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At least 1,135 records · Page 63Linked to original sources
USGS2010

Origins of large-volume, compositionally zoned volcanic eruptions: New constraints from U-series isotopes and numerical thermal modeling for the 1912 Katmai-Novarupta eruption

We present the results of a combined U-series isotope and numerical modeling study of the 1912 Katmai-Novarupta eruption in Alaska. A stratigraphically constrained set of samples have compositions that range from basalt through basaltic andesite, andesite, dacite, and rhyolite. The major and trace element range can be modeled by 80–90% closed-system crystal fractionation over a temperature interval from 1279°C to 719°C at 100 MPa, with an implied volume of parental basalt of ∼65 km 3 . Numerical models suggest, for wall rock temperatures appropriate to this depth, that 90% of this volume of magma would cool and crystallize over this temperature interval within a few tens of kiloyears. However, the range in 87 Sr/ 86 Sr, ( 230 Th/ 238 U), and ( 226 Ra/ 230 Th) requires open-system processes. Assimilation of the host sediments can replicate the range of Sr isotopes. The variation of ( 226 Ra/ 230 Th) ratios in the basalt to andesite compositional range requires that these were generated less than several thousand years before eruption. Residence times for dacites are close to 8000 years, whereas the rhyolites appear to be 50–200 kyr old. Thus, the magmas that erupted within only 60 h had a wide range of crustal residence times. Nevertheless, they were emplaced in the same thermal regime and evolved along similar liquid lines of descent from parental magmas with similar compositions. The system was built progressively with multiple inputs providing both mass and heat, some of which led to thawing of older silicic material that provided much of the rhyolite.

Alaska↗
USGS2010

Variability and trends in dry day frequency and dry event length in the southwestern United States

Daily precipitation from 22 National Weather Service first-order weather stations in the southwestern United States for water years 1951 through 2006 are used to examine variability and trends in the frequency of dry days and dry event length. Dry events with minimum thresholds of 10 and 20 consecutive days of precipitation with less than 2.54 mm are analyzed. For water years and cool seasons (October through March), most sites indicate negative trends in dry event length (i.e., dry event durations are becoming shorter). For the warm season (April through September), most sites also indicate negative trends; however, more sites indicate positive trends in dry event length for the warm season than for water years or cool seasons. The larger number of sites indicating positive trends in dry event length during the warm season is due to a series of dry warm seasons near the end of the 20th century and the beginning of the 21st century. Overall, a large portion of the variability in dry event length is attributable to variability of the El Niño–Southern Oscillation, especially for water years and cool seasons. Our results are consistent with analyses of trends in discharge for sites in the southwestern United States, an increased frequency in El Niño events, and positive trends in precipitation in the southwestern United States.

Journal of Geophysical Research D: Atmospheres↗
USGS2009

Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration

[1] The Noachian terrain west of the Isidis basin hosts a diverse collection of alteration minerals in rocks comprising varied geomorphic units within a 100,000 km 2 region in and near the Nili Fossae. Prior investigations in this region by the Observatoire pour l'Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) instrument on Mars Express revealed large exposures of both mafic minerals and iron magnesium phyllosilicates in stratigraphic context. Expanding on the discoveries of OMEGA, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) has found more spatially widespread and mineralogically diverse alteration minerals than previously realized, which represent multiple aqueous environments. Using CRISM near-infrared spectral data, we detail the basis for identification of iron and magnesium smectites (including both nontronite and more Mg-rich varieties), chlorite, prehnite, serpentine, kaolinite, potassium mica (illite or muscovite), hydrated (opaline) silica, the sodium zeolite analcime, and magnesium carbonate. The detection of serpentine and analcime on Mars is reported here for the first time. We detail the geomorphic context of these minerals using data from high-resolution imagers onboard MRO in conjunction with CRISM. We find that the distribution of alteration minerals is not homogeneous; rather, they occur in provinces with distinctive assemblages of alteration minerals. Key findings are (1) a distinctive stratigraphy, in and around the Nili Fossae, of kaolinite and magnesium carbonate in bedrock units always overlying Fe/Mg smectites and (2) evidence for mineral phases and assemblages indicative of low-grade metamorphic or hydrothermal aqueous alteration in cratered terrains. The alteration minerals around the Nili Fossae are more typical of those resulting from neutral to alkaline conditions rather than acidic conditions, which appear to have dominated much of Mars. Moreover, the mineralogic diversity and geologic context of alteration minerals found in the region around the Nili Fossae indicates several episodes of aqueous activity in multiple distinct environments.

