Heat Flow in the Arctic

Defines heat flow as the flux at the earth's solid surface of heat conducted from the interior; the heat-flow-unit (hfu) is on the order of 1-millionth calorie through each sq cm of the surface/sec, which is enough to melt a 4-mm layer of ice over the earth's surface/yr. Earth heat originates from radioactive decay of U, Th and K in the crust and mantle. Although land heat-flow measurements in the Arctic are too few for regional interpretation, those from Cape Thompson, Barrow and Cape Simpson, Northern Alaska are discussed and figured to show what they contribute to understanding of permafrost, climatic change and shoreline movements. Measuring thermal conductivity and gradient is much simpler in ocean basins than on land. Locations of such measurements are mapped, the results for the Alaskan quadrant in more detail. The sharp change in heat flow at the edge of the Alpha Cordillera, shown in a geothermal model, suggests that this feature is a huge accumulation of basalt, rather than mantle material or remnant of a foundering continent as previously postulated. Future Arctic heat flow studies are discussed

A 200-year 210Pb record from Greenland

A continuous profile of 210Pb activity extending back to 1766 has been developed for a firn/ice core collected at Site D in central Greenland in 1984. Unexpectedly high activities of 210Pb were found at the base of this core (0.032 pCi kg−1 in samples more than 200 years old), calling into question the common assumption that supported 210Pb can be neglected when constructing chronologies in glacial snow and ice. It is problematic to assert that all of the 210Pb measured at depth should be attributed to the supported fraction, given previous estimates of dust loading in Greenland ice cores. However, even if an estimated constant value of 0.032 pCi supported210Pb kg−1 is subtracted from the measured values to estimate excess 210Pb, the 210Pb chronology for Site D yields ages that are significantly younger (mean accumulation rate too high) than an independent depth-age scale based on annual layer counting. It is apparent that the flux of excess and/or supported 210Pb to this site must have decreased over the past 2 centuries, with decreasing trends in both fractions most likely. Previously published 210Pb profiles for cores from Summit and Dye 3, Greenland, show similar trends, which had been interpreted as decreasing fluxes of excess 210Pb only. For all three sites, it is not possible to separate variations in the fluxes of the excess and supported fractions of 210Pb, but variations in the total 210Pb flux will impact 210Pb-based chronologies generally if these variations have not been restricted to the Greenland ice sheet

Stress dependent thermal pressurization of a fluid-saturated rock

Temperature increase in saturated porous materials under undrained conditions leads to thermal pressurization of the pore fluid due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. This increase in the pore fluid pressure induces a reduction of the effective mean stress and can lead to shear failure or hydraulic fracturing. The equations governing the phenomenon of thermal pressurization are presented and this phenomenon is studied experimentally for a saturated granular rock in an undrained heating test under constant isotropic stress. Careful analysis of the effect of mechanical and thermal deformation of the drainage and pressure measurement system is performed and a correction of the measured pore pressure is introduced. The test results are modelled using a non-linear thermo-poro-elastic constitutive model of the granular rock with emphasis on the stress-dependent character of the rock compressibility. The effects of stress and temperature on thermal pressurization observed in the tests are correctly reproduced by the model

Application of Two-Part Statistics for Comparison of Sequence Variant Counts

Investigation of microbial communities, particularly human associated communities, is significantly enhanced by the vast amounts of sequence data produced by high throughput sequencing technologies. However, these data create high-dimensional complex data sets that consist of a large proportion of zeros, non-negative skewed counts, and frequently, limited number of samples. These features distinguish sequence data from other forms of high-dimensional data, and are not adequately addressed by statistical approaches in common use. Ultimately, medical studies may identify targeted interventions or treatments, but lack of analytic tools for feature selection and identification of taxa responsible for differences between groups, is hindering advancement. The objective of this paper is to examine the application of a two-part statistic to identify taxa that differ between two groups. The advantages of the two-part statistic over common statistical tests applied to sequence count datasets are discussed. Results from the t-test, the Wilcoxon test, and the two-part test are compared using sequence counts from microbial ecology studies in cystic fibrosis and from cenote samples. We show superior performance of the two-part statistic for analysis of sequence data. The improved performance in microbial ecology studies was independent of study type and sequence technology used

