The effects of particle breakage and abrasion on the magnetic properties of two soils

As mineral magnetism is used as a tracer for sourcing river and dam sediments, changes in the magnetic properties that may occur during transport between the source and sink must be considered. Abrasion and breakage of particles will occur during transport. These processes were examined in simulation experiments with a granitic and a sedimentary soil. The effects of these processes on the magnetic properties of a granitic and a sedimentary soil were examined using a simulated breakage/abrasion experiment. Breakage and/or abrasion had substantial effects on the magnetic properties of both soils. All particle sizes were affected, but the magnitude varied through the size range of derived particles. The major effect was on the concentration of magnetic minerals, with differences between the concentrations in particle sizes of the original material and those generated by the experiments being as much as 20 times. The effect on the granite soil was to reduce the concentrations, i.e. derived material was less than original material; whereas for the sedimentary soil the derived particles had higher concentrations. The effect on magnetic grain size, as indicated by the magnetic ratios, was less than the effect on the mass magnetic properties, but still substantial for some ratios for some sizes. © 1998 John Wiley & Sons, Ltd.     

Proximity to ice fields and lake depth as modulators of paleoclimate records: a regional study from southwest Yukon,Canada

Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted.     

Macrofaunal Spatial Patterns in Relationship to Environmental Variables in the Richibucto Estuary,New Brunswick,Canada

Estuarine macrobenthos respond to a variety of environmental gradients such as sediment type and salinity, and organic enrichment. A relatively new influence, organic loading from suspended bivalve culture, has the potential to alter this response. A study on soft-bottom macrobenthic communities was carried out in the Richibucto estuary (46°40′N, 64°50′W), New Brunswick, Canada, with samples collected from 18 stations in late September and early October 2006. The site consisted of a large tidal channel originating upstream in a small river. The channel was punctuated by bag culture of oysters along its length. A total of 88 species were recorded. The mean values of abundance, species richness, and diversity (H′) of macrofauna were 11,199 ind. m⁻² (ranged from 4,371 to 19,930 ind. m⁻²), 23.4 species grab⁻¹ and 3.29 grab⁻¹, respectively. In general species richness and H′ increased from the upper estuary to the estuarine mouth. Multivariate analyses clearly exhibited the spatial distribution in community structure, which coincided with the locations along the estuary (the upper, the lower and the mouth), as well as inside and outside the channel. Species richness and diversity H′ showed strong positive correlations with salinity (21.2–25.2 ppt), and abundance was positively correlated with water depth (1.0–4.5 m). Abundance and species richness were negatively correlated with both of silt–clay fraction (3.3–24.8%) and sorting (σ_I). Species richness was also negatively correlated with organic content (1.9–12.7%). The BIO-ENV analyses identified silt–clay fraction, σ_I and salinity as the major environmental variable combination influencing the macrofaunal patterns, and silt–clay fraction as the single best-correlated variable.     

Complex Archean lower crustal structure revealed by COCORP crustal reflection profiling in the Wind River Range,Wyoming

A COCORP deep crustal reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24–31 km in an area where depth relations in Precambrian rocks can be inferred. The different levels of exposure across the beveled plunge of the Wind River uplift reveal supracrustal rocks at shallower levels with migmatites and pyroxene granulites at deeper levels. For the first time, deep crustal structure from reflection profiling may be interpreted in terms of exposed basement geology. A folded, multilayered deep structure shown by relfection data resembles multiply folded pyroxene granulite interlayered with granitic gneiss exposed in the central Wind River uplift; isoclinal folding is suggested in the folded layered seismic structure. Earlier seismic reflection studies suggested a simpler lower crust. These data indicate that lower crustal structure may have a complexity similar to deeply eroded Precambrian granulite-facies rocks. If this seismic feature represents folded metamorphic rocks, it seems unlikely that this Archean crust could have been thickened by underplating after 2.7 b.y. B.P. and the crust would have to be at least 30 km thich when this structure was formed.     

