The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry.

Mayr, M.J., Zimmermann, M., Dey, J., Wehrli, B., Bürgmann, H., 2020. Lake mixing regime selects methane- oxidation kinetics of the methanotroph assemblage. Biogeosciences Discussions 2020, 1-15.

https://www.biogeosciences-discuss.net/bg-2019-482/ In freshwater lakes, large amounts of methane are produced in anoxic sediments. Methane-oxidizing bacteria effectively convert this potent greenhouse gas into biomass and carbon dioxide. These bacteria are present throughout the water column where methane concentrations can range from nanomolar to millimolar concentrations. In this study, we tested the hypothesis that methanotroph assemblages in seasonally stratified lakes are adapted to the contrasting methane concentrations in the epi- and hypolimnion. We further hypothesized that lake overturn would change the methane oxidation kinetics as more methane becomes available in the epilimnion. Together with the change of methane oxidation kinetics, we investigated changes in the transcription of genes encoding methane monooxygenase, the enzyme responsible for the first step of methane oxidation, with metatranscriptomics. We show that the half-saturation constant (K ) for methane, m obtained from laboratory experiments with the natural microbial community, differed by two orders of magnitude between epi- and hypolimnion during stable stratification. During lake overturn, however, the kinetic constants in the epi- and hypolimnion converged along with a change of the transcriptionally active methanotroph assemblage. Conventional particulate methane monooxygenase appeared to be responsible for methane oxidation under different methane concentrations. Our results suggest that methane availability is important for creating niches for methanotroph assemblages with well-adapted methane-oxidation kinetics. This rapid selection and succession of adapted lacustrine methanotroph assemblages allows high methane removal efficiency of more than 90 %to be maintained even under rapidly changing conditions during lake overturn. Consequently, only a small fraction of methane stored in the anoxic hypolimnion is emitted to the atmosphere.

McCaig, A.M., Früh-Green, G.L., Kelemen, P., Teagle, D.A.H., 2020. Serpentinite in the Earth system. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378, 20190332. https://doi.org/10.1098/rsta.2019. 0332

Olivine ((MgFe) SiO ) is the most abundant mineral in the upper mantle, which together with the crust 2 4 comprises the upper 660 km of Earth. It is also a major component of stony meteorites and planetary bodies including Icy Worlds. When olivine comes into contact with water at temperatures less than 500°C, it alters to serpentine (Mg,Fe) Si O (OH) . This hydration reaction is often accompanied by a redox reaction producing 3 2 5 4 3+ 2+ Fe from Fe , and H from water. H is the first building block of life, and hence there is much attention on 2 2 environments where H and organic compounds can be produced abiotically. 2 Serpentine is also important in the Earth system in several other ways: it is a much weaker mineral than olivine and can facilitate slip on faults. Dehydration of serpentine increases pore water pressure and reduces normal