The prediction of mineral solubilities in natural waters: the NaKMgCaClSO4H2O system from zero to high concentration at 25° C

Sea salt aerosol (SSA) exerts a major influence over a broad reach of geophysics. It is important to the physics and chemistry of the marine atmosphere and to marine geochemistry and biogeochemistry generally. It affects visibility, remote sensing, atmospheric chemistry, and air quality. Sea salt aerosol particles interact with other atmospheric gaseous and aerosol constituents by acting as sinks for condensable gases and suppressing new particle formation, thus influencing the size distribution of these other aerosols and more broadly influencing the geochemical cycles of substances with which they interact. As the key aerosol constituent over much of Earth's surface at present, and all the more so in pre-industrial times, SSA is central to description of Earth's aerosol burden.

Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models - A Critical Review

The transition from the Eocene to the Oligocene epoch was the most significant event in Earth history since the extinction of dinosaurs. As the first Antarctic ice sheets appeared, major extinctions and faunal turnovers took place in the land and in the sea, eliminating forms adapted to a tropical world and replacing them with the ancestors of most of our modern animal and plant life. Through a detailed study of climatic conditions and of organisms buried in Eocene-Oligocene sediments, this volume shows that the separation of Antarctica from Australia was a critical factor in changing oceanic circulation and ultimately world climate. In this book, contributors examine the full range of Eocene and Oligocene phenomena. Their articles cover nearly every major group of organisms in the ocean and on land and include evidence from palaeontology, stable isotopes, sedimentology, seismology and computer climatic modelling. The volume concludes with an update of the geochronological framework of the late Palaeogenic period.

Eocene-Oligocene Climatic and Biotic Evolution

To study the process of microbial-mediated dolomite formation, growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in Lagoa Vermelha, a hypersaline lagoon in Brazil where dolomite precipitation occurs. Specifically, we report the isolation of a particular strain of sulfate-reducing bacteria, LVform6, from Lagoa Vermelha sediment, which apparently promotes the formation of nonstoichiometric dolomite. Sulfate-reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 °C. The precipitates have morphologies similar to those observed in Lagoa Vermelha sediment. Our results demonstrate that sulfate-reducing bacteria can influence dolomite precipitation under controlled lowtemperature, anoxic conditions, and imply that anaerobic microorganisms can play an important role in carbonate sedimentation. They may have been particularly significant in Earth's earliest history when a more reducing atmosphere existed.

Bacterially induced dolomite precipitation in anoxic culture experiments

Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits

The High Magadi beds can be divided into a lower and an upper sequence, separated by a bed rich in Tilapia nilotica remains. The lower sequence contains one or more magadiite horizons. Through digging of trenches it has been possible to establish that the cherts of the “Chert Series” are stratigraphically equivalent to the magadiite horizons. The transition magadiite-chert has been observed in many places and may occur over a distance of less than 50 cm. It is accomplished by removal of sodium by percolating waters. The resulting bedded chert may preserve the fine lamination present in magadiite. The conversion is accompanied by many textural and structural features such as large desiccation polygons, buckling, reticulation, extrusion, casts of mud-cracks and calcite. The precipitation of magadiite is postulated to have taken place in annual increments in a stratified lake at the brine-epilimnion interface. The maximum duration is estimated at 4,000–6,000 years. Solubility measurements indicate that most Magadi waters at the present time are supersaturated with respect to magadiite and kenyaite, though no precipitation has been observed. The presence of chert chips in intraformational gravels indicates that the conversion to chert was already initiated during the magadiite precipitation interval. The inorganic mechanism for the formation of bedded chert observed at Magadi is suggested to apply also to the Dales Gorge Member of the Precambrian Brockman Iron Formation of W. Australia. Evidence for magadiite as chert precursor is found in the depositional rates, the presence of riebeckite and in textures such as macules and cross-pods. The proposed model is extended to include Precambrian banded iron formations in general and the restriction of these formations to the early history of the earth is explained by the likely compositions of waters from closed basins.

