Deciphering degassing mechanisms of He and H2 at the sedimentary basin‐basement interface by surface geophysics and gas geochemistry

With the increasing importance of the carbon footprint of transport, new sources, closer to consumers, of H₂ and He are explored. Within this context, we present a combined near‐surface geophysical imaging, soil gas sampling, and bubbling well gas sampling approach to study fluid and gas pathways near a fault system in the Morvan massif, part of the French Massif Central, in the south‐east of the Paris Basin. Electrical resistivity and seismic refraction tomography profiles allowed identifying a fault network. The co‐located soil gas sampling shows a He hot‐spot clearly linked to a section of one fault, suggesting a preferential pathway via water advection. Very high He concentrations, are also measured in N₂‐dominated free gas from two bubbling wells very close to the soil He hot‐spot. Evidence for a water reservoir with high N₂‐He gas bubbles in the very shallow basement‐sediment cover interface is obtained through our geophysical data. In contrast, H₂ spreads more widely, pointing to biological production and consumption coupled to soil aeration, as well as a possible geological seep with diffusion processes controlled by clay/marls. The very distinct spatial variability observed for He and H₂ results from these different transport processes. A simple geochemical model is proposed to explain the geochemical signature of bubble gas, rich in N₂ and He, through exsolution of air N₂ dissolved at recharge, where radiogenic He originates from the granitic basement

NanoSr – A New Carbonate Microanalytical Reference Material for In Situ Strontium Isotope Analysis

The in situ measurement of Sr isotopes in carbonates by MC-ICP-MS is limited by the availability of suitable microanalytical reference materials (RMs), which match the samples of interest. Whereas several well-characterised carbonate reference materials for Sr mass fractions > 1000 μg g−1 are available, there is a lack of well-characterised carbonate microanalytical RMs with lower Sr mass fractions. Here, we present a new synthetic carbonate nanopowder RM with a Sr mass fraction of ca. 500 μg g−1 suitable for microanalytical Sr isotope research (‘NanoSr’). NanoSr was analysed by both solution-based and in situ techniques. Element mass fractions were determined using EPMA (Ca mass fraction), as well as laser ablation and solution ICP-MS in different laboratories. The 87Sr/86Sr ratio was determined by well-established bulk methods for Sr isotope measurements and is 0.70756 ± 0.00003 (2s). The Sr isotope microhomogeneity of the material was determined by LA-MC-ICP-MS, which resulted in 87Sr/86Sr ratios of 0.70753 ± 0.00007 (2s) and 0.70757 ± 0.00006 (2s), respectively, in agreement with the solution data within uncertainties. Thus, this new reference material is well suited to monitor and correct microanalytical Sr isotope measurements of low-Sr, low-REE carbonate samples. NanoSr is available from the corresponding author

Importance of Cadmium Sulfides for Biogeochemical Cycling of Cd and Its Isotopes in Oxygen Deficient Zones-A Case Study of the Angola Basin

Understanding oceanic cadmium (Cd) cycling is paramount due to its micronutrient-like behavior in seawater, which has been inferred from its similarity to phosphate (PO4) cycling. Cadmium concentrations follow a nutrient-like consumption-regeneration cycle in the top of the water column and are mainly controlled by water mass mixing and circulation in the deep ocean. However, an additional scavenging mechanism through cadmium sulfide (CdS) precipitates, occurring within sinking biogenic particles in oxygen deficient zones (ODZ), has been proposed. In this study, we report Cd stable isotope and concentration data for seven vertical seawater profiles sampled during GEOTRACES cruise GA08 in the northern Cape and Angola Basins, which feature a significant ODZ along their eastern margins. Outside the ODZ, Cd cycling is similar to that previously reported for the South Atlantic. While water mass mixing largely controls deep ocean Cd isotope signatures, Cd isotope fractionation in surface waters can be modeled as an open system at steady state buffered by organic ligand complexation. In the ODZ, stronger Cd depletion relative to PO4 is associated with a shift in delta Cd-114 toward heavier values, which is indicative of CdS precipitation. Our interpretation is supported by experimental CdS precipitation data and a size-resolved particle model involving bacterial sulfate reduction as a precursor of CdS. Our estimates of the CdS flux to the seafloor (10(7) to 10(9) mol/yr) indicate that CdS precipitation is a significant process of Cd removal and constitutes a nonnegligible Cd sink that needs to be better quantified by Cd isotope analyses of marine sediments

