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 Early organic carbon got deep burial in mantle
25.04.2017 | Rice University

nachricht New atlas provides highest-resolution imagery of the Polar Regions seafloor
25.04.2017 | British Antarctic Survey

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

7th International Conference on Crystalline Silicon Photovoltaics in Freiburg on April 3-5, 2017

03.04.2017 | Event News

 
Latest News

NASA's Fermi catches gamma-ray flashes from tropical storms

25.04.2017 | Physics and Astronomy

Researchers invent process to make sustainable rubber, plastics

25.04.2017 | Materials Sciences

Transfecting cells gently – the LZH presents a GNOME prototype at the Labvolution 2017

25.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>