Aalto Rolf, Maurice-Bourgoin Laurence, Dunne Thomas, Montgomery David R, Nittrouer Charles A, Guyot Jean-Loup
Quaternary Research Center University of Washington, Seattle, Washington 98195, USA.
Nature. 2003 Oct 2;425(6957):493-7. doi: 10.1038/nature02002.
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 described, and flood-plain accumulation has been determined at decadal timescales, but it has not been possible to identify discrete events or to resolve deposition on near-annual timescales. Here we analyse (210)Pb 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.
河床为沙地的大陆尺度河流，会将很大一部分泥沙负载截留在洪泛平原中。此类沉积物的空间范围和深度已有描述，且已确定了洪泛平原在年代际时间尺度上的堆积情况，但一直无法识别离散事件或解析近年度时间尺度上的沉积情况。在此，我们分析了取自原始的贝尼河和马莫雷河流域沉积物岩芯的（210）Pb活度剖面，这两个流域共同构成了72万平方公里的亚马孙河流域，以研究安第斯 - 亚马孙前陆的沉积物堆积模式。我们发现，在大多数地点，沉积物地层主要由年龄一致的离散沉积物包组成，这些沉积物包通常厚20 - 80厘米，全系统的重现间隔约为8年，这表明沉积事件相对罕见且具有偶发性。海洋温度和水流记录将这些偶发事件与与拉尼娜事件相关的迅速上涨的洪水联系起来，这些洪水会从安第斯山脉带出大量沉积物。我们得出结论，由厄尔尼诺/南方涛动周期驱动的瞬态过程控制着玻利维亚洪泛平原的形成，并调节着沉积物以及相关碳、养分和污染物向下游输送至亚马孙干流的过程。
