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The history of the growth of continental crust is uncertain, and several different models that involve a gradual, decelerating, or stepwise process have been proposed.sup.1-4. Even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea (subaerial landmasses), with estimates ranging from about one billion to three billion years ago.sup.5-7. The area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans.sup.8, and therefore connects Earth's crustal evolution to surface environmental conditions.sup.9-11. Here we use the triple-oxygen-isotope composition of shales from all continents, spanning 3.7 billion years, to provide constraints on the emergence of continents over time. Our measurements show a stepwise total decrease of 0.08 per mille in the average triple-oxygen-isotope value of shales across the Archaean-Proterozoic boundary. We suggest that our data are best explained by a shift in the nature of water-rock interactions, from near-coastal in the Archaean era to predominantly continental in the Proterozoic, accompanied by a decrease in average surface temperatures. We propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near-modern average elevation and aerial extent roughly 2.5 billion years ago.The use of triple-oxygen-isotope data from continental shales spanning the past 3.7 billion years suggests that continental crust with near-modern average elevation and extent emerged about 2.5 billion years ago., Author(s): I. N. Bindeman [sup.1] , D. O. Zakharov [sup.1] , J. Palandri [sup.1] , N. D. Greber [sup.2] , N. Dauphas [sup.3] , G. J. Retallack [sup.1] , A. [...]

The results of an experimental study of the processes of ion exchange sorption and desorption of copper and zinc ions on a composite sorbent are presented. The sorbent is obtained from modified sawdust and chitosan at a molar ratio of 1 : 0.4. The average particle size is 1.6 mm. The sorbent has a macroporous structure and contains carboxyl, phenolic hydroxyl groups and amino groups capable of ion exchange with heavy metal ions from solutions. The experimental study is а passing the initial solution through a stationary annular sorbent layer and reading the output curves of ion exchange. The sorbent with a volume of 9.8×10–4 m3 was placed in an annular adsorber between the inner and outer gratings. In the study of the direct ion exchange process, solutions of copper and zinc sulfates with a concentration of 0.05 N with volume velocity 8.1×10–6 m3/s and a sorbent in Na- form were used. Sorbent was regenerated from heavy metal ions with 0.08 N sodium hydroxide solution with volume velocity 3.2×10–6 m3/s. The full exchange capacities of the sorbent, the degree of regeneration of the sorbent and the specific consumption of the regeneration solution were calculated based on the output curves of ion exchange. The analysis of experimental data showed a decrease in the exchange capacity of the sorbent during its repeated use. After five sorption-desorption cycles, the value of the total dynamic exchange capacity of the sorbent decreased for copper ions by 30% and for zinc ions by 25%. The decrease in the exchange capacity is associated with a decrease in the amount of carboxyl and phenolic hydroxyl groups in the sorbent, as well as with the compaction of its structure and, consequently, a decrease in the availability of functional groups. [ABSTRACT FROM AUTHOR]

Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth's history remain poorly constrained. Here we show how the climate-sensitive δ18O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ18O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ18O values as low as -8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ18O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ17O approach, these rocks reveal that the precipitation had a δ18O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes. [ABSTRACT FROM AUTHOR]