Sensitivity of carbon isotopic proxies to paleoclimatic forcing: A case study from Lake Bosumtwi, Ghana, over the last 32,000 years

Analyses of carbon isotopic ratios in concomitant bulk sediment samples and fossil grass epidermal fragments from Lake Bosumtwi, Ghana, demonstrate that both records reflect paleohydrologic variability. However, the bulk sediment signal is dominated by within-lake processes, whereas the fossil grass epidermal record provides the terrestrial vegetation response to changes in available moisture. The direction of change is similar, but the magnitude and timing of response are different. During the terminal Pleistocene, the aquatic record shows a dramatic, flip-flop behavior (bulk sediment δ¹³C: -4 to -32‰), while the terrestrial grass epidermis record is much more muted (δ¹³C shifts from -11 to -15‰). Furthermore, during the transition to the relatively moister conditions of the Holocene, a dominance of C₄ plants persisted for at least 80000 years after limnological changes began. On the other hand, the epidermal isotopic record shows a much more dramatic response than that provided by the bulk sediment to the onset of drier, more seasonally contrasted conditions during the late Holocene. These results emphasize the need to consider varying response times of the biogeochemical systems that control the production of proxies, especially when attempting to correlate widely separated records based upon fundamentally different proxies.