A High-Resolution Lateglacial Climate Record From Lake Sediments in the Ethiopian Bale Mountains

The Bale Mountains in Ethiopia reach an altitude of ~4300 m. They were glaciated during the Last Glacial Maximum. After deglaciation, several small depressions on the Sanetti Plateau turned into shallow lakes. A 2.5 m deep pit was dug down to the base in one of the presently desiccated depressions and sampled at high resolution (2 cm) for geochemical analysis. A high-resolution tropical record of paleo-environmental changes has thus been made available, spanning the lateglacial period from ~15.5 to 11.5 cal. ka BP (based on 10 radiocarbon ages). The total organic carbon content (TOC) was most likely determined by temperature-controlled lacustrine productivity at this altitude. It reveals relatively high and constant values (~7%) from the deglaciation at 15.5 to 14 ka BP. Then a first sudden drop in TOC indicates a change in environmental conditions, coincident with the Oldest Dryas. Subsequent minima in TOC also seem to correlate with cold phases in the northern hemisphere: the Older Dryas at ~13.5 ka BP, the Gerzensee fluctuation at ~13 ka BP and the onset of the Younger Dryas (~12.5 ka BP). Other geochemical parameters, e.g. C/N, HI (Rock Eval), the carbon preference index (alkanes), and carbon isotopes show that degradation of lacustrine organic material due to eventual periodical desiccation did probably not influence the TOC signal. Besides, major dilution effects due to inorganic aeolian input can be excluded: there is no obvious correlation of TOC to grain size parameters. Therefore, we interpret the TOC content as temperature proxy, indicating synchrony of tropical and northern-hemispheric temperature changes during the lateglacial. The grain size analysis reveal an increased input of coarser aeolian material during the Oldest Dryas, indicating a more intensive wind strength. On the other hand, fine silts dominate before 14 ka BP and again with the onset of the Younger Dryas. This finding is consistent with previous studies suggesting reduced monsoonal intensity during the LGM and during the Younger Dryas event.