A Tropical 'Rain Gauge'

Revealing Secrets in the Depths of South America's Largest Lake p.2

Over the centuries, Native American cultures like the Aymara's have learned to adapt to this seasonal rainfall pattern. Innovation seemed to have reached a high art in the now abandoned city of Tiwanaku, located near Lake Titicaca, where archaeological excavations suggest both an architecture and agriculture that stressed water management.

Tiwanaku's decline and eventual disappearance after about AD 1200 may have been linked to an extended drought, according to some experts.

What were conditions like in the Amazon when Tiwanaku was being abandoned? For that matter, what were they like in the jungle during the last Ice Age, when higher latitudes of both Earth's northern and southern hemispheres were buried by thick glaciers?

While it would seem logical to go to the Amazon for those answers, conditions in that wet, ever-growing steambath environment make it hard to gather enough reliable clues, Baker said. That's why he and fellow investigators are using the much higher and drier lake as a proxy field site.

Geoffrey Seltzer, an associate professor of earth sciences at Syracuse University, has been bouncing shock waves off the underwater lake bed - a technique called "seismic reflection profiling" - to discern the presence of now submerged former shorelines.

Sherilyn Fritz, an associate geology professor at the University of Nebraska at Lincoln, and her graduate student Pedro Tapia, have been studying remains of tiny fossilized plants called diatoms. These silica-encrusted algae, now embedded in lake bed sediments, vary according to how deep or shallow, salty or fresh, Lake Titicaca was in the past.

Baker has been evaluating the telltale chemistries and magnetic properties of layer upon layer of those sediments, which collect when aquatic plants die and settle, or as soil and stones wash down from the land. He also studies the water trapped within all that mud and debris.

"I think of Lake Titicaca as a little ocean," he said. "Scientifically, we have to figure out how it works. How much water comes in? How much of it evaporates off? How does the chemistry change with depth? How does the circulation change? Generations of people have figured out how the ocean works. We need to know that in Lake Titicaca if we want to interpret what the sediment tells about the climate."

Collecting mud from lake beds requires teamwork. Baker, Fritz, Seltzer and others have teamed up to extract tubes of ancient sediments from Tale Titicaca through a process known as "coring." In its simplest form coring involves forcing a hollow pipe into the sediments, then pulling up samples trapped inside for study.

But coring isn't usually so simple. For one thing, the bottom may lie far below the vessel where scientists are working. That means linking and lowering strings of heavy pipes to reach and penetrate the sediments, and finding room to stack all that pipe up on the deck between use.

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