Cosmogenic nuclides and tropical erosion

In the highlands of central Sri Lanka the sediment suspended in rivers suggest rates of soil loss from agricultural land of the order of up to 7000 tonnes per km2 each year.  However, it is difficult to judge how much would be eroded under natural conditions, compared with the probable loss as a result of deforestation and human activities, particularly from very rugged landscapes where seasonal rainfall is high..  Radionuclides produced by cosmic-ray bombardment of minerals exposed to them, such as 10Be and 26Al, accumulate in soil that is being eroded at a rate that is inversely proportional to the rate of erosion.  The nuclides form in the top 0.6 m of soil, which is the depth within which cosmic rays are normally absorbed.  So erosion rates that can be calculated from the cosmogenic nuclides in minerals, such as quartz, in river sediments apply to the times taken to remove that depth of soil.  Essentially, the rates that are measured represent the long-term erosion within a catchment basin.  Swiss and Sri Lankan geoscientists have applied the technique to rivers in central Sri Lanka, whose catchments have different vegetation cover and land usage (Hewawasam, T. et al. 2003.  Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides.  Geology, v. 31, p. 597-600), such as forest reserves, rice terraces, tea plantations, areas of slash and burn agriculture, and various levels of degraded land.  The unmodified forest catchments give the lowest long-term erosion rates of 5-11 mm per 100 ka (13-30 tonnes per km2 per year) as expected, but this is about a quarter of the rate of erosion measured by the same method throughout the highland region.  That probably reflects the antiquity of erosion induced by agriculture, yet current rates measured from sediments being carried by rivers suggests that soil erosion is now between 10 and 100 times faster than would occur under natural conditions.

Remote signs of earthquakes

All manner of ground-based observations have been tried as means of timely predictors of pending earthquakes, ranging from strange behaviour of wildlife to emissions of radon from wells (see Radon emissions and earthquakes, July 2003 issue of EPN).  So far, none of them have been universally useful, although there have been successful evacuations of threatened populations, principally in China, whose seismologists have focused on a wide range of signals.  Ideally, what is needed is some kind of global monitoring, and as with attempts to predict volcanic eruptions the only realistic means is from satellite surveillance.  Long ago, Doug Shearman of the Royal School of Mines at Imperial College, London introduced me to the peculiar properties of the mineral dolomite, as discovered by the man whose name it takes, Count Deodar de Dolomieu.  If you rub two lumps of dolomite together in a darkened room, they emit a sinister glow, and so do other minerals, such as quartz and even sugar.  Excellent for amusing the kids.  But then I learnt of “earth lights”, which had been photographed by Japanese observers just before earthquakes, in the vicinity of active faults – previously they were supposed to be as mythological as the fire balls during thunder storms (also a proven fact now).  At the time, the Landsat remote sensing satellite captured images during its night-time overpasses, on request.  A nice, if a little “blue skies” research project.  I submitted a brief proposal to my department’s research committee for ranking along with other studentship projects.  Perhaps my wry attitude to what had become somewhat dominated by other disciplines than remote sensing coloured my efforts; it was rejected.  So it was with some glee, a decade later, to find that NASA and the US Federal Emergency Management Agency had been testing the idea using weather satellites and the MODIS instrument carried by the Terra platform since 2000 (Enriquez, A. 2003.  The shining.  New Scientist, 5 July 2003, p. 26-29).  Encouragingly, though not for their victims, the devastating 1999 Izmit and 2001 Gujarat earthquakes were preceded by increased infrared emissions, detected from space, 5 days before the event.  Experiments show that when rock is stressed, emissions build up, and then vanish once the rocks fails, as in an earthquake, so the method looks very promising.

Another seismic phenomenon is changing magnetic fields around the site of failure.  This was first noticed from magnetometer records on the ground before the 1989 Loma Prieta earthquake that damaged large tracts of northern California.  Magnetic field variations too can be monitored from orbit.  The privately funded QuakeSat, launched on 30 June 2003 aims to test this possibility, as will a more ambitious French satellite, due to reach orbit in April next year (Reichhardt, T. 2003.  Satellites aim to shake up quake prediction.  Nature, v. 424, p. 478).


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