Journal of Geophysical Research E: Planets↗
USGS2010

Seasonal H2O and CO2 ice cycles at the Mars Phoenix landing site: 1. Prelanding CRISM and HiRISE observations

The condensation, evolution, and sublimation of seasonal water and carbon dioxide ices were characterized at the Mars Phoenix landing site from Martian northern midsummer to midspring (L s ∼ 142° – L s ∼ 60°) for the year prior to the Phoenix landing on 25 May 2008. Ice relative abundances and grain sizes were estimated using data from the Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment aboard Mars Reconnaissance Orbiter and a nonlinear mixing model. Water ice first appeared at the Phoenix landing site during the afternoon in late summer (L s ∼ 167°) as an optically thin layer on top of soil. CO 2 ice appeared after the fall equinox. By late winter (L s ∼ 344°), the site was covered by relatively pure CO 2 ice (∼30 cm thick), with a small amount of ∼100 μ m diameter water ice and soil. As spring progressed, CO 2 ice grain sizes gradually decreased, a change interpreted to result from granulation during sublimation losses. The combined effect of CO 2 sublimation and decreasing H 2 O ice grain sizes allowed H 2 O ice to dominate spectra during the spring and significantly brightened the surface. CO 2 ice disappeared by early spring (L s ∼ 34°) and H 2 O ice by midspring (L s ∼ 59°). Spring defrosting was not uniform and occurred more rapidly over the centers of polygons and geomorphic units with relatively higher thermal inertia values.

Journal of Geophysical Research E: Planets↗
USGS2010

Mineralogy and stratigraphy of phyllosilicate-bearing and dark mantling units in the greater Mawrth Vallis/west Arabia Terra area: Constraints on geological origin

Analyses of MRO/CRISM images of the greater Mawrth Vallis region of Mars affirm the presence of two primary phyllosilicate assemblages throughout a region ∼1000 × 1000 km. These two units consist of an Fe/Mg-phyllosilicate assemblage overlain by an Al-phyllosilicate and hydrated silica assemblage. The lower unit contains Fe/Mg-smectites, sometimes combined with one or more of these other Fe/Mg-phyllosilicates: serpentine, chlorite, biotite, and/or vermiculite. It is more than 100 m thick and finely layered at meter scales. The upper unit includes Al-smectite, kaolin group minerals, and hydrated silica. It is tens of meters thick and finely layered as well. A common phyllosilicate stratigraphy and morphology is observed throughout the greater region wherever erosional windows are present. This suggests that the geologic processes forming these units must have occurred on at least a regional scale. Sinuous ridges (interpreted to be inverted channels) and narrow channels cut into the upper clay-bearing unit suggesting that aqueous processes were prevalent after, and possibly during, the deposition of the layered units. We propose that layered units may have been deposited at Mawrth Vallis and then subsequently altered to form the hydrated units. The Fe/Mg-phyllosilicate assemblage is consistent with hydrothermal alteration or pedogenesis of mafic to ultramafic rocks. The Al-phyllosilicate/hydrated silica unit may have formed through alteration of felsic material or via leaching of basaltic material through pedogenic alteration or a mildly acidic environment. These phyllosilicate-bearing units are overlain by a darker, relatively unaltered, and indurated material that has probably experienced a complex geological history.

Journal of Geophysical Research↗
USGS2010

Detection and mapping of hydrocarbon deposits on Titan

We report the identification of compounds on Titan's surface by spatially resolved imaging spectroscopy methods through Titan's atmosphere, and set upper limits to other organic compounds. We present evidence for surface deposits of solid benzene (C 6 H 6 ), solid and/or liquid ethane (C 2 H 6 ), or methane (CH 4 ), and clouds of hydrogen cyanide (HCN) aerosols using diagnostic spectral features in data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). Cyanoacetylene (2-propynenitrile, IUPAC nomenclature, HC 3 N) is indicated in spectra of some bright regions, but the spectral resolution of VIMS is insufficient to make a unique identification although it is a closer match to the feature previously attributed to CO 2 . We identify benzene, an aromatic hydrocarbon, in larger abundances than expected by some models. Acetylene (C 2 H 2 ), expected to be more abundant on Titan according to some models than benzene, is not detected. Solid acetonitrile (CH 3 CN) or other nitriles might be candidates for matching other spectral features in some Titan spectra. An as yet unidentified absorption at 5.01- μ m indicates that yet another compound exists on Titan's surface. We place upper limits for liquid methane and ethane in some locations on Titan and find local areas consistent with millimeter path lengths. Except for potential lakes in the southern and northern polar regions, most of Titan appears “dry.” Finally, we find there is little evidence for exposed water ice on the surface. Water ice, if present, must be covered with organic compounds to the depth probed by 1–5- μ m photons: a few millimeters to centimeters.

