Microbes complicate the K-T mystery.

Microbes complicate the K-T mystery

A new problem has surfaced for those trying to decipher whether a meteorite slammed into the Earth 65 million years ago, killing off a large fraction of species, including the last remaining dinosaurs. Ancient bacteria and fungi may have tampered with the principal pieces of evidence from that time, caution researchers who have performed experiments with modern microorganisms.

"This could have a really important role in deciding what the nature of the catastrophe was," says biologist Betsey D. Dyer from Wheaton College in Norton, Mass. She and her colleagues describe their experiments in the November GEOLOGY. Dyer says the study is the first to demonstrate that microorganisms can both enhance and erase the iridium concentration in rock.

Iridium has played a primary role in uncovering the drama at the junction between the Cretaceous and Tertiary periods in Earth's history, which geologists call the K-T boundary. In 1980, after discovering extremely high iridium concentrations in a section of sedimentary rocks from Gubbio, Italy, scientists from the University of California, Berkeley, proposed the impact theory to explain the mass extinctions. Iridium is scarce in rocks from Earth's crust but more concentrated in deeper layers as well as in meteorites.

Berkeley researchers Luis and Walter Alvarez, Frank Asaro and Helen Michel proposed that the impact of a meteorite or comet sent up a global dust cloud that blocked out sunlight and caused a series of catastrophic changes that could explain the K-T extinctions. According to their theory, the impact body, called a bolide, vaporized after hitting, and extraterrestrial iridium rained down around the world to form a thin, highly concentrated layer. From the amount of iridium, the group estimated the bolide measured 10 kilometers across.

Since the discovery of the iridium layer at Gubbio, scientists have found the element at more than 100 K-T boundary sites worldwide. Some researchers maintain it came from a series of volcanic eruptions rather than an impact (SN: 6/10/89, p.356). Part of the debate over its origin has focused on characteristics, such as the layer's thickness, that vary from one site to another.

Dyer's group now reports results suggesting microbes could have severely altered the "original" iridium layer. In one experiment, the researchers compared pieces of a nickel-iron meteorite immersed in a bacteria-containing solution with pieces in a sterile solution. The bacteria caused more iridium to leave the meteorite and enter solution, they found. This suggests that microbes could have erased part of the original iridium layer or spread it into deeper sediments, Dyer says. In another trial, fungi and bacteria actually concentrated iridium that was dissolved in water. This indicates microbes could have enhanced the iridium layer, she says.

Ancient organisms may complicate the job of interpreting the iridium record, the scientists conclude. Previous estimates on the size of the bolide might be too large or too small, they note.

Walter Alvarez says the calculations of bolide size were always regarded as rough, so he does not foresee a change in those estimates. Yet he praises the study, saying, "I think this is something we've needed for a long time."

Scientists over the last decade have found substantial variations in iridium concentrations at different K-T sites. At Gubbio, for instance, researchers last year discovered high levels of iridium both above and below the main layer. Some say this indicates the iridium accumulated over a period of tens of thousands of years, suggesting prolonged volcanic eruptions or perhaps a series of impacts. Walter Alvarez says the microbial factor now offers another explanation for these and other iridium variations.