ScienceWeek

SCIENCEWEEK

January 26, 2007

Vol. 11 - Number 4

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We still stand in the presence of riddles, but not without hope of solving them. And riddles with the hope of solution -- what more can a scientist desire?

-- Hans Spemann (1869-1941)

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Contents (full text below):

1. Neuroscience: On Excess Neurogenesis

2. Geochemistry: Seismic Waves and Water in the Mantle

3. Climatology: Climate Change and Asian Monsoons

4. Cosmology: On Dark Matter Structure

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New Books Noted:

Conceptions of Cosmos From Myths to the Accelerating Universe: A History of Cosmology. Helge S. Kragh. Oxford University Press, Oxford, 2007 Hardback: 284 pp., illus. ISBN 9780199209163. More information at: http://www.amazon.com/exec/obidos/ASIN/0199209162/scienceweek

Decoding the Universe How the New Science of Information Is Explaining Everything in the Cosmos, from Our Brains to Black Holes. Charles Seife. Penguin, New York, 2007 Paperback: 304 pp., illus. ISBN 9780143038399. More information at: http://www.amazon.com/exec/obidos/ASIN/0143038397/scienceweek

The Man Who Knew Too Much Alan Turing and the Invention of the Computer. David Leavitt. Atlas (Norton), New York, 2006 Paperback: 329 pp., illus. ISBN 9780393329094. More information at: http://www.amazon.com/exec/obidos/ASIN/0393329097/scienceweek

The March of Unreason Science, Democracy, and the New Fundamentalism. Dick Taverne. Oxford University Press, Oxford, 2007 Paperback: 320 pp. ISBN 9780199205622. More information at: http://www.amazon.com/exec/obidos/ASIN/0199205620/scienceweek

Richter's Scale Measure of an Earthquake, Measure of a Man. Susan Elizabeth Hough. Princeton University Press, Princeton, NJ, 2007 Hardback: 349 pp., illus. ISBN 9780691128078. More information at: http://www.amazon.com/exec/obidos/ASIN/0691128073/scienceweek

The Standard Model A Primer. C. P. Burgess and Guy D. Moore. Cambridge University Press, Cambridge, 2007 Hardback: 558 pp., illus. ISBN 9780521860369. More information at: http://www.amazon.com/exec/obidos/ASIN/0521860369/scienceweek.


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1. NEUROSCIENCE: ON EXCESS NEUROGENESIS

The following points are made by H.E. Scharfman and R. Hen (Science 2007 315:336):

1) For most of the 20th century, neurobiologists believed that the adult mammalian brain could not generate new neurons. But during the 1990s that concept changed. Evidence of the birth of new neurons in adult mammals, including humans, raised expectations for improved treatment for patients with central nervous system injury or illness. But this enthusiasm has been tempered since then, as more recent studies indicate that excess adult neurogenesis can be as detrimental as a deficit. In some cases, the clinical relevance of increasing neurogenesis may need to be reconsidered.

2) Neurogenesis in the normal adult mammalian brain is primarily limited to three areas: the subventricular zone, hippocampal dentate gyrus, and olfactory bulb (1). The identification that this is true in humans, at least in the hippocampus (2), together with the findings that neurogenesis can be increased in laboratory animals by learning, exercise, and antidepressants, and decreased by stress and aging (1), reinforced the expectation that neurogenesis might be clinically beneficial. Moreover, additional sites in the adult brain -- the cortex and hypothalamus -- demonstrate ongoing neurogenesis (3,4), although this remains controversial (5). However, we now know that neurogenesis in the adult brain occurs at a very low rate after maturity, and many of the new neurons do not survive for long.

3) Thus, new neurons born in the adult brain may support plasticity on an acute time scale because of their increased excitability but have limited long-term restorative ability. Such transient existence of new neurons should not necessarily dampen therapeutic potential. Survival of new neurons increases with benign interventions such as learning and enriching the environment (1). Dormant stem cells may also exist throughout the brain. These cells could potentially be stimulated to mature in pathological situations or after pharmacological interventions. Indeed, a possible reason for the beneficial effects of rehabilitation or psychotherapy may be that treatment increases survival of new neurons.

