The radioactive gas radon-222, a fluid and aerosol tracer in Earth's lithosphere and atmosphere, can also reveal subtle rock physics processes in extraterrestrial environments, such as those involving water, but remains poorly constrained in planetary bodies due to the limited number of samples available. Here we measure the effective radium-226 concentration (ECRa) of six Martian and nine lunar meteorites to derive radon source terms for Martian and lunar rocks. ECRa values are 0.029-0.78 and 0.045-0.80 Bq kg-1 for Martian and lunar meteorites, respectively (0.041 +/- 0.003 Bq kg-1 for falls and 0.28 +/- 0.02 Bq kg-1 for finds), lower than most terrestrial rocks but similar to other meteorites and terrestrial primitive basalts. The effect of terrestrial alteration on ECRa and its temperature sensitivity are also determined experimentally. Radon emanation coefficient values are 2.1-17% (mean: 8.1 +/- 2.5%) for Martian meteorites and 0.43-11% (mean: 5.5 +/- 1.0%) for lunar meteorites. Mean estimated surface radon fluxes for Mars and the Moon are 0.16-0.60 and 0.33-0.44 mBq m-2 s-1 (78-280 and 160-210 atoms m-2 s-1), respectively, much lower than on Earth (21 mBq m-2 s-1 or 104 atoms m-2 s-1). Our meteorite analyses constrain radon emanation on Mars and the Moon and provide a basis for current and future in-situ measurements.

Radon-222, a radioactive noble gas with a half-life of 3.8 days produced by radium-226, is a health hazard in caves, but also a powerful tracer of atmospheric dynamics. Here we show how airborne radon-222 can be analysed in a cave with multiple openings, the Pech Merle Cave in South-West France. This two-level cave hosts prehistoric remains and Gravettian paintings in its lower level. Radon concentration, monitored at 15 points with one-hour sampling intervals for more than one year, including two points for more than three years, showed mean values from 1274 +/- 11 to 5281 +/- 20 Bq m-3, with transient values above 15,000 Bq m-3. Seasonal variations were observed, with a weak normal cycle (low in winter) at two points in the upper level and a pronounced inverse seasonal cycle (low in summer) at the other points in the cave. The radon-222 source (effective radium-226 concentration, ECRa) was measured in the laboratory for floor deposits, soil and rock samples. While ECRa values obtained for rocks and speleothems are smaller than 1 Bq kg- 1, most ECRa values for soils are larger than 10 Bq kg- 1. Quantitative modelling confirms that the floor fillings inside the cave are responsible for the stationary lower concentrations, while the higher concentrations observed in winter are explained by percolation of outside air, which collects radon-222 as it passes through the soil layers. In addition, Stored Available Radon (SAR) is sufficient to account for transient variations. While air currents occur when visitors enter the cave or when the cave is deliberately ventilated, the climatic processes revealed by their radon222 signatures appear to be essentially natural. These processes, enhanced by global climate change, could cause or accelerate the deterioration of prehistoric paintings. Radon-222 source analysis using ECRa-based modelling and SAR appears essential for the preservation of underground heritage.

Resistor networks are increasingly being considered in heuristic research as models for natural or artificial matter. The equivalent resistance between two nodes, the Two-Point Resistance (TPR), can be calculated using a variety of methods. The transfer matrix (TM) method was originally considered as a numerical tool for estimating percolation thresholds in random networks with a repeating pattern. The TM method is revisited here as an efficient tool to obtain, in a fast and elegant way, iteration relations and exact explicit expressions for leading TPRs that include a node in the last repeated pattern. Several rotationally invariant networks are studied, such as simple cylindrical networks, spider web networks and cylindrical networks with a central resistive axis, in which case the TM matrices are circulant matrices. Examples of explicit expressions are given for orders of rotation <= 4 or 5, depending on the case. The method can be applied in a similar way to networks with less symmetry, such as grids. The general expressions of TPRs obtained using the TM method can provide quantitative guidelines for resistor networks developed in materials science, environmental issues or industrial applications.

Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important constituents of sedimentary sequences deposited on passive margins through Earth's history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e. impure limestones and dolostones, marls sensu stricto and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate-silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on (i) the main decarbonation reactions, (ii) the P-T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15-40% carbonate is significant (>5.5 +/- 1.0 CO2 wt% and up to 10.5 +/- 1.5 CO2 wt%), whereas for carbonate contents above 60-70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.

Resistor networks, used to model new types of natural or artificial matter, also provide generic examples for practising the methods of physics for obtaining estimates, revealing the main properties of a system and deriving exact expressions. Symmetric bracelet resistor networks are constructed by connecting n identical resistors in a circle, and then connecting two such circles by another set of n identical resistors. First, using van Steenwijk’s method, we establish that the equivalent resistance or two-point resistance (TPR) between any two nodes is derived when the layer-to-layer resistance R 0 n is known. We then determine R 0 n by an elementary recurrence relation which converges rapidly to its large n limit. Using this reference value of R 0 n , accurate estimates of other TPRs follow for all values of n , characterised by a leading 1/ n variation. In addition, exact explicit expressions of the TPRs can be calculated for any value of n . These networks, prototypes of three-dimensional networks considered in research, can be used to illustrate the diversity of the physical approach, the power of elementary methods, and to learn to be comfortable with approximations. Easy to make and use for experimental tests, they can support hands-on activities and conceptual changes.

Besides carbonates, carbonaceous material in meta-sediments represents an important carbon reservoir on Earth. In orogenic belts, carbon can be stored as graphite, a contribution to the global carbon cycle that remains insufficiently constrained. Here, we focus on the Himalayan belt where moderately crystalline graphite is found at the footwall of the Main Central Thrust. Using various analytical methods, we investigate metamorphic conditions of graphite at increasing spatial scales in the Upper Lesser Himalayan Sequence (upper-LHS) phyllites of Nepal. First, we develop and validate a new SEM-BSE technique to measure total organic carbon (TOC) content. Graphite shows mostly inter- and intra-granular flaky crystals associated with carbonates. Elemental carbon analyses yield large range of TOC content in phyllites with a mean of 3.7 +/- 0.4 vol.%. Inferred from micro-Raman spectroscopy, peak metamorphic temperature (peak-T) ranges from 519 +/- 9 to 590 +/- 8 degrees C. We observe a significant coherence of metamorphic conditions of graphitic phyllites at metric, decametric and kilometric scales using extensive data obtained in the Upper Trisuli valley, Central Nepal (n = 77), and, at larger spatial scales, using data from Far-Western to Eastern Nepal (n = 67). Mean carbon isotopic composition (delta 13C) of -26.3 +/- 0.1 parts per thousand indicates biogenic source. Less negative delta 13C values, also reported in other orogenic belts, suggest interaction with pre-existing sedimentary carbonates, probably related to metamorphic isotopic reequilibration. First-order extrapolations at the upper-LHS scale suggest meta-sediment organic carbon may represent up to 20 % of the total amount of carbon stored in orogens. Central Nepal segment, with higher TOC content and peak-T, is also characterized by the largest current metamorphic carbon dioxide (CO2) emissions of the chain. This hardly coincidental association, at the scale of the Himalayan belt, between organic carbon stored as graphite and mobile inorganic carbon released as CO2, represents an important clue to investigate further for global carbon budget estimations.

Convergent plate boundaries are essential components of the global carbon dioxide (CO 2 ) balance of the Earth. Their role of atmospheric CO 2 sink is controlled by silicates weathering. The Nepal Himalaya appears as a natural laboratory for the study of current spatial and temporal variations of CO 2 release at large scale. This chapter presents an up-to-date overview of the known hydrothermal systems in the Nepal Himalaya. It introduces the main hydrothermal sites and presents the associated results separately for thermal springs and gaseous emission zones from Far-Western to Eastern Nepal in the vicinity of the Main Central Thrust and Main Frontal Thrust. The chapter recalls the different measurement techniques used to identify, detects and measures various parameters in thermal spring waters and gaseous emissions.

