Volume 3, Number 2

Abstract:
Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (δ⁷Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive δ⁷Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO₂ degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO₂ drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.

Abstract:
The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans, due to the oxygen demand of the evolving animal ecosystems. However, how early animals may have regulated Earth’s surface oxygen budget via self-stabilising feedbacks is poorly understood. Here, we report parallel positive uranium, carbon, and sulphur isotope excursions from carbonate successions in Siberia that document a brief global oxygenation episode 521–520 Myr ago, at the onset of diversification of larger arthropods known from the fossil record. Our data and model indicate that an abrupt increase in the sinking rate of marine organic matter expanded the oxygenated zone in the oceans and that reducing conditions returned 1.3 ± 0.8 Myr after the onset of this transient oxygenation episode, necessitating a strong negative feedback to the increasing levels of oxygen. We speculate that larger zooplankton could have sourced both oxygen and food to the seafloor, fueling bioturbation over wider areas and, thereby, stabilising O₂-rich habitats in the oceans. Thus, this reorganisation exemplifies how animal ecosystems might have influenced oxygen availability in Earth's surface environment soon after their establishment.

Abstract:
Mixing and juxtaposition of chemically distinct magmatic systems are key processes for the evolution of Earth’s crust. Yet, the physicochemical nature at mixing interfaces remains poorly described, as crystallisation, melting, heat transfer, and diffusion are interconnected and lead to complex mass transfer processes driving unique patterns of element fractionation. Here, we use diffusion couple experiments between felsic and mafic magmas (melt + crystals ± volatiles) to document the formation of large gradients in oxygen fugacity at the magma-magma mixing interface. Reducing and oxidising boundary layers at the interface develop rapidly and remain in dynamic disequilibrium for days to possibly weeks. We suggest that the observed transient redox gradient is caused by cation transfer across the interface where the required counter flux of electron holes is insufficient to compensate an evolving electron hole gradient. Such boundary layer redox effects may control fractionation of polyvalent and chalcophile elements and moderate, for example, Cu/Au ratios in arc-related porphyry ore deposits.

Abstract:
The isotopic similarity of enstatite chondrites and Earth has often been cited as evidence that the Earth is made of enstatite chondrite-like material. Here we show, however, that enstatite chondrites exhibit nucleosynthetic molybdenum (Mo) isotope anomalies and, therefore, cannot represent the sole building blocks of the Earth. Enstatite and ordinary chondrites together with the Earth’s mantle plot on a cosmic Mo-Nd isotope correlation line that reflects varying proportions of s-process matter in these samples. This correlation indicates that the nucleosynthetic makeup of Earth’s building material did not change over time and that Earth, on average, accreted from bodies that originated closer to the Sun and were enriched in s-process matter compared to known chondrites. As such, any contribution of chondrites to Earth’s accreting material must be compensated by the addition of s-process enriched bodies. This material is not present in our meteorite collections, but may have been sampled by Venus or Mercury. The s-process enriched nature of the Earth can fully account for its higher ¹⁴²Nd compared to chondrites, which therefore does not require an early differentiation of Earth’s mantle.

Abstract:
The isotopic and elemental compositions of noble gases constitute a powerful tool to study volatile origin and evolution, due to their inertness, and can thus provide crucial information about the early stage of planetary formation. Two models are proposed to explain the light noble gas origin on Earth: the solar wind implantation model and the solar nebula gas dissolution model. However, noble gas measurements often show addition of air to the mantle-derived gas, which complicates the determination of mantle isotopic ratios. We analysed the noble gas isotopic compositions of single vesicles in samples from the Galápagos hotspot with laser ablation, in order to understand and remove this atmospheric component, as well as discriminate between the two scenarios. Based on the new high precision results and a new statistical approach, we show that the solar wind implantation model is more likely to explain the terrestrial He, Ne and Ar composition. This scenario could bring important constraints on the solar system environment during the early stage of planetary formation.

Abstract:
Reply to Comment on “A cometary origin for martian atmospheric methane” by Crismani et al., 2017
First, I would like to extend thanks to Crismani et al.. for their commentary, which highlights an important uncertainty acknowledged in Fries et al.. (2016), namely whether the total mass of infall material required to produce the observed methane is on par with that available from meteor showers. I would like to disagree on two points presented in the letter, and then discuss the implications of their findings.

Abstract:
Weathering in carbonate rocks is often thought to be governed by chemical dissolution. However, recent studies have shown that mechanical detachment of tiny grains contributes significantly to the overall surface retreat. Whether this detachment is caused by shear forces acting at the surface, or repulsive forces acting between grains, was not known. In this study, we used atomic force microscopy to examine the mechanism of grain detachment and we demonstrate that it occurs even in the absence of shearing fluid flow. This suggests that the removal of grains from rock surfaces can be caused by repulsive forces between calcite grains. Although these repulsive forces are expected to be sensitive to the ionic strength of the solution, we did not find enough evidence to demonstrate a correlation between salinity and the frequency of grain detachment. Importantly, our findings suggest that grain detachment occurs during water-rock interaction under low flow conditions over a range of salinities, with potential impacts on geological carbon sequestration and enhanced oil recovery in carbonate formations.

Abstract:
Recent detection of indigenous hydrogen in a diversity of lunar materials, including volcanic glass (Saal et al., 2008), melt inclusions (Hauri et al., 2011), apatite (Boyce et al., 2010; McCubbin et al., 2010), and plagioclase (Hui et al., 2013) suggests water played a role in the chemical differentiation of the Moon. Water contents measured in plagioclase feldspar, a dominant mineral in the ancient crustal lunar highlands have been used to predict that 320 ppm water initially existed in the lunar magma ocean (Hui et al., 2013) whereas measurements in apatite, found as a minor mineral in lunar rocks, representing younger potassium-enriched melt predict a bulk Moon with <100 ppm water. Here we show that water in alkali feldspar, a common mineral in potassium-enriched rocks, can have ~20 ppm water, which implies magmatic water contents of ~1 wt. % in chemically evolved rhyolitic magmas. The source for these wet, potassium-rich magmas probably contained ~1000 ppm H₂O. Thus, lunar granites with ages from 4.3–3.9 Ga (Meyer et al., 1996) likely crystallised from relatively wet melts that degassed upon crystallisation. Geochemical surveys by the Lunar Prospector (Jolliff et al., 2011) and Diviner Lunar Radiometer Experiment (Glotch et al., 2010; Jolliff et al., 2011) indicating the global significance of evolved igneous rocks suggest that the formation of these granites removed water from some mantle source regions, helping to explain the existence of mare basalts with <10 ppm water, but must have left regions of the interior relatively wet as seen by the water content in volcanic glass and melt inclusions. Although these early-formed evolved melts were water-rich, their petrogenesis supports the conclusion that the Moon’s mantle had <100 ppm water for most of its history.