Fractures as the main pathways of water flow in temperate glaciers

Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaci?ren, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.     

Antarctic climate cooling and terrestrial ecosystem response.

The average air temperature at the Earth's surface has increased by 0.06 degrees C per decade during the 20th century, and by 0.19 degrees C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarctica's peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 degrees C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6-9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.     

An intracellular P2X receptor required for osmoregulation in Dictyostelium discoideum

P2X receptors are membrane ion channels gated by extracellular ATP that are found widely in vertebrates, but not previously in microbes. Here we identify a weakly related gene in the genome of the social amoeba Dictyostelium discoideum, and show, with the use of heterologous expression in human embryonic kidney cells, that it encodes a membrane ion channel activated by ATP (30-100 muM). Site-directed mutagenesis revealed essential conservation of structure-function relations with P2X receptors of higher organisms. The receptor was insensitive to the usual P2X antagonists but was blocked by nanomolar concentrations of Cu2+ ions. In D. discoideum, the receptor was found on intracellular membranes, with prominent localization to an osmoregulatory organelle, the contractile vacuole. Targeted disruption of the gene in D. discoideum resulted in cells that were unable to regulate cell volume in hypotonic conditions. Cell swelling in these mutant cells was accompanied by a marked inhibition of contractile vacuole emptying. These findings demonstrate a new functional role for P2X receptors on intracellular organelles, in this case in osmoregulation.     

Filtered Backprojection Reconstruction with Depth-Dependent Filtering

A direct filtered-backprojection(FBP) reconstruction algorithm is presented for circular cone-beam computed tomography(CB-CT) that allows the filter operation to be applied efficiently with shift-variant band-pass characteristics on the kernel function.Our algorithm is derived from the ramp-filter based FBP method of Feldkamp et al.and obtained by decomposing the ramp filtering into a convolution involving the Hilbert kernel(global operation) and a subsequent differentiation operation(local operation).The d...