The Great Acid Lakes

Ancient lakes across a huge portion of the western United States may have been so acidic their waters would have dissolved a person's skin. The discovery, reported in the 30 April Nature, may force a reevaluation of some of Earth's old watering holes.

Scientists had assumed that a series of ancient lakes once scattered from Kansas to North Dakota were largely alkaline, because of the large quantities of salt found in the area's sandy reddish rocks that were formed when the lake evaporated about 270 million years ago. (Sea salt keeps today's oceans alkaline for instance.) But several years ago, Robert Goldstein, a geologist at the University of Kansas in Lawrence, and Robert Burrus of the U.S. Geological Survey happened to test salt from one of these rocks with a laser spectrometer and found that it contained something surprisingly acidic.

Kathleen Benison, then Goldstein's graduate student, has now evaluated these rocks in detail. The acidity, she says, comes from tiny drops trapped in the salt deposits, which were left behind when the lakes evaporated. The drops contained large amounts of dissolved sulphate and acidic sulpher complex HSO4-. Benison, now at Central Michigan University in Mt. Pleasant, estimates that the lakes and groundwater in this part of the Midwest must have been highly acidic, with a pH between 0 and 1. Her analysis assumes the preserved droplets are identical to the ancient lake water.

How such vast bodies of highly acidic water could have formed is a mystery. Although small quantities of acid often develop naturally, large pools of highly acidic water are rare because there are usually some rocks or minerals containing carbonate to neutralize the acid. "As far as we know, this is the most acidic lake and groundwater system ever found outside of a volcanic area," she says, adding that there's no evidence of ancient volcanism in the U.S. Midwest.

"These are interesting and unexpected results," says sedimentologist Russell Dubiel of the U.S. Geological Survey in Denver, Colorado, it "goes against what the classical interpretations are, and ought to be followed up."