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

Continental rift systems are often characterized by geothermal activity and associated discharge of hot groundwater, which can have a substantial impact on water, solute and sediment budgets of rift-valley lakes. Hot-spring inflow can result in aberrant geochemical characteristics, but at the same time can buffer against desiccation of lakes in dry climate regimes. Consequently, hydrothermally fed lakes can provide continuous sedimentary records from regions where other paleoenvironmental archives are lacking. This is illustrated by Nasikie Engida, a shallow hypersaline and alkaline (soda) lake in the semiarid Rift Valley of southern Kenya. Here, inflow of hot-spring water has maintained a shallow but permanent water body and continuous deposition of sediments through past climatic episodes when some larger lakes in the Kenyan rift stood dry. We present the first data on late-Holocene sedimentation in this remarkable lacustrine system, marked by authigenic nahcolite [NaH(CO3)] formation during part of its recorded history. Our data include analyses of bulk-sediment and mineralogical composition, clastic-mineral grain size and magnetic susceptibility, all corrected for the systematic anomalies induced by large amounts of sedimentary sodium carbonate. We also present exploratory time series of the stable-isotope composition of bulk organic matter and authigenic nahcolite. The result is a first paleoenvironmental reconstruction of the history of Nasikie Engida from ca. 2,670 cal yr BP to the present. Nasikie Engida has accumulated finely laminated sediments continuously during that period, which is remarkable given its current maximum depth of only 1.6 m. Synsedimentary nahcolite appears abruptly around 2,240 cal yr BP and since then has regularly been deposited in the form of distinct pure layers up to several cm thick. Its formation has been enabled by high pCO2, probably from geothermal activity and decaying organic matter, accumulating in a high-density brine. Variations in nahcolite deposition and bulk-sediment composition suggests multi-decadal to centennial oscillations in climate-driven water-column stability and stream inflow. Furthermore, Nasikie Engida is identified as a unique modern analogue for the interpretation of ancient nahcolite-bearing salt-lake deposits.