古印度人定居北美早于预计时间

Researchers excavating（挖掘） a creek（小溪） bed in central Texas have found evidence suggesting humans settled in North America some two thousand years earlier than previously estimated. The findings are reported in the March 25 issue of Science.

Earth scientists at the University of Illinois at Chicago determined the age using an optical dating technique. They linked sediment and mineral samples to human artifacts（史前古器物） and tools found in a single stratigraphic layer located below younger, previously dated Paleo-Indian Clovis-culture artifacts.

Texas A&M University anthropologist（人类学家） Michael Waters led the archaeological dig in the Buttermilk Creek complex, near Austin, which has been excavated for years. While artifacts suggesting earlier settlement had been found at the site before, they couldn't be verifiably dated.

UIC earth and environmental sciences professor Steven Forman worked with Waters and a multi-university team of researchers, finding buried beneath the Clovis layer tools such as blades and cores -- technologies unique to the Clovis culture.

Forman gathered about 50 core samples from two sites at Buttermilk Creek for luminescence（发冷光） dating analysis at UIC. Carbon-14 dating couldn't be used to date the pre-Clovis artifact layer because that layer didn't contain any organic matter.

"We dated the sediments by a variety of optical methods," Forman said. "We also dated different mineral fractions as well, and we consistently got the same ages. We looked at the age structure of the sediment by many different ways and got the same answers."

This is the first time that optical dating has been used to constrain a paleoculture within a date range, Forman said.

Luminescence dating, a relatively new technique, measures light energy trapped in minerals such as feldspar（长石） and quartz formed centuries ago. Samples must be carefully handled to avoid any exposure to light, which would contaminate（污染） the readings. They were analyzed in a darkroom laboratory at UIC.

"We found Buttermilk Creek to be about 15,500 years ago -- a few thousand years before Clovis," Forman said. "It's the first identification of pre-Clovis lithic technology (stone tool technology) in North America."