A framework for understanding the hydroecology of impacted wet meadows in the Sierra Nevada and Cascade Ranges, California, USA

Meadows of the Sierra Nevada and Cascade mountains of California, USA, support diverse and highly productive wet-meadow vegetation dominated by sedges, rushes, grasses, and other herbaceous species. These groundwater–dependent ecosystems rely on the persistence of a shallow water table throughout the dry summer. Case studies of Bear Creek, Last Chance, and Tuolumne meadow ecosystems are used to create a conceptual framework describing groundwater–ecosystem connections in this environment. The water requirements for wet-meadow vegetation at each site are represented as a water-table-depth hydrograph; however, these hydrographs were found to vary among sites. Causes of this variation include (1) differences in soil texture, which govern capillary effects and availability of vadose water and (2) elevation-controlled differences in climate that affect the phenology of the vegetation. The field observations show that spatial variation of water-table depth exerts strong control on vegetation composition and spatial patterning. Groundwater-flow modeling demonstrates that lower hydraulic-conductivity meadow sediments, higher groundwater-inflow rates, and a higher ratio of lateral to basal-groundwater inflow all encourage the persistence of a high water table and wet-meadow vegetation, particularly at the margin of the meadow, even in cases with moderate stream incision.     

Holocene lake-level trends in the Rocky Mountains, U.S.A.

The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability.     

A non-magmatic iron projectile for the Gardnos impact event

The goal of this study is to identify the type of projectile responsible for the formation of the late Precambrian Gardnos impact structure in Norway. Fifteen impactite samples, predominantly impact breccias and suevites from the central and northeastern part of the structure, were analyzed for platinum group elements (PGE) and Au using nickel-sulfide fire assay combined with inductively coupled plasma mass spectrometry (ICP-MS). Major and trace elements were measured in the same samples using X-ray fluorescence (XRF). In addition, the concentrations of siderophile elements Ni, Cr, and Co were determined by ICP-MS after acid digestion. The samples collected at the contact between suevite and the sedimentary infill yielded the highest PGE concentrations (Ir = 1.926 ng/g, Ru = 3.494 ng/g, Pt = 4.716 ng/g, Rh = 0.766 ng/g, Pd = 2.842 ng/g for GC6). The CI-normalized PGE patterns are characterized by Ru and Rh enrichments suggesting a non-chondritic impactor. Concentration plots of the different PGE display an excellent correlation (R > 0.99), indicative of a single source for the PGE enrichment. The Ni/Cr ratio of the Gardnos impactor (2.56 ± 0.20) agrees with that of chondrites (2 to 7), whereas Ir is depleted relative to Ni in this projectile (Ni/Ir ratio of 92 000 ± 8000 compared to an average Ni/Ir ratio of 23 150 ± 4250 for chondrites). There is no clear indication of selective post-depositional remobilization of the characteristic highly siderophile elements. The Ni/Ir and Cr/Ir data combined with the non-chondritic PGE ratios probably indicate a differentiated projectile. Based on (1) the similarity of the inter-element ratios of the impactor with the iron phase of non-magmatic iron meteorites and (2) the presence of characteristics of both chondrites and iron meteorites (Ni/Cr and Ni/Ir ratios), an IA or IIIC non-magmatic iron meteorite is a very plausible impactor.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Terrestrial vegetation dynamics and global climate controls

Monthly data from the moderate resolution imaging spectroradiometer (MODIS) and its predecessor satellite sensors was used to reconstruct vegetation dynamics in response to climate patterns over the period 1983–2005. Results suggest that plant growth over extensive land areas of southern Africa and Central Asia were the most closely coupled of any major land area to El Niño–southern oscillation (ENSO) effects on regional climate. Others land areas strongly tied to recent ENSO climate effects were in northern Canada, Alaska, western US, northern Mexico, northern Argentina, and Australia. Localized variations in precipitation were the most common controllers of monthly values for the fraction absorbed of photosynthetically active radiation (FPAR) over these regions. In addition to the areas cited above, seasonal FPAR values from MODIS were closely coupled to rainfall patterns in grassland and cropland areas of the northern and central US. Historical associations between global vegetation FPAR and atmospheric carbon dioxide (CO₂) anomalies suggest that the terrestrial biosphere can contribute major fluxes of CO₂ during major drought events, such as those triggered by 1997–1998 El Niño event.     