Journal of Geophysical Research - Planets↗
USGS2010

A comparison of Martian north and south polar cold spots and the long‐term effects of the 2001 global dust storm

In the 1970s, Viking and Mariner observed areas in the polar regions of Mars with winter brightness temperatures below the expected kinetic temperatures for CO 2 ice sublimation. These areas have since been termed “cold spots” and have been identified as surface deposits of CO 2 atmospheric condensates and, occasionally, active CO 2 storms. Three Mars years of data from the Mars Global Surveyor Thermal Emission Spectrometer were used to observe autumn and winter cold spot activity. In this study, cold spots that occur near and on the southern perennial cap were compared to those found near or on the northern perennial cap. On the southern perennial cap, cold spots associated with topographic features (induced by orographic lifting) were less common than cold spots independent of topography, similar to the north. However, the cold spots in the south lasted longer than those observed in the north. There is also evidence that cold spot formation in the south was affected by the global dust storm of 2001, even though the dust storm occurred during the southern spring and summer seasons. Prior to the dust storm, the amount of overall cold spot activity closer to the perennial cap increased and the average CO 2 grain size for most of the cold spots increased as well. Following the dust storm, the majority of cold spots in the south increased in size and duration but they did not form north of 62°S latitude, whereas, in other years, cold spots formed as far north as 48°S.

Journal of Geophysical Research E: Planets↗
USGS2010

Crater population and resurfacing of the Martian north polar layered deposits

Present‐day accumulation in the north polar layered deposits (NPLD) is thought to occur via deposition on the north polar residual cap. Understanding current mass balance in relation to current climate would provide insight into the climatic record of the NPLD. To constrain processes and rates of NPLD resurfacing, a search for craters was conducted using images from the Mars Reconnaissance Orbiter Context Camera. One hundred thirty craters have been identified on the NPLD, 95 of which are located within a region defined to represent recent accumulation. High Resolution Imaging Science Experiment images of craters in this region reveal a morphological sequence of crater degradation that provides a qualitative understanding of processes involved in crater removal. A classification system for these craters was developed based on the amount of apparent degradation and infilling and where possible depth/diameter ratios were determined. The temporal and spatial distribution of crater degradation is interpreted to be close to uniform. Through comparison of the size‐frequency distribution of these craters with the expected production function, the craters are interpreted to be an equilibrium population with a crater of diameter D meters having a lifetime of ∼30.75 D 1.14 years. Accumulation rates within these craters are estimated at 7.2 D −0.14 mm/yr, which corresponds to values of ∼3–4 mm/yr and are much higher than rates thought to apply to the surrounding flat terrain. The current crater population is estimated to have accumulated in the last ∼20 kyr or less.

Journal of Geophysical Research E: Planets↗
USGS2010

Bathymetry and absorptivity of Titan's Ontario Lacus

Ontario Lacus is the largest and best characterized lake in Titan's south polar region. In June and July 2009, the Cassini RADAR acquired its first Synthetic Aperture Radar (SAR) images of the area. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study the lake's nearshore bathymetry and complex refractive properties. Average radar backscatter is observed to decrease exponentially with distance from the local shoreline. This behavior is consistent with attenuation through a deepening layer of liquid and, if local topography is known, can be used to derive absorptive dielectric properties. Accordingly, we estimate nearshore topography from a radar altimetry profile that intersects the shoreline on the East and West sides of the lake. We then analyze SAR backscatter in these regions to determine the imaginary component of the liquid's complex index of refraction (κ). The derived value, κ = (6.1 −1.3 +1.7 ) × 10 −4 , corresponds to a loss tangent of tan Δ = (9.2 −2.0 +2.5 ) × 10 −4 and is consistent with a composition dominated by liquid hydrocarbons. This value can be used to test compositional models once the microwave optical properties of candidate materials have been measured. In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming the index of refraction is constant throughout the liquid. Accordingly, we construct a coarse bathymetry map for the nearshore region by measuring bathymetric slopes for eleven additional areas around the lake. These slopes vary by a factor of ∼5 and correlate well with observed shoreline morphologies.

