A history of violence: magma incubation,timing and tephra distribution of the Los Chocoyos supereruption (Atitlán Caldera,Guatemala)

The climactic Los Chocoyos (LCY) eruption from Atitlán caldera (Guatemala) is a key chronostratigraphic marker for the Quaternary period given the extensive distribution of its deposits that reached both the Pacific and Atlantic Oceans. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. Using zircon (U–Th)/He geochronology, we present the first radioisotopically determined eruption age for the LCY of 75 ± 2 ka. Additionally, the youngest zircon crystallization ²³⁸U–²³⁰Th rim ages in their respective samples constrain eruption age maxima for two other tephra units that erupted from Atitlán caldera, W-Fall (130 ⁺¹⁶/₋₁₄ ka) and I-Fall eruptions (56 ^+8.2/_−7.7 ka), which under- and overlie LCY tephra, respectively. Moreover, rim and interior zircon dating and glass chemistry suggest that before eruption silicic magma was stored for >80 kyr, with magma accumulation peaking within ca. 35 kyr before the LCY eruption during which the system may have developed into a vertically zoned magma chamber. Based on an updated distribution of LCY pyroclastic deposits, a new conservatively estimated volume of ~1220 ± 150 km³ is obtained (volcanic explosivity index VEI > 8), which confirms the LCY eruption as the first-ever recognized supereruption in Central America.     

Far-infrared photometry of Titan and Iapetus

Titan was observed in four broad passbands between 35 and 150 μm. The brightness temperature in this interval is roughly constant at 76 ± 3°K. Integrating Titan's spectrum from 5 to 150 μm yields an effective temperature of 86 ± 3°K. Both the bright and dark hemispheres of Iapetus were observed in one broadband filter with λ_e ～ 66 μm. The brightness temperatures for these two sides of Iapetus are 96 ± 9°K and 114 ± 10°K, respectively. The bright-side Bond albedo is calculated to be 0.61_?0.22^+0.16.     

Eolian dust from the lower atmosphere of the eastern Atlantic and Indian Oceans,China Sea and Sea of Japan

Dust-loadings in the lower atmosphere of the Atlantic and Indian Oceans and the China Sea and Sea of Japan are given. Average dust-loadings decrease in the following order: North Atlantic (northeast trades, ～ 7.7 μg/m³ of air) > northern Indian Ocean (～ 1.2 μg/m³ of air) > South Atlantic (southeast trades, ～ 0.78 μg/m³ of air) = southern Indian Ocean (～ 0.68 μg/m³ of air) > China Sea (～ 0.21 μg/m³ of air).There are differences in the clay mineralogies of dusts transported in similar latitudes in the North Atlantic and northern Indian Oceans. Dusts in the Atlantic northeast trades are dominated by kaolinite from the soils of equatorial Africa; dusts in the northeast monsoons of the northern Indian Ocean have a source in the arid regions of the Rajasthan desert where illite is the principal clay mineral.In the regions investigated quartz occurs in larger amounts in the dusts of the Northern Hemisphere (～ 7% quartz) than those of the Southern Hemisphere (～ 3% quartz) over both the Atlantic and Indian Oceans.From the results given in the present paper and those reported in the literature a map is presented illustrating the order of magnitude of lower atmospheric dust-loadings in part of the world ocean.     

Preferential flow and segregation of porewater solutes in Wetland sediment

Sediment macropores (with effective diameters larger than 100 μm) comprise 11% of the bulk sediment volume in a tidal freshwater wetland vegetated withPeltandra virginica. In order to determine effects of macroporous sediment structure on solute transport, we conducted a solute tracer experiment in the sediment. The effective transport volume (θ_eff, the volume of sediment through which solute was transported normalized to sediment bulk volume) was 0.15 cm³ cm^?3, which is considerably smaller than the total pore space that is potentially available for transport (porosity of sediment is 0.63 cm³ cm^?3). A mean transport time of 13 d was required to flush preferential flow paths inPeltandra hummocks; hydrologic turnover of the volumetrically dominant matrix pores (0.53 cm³ cm^?3) was apparently much slower. Based on porewater sampler design and hydrological principles, we suggest that N₂-purged tension solution samplers and diffusion equilibrators preferentially sample porewater from macropore and matrix domains, respectively. Dissolved ammonium and orthophosphate concentrations were three-fold higher in matrix pores compared to macropores, which is consistent with our finding that more rapid hydrological flushing occurred in macropores compared to matrix pores. Further evaluation of porewater sampler designs in macroporous sediment is needed to improve studies of hydrologic transport and biogeochemical cycling in wetlands.     

