Holocene floodplain lake sediments in the Amazon: 14C dating and palaeoecological use

Abstract

The Holocene biostratigraphic record of Amazon lowland rain forests is fragmentary and is based on restricted number of cores, the majority representing sedimentation in a floodplain environment. These biostratigraphic data have often been interpreted as demonstrating climatic oscillation between wetter and drier conditions, and the subsequent vegetation changes. Here, we survey and 14C date a further six western Amazon floodplain lakes and examine their fluvially controlled sedimentation processes. The lakes, which are of the cutoff type, are located along the rivers Manu, Tambopata and Amazon (Solimões) in the Peruvian Amazon lowlands. All the lakes cored are under the influence of current overbank sedimentation of the parent suspension-rich whitewater rivers and receive an annual fill of allochthonous inorganic suspension during the flood. Most of the cores collected were rich in minerogenic sediments (gyttja clay, loss on ignition < 5% dry weight) and gave 14C ages for the cutoff events of the lakes ranging from 2000 to 6000 BP. However, the high channel migration rates of the Manu and Amazon, as well as observations on the marginal, young successional floodplain forest structure, cast doubt on these dates, suggesting that they are too old. Two cores, however, yielded considerably younger, and more reliable, dates for the cutoff events, one from a more organic-rich sediment layer (190 ± 80 BP) and one from a wood fragment (1300 ± 90 BP, a 14C accelerator date). We think that the observed bias towards older 14C ages documented in the gyttja clays is caused by the high proportion of old (fine particulate) organic matter being recirculated and resedimented within the suspension load. The strong fluvial control of lake sedimentation and the unreliability of the 14C dates from the Amazon suspension-rich floodplains call for caution in palaeoecological interpretation. Furthermore, owing to the low number of within-lake replicate cores in the Amazon Holocene stratigraphy analyses, we consider the postulated Holocene climatic oscillation schemes to be tentative only.