PALEOENVIRONMENTAL CONTROLS ON THE ISOTOPIC COMPOSITION OF ORGANIC CARBON IN TROPICAL AFRICAN LAKES

The isotopic composition of organic carbon (δ¹³C_om) in lake sediments is one of the most commonly used proxies in paleolimnology. Yet δ¹³C_om is controlled by a complex array of terrestrial and limnological processes that complicate the paleoenvironmental and paleoclimatic interpretation of δ¹³C_om profiles. Efforts to unravel the paleoenvironmental controls on δ¹³C_om in tropical African lakes were spearheaded by Michael R. Talbot, who hypothesized that variations in the relative importance of material from plants using C4 versus C3 photosynthesis mainly control δ¹³C_om.

Here I test this hypothesis by comparing variations in the δ¹³C of bulk organic matter with that of terrestrial leaf waxes in a suite sediment cores from tropical African lakes of varying size, productivity, and chemistry. These data demonstrate that the relative abundance of C3 and C4 plants growing in the lake catchment is indeed the principal control on the carbon isotopic composition of bulk organic matter in these lakes, despite numerous indicators that the bulk of the organic carbon accumulating in lakes sediments is derived from aquatic organisms. The mechanisms by which the δ¹³C of terrestrial organic carbon is “transferred” to lake remain unclear, but are likely founded in microbial degradation of terrestrial dissolved organic carbon.

These results simplify the interpretation of δ¹³C_om, yet the paleoclimatic interpretation of δ¹³C_om is not simple. By comparing trends in bulk and compound-specific δ¹³C to other paleoclimate proxies in these same lakes, I show that multiple environmental forcings control variations in the abundance of C3 and C4 plants, and the δ¹³C. These include atmospheric CO₂ concentrations, air temperature, precipitation, and, at least during the past millennium, human land clearance and agriculture. Multiproxy data are therefore necessary to constrain the dynamic interplay between long-term variations in climate and vegetation.