1950, with lowest source strength at the eroding sites (13 to 24 gC m-2 yr-1), intermediate at the ridge top (34 gC m-2 yr-1), and highest at the depositional sites (42 to 49 gC m-2 yr-1). During this period, erosion reduced carbon emissions via dynamically replacing surface soil with subsurface soil that had lower SOC contents (quantity change) and higher passive SOC fractions (quality change). Soils at all landscape positions became carbon sinks from 1950 to 1997 due to changes in management practices (e.g., intensification of fertilization and crop genetic improvement). The sink strengths were highest at the eroding sites (42 to 44 gC m-2 yr-1 , intermediate at the ridge top (35 gC m-2 yr-1), and lowest at the depositional sites (26 to 29 gC m-2 yr-1). During this period, erosion enhanced carbon uptake at the eroding sites by continuously taking away a fraction of SOC that can be replenished with enhanced plant residue input. Overall, soil erosion and deposition reduced CO2 emissions from the soil into the atmosphere by exposing low carbon-bearing soil at eroding sites and by burying SOC at depositional sites. The results suggest that failing to account for the impact of soil erosion and deposition may potentially contribute to an overestimation of both the total historical carbon released from soils owing to land use change and the contemporary carbon sequestration rates at the eroding sites.

Global Biogeochemical Cycles↗
USGS2003

Use of hydraulic head to estimate volumetric gas content and ebullition flux in northern peatlands

Hydraulic head was overpressured at middepth in a 4.2‐m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free‐phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2‐m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous‐phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m −2 d −1 . These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas‐flux chambers or tower‐mounted sensors.

Water Resources Research↗
USGS2006

Wildfires threaten mercury stocks in northern soils

With climate change rapidly affecting northern forests and wetlands, mercury reserves once protected in cold, wet soils are being exposed to burning, likely triggering large releases of mercury to the atmosphere. We quantify organic soil mercury stocks and burn areas across western, boreal Canada for use in fire emission models that explore controls of burn area, consumption severity, and fuel loading on atmospheric mercury emissions. Though renowned as hotspots for the accumulation of mercury and its transformation to the toxic methylmercury, boreal wetlands might soon transition to hotspots for atmospheric mercury emissions. Estimates of circumboreal mercury emissions from this study are 15-fold greater than estimates that do not account for mercury stored in peat soils. Ongoing and projected increases in boreal wildfire activity due to climate change will increase atmospheric mercury emissions, contributing to the anthropogenic alteration of the global mercury cycle and exacerbating mercury toxicities for northern food chains. Copyright 2006 by the American Geophysical Union.

Geophysical Research Letters↗
USGS2007

A user-friendly one-dimensional model for wet volcanic plumes

This paper presents a user-friendly graphically based numerical model of one-dimensional steady state homogeneous volcanic plumes that calculates and plots profiles of upward velocity, plume density, radius, temperature, and other parameters as a function of height. The model considers effects of water condensation and ice formation on plume dynamics as well as the effect of water added to the plume at the vent. Atmospheric conditions may be specified through input parameters of constant lapse rates and relative humidity, or by loading profiles of actual atmospheric soundings. To illustrate the utility of the model, we compare calculations with field-based estimates of plume height (∼9 km) and eruption rate (>∼4 × 10 5 kg/s) during a brief tephra eruption at Mount St. Helens on 8 March 2005. Results show that the atmospheric conditions on that day boosted plume height by 1–3 km over that in a standard dry atmosphere. Although the eruption temperature was unknown, model calculations most closely match the observations for a temperature that is below magmatic but above 100°C.

Geochemistry, Geophysics, Geosystems↗
USGS2009

The source of infrasound associated with long-period events at mount St. Helens

During the early stages of the 2004-2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from , the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the "valve" sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.

Washington↗
USGS2008

Sources, transformations, and hydrological processes that control stream nitrate and dissolved organic matter concentrations during snowmelt in an upland forest

We explored catchment processes that control stream nutrient concentrations at an upland forest in northeastern Vermont, USA, where inputs of nitrogen via atmospheric deposition are among the highest in the nation and affect ecosystem functioning. We traced sources of water, nitrate, and dissolved organic matter (DOM) using stream water samples collected at high frequency during spring snowmelt. Hydrochemistry, isotopic tracers, and end‐member mixing analyses suggested the timing, sources, and source areas from which water and nutrients entered the stream. Although stream‐dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) both originated from leaching of soluble organic matter, flushing responses between these two DOM components varied because of dynamic shifts of hydrological flow paths and sources that supply the highest concentrations of DOC and DON. High concentrations of stream water nitrate originated from atmospheric sources as well as nitrified sources from catchment soils. We detected nitrification in surficial soils during late snowmelt which affected the nitrate supply that was available to be transported to streams. However, isotopic tracers showed that the majority of nitrate in upslope surficial soil waters after the onset of snowmelt originated from atmospheric sources. A fraction of the atmospheric nitrogen was directly delivered to the stream, and this finding highlights the importance of quick flow pathways during snowmelt events. These findings indicate that interactions among sources, transformations, and hydrologic transport processes must be deciphered to understand why concentrations vary over time and over space as well as to elucidate the direct effects of human activities on nutrient dynamics in upland forest streams.