4) But an increase in neurogenesis may not always result in improved function. Recent studies show surprising limitations in the ways new neurons in the adult brain can improve function. For example, dentate gyrus neurogenesis in laboratory animals influences some hippocampal-dependent behaviors but not other behaviors. Specifically, there are positive effects on trace and contextual fear conditioning, but not on spatial learning. Animals without new neurons also perform better in certain working memory paradigms. Specifically, mice that are devoid of neurogenesis due to irradiation or genetic ablation display improved memory in a radial maze, but only when repetitive information is presented. Therefore, manipulations that increase neurogenesis may have positive effects on some behaviors but negative effects on other behaviors. In addition, improved function may not always be caused by increased neurogenesis. For example, some of the behavioral effects of enriched environment and antidepressants are independent of their influence on hippocampal neurogenesis. So despite increasing experimental support for an influence of neurogenesis on specific behaviors, it is not yet clear how these effects may translate into clinical benefits.

References (abridged):

1. G. Kempermann, Adult Neurogenesis: Stem Cells and Neuronal Development in the Adult Brain (Oxford Univ. Press, New York, 2006).

2. P. S. Eriksson et al., Nat. Med. 4, 1313 (1998).

3. M. V. Kokoeva, H. Yin, J. S. Flier, Science 310, 679 (2005).

4. A. G. Dayer, K. M. Cleaver, T. Abouantoun, H. A. Cameron, J. Cell. Biol. 168, 415 (2005).

5. P. Rakic, Nat. Rev. Neurosci. 3, 65 (2002).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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2. GEOCHEMISTRY: SEISMIC WAVES AND WATER IN THE MANTLE

The following points are made by Nathalie Bolfan-Casanova (Science 2007 315:338):

1) Our understanding of plate tectonics relies on the concept of relatively rigid rocky plates moving on a more ductile shallow mantle called the asthenosphere (1). The word asthenosphere comes from the Greek "a-sthenos" meaning "without strength." This lack of strength especially affects seismic waves, which slow down when entering the asthenosphere. For decades, Earth scientists have tried to understand the reason for this seismic wave deceleration. New work (2) reports new experimental findings on the maximum amount of water that can be stored by the shallow mantle. These results may solve a number of riddles, including the cause of the seismic slowdown.

2) Why is water important? Water not only is essential to life but also controls the dynamics of Earth's interior (3). Since the 1990s, geologists have recognized with increasing certainty that mantle minerals can hold substantial amounts of water. This implies that the oceans may no longer be the main water reservoir of Earth. But water does not necessarily have to be fluid to be stored in the deep Earth. Rather, it dissolves as hydroxyl (OH-) in anhydrous minerals (such as olivine, pyroxenes, garnet, and their high-pressure forms) as a result of the association of a proton (H+) with oxygen of the mineral lattice. This creates a defect in the lattice and thereby speeds up the kinetics of physical properties that depend on the concentration of defects. Even at very low concentrations -- lower than 1% by weight -- the presence of water has many consequences for mantle properties such as creep and electrical conductivity (4,5). When present in minerals as a defect, water will enhance the deformation of rocks and make them more ductile. Dissolved as H+ in minerals, water will also increase the electrical conductivity of the mantle by adding mobile charges. Water also lowers the melting point of mantle rocks and allows melting at greater depths than in the absence of water.

3) To understand how water affects mantle properties, we need to know how much water can be stored in mantle minerals and how this storage capacity varies with increasing depth. Researchers have firmly established that the solubility of water in minerals increases with pressure and water partial pressure. The water storage capacity of Earth's upper mantle (extending from the base of the crust down to the transition zone at 410 km depth) was thought to increase monotonically with depth. Moreover, in a mantle consisting of 60% by volume of olivine, this mineral was believed to be the one that dictates the water budget.

