Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Amazon Basin sediment accumulation influenced by La Niña

24.11.2003


Enormous quantities of sediment are deposited in the flood-plains traversed by the Amazon and its tributaries in times of flooding. Scientists have hitherto considered the sedimentation rate to be generally constant with time.



Research conducted jointly by the IRD, the Universities of Washington1 and California2 and the Bolivian National Meteorology and Hydrology Service (SENAMHI) of La Paz, on two Bolivian rivers shows on the contrary that such events are irregular and less frequent than has been thought. These results, just published in Nature, emphasize that, in this Andean-Amazonian foreland, sedimentation is closely dependent on the flood amplitude, in turn linked to climatic variability, and particularly to La Niña, the cold phase of the ENSO (El Niño Southern Oscillation).

Continental-scale rivers can transport enormous sediment loads. In periods of flood, a proportion of these is deposited on flood-plains. In the Amazon Basin, crossed by the Earth’s largest river, great volumes of such sequestered sediment accumulations occur. This is especially so in the Llanos, the Bolivian lowland flood-plains which stretch from the foot of the Andes. An estimated 100 to 150 million tonnes of sediment are deposited each year respectively in the Rio Beni and the Rio Mamore flood-plains. These are the two Andean tributaries of the Rio Madeira, one of the Amazon’s main tributaries and source of more than half the sediment load transported by that river.


The inter-annual sediment accumulation rate has up to now been considered to be generally constant. Now a study conducted in the floodplain which receives these two tributaries refutes this. It is the first to reveal an episodic pattern. During the past century, large sediment accumulation events indeed occurred only quite infrequently (11 events recorded over 90 years of analysis), corresponding to an average recurrence interval of eight years.

These results were the fruit of investigations forming part of the HyBAm (Hydrogéodynamique du Bassin amazonien) programme, conducted by a joint research team involving the IRD (working in the combined research unit UMR LMTG-CNRS-IRD- Paul Sabatier University of Toulouse), the Universities of Washington1 and California2, and the Bolivian National Meteorology and Hydrology Service (SENAMHI) of La Paz. 210Pb3 activity profile analysis on 300 sediment cores sampled from the Beni and Mamore basin flood-plains, interpreted using a new geochronological model developed by the University of Washington,1 enabled them to date discrete sedimentary packages to near-annual resolution. They revealed evidence of an episodic pattern in the main sedimentation events.

Why does sediment accumulation in this part of the Amazon Basin show an episodic pattern? Climatic variability plays a prime role. The team established a significant correlation between these periods of mass sediment deposition and La Niña, the cold phase of the ENSO (El Niño Southern Oscillation) climatic cycle. During most La Niña years over the past century, the Andean relief has been subjected to torrential rainfall generating fierce flooding and intensive mechanical erosion on the Andean slopes. Recordings from a hydrological station situated at the foot of the Andes showed this.

Measurements were continued and followed up in the Hybam programme. When these floods occur, huge volumes of sediment are eroded from the Andean piedmont sub-basins and transported towards the floodplain. For a proportion of these sediments to have been deposited there (for example up to 40% in the Beni Basin flood-plain), the researchers estimate that the water-level rise, during these years of high sedimentation rate, must have been extremely rapid, in excess of 8 000 m3/s. The floods would then have reached the force necessary to cut crevasses in small natural levees along the main stems of the Beni and Mamore rivers and inundate a large expanse of the plain.

With the new data the research team also studied the century-long depositional history in the floodplain of mercury associated with fine particles and the role of these in trapping metal elements transported by the great Amazonian rivers and their tributaries. Also brought to light was a strong increase, over the past 30 years, of mercury concentrations in the sediment particles deposited in the Rio Beni plain. This corresponds to the recent "boom" in gold prospecting, now at an end, but also to the colonization of new arable land on the steep flanks of the Andean piedmont.

Marie Guillaume | alfa
Further information:
http://www.ird.fr/fr/actualites/fiches/

More articles from Earth Sciences:

nachricht Climate change weakens Walker circulation
20.10.2017 | MARUM - Zentrum für Marine Umweltwissenschaften an der Universität Bremen

nachricht Shallow soils promote savannas in South America
20.10.2017 | Senckenberg Forschungsinstitut und Naturmuseen

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Neutron star merger directly observed for the first time

University of Maryland researchers contribute to historic detection of gravitational waves and light created by event

On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

 
Latest News

NRL clarifies valley polarization for electronic and optoelectronic technologies

20.10.2017 | Interdisciplinary Research

Metallic nanoparticles will help to determine the percentage of volatile compounds

20.10.2017 | Materials Sciences

Shallow soils promote savannas in South America

20.10.2017 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>