Inorganic bedded cherts from the Magadi area, Kenya

Van't Hoff's predictions of the mineral sequences to be expected on the evaporation of seawater are based on an oversimplified system. New computer calculation that include calcium-bearing phases are in better agreement with natural occurrences, removing discrepancies that have plagued evaporite geologists for nearly a century. A new hydrologic model that combines equilibrium batch evaporation with fractionation between successive batches is proposed to account for the mineral sequences observed in such classic deposits as the German Zechstein.

https://science.sciencemag.org/content/sci/208/4443/498.full.pdf

Evaporation of seawater: calculated mineral sequences.

Rivers, springs and brines of the Lake Magadi, Kenya, basin in the Eastern Rift Valley have been analyzed for Na, K, Si02, Cl, COa, HCOa, SO" F, Br and B. Na/CI ratios are remarkably constant for all waters, except the most concentrated brines, and this indicates that the river waters may be the recharge for a hot ground water reservoir from which the springs are fed. The concentration factor from rivers to springs is 1 to 1000 and is probably achieved through evaporative concentration throughout the basin floor. During this process most of the total carbonate and potassium is lost, the former mainly by precipitation of alkaline earth carbonates, the latter probably through ion exchange. Fluoride and sulfate show straight evaporative concentration, because saturation in fluorite and gypsum does not occur in this calcium-poor environment. The hot springs (up to 86°C) discharge saline alkaline waters into the lake, and their compositions and temperatures have remained constant for over 50 years. Individual springs differ greatly in concentration, temperature and carbonatebicarbonate ratio. Good correlations are obtained by assuming the mixing of three water types: (1) dilute, cold, bicarbonate-rich river water, (2) saline, hot bicarbonate-rich ground water, (3) saline, cold, carbonate-chloride enriched surface brine. Mixing ratios differ greatly from spring to spring. The well-mixed ground water reservoir may be located at a depth of as much as 2 km. The surface brine seems to enter the hydrologic paths from near the south end of the lake. Springs feed directly into lagoons, which act as preconcentration basins for the lake brines. Crystallization of trona (Na,COa·NaHCOa·2H20) leads to bicarbonate depletion and chloride enrichment in the brines. Saturation also occurs with respect to villiaumite (NaF), mirabilite (Na2SO.·10 H20) and thermonatrite (Na2COa·H20). Inflow and recirculation estimates lead to a time span of at least 6000 years for the accumulation of the trona now present in the lake. Yearly net accumulation amounts to about 0.3 em trona. The salient hydrological features of Magadi are generalized into a model salt lake. This lake forms in a tectonically active area in an arid to semiarid climate. Large vertical relief insures the presence of perennial rivers, but these rivers disappear underground before they reach the lake. The lake itself is fed by groups of springs, which derive from a ground water reservoir whose salinity and temperature depends upon the circulation pattern. Lake Magadi is pictured as a typical salt lake at the peak of its productivity.

Chemistry and origin of the brines of Lake Magadi, Kenya

Analysis of sodium carbonate-bicarbonate brines from closed basins in volcanic terranes of Oregon and Kenya reveals silica contents of up to 2700 parts per million at p H's higher than 10. These high concentrations of SiO2 can be attributed to reaction of waters with silicates, and subsequent evaporative concentration accompanied by a rise in p H. Supersaturation with respect to amorphous silica may occur and persist for brines that are out of contact with silicate muds and undersaturated with respect to trona; correlation of SiO2 with concentration of Na and total CO2 support this interpretation. Addition of more-dilute waters to alkaline brines may lower the p H and cause inorganic precipitation of substantial amounts of silica.

Hans P. Eugster

Papers

Inorganic bedded cherts from the Magadi area, Kenya

Evaporation of seawater: calculated mineral sequences.

Chemistry and origin of the brines of Lake Magadi, Kenya

Silica in alkaline brines.

Origin and deposition of trona