Journal of Geophysical Research E: Planets↗
USGS2010

Complexities in barrier island response to sea level rise: Insights from numerical model experiments, North Carolina Outer Banks

Using a morphological-behavior model to conduct sensitivity experiments, we investigate the sea level rise response of a complex coastal environment to changes in a variety of factors. Experiments reveal that substrate composition, followed in rank order by substrate slope, sea level rise rate, and sediment supply rate, are the most important factors in determining barrier island response to sea level rise. We find that geomorphic threshold crossing, defined as a change in state (e.g., from landward migrating to drowning) that is irreversible over decadal to millennial time scales, is most likely to occur in muddy coastal systems where the combination of substrate composition, depth-dependent limitations on shoreface response rates, and substrate erodibility may prevent sand from being liberated rapidly enough, or in sufficient quantity, to maintain a subaerial barrier. Analyses indicate that factors affecting sediment availability such as low substrate sand proportions and high sediment loss rates cause a barrier to migrate landward along a trajectory having a lower slope than average barrier island slope, thereby defining an “effective” barrier island slope. Other factors being equal, such barriers will tend to be smaller and associated with a more deeply incised shoreface, thereby requiring less migration per sea level rise increment to liberate sufficient sand to maintain subaerial exposure than larger, less incised barriers. As a result, the evolution of larger/less incised barriers is more likely to be limited by shoreface erosion rates or substrate erodibility making them more prone to disintegration related to increasing sea level rise rates than smaller/more incised barriers. Thus, the small/deeply incised North Carolina barriers are likely to persist in the near term (although their long-term fate is less certain because of the low substrate slopes that will soon be encountered). In aggregate, results point to the importance of system history (e.g., previous slopes, sediment budgets, etc.) in determining migration trajectories and therefore how a barrier island will respond to sea level rise. Although simple analytical calculations may predict barrier response in simplified coastal environments (e.g., constant slope, constant sea level rise rate, etc.), our model experiments demonstrate that morphological-behavior modeling is necessary to provide critical insights regarding changes that may occur in environments having complex geometries, especially when multiple parameters change simultaneously.

North Carolina↗
USGS2010

Estimating the timing and location of shallow rainfall-induced landslides using a model for transient, unsaturated infiltration

Shallow rainfall-induced landslides commonly occur under conditions of transient infiltration into initially unsaturated soils. In an effort to predict the timing and location of such landslides, we developed a model of the infiltration process using a two-layer system that consists of an unsaturated zone above a saturated zone and implemented this model in a geographic information system (GIS) framework. The model links analytical solutions for transient, unsaturated, vertical infiltration above the water table to pressure-diffusion solutions for pressure changes below the water table. The solutions are coupled through a transient water table that rises as water accumulates at the base of the unsaturated zone. This scheme, though limited to simplified soil-water characteristics and moist initial conditions, greatly improves computational efficiency over numerical models in spatially distributed modeling applications. Pore pressures computed by these coupled models are subsequently used in one-dimensional slope-stability computations to estimate the timing and locations of slope failures. Applied over a digital landscape near Seattle, Washington, for an hourly rainfall history known to trigger shallow landslides, the model computes a factor of safety for each grid cell at any time during a rainstorm. The unsaturated layer attenuates and delays the rainfall-induced pore-pressure response of the model at depth, consistent with observations at an instrumented hillside near Edmonds, Washington. This attenuation results in realistic estimates of timing for the onset of slope instability (7 h earlier than observed landslides, on average). By considering the spatial distribution of physical properties, the model predicts the primary source areas of landslides.

Journal of Geophysical Research F: Earth Surface↗
USGS2011

Flow structures and sandbar dynamics in a canyon river during a controlled flood, Colorado River, Arizona

In canyon rivers, debris fan constrictions create rapids and downstream pools characterized by secondary flow structures that are closely linked to channel morphology. In this paper we describe detailed measurements of the three‐dimensional flow structure and sandbar dynamics of two pools along the Colorado River in the Grand Canyon during a controlled flood release from Glen Canyon Dam. Results indicate that the pools are characterized by large lateral recirculation zones (eddies) resulting from flow separation downstream from the channel constrictions, as well as helical flow structures in the main channel and eddy. The lateral recirculation zones are low‐velocity areas conducive to fine sediment deposition, particularly in the vicinity of the separation and reattachment points and are thus the dominant flow structures controlling sandbar dynamics. The helical flow structures also affect morphology but appear secondary in importance to the lateral eddies. During the controlled flood, sandbars in the separation and reattachment zones at both sites tended to build gradually during the rising limb and peak flow. Deposition in shallow water on the sandbars was accompanied by erosion in deeper water along the sandbar slope at the interface with the main channel. Erosion occurred via rapid mass failures as well as by gradual boundary shear stress driven processes. The flow structures and morphologic links at our study sites are similar to those identified in other river environments, in particular sharply curved meanders and channel confluences where the coexistence of lateral recirculation and helical flows has been documented.

