Hydroclimatic change and vegetation response in Tropical African alpine environments over the Holocene

Abstract

Future precipitation is highly uncertain, particularly in East Africa where model projections contradict modern observational trends. Climate changes in East Africa's mountains are even less well understood, yet these mountains are threatened by warming, extreme flooding, fires, and the rapid disappearance of glaciers. Paleoclimatic and paleoenvironmental records can contextualize ongoing and future changes; however, proxy records from East African mountains disagree with each other and, in some cases, differ from nearby lowland sites. It is unclear if these discrepancies are due to geographic or altitudinal gradients in climate or processes that impact the fidelity of climate proxies in these mountains. To better understand past mountain precipitation and vegetation change, we produced three new records of the hydrogen and carbon isotopic composition of terrestrial leaf waxes (δ2Hwax and δ13Cwax) spanning the Holocene using lake sediment cores collected from Lake Mahoma (2990 m asl), Lake Africa (3895 m asl), and Lake Kopello (4017 m asl) in the Rwenzori Mountains, Uganda-Democratic Republic of Congo. We find similar trends at all three sites in reconstructed precipitation hydrogen isotopic composition (δ2Hprecip), which we interpret to reflect precipitation amount. Our records show drying occurred during the Younger Dryas (∼12 cal ka BP), enhanced precipitation during the African Humid Period (∼10-5 cal ka BP) and drying in the mid to late Holocene (∼5-0 cal ka BP), similar to adjacent lowland sites. We observe a gradual termination of the African Humid Period in the Rwenzori, consistent with many regional records. We also observe distinct patterns of Holocene precipitation change in Eastern and Western Africa, indicating differences in the delivery of moisture from the Atlantic and Indian Oceans. Despite changes in precipitation over the Holocene, the carbon isotopic composition of the waxes in Rwenzori lakes indicates constant C3 vegetation and eco-hydrological stability, in contrast to sites from lower elevations. This likely indicates climate remained humid and cool enough throughout the Holocene to maintain forest and alpine plant communities at these high-elevation mountain sites.