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Geoarchaeology
Warming opened Americas to humans

About 18,000 years ago, life for hunter-gatherers in eastern Asia started to change: Food became scarce, and winters grew brutal. But instead of heading south for a cozier climate, these people, strangely, headed north. The comparatively luxuriant Americas beckoned by way of present-day Alaska, with warmer climes and plenty of fish and game. In fur-and-skin canoes, small groups paddled along the shores of the Bering land bridge. They became the first Americans.

This northward coastal migration scenario, though uncertain in timing and mechanism, is gaining support in the archaeological community. To help address the long-standing mystery of how and when the first humans reached the New World, geologists have examined the temperature record of the northwest Pacific Ocean, finding that a series of warming events may have created ice-free coastlines for nearshore navigation into North America.

Archaeologists studying the early Americans, called paleoindians, struggle with a long-standing paradox: While the easiest way for people to reach North America was across a now-submerged land bridge from Siberia to Alaska about 13,000 years ago, some archaeological remains place humans in the Americas long before impassable ice sheets across the bridge’s interior retreated at the end of the last ice age. “It’s probably the major unanswered question in archaeology, and very contentious too,” says John Hoffecker, an archaeologist and paleoecologist at the University of Colorado in Boulder.

Tracing the footsteps of paleoindians has proven difficult. They left behind few traces, and rising sea levels have washed away anything deposited in the area known as Beringia, the vast tract of land that once connected Siberia and Alaska. Some of the best remaining evidence may be in sediments at the seafloor, where tiny marine organisms called foraminifera (forams) record past temperatures.

As published in the March issue of Geology, Michael Sarnthein of the University of Kiel in Germany and his co-workers examined forams that revealed a climate seesaw of sorts during three periods between 18,000 and 15,000 years ago, after the last ice age had passed its peak. While eastern Asia was becoming cool and exceptionally arid, the coastal Pacific was warming in the area of the proposed paleoindian crossing. The warm times in the Pacific coincided with the last major Heinrich event, an abrupt cold and dry phase in Europe and the North Atlantic. Each warm spell, Sarnthein says, lasted 500 to 1,500 years, enough time for many small groups to make the crossing.

The team arrived at the 20,000-year temperature record using three measures of the forams: their magnesium to calcium ratio, species composition and oxygen isotopic composition. “These three records all tell us the same thing,” Sarnthein says. “There were marked warm phases in antiphase to the North Atlantic.”

This record provides some of the first physical evidence for the northern seesaw effect, according to Andreas Schmittner, a climatologist at Oregon State University in Corvallis, who also studies climate swings in the region. And good temperature histories of the North Pacific are hard to come by.

Accurate radiocarbon dating — necessary for establishing a timeline — requires that organisms were exposed during their lifetimes to new carbon-14, produced continually in the atmosphere. However, marine organisms such as forams are submerged and thus are cut off from atmospheric carbon-14. By taking in carbon-14 only from the ocean, their radiocarbon dates, uncorrected, are older than they should be.

To correct for the local ocean radiocarbon effect, Sarnthein’s group calibrated dates by comparing features in their marine history to established land- and ocean-based radiocarbon histories. This method is gaining acceptance as a useful tool for dating marine samples, says paleoclimatologist John Andrews of the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado.

When Sarnthein’s team examined dates of archaeological sites throughout North and South America, they found a surprisingly close alignment between the timing of warm phases and the ages of human remains scattered across the continents. However, the dates assigned to some of these archaeological sites are in dispute, making a precise alignment of warm climate pulses with human activities difficult. “It makes sense, but the archaeological data just isn’t there,” Hoffecker says. “We need more sites.”

To find those sites along the now-submerged Beringian coastline, scientists may need to look underwater. Geoarchaeologist James Dixon, also at INSTAAR, mounted an underwater archaeological expedition of the Bering Sea in the early 1970s. He says that current technologies such as side-scanning sonar, remotely operated vehicles and sub-bottom density profiling could help locate stone tools or other human castoffs. In the meantime, Dixon says, Berinigian paleoenvironment data such as Sarnthein’s climate reconstruction provide “major breakthroughs” in understanding the timing of the first Americans.

Erika Engelhaupt
Geotimes contributing writer

Links:
"The Ice-Free Corridor Revisited," Geotimes, February 2004
"Quest for the Lost Land," Geotimes, February 2004

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