Image: Peter West, National Science Foundation

In the Dry Valleys of Antarctica, where constant snow and ice aren't the norm, the exposed soils contain a surprisingly large amount of salt. Over the years researchers have come up with a number of possible explanations: an ancient sea once covered the valleys; fierce winds carry the salt there from the surrounding oceans; the region sees a lot of hydrothermal activity; chemical or physical weathering of the rocks raises the concentration. As it turns out, though, none of these reasons are very important. A new study, appearing in today's issue of Nature, instead shows that biological sulfur emissions are to blame.

James G. Bockheim, a scientist from the University of Wisconsin, collected soil samples in Taylor Valley (right), near the U.S. Antarctic station, McMurdo. He then worked with geochemists and chemists from the University of California at San Diego, who extracted sulfates from the soils for analysis. What the team discovered was an odd oxygen isotope telling them that the sulfates came from gases that had undergone atmospheric reactions en route to the valley. And because the valley is thousands of miles away from any sources of man-made sulfur gases, they concluded that those gases could only have come from sulfur-producing marine algae.

The sulfates make up different portions of the total salt content in different areas of Antarctica. Inland, the scientists found it to be a larger fraction than near the coast, where perhaps wind-blown sea salt makes a greater contribution. Also, they discovered higher concentrations of the biologically produced sulfates deeper down in the soil. Lead author Huiming Bao attributes this difference to the fact that sea salts are bigger and so are less able to penetrate the soil's surface. "By studying the soil of the Dry Valleys, you really have a good glimpse of what can happen on Mars," adds co-author Mark H. Thiemens, dean of UCSD's Division of Physical Sciences. "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, because these sulfates may migrate.