EU/Th AND 14C isotope dating of lake sediments from sacred lake and lake Nkunga, Kenya

In the tropical regions, lake and swamp sediment core chronologies have traditionally been established solely by radiocarbon dating. In several instances, however, the radiocarbon sampling resolution has been coarse, entailing extrapolations over time periods where there may have been considerable change in sedimentation rates related, for example, to significant, albeit abrupt, palaeoclimatic and palaeoenvironmental change. Moreover, some cores may age-wise exceed the radiocarbon dating limit of ca.40,000 yr, thus entailing tenuous extrapolations of radiocarbon dates obtained in the younger sections of the core in order to obtain a whole corechronology. In this paper, the chronology of lake sediment cores retrieved from Sacred Lake and Lake Nkunga on the north-eastern flank of Mount Kenya is established using a combination of highresolution radiocarbon dating and experimental U/Th dating to circumvent the drawbacks mentionedabove. The derived chronosequences, which show that these sediment records span almost the whole of the late Quaternary period, demonstrate the efficacy and synergism of these dating techniques

Late Holocene sedimentology and palaeoenvironment of Kiluli Swamp, Mount Kenya

Kiluli Swamp is an extensive valley swamp near the lower limit of the montane forest on the eastern slopes of Mount Kenya, East Africa. The swamp is fed by a small spring on the northeastern margin, and the water table lies a few centimetres below the surface. The swamp's sediments modify water chemistry: the Na-Mg-HCO3 water-type at the input changes to a Ca- Mg-HCO3 water-type in the central parts of the swamp. A short sediment core (2.12m) was retrieved from the central part of the swamp using a modified Livingstone piston corer. The sediments were mainly composed of silty organic mud, silty clay and coarse silt. Three radiocarbon dates were obtained. A suite of sedimentological analyses was carried out in order to reconstruct the palaeoenvironmental history of the area, and these included: mineral magnetic characteristics (susceptibility, IRM etc.); total organic carbon (TOC); total nitrogen (TN), and stable carbon isotopes. The sediment record stretches from about 4,000 yr BP (before present) to present. Indications are that the valley was initially dry and the catchment vegetation was characterised by dominant C4-type grassland. The initiation of true swamp conditions occurred at ca.470 yr BP immediately following a phase of deep ponding and high diatom productivity within the swamp between 600 and 470 yr BP. A high incidence of charcoal from 470 to 0 yr BP probably marks the period of persistent anthropogenic activities within the catchment. There is a change in vegetation type from a predominantly C4-type to predominantly C3-type at about 130 yr BP that is attributed to crop cultivation within the swamp rather than due to climate change, since the arid phase which marks this zone would have, under natural conditions, abetted the continued dominance of C4 plants which are more drought-resistant than C3 plants. The changes observed are broadly synchronous with other palaeoenvironmental records from Mount Kenya and the surrounding region. African Journal of Science and Technology Vol.4(2) 2003: 12-2

Linear and non-linear responses of vegetation and soils to glacial-interglacial climate change in a Mediterranean refuge

The impact of past global climate change on local terrestrial ecosystems and their vegetation and soilorganic matter (OM) pools is often non-linear and poorly constrained. To address this, we investigatedthe response of a temperate habitat influenced by global climate change in a key glacial refuge, LakeOhrid (Albania, Macedonia). We applied independent geochemical and palynological proxies to asedimentary archive from the lake over the penultimate glacial-interglacial transition (MIS 6–5) andthe following interglacial (MIS 5e-c), targeting lake surface temperature as an indicator of regionalclimatic development and the supply of pollen and biomarkers from the vegetation and soil OM poolsto determine local habitat response. Climate fluctuations strongly influenced the ecosystem, however,lake level controls the extent of terrace surfaces between the shoreline and mountain slopes and hencelocal vegetation, soil development and OM export to the lake sediments. There were two phases oftransgressional soil erosion from terrace surfaces during lake-level rise in the MIS 6–5 transition thatled to habitat loss for the locally dominant pine vegetation as the terraces drowned. Our observationsconfirm that catchment morphology plays a key role in providing refuges with low groundwater depthand stable soils during variable climate