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Clam Shells Reveal Evidence of Ancient, Climate-Induced Downfall, UA Researchers Say

Editor’s Note: Prior to April 1, Andrus is available via e-mail, cell phone and Skype.   

Seashells sit on a glass slide in a UA laboratory. By analyzing the smaller shells, left, researchers are learning more about historical impacts of climate change.

Seashells sit on a glass slide in a UA laboratory. By analyzing the smaller shells, left, researchers are learning more about historical impacts of climate change on residents of 6th-century Peru.

TUSCALOOSA, Ala. – Analyses of clam shells used in ancient funeral ceremonies offer additional evidence as to how climate change may have contributed to the gradual collapse of an early South-American civilization, according to University of Alabama scientists.

The research, publishing online earlier this month in the scientific journal Geology, indicates El Niño, a temporary, cyclical change in the Pacific’s circulation, and an intertwined ocean phenomenon, known as upwelling, likely contributed to the 6th century downfall of an advanced civilization called the Moche, said Dr. Fred Andrus, a UA associate professor of geological sciences and co-author of the research article.

The Moche once flourished along the northern coast of Peru.

“This gives different insight into how climate change looks in terms of human impact,” said Andrus. “We often view the impacts of climate change – of any form – in terms of dramatic destruction – dramatic sea-level rise inundating a city overnight or a terrible hurricane wiping a city off a map. Those things certainly do happen. But, climate change can also bring more subtle impacts that will still have profound, indirect consequences.

“In the case of the Moche, much of their protein was derived from the sea. They had specialized fishermen with trade and transfer of fish between people. If you change the fishery, like we think occurred here, that is going to have a domino effect through the people, ending on their dinner table.”

The new research illustrates some climate-related challenges faced by the Moche around 540 A.D., prior to their collapse. It indicates that upwelling (a wind-driven act that brings deep water and nutrients back to the ocean’s surface) diminished during this time-frame because of changes in El Niño patterns. This would have resulted in less seafood for the Moche and may have changed the types of fish species common in the area.

For example, anchovies, a key species in the region today, may have become less common at that time.

The article’s lead author is Dr. Miguel Etayo-Cadavid, one of Andrus’ former doctoral students who earned his Ph.D. from UA in 2010. Additional co-authors include representatives from the U.S. Geological Survey, the Arizona Accelerator Spectrometry Facility, the University of Arizona, the University of Maine, the University of Trujillo in Peru and Harvard University.

Dr. Miguel Etayo-Cadavid, a former UA doctoral student, uses a syringe to inject acide into a vial containing samples for isotopic analysis.

Etayo-Cadavid, a former UA doctoral student, uses a syringe to inject acid into a vial containing samples for isotopic analysis.

The National Science Foundation funded the research with a grant to Andrus and his colleagues from Arizona and Maine.

Key to the research was the recovery and analysis of clam shells the Moche presented to their dead during funeral ceremonies held nearly 1,500 years ago, Andrus said.

Clam shells used in the research were recovered from well preserved Moche burial sites at Huaca de la Luna, near the modern city of Trujillo in northern Peru. Through radiocarbon dating of the shells and other materials left in the excavated tombs and by studying the archaeological contexts of the burial tombs, the scientists can estimate the shells’ age and the relative amount of upwelling that occurred when the shells grew.

Through chemical analysis of the shells’ growth bands – which form somewhat similarly to tree rings – the scientists are able to determine what some of the environmental conditions were like during the lifespan of the clams. The process of combining the shells’ known age with the derived environmental conditions occurring during its lifetime to develop conclusions of long ago conditions is an example of a field of a science known as sclerochronology.

Andrus said discovering the information the shells “recorded” during their lifetimes is somewhat analogous to the “black boxes” housed in airplanes.

“You have tape recorders slowly adding data throughout the entire flight,” Andrus said. “After something happens in the flight, if you can get your hands on that record, you can review the timed series of events.”

Some organisms constantly add new skeleton on top of old, which can be analyzed to create a time series record.

“Of course, climate is very time-dependant, particularly El Niño. We’re interested in how often it comes and how long it lasts. When you combine our data with data from flood sediments and the archaeological data, we can assemble a picture of a period in which El Niño was behaving profoundly different than it does now.”

Andrus, sitting, and, from left, Joe Lambert, environmental isotopes specialist, and Etayo-Cadavid work in a UA lab.

Andrus, sitting, and, from left, Dr. Joe Lambert, environmental isotopes specialist, and Etayo-Cadavid work in a UA lab.

Research indicates the Moche faced periods of more frequent, more intense El Niño conditions with cycles lasting longer than what commonly occurs now.  The deep water upwellings, which can make fish more plentiful, were likely reduced by the extended El Niño conditions, Andrus said.

“Perhaps this upwelling shift didn’t strike people as equal importance as the flood waters washing their fields away, but rather was a subtle and fairly long-lasting change that would have contributed to social, economic and political unrest at the time.

“It was a gradual, ugly, painful transition that really was an economic and political adaptation to climate change that occurred over generations,” said Andrus of the collapse.

“The Moche example shows how unforeseen changes to the climate and environment under which a society has flourished may trigger profound political transformations within it,” the authors write in the journal article.

Additional background on the research project is contained in this 2010 feature story.

UA’s department of geological sciences is part of the College of Arts and Sciences, the University’s largest division and the largest liberal arts college in the state. Students from the College have won numerous national awards including Rhodes Scholarships, Goldwater Scholarships and memberships on the USA Today Academic All American Team.

The University of Alabama, a student-centered research university, is experiencing significant growth in both enrollment and academic quality. This growth, which is positively impacting the campus and the state's economy, is in keeping with UA's vision to be the university of choice for the best and brightest students. UA, the state's flagship university, is an academic community united in its commitment to enhancing the quality of life for all Alabamians.