Structure-sensitive physical properties of partially melted rocks

Structure-sensitive physical properties (or generalized conductivity) such as electrical conductivity, heat conduction, viscosity and elastic properties significantly change on partial melting. Though the connectivity of a melts is of decisive importance, these properties are also considerably affected by the size, surface properties, shape and orientation of inclusions. Lateral discretness of the asthenosphere can be explained as being a result of inversion in melt distribution pattern, a transition from isolated inclusions and films to interconnected films, which causes drastic changes of the generalized conductivity.     

A comprehensive model current system for high-latitude magnetic activity -II. The substorm component

Summary. In paper I of this series (Hughes & Rostoker) we have presented a composite model three-dimensional current system whose magnetic perturbation pattern is in good agreement with that observed at high latitudes during periods of moderate magnetospheric activity. In this paper we extend the model current system to demonstrate its ability to reproduce the magnetic perturbation pattern associated with the magnetospheric substorms. In agreement with previous workers, we find it necessary to include a region of intense equatorward current flow at the western edge of the westward travelling surge. For the particular substorm event presented in this paper, we find that the main eastward and westward electrojets intensified slightly before the appearance of the surge form, and we are able to evaluate quantitatively the amount of change involved.     

Comparison of the results of quantitative and non-quantitative tracer tests for determination of Karst conduit networks: An example from the Traligill Basin,Scotland

The relative advantages of fluorescent dyes and Lycopodium spores as tracers are discussed. The major advantage of fluorescent dyes is that they may be detected quantitatively. Thus, in combination with discharge measurements, a dye mass balance can be prepared for tracer tests in karst conduits, which permits elucidation of the underground network. The advantages of this procedure are illustrated by comparison of the networks derived from non-quantitative (Lycopodium and dye) and quantitative (fluorescent dye) tracer methods in the Traligill Basin, Scotland. These tests also suggest that Lycopodium does not give a true indication of travel time, due to sedimentation underground. This could also cause contamination problems in later tests. For non-quantitative tracer tests, sensitive methods are necessary if incorrect inferences on conduit networks are to be avoided. In general, however, quantitative tests give much less ambiguous results, and are therefore to be preferred.     

Mode coupling due to twisting of magnetic field lines

A new approach to the theory of mode coupling in an inhomogeneous, birefringent medium is used to treat mode coupling in a magnetized plasma with a twisted magnetic field. The twist introduces a resonance, corresponding to the rate of twisting being equal to the rate of generalized Faraday rotation. When this resonance occurs it introduces a new regime of strong mode coupling. The implications of this regime are discussed in connection with the long-standing problem in solar radiophysics that mode coupling appears to be stronger than theory implies, but no obvious resolution of the problem is found.     

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.