Toward the snowball earth deglaciation…

The current state of knowledge suggests that the Neoproterozoic snowball Earth is far from deglaciation even at 0.2 bars of CO₂. Since understanding the termination of the fully ice-covered state is essential to sustain, or not, the snowball Earth theory, we used an Atmospheric General Climate Model (AGCM) to explore some key factors which could induce deglaciation. After testing the models’ sensitivity to their parameterizations of clouds, CO₂ and snow, we investigated the warming effect caused by a dusty surface, associated with ash release during a mega-volcanic eruption. We found that the snow aging process, its dirtiness and the ash deposition on the snow-free ice are key factors for deglaciation. Our modelling study suggests that, under a CO₂ enriched atmosphere, a dusty snowball Earth could reach the deglaciation threshold.     

Kinetics of pyrite to pyrrhotite reduction by hydrogen in calcite buffered solutions between 90 and 180 °C: Implications for nuclear waste disposal

The kinetics of abiotic redox reactions induced by hydrogen are poorly documented although it represents a growing area of interest in terms of both nuclear waste storage assessment and the comprehensive study of hydrogen-rich fluid in mid-ocean ridge hydrothermal systems. We present an experimental kinetics study of pyrite reduction into pyrrhotite under significant H₂ pressure and mid-hydrothermal conditions. We describe the mechanism and kinetic behavior of this reaction by combining textural and solution analyses under various conditions of temperature, pyrite particles size, H₂ pressure and pH. When pH is controlled by calcite, the reaction presents all the characteristics of a coupled dissolution-precipitation mechanism occurring at the pyrite-pyrrhotite interface. By considering the chemical affinity of the coupled reaction as a function of reaction extent, we demonstrate that the spatial coupling is induced both by pyrite as a substrate for pyrrhotite nucleation and by the role of fluid chemistry at the reaction front. Far from equilibrium with respect to pyrite, the kinetics of sulfide production associated with the reaction are linearly related to the square root of time with an activation energy of 53 kJ/mol. This value is higher than what is expected for a diffusion-controlled kinetic regime. We suggest that the reaction rate is controlled both by pyrite reductive dissolution and by sulfide diffusion through the porous pyrrhotite microstructure. We provide a simple sulfide production-rate expression on the basis of our measured rate constants that can be used in geochemical modeling to further evaluate the impact of hydrogen on pyrite under nuclear waste disposal conditions.     

Stratigraphic response to spatiotemporally varying tectonic forcing in rifted continental basin: Insight from a coupled tectonic‐stratigraphic numerical model

The role of spatiotemporally varying tectonic forcing in the development of stratigraphic patterns along passive margins and continental rift basins has been recognized for decades, but the exact nature of the stratigraphic response is still debated. This study develops a coupled tectonic‐stratigraphic numerical model with a fixed absolute lake level and constant climate conditions to quantify the signatures of spatiotemporally varying tectonic forcing on the stratigraphic record. This model consists of a three‐dimensional rift basin with a range of geomorphic features and produces a number of well‐recognized stratigraphic patterns, which are commonly interpreted to be caused by lake‐/sea‐level or climate fluctuations. This study demonstrates that the shoreline and grain‐size front are decoupled through the adjustment of the depositional slope and sediment dispersal under spatiotemporally varying tectonic forcing, especially in underfilled basins. Under such a decoupled situation, the pathway of the migrating subsidence centre correlates with the pathway of the grain‐size front, a result of competition between spatiotemporally varying tectonic forcing and autogenic sediment transport. The model results also highlight the significance of three‐dimensional variability in the stratigraphic response to tectonic forcing, which may be overlooked or misinterpreted and suggests a high degree of uncertainty in re‐establishing the base‐level cycles from the stratigraphic record alone. Moreover, spectral analysis of the modelled stratigraphy and tectonic forcing suggests that low‐frequency tectonic signals are more likely to be recorded in the stratigraphy with a lag time, whereas high‐frequency tectonic signals are likely to be shredded, mixed with autogenic signals, or buffered through sediment‐routing systems. Finally, quantitative measurements of the stratigraphic architecture of the Nanpu sag in the Bohai Bay Basin, China are used to tune the numerical model of this study to illustrate how to evaluate the role of tectonic forcing on the development of characteristic stratigraphic sequences.     

Constraining groundwater modeling with magnetic resonance soundings

Magnetic resonance sounding (MRS) is a noninvasive geophysical method that allows estimating the free water content and transmissivity of aquifers. In this article, the ability of MRS to improve the reliability of a numerical groundwater model is assessed. Thirty-five sites were investigated by MRS over a ～5000 km(2) domain of the sedimentary Continental Terminal aquifer in SW Niger. Time domain electromagnetic soundings were jointly carried out to estimate the aquifer thickness. A groundwater model was previously built for this section of the aquifer and forced by the outputs from a distributed surface hydrology model, to simulate the observed long-term (1992 to 2003) rise in the water table. Uncertainty analysis had shown that independent estimates of the free water content and transmissivity values of the aquifer would facilitate cross-evaluation of the surface-water and groundwater models. MRS results indicate ranges for permeability (K = 1 × 10(-5) to 3 × 10(-4) m/s) and for free water content (w = 5% to 23% m(3) /m(3) ) narrowed by two orders of magnitude (K) and by ～50% (w), respectively, compared to the ranges of permeability and specific yield values previously considered. These shorter parameter ranges result in a reduction in the model's equifinality (whereby multiple combinations of model's parameters are able to represent the same observed piezometric levels), allowing a better constrained estimate to be derived for net aquifer recharge (～22 mm/year).     

