Bulk composition of sedimentary organic matter used in palaeoenvironmental reconstructions: examples from the tropical belt of South America and Africa

Abstract

The organic matter of lake sediments contains information that allows for the reconstruction of past environmental and climatic changes. The elemental composition and isotopic characteristics of sedimentary organic matter enable discrimination between allochthonous (vegetal matter from within the catchment) and autochthonous (phytoplanktonic and fixed algae) components. This approach makes it possible to distinguish between the inputs of organic matter from C 3- vs. C 4-type land plants. Sedimentary records from South America and Africa provide examples of applications of organic bulk composition in palaeoenvironmental reconstructions. The records from two African lakes show that, during the Last Glacial Maximum (LGM), the distribution of C org/N org– δ 13C pairs indicates the dominance of organic material having a C 4 terrestrial origin. These inferences are supported by pollen analysis, which indicates opening up of the forest at both sites, consistent with drier climatic conditions than present. The record from Siberia Lake (Bolivia), ca. 3000 m altitude, shows that, during the LGM, the sedimentary organic matter is dominated by the phytoplanktonic fraction, as indicated by the C org/N org– δ 13C pairs. The petrographic analyses show that this material is not well preserved, suggestive of drier conditions, consistent with interpretations from the pollen record. The beginning of the Holocene is marked by a shift from a scattered C 4 signal to one with a greater terrestrial C 3 and aquatic phytoplanktonic influence. These results indicate climatic amelioration towards more humid and warmer climatic conditions, leading to forest development in the lake catchment, together with increasing lake levels conducive to greater phytoplankton production. These conditions persisted during the Holocene, as shown by the C org/N org– δ 13C pairs in all lakes, with a terrestrial C 3 signal predominating at the Carajas and Siberia lakes. Only Dom Helvécio Lake shows a different pattern during the beginning of the Holocene in the distribution of C org/N org– δ 13C pairs. This pattern indicates that the sedimentary organic material is dominated by C 4 terrestrial material coming from savanna expansion in the lake catchment during a period of relative aridity in central Brazil.