•Temperature variations induced by atmospheric pressure variations visible in caves.•Frequency dependent transfer functions (TFs) studied in four caves in France.•TFs inferred from 8 × 10-7 to 8 × 10−4 Hz, amplitude at 12 h from 2 to 14 × 10−3 °C/hPa.•Air to rock surface temperature TFs also evaluated at selected locations.•TF interpreted with a model including heat exchange, phase changes and air motion

This chapter presents an overview of the physico-chemical properties of thermal springs and gaseous emission zones in Nepal, with an emphasis on dissolved inorganic carbon and gaseous CO 2 emissions. It discusses the spatial organization and earthquake-induced temporal variations of the deep metamorphic carbon release, and identifies research perspectives for the coming years. A total of 227 thermal springs have been studied so far in Nepal. Among them, about 20 have enough flow rate and temperature to be used efficiently by the population, and remain therefore important pilgrimage sites. In a given region of the MCT zone, such as the upper Trisuli valley, sites separated by 5–10 km show similar characteristics of the crustal fluid release. This suggests that hydrology and topography locally are not the predominant controlling factors of the small-scale spatial heterogeneity of the CO 2 release.

The inert radioactive gas radon-222, ubiquitous in the environment, is the first cause of lung cancer in non-smokers and a powerful asset to trace geological fluids and constrain atmospheric transport models. In all of these applications, radon flux (RF) from the ground needs to be estimated. However, obtaining a large-scale representative estimate of RF in continental land is a challenging task and a starting basis is to focus on large countries in the world. Here, we collected a total of 2622 direct RF measurements carried out in continental China from 1988 to 2021 using 69 publications in a 494-document pool. Over the whole dataset, the RF results were classified into geographical provinces, time periods and substratum types. Mean RF values yield 180±32, 36.9±4.8, 31.1±1.0, 66.1±1.6 and 1510±100 mBq m−2 s−1 in metamorphic and igneous (n=71), sedimentary (n=24), Quaternary (n=585), faulted areas (n=832) and uranium-related sites (n=898), respectively. Considering geology only, mean RF for China (66±11 mBq m−2 s−1) is larger than most large-scale estimates worldwide. To account for the distribution of soil thickness, RF values were extrapolated to representative soil thickness values per lithology using a scaling law inferred from natural sites. The inferred corrected mean RF for China yields 20.2±6.1 mBq m−2 s−1, corresponding to a total radon-222 emission of (191±43)×109 Bq s−1, and appears compatible with available large-scale estimates in Europe or at global scale. Potential sources of systematic bias and variability, such as vegetation cover, meteorological and seasonal effects, remain. The contribution of urban areas, estimated for the first time, was found significant. Our study suggests future research directions to better constrain the RF source to the atmosphere in China, and, more generally, to extract robust global-scale representative RF values from direct RF measurements.

