Quaternary history of the Lake Magadi Basin, southern Kenya Rift: Tectonic and climatic controls

Owen, R. Bernhart; Renaut, Robin W.; Muiruri, Veronica M.; Rabideaux, Nathan M.; Lowenstein, Tim K.; McNulty, Emma P.; Leet, Kennie; Deocampo, Daniel M.; Luo, Shaohua; Deino, Alan L.; Cohen, Andrew S.; Sier, Mark J.; Campisano, Christopher J.; Shen, Chuan Chou; Billingsley, Anne L.; Mbuthia, Anthony; Stockhecke, Mona

doi:10.1016/j.palaeo.2019.01.017

Cited by 45 publications

(11 citation statements)

References 62 publications

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“…However, when we collected water samples in August, there were still some populations of species such as Potamogeton distinctus, Hydrilla verticillata, Nelumbo nucifera Gaertn, Najas minor, Zizania caduciflora and Phragmitis communis Trin, which could indicate that the water quality was good and that aquatic organisms developed well in Dongping Lake [22,38]. This is consistent with the fact that Fragilaria and Ulnaria prefer areas with abundant light and high transparency [39]. Submerged plants, such as Hydrilla verticillata, Vallisneria spiralis, Potamogeton malaianus, Ceratophyllum demersum and Myriophyllum spicatum, grow seasonally in different areas of the lake, and the transparency and chlorophyll content were significantly higher in these regions.…”

Section: Relationship Between Diatom Community and Environmental Factorssupporting

confidence: 55%

Surface Sediment Diatom Assemblages Response to Water Environment in Dongping Lake, North China

Liu,

Yao,

Chen

et al. 2021

Water

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The relationship between the diatom taxa preserved in surface lake sediments and environmental variables in Dongping Lake was explored using multivariate statistical methods. The statistical analysis showed that the lake was eutrophicated in all seasons. Transparency, chlorophyll a (Chla) and total phosphorus (TP) were the dominant environmental factors in spring and summer, and NH4+-N and chemical oxygen demand (COD) were the dominant environmental factors in autumn and winter. Sixteen genera and 43 species of diatom were found in the surface sediments, and the dominant diatom genera were Aulacoseira, Ulnaria, Cyclotella, Navicula and Fragilaria. A redundancy analysis (RDA) and Monte Carlo permutation 20 test revealed that COD, pH, TP, conductivity and transparency were significant factors influencing diatom assemblage change, meaning that the distribution of the diatom assemblages were mostly influenced by nutrient composition, light intensity and ion concentrations.

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Section: Relationship Between Diatom Community and Environmental Factorssupporting

confidence: 55%

Surface Sediment Diatom Assemblages Response to Water Environment in Dongping Lake, North China

Liu,

Yao,

Chen

et al. 2021

Water

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“…The kerogen matrix can form an effective shield against oxidation, biodegradation, and thermal maturation, thus promoting the preservation of bound compounds over geological time. It has been shown that archaeal lipids can be bound rapidly into macromolecular networks in nonhydrothermal marine sediments (Pancost et al, 2008). We cannot completely exclude, however, that some isoprenoids were released from high-molecular-weight polymers that are not part of the kerogen.…”

Section: Organic Signatures From the Magadi Chertsmentioning

confidence: 88%

“…Today, trona (Na 3 (HCO 3 )(CO 3 ) • H 2 O) is precipitating in large areas of the residual lake (evaporite series; Baker, 1958), and Lake Magadi is the type locality for cherts based on the sodium silicate mineral magadiite (NaSi 7 O 13 (OH) 3 •4(H 2 O); Eugster, 1967Eugster, , 1969Eugster and Jones, 1968;Hay, 1968). Lake Magadi has strongly been influenced by changes in the local climate and tectonics (Owen et al, 2019). Today the Magadi basin represents an evaporation pan with a closed hydrological cycle (i.e., no outflow) that is only recharged by ephemeral runoff and hydrothermal springs (ca.…”

