Late Holocene sedimentology and palaeoenvironment of Kiluli Swamp, Mount Kenya

Abstract

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.