February 12, 2009 | By Tim Steury | 1 Comment »
Categories: Earth sciences
Tags: archaeology, electron probe microanalysis, Franklin Foit, tephra, tephrochronology, volcanoes, Washington State University
Geologist Franklin (Nick) Foit uses a million dollar time machine called an electron probe micro-analyzer to identify the chemical fingerprints of ancient volcanoes. Among other applications, those fingerprints can then be used to date adjacent soil layers and archaeological objects.
Foit recently worked with Robert Mierendorf, a student of Foit’s some years ago who is now the archaeologist with the North Cascades National Park, to date an archaeological site at Cascade Pass, a 5,400-foot pass that has been used by area Indians for ages. Using both tephrochronology and radiocarbon dating, they established the oldest human usage of the site as occurring more than 9,000 years ago.
The Cascade Range, which reaches from British Columbia to northern California, is full of volcanoes, many of them active. The ash, or tephra, they expel during eruptions can travel thousands of miles before settling to earth. Foit and others try to identify those ash layers in order to better understand both the geologic and human history of the area.
The tephra from every volcanic eruption has its own chemical makeup. Foit identifies their origin by first preparing samples of tephra for analysis by sealing them in a thin glass slide. An electron probe in the micro-analyzer measures the amounts of sodium, magnesium, aluminum, silicon, iron, calcium titanium, potassium, chlorine, and occasionally other elements. Once he gets an analysis, he calculates a “similarity coefficient,” which indicates how close a match the sample is to one in his database of known tephras.
Although that analysis might correlate to a known source, alone it does not date an ash layer. Doing so involves analyzing the context, the ash stratigraphy right next to the volcano. Often, says Foit, the tephra will fall on vegetation, which may be preserved in the tephra layer.
Living things absorb an isotope of carbon, carbon-14. When an organism dies, it stops taking in carbon-14, and the isotope begins to decay. That rate of decay is known and is used to determine the age of the organic material.
If the strata have been undisturbed, the age of the tephra can be determined by the age of the organic material.
Foit says as many as ten tephras are represented in the meter of sediment from Cascade Pass. The best preserved were from several eruptions of Mount St. Helens, two from Glacier Peak (2,010, 5,800 years old), one from Mount Baker (7,200 years old), and one from Mount Mazama.
Mazama is the eruption that resulted in Oregon’s Crater Lake 7,600 years ago. Tephra from Mazama, which was 42 times as powerful as the 1980 St. Helens eruption, spread to the northeast, covering over a million square miles. Its distinctive tephra provides a chronological marker across much of the Western United States, British Columbia, Alberta, and Saskatchewan.
Foit is one of only two people in the United State who perform this kind of analysis. Foit has built a database of 1,700 analyses over the past 25 years, including analyses from others as well as his own.
Even with such a substantive database, however, Foit has plenty of detective work remaining. Only recently, for example, did he and colleagues solve a tephrochronological mystery.
Mount St. Helens has erupted many times. Even though each tephra is unique, there are subtle compositional differences that are difficult to tease out. In this instance it involved discriminating tephras from two more recent eruptions from Mount St. Helens critical to dating archaeological sites in the Park.
Barely a month ago, a former student, Steven Kuehn, who is a post doc at the University of Alberta, was able to chemically distinguish tephras from two eruptions of Glacier Peak that occurred closely together 13,550 years ago. Trace element analysis revealed that the older layer matched perfectly what Foit had collected in the North Cascades, providing one more solid piece of information in understanding the long geologic narrative of the Pacific Northwest.