Geochronology Lab Dates 4.5 Million-Year-Old Hominid Fossils, Feb. 7, 2005

Dr. William McIntosh

by George Zamora

SOCORRO, N.M., Feb. 7, 2005 – A recent article published in the prestigious science journal Nature that reports on new Early Pliocene hominid discoveries in Gona, Ethiopia, has a strong connection to a research lab based in Socorro, New Mexico.

Volcanic sediments encasing the 4.5-million-year-old fossilized remains of Ardipithecus ramidus that were recently discovered in Ethiopia were accurately dated at the New Mexico Geochronology Research Laboratory (NMGRL) at New Mexico Tech.

William C. McIntosh, a volcanologist and geochronologist with the university’s New Mexico Bureau of Geology and Mineral Resources was cited as a co-author of the research paper which was written by an international team of scientists. McIntosh serves as a co-director of the NMGRL and is also a faculty member at the state-supported research university.

The NMGRL, which has been in operation for about 12 years, uses a precise method of argon isotope dating to calculate when rocks cooled from high temperatures, thereby providing ages, for example, for when lava flows solidified, or when mountain ranges were uplifted. Argon, an inert gas, accumulates over time when it is produced by the decay of naturally radioactive potassium within rocks.

The basalt specimens that were dated by the NMGRL in the recent hominid fossil study, which was led by paleo-archeologists at the CRAFT Stone Age Institute, were collected from lava flows interlayered with fossiliferous sediments in drainage basins in the Ethiopian rift, site of several important hominid fossil finds in recent years.

“There are several layers of lava and volcanic ash in the fossil site area,” said McIntosh. “These hominids were probably living in areas adjacent to active volcanoes.

“At the same time the volcanoes were active, river valleys were accumulating sediments, resulting in alternating sediments, lavas, and ash layers,” he added.

“The key to dating the fossils within the sediments is to date the overlying or underlying volcanic units,” McIntosh explained.

McIntosh and his fellow researchers at the NMGRL conducted two independent analyses of different samples of basalt in conjunction with the fossil hominid study and found both samples to be about 4.5 million years old.

“The similarity between the two age determinations increases confidence in their accuracy,” McIntosh said.

Geochronology analyses on volcanic ashes from the same archeological dig also were performed at the Berkeley Geochronology Center in California, and results from that testing further corroborated the ages that had been determined by the NMGRL.

The recently discovered A. ramidus fossils from Gona, Ethiopia included a piece of a toe bone that curves in a manner indicative of upright walking as a means of locomotion for what is thought to be an apelike common ancestor of both humans and modern chimpanzees.

Gona first gained scientific fame as the field site where the earliest known tools — dating back 2.5 million years — were found.

“Most of the work I’ve done in the New Mexico Geochronology Research Laboratory over the past 12 years has been focused on volcanoes and related topics, but we also have been involved with some early man studies — ranging from Ethiopia to Java,” McIntosh noted. “Early man studies tend to be more controversial than volcanoes.”

In hopes of expanding the capabilities of the NMGRL, McIntosh’s fellow co-director Matthew Heizler and New Mexico Tech adjunct professor Shari Kelley recently submitted a proposal to the National Science Foundation to add uranium, thorium, and helium isotope dating capabilities to the lab, including new equipment such as a state-of-the-art mass spectrometer and a laser extraction line.

“The addition of this new equipment would help us ‘fill in the gaps’ in our geochronology dating abilities, helping us draw a more accurate picture of recent uplifts and faulting, which can have a direct impact on today’s society when addressing issues such as management of water resources or land development,” McIntosh said.

McIntosh credited NMGRL technicians Lisa Peters and Rick Esser for “doing most of day-to-day work in the lab for the past ten years,” including the recent analyses of the basalts from Gona.