Editors' Choice

Science  11 Jun 2004:
Vol. 304, Issue 5677, pp. 1567
  1. CLIMATE

    Means of Production

    Millennial-scale changes in climate during the last glacial period, discovered in ice cores from Greenland, are also clearly recorded by laminated marine sediments from the Santa Barbara Basin. The variations seen in the sediments were thought to have been caused by changes in deep ocean convection originating in the North Pacific. Ortiz et al., using a 52,000-year-long sedimentary core from a location 1500 km to the south, off the coast of Baja California, Mexico, see the same millennial variations but ascribe them to an entirely different cause: surface ocean circulation changes driven by processes rooted in the tropics. Increases in the abundance of benthic forams as great as 100-fold occurred during warm periods, and the authors suggest that they resulted from more intense marine productivity and a concomitant increase in the delivery of organic carbon to the floor of the ocean. Greater productivity would have required more nutrients to upwell from deeper waters, which could have been caused by a shift in the mean state of the ocean toward more El Niño-like conditions, with a regionally deepened nutricline in place during cool climate periods and a shallower one during warm intervals. — HJS

    Geology 32, 521 (2004).

  2. MEDICINE

    Fragile Bones Redefined

    One of the most important predictors of osteoporotic fractures in postmenopausal women is bone mineral density (BMD). There has been considerable debate about the BMD threshold at which therapies to reduce the risk of fracture (lifestyle changes and/or pharmacological agents) should be initiated. Currently, therapy is recommended primarily for women whose severe bone loss classifies them as “osteoporotic,” whereas “osteopenic” women, who show moderate bone loss, are not treated.

    New evidence suggests that osteopenic women are at increased risk for fracture and raises the possibility that they too might benefit from therapeutic intervention. In a large study of postmenopausal Caucasian women in the United States, Siris et al. found that 82% of the women who experienced bone fractures within 1 year of study entry had BMD values in the osteopenic rather than osteoporotic range. Using data from the same cohort of women, Miller et al. designed a classification tool that combines BMD with other risk factors and correctly identified about 75% of the osteopenic women who had sustained a fracture. Current screening strategies for osteoporosis, whose prevalence is rising dramatically with the aging of baby boomers, may thus merit re-examination. — PAK

    Arch. Intern. Med. 164, 1108; 1113 (2004).

  3. ECOLOGY

    Eats, Leaves and Roots

    It is becoming increasingly evident that leaf-eating herbivores can exert influences on soil biology and chemistry. In particular, herbivore pressure can alter patterns of nutrient cycling by changing carbon inputs to the soil, thereby influencing microbial biomass and nitrogen mineralization. However, the mechanisms linking above-ground herbivory to below-ground processes are often obscure. Ayres et al. performed mesocosm experiments in which they simulated herbivory by clipping leaves of beech and fir seedlings and monitored the effects on soil biota and nutrient dynamics over two growing seasons. The two species had different responses: Beech, but not fir, responded to simulated herbivory by increasing leaf production and photosynthetic rate; conversely, fine root biomass in fir, but not beech, was reduced by the treatment. In both cases, however, simulated herbivory led to increased N mineralization in the soil, indicating that physiological responses mediated through plant roots can directly affect nutrient dynamics. — AMS

    Ecol. Lett. 7, 469 (2004).

  4. MATERIALS SCIENCE

    Dissecting a Breakup

    When a material breaks into more than one piece, the cleavage occurs through some combination of brittle and ductile failure. During brittle failure, bonds are ruptured and there is little overall stretching of the material. During ductile failure, atoms or molecules slide past each other and the material deforms plastically before finally breaking. Glass is typically thought to break only through brittle failure, although it is known that plastic deformation will occur when glass is indented or scratched. Recent observations also suggested that cavities formed during glass fracture, implying that brittle materials also fracture via plastic flow instead of via bond rupture.

    Guin and Wiederhorn used atomic force microscopy to study the topology of slowly fractured silica and soda-lime-silicate glasses. Mapping the high-contrast points from one of the fracture surfaces revealed that they mirror those on the opposing surface. Line scans of the two surfaces revealed that the vertical error between the profiles, i.e., the extent to which they don't overlap, was less than 0.3 nm, and the horizontal error was less than 5 nm. Within the limits of the resolution of the atomic force microscope, the authors conclude that glass fracture only occurs via the local rupture of atomic bonds. — MSL

    Appl. Phys. Lett. 92, 215502 (2004)

  5. GEOPHYSICS

    Tsunami and Its Shadow

    Tsunamis are long-wavelength and long-period surface waves generated by coastal earthquakes, landslides, or volcanoes. Tsunamis can travel great distances without losing much energy, e.g. from Chile to Japan or Alaska to Hawaii, yet they are undetectable in the open ocean because of their slow speed and negligible amplitudes. When a tsunami reaches shallow water near land, it slows down further, but grows in height, typically to 2 to 3 m, although devastating tsunamis as high as 30 m have been recorded.

    A tsunami shadow is a darkened strip of water observed ahead of the wave on rare occasions. Godin determined that a tsunami creates perturbations in the wind velocity along a thin layer of air above the sea. The perturbations enhance the roughness of the sea surface and create a darker strip parallel to the wave front between the troughs and crests. With this model of the shadow, a tsunami can be detected in the deep ocean, far away from land, with airborne and satellite-based radars and radiometers. This should allow tsunami warnings in distant coastal areas to be more accurate and more effective. — LR

    J. Geophys. Res. 109, C05002 (2004).

  6. APPLIED PHYSICS

    A Bright Idea for Nanotubes?

    The design of the incandescent electric lightbulb, invented over 130 years ago, has, on the whole, not changed much since then. A metal filament is enclosed within an evacuated glass bulb and an electrical current is passed through it. The inefficient conversion into light is a problem, however, as is evident from more efficient lightbulb technologies entering the market. Wei et al. now bring some nanotechnology onto the scene by replacing the tungsten filament with strands of single-walled and multiwalled carbon nanotubes. Carbon nanotube filaments light up at lower voltages and are more efficient than their tungsten filament counterparts. Perhaps this simple demonstration is a first clearly visible example of nanotechnology delivering on the promises made for it in the production of more efficient technologies. — ISO

    Appl. Phys. Lett. 84, 4869 (2004).

  7. STKE

    Norepinephrine and Antidepressants

    It is important to understand the role of various neurotransmitter pathways in the efficacy of antidepressants. Cryan et al. studied mice deficient for the norepinephrine (and epinephrine) biosynthetic enzyme dopamine β-hydroxylase (Dbh−/−). The acute behavioral effects of different classes of antidepressants were dependent on norepinephrine as well as on the known changes in serotonergic or dopaminergic signaling. Drugs that were not selective for a specific neurotransmitter and inhibited the reuptake or metabolism of serotonin, dopamine, and norepinephrine were effective in heterozygous littermates, but not in Dbh−/− mice, in an assay for the acute effects of antidepressant drugs. Two of the three drugs in the serotonin selective reuptake class were not effective in Dbh−/− mice. Only citalopram, which is the most selective and does not increase norepinephrine concentrations in vivo, was effective in the Dbh−/− mice. Restoration of norephinephrine by administration of a metabolic precursor restored responsiveness to each of the antidepressants tested. Thus, most antidepressants appear to have a requirement for noradrenergic signaling, with the exception of citalopram. — NG

    Proc. Natl. Acad. Sci. U.S.A. 101, 8186 (2004).

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