The outstanding preservation of Paleolithic decorated caves is related to the buffering properties of their karstic environment. However, long-term monitoring of air/wall temperatures and gas compositions has recently revealed disruption signs in cave microclimates that had been maintained stable for hundreds of centuries. High precision and continuous temperature data records are currently monitored in various prehistoric caves in the South of France. Such operations have been promoted since the late 1990s by the French government for risk assessment and conservation. The most striking feature is the positive drift of underground temperatures (air and wall) which is now obvious in most sites except for Niaux Cave (> 300 m undersurface) and in the deepest parts of Mas-d’Azil and Chauvet Caves (> 50 m undersurface). In tourist caves (Pech-Merle, Mas-d’Azil, Gargas, Villars), the positive thermal trends could not be related to the energy increase brought by visitors which number is now stable, nor to the lighting systems whose energy demand was strongly reduced. In addition, the underground thermal drift nearly starts at the same time in many caves with uncertainties of +/- 1 year: 2012 for Chauvet with +0.4 °C/decade, 2011 for Pech Merle with +0.32 °C/decade, 2011 for Marsoulas Cave with +1.09 to +0.36 °C/decade from the entrance to the deep gallery, 2011 for Gargas with +0.69 °C, +0.54 °C and +0.36 °C for the deeper station. It is worth noting that a 0.3-0.4 °C thermal drift is consistent with that predicted from global warming in these regions. The thermal drifts were already in progress when monitoring began in Villars Cave in 1996 (+0.17 °C to +0.39 °C/decade), in Mas-d’Azil in 2012 and in Bruniquel in 2015. Marsoulas (+1.09 °C/decade) and Mas-d’Azil (more than +1 °C/decade in 6 of the 16 stations) present a much higher drift rate compared with that of surface, which suggests a thermal amplification process. As measurements are performed in heterothermal zones, the long-term thermal drifts are modulated by persisting smoothed and out-of-phase yearly variations. A notable exception is the case of Bruniquel main gallery where the temperature records show a quasi-linear increase. In that case, the decadal evolutions of temperature +0.31 °C, +0.175 °C, and +0.24 °C, are not related to the depth of monitoring stations (32 m, 55 m, and 38 m, respectively) nor to their distances from the entrance. In 2018, those drift rates induced a permanent inversion of thermal gradient in the main gallery. In Gargas, the drift rate is more pronounced in the outer parts of the karst body, thus inducing a continuous evolution of the thermal gradients within the galleries. Such underground microclimate disruption of patrimonial caves is a warning signal of direct threat on the preservation of remains. Karst physical organization and its related underground environment are themselves legacies of past climates; the current functioning of transfer zones of karst aquifers which includes the caves, are directly dependent on the outside climate. A more comprehensive approach and modelling of possible tipping points are urgently needed for conservation issues.

SUMMARY The Mw 7.9 2015 April 25 Gorkha earthquake is the latest of a millenary-long series of large devastating Himalayan earthquakes. It is also the first time a large Himalayan earthquake and its aftershocks were recorded by a local network of seismic stations. In the 5 yr following the main shock, more than 31 000 aftershocks were located by this permanent network within the ruptured area, including 14 362 events with ML greater than 2.5, 7 events with ML > 6, including one large aftershock with Mw 7.2 on 2015 May 12. In 2020, 5 yr after the main shock, the seismicity rate along the ruptured fault segments was still about 5 times higher than the background seismicity before the Gorkha earthquake. Several bursts of earthquakes, sometimes organized in clusters, have been observed from a few days to several years after the main shock. Some of these clusters were located at the same place as the clusters that happened during the decades of interseismic stress build-up that preceded the large earthquake. They also happened in the vicinity of the high frequency seismic bursts that occurred during the main shock. These heterogeneities contribute to a persistent segmentation of the seismicity along strike, possibly controlled by geological structural complexities of the Main Himalayan Thrust fault. We suggest that these pre-2015 clusters revealed the seismo-geological segmentation that influences both the coseismic rupture and the post-seismic relaxation.