Cloud Forest Conservation in the Central Highlands of Guatemala Hinges on Soil Conservation and Intensifying Food Production

Soil erosion threatens long-term soil fertility and food production in Q’eqchi’ communities native to the Sierra Yalijux and Sierra Sacranix mountain ranges in the central highlands of Guatemala. Environmental factors such as steep topography, erodible soils, and intense precipitation events, combined with land subdivision and reduced fallow periods as a consequence of population growth, contribute to severe erosion and strain soil resources. The preservation of the region's cloud forests hinges on enhancing production of staple crops through agricultural intensification while maintaining soil fertility through implementation of soil conservation measures.     

A lower limit to the field strength in magnetic reconnection sites in solar flares inferred from hard X-ray bursts

It is shown how the hard X-ray burst count rate and itse-folding ime can be used to estimate the minimum magnetic fieldB _min required in a flare magnetic reconnection site for the burst to be interpreted in terms of a thick target model. Application of the method to data from the Solar Maximum Mission (HXRBS) indicates absolute minimum fields well in excess of 100 G, and impossibly high values for some reconnection geometries.     

Microwave Remote Sensing of the Temperature and Distribution of Sulfur Compounds in the Lower Atmosphere of Venus

A multi-wavelength radio frequency observation of Venus was performed on April 5, 1996, with the Very Large Array to investigate potential variations in the vertical and horizontal distribution of temperature and the sulfur compounds sulfur dioxide (SO₂) and sulfuric acid vapor (H₂SO₄(g)) in the atmosphere of the planet. Brightness temperature maps were produced which feature significantly darkened polar regions compared to the brighter low-latitude regions at both observed frequencies. This is the first time such polar features have been seen unambiguously in radio wavelength observations of Venus. The limb-darkening displayed in the maps helps to constrain the vertical profile of H₂SO₄(g), temperature, and to some degree SO₂. The maps were interpreted by applying a retrieval algorithm to produce vertical profiles of temperature and abundance of H₂SO₄(g) given an assumed sub-cloud abundance of SO₂. The results indicate a substantially higher abundance of H₂SO₄(g) at high latitudes (above 45°) than in the low-latitude regions. The retrieved temperature profiles are up to 25 K warmer than the profile obtained by the Pioneer Venus sounder probe at altitudes below 40 km (depending on location and assumed SO₂ abundance). For 150 ppm of SO₂, it is more consistent with the temperature profile obtained by Mariner 5, extrapolated to the surface via a dry adiabat. The profiles obtained for H₂SO₄(g) at high latitudes are consistent with those derived from the Magellan radio occultation experiments, peaking at around 8 ppm at an altitude of 46 km and decaying rapidly away from that altitude. At low latitudes, no significant H₂SO₄(g) is observed, regardless of the assumed SO₂ content. This is well below that measured by Mariner 10 (Lipa and Tyler 1979, Icarus39, 192-208), which peaked at ∼14 ppm near 47 km. Our results favor ≤100 ppm of SO₂ at low latitudes and ≤50 ppm in polar regions. The low-latitude value is statistically consistent with the results of Bézard et al. (1983, Geophs. Res. Lett.20, 1587-1590), who found that a sub-cloud SO₂ abundance of 130±40 ppm best matched their observations in the near-IR. The retrieved temperature profile and higher abundance of H₂SO₄(g) in polar regions are consistent with a strong equatorial-to-polar, cloud-level flow due to a Hadley cell in the atmosphere of Venus.     

Radar observations of asteroid 1998 ML14

Abstract— Goldstone and Arecibo delay‐Doppler radar imaging of asteroid 1998 ML 14 shortly after its discovery reveals a 1 km diameter spheroid with prominent topography on one side and subdued topography on the other. The object's radar and optical properties are typical for S‐class near‐Earth asteroids. The gravitational slopes of a shape model derived from the images and assumed to have a uniform density are shallow, exceeding 30° over only 4% of the surface. If 1998 ML14's density distribution is uniform, then its orbital environment is similar to a planetary body with a spheroidal gravitational field and is relatively stable. Integration of a radar‐refined orbit reveals that the 1998 apparition was the asteroid's closest approach to Earth since at least 1100 and until 2283, when it approaches to within 2.4 lunar distances. Outside of that time interval, orbit uncertainties based on the present set of observations preclude reliable prediction.