4) The results of Mierdel et al (2) completely change the picture: Water storage capacity in Earth's shallow mantle is controlled by orthopyroxene, a less abundant phase than olivine, because water solubility in this phase is more than two orders of magnitude higher than in olivine. The reason for this is composition. The enhanced affinity of pyroxenes for water is indeed aided by aluminum through the coupled substitution of 2Al3+ + 2H+ for 2Mg2+ + Si4+,which is a very efficient way to store up to 1 weight % water in MgSiO3 orthopyroxene. Mierdel et al (2) also show that the curve of water saturation versus depth has a pronounced minimum between 100 and 200 km. Indeed, the water storage capacity of pyroxene with substituted aluminum is dependent on the acceptance of the large aluminum cation into the small tetrahedral site of silicon, the size of which diminishes as a function of increasing pressure because of atomic compaction. This leads to the drastic change of water solubility in pyroxene at pressures between 3 and 5 GPa, corresponding to depths of 100 to 175 km. Depending on the tectonic environment and the temperature, the minimum in solubility is shallow in the case of the oceanic mantle but deepens in the case of the colder continental mantle.

References (abridged):

1. D. L. Anderson, Theory of the Earth (Blackwell Scientific, Boston, 1989).

2. K. Mierdel, H. Keppler, J. R. Smyth, F. Langenhorst, Science 315, 364 (2007).

3. N. Bolfan-Casanova, Mineral. Mag. 69, 229 (2005).

4. D. L. Kohlstedt et al., in Physics and Chemistry of Partially Molten Rocks, N. Bagdassarov, D. Laporte, A. B. Thompson, Eds. (Kluwer Academic, Dordrecht, Netherlands, 2000).

5. D. Wang et al., Nature 443, 977 (2006).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. CLIMATOLOGY: CLIMATE CHANGE AND ASIAN MONSOONS

The following points are made by J.T. Overpeck and J.E. Cole (Nature 2007 445:270):

1) As Earth's climate continues to warm, understanding the dimensions of our vulnerability to present and future changes is crucial if we are to plan and adapt. Studies of palaeoclimate have an important role here in helping us to uncover the full range of past climate variability, and so avoid future surprises. New work (1) presents a study of past climate change in Indonesia that expands our view of the pivotal climatological influences in that region to include a geographically distant player: the Asian monsoon.

2) Indonesia's climate is known to vary significantly from year to year as a result of the El Niño/Southern Oscillation (ENSO) system. This system is associated with changes in sea surface temperature and atmospheric pressure across the tropical Pacific. When the central tropical Pacific to the east of Indonesia is warm (an El Niño phase), the normally abundant rainfall in Indonesia moves eastward, leaving much of the island nation in drought. In the west of the country, drought is also brought about by another coupled oscillation in ocean–atmosphere conditions, the "Indian Ocean Dipole", as a result of cool sea surface temperatures off Sumatra, the most westerly of Indonesia's principal islands.

3) Abram and colleagues (1) exploit the fact that climate information is preserved in the geochemistry of huge, rapidly growing corals off Sumatra to study past dipole events in the Indian Ocean. Different aspects of coral geochemistry reflect variations in temperature and in the hydrological balance (the difference between levels of precipitation and evaporation). By analysing several geochemical tracers -- oxygen isotopic ratios and the ratio of strontium to calcium -- in annually banded coral skeletons, the authors can reconstruct month-by-month changes in temperature and drought. Using fossil corals from the mid-Holocene (between around 6,500 and 4,000 years ago), when the Asian monsoon was stronger and ENSO seemingly weaker than today, they demonstrate that the cool ocean temperatures persisted longer -- for five months, instead of three -- and were accompanied by longer droughts than has been the case in modern times.

4) Results of climate simulations for 6,000 years ago agree with these observations and suggest a mechanism for the change. First, a stronger Asian monsoon generates anomalies in the easterly winds that would cool the eastern Indian Ocean, predisposing cooler, deep-ocean water in this area to move upward earlier during dipole events. Cooler sea surface temperatures would lead to anomalous downward movement and outflow of air from the region. The resultant weakening of the degree to which moist air converges, together with the atmospheric vertical motion, would result in reduced precipitation, and drought over adjoining land areas.