Water Resources Research↗
USGS2020

Step increase in eastern U.S. precipitation linked to Indian Ocean warming

A step increase in annual precipitation over the eastern U.S. in the early 1970’s commenced five decades of invigorated hydroclimate, with ongoing impacts on streamflow and water resources. Despite its far-reaching impacts, the dynamical origin of this change is unknown. Here, analyses of a century of atmospheric and oceanic data trace the dynamics to changes in the Indian Ocean. Spring and fall precipitation explain more than half the annual eastern-U.S. precipitation variance over the century, and changes in fall are predominantly responsible for the step increase. The driving mechanism is emergence of a pan-Pacific atmospheric wave emanating from deep convection over the warming Indian Ocean. Documentation of this fall teleconnection draws attention to projected anthropogenic increases in tropical oceanic heat content, and their potential impacts on hydroclimate of the midlatitudes.

Geophysical Research Letters↗
USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗
USGS2012

Laboratory studies of volcanic jets

The study of the fluid dynamics of violent volcanic eruptions by laboratory experiment is described, and the important fluid-dynamic processes that can be examined in laboratory models are discussed in detail. In preliminary experiments, pure gases are erupted from small reservoirs. The gases used are Freon 12 and Freon 22, two gases of high molecular weight and high density that are good analogs of heavy and particulate-laden volcanic gases; nitrogen, a moderate molecular weight, moderate density gas for which the thermodynamic properties are well known; and helium, a low molecular weight, lowdensity gas that is used as a basis for comparison with the behavior of the heavier gases and as an analog of steam, the gas that dominates many volcanic eruptions. Transient jets erupt from the reservoir into the laboratory upon rupture of a thin diaphragm at the exit of a convergent nozzle. The gas accelerates from rest in the reservoir to high velocity in the jet. Reservoir pressures and geometries are such that the fluid velocity in the jets is initially supersonic and later decays to subsonic. The measured reservoir pressure decreases as the fluid expands through repetitively reflecting rarefaction waves, but for the conditions of these experiments, a simple steady-discharge model is sufficient to explain the pressure decay and to predict the duration of the flow. Density variations in the flow field have been visualized with schlieren and shadowgraph photography. The observed structure of the jet is correlated with the measured pressure history. The starting vortex generated when the diaphragm ruptures becomes the head of the jet. Though the exit velocity is sonic, the flow head in the helium jet decelerates to about one-third of sonic velocity in the first few nozzle diameters, the nitrogen head decelerates to about three-fourths of sonic velocity, while Freon maintains nearly sonic velocity. The impulsive acceleration of reservoir fluid into the surrounding atmosphere produces a compression wave. The strength of this wave depends primarily on the sound speed of the fluid in the reservoir but also, secondarily with opposite effect, on the density: helium produces a relatively strong atmospheric shock while the Freons do not produce any optically observable wave front. Well-formed N waves are detected with a microphone far from the reservoir. Barrel shocks, Mach disks, and other familiar features of steady underexpanded supersonic jets form inside the jet almost immediately after passage of the flow head. These features are maintained until the pressure in the reservoir decays to sonic conditions. At low pressures the jets are relatively structureless. Gas-particle jets from volcanic eruptions may behave as pseudogases if particle concentrations and mass and momentum exchange between the components are sufficiently small. The sound speed of volcanic pseudogases can be as large as 1000 m s −1 or as small as a few tens of meters per second depending on the mass loading and initial temperature. Fluids of high sound speed produce stronger atmospheric shock waves than do those of low sound speed. Therefore eruption of a hot gas lightly laden with particulates should produce a stronger shock than eruption of a cooler or heavily laden fluid. An empirical expression suggests that the initial velocity of the head of supersonic volcanic jets is controlled by the sound speed and the ratio of the density of the erupting fluid to that of the atmosphere. The duration of gas or pseudogas eruptions is controlled by the sound speed of the fluid and the ratio of reservoir volume to vent area.

