Late-Holocene sedimentation and sodium-carbonate deposition in saline alkaline Nasikie Engida, southern Kenya Rift Valley

Continental rifts are often characterised by geothermal activity and associated hydrothermal discharge, phenomena which can play a substantial role in depositional systems on the rift-basin floor. Water and solute budgets of hydrothermally fed lakes are often complex. At the same time, hydrothermal inflow can be a major buffer against desiccation, especially in shallow closed lake basins in arid environments. This is because, even though especially shallow groundwater flow is ultimately dependent on replenishment by meteoric waters, its lagged response time can attenuate climatic fluctuations on shorter time scales. Therefore, hydrothermal lakes are potential candidates to provide paleoenvironmental records in regions where other archives might be compromised by frequent drought. This is illustrated in Nasikie Engida, a shallow hypersaline lake in the arid southern Kenya Rift Valley, where geothermal groundwater has maintained a shallow (currently 1.5 m) but permanent water column and continuous deposition of highly authigenic laminated sediments during times when larger and deeper Rift-Valley lakes stood dry. Here we present the first data on the late-Holocene sediment composition of this remarkable system, including a history of the lacustrine deposition of the sodium-carbonate mineral nahcolite (NaHCO3). Our data includes bulk-sediment and mineralogical composition, the former corrected for systematic anomalies induced by large amounts of sedimentary sodium carbonate. We also present a stable-isotope time series of δ13C and δ18O of authigenic nahcolite, and of δ13C and δ15N of bulk organic matter. The potential roles of hydroclimate change and geothermal activity as driving processes behind the observed patterns are discussed, resulting in a first paleoenvironmental reconstruction from Nasikie Engida over the time period from ca. 2,700 cal yr BP to the present.