5) But what about ENSO? During El Niño conditions, the eastward migration of rainfall and warm ocean temperatures in the tropical Pacific lead to drought in Indonesia. But it has been suggested (2,3) that the mid-Holocene experienced background conditions that may have more closely resembled La Niña conditions. La Niña brings cooler temperatures to the central tropical Pacific, and Indonesia generally receives enhanced rainfall during these periods. So why were droughts more prominent during the mid-Holocene? The implication of Abram and colleagues' work (1) is that the Asian monsoon trumps ENSO and generates prolonged droughts in Indonesia through its influence on the Indian Ocean Dipole. Whether the more frequent droughts associated with interannual variations in ENSO are similarly affected by a stronger monsoon remains unexplored.(4,5)

References (abridged):

1. Abram, N. J. et al. Nature 445, 299–302 (2007).

2. Clement, A. C., Seager, R. & Cane, M. A. Paleoceanography 15, 731–737 (2000).

3. Cole, J. E. Science 291, 1496–1497 (2001).

4. Myers, N., Mittermeier, R. A., Mittermeier, C. G., da Fonseca, G. A. B. & Kent, J. Nature 403, 853–858 (2000).

5. Morrill, C., Overpeck, J. T. & Cole, J. E. Holocene 13, 465–476 (2003).

Nature http://www.nature.com/nature

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4. COSMOLOGY: ON DARK MATTER STRUCTURE

The following points are made by Eric V. Linder (Nature 2007 445:273):

1) Astronomers use every resource at their disposal to construct an image of the Universe: from the microwave radiation of the cooling Big Bang, past the visible wavelengths of light seen by the casual viewer of the night sky, to the gamma-rays of the most powerful explosions of collapsing stars and ravenous black holes. They don't even have to see an object directly to detect its presence. In a manner akin to the Polynesian seafarers who sense islands out of their sight through the deflected direction of ocean waves, cosmologists can map a concentration of the Universe's unseen mass through the gravitational deflection of light coming from sources behind it. New work (1) reports the latest and most detailed atlas of these "dark-matter" congregations.

2) Dark matter is thought to comprise as-yet undiscovered elementary particles, and is a central prediction of the current "concordance" model of the Universe's make-up. Here, it acts as a skeleton around which bright matter -- galaxies and clusters of galaxies -- assembles. There is abundant indirect evidence for structures made of dark matter. The gravitational deflection, or lensing, technique has already been used to make maps of dark matter around individual galaxies and clusters, most dramatically by Clowe et al (2). Low-resolution reconstructions of the skeleton of a larger chunk of the nearby Universe have also been carried out using ground-based telescopes.

3) Massey et al (1) present an analysis of gravitational-lensing data from the Cosmic Evolution Survey, COSMOS, that were acquired using the Hubble Space Telescope. These data cover an area of sky about eight times the size of the full Moon, and represent the first wide-sky, space-based survey of dark matter. This is a great step forward, as observations from space avoid the distortions and time variations that Earth's atmosphere imposes on the astronomical signal. (The effect of these perturbations is rather as though our seafarer friend had to contend with a nearby canoe's paddlers roiling the ocean when studying his waves.) The new maps consequently have a much higher resolution than the best ground-based observations, with four times the density of sources than in previous studies. They also go deeper into the Universe, looking back to a time when the Universe was about half the age it is now, at a redshift z=1. (The redshift is a measure of the expansion of the Universe; the higher the redshift of a cosmological object, the smaller and younger the Universe was when the object sent out its signal.)

4) To assess the third dimension of their map, the depth of the field of view, astronomers cannot move their canoe and use triangulation. They rely instead on mathematics to derive the distance of the gravitational lens by means of its focal length -- which is known from Einstein's general theory of relativity --and the distance to the background source galaxy whose light is probing the dark matter. Massey et al (1) combine the COSMOS observations with follow-up observations from ground-based telescopes in 15 wavelength bands, and so are able to estimate redshifts of the galaxy sources all the way back to z=3, when the Universe was just a sixth of its present age. With this information, the pattern of dark-matter concentrations can be mapped crudely in three dimensions by slicing the data into three redshift shells. Although this is just a first step in "cosmic tomography", the advantage of using a space telescope is that the source density in each slice of this three-dimensional map is nearly as high as for the "unsliced" maps -- two-dimensional projections -- expected from the next generation of ground-based surveys. (2-4)

References:

1. Massey, R. et al. Nature 445, 286–290 (2007).

2. Clowe, D. et al. Astrophys. J. Lett. 648, 109–113 (2006).

3. http://www.cfht.hawaii.edu/Science/CFHLS

4. http://snap.lbl.gov

Nature http://www.nature.com/nature

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