Journal of Geophysical Research Solid Earth↗
USGS2012

High-resolution reflectance spectra of Mars in the 2.3-μm region: evidence for the mineral scapolite

patially resolved reflectance spectra of Mars in the 2.2- to 2.4-μm spectral region were obtained in August 1988 using the NASA 3-m Infrared Telescope Facility. The spectra show weak absorption features due to Martian atmospheric carbon monoxide and a surface mineral. Both CO and the mineral absorptions are composed of overlapping narrow features, but in many locations, such as Hellas, Chryse, Eden, and Moab, the mineral absorptions are quite strong, at least 3 times stronger than at the most absorbing wavelengths of CO near 2.33 μm. Therefore CO complicates the analysis of the surface mineral but does not always overwhelm its signature. Model removal of the Martian atmospheric CO has been performed, and the remaining absorption bands are identified as scapolite. Relatively strong absorptions that match bands in the spectrum of scapolite and have little or no CO absorption interference are seen near 2.41, 2.39, and 2.29 μm. Absorption also occurs at the scapolite bands at 2.36 and 2.33 μm, but the analysis is complicated by uncertainty in the atmospheric CO removal at these wavelengths. Weaker scapolite bands are seen at 2.44 and 2.23 μm where there is virtually no atmospheric interference. The scapolite bands observed on Mars are due to HCO 3 − and HSO 4 − ions in the scapolite structure. The bicarbonate and bisulfate contents appear to vary with location: the scapolite in Hellas is more bisulfate-rich relative to that in the Chryse/Moab/Eden area. Other locations contain little (Arabia, Syrtis Major, Hellespontica, and Isidis) or no scapolite (e.g., Margaritifer, Ausonia, and Erythraeum). The calculated abundances are unconstrained because the amounts of HCO 3 − and HSO 4 − in the Martian scapolites as well as their grain sizes are not known. If the scapolites contain about 3 wt % of each, near the maximum possible, the scapolite abundances probably range from about 5 wt % scapolite at Eden and Hellas; 3–5% at Chryse, Moab, and Oxia Palus; 2–3% at Arabia, Syrtis Major, and Isidis; to less than 2% at Hellespontica, Syrtis Minor, and Margaritifer, assuming a relatively large grain size of 50–100 μm. If the characteristic grain sizes are smaller or the HCO 3 − and HSO 4 − contents are lower, the scapolite abundances required to match the observed band depths would be higher. The mineral bands are apparent in many of the Mars spectra measured, so it appears to be widely but not uniformly distributed. The newly observed fine structure also varies greatly in both depth and spectral detail with location on Mars. Thus there appears to be regional variations in composition. The mineral phases appear to reflect local or regional geology and are not primarily contained in the homogeneous, globally redistributed aeolian dust. Higher spectral resolution Martian spectra in the 2.3-μm region as well as at 3.9 μm are needed to confirm the scapolite identification and to constrain its abundance.

Journal of Geophysical Research↗
USGS1998

Effect of interannual climate variability on carbon storage in Amazonian ecosystems

The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna. The forests account for about 10 per cent of the world's terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems, and short-term field measurements suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Nino episodes in recent decades. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage. Here we use a transient process-based biogeochemical model of terrestrial ecosystems to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Nino years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation in the Amazon Basin in the early 1990s. Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

Nature↗
USGS2005

Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan

Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1) and, similar to the Earth, N 2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 10 years (ref. 3). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure ???30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.

Nature↗
USGS2015

Large divergence of satellite and Earth system model estimates of global terrestrial CO 2 fertilization

Atmospheric mass balance analyses suggest that terrestrial carbon (C) storage is increasing, partially abating the atmospheric [CO 2 ] growth rate , although the continued strength of this important ecosystem service remains uncertain . Some evidence suggests that these increases will persist owing to positive responses of vegetation growth (net primary productivity; NPP) to rising atmospheric [CO 2 ] (that is, ‘CO 2 fertilization ’ ) . Here, we present a new satellite-derived global terrestrial NPP data set , which shows a significant increase in NPP from 1982 to 2011. However, comparison against Earth system model (ESM) NPP estimates reveals a significant divergence, with satellite-derived increases (2.8 ± 1.50%) less than half of ESM-derived increases (7.6   ±   1.67%) over the 30-year period. By isolating the CO 2 fertilization effect in each NPP time series and comparing it against a synthesis of available free-air CO 2 enrichment data , we provide evidence that much of the discrepancy may be due to an over-sensitivity of ESMs to atmospheric [CO 2 ], potentially reflecting an under-representation of climatic feedbacks and/or a lack of representation of nutrient constraints . Our understanding of CO 2 fertilization effects on NPP needs rapid improvement to enable more accurate projections of future C cycle–climate feedbacks; we contend that better integration of modelling, satellite and experimental approaches offers a promising way forward.

Nature Climate Change↗