[1] Piercement structures such as mud volcanoes, hydrothermal vent complexes, pockmarks and kimberlite pipes, form during the release of pressurized fluids. The goal of this work is to predict under which conditions piercement structures form from the insights gained by sand box experiments injecting compressed air through an inlet of width w at the base of a bed of glass beads of height h. At an imposed critical velocity v f , a fluidized zone consisting of a diverging cone-like structure formed with morphological similarities to those observed in nature. Dimensional analysis showed that v f is correlated to the ratio of h over w. In addition, we derived an analytical model for v f which is compared to the experimental data. The model consists of a force balance between the weight and the seepage forces imparted to the bed by the flowing gas. The analytic model reproduces the observed correlation between v f and h/w, although a slight underestimate was obtained. The results suggest that the gas-particle seepage force is the main triggering factor for fluidization and that the commonly used proxy, which the fluid pressure must equal or exceed the lithostatic weight, needs to be reconsidered. By combining the experiments and the model, we derived critical pressure estimates which were employed to a variety of geological environments. Comparing the estimated and measured pressures prior to the Lusi mud volcano shows that the presented model overestimates the critical pressures. The model paves the way for further investigations of the critical conditions for fluidization in Earth systems.

[1] Sedimentary rocks represent vast reservoirs for hydrous and carbonaceous fluids (liquid or gas) that can be generated and released during contact metamorphism following the emplacement of igneous sill intrusions. A massive release of these fluids may impose perturbations in the global climate. In this study we assess the influence of varying host-rock compositions on the magnitude and type of fluids generated from thermal devolatilization, with particular emphasis on carbon and halogens released from heated limestone, coal and rock salt, and the different timescales of metamorphism. In limestones the generated fluids are dominated by H 2 O with limited CH 4 and CO 2 production on a time-scale of 600-3000 years. Cracking of organic matter and CO 2 production (8000-28,000 years) dominates the fluid products from a coal sequence. In the case of evaporites, the presence of reactive organic matter or petroleum results in the generation of CH 4 and CH 3 Cl (260-1000 years). In order to compare the basin scale impacts of the differing host-rocks, two plausible scenarios are explored in which a 100 m thick and 50 000 km 2 large sill is emplaced into 1) organic-rich shale and coal, and 2) limestones and rock salt. The results show the formation of 1) >1600 Gt CH 4 , and 2) >700 Gt of CH 3 Cl, demonstrating that the sill emplacement environment (i.e., the composition of the host rocks) is of major importance for understanding both gas generation in sedimentary basins and the environmental impact of a Large Igneous Province. By evaluation of the isotopic signature of carbonaceous fluids from shales and coals, we show that intrusions into coal-rich sediments are potentially of much less importance for perturbing the atmospheric carbon isotope values than shales.

The geochemistry of hot springs and the availability of oxidants capable of supporting microbial metabolisms are influenced by subsurface processes including the separation of hydrothermal fluids into vapor and liquid phases. Here, we characterized the influence of geochemical variation and oxidant availability on the abundance, composition, and activity of hydrogen (H )-dependent chemoautotrophs along the outflow channels of two-paired hot springs in Yellowstone National Park. The hydrothermal fluid at Roadside East (RSE; 82.4°C, pH 3.0) is acidic due to vapor-phase input while the fluid at Roadside West (RSW; 68.1°C, pH 7.0) is circumneutral due to liquid-phase input. Most chemotrophic communities exhibited net rates of H oxidation, consistent with H support of primary productivity, with one chemotrophic community exhibiting a net rate of H production. Abundant H -oxidizing chemoautotrophs were supported by reduction in oxygen, elemental sulfur, sulfate, and nitrate in RSW and oxygen and ferric iron in RSE; O utilizing hydrogenotrophs increased in abundance down both outflow channels. Sequencing of 16S rRNA transcripts or genes from native sediments and dilution series incubations, respectively, suggests that members of the archaeal orders Sulfolobales, Desulfurococcales, and Thermoproteales are likely responsible for H oxidation in RSE, whereas members of the bacterial order Thermoflexales and the archaeal order Thermoproteales are likely responsible for H oxidation in RSW. These observations suggest that subsurface processes strongly influence spring chemistry and oxidant availability, which in turn select for unique assemblages of H oxidizing microorganisms. Therefore, these data point to the role of oxidant availability in shaping the ecology and evolution of hydrogenotrophic organisms.

Images returned by the spacecraft Clementine have been used to produce a quantitative illumination map of the north pole of the Moon, revealing the percentage of time that points on the surface are illuminated during the lunar day. We have used this map to identify areas that are constantly illuminated during a lunar day in summer and which may therefore be in permanent sunlight. All are located on the northern rim of Peary crater, close to the north pole. Permanently sunlit areas represent prime locations for lunar outpost sites as they have abundant solar energy, are relatively benign thermally (when compared with equatorial regions), and are close to permanently shadowed regions that may contain water ice.

Summary Hydrogen (H2) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H2 concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H2 that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids. Variable vapour input leads to differences in H2 concentration among springs. Analysis of 50 metagenomes from a variety of YNP springs reveals that genes encoding oxidative hydrogenases are enriched in communities inhabiting springs sourced with vapour‐phase gas. Three springs in the Smokejumper (SJ) area of YNP that are sourced with vapour‐phase gas and with the most H2 in YNP were examined to determine the fate of H2. SJ3 had the most H2, the most 16S rRNA gene templates and the greatest abundance of culturable hydrogenotrophic and autotrophic cells of the three springs. Metagenomics and transcriptomics of SJ3 reveal a diverse community comprised of abundant populations expressing genes involved in H2 oxidation and carbon dioxide fixation. These observations suggest a link between geologic processes that generate and source H2 to hot springs and the distribution of organisms that use H2 to generate energy.