Lake archives from tropical Africa: Reconstructing the paleoclimate leading to the African Rainforest Crisis 3000 years ago

The second largest rainforest biome on Earth lies in the Congo Basin of central Africa. Due to increasing pressure associated with slash-and-burn farming practices as well as climate change, the rainforest may loose its ability to absorb atmospheric carbon. Interestingly, former studies have found evidence for a similar so-called “African Rainforest Crisis” that took place in the late Holocene, around 3000 years ago (Brncic et al. 2009; Garcin et al. 2018). There are some indications that this crisis was caused by the expansion of the Bantu people, who are believed to have migrated to this area to pursue extensive farming at this time. Pollen records, to the contrary, suggest a shift towards a drier climate as the primary mechanism inducing the crisis. Since data from this area are scarce, more information is needed to resolve the exact causes of the African Rainforest Crisis, especially since it is a likely analog for ongoing and future rainforest contraction.

To provide such information, we collected a total of about 30 m of core from 13 lakes along the rainforest-savannah boundary in the Kasaï Basin, Democratic Republic of Congo, which constitutes the southwest portion of the Congo Basin. We developed preliminary age models of the sediment records using bulk radiocarbon measurements to compare the estimated sedimentation rates. Two periods of reduced sedimentation rates around 700 and 3500 years before present have been identified. These can indicate the timing and spatial extent of erosive events usually linked to drying and a shift in vegetation. In parallel, we analyse the catchment’s vegetation through time using stable-carbon istopes to distinguish between the abundance of C₃ and C₄ plants. Additionally, bulk XRF and XRD data offers insights into the underlying mineralogy. First results suggest kaolinitic and quartz-rich sediments, indicating high wethering intensity. By using mineralogy as an indicator of weathering intensity, we expect to draw conclusions on the hydrology and temperature throughout the late Holocene. To further constrain the paleoclimate reconstructions, triple-oxygen isotope analysis on the clay-size fraction and pollen and charcoal analyses are planned. A holistic interpretation will aim to reveal the role of climate vs. land-use change as a trigger of the African Rainforest Crisis, which serves as an analog to better predict how the Congo rainforest will respond to today’s land use- and climate-triggered challenges.

 

REFERENCES

Brncic, T. M. et al. (2009) Holocene 19, 79–89.

Garcin, Y. et al. (2018) PNAS 115, 3261–3266.