Radar observations and a physical model of contact binary Asteroid 4486 Mithra

Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) delay-Doppler radar images obtained in July and August of 2000 reveal that 4486 Mithra is an irregular, significantly bifurcated object, with a central valley ∼380 m deep and a long axis potentially exceeding 2 km. With its bimodal appearance, Mithra is a strong candidate for a contact binary asteroid. Sequences of Goldstone images spanning up to 3 h per day show very little rotation and establish that Mithra is an unusually slow rotator. We used Goldstone and Arecibo data to estimate Mithra’s 3D shape and spin state. We obtain prograde (λ = 337°, β = 19°) and retrograde (λ = 154°, β = −19°) models that give comparable fits, have very similar shapes roughly resembling an hourglass, and have a rotation period of 67.5 ± 6.0 h. The dimensions of these two models are very similar; for the prograde solution the maximum dimensions are X = 2.35 ± 0.15 km, Y = 1.65 ± 0.10 km, Z = 1.44 ± 0.10 km. Dynamical analysis of our models suggests that in the past, Mithra most likely went through a period of even slower rotation with its obliquity close to 90°. The spin rate is predicted to be increasing due to thermal torque (YORP), while the obliquity, which is currently +68° and +106° for the prograde and retrograde models, respectively, is predicted to move away from 90°.     

Gem-quality transparent diaspore (zultanite) in bauxite deposits of the İlbir Mountains, Menderes Massif, SW Turkey

Several m-thick, karst-unconformity-type metabauxite horizons in the İlbir Mountains of SW Turkey host open-space mineralization of gem-quality diaspore (trademarked as zultanite), associated with muscovite, hematite, ilmenite, chloritoid, and younger calcite. The hydrothermal-metamorphogenic mineralization occurs in fracture zones (veins and open structures) that crosscut the metabauxite horizons, but does not extend into the marble host rocks. The white to dark gray marble sequence (over 2,000 m in thickness) is of Cretaceous depositional age and was affected by Alpine (Paleogene) tectonometamorphism which caused the hydrothermal remobilization of primary bauxite components into crosscutting structures.     

Europium sorption to sediments in the presence of natural organic matter: A laboratory and modeling study

Cellulosic materials, such as wood, paper products and cardboard that have been co-disposed with low-level nuclear waste have been shown to produce leachate with natural organic matter (NOM) concentrations of hundreds of mg/L C and, as such, have the potential to influence the fate and transport of radionuclides in the subsurface environment. The objective of this study was to examine the influence of NOM on the sorption of Eu (an analogue for trivalent radionuclides) to two coastal plain sediments from the US Department of Energy’s Savannah River Site. Particular attention was directed at quantifying Eu interactions with NOM sorbed to sediments (NOM_sed) in laboratory experiments and developing conditional stability constants for that interaction using the thermodynamic equilibrium speciation model MINTEQA2. Europium sorption to the two sediments systematically increased as pH increased from 3.9 to 6.7. With increasing additions of NOM to the aqueous phase from 0 to 222 mg/L C, Eu sorption initially increased to a maximum at 10 mg/L C NOM_aq and then decreased with increasing NOM_aq concentrations. Increases in Eu sorption at low NOM additions was attributed to the sorption of NOM to the sediment surface increasing the number of sorption sites on the low cation-exchange capacity sediments and/or increasing the association constant (log K) for the Eu-sediment surface reaction. Decreases in Eu sorption at higher NOM levels was attributed to Eu_aq complexation to NOM_aq being more favored than Eu sorption to the solid phase. A component additivity model was developed to describe the Eu–NOM-sediment system by the additive effects of the three binary system models: Eu–NOM, Eu-sediment and NOM-sediment. The model generally captured the data trends in the ternary system. Conditional stability constants developed from the experimental data for the complexation of Eu to NOM_sed were as much as four orders of magnitude greater than Eu complexation with NOM_aq, presumably due to the NOM_sed deriving additional negative (attractive) charge from the sediment surface. At high initial NOM_aq levels, >99 mg/L C, the model captured the trend of reduced Eu sorption but tended to over-estimate Eu sorption. The additivity approach of combining binary models to form a ternary model was only successful when the unique complexation properties of the NOM_sed were properly calculated.     

Geophysical identification of unmarked historic graves

Down‐hole magnetic susceptibility techniques were explored as a means of improving near‐surface geophysical surveys in historic grave detection. These techniques were used to document distinctive magnetic characteristics of grave shafts at three historic cemeteries first surveyed using various near‐surface geophysical methods. Tests revealed a low magnetic susceptibility signature that soil magnetic studies indicated was largely related to differential soil compaction associated with the excavation and refilling of grave shafts. Most apparent at depths beyond those reached by soil penetrometers, this magnetic signature offers a way to assess anomalies identified by near‐surface techniques that potentially represent graves. At one cemetery, magnetic studies of the interments themselves suggested spatially patterned, magnetically enhanced zones that might also aid in burial identification in certain contexts. While down‐hole techniques will not be foolproof, they do provide another geophysical tool that can be used to improve grave detection. © 2010 Wiley Periodicals, Inc.