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Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation


Continental-scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains. The spatial extent and depths of such deposits have been described1,2, and flood-plain accumulation has been determined at decadal timescales3,4,5, but it has not been possible to identify discrete events or to resolve deposition on near-annual timescales. Here we analyse 210Pb activity profiles from sediment cores taken in the pristine Beni and Mamore river basins, which together comprise 720,000 km2 of the Amazon basin, to investigate sediment accumulation patterns in the Andean–Amazonian foreland. We find that in most locations, sediment stratigraphy is dominated by discrete packages of sediments of uniform age, which are typically 20–80 cm thick, with system-wide recurrence intervals of about 8 yr, indicating relatively rare episodic deposition events. Ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with La Niña events, which debouch extraordinary volumes of sediments from the Andes. We conclude that transient processes driven by the El Niño/Southern Oscillation cycle control the formation of the Bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon, nutrients and pollutants to the Amazon main stem.

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Figure 1: The Beni and Mamore river flood plains within the Llanos, northern Bolivia.
Figure 2: Unsupported 210Pb activity profiles (circles) and clay abundance (crosses) from representative flood-plain cores.
Figure 3: Spatial distribution of Beni flood-plain accumulation rates, averaged for five distance ranges, with best exponential fit (r2 = 0.82).
Figure 4: Temporal distribution of fluvial processes.


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This work was supported by an NSF research grant, a NASA Earth Systems Science Graduate Fellowship to R.A., and by the research collaboration (HYBAM Project) between IRD, SENAMHI (Bolivia), and the Universidad Mayor de San Andres, Bolivia. Laboratory assistance was provided by J. Staly, K. Sauers, G. Smith, J. Nittrouer and C. Gardner. Suggestions from C. Paola and A. Aufdenkampe improved the manuscript.

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Correspondence to Rolf Aalto.

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Aalto, R., Maurice-Bourgoin, L., Dunne, T. et al. Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation. Nature 425, 493–497 (2003).

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