Journal of Geophysical Research F: Earth Surface↗
USGS2010

A universal approximation to grain size from images of non-cohesive sediment

The two-dimensional spectral decomposition of an image of sediment provides a direct statistical estimate, grid-by-number style, of the mean of all intermediate axes of all single particles within the image. We develop and test this new method which, unlike existing techniques, requires neither image processing algorithms for detection and measurement of individual grains, nor calibration. The only information required of the operator is the spatial resolution of the image. The method is tested with images of bed sediment from nine different sedimentary environments (five beaches, three rivers, and one continental shelf), across the range 0.1 mm to 150 mm, taken in air and underwater. Each population was photographed using a different camera and lighting conditions. We term it a “universal approximation” because it has produced accurate estimates for all populations we have tested it with, without calibration. We use three approaches (theory, computational experiments, and physical experiments) to both understand and explore the sensitivities and limits of this new method. Based on 443 samples, the root-mean-squared (RMS) error between size estimates from the new method and known mean grain size (obtained from point counts on the image) was found to be ±≈16%, with a 95% probability of estimates within ±31% of the true mean grain size (measured in a linear scale). The RMS error reduces to ≈11%, with a 95% probability of estimates within ±20% of the true mean grain size if point counts from a few images are used to correct bias for a specific population of sediment images. It thus appears it is transferable between sedimentary populations with different grain size, but factors such as particle shape and packing may introduce bias which may need to be calibrated for. For the first time, an attempt has been made to mathematically relate the spatial distribution of pixel intensity within the image of sediment to the grain size.

Journal of Geophysical Research F: Earth Surface↗
USGS2010

The perfect debris flow? Aggregated results from 28 large-scale experiments

Aggregation of data collected in 28 controlled experiments reveals reproducible debris-flow behavior that provides a clear target for model tests. In each experiment ∼10 m 3 of unsorted, water-saturated sediment composed mostly of sand and gravel discharged from behind a gate, descended a steep, 95-m flume, and formed a deposit on a nearly horizontal runout surface. Experiment subsets were distinguished by differing basal boundary conditions (1 versus 16 mm roughness heights) and sediment mud contents (1 versus 7 percent dry weight). Sensor measurements of evolving flow thicknesses, basal normal stresses, and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated, coarse-grained, high-friction snouts, followed by bodies of nearly liquefied, finer-grained debris. Mud enhanced flow mobility by maintaining high pore pressures in flow bodies, and bed roughness reduced flow speeds but not distances of flow runout. Roughness had these effects because it promoted debris agitation and grain-size segregation, and thereby aided growth of lateral levees that channelized flow. Grain-size segregation also contributed to development of ubiquitous roll waves, which had diverse amplitudes exhibiting fractal number-size distributions. Despite the influence of these waves and other sources of dispersion, the aggregated data have well-defined patterns that help constrain individual terms in a depth-averaged debris-flow model. The patterns imply that local flow resistance evolved together with global flow dynamics, contradicting the hypothesis that any consistent rheology applied. We infer that new evolution equations, not new rheologies, are needed to explain how characteristic debris-flow behavior emerges from the interactions of debris constituents.

Journal of Geophysical Research F: Earth Surface↗
USGS2010

Iceberg calving as a primary source of regional‐scale glacier‐generated seismicity in the St. Elias Mountains, Alaska

Since the installation of the Alaska Regional Seismic Network in the 1970s, data analysts have noted nontectonic seismic events thought to be related to glacier dynamics. While loose associations with the glaciers of the St. Elias Mountains have been made, no detailed study of the source locations has been undertaken. We performed a two-step investigation surrounding these events, beginning with manual locations that guided an automated detection and event sifting routine. Results from the manual investigation highlight characteristics of the seismic waveforms including single-peaked (narrowband) spectra, emergent onsets, lack of distinct phase arrivals, and a predominant cluster of locations near the calving termini of several neighboring tidewater glaciers. Through these locations, comparison with previous work, analyses of waveform characteristics, frequency-magnitude statistics and temporal patterns in seismicity, we suggest calving as a source for the seismicity. Statistical properties and time series analysis of the event catalog suggest a scale-invariant process that has no single or simple forcing. These results support the idea that calving is often a response to short-lived or localized stress perturbations. Our results demonstrate the utility of passive seismic instrumentation to monitor relative changes in the rate and magnitude of iceberg calving at tidewater glaciers that may be volatile or susceptible to ensuing rapid retreat, especially when existing seismic infrastructure can be used.

