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The role of hydrothermal fluids in sedimentation in saline alkaline lakes : evidence from Nasikie Engida, Kenya Rift Valley

(2021) SEDIMENTOLOGY. 68(1). p.108-134
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Abstract
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.
Keywords
Stratigraphy, Geology, Evaporites, geothermal, Kenya Rift, nahcolite, saline alkaline lake, siliceous gel, trona, HOT-SPRINGS, EAST-AFRICA, MAGADI, BASIN, ORIGIN, DEPOSITION, BOGORIA, CRYSTALLIZATION, HYDROCHEMISTRY, BICARBONATE

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MLA
Renaut, Robin W., et al. “The Role of Hydrothermal Fluids in Sedimentation in Saline Alkaline Lakes : Evidence from Nasikie Engida, Kenya Rift Valley.” SEDIMENTOLOGY, vol. 68, no. 1, 2021, pp. 108–34, doi:10.1111/sed.12778.
APA
Renaut, R. W., Owen, R. B., Lowenstein, T. K., De Cort, G., Mcnulty, E., Scott, J. J., & Mbuthia, A. (2021). The role of hydrothermal fluids in sedimentation in saline alkaline lakes : evidence from Nasikie Engida, Kenya Rift Valley. SEDIMENTOLOGY, 68(1), 108–134. https://doi.org/10.1111/sed.12778
Chicago author-date
Renaut, Robin W., R. Bernhart Owen, Tim K. Lowenstein, Gijs De Cort, Emma Mcnulty, Jennifer J. Scott, and Anthony Mbuthia. 2021. “The Role of Hydrothermal Fluids in Sedimentation in Saline Alkaline Lakes : Evidence from Nasikie Engida, Kenya Rift Valley.” SEDIMENTOLOGY 68 (1): 108–34. https://doi.org/10.1111/sed.12778.
Chicago author-date (all authors)
Renaut, Robin W., R. Bernhart Owen, Tim K. Lowenstein, Gijs De Cort, Emma Mcnulty, Jennifer J. Scott, and Anthony Mbuthia. 2021. “The Role of Hydrothermal Fluids in Sedimentation in Saline Alkaline Lakes : Evidence from Nasikie Engida, Kenya Rift Valley.” SEDIMENTOLOGY 68 (1): 108–134. doi:10.1111/sed.12778.
Vancouver
1.
Renaut RW, Owen RB, Lowenstein TK, De Cort G, Mcnulty E, Scott JJ, et al. The role of hydrothermal fluids in sedimentation in saline alkaline lakes : evidence from Nasikie Engida, Kenya Rift Valley. SEDIMENTOLOGY. 2021;68(1):108–34.
IEEE
[1]
R. W. Renaut et al., “The role of hydrothermal fluids in sedimentation in saline alkaline lakes : evidence from Nasikie Engida, Kenya Rift Valley,” SEDIMENTOLOGY, vol. 68, no. 1, pp. 108–134, 2021.
@article{8676546,
  abstract     = {{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.}},
  author       = {{Renaut, Robin W. and Owen, R. Bernhart and Lowenstein, Tim K. and De Cort, Gijs and Mcnulty, Emma and Scott, Jennifer J. and Mbuthia, Anthony}},
  issn         = {{0037-0746}},
  journal      = {{SEDIMENTOLOGY}},
  keywords     = {{Stratigraphy,Geology,Evaporites,geothermal,Kenya Rift,nahcolite,saline alkaline lake,siliceous gel,trona,HOT-SPRINGS,EAST-AFRICA,MAGADI,BASIN,ORIGIN,DEPOSITION,BOGORIA,CRYSTALLIZATION,HYDROCHEMISTRY,BICARBONATE}},
  language     = {{eng}},
  number       = {{1}},
  pages        = {{108--134}},
  title        = {{The role of hydrothermal fluids in sedimentation in saline alkaline lakes : evidence from Nasikie Engida, Kenya Rift Valley}},
  url          = {{http://dx.doi.org/10.1111/sed.12778}},
  volume       = {{68}},
  year         = {{2021}},
}

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