Peruvian Mummies, Seashells Share Historical Climate Insight

Submitted by coldrum on Tuesday, 27 April 2010 Page Views: 1621

Shells from mollusks presented to the dead during ancient funeral ceremonies are keys to helping a geologist gauge ocean movements near the Peruvian coast from as much as 13,000 years ago.
No, really.

These unlikely sources serve almost as historical calendars filled with data about the conditions of the portions of the ocean in which they lived – conditions which may well have been vital to the well being of ancient Peruvian cultures, including the Moche, according to Dr. Fred Andrus, an assistant professor in UA’s geological sciences department.

Andrus and colleagues at the University of Arizona and the University of Maine, were awarded a $600,000 National Science Foundation grant to develop a better understanding of a deep ocean phenomenon known as upwelling and its impact on the climate and the economy of the people who lived in Peru over the past 13,000 years.

The work revolves around radiocarbon dating of the shells, but draws from anthropology, archaeology, chemistry, forensics and geology. It’s providing insight into climate history and a key contributor to that history, El Niño events, as well as, perhaps, significant developments in culture shifts that may have resulted from those long ago climate changes.

In 2002, Andrus co-authored a paper publishing in Science describing a change in El Niño-related ocean temperatures 5,000 years ago. The research suggested the climate shift may have contributed to increased economic complexities among cultures. Suggesting the one led to the other was, and is, a contentious idea, Andrus says.

“What does seem to be pretty well accepted is that you had a major change in El Niño about five-to-six thousand years ago, and then you had this huge fluorescence in civilization,” Andrus says. “The question has long been, is that just a coincidence, or is it linked? My mind was drawn to see if changes in upwelling changed coastal production (related to fishing) because those things can be directly linked to El Niño.”

Upwelling is the wind-driven act that brings deep water back to the ocean’s surface. As the ocean’s deeper waters contain more nitrogen and phosphorous, more marine life lives in the upwelled water. So, deep water upwelling off Peru’s coast makes fishing more plentiful, except when it doesn’t occur during El Niño events.

Upwelling is part of the El Niño cycle, but the interaction is complex. El Niño, a temporary, cyclical climate change initiating in portions of the Pacific Ocean near the equator, is devastating to Peruvians and impacts the rest of the world, Andrus says. “If El Niño changes, global climate changes.”

“So, many people, including myself, have been interested to see how El Niño has changed over time. I want to figure out how upwelling has changed because that will tell me, on one hand, how the Pacific Ocean circulated back then and, two, how these civilizations may have been able to make a living.”

And, that’s where the mollusk shells come in.

“If you can find a shell that you already know how old it is, you can measure the radiocarbon, and it will tell about past upwelling there.”
And dating those shells? That’s where the ancient funerals – and the mummified remains within Peruvian tombs – come in.

Archaeological sites are numerous along the coast of Peru. And for many of these cultures, including the Moche, the dry climate assists in the preservation of their artifacts. “You have these really well-preserved burials” Andrus says. The [burial] offerings sometimes include food, such as the sea creatures and their shells, and/or jewelry, weapons and tools.

Andrus and colleagues radiocarbon date the cotton, corn or other offerings to obtain an accurate estimate of the mummies’ ages.

“Then, we compare that to what the archaeologists think based on the stratigraphy (the study of sediment layers in the tombs) and artifacts. When we are confident we know the age when someone was buried, then we measure the radiocarbon in the shell, and it tells us what the ocean upwelling was like then. It’s a very direct measure.

Ok, so that explains how scientists know how old the shells are, but how does knowing their age help in determining the conditions of the waters they once lived in? To understand that, you have to explore the basics of radiocarbon dating.

Radiocarbon forms in the atmosphere and rapidly mixes throughout it. It takes, on average, about 400 years for this radiocarbon to mix into the ocean. After diffusion at the ocean’s surface, it may travel downward with sinking currents. In the North Atlantic, the ocean’s cold, salty waters sinks near to the ocean’s floor, taking the radiocarbon with it, Andrus explains. “In the ocean, water that sinks must come back up to the surface.”

This rising to the surface, or upwelling, occurs in zones, including one near Peru. The longer the water has stayed beneath the ocean’s surface, the more the radiocarbon within it decays. “Basically, you can date the carbon dissolved in water and learn how long that current has been down deep,” Andrus said.

Shells contain growth bands – rings similar to those found in trees. These rings provide hints as to what was happening during a certain portion of the shell’s life like a calendar.

“We would take multiple samples in a single shell, from birth to death, and it would tell us, at this particular moment in a shell’s life, it has x amount of radiocarbon, and, in this part, it has y amount of radiocarbon, and it would vary through the life of the shell – up or down along with upwelling. This is like a little key.

“We know how old the mummies are due to archaeological contexts and other types of dating techniques. We have a shell, and we know how old it is. We measure the radiocarbon in the shell and the difference between its real age and its radiocarbon age is, at least in part, a function of upwelling.

“We can go to these archaeological sites, collect shells and measure fairly directly how much upwelling was happening at the time the people lived there, and then infer how abundant fishes were, what the coastal ecosystem was like and, since these people were depending upon fishing, maybe how hard it was for them economically.”

Andrus is awaiting final data from the research, but is intrigued by early indications. “We are detecting changes in upwelling through time. Sometimes they are very sudden changes and happen very quickly.

For more, see www.newswise.com.

Note: Shells contain growth bands – rings similar to those found in trees. These rings provide hints as to what was happening during a certain portion of the shell’s life like a calendar.

Contributor Bio for coldrum: Based in Milkyway, contributing since 2002. Favourite site: Avebury.

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