Central Asian modulation of Northern Hemisphere moisture transfer over the Late Cenozoic

Abstract Earth’s climatic evolution over the last 5 million years is primarily understood from the perspective of marine mechanisms, however, the role of terrestrial feedbacks remains largely unexplored. Here we reconstruct the last 5 million years of soil moisture variability in Central Asia using paleomagnetism data and isotope geochemistry of an 80 m-thick sedimentary succession at Charyn Canyon, Kazakhstan. We identify a long-term trend of increasing aridification throughout the period, along with shorter-term variability related to the interaction between mid-latitude westerlies and the Siberian high-pressure system. This record highlights the long-term contribution of mid-latitude Eurasian terrestrial systems to the modulation of moisture transfer into the Northern Hemisphere oceans and back onto land via westerly air flow. The response of Earth-surface dynamics to Plio-Pleistocene climatic change in Central Asia likely generated terrestrial feedbacks affecting ocean and atmospheric circulation. This missing terrestrial link elucidates the significance of land-water feedbacks for long-term global climate

A multi-isotope and modelling approach for constraining hydro-connectivity in the East African Rift System, southern Ethiopia

During the last African Humid Period (AHP; 15–5 ka), many lakes in the East African Rift System (EARS) experienced pronounced lake-level variations that dramatically transformed the hydrological landscape. Currently dry, saline or marshy-wetland terminal lakes became vast waterbodies, interconnected via overflow sills resulting in the formation of a several thousand-kilometre-long chain of lakes in the EARS. A quantitative, process-based understanding of these hydrological systems can advance our interpretation of past hydroclimate variability from proxy records. Here, we provide a critical modern hydrological dataset for the data-sparse Lake Chew Bahir basin in southern Ethiopia. Driven by modern data, an isotope-enabled hydro-balance model was developed to assess how increases in rainfall modulate δ18O and 87Sr/86Sr variability. Considering a terminal Lake Chew Bahir scenario, humid conditions resulted in higher lake δ18O (∼+14‰) due to increased evaporation and longer water residence times. At the same time 87Sr/86Sr decreased from 0.7064 to 0.7061 due to an increased riverine Sr flux characterised by lower, unradiogenic 87Sr/86Sr ratios. In a modelling scenario where Lake Chew Bahir became a flow-through system with interconnectivity between lakes Abaya, Chamo, Chew Bahir and Turkana, higher lake δ18O (∼+12‰) relative to present was found, but δ18O was lower than in the terminal lake scenario. The lake water 87Sr/86Sr ratios (<0.7061) were also slightly lower. A moderate concomitant change in rainfall input δ18O of −1‰ in step with hydrological reorganisation resulted in the lowest lake δ18O (∼+5‰). Modelled δ18O values were similar to the δ18O range of endogenic carbonates from sedimentary cores from Lake Chew Bahir at the onset of the AHP, supporting the validity of our model, and suggesting that evaporation and the lake water residence time strongly influence lake water δ18O. However, the reported 87Sr/86Sr of fossil carbonates from Lake Chew Bahir during the AHP (0.7065–0.7060) could not be reproduced by our modelled scenarios without adjusting the surface-water-to-groundwater ratio, highlighting the potential role of groundwater as a water source in semi-arid regions. These results demonstrate the insights that can be gained from applying a process-based approach using O and Sr isotope hydro-balance modelling to aid interpretation of past hydro-balance and lake interconnectivity from lacustrine sedimentary records

Characterization of Saharan and Sahelian dust sources based on geochemical and radiogenic isotope signatures