Local relief and the height of Mount Olympus

A three‐dimensional assessment of the net volume of rock differentially eroded from below mountain tops to form valleys yields a range‐wide constraint on feedback between valley development and the height of mountain peaks. The ‘superelevation’ of mountain peaks potentially attributable to differential removal of material from below peaks in the Olympic Mountains, Washington, was constrained by fitting a smoothed surface to the highest elevation points on a 30 m grid digital elevation model of the range. High elevation areas separate into two primary areas: one centred on Mount Olympus in the core of the range and the other at the eastern end of the range. The largest valleys, and hence areas with the greatest volume of differentially eroded material, surround Mount Olympus. In contrast, the highest mean elevations concentrate in the eastern end of the range. Calculation of the isostatic rebound at Mount Olympus attributable to valley development ranges from 500 to 750 m (21 to 32 per cent of its height) for a 5 to 10 km effective elastic thickness of the crust. Comparison of cross‐range trends in mean and maximum elevation reveals that this calculated rebound for Mount Olympus corresponds well with its ‘superelevation’ above the general cross‐range trend in mean elevation. It therefore appears that the location of the highest peak in the Olympics is controlled by the deep valleys excavated in the centre of the range. Copyright © 2000 John Wiley & Sons, Ltd.     

Long-term changes in ground water chemistry at a phytoremediation demonstration site

A field-scale demonstration project was conducted to evaluate the capability of eastern cottonwood trees (Populus deltoides) to attenuate trichloroethene (TCE) contamination of ground water. By the middle of the sixth growing season, trees planted where depth to water was <3 m delivered enough dissolved organic carbon to the underlying aquifer to lower dissolved oxygen concentrations, to create iron-reducing conditions along the plume centerline and sulfate-reducing or methanogenic conditions in localized areas, and to initiate in situ reductive dechlorination of TCE. Apparent biodegradation rate constants for TCE along the centerline of the plume beneath the phytoremediation system increased from 0.0002/d to 0.02/d during the first six growing seasons. The corresponding increase in natural attenuation capacity of the aquifer along the plume centerline, from 0.0004/m to 0.024/m, is associated with a potential decrease in plume-stabilization distance from 9680 to 160 m. Demonstration results provide insight into the amount of vegetation and time that may be needed to achieve cleanup objectives at the field scale.     

Comparison of Ground-Based and Space-Based Longitudinal Magnetograms

We compare photospheric line-of-sight magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) instrument with observations from the 150-foot Solar Tower at Mt. Wilson Observatory (MWO), the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We find very good agreement between VSM and the other data sources for both disk-averaged flux densities and pixel-by-pixel measurements. We show that the VSM mean flux density time series is of consistently high signal-to-noise ratio with no significant zero offsets. We discuss in detail some of the factors ?C spatial resolution, flux dependence, and position on the solar disk ?C affecting the determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The VSM flux densities agree well with spatially smoothed data from MDI and HMI, although the scaling factors show a clear dependence on flux density. The factor to convert VSM to HMI increases with increasing flux density (from ??1 to ??1.5). The nonlinearity is smaller for the VSM vs. SOHO/MDI scaling factor (from ??1 to ??1.2).     

Thermal conductivity of carbonate rocks

The thermal conductivities of several well-defined carbonate rocks were determined near 40°C. Values range from 1.2 W m^?1C^?1 for a highly porous chalk to 5.1 W m^?1C^?1 for a dolomite. The thermal conductivity of magnesite (5.0) is at the high end of the range, and that for Iceland Spar Calcite (3.2) is near the middle. The values for limestones decrease linearly with increasing porosity. Dolomites of comparable porosity have greater thermal conductivities than limestones.Water-sorbed samples have expected greater thermal conductivities than air-saturated (dry) samples of the same rock. An anomalously large increase in the thermal conductivity of a water-sorbed clayey dolomite over that of the same sample when dry is attributed to the clay fraction, which swells during water inhibition, causing more solid-to-solid contacts within the dolomite framework.Measurements were made with a Colora Thermoconductometer. Chemical and mineralogical analyses were made and tabulated. Porosity of the rocks was determined by mercury porosimetry and also from density measurements. The Iceland Spar Calcite and magnesite were included for reference.