Alaska↗
USGS2010

Testing statistical self-similarity in the topology of river networks

Recent work has demonstrated that the topological properties of real river networks deviate significantly from predictions of Shreve's random model. At the same time the property of mean self-similarity postulated by Tokunaga's model is well supported by data. Recently, a new class of network model called random self-similar networks (RSN) that combines self-similarity and randomness has been introduced to replicate important topological features observed in real river networks. We investigate if the hypothesis of statistical self-similarity in the RSN model is supported by data on a set of 30 basins located across the continental United States that encompass a wide range of hydroclimatic variability. We demonstrate that the generators of the RSN model obey a geometric distribution, and self-similarity holds in a statistical sense in 26 of these 30 basins. The parameters describing the distribution of interior and exterior generators are tested to be statistically different and the difference is shown to produce the well-known Hack's law. The inter-basin variability of RSN parameters is found to be statistically significant. We also test generator dependence on two climatic indices, mean annual precipitation and radiative index of dryness. Some indication of climatic influence on the generators is detected, but this influence is not statistically significant with the sample size available. Finally, two key applications of the RSN model to hydrology and geomorphology are briefly discussed.

Journal of Geophysical Research F: Earth Surface↗
USGS2009

Fluorescence-based proxies for lignin in freshwater dissolved organic matter

Lignin phenols have proven to be powerful biomarkers in environmental studies; however, the complexity of lignin analysis limits the number of samples and thus spatial and temporal resolution in any given study. In contrast, spectrophotometric characterization of dissolved organic matter (DOM) is rapid, noninvasive, relatively inexpensive, requires small sample volumes, and can even be measured in situ to capture fine-scale temporal and spatial detail of DOM cycling. Here we present a series of cross-validated Partial Least Squares models that use fluorescence properties of DOM to explain up to 91% of lignin compositional and concentration variability in samples collected seasonally over 2 years in the Sacramento River/San Joaquin River Delta in California, United States. These models were subsequently used to predict lignin composition and concentration from fluorescence measurements collected during a diurnal study in the San Joaquin River. While modeled lignin composition remained largely unchanged over the diurnal cycle, changes in modeled lignin concentrations were much greater than expected and indicate that the sensitivity of fluorescence-based proxies for lignin may prove invaluable as a tool for selecting the most informative samples for detailed lignin characterization. With adequate calibration, similar models could be used to significantly expand our ability to study sources and processing of DOM in complex surface water systems.

Journal of Geophysical Research G: Biogeosciences↗
USGS2009

Projected changes in atmospheric heating due to changes in fire disturbance and the snow season in the western Arctic, 2003–2100

In high latitudes, changes in climate impact fire regimes and snow cover duration, altering the surface albedo and the heating of the regional atmosphere. In the western Arctic, under four scenarios of future climate change and future fire regimes (2003–2100), we examined changes in surface albedo and the related changes in regional atmospheric heating due to: (1) vegetation changes following a changing fire regime, and (2) changes in snow cover duration. We used a spatially explicit dynamic vegetation model (Alaskan Frame-based Ecosystem Code) to simulate changes in successional dynamics associated with fire under the future climate scenarios, and the Terrestrial Ecosystem Model to simulate changes in snow cover. Changes in summer heating due to the changes in the forest stand age distributions under future fire regimes showed a slight cooling effect due to increases in summer albedo (mean across climates of −0.9 W m −2 decade −1 ). Over this same time period, decreases in snow cover (mean reduction in the snow season of 4.5 d decade −1 ) caused a reduction in albedo, and a heating effect (mean across climates of 4.3 W m −2 decade −1 ). Adding both the summer negative change in atmospheric heating due to changes in fire regimes to the positive changes in atmospheric heating due to changes in the length of the snow season resulted in a 3.4 W m −2 decade −1 increase in atmospheric heating. These findings highlight the importance of gaining a better understanding of the influences of changes in surface albedo on atmospheric heating due to both changes in the fire regime and changes in snow cover duration.

Journal of Geophysical Research: Biogeosciences↗