Carbon cycling within an East African lake revealed by the carbon isotope composition of diatom silica: a 25-ka record from Lake Challa, Mt. Kilimanjaro

The carbon cycle of a lake is a balance between supply from the atmosphere and catchment, and the net demand exerted by primary producers, minus losses back to the atmosphere and to sediment storage. Evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi-proxy approach. One promising technique is to explore the carbon-isotope composition (δ13Cdiatom) of organic matter incorporated within the silica frustules of diatom algae. Here we present a 25,000-year record of δ13Cdiatom from the sediments of crater Lake Challa on the eastern flank of Mt. Kilimanjaro, and along with other proxy data we make inferences about the three major phases in the history of the lake's carbon cycle. From 25 ka to 15.8 ka years BP, δ13Cdiatom is positively correlated with the δ13C of bulk sediment organic matter (δ13Cbulk), indicating that high diatom productivity, as recorded by high % biogenic silica at this time, was preferentially removing 12C and enriching the δ13C of lake-water dissolved inorganic carbon. From 15.8 to 5.5 ka the correlation between δ13Cdiatom and δ13Cbulk breaks down, suggesting carbon supply to the lake satisfied or exceeded the demand from productivity. From 5.5 ka BP the positive correlation resumes, indicating an increase in the internal demand for carbon relative to external supply. Diatom frustule-bound carbon isotopes offer an original tool in examining long-term fluctuations in a lake's carbon budget and how the balance between supply and demand has changed through time.