ScienceWeek

EARTH SCIENCE: ON THE FORMATION OF TIBET

The following points are made by A. Mulch and C.P. Chamberlain (Nature 2006 439:670):

1) Is Everest now at its highest point, or was it once even loftier? What was the greatest height attained by the vast highlands of the Tibetan plateau, and when did this occur? New work[1] shows these questions can be tackled -- if not yet answered definitively -- by analysing the isotopic composition of ancient raindrops. With this approach, the authors show that Tibet continuously grew northward over millions of years in response to the thickening of Earth's crust associated with the collision of the Indian and Asian continental plates. The driving forces for this collision are generated deep in Earth's mantle. But the key to unravelling the uplifting history of the central Tibetan plateau is found in lake sediments on the plateau, some of which formed as long ago as 40 million years.

2) In these lakes and their surrounds, changes in the oxygen-isotope composition of surface water (which is controlled by regional climate and elevation) are recorded in sediments. Systematic variations in oxygen-isotope composition across the plateau reveal that spatially variable uplift of the plateau to 4000 meters or more above sea level was intimately linked to the timing and rates of convergence of India and Asia. Rowley and Currie[1] estimate that uplift to 4000 meters was initiated as long ago as 40 million to 50 million years, in the early stages of that convergence.

3) The evolution of mountain topography reflects the balance between tectonic forces in Earth's crust and upper mantle, and climatically driven erosion at Earth's surface. Their relative role in controlling the rise of mountains remains unclear[2,3], but the problem can be approached by reconstructing the elevation history of large continental plateaux. Such studies can improve our understanding of the coupling between tectonics and long-term climate change. For example, the Tibetan plateau -- the largest continental highland on Earth -- is a major barrier to air flow in the atmosphere, and it has been suggested that uplift of the plateau triggered the onset of the Indian summer monsoon[3].

4) The chemical fingerprint of rain and snow that precipitated on the Tibetan plateau is found in the oxygen-isotope composition of calcareous minerals in Tibetan lake sediments. This is expressed as delta-18O, which is the 18O/16O ratio in the soils or sediments relative to the 18O/16O ratio in sea water. For example, the delta-18O of calcite formed in soils is related to the delta-18O of soilwater or groundwater by a temperature-dependent fractionation factor; so delta-18O in calcite formed in soil is a sensitive tracer of surface water that stems from precipitation. The underlying principle of oxygen-isotope altimetry is that water that precipitates as rain or snow becomes increasingly depleted in 18O the higher up a mountain range that it falls[4]; systematic changes in delta-18O with elevation can then be used to infer relative elevation differences between the water source in the ocean and the elevation at which the rain or snow fell[5].

References (abridged):

1. Rowley, D. B. & Currie, B. S. Nature 439, 677-681 (2006)

2. Molnar, P. & England, P. Nature 346, 29-34 (1990)

3. Molnar, P. , England, P. & Martinod, J. Rev. Geophys. 31, 357-396 (1993)

4. Dansgaard, W. Tellus 16, 436-468 (1964)

5. Rowley, D. B. , Pierrehumbert, R. T. & Currie, B. S. Earth Planet. Sci. Lett. 188, 253-268 (2001)

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

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EARTH SCIENCE: ON WATER AND PLATE TECTONICS

The following points are made by Rob Evans (Nature 2004 429:356):

1) In places such as the Pacific Rim, the sea floor collides with and is pushed below the land masses that border the ocean. This process of "subduction", and its consequences in the form of earthquakes and volcanism, are the result of plate tectonics, in which the formation of sea floor at mid-ocean ridges is accommodated by the recycling of tectonic plates in the Earth's interior. But the recycling is a complex process: the interaction of the subducting plate, or slab, with the surrounding material itself produces volcanism and, in some settings, new sea floor.

2) Water plays an important role in subduction recycling: its release from the oceanic plate can affect the rheology of the rock as it moves into the mantle, below the Earth's crust, and also increases rock melting(2). Yet the amount of water released, the depth of release and the pathways it takes are poorly defined, as are the depths of melting and the route that the melt takes as it moves from close to the subducting plate to its eruption in the overlying volcanic arc.

3) Seismic techniques for looking into the Earth are familiar, but there are other approaches, one being the magnetotelluric (MT) method, which offers promise for locating melt and identifying the distribution of water in the mantle. This technique uses naturally occurring electric currents in the ionosphere, created by the capture of charged particles by the planet's magnetic field, to measure Earth's electrical conductivity. That conductivity depends partly on composition and temperature. But it can be dramatically increased by small amounts of partial melt, provided that the melt forms an interconnected network(3). And it can also be affected by water in the mantle, in the form of dissolved hydrogen, both above and below the transition that occurs globally at around 410 km, where the mantle composition undergoes a change from one mineral phase (olivine(4)) to another (wadsleyite(5)). MT has been used only infrequently to address subduction-zone processes, mostly because these systems are close to the ocean and require complex onshore-offshore investigations.

4) Booker et al(1) have presented dramatic models of the subduction system beneath the Andes that suggest that the recycling processes run deeper than previously thought. Their interpretation depends on new data, which in themselves are striking but which also allow the authors to make some inferences about the influence of water at depth. Not only are the results of Booker et al(1) dramatic in their own right, they also represent a significant advance in MT imaging of subduction systems.

References (abridged):

1. Booker, J. R., Favetto, A. & Pomposiello, M. C. Nature 429, 399-403 (2004)

2. Hirth, G. & Kohlstedt, D. L. Earth Planet. Sci. Lett. 144, 93-108 (1996)

3. Roberts, J. J. & Tyburczy, J. A. J. Geophys. Res. 104, 7055-7066 (1999)

4. Karato, S. Nature 347, 272-273 (1990)

5. Xu, Y. et al. Science 280, 1415-1418 (1998)

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

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ON CONTINENTAL DRIFT AND PLATE TECTONICS

The following points are made by Naomi Oreskes (citation below):

1) Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the continental edges. Since the 19th century, geologists have known that some fossil plants and animals are extraordinarily similar across the globe, and some sequences of rock formations in distant continents are also strikingly alike. At the turn of the 20th century, Austrian geologist Eduard Suess (1831-1914) proposed the theory of Gondwanaland to account for these similarities: that a giant supercontinent had once covered much or all of the Earth's surface before breaking apart to form continents and ocean basins.

2) A few years later, German meteorologist Alfred Wegener (1880-1930) suggested an alternative explanation: continental drift. The paleontological patterns and jigsaw-puzzle fit could be explained if the continents had migrated across the Earth's surface, sometimes joining together, sometimes breaking apart. Wegener argued that for several hundred million years during the late Paleozoic and Mesozoic eras (200 million to 300 million years ago), the continents were united into a supercontinent that he labeled "Pangea" -- all Earth. Continental drift would also explain paleoclimate change, as continents drifted through different climate zones and ocean circulation was altered by the changing distribution of land and sea, while the interactions of rifting and drifting land masses provided a mechanism for the origins of mountains, volcanoes, and earthquakes. Continental drift was not accepted when first proposed, but in the 1960s it became a cornerstone of the new global theory of plate tectonics. The motion of land masses is now explained as a consequence of moving 'plates' -- large fragments of the Earth's surface layer in which the continents are embedded.

Adapted from: Naomi Oreskes (ed.): Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview Press, Cambridge MA 2001, p.3. More information at: http://www.amazon.com/exec/obidos/ASIN/0813339812/scienceweek

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