Palaeolimnology of high and low altitude sites in Ethiopia

: Diatom and geochemistry records from Garba Guracha and Lake Babogaya

Crynodeb

Palaeoenvironmental studies at high altitude in eastern Africa are rare. As a result, our understanding of contemporary climate change in the region is limited. This study presents high-resolution, multi-proxy (diatoms and core geochemistry) palaeolimnological reconstructions at sites located at contrasting altitudes: Garba Guracha a cirque lake >3950 m above sea level spanning the last 16,000 years, and since the mid-Holocene at Lake Babogaya a crater lake on the western escarpment of the Main Ethiopian Rift (1800 m above sea level). These palaeo-records broadly exhibit the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from across eastern Africa. However, these shifts are more muted at Garba Guracha. Furthermore, the precise timing and expression of these climatic events is not always synchronous between the Lake Babogaya and Garba Guracha records as well as other records across eastern Africa. The Garba Guracha sediment geochemistry and diatom record is typical of a dry, post-glacial and sparsely vegetated environment with the dominance of Fragilarioid taxa and high terrigenous input. Due to the poorly developed postglacial landscape at Garba Guracha, any Younger Dryas-like drying signal would have been superimposed on to high soil erosion and a poorly productive lake environment. The retreat of a nearby ice mass may also have over-ridden any catchment development related to the start of the African Humid Period. However, in response to increased moisture availability and warming at Garba Guracha after 10,250 cal BP, catchment productivity considerably increases with a large increase in diatom productivity and organic matter content and remains high for the majority of the remaining African Humid Period interval. The termination of the African Humid Period is clear in the Garba Guracha diatom record at around 5,500 cal BP, with a second phase of change to an environment similar to the modern day after 4,500 cal BP, potentially representing the Meghalayan. Drier conditions around the Meghalayan is also documented in the Lake Babogaya record with the deposition of a Gastropod shell layer. A reduction in organic matter content following the deposition of this layer likely represents reduced productivity and a destabilising catchment under a drier climate. These drier conditions culminate in the deposition of coarse grained, terrigenous material between 3,600 and 3,300 cal BP. Following high lake nutrient content and deepening of the lake between 3,300 cal BP and 1,750 cal BP, the Lake Babogaya record suggests a relatively stable, deep lake, with only brief intervals of shallower waters, until the top of the core (~600 cal BP). The diatom record after 560 cal BP at Garba Guracha is characterised by a dominance of aerophilous taxa such as Nitzschia amphibia and Cymbelloid taxa driven by an increased growing season and nutrient loading under a drier climate. These records further reinforce the reported heterogeneous pattern of climate across the region and the significance of site-specific dynamics in the responses of catchments to regional drivers. The data produced in this study may be used to inform future climate modelling and perform more complete regional downscaling, while also furthering our understanding of palaeoenvironmental change in eastern Africa, at a variety of altitudes

Dyddiad Dyfarnu	2020
Iaith wreiddiol	Saesneg
Sefydliad Dyfarnu	Prifysgol Aberystwyth
Goruchwyliwr	Sarah Davies (Goruchwylydd) & Henry Lamb (Goruchwylydd)