Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

La Niña Takes Bolivian Andes on a Sedimental Journey

02.10.2003


Rolf Aalto stands on the cracked mud of a sediment deposit along the Mamoré River in Bolivia. The deposit is part of an estimated annual average of 3 billion tons of sediment washed into the Amazon River basin from the Andes Mountains.
Credit: Rolf Aalto/University of Washington

Conventional wisdom says a river’s flood plain builds bit by bit, flood after flood, whenever the stream overflows its banks and deposits new sediment on the flood plain. But for some vast waterways in South America’s Amazon River basin, that wisdom doesn’t hold water, according to scientists funded by the National Science Foundation (NSF). Results of their research are published in the October 2nd issue of the journal Nature.

Seasonal rains wash billions of tons of rock and soil from the Andes Mountains each year. But new evidence suggests that it’s only about once every eight years, when the equatorial climate phenomenon known as La Niña is in full swing, that water rises rapidly enough to move huge amounts of that sediment to the flood plains.

"Because this research provides new insights into fundamental issues of fluvial rates and processes, the results will be applicable to many of the world’s river systems," said Walt Snyder, section head in NSF’s division of earth sciences, which funded the research. "This paper wonderfully underscores that understanding Earth processes is highly dependent on our ability to measure the rates at which the processes occur and the spatial extent of their products."

Rolf Aalto, a University of Washington (UW) geologist and lead author of the Nature paper, uses a lightweight tubelike device to extract core samples of such sediment deposits across the remote, thickly forested and unspoiled flood plains of northern Bolivia. He measures the amount and activity of lead-210, a radioactive isotope produced by the decay of naturally occurring radon. The isotope is deposited on the flood plain by fresh river sediment and by rainfall, and it can be used to date successive layers of sediment, for as long as a century after it is deposited, as the lead-210 decays further.

But when Aalto examined samples from the Beni River, which flows northward from the Andes into a Mississippi-sized Amazon tributary called the Rio Madeira, he was puzzled.

"I wasn’t seeing a signal that looked anything like what I expected from previous studies on other river flood plains. I was befuddled," he said. "I would see a lot of lead-210 on the surface, and then the activity would drop down to an unchanging level of activity for a long way down, until the next big decrease."

The multitude of thin layers and gradually declining lead210 activity he expected to see simply didn’t show up. The reason: A few thick sediment layers were filling most of the core samples. Aalto devised a new dating method to better reflect a more complex flood plain environment, then analyzed new samples that showed similar large deposits at various intervals going back nearly 100 years. Each of the large layers was deposited at a time that correlated with a La Niña event in the equatorial Pacific Ocean. La Niña is the cold phase of the El Niño Southern Oscillation climate phenomenon that now is recognized as having wide affects on weather throughout the Western Hemisphere.

"The whole system is being raised," Aalto said. Sediment is moved and deposited throughout the river system in episodes orchestrated by the El Niño cycle."

Thousands of years ago, the Beni flowed into the Mamore’ near the Andes. But its course has changed many times through the millennia and the main channel now is hundreds of miles away from the Mamore’. Major floods in which a river shifts course don’t happen often on human time scales, but these Bolivian rivers still can migrate large distances during a single flood, he said.

Aalto believes such active channel migration, periodic large sediment deposits on the flood plain and large floods might all be part of the general mechanics of large rivers. However, that theory cannot be fully explored in the United States because few rivers of appreciable size still function naturally. He hopes that studying largely unspoiled rivers such as the Beni and Mamoré will help to build a better understanding of how rivers behave in completely natural conditions, and that understanding can be applied to disturbed river systems in the United States and elsewhere.

"Rivers are the arteries of continents," said Aalto. "And the Beni is about as close to the heart of the Amazon as you can get."

Other authors of the Nature paper are Laurence Maurice Bourgoin and Jean-Loup Guyot of Institut de Recherche pour le Développement (IRD) in France and Brazil, who have conducted extensive field studies in Bolivia and Brazil; Thomas Dunne of the University of California, Santa Barbara; and David Montgomery and Charles Nittrouer, both scientists at UW. The research is funded by NSF and IRD.

Cheryl Dybas | NSF
Further information:
http://www.nsf.gov/
http://www.nsf.gov/od/lpa

More articles from Earth Sciences:

nachricht Six-decade-old space mystery solved with shoebox-sized satellite called a CubeSat
15.12.2017 | National Science Foundation

nachricht NSF-funded researchers find that ice sheet is dynamic and has repeatedly grown and shrunk
15.12.2017 | National Science Foundation

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First-of-its-kind chemical oscillator offers new level of molecular control

DNA molecules that follow specific instructions could offer more precise molecular control of synthetic chemical systems, a discovery that opens the door for engineers to create molecular machines with new and complex behaviors.

Researchers have created chemical amplifiers and a chemical oscillator using a systematic method that has the potential to embed sophisticated circuit...

Im Focus: Long-lived storage of a photonic qubit for worldwide teleportation

MPQ scientists achieve long storage times for photonic quantum bits which break the lower bound for direct teleportation in a global quantum network.

Concerning the development of quantum memories for the realization of global quantum networks, scientists of the Quantum Dynamics Division led by Professor...

Im Focus: Electromagnetic water cloak eliminates drag and wake

Detailed calculations show water cloaks are feasible with today's technology

Researchers have developed a water cloaking concept based on electromagnetic forces that could eliminate an object's wake, greatly reducing its drag while...

Im Focus: Scientists channel graphene to understand filtration and ion transport into cells

Tiny pores at a cell's entryway act as miniature bouncers, letting in some electrically charged atoms--ions--but blocking others. Operating as exquisitely sensitive filters, these "ion channels" play a critical role in biological functions such as muscle contraction and the firing of brain cells.

To rapidly transport the right ions through the cell membrane, the tiny channels rely on a complex interplay between the ions and surrounding molecules,...

Im Focus: Towards data storage at the single molecule level

The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.

Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

See, understand and experience the work of the future

11.12.2017 | Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

AKL’18: The opportunities and challenges of digitalization in the laser industry

07.12.2017 | Event News

 
Latest News

Engineers program tiny robots to move, think like insects

15.12.2017 | Power and Electrical Engineering

One in 5 materials chemistry papers may be wrong, study suggests

15.12.2017 | Materials Sciences

New antbird species discovered in Peru by LSU ornithologists

15.12.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>