International audienceMineral dust can significantly impact climate and biogeochemical cycles on Earth. To understand dust provenance, an accurate characterization of dust sources and emission regions is required. In this study, we combine rare-earth element patterns, elemental ratios, and radiogenic Sr-Nd-Pb isotopes to discriminate dust sources from key regions in North Africa responsible for ∼55% of the total dust load emitted annually on Earth. This new dataset, based on fifty-nine analyses of deflatable fine soil material, improves our current knowledge about North African dust sources, especially those underrepresented in Saharan-Sahelian regions. Six potential source areas (PSA) - Libya-Algeria-Mali (PSALAM), Libya-Egypt (PSALE), Bodélé Depression (PSABD), Mali Center (PSAMC), West African Coast (PSAWAC) and Mauritania (PSAMa) - are defined based on several lithogenic tracers as well as on the geological subdivision of North African geological provinces, providing a unique chemical and isotope fingerprint for each PSA. For example, the PSABD - the main dust activation area in North Africa - is clearly distinguished from western African sources using Pb isotopes. Major elements show a large variability within each PSA and are not alone diagnostically useful. Comparison of the newly defined PSA with aerosols collected in remote locations over the North Atlantic shows that their sources are predominantly from western African regions (PSALAM and PSAWAC). Bodélé-derived dust has a limited impact on the bulk dust transported over the tropical North Atlantic, regardless of the season and altitude of dust transport. The low impact of the Bodélé Depression can be explained by a high deposition rate (wet and dry) of aerosols along its southwestward trajectory. A detailed analysis of dust collected over South America during winter will be essential to confirm this observation at a global scale. The present dataset can be efficiently used to track modern dust emissions as well as their changes at the scale of glacial/interglacial cycles or during the development of African Humid Periods as stored in sedimentary archives

Lateritic Cenozoic paleoenvironmental and paleoclimatic conditions in the Central Amazon basin, Brazil, inferred from mineralogy, geochemistry and geochronology

International audienceLateritic profiles are natural archives recording the weathering history in the tropics over million-year time-scales. This study combines oxygen, hydrogen, and silicon isotope data with mineralogical, geochronological, and geochemical analyses of lateritic secondary mineral assemblages to constrain the environmental and climatic conditions prevailing in the central Amazon Basin during the Cenozoic. The studied lateritic profile was developed over the sedimentary succession of the Alter do Chao Formation in Manaus, Brazil. Three distinct weathering episodes were identified, each constrained by (U-Th)/He and EPR ages. Dated to the Oligocene, the earliest (Oligocene) and long-lasting (duration &gt;10 Ma) weathering episode involves the formation of well-ordered kaolinites through in situ chemical weathering of parent minerals under well-drained conditions, indicative of a tropical climate with restricted seasonality. The second episode, occurring in the mid-Miocene (similar to 16 +/- 3 Ma), involves the formation of a ferruginous duricrust. Most probably resulting from lateral iron migration and precipitation at the oxidizing front of an oscillating water table, the duricrust developed in a context of rising sea levels. The third episode, during the Upper Miocene (similar to 10 Ma), led to the replacement of the kaolinite initially precipitated at the top of the profile by lower-crystallinity kaolinite formed at faster kinetics. This episode coincides with the final phase of the Andean uplift and the onset of the transcontinental Amazon River, reflecting enhanced water drainage under a "monsoon-type" climate

Deciphering Degassing Mechanisms of He and H2 H2{\mathbf{H}}_{\mathbf{2}} at the Sedimentary Basin‐Basement Interface by Surface Geophysics and Gas Geochemistry

International audienceWith the increasing importance of the carbon footprint of transport, new sources, closer to consumers, of and He are explored. Within this context, we present a combined near‐surface geophysical imaging, soil gas sampling, and bubbling well gas sampling approach to study fluid and gas pathways near a fault system in the Morvan massif, part of the French Massif Central, in the south‐east of the Paris Basin. Electrical resistivity and seismic refraction tomography profiles allowed identifying a fault network. The co‐located soil gas sampling shows a He hot‐spot clearly linked to a section of one fault, suggesting a preferential pathway via water advection. Very high He concentrations, are also measured in ‐dominated free gas from two bubbling wells very close to the soil He hot‐spot. Evidence for a water reservoir with high ‐He gas bubbles in the very shallow basement‐sediment