A semi-empirical wind set-up forecasting model for Lake Champlain

The precision of Lake Champlain's water level estimation is a key component in the flood forecasting process for the Richelieu River. Hydrological models do not typically take into consideration the effects of the wind on the water level (also known as the wind set-up). The objective of this study is to create an empirical wind set-up forecast model for Lake Champlain during high wind events. The proposed model uses wind speed and direction across the Lake, as well as wind gusts as inputs. The model is calibrated to a subset of observations and evaluated on an independent sample, considering four wind speed bins. It is tested and compared to a variant of the Zuider Zee equation on 20 wind set-up events that occurred between 2017 and 2019 using hindcast data from five different numerical weather prediction systems (GDPS, RDPS, HRDPS, NOAA and ECMWF). A quantile mapping-based forecast calibration scheme is implemented for each of the forecast products to correct their biases. Results show that events are successfully predicted by the proposed model at least 72 h in advance. These results are better than the other comparative models found in the literature and tested herein. Overall, significant improvements are obtained by including wind speed and wind gusts from different weather stations.     

Oxygen isotope disequilibrium during serpentinite dehydration

Talc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition.     

The Multiscale TROPIcal CatchmentS critical zone observatory M-TROPICS dataset II: Land use,hydrology and sediment production monitoring in Houay Pano,northern Lao PDR

Mountain regions of the humid tropics are characterized by steep slopes and heavy rains. These regions are thus prone to both high surface runoff and soil erosion. In Southeast Asia, uplands are also subject to rapid land-use change, predominantly as a result of increased population pressure and market forces. Since 1998, the Houay Pano site, located in northern Lao PDR (19.85°N 102.17°E) within the Mekong basin, aims at assessing the long-term impact of the conversion of traditional slash-and-burn cultivation systems to commercial perennial monocultures such as teak tree plantations, on the catchment hydrological response and sediment yield. The instrumented site monitors hydro-meteorological and soil loss parameters at both microplot (1 m²) and small catchment (0.6 km²) scales. The monitored catchment is part of the network of critical zone observatories named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Houay Pano are (1) rainfall, (2) air temperature, air relative humidity, wind speed, and global radiation, (3) catchment land use, (4) stream water level, suspended particulate matter, bed particulate matter and stones, (5) soil surface features, and (6) soil surface runoff and soil detachment. The dataset has already been used to interpret suspended particulate matter and bed particulate matter sources and dynamics, to assess the impact of land-use change on catchment hydrology, soil erosion, and sediment yields, to understand bacteria fate and weed seed transport across the catchment, and to build catchment-scale models focused on hydrology and water quality issues. The dataset may be further used to, for example, assess the role of headwater catchments in large tropical river basin hydrology, support the interpretation of new variables measured in the catchment (e.g., contaminants other than faecal bacteria), and assess the relative impacts of both climate and land-use change on the catchment.     

Contribution of magmatism,partial melting buffering and localized crustal thinning on the late Variscan thermal structure of the Agly massif (French Pyrenees)

Low pressure-high temperature (LPHT) metamorphism, with geothermal gradients in the order of 50–100°C/km, is a common feature of the late evolution of collisional orogens. These abnormal thermal conditions may be the results of complex interactions between magmatism, metamorphism and deformation. The Agly massif, in the French Pyrenees, preserves the metamorphic footprints of the late Variscan thermal structure of an almost continuous section from the upper and middle continental crust. The upper crust is characterized by a very high geothermal gradient of ~55°C/km, evolving from greenschist to amphibolite facies, while the middle crust, exposed in a gneissic core, exhibits granulite facies conditions with a near isothermal geothermal gradient (<8°C/km) between 740 and 790°C. The abnormal and discontinuous crustal geothermal gradient, dated at c. 305 Ma on syn-granulitic monazite by LA-ICP-MS, is interpreted to be the result of magmatic intrusions at different structural levels in the crust: the Ansignan charnockite (c. 305 Ma) in the deepest part of the gneissic core, the Tournefort granodiorite (c. 308 Ma) at the interface between the gneissic core and the upper crust and the Saint-Arnac granite (c. 304 Ma) in the upper section of the massif. The heat input from these magmas combined with the thermal buffering effect of the biotite dehydration-melting reaction resulted in the near isothermal geothermal gradient in the gneissic core (melt-enhanced geotherm). The higher geothermal gradient (>50°C/km) in the upper crust is only due to conduction between the hot middle crust and the Earth's surface. The estimated maximum finite pressure range suggests that ~10 to 12 km of crust are exposed in the Agly massif while the present-day thickness does not exceed 5–6 km. This pressure/depth gap is consistent with the presence of several normal mylonitic shear zones that could have contributed to the subtraction of ~5 km of the rock pile. Monazite U–Th–Pb ages carried out on monazite overgrowths from a highly mylonitized sample suggest that this vertical thinning of the massif occurred at c. 296–300 Ma. This later Variscan extension might have slightly perturbed the 305 Ma geothermal gradient, resulting in an apparent higher conductive geothermal gradient in the upper crust. Although the Agly massif has been affected by Cretaceous extension and Eocene Alpine compression, we suggest that most of the present-day thickness of the column rock was acquired by the end of the Palaeozoic.