In underground cavities, temperature variations of the order of 10−3 °C are permanently induced by the variations of atmospheric pressure, even at great depths, with couplings of the order of 0.2 to 20 × 10−3 °C/hPa depending on frequency. In the first part of this study, we established the atmospheric pressure to temperature transfer function (TF) as a function of frequency from 8 × 10−7 to 8 × 10−4 Hz. Here, we use this TF to calculate the expected PIT variations, which, after being subtracted from the observed time-series, provide residual temperature time-series. We calculated such temperature residuals in four natural caves in France: Esparros, Aven d'Orgnac, Pech Merle and Chauvet-Pont d'Arc Caves, the last two containing unique prehistoric wall paintings. Temperature signals, as small as a few 10−3 °C, due to human presence, are then conspicuous, with evidence of relaxation longer than several days and long-term cumulative effects. In addition, we observe temperature signals suggesting non-stationary states characterized by several processes which are not necessarily easy to separate, such as transient air currents, due to barometric winds or locally semi-confined convection cells, transient infiltration, or energy dissipation by evaporation-condensation at the rock surface. This background thermal agitation displays a scale-free amplitude spectrum, from 2 × 10−5 to 4 × 10−4 Hz, of the form f−α, with α varying from 0.1 to 0.6 depending on the site. Furthermore, at the Chauvet Cave, a weak but unambiguous peak emerges during some months at a period of about 82.2 ± 0.8 minutes, suggesting a Helmholtz-type resonance. Small but significant temperature signals are therefore detected in underground cavities once the effect of atmospheric pressure variations is corrected for. These signals reveal subtle coupled processes whose knowledge is essential to evaluate preservation strategies and to establish conditions for resilience of underground systems under artificial or natural influence including climate change.

The rise of novel materials such as graphene compounds or carbon nanotubes recently revived the interest on the role of symmetries upon electrical properties. Among possible solids, symmetrical polytopes, widely realized in natural and artificial matter, deserve particular attention. When equal resistors are attached between neighbouring nodes of Perfect (Platonic) solids (PS) along the edges, the two-point resistance between any pair of nodes was established in the 1990s by van Steenwijk using an elegant and efficient method. Using van Steenwijk's method, Moddy and Aravind subsequently derived the resistance values for two Archimedean and two Catalan solids. Here, with the same method, we derive the exact expression for the two-point resistance between any two nodes for resistor networks made of equal resistors placed along the edges of the thirteen Archimedean solids (AS). While the calculation remains elementary for the simplest AS, a dedicated method using computer assistance was developed for the Snub Cube, the Snub Dodecahedron and the three AS with three different rotation symmetries (the Great Rhombicuboctahedron, the Small Rhombicosidodecahedron and the Great Rhombicosidodecahedron). The largest resistance value (191/90) is obtained between the two opposite nodes of the Truncated Dodecahedron, and the second largest (42815/21114) is obtained between the two opposite nodes of the Great Rhombicosidodecahedron. The smallest resistance value (14137/38016) is obtained between some of the neighbouring nodes of the Snub Cube, whereas the resistance between neighbouring nodes of the Icosahedron (11/30) is slightly smaller. Some general symmetry relations between two-point resistances in AS networks are also derived, as well as some relations with two-point resistances in PS networks. The complete set of exact two-point resistance values for PS and AS networks can be used to check numerical codes or to evaluate the capacity of regular solids to provide appropriate models for given experimental situations.

Substantial terrestrial gas emissions, such as carbon dioxide (CO 2 ), are associated with active volcanoes and hydrothermal systems. However, while fundamental for the prediction of future activity, it remains difficult so far to determine the depth of the gas sources. Here we show how the combined measurement of CO 2 and radon-222 fluxes at the surface constrains the depth of degassing at two hydrothermal systems in geodynamically active contexts: Furnas Lake Fumarolic Field (FLFF, Azores, Portugal) with mantellic and volcano-magmatic CO 2 , and Syabru-Bensi Hydrothermal System (SBHS, Central Nepal) with metamorphic CO 2 . At both sites, radon fluxes reach exceptionally high values (> 10 Bq m −2 s −1 ) systematically associated with large CO 2 fluxes (> 10 kg m −2 day −1 ). The significant radon‒CO 2 fluxes correlation is well reproduced by an advective–diffusive model of radon transport, constrained by a thorough characterisation of radon sources. Estimates of degassing depth, 2580 ± 180 m at FLFF and 380 ± 20 m at SBHS, are compatible with known structures of both systems. Our approach demonstrates that radon‒CO 2 coupling is a powerful tool to ascertain gas sources and monitor active sites. The exceptionally high radon discharge from FLFF during quiescence (≈ 9 GBq day −1 ) suggests significant radon output from volcanoes worldwide, potentially affecting atmosphere ionisation and climate.

