Redox-sensitive trace metals and their isotopes have emerged as important tools that are used to reconstruct the redox-evolution of the ocean-atmosphere system. However, reliability of such reconstructions ultimately depends on a solid understanding of the proxies in the present-day oceanic system and their archival potential in sediments. This study compares isotope fractionation of molybdenum (Mo) and uranium (U) during their removal from seawater and deposition into sediments by investigating sites at various depths of the presently two largest restricted anoxic oceanic basins: The Black Sea and the Cariaco Basin. In support of previous investigations, our data indicate that Mo scavenging and isotope fractionation are mainly controlled by water column sulfide levels. In contrast to Mo, U reduction and immobilization appears to occur mainly at the sediment-water interface and within the uppermost few cm of the sediment pile in both basins. In the Black Sea, 238 238 decreasing δ U of surface sediments with increas ing water depth correlate with trends for water column δ U, implying constant U isotope fractionation between water and sediment. However, increasing U concentrations 238 and δ U within the uppermost few cm of the sediment pile of both basins indicate additional U reduction with depth. Despite the different mechanisms for Mo and U removal and associated isotope fractionations, a similar inverse 98 238 correlation between δ Mo and δ U is observed for sediments of both basins, which translates in a positive correlation of Mo and U isotope fractionation between the sediments and open seawater. The correlation of 98 238 δ Mo and δ U indicates a similar response of isotope fractionation to the efficiency of Mo and U removal that is mainly controlled by sulfate reduction rates. High dissolved sulfide concentrations and sulfate reduction rates are responsible for very effective Mo and U removal and corresponding minor Mo and U isotope fractionation relative to seawater. Further, high dissolved sulfide concentrations also correlate positively with deep water 238 renewal times, resulting in an isotopically fractionated water column with low δ U (and somewhat higher 98 δ Mo) in restricted basins with sluggish ventilations, such as the Black Sea. Both mechanisms result in 98 238 98 238 negatively correlated δ Mo and δ U with high δ Mo and low δ U in sediments under strong euxinic conditions. The particularly strong correlation observed for Cariaco Basin sediments may indicate that its water column was variably stratified in the past. 98 238 The observed δ Mo and δ U correlation of both basins can be reproduced in a simple coupled water column 98 238 and sediment reactive transport model. Different slopes in δ Mo and δ U trends can be linked to varying degree of basin restriction, sulfate reduction rates, and isotope compositions of the respective water columns. 98 238 The offset towards lower δ Mo (and δ U), observed for Cariaco Basin sediments compared to those from the Black Sea, may be the result of inefficient Mo reduction with high Mo isotope fractionation or isotopically light Mo from a particulate Fe-Mn oxide shuttle. The results of this study will help to interpret sedimentary Mo and 98 238 U isotope values, while showing that coupling of δ Mo and δ U in sedimentary archives may be useful for paleo-reconstruction work.

Bryanskaya, A.V., Berezhnoy, A.A., Rozanov, A.S., Serdyukov, D.S., Malup, T.K., Peltek, S.E., 2020. Survival of halophiles of Altai lakes under extreme environmental conditions: implications for the search for Martian life. International Journal of Astrobiology 19, 1-15.

https://doi.org/10.1017/S1473550419000077 Mars is considered to be one of the most favourable places in the Solar System to search for past and present life. In the past Mars was warmer and wetter, so terrestrial halophiles can be regarded as analogues of hypothetical ancient Martian halophiles. In this study we used microorganisms from unique Altai region (Russia) to estimate the capability of terrestrial bacteria and archaea to survive at low temperatures and high concentration of salts and metals, similar to the Martian environment. The current report demonstrates that both halophilic archaea and halotolerant bacteria from saline lakes of the Altai region may be considered as analogues of ancient Martian organisms, since they are able to withstand conditions that hypothetically existed in subsurface layers of the early Mars (low temperatures, salt solutions with a high content of NaCl) with only slight decrease in viability. We also found that the studied microorganisms can use some organic substances found in meteorites. We consider that transfer of unicellular halophiles from Earth to Mars was possible, and, moreover, they could successfully survive and grow on early Mars. Adjusting our growth media to the chemical composition of the lakes, from which the studied strains were isolated, resulted in significant increase in survival and growth rates. Certain strains could survive several freeze -thaw cycles at -70 °C typical for Martian nights.