Section: Introductionmentioning

confidence: 99%

Organic signatures in Pleistocene cherts from Lake Magadi (Kenya) – implications for early Earth hydrothermal deposits

et al. 2019

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Abstract. Organic matter in Archean hydrothermal cherts may provide an important archive for molecular traces of the earliest life on Earth. The geobiological interpretation of this archive, however, requires a sound understanding of organic matter preservation and alteration in hydrothermal systems. Here we report on organic matter (including molecular biosignatures) enclosed in hydrothermally influenced cherts of the Pleistocene Lake Magadi (Kenya; High Magadi Beds and Green Beds). The Magadi cherts contain low organic carbon (< 0.4 wt %) that occurs in the form of finely dispersed clots, layers, or encapsulated within microscopic carbonate rhombs. Both extractable (bitumen) and non-extractable organic matter (kerogen) were analyzed. The bitumens contain immature “biolipids” like glycerol mono- and diethers (e.g., archaeol and extended archaeol), fatty acids, and alcohols indicative for, inter alia, thermophilic cyanobacteria, sulfate reducers, and haloarchaea. However, co-occurring “geolipids” such as n-alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs) indicate that a fraction of the bitumen has been thermally altered to early or peak oil window maturity. This more mature fraction likely originated from defunctionalization of dissolved organic matter and/or hydrothermal petroleum formation at places of higher thermal flux. Like the bitumens, the kerogens also show variations in thermal maturities, which can partly be explained by admixture of thermally pre-altered macromolecules. However, findings of archaea-derived isoprenoid moieties (C20 and C25 chains) in kerogen pyrolysates indicate rapid sequestration of some archaeal lipids into kerogen while hydrothermal alteration was active. We posit that such early sequestration may enhance the resistance of molecular biosignatures against in situ hydrothermal and post-depositional alteration. Furthermore, the co-occurrence of organic matter with different thermal maturities in the Lake Magadi cherts suggests that similar findings in Archean hydrothermal deposits could partly reflect original environmental conditions and not exclusively post-depositional overprint or contamination. Our results support the view that kerogen in Archean hydrothermal cherts may contain important information on early life. Our study also highlights the suitability of Lake Magadi as an analog system for hydrothermal chert environments on the Archean Earth.

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“…It is also noteworthy that lacustrine cherts occur bedded with either trona formed in an evaporative environment 3,4 or with dolomite formed in a shallow saline-lake environment 7,8 . Upper Pleistocene lacustrine chert–trona deposits exposed around the highly alkaline Lake Magadi in the East African rift valley 3,9–12 are considered the prime example of the first type, and similar deposits (referred to as Magadi-type cherts) have been reported from several other localities worldwide 4,13–15 . Previous studies concluded that dilution of alkaline brines by fresh-water input could decrease the pH of the lake water and result in precipitation of magadiite (NaSi 7 O 13 (OH) 3 ·3H 2 O), which probably converted initially to kenyaite (NaSi 11 O 20.5 (OH) 4 ·3H 2 O) and eventually to chert (6SiO 2 ·H 2 O) by interaction with percolating waters.…”

Section: Introductionmentioning

confidence: 97%

Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation

Kuma

Hasegawa

Yamamoto

et al. 2019

Sci Rep

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Rhythmically bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Our study of alternating chert–dolomite beds in the Eocene Green River Formation, Utah, USA reveals dense accumulations of organic-matter spheres (30–50 μm diameter) of probable algal cyst origin in the chert layers, and centennial- to millennial-scale periodicities in chert layer deposition. A positive correlation between the degree of degradation of the organic spheres and Si distribution implies decomposition of algal organic matter lead to precipitation of lacustrine chert. As both alkalinity and dissolved silica were likely high in the palaeo-lake waters of the Green River Formation, we hypothesize that decomposition of algal organic matter lowered the pH of sediment pore waters and caused silica precipitation. We propose a formation model in which the initial abundance of algal organic matter in sediment varies with productivity at the lake surface, and the decomposition of this algal matter controls the extent of silica precipitation in sediment. The formation of rhythmically bedded chert–dolomite may be linked to centennial- to millennial-scale climatic/environmental factors that modulate algal productivity, which are possibly tied to solar activity cycles known to have similar periodicities.

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Quaternary history of the Lake Magadi Basin, southern Kenya Rift: Tectonic and climatic controls

Cited by 45 publications

References 62 publications

Surface Sediment Diatom Assemblages Response to Water Environment in Dongping Lake, North China

Surface Sediment Diatom Assemblages Response to Water Environment in Dongping Lake, North China

Organic signatures in Pleistocene cherts from Lake Magadi (Kenya) – implications for early Earth hydrothermal deposits

Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation

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