Carbon sedimentation at Lago Grande de Curuai, a floodplain lake in the low Amazon region: insights into sedimentation rates

Abstract

We studied recent organic carbon sedimentation in an Amazonian floodplain using sediment trap samples and one core. Sediment traps were placed in several lakes of the “Lago Grande de Curuai” floodplain during different water-level phases: the end of the period of water-level reduction (‘falling water’) and the start of the period of water-level increase (‘rising water’). Vertical seasonal fluxes of settling particles (SP) are very difficult to estimate and interpret, especially during ‘falling water’ periods, when the concentration of suspended matter in the water column is greater than during ‘rising water’ periods, probably due to resuspension processes. Vertical fluxes of settling particles (SP) varied between ca. 4 and 2000 g m−2 day−1. Fluxes of organic carbon ranged between (5–50 g C m−2 day−1) during the ‘falling water’ and between (0.041–4 g C m−2 day−1) during the ‘rising water’. The core is characterised by two laminated, sedimentological units. Sediments in the upper unit, which generally coarsens upwards, are interpreted as a progradation sequence of the sand and silt flat at the entrance of an inflow channel where the core was collected. This unit is composed of three sub-units: a superficial layer reworked by bioturbation, a lower sub-unit characterised by horizontal and sub-horizontal laminations of fine sands and silts, and a basal sub-unit composed of horizontal laminations of fine silts and clays. The lower unit is characterised by horizontal and sub-horizontal laminations of fine sands and silts and corresponds to the top of an older accumulating sequence. These changes show that the sedimentary environment is very dynamic at the centennial scale. Sedimentation rates generally ranged between 0.42 and 1.34 cm year−1, and were estimated to range from 0.56 to 1.35 cm year−1 after normalisation for clay-sized particles (<4 μm). Floodplain sediment accumulation generally varies between 0.42, 1.16, and 1.10 g m−2 year−1, within the first, second and third sub-units, respectively. In the second unit the non-supported 210Pb date is close to the detection limit. Carbon content is relatively low, around 1% sediment weight through the core, although mean carbon accumulation rates are very high, generally 100 g m−2 year−1 and peaking at 250 g m−2 year−1. These results suggest that Amazon floodplain lakes may constitute an important sink for organic carbon.