New technique dates Saharan groundwater as million years old
ARGONNE, Ill. (March 1, 2004) The Sahara Desert was once a lush,
green landscape dotted with lakes and ponds. Evidence of this past verdancy
lies hidden beneath the sands of Egypt and Libya, in the form of a huge
aquifer of fresh groundwater. An international team of geologists and
physicists has found that this groundwater has been flowing slowly northward
(at about the rate grass grows) for the past million years. Their findings
are published in the March issue of Geophysical Research Letters.
Determining residence times and flow velocities of groundwater in aquifers
is a vexing challenge. The extremely rare radioactive isotope krypton-81
(half-life of 229,000 years), which is produced by cosmic rays in the
atmosphere, could be an ideal groundwater chronometer on the million-year
time scale, but it has been nearly impossible to measure because of its
low abundance. There is only one atom of krypton-81 in a trillion atoms
of atmospheric krypton, a rare gas to begin with, and krypton is absorbed
only slightly by water.
Recently, scientists at the U.S. Department of Energy's Argonne National
Laboratory have developed a laser-based method to count individual atoms
of krypton-81 and measure its abundance accurately. Known as ATTA, for
Atom-Trap Trace Analysis, this new, ultrasensitive method could revolutionize
the scientific study of such topics as continental groundwater flow,
long-term glacier motion, and seawater circulation through the ocean
floor, say the researchers.
In their Geophysical Research Letters paper, Neil C. Sturchio (University of Illinois at Chicago),
Zheng-Tian Lu (Argonne National Laboratory), Roland Purtschert (University of Bern), Mohamed
Sultan (State University of New York
at Buffalo), and others report the most extensive measurements yet
made for krypton-81 in groundwater. In the first application of the ATTA
method to a groundwater investigation, this team visited the Western
Desert of Egypt to sample krypton from the Nubian Aquifer groundwater,
which was reputedly old but of unknown age.
To obtain a sufficient amount of krypton for the ATTA measurements,
the team had to extract dissolved gases from thousands of liters of groundwater
in the field, using a device invented by the Swiss members of the team.
The gas extracted from each well was compressed into steel containers
and shipped to Bern, where the trace amount of krypton in each sample
was purified and delivered to Argonne for analysis of krypton-81. ATTA
measured the ratios of krypton-81 to ordinary krypton, which ranged from
about five to 53 percent of that in the air, corresponding to groundwater
ages of 200,000 to 1,000,000 years.
Based on these data, it is possible to estimate the direction and velocity
of the groundwater flow, which is about one-to-two meters [yards] per
year toward the north, and to determine the recharge location in southwest
Egypt. This confirms results from some previous numerical hydrologic
models, but refutes others. Isotopic characteristics of the water itself
indicate that it was transported by air masses traveling long distances
over North Africa from the Atlantic Ocean, thus reflecting climate conditions
much different from the present during the past million years. Changing
climate patterns turned this green oasis into today's desert.
The research was funded primarily by the National
Science Foundation and the U.S. Department of Energy.
The nations first national laboratory, Argonne National Laboratory
conducts basic and applied scientific research across a wide spectrum
of disciplines, ranging from high-energy physics to climatology and biotechnology.
Since 1990, Argonne has worked with more than 600 companies and numerous
federal agencies and other organizations to help advance America's scientific
leadership and prepare the nation for the future. Argonne is operated
by the University of Chicago for
the U.S. Department of Energy's Office
of Science.
For more information, please contact Catherine Foster (630/252-5580
or media@anl.gov) at Argonne.
|