A 2700 cal yr BP extreme flood event revealed by sediment accumulation in Amazon floodplains
Introduction
Along its course from the Andes of Peru to the Atlantic Ocean, the Amazon River and its main tributaries are lined by extensive floodplain-connected lakes that periodically oscillate between terrestrial and aquatic phases, with sediments constantly exchanging between river channels and floodplains. The main sedimentary balances in the Amazon Basin, from the upstream to the downstream, allow identification of the zones of transfer, erosion and deposition (Mertes et al., 1996, Gautier et al., 2010, Aalto et al., 2003, Dunne et al., 1998, Baby et al., 2009, Gautier et al., 2010). All these authors highlight the importance of these floodplain lakes to sediment accumulation. Sediment storage can occur at different timescales (from hundreds to thousands of years) (Dunne et al., 1998, Behling and Costa, 2000, Moreira-Turcq et al., 2004), and it is strongly influenced by the hydrodynamics of the Amazon River (Irion et al., 2006, Irion et al., 2010, Moreira et al., 2012, Moreira et al., 2013).
Mertes et al. (1996) estimated that 80% of the material being transported by the Amazon passes through the floodplain lake systems. Most of these suspended sediments in the Amazon River are derived from the tributaries draining the Andes, where high rainfall and steep slopes contribute to the release of stored sediment (Meade et al., 1985). When the river rises, water enters through channels from the downstream end of the islands into the lakes and transports fine-grained suspended loads into the lake basin (Irion et al., 2010). At high water, when most of the floodplain is inundated, relatively coarse sediment is transported into the lakes by the high river flow velocities. When the water level drops and the water drains slowly through the downstream channels, the fine-grained suspended matter, still present in the lake water, is deposited (Irion et al., 2010). Sedimentation rates in the Amazonian floodplains can vary considerably, both temporally and spatially, and are directly related to the geographic location, proximity to the mainstream, channel morphology and duration of connection between floodplain lakes and the river (Aalto et al., 2003, Moreira-Turcq et al., 2004, Gautier et al., 2009).
Gautier et al. (2010) proposed that the construction of the floodplain lakes in the Beni River (Bolivian Amazonia) is the result of two different sedimentary processes that can occur at different times. The first one occurs during the “normal” hydrological years when the suspended matter is transferred by advection from the river towards lakes. These sediments are fine-grained and relatively small in quantity, but the sedimentation is made in a regular way. The second process takes place during very large floods. In this case, a great part of the sediments can arrive by overflowing of the main channel on banks (overbank deposition), which can transport coarser material and consequently the sedimentation rates can be very high. Aalto et al. (2003) related sedimentation in the floodplains to climatic changes, more specifically to “La Niña events”, which in Amazonia are responsible for heavy rain, and they show that sedimentation is irregular and very dependent on the intensity of the floods. Finally, all these studies show that sediment deposition in Amazonia floodplain lakes is not constant over time and can be influenced strongly by hydrological factors dependent on climate conditions.
The Amazon seasonal flooding is related to the successive floods of the northern tributaries, principally the Branco and Negro rivers, that reflect the heavy precipitation in these regions from May to July and of the southern tributaries, largely the Madeira River, that have heavy rainfall from December to March. Downstream, the annual and regular flood of the Amazon River occurs from April to July (Molinier et al., 1997). Droughts in Amazon discharge have been related to Sea Surface Temperature (SST) anomalies in the tropical Atlantic and to El Niño Southern Oscillation (ENSO) events (Espinoza et al., 2011). On the other hand, high discharge events also occur during heavy La Niña rains in the north and northeast of the Amazon region (Ronchail et al., 2002, Espinoza et al., 2009) and are also influenced by enhancement of South American Monsoon circulation and southward shifts of the Intertropical Convergence Zone (ITCZ) and associated heavy precipitation (Marengo et al., 2011).
Our objective was to study the histories of floodplain sedimentation in two Amazon lakes during the late Holocene, to interpret them in terms of paleohydrology, and to relate them to the climate changes reported for that period.
Section snippets
Regional setting
The Amazon Basin (Fig. 1) extends between 5°N and 20°S and from the Andes to the Atlantic Ocean, covering approximately 6,500,000 km2. Wet and dry seasons in the Amazon basin are induced by the South American monsoon system and fluctuations in the position of the ITCZ (Garcia and Kayano, 2011). Precipitation ranges from < 2000 mm yr− 1 in the extreme northeastern and southern parts of the basin and to 7000 mm yr− 1 on the east side of the Andes (Salati et al., 1979). Interannual rainfall variability in
Material and methods
The cores TLS1 (Salé Lake), TLP2 and TLP3 (Poção Lake) and TLG1 (Grande Lake) were collected using a “vibra-core”, and the MAR1 core (Maracá Lake) was collected manually using a 2.0 m-long PVC tube (10 cm of diameter). The cores were opened, described and sampled in the laboratory. Soon after opening, aluminum U-channels were collected to determine sediment bulk density and water content. Sub-samples were taken contiguously at 1 cm intervals along the cores, and the water content was measured
Lithology and chronology
The TLS1, TLP2, TLP3, TLG1 and MAR1 sediment cores from Curuai floodplain and Maracá Lake consist mainly of clays and silts (Table 1 and Fig. 2). The TLP2, TLP3, TLG1 and MAR1 cores contain organic-rich clay layers. The TLS1 and TLG1 cores show a pattern of horizontal lamination, particularly at the base of the cores that are the older sediment sampled. In all the cores we observed 10 to 80 cm-thick heterogeneous layers with variegated color ranging from gray to dark brown and yellowish brown (
Discussion
Sedimentation processes in Amazon floodplain lakes are strongly dependent on the hydrology of the river (Behling et al., 2001, Irion et al., 2006, Moreira et al., 2012, Moreira et al., 2013). Nowadays, the annual water-level fluctuations in floodplain lakes are approximately synchronous with those of the Amazon River (Bonnet et al., 2008). When the Amazon River level rises, the water and sediments flow into the lakes through channels, transporting a mostly fine-grained suspended load into the
Conclusions
The Late Holocene sedimentation in the studied floodplains is dominated by the fast deposition of sediment packages that likely correspond to one or several successive extreme flood events of the Amazon River. These packages are characterized by a variegated color and a uniform 14C age. Their density, granulometry and composition do not differ markedly from the rest of the sediments indicating that the source of material is always the Amazon River. The sediment packages are probably produced by
Acknowledgments
This research was supported by the French Research Institute for Development (IRD), by the HYBAM Research Program (Hydrology and Geochemistry of the Amazonian Basin, http://www.mpl.ird.fr/hybam/) in the frame of its cooperation agreement with the Brazilian Research Centre (CNPq process nos. 492685/2004–05 and 690139/2003–09). This project was also supported by the project INSU Paleo2–PASCAL (Past Climate Change Impacts on Carbon Accumulation in Amazonia Floodplain lakes (2010–2012)) and ANR
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