Climate and hydrologic controls on late Holocene sediment supply to an Amazon floodplain lake

Abstract

Organic and inorganic geochemical analyses on sediments from Preto Lake, a central Amazon basin floodplain water body, were used to document hydrological changes of the Solimoes River during the late Holocene. Between 3600 and 400 cal yr BP, Preto Lake received smectite-rich sediment from the Solimoes River, with high concentrations of Al (~ 53 × 103 ppm) and Si (~ 210 × 103 ppm). The high detrital input suggests there was a direct connection between Preto Lake and the main river channel. High river inflow maintained lake stage and was responsible for the contribution to sediments of phytoplankton-derived organic matter. Low sediment organic carbon concentrations characterized this period (mean ~ 3.8 wt%), probably because of dilution by river-borne lithogenic matter. Although the river inputs remained high, abrupt increases in TOC content around 1800 and 1200 cal yr BP suggest brief increases in fluvial nutrient supply to Preto Lake. During the last 400 cal yr, substantial declines in smectite (mean 40%), Al (~ 29 × 103 ppm) and Si (138 × 103 ppm) suggest the establishment of a semi-isolated lake, with reduced river inflow. A large proportion of C3-plant-derived organic matter, supplied by runoff from the kaolinite-rich watershed, was observed during this period, and was accompanied by high autochthonous primary production, driven by development of an aquatic macrophyte community. This change in sediment organic matter source accounted for the greater TOC content (~ 20%) of sediments deposited during this period. Although Holocene climate change was an important control on fluvial sediment supply to upper and lower Amazon Basin floodplain lakes, sedimentation processes in Preto Lake can also be linked to changes in the river’s course. The transition to a semi-isolated lake could have been a consequence of lateral and vertical sediment accretion, which formed a natural levee that blocked fluvial input to Preto Lake.