Dead in the water

What killed Fossil Lake?

SINCE the early 19th century, Fossil Lake, a 52m-year-old site in south-west Wyoming, has been known for its fish, insects, reptiles, birds and mammals. It contains millions of them, beautifully preserved in layers of limestone that are interspersed with volcanic ash. Yet this palaeontological paradise holds a dark secret: the mass deaths were not caused by a single event. The interspersing layers of ash show they were a regular occurrence. Until now, though, nobody has worked out what happened.

Jo Hellawell of Trinity College, Dublin, and her colleagues in the Organic Geochemistry Unit at Bristol University think that they have solved the mystery. In doing so, they adopted Sherlock Holmes’s maxim that when you have eliminated the impossible, whatever remains, however improbable, must be the truth.

One suggestion—that the eruptions which laid down the ash were responsible—is easily ruled out because the ash layers do not correlate with the fossil-rich ones. Severe storms or floods look equally unlikely causes. They would have washed vast quantities of rock into the lake at the time the animals died—but the fossils are surrounded only by finely layered silt. Droughts, too, look scarcely credible as culprits. They would leave subtle clues in the isotopic composition of the limestone by shifting the ratios of light to heavy carbon and oxygen atoms in its calcium carbonate.

Having shown that climatic or environmental events were improbable explanations, Ms Hellawell and her colleagues started analysing the sediments in more detail. First, they considered the possibility that seasonal upwellings of toxic or oxygen-poor bottom water were responsible. Changing temperatures in the lake during the winter could have brought stagnant water to the surface, thus asphyxiating entire schools of fish. They concluded that such upwellings might indeed account for the catastrophic fish deaths, but that they would not explain the deaths of insects, reptiles, birds and mammals, since these animals breathe air rather than relying on dissolved oxygen.

The only possibility that remained was that the lake itself had somehow become an enormous pot of poison, intoxicating anything that drank the waters or ate animals found within. How such a thing could happen on a regular basis was, at first, perplexing. But Ms Hellawell revealed on September 23rd, at the Society of Vertebrate Palaeontology’s annual meeting, that she and her team had found evidence that toxic algae were responsible for the mass deaths.

The team analysed the organic compounds in the rocks of Fossil Lake. This analysis detected 4-methyl steranes—chemicals often made by tiny algae known as dinoflagellates. Many dinoflagellates are harmless, but some produce neurotoxins. In several parts of the world, such as the seas off the coast of Florida, toxic dinoflagellates sometimes develop into enormous blooms called red tides. These release vast quantities of neurotoxin and kill almost everything that comes near.

Toxic blooms also occur, albeit on a smaller scale, in bodies of freshwater—and that, Ms Hellawell thinks, is the answer to the mysterious case of Fossil Lake. The killer has been caught, as it were, red-handed.

Source of Information : The Economist 2009-10-03