Late-Holocene and recent hydroclimatic variability in the central Kenya Rift Valley: The sediment record of hypersaline lakes Bogoria, Nakuru and Elementeita
Abstract The sedimentology, salt mineralogy and stable oxygen- and carbon-isotope signatures of a mid-lake sediment sequence from hypersaline Lake Bogoria in Kenya reveal marked climate-driven changes in water-column mixing regime and salinity over the past 1700 years. Combined with sedimentological data on short sediment sequences from nearby lakes Nakuru and Elementeita, this results in a preliminary reconstruction of hydroclimatic variability in the central Kenya Rift Valley since ca. 300 AD. Stratigraphic analyses of bulk sediment composition, texture and magnetic susceptibility of all three cores; and smear slide, XRD and stable-isotope analyses on carbonate minerals in the Bogoria sequence allowed to define a succession of sedimentary units, corresponding to distinct phases in lake history. In the lowermost unit, four characteristic trona layers (Na3(HCO3)(CO3)·2H2O) are attributed to predominantly dry conditions during the second half of the first millennium AD and the period equivalent to the Medieval Climate Anomaly, until the first half of the 12th century AD. Lake Bogoria was probably polymictic at that time, its surface level standing significantly lower than today. The second unit displays uniform deposition of nahcolite (NaHCO3), indicative of a strongly stratified lake where a layer of less saline surface water (near-)permanently covers a hypersaline water mass with high pCO2. We propose that this depositional phase reflects the rising lake level related to the transition from dry to more humid climate conditions at the onset of the Little Ice Age, as previously documented for other parts of equatorial East Africa. The third, uppermost Bogoria unit contains only sporadic depositions of magadiite (Na2Si14O29(OH)3·11H2O) and various sodium carbonate minerals. This, together with low magnetic susceptibility, is interpreted to indicate humid Little Ice Age conditions, with highest lake level and reduced salinity around the late-15th to early-16th century AD. From the late-18th to early-19th century and again in the 1870's, the region experienced two episodes of drought more severe than any recorded in historical time. Tentative evidence for these events in the Bogoria record is not well constrained in time, but dated desiccation surfaces in the Nakuru and Elementeita records confirm their widespread nature across eastern equatorial Africa.