Radon-222 (222Rn) is a well-known tracer of atmospheric, environmental and geological processes. In a recent reviews of radon-222 flux (RF) from ground surface at continental scale, or in recent observations of RF in association with earthquakes, the question of the influence of vegetation cover emerges repeatedly. In this study, a total of 58 RF flux (RF) measurement were performed from ground surface in September 2021 at the Sapine drainage in the Mont Lozère (French Central Massif). The micro-observations site was located at the south slope of the granitic context between a forest and pasture. No significant difference was observed between the RF in pasture (225±63 mBq m-2 s-1) and forest (247±80 mBq m-2 s-1). These results are compared with other recent RF results obtained in granitic areas in France, and to experimental evidence on radium-226 distribution obtained in soils and in vegetation. Other systematic effects on RF, such as soil humidity, soil pH or soil temperature, and their potential consequences on transport processes are discussed, as well as their impact on various problems in geosciences.

&lt;p&gt;Radon-222 (&lt;sup&gt;222&lt;/sup&gt;Rn) is a well-known tracer of atmospheric, environmental and geological processes. In a recent reviews of radon-222 flux (RF) from ground surface at continental scale, or in recent observations of RF in association with earthquakes, the question of the influence of vegetation cover emerges repeatedly. In this study, a total of 58 RF flux (RF) measurement were performed from ground surface in September 2021 at the Sapine drainage in the Mont Loz&amp;#232;re (French Central Massif). The micro-observations site was located at the south slope of the granitic context between a forest and pasture. No significant difference was observed between the RF in pasture (225&amp;#177;63 mBq m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;) and forest (247&amp;#177;80 mBq m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;). These results are compared with other recent RF results obtained in granitic areas in France, and to experimental evidence on radium-226 distribution obtained in soils and in vegetation. Other systematic effects on RF, such as soil humidity, soil pH or soil temperature, and their potential consequences on transport processes are discussed, as well as their impact on various problems in geosciences.&lt;/p&gt;

The Nepal Himalayas result from the India-Eurasia collision and the actual shortening is accommodated by a detachment ramp, the Main Himalayan Thrust (MHT). Separating high-grade metamorphic rocks from the Greater Himalayan Sequence to the north and low-grade metamorphic rocks from the Lesser Himalayan Sequence to the south, the Main Central Thrust (MCT) shear zone, is related to the MHT at depth where large Himalayan earthquakes nucleate. The MCT zone occurs from Far-Western to Eastern Nepal, associated at mid-crustal depth with active seismicity and high electrical conductivity; it exhibits carbon-rich rock layers and numerous active hydrothermal systems. Here, based on a multidisciplinary approach that includes geology, geochemistry and geophysics, we study the various sulphur and carbon signatures in the MCT zone in the Nepal Himalayas. First, we characterise the upper LHS rocks that include alternation of graphite-rich mica-schists (the so-called “black schists”) and carbonates (mainly siliceous dolomite). In the laboratory, we determine organic and inorganic carbon contents, as well as complex electrical conductivity. Second, we concentrate on numerous thermal springs in which we measure dissolved carbon and sulphur concentrations and their isotopic compositions (δ13C and δ34S). Third, we study the surface gaseous emissions, directly observed in the vicinity of hot springs, with the measurements of carbon dioxide (CO2) and hydrogen sulphide (H2S) fluxes and isotopic compositions. By comparing the signatures of carbon and sulphur sequestration and carbon and sulphur release at a large spatial scale, our work provides insights into the carbon source-to-sink duality of large orogens, the metamorphic processes and the carbon and sulphur geochemical cycles.