Pavel V. Medvedev

Two-billion-year-old evaporites capture Earth’s great oxidation

A strongly oxidizing Paleoproterozoic era Two billion years ago, marine sulfate concentrations were around one-third as high as modern ones, constituting an oxidizing capacity equivalent to more than 20% of that of the modern ocean-atmosphere system. Blättler et al. found this by analyzing a remarkable evaporite succession more than 1 billion years older than the oldest comparable deposit discovered to date. These quantitative results, for a time when only more qualitative information was previously available, provide a constraint on the magnitude and timing of early Earths response to the Great Oxidation Event 2.3 billion years ago. Science, this issue p. 320 The oxidizing capacity of the ocean was one-fifth of modern values in the Paleoproterozoic era. Major changes in atmospheric and ocean chemistry occurred in the Paleoproterozoic era (2.5 to 1.6 billion years ago). Increasing oxidation dramatically changed Earth’s surface, but few quantitative constraints exist on this important transition. This study describes the sedimentology, mineralogy, and geochemistry of a 2-billion-year-old, ~800-meter-thick evaporite succession from the Onega Basin in Russian Karelia. The deposit consists of a basal unit dominated by halite (~100 meters) followed by units dominated by anhydrite-magnesite (~500 meters) and dolomite-magnesite (~200 meters). The evaporite minerals robustly constrain marine sulfate concentrations to at least 10 millimoles per kilogram of water, representing an oxidant reservoir equivalent to more than 20% of the modern ocean-atmosphere oxidizing capacity. These results show that substantial amounts of surface oxidant accumulated during this critical transition in Earth’s oxygenation.

Impact of thermal diffusion and other abundance anomalies on cosmological uses of galaxy clusters

Depending on the topology of the magnetic field and characteristics of turbulent motions, diffusion can significantly affect the distribution of elements, in particular helium, in the intracluster medium (ICM). As has been noted previously, an incorrect assumption about the helium abundance will lead to an error in the iron abundance determined from X-ray spectroscopy. The corresponding effect on the temperature measurement is negligibly small. An incorrectly assumed helium abundance will also lead to a systematic error in angular distance measurements based on X-ray and Sunyaev–Zeldovich (SZ) observations of clusters of galaxies. Its magnitude is further amplified by the associated error in the metal abundance determination, the impact being larger at lower ICM temperatures. Overall, a factor of 2–5 error in the helium abundance will lead to an � 10–25% error in the angular distance. We solve the full set of Burgers equations for a multi-component intracluster plasma to determine the maximal effect of diffusion on the interpretation of X-ray and microwave observations of clusters of galaxies. For an isothermal cluster, gravitational sedimentation can lead to up to a factor of � 5–10 enhancements of helium and metal abundances in the cluster center on a � 3–7 Gyr timescale. In cool-core clusters on the contrary, thermal diffusion can counteract gravitational sedimentation and effectively remove helium and metals from the cluster inner core. In either case, a significant, up to � 40%, error in the metal abundances determined by means of Xray spectroscopy is possible. The angular distance determined from X-ray and SZ data can be underestimated by up to � 10–25%.

Helium diffusion during formation of the first galaxies

We investigate the possible impact of diffusion on the abundance of helium and other primordial elements during formation of the first structures in the early Universe. We consider the primary collapse of a perturbation and subsequent accretion of matter onto the virialized halo, restricting our consideration to halos with masses considerably above the Jeans limit. We find that diffusion in the cold and nearly neutral primordial gas at the end of the Dark Ages could raise the abundance of primordial elements relative to hydrogen in the first virialized halos: helium enrichment could reach

4.3 The Onega Basin

\delta Y_p/Y_p \sim 10^{-4}

On the age of sediments from the Sredni, Rybachy Peninsulars and Kildin Island (the Kola region) in connection with the finding of strata stromatolites

in the first star-forming minihalos of

Application of Raman Spectroscopy and High-Precision Geochemistry for Study of Stromatolites

\sim 10^5-10^6 M_{\odot}

Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–`red beds' association in a global context: a case for the world-wide signal enhanced by a local environment

. A moderate (to ~ 100 K) preheating of the primordial gas at the beginning of cosmic reionization could increase this effect to

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

\delta Y_p/Y_p \sim 3\times 10^{-4}

Petroleum surface oil seeps from a Palaeoproterozoic petrified giant oilfield

for

Palaeoproterozoic magnesite-stromatolite-dolostone-'red bed' association, Russian Karelia: palaeoenvironmental constraints on the 2.0 Ga-positive carbon isotope shift

\sim 10^6 M_{\odot}