Biogeochemical Proxies of Environmental and Climate Change on Mount Kenya

Abstract

The three crater lakes from Mount Kenya (Nkunga, Sacred and Rutundu) were studied to infer Late Holocene climate and environmental changes. These crater lakes are small closed lake basins with well-defined catchments that are sensitive to seasonal, inter-annual and long-term fluctuations. Consequently, they are promising archives for high-resolution reconstruction of climate and environmental change in Kenya. Already published sedimentary records on the palaeoclimatic and palaeoenvironmental history of east Africa show that the region exhibits variability at various timescales, but matching them across space and time has been problematic either because the records were of low resolution or because the chronology has been too coarse. Consequently, the aim of this study was to elucidate the Late Holocene history of climate and environmental changes on Mount Kenya using multi-proxy palaeo-indicators in soil and lake sediments. By using a multi-proxy suite of indicators, coupled with a good chronology based on radiocarbon dating, it was possible to capture a diverse array of climatic and environmental changes that may not be apparent from one or a limited number of the traditionally used indicators, such as pollen. The multi-proxy analysis that was carried out on the Mount Kenya soils and lake sediments comprised traditional sedimentological (XRD, XRF and Xlf) and relatively novel organic geochemical analyses (%C, %N, δ13C, δ15N, n-alkanes and GDGT). The occurrence and timing of different events were established by AMS 14C dating of the cores. From these, it was possible to tease out a high-resolution record that reflects both local and regional changes, thus capturing also the spatial heterogeneity that has been observed in the different east African lake records. The results from our crater lakes cover the last 4770 cal yr. BP to present. There are significant changes in lake ecosystems and hydrology that have occurred during the Late Holocene, which are coincident with large sediment and organic matter influx to the lakes. At the millennial scale, a wet early Holocene followed by a drier mid to late Holocene is observed. The Holocene is punctuated by major dry spells separated by abrupt transitions to wet periods. During the Late Holocene two key dry spells at ca. 4200 and 2800 cal yr. BP occur in the shallow lake phases at lakes Rutundu and Sacred. There is also evidence that describes a wet early Little Ice Age (at Lakes Nkunga and Sacred) followed by drier conditions during the late phase of the Little Ice Age (Lake Nkunga). The multiproxy approach has therefore also allowed the identification of local catchment-scale effects on the individual lakes in addition to the observed regional climate effects, reflecting their sensitivity to climate perturbations and related localised environmental responses.