Orbital control of Pleistocene euxinia in Lake Magadi, Kenya

Lake Magadi is an internally drained, saline and alkaline terminal sump in the southern Kenya Rift. Geochemistry of samples from an ~200 m core representing the past ~1 m.y. of the lake’s history shows some of the highest concentrations of transition metals and metalloids ever reported from lacustrine sediment, including redox-sensitive elements molybdenum, arsenic, and vanadium. Elevated concentrations of these elements represent times when the lake’s hypolimnion was euxinic—that is, anoxic, saline, and sulfide-rich. Euxinia was common after ca. 700 ka, and after that tended to occur during intervals of high orbital eccentricity. These were likely times when high-frequency hydrologic changes favored repeated episodes of euxinia and sulfide precipitation. High-amplitude environmental fluctuations at peak eccentricity likely impacted water balance in terrestrial habitats and resource availability for early hominins. These are associated with important events in human evolution, including the first appearance of Middle Stone Age technology between ca. 500 and 320 ka in the southern Kenya Rift.

Supplemental Material: Orbital control of Pleistocene euxinia in Lake Magadi, Kenya

Comparison of Lake Magadi euxinia with global paleoclimate and orbital parameters, photography of selected core intervals, geochemical data, and statistical analyses.<br>

Supplemental Material: Orbital control of Pleistocene euxinia in Lake Magadi, Kenya

Comparison of Lake Magadi euxinia with global paleoclimate and orbital parameters, photography of selected core intervals, geochemical data, and statistical analyses.<br>

Labyrinth patterns in Magadi (Kenya) cherts: Evidence for early formation from siliceous gels

Sedimentary cherts, with well-preserved microfossils, are known from the Archean to the present, yet their origins remain poorly understood. Lake Magadi, Kenya, has been used as a modern analog system for understanding the origins of nonbiogenic chert. We present evidence for synsedimentary formation of Magadi cherts directly from siliceous gels. Petrographic thin-section analysis and field-emission scanning electron microscopy of cherts from cores drilled in Lake Magadi during the Hominin Sites and Paleolakes Drilling Project in 2014 led to the discovery of two-dimensional branching "labyrinth patterns" in chert, which are a type of fractal "squeeze" pattern formed at air-liquid interfaces. Labyrinth patterns preserved in chert from Lake Magadi cores indicate invasion of air along planes in dewatering gels. These patterns support the precipitation of silica gels in the saline-alkaline Lake Magadi system and syndepositional drying of gels in contact with air as part of chert formation. Recognizing cherts as syndepositional has been critical for our use of them for U-Th dating. Identification of labyrinth patterns in ancient cherts can provide a better understanding of paleoenvironmental and geochemical conditions in the past.

Monitoring mineral precipitation sequence of Lake Magadi soda lake: A multi-technical approach

&lt;p&gt;Lake Magadi is a saline soda lake in East African Rift Valley, occupying the axial trough of Southern Kenyan Rift. Its fed by perennial saline hot/warm springs, which evolve into the soda and saline chemistry of the lake. The main processes thought to cause the enrichment of the lake in Na&lt;sup&gt;+&lt;/sup&gt;, CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;, Cl&lt;sup&gt;-&lt;/sup&gt;, HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; and SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt; are evaporative concentration, mineral precipitation and fractional dissolution [1]. Lake Magadi is considered an analogous environment to the early Earth [2]. The high pH, silica and carbonate content of Lake Magadi allows the formation of silica and carbonate induced self-assembled mineral structures [3,4]. Revealing the mineral precipitation sequence of Lake Magadi have implications in understanding the geochemistry of evaporative rift settings and soda oceans. We have experimentally investigated the mineral precipitation sequence during evaporation at 25 &amp;#176;C. The sequence of mineral precipitation was recorded by using in-situ video microscopy. The mineral patterns observed in video microscopies were identified by spectroscopic, diffraction and electron microscopy techniques. The mineralogy and elemental composition of the precipitates were determined by using Raman spectroscopy, powder X-ray diffractions and scanning electron microscopy coupled with energy dispersive X-ray analyser. The results of the ex-situ analyses were compared with the in-situ X-ray diffraction. In-situ X-ray diffractions were performed on acoustically levitated droplets in the &amp;#956;Spot beamline at BESSY II synchrotron (Berlin, Germany). Finally, thermodynamic evaporation simulation was performed by using PHREEQC code with Pitzer database. Ex-situ and in-situ experiments revealed that mineral precipitation begins with trona, followed by halite and finally thermonatrite. In PHREEQC simulations, natron was observed instead of thermonatrite, suggesting the role of kinetics in the mineral assemblages. This multi-technical approach of in-situ monitoring and ex-situ characterization is a powerful approach to unveil mineral precipitation patterns and the resulting geochemical evolution in evaporative rift settings.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgments&lt;/strong&gt;&lt;strong&gt;: &lt;/strong&gt;We acknowledge funding from the European Research Council under grant agreement no. 340863, from the Ministerio de Econom&amp;#237;a y Competitividad of Spain through the project CGL2016-78971-P and Junta de Andaluc&amp;#237;a for financing the project P18-FR-5008. M.G. acknowledges Grant No. BES-2017-081105 of the Ministerio de Ciencia, Innovacion y Universidades of the Spanish government.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1] Eugster, H.P. (1970). Chemistry and origin of the brines of Lake Magadi, Kenya. Mineralogical Society of America Special Papers, 3, 213&amp;#8211;235.&lt;/p&gt;&lt;p&gt;[2] Kempe, S.; Degens, E.T. (1985). An early soda ocean?.&amp;#160;Chem. Geol.&amp;#160;&amp;#160;53, 95&amp;#8211;108&lt;/p&gt;&lt;p&gt;[3] Getenet, M.; Garc&amp;#237;a-Ruiz, J.M.; Verdugo-Escamilla, C.; Guerra-Tschuschke, I (2020). Mineral Vesicles and Chemical Gardens from Carbonate-Rich Alkaline Brines of Lake Magadi, Kenya, Crystals, 10, 467.&lt;/p&gt;&lt;p&gt;[4] Garc&amp;#237;a-Ruiz J.M., van Zuilen M.A., Bach W. (2020) Mineral self-organization on a lifeless planet. Phys Life Rev, 34&amp;#8211;35,62&amp;#8211;82&lt;/p&gt;