September 27, 2000
Contacts: Mark
Thiemens (858) 534-6882, Huiming
Bao (858) 534-6053,
Media Contact: Kim
McDonald (858) 534-7572,
Credit for images: Peter West,
National Science Foundation
ORIGIN
OF SALTS IN THE 'DRY VALLEYS' OF ANTARCTICA
PROVIDE CLUES TO ATMOSPHERIC
DEPOSITION ON MARS
Chemists
at the University of California, San Diego have discovered that the
mysteriously high salt concentrations in exposed soils of Antarctica's
Dry Valleys are due in large part to biological sulfur emissions in the
oceans surrounding the continent.
Writing in the September 28
issue of Nature, the UCSD scientists said they discovered an
unmistakable chemical signature in soil samples from this Mars-like
region that suggested that atmospheric deposition of sulfates from
sulfur-emitting marine algae is a significant contributor to the high
salt concentrations in that region.
For
decades, researchers have speculated that the high salt concentrations
were due primarily to an ancient sea that once covered the region, to
sea salt carried to the continent by fierce Antarctic winds, to
biologically produced sulfates, to the weathering of rocks or to
hydrothermal activity.
The UCSD chemists shed light on
this question by discovering an oxygen-isotope anomaly in the sulfates
they chemically retrieved from the soils of this region, located near
the main U.S. Antarctic station, McMurdo, and unique to Antarctica
because portions of it are perennially devoid of ice and snow cover.
Their discovery enabled them to conclude that the sulfates came from
sulfur gases that had undergone chemical reactions in the atmosphere and
were transported into the Dry Valleys. Because Antarctica is thousands
of miles from sources of man-made sulfur gases due to fossil-fuel
burning, the scientists were further able to conclude that these
sulfates came largely from sulfur-producing algae abundant in the ocean
surrounding the continent.
"This provides the first
concrete evidence that a major portion of the salt in the Dry Valleys
came from biological activity," said Huiming Bao, a geochemist at
UCSD and the principal author of the paper. "It looks like
wind-blown sea salt, sea-water intrusions, chemical or physical
weathering of the local rocks, or hydrothermal activity are not all that
important."
Other co-authors of the paper
were UCSD chemists Douglas A. Campbell and Mark H. Thiemens, and James
G. Bockheim, a soil scientist at the University of Wisconsin at Madison
who obtained the soil samples under the auspices of the U.S. Antarctic
Program, which is managed by the National Science Foundation. The study
was financed by the NSF and the National Aeronautics and Space
Administration.
The scientists discovered that
in areas of the Dry Valleys closer to the coast, the biologically
produced sulfates constituted a smaller fraction of the total salt
content, but that further inland, the biologically produced fraction
increased substantially. The researchers said this suggested that
wind-blown sea spray played a larger role in the overall salt content
closer to the coast, but that further inland, biologically produced
sulfates were the major contributor.
The scientists also found that
digging more deeply into the soil of the Dry Valleys yielded
substantially higher concentrations of biologically produced sulfates.
Bao says this may be due to the fact that the salts from sea spray are
larger in size and are unable to migrate through the soil as far or fast
as the smaller biologically produced sulfates.
Such observations have
important implications in the search for evidence of past or present
life on Mars, as well as on understanding the chemical interactions
between the Martian atmosphere and the red planet's surface.
"What this tells us is
that when we go to Mars to retrieve soil samples, we're going to have to
go beneath the surface to retrieve samples, because these sulfates may
migrate," said Thiemens, a professor of chemistry and dean of
UCSD's Division of Physical Sciences.
"By studying the soil of
the Dry Valleys, you really have a good glimpse of what can happen on
Mars," he added. "The conditions of the Dry Valleys are about
as close as you're going to get on Earth to the conditions on Mars. So
if you really want to understand in a controlled fashion what can happen
in an extreme environment like Mars, this is it."
"Studying processes like
this on Earth is very important because it gives us clues to how similar
processes may have worked on Mars when there was more liquid water
available," said Scott Borg, who manages NSF's Antarctic geology
and geophysics program. "It is also very important in helping us to
design experiments for spacecraft that may one day visit other
planets."
Photo Captions: Two views of
the Taylor Valley show the exposed soil in the Dry Valleys of
Antarctica, where the high salt content has been shown by UCSD chemists
to be due largely to biologically produced sulfates. One photo shows the
tents of researchers at the National Science Foundation's Lake Hoare
camp. The other photo shows an aerial view of the Taylor Valley. Credit:
Peter West, National Science Foundation
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