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In eastern Africa, there are few long, high-quality records of environmental change at high altitudes, inhibiting a broader understanding of regional climate change. We investigated a Holocene lacustrine sediment archive from Lake Garba Guracha, Bale Mountains, Ethiopia, (3,950 m asl), and reconstructed high-altitude lake evaporation history using δ18O records derived from the analysis of compound-specific sugar biomarkers and diatoms. The δ18Odiatom and δ18Ofuc records are clearly correlated and reveal similar ranges (7.9‰ and 7.1‰, respectively). The lowest δ18O values occurred between 10–7 cal ka BP and were followed by a continuous shift towards more positive δ18O values. Due to the aquatic origin of the sugar biomarker and similar trends of δ18Odiatom, we suggest that our lacustrine δ18Ofuc record reflects δ18Olake water. Therefore, without completely excluding the influence of the 'amount-effect' and the 'source-effect', we interpret our record to reflect primarily the precipitation-to-evaporation ratio (P/E). We conclude that precipitation increased at the beginning of the Holocene, leading to an overflowing lake between ca. 10 and ca. 8 cal ka BP, indicated by low δ18Olake water values, which are interpreted as reduced evaporative enrichment. This is followed by a continuous trend towards drier conditions, indicating at least a seasonally closed lake system. [ABSTRACT FROM AUTHOR]

Abstract: Lake Ashenge, a closed-basin lake near the northernmost penetration of summer monsoon rains, is well placed to provide a continental record of past changes in the strength of the African monsoon system. Diatom and oxygen isotope analyses of the lake sediments confirm that the overall trend of climate change during the past 17,000 years was driven by precessional forcing, punctuated by abrupt shifts that may be linked to changes in Atlantic surface temperatures. The lake level was low from at least 17.2 to 16.2 cal kyr BP, and then rose between 16.2 and 15.2 cal kyr BP, which may represent a temporary reactivation of the monsoonal circulation system following its reduced activity during the Last Glacial Maximum. The lake was significantly low between 13.6 and ~11.8 cal kyr BP coinciding approximately with the Younger Dryas, but beginning 900 years before its recognised onset in the Greenland ice-core record. A major sedimentary hiatus, covering the interval ~11.8 to 7.6 cal kyr BP, was probably caused by an early Holocene lowstand, the precise timing of which cannot be determined because pre-lowstand sediments were eroded from the core site. The lake filled to its overflow from 7.6 cal kyr BP until 5.6 cal kyr BP, when the sediments record an abrupt lake response to the regional transition to arid conditions that mark the end of African Humid Period. Evidence is also presented for climate changes which may have been associated with the rise and fall of Aksum, Ethiopia''s first great civilisation. [Copyright &y& Elsevier]

Abstract Multi-proxy analyses of an 8 m sediment core from Lake Hayk, a closed, freshwater lake in the north-central highlands of Ethiopia, provide a record of changing lake level and inferred regional climatic change for the last 15.6 cal ka years. Between ca. 15.6–15.2 cal ka BP, a lowstand was synchronous with Heinrich Event 1 and an intense drought across Eastern Africa. At ca. 15.2 cal ka BP a lake began to develop at the core site in response to wetter conditions, at the onset of the African Humid Period (AHP). However, in contrast to other lakes in eastern Africa, Hayk lake level fell around ca. 14.8 cal ka BP, indicating a climate shift towards aridity. The lake began filling again at ca. 12.3 cal ka BP and reached maximum water depth between ca. 12.0–10.0 cal ka BP. Lake level declined slowly during the Holocene, culminating in the termination of the AHP at Hayk between ca. 5.2–4.6 cal ka BP. In the late Holocene, ca. 2.2–1.3 cal ka BP, Lake Hayk was again deep and fresh with some evidence of short-term lake level variability. The palaeo-record from Lake Hayk indicates that while it experienced, to a broad degree, the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from eastern Africa, there are offsets in timing and rate of response. These differences reflect chronological discrepancies between records, as well as the varying climate sensitivities and site-specific factors of individual lake basins. This record highlights the different responses by lakes in a climatically vulnerable area of Ethiopia. Highlights • Discrepancies in the timing of the Younger Dryas Stadial proper. • No evidence for the catastrophic shift in climate centred at 8.2 ka. • Diatom evidence suggesting relatively rapid AHP termination. • High-resolution evidence of millennial to multi-decadal variability at Lake Hayk. • Regional discrepancies in the timing and expression of climatic events. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution diatom, sediment and pollen analyses of two sections from the annually laminated SG06 core from Lake Suigetsu were used to study the onset and termination of the late-glacial climate reversal in central Japan. Its broadly recognised counterpart is the Younger Dryas or Greenland Stadial-1 (ca. 12.85–11.65cal. kyr BP based on the NGRIP ice core records). Our study suggests that accumulation of the analysed late-glacial sediments occurred in a deep and relatively cold water meso-eutrophic lake with a strong mixing regime and relatively high silica content. Combining these results together with available pollen-based environmental reconstructions we suggest that climate cooling, together with an intensified winter monsoon and thicker snow cover could influence changes in regional vegetation, sedimentation processes and trophic status of the lake during the transition from the last interstadial to stadial around Lake Suigetsu. A decrease in total pollen concentration and increase in Fagus pollen percentage indicate local vegetation stress/disturbances and suggest that cooling started at least 2–3 decades prior to the major shift in the inorganic sediment (accumulation of detrital layers) and in diatom assemblages (change from Aulacoseira ambigua to Aulacoseira subarctica dominance), which took about 10years. The transition from the last stadial to the Holocene again shows that vegetation in the lake catchment area reacted first to the regional climate change, i.e. to the weakening of the winter monsoon and decrease in winter snow accumulation. The increase in the vegetation cover density and reduced volume of surface runoff associated with the decrease in melt water supply is likely responsible for the reduced soil erosion activity which caused the cessation in detrital layer accumulation and consequent decrease in the amount of nutrients brought to the lake and lowering of the water nutrient status. The latter process finally influenced changes in the diatom assemblages, including the return to dominance of A. subarctica ca. 30years after the virtual disappearance of detrital layers from the sediment. Our results demonstrate the rapid response of the Lake Suigetsu system to the global cooling and subsequent warming, and allow clear definition of the onset and termination of the late-glacial climate reversal. Despite the fact that the lake system shows a more abrupt shift from the warm to cold (and cold to warm) environments than terrestrial records of vegetation demonstrate, we do not see any delayed response of local vegetation to the climate change. This last conclusion is of particular importance for application of the SG06 pollen record for quantitative climate reconstruction. [Copyright &y& Elsevier]