Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi-arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate-dominated and chloride-dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot-spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom-nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.
Bibliographical noteFunding Information:
This research was funded by NSERC (Canada) (Grants GP629 to RWR, and PGS-D and Postdoctoral Fellowship to JJS), the Hong Kong Research Grants Council (Grants 12300815 and 12304018 to RBO and RWR), and National Science Foundation grant NSF-1338553 to TKL. GDC acknowledges funding from the Belgian Science Policy (BR/121/A2/PAMEXEA) and the Research Foundation Flanders (FWO). Special thanks are owed to Tata Chemicals Magadi and the National Oil Corporation of Kenya, especially John Ego for logistical support, and to the local Maasai communities. The late Jean-Jacques Tiercelin contributed to fieldwork in June 2013. Research was undertaken under several permits issued by the Office of the President and the Ministry of Science and Technology, Republic of Kenya, which we gratefully acknowledge. We thank the four reviewers, including Dr Paul Wright and Editor, Dr Giovanna Della Porta, for their comments that helped us to improve the manuscript.
© 2020 The Authors. Sedimentology © 2020 International Association of Sedimentologists
Copyright 2020 Elsevier B.V., All rights reserved.
- Kenya Rift
- saline alkaline lake
- siliceous gel
Continental Scientific Drilling Facility tags