Development and geometry of isotropic and directional shrinkage crack patterns

We have studied shrinkage crack patterns which form when a thin layer of an alumina/water slurry dries. Both isotropic and directional drying were studied. The dynamics of the pattern formation process and the geometric properties of the isotropic crack patterns are similar to what is expected from recent models, assuming weak disorder. There is some evidence for a gradual increase in disorder as the drying layer become thinner, but no sudden transition, in contrast to what has been seen in previous experiments. The morphology of the crack patterns is influenced by drying gradients and front propagation effects, with sharp gradients having a strong orienting and ordering effect.Comment: 8 pages, 11 figures, 8 in jpg format, 3 in postscript. See also http://mobydick.physics.utoronto.ca/mud.htm

Moving water well: comparing hydraulic efficiency in twigs and trunks of coniferous, ring-porous, and diffuse-porous saplings from temperate and tropical forests

• Coniferous, diffuse‐porous and ring‐porous trees vary in their xylem anatomy, but the functional consequences of these differences are not well understood from the scale of the conduit to the individual. • Hydraulic and anatomical measurements were made on branches and trunks from 16 species from temperate and tropical areas, representing all three wood types. Scaling of stem conductivity (Kh) with stem diameter was used to model the hydraulic conductance of the stem network. • Ring‐porous trees showed the steepest increase in Kh with stem size. Temperate diffuse‐porous trees were at the opposite extreme, and conifers and tropical diffuse‐porous species were intermediate. Scaling of Kh was influenced by differences in the allometry of conduit diameter (taper) and packing (number per wood area) with stem size. • The Kh trends were mirrored by the modeled stem‐network conductances. Ring‐porous species had the greatest network conductance and this value increased isometrically with trunk basal area, indicating that conductance per unit sapwood was independent of tree size. Conductances were lowest and most size‐dependent in conifers. The results indicate that differences in conduit taper and packing between functional types propagate to the network level and have an important influence on metabolic scaling concepts.Keywords: sapling, vessel, tracheid, conduit frequency, hydraulic architecture, network efficienc

Malaria vaccine efficacy: the difficulty of detecting and diagnosing malaria

New sources of funding have revitalized efforts to control malaria. An effective vaccine would be a tremendous asset in the fight against this devastating disease and increasing financial and scientific resources are being invested to develop one. A few candidates have been tested in Phase I and II clinical trials, and several others are poised to begin trials soon. Some studies have been promising, and others disappointing. It is difficult to compare the results of these clinical trials; even independent trials of the same vaccine give highly discrepant results. One major obstacle in evaluating malaria vaccines is the difficulty of diagnosing clinical malaria. This analysis evaluates the impact of diagnostic error, particularly that introduced by microscopy, on the outcome of efficacy trials of malaria vaccines and make recommendations for improving future trials

Structure of the Dead Sea Pull-Apart Basin From Gravity Analyses

Analyses and modeling of gravity data in the Dead Sea pull-apart basin reveal the geometry of the basin and constrain models for its evolution. The basin is located within a valley which defines the Dead Sea transform plate boundary between Africa and Arabia. Three hundred kilometers of continuous marine gravity data, collected in a lake occupying the northern part of the basin, were integrated with land gravity data from Israel and Jordan to provide coverage to 30 km either side of the basin. Free-air and variable-density Bouguer anomaly maps, a horizontal first derivative map of the Bouguer anomaly, and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin. The basin is a long (132 km), narrow (7-10 km), and deep (≤10 km) full graben which is bounded by subvertical faults along its long sides. The Bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends, suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends. The surface expression of the basin is wider at its center (≤16 km) and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin. These blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened. The collapse was probably facilitated by movement along the normal faults that bound the transform valley. We present a model in which the geometry of the Dead Sea basin (i.e., full graben with relative along-axis symmetry) may be controlled by stretching of the entire (brittle and ductile) crust along its long axis. There is no evidence for the participation of the upper mantle in the deformation of the basin, and the Moho is not significantly elevated. The basin is probably close to being isostatically uncompensated, and thermal effects related to stretching are expected to be minimal. The amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1.19. If the rate of crustal stretching is similar to the rate of relative plate motion (6 mm/yr), the basin should be ~3.5 m.y. old, in accord with geological evidence