Hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators
Abstract Optical and geochemical techniques were applied to organic matter of Late Quaternary sediments from Lakes Victoria and Rukwa, East Africa. Variations in total organic carbon (TOC) and hydrogen content (expressed as Hydrogen Index, HI) can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences. TOC and HI both decline as an exposure surface is approached, mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation. Inert organic material of very low HI is typically concentrated at the overlying transgression surface. The carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces. Although weathering may be partially responsible, isotopic variations are mainly due to changes in the relative importance of material from plants using C4 photosynthesis, such as grasses and sedges, growing around the shore and on the exposed lake muds, and of material from planktonic algae and terrestrial C3 plants.