Shoreline fluctuations of Lake Hayk (northern Ethiopia) during the last 3500 years: Geomorphological, sedimentary, and isotope records

This paper presents a stratigraphical and geomorphological analysis supported with geochronological dating aiming to reconstruct the Late Holocene lake-level fluctuations and paleohydrological history of Lake Hayk. This lake (23 km2 in surface) is located in the eastern side of the northern Ethiopia highlands, at an altitude of around 2000 m above sea level. Along the northern margin of the lake, which is mainly characterized by steep rocky slopes, the studied succession consists of colluvial and stromatolitic deposits. Along the southern margin, which is characterized by a wide coastal plain drained by the main watercourse feeding the lake (the Ankarka River), the study deposits are mainly palustrine, deltaic and fluvial in origin. Geomorphological and sedimentological data, integrated with a radiocarbon chronology, highlighted that during the last 3500 years the lake was affected by three main highstand phases, which occurred at about 3250–3000, 2600–950 and 650–160 cal yrs BP, respectively. These highstands are documented by aggradation of siliciclastic systems both along the southern and northern margins, whereas isotope data highlight that most of the stromatolitic deposits developed during phases of intense evaporation. This peculiar feature could be differently explained: i) stromatolite growth was triggered by rapid lacustrine rises, but mainly occurred during the long-lasting lacustrine falls; ii) stromatolite developed during tectonic-induced transgressions, which occurred during phases of intense evaporation. Despite the possible effects of local tectonics, the Lake Hayk record shows a good correlation with those of several late Holocene lakes of East Africa lakes (Naivasha, Tanganyika, Victoria, Turkana, Abhè, Ziway-Shala, Bosumtwi and Abyiata), highlighting that it can be considered as a suitable climatic proxy for late Holocene time span. In particular, by both siliciclastic and stromatolitic deposits occurring in the uppermost part of the sedimentary succession record high-frequency oscillations, which have been ascribed to three main droughts that occurred during the Little Ice Age. These droughts were previously documented only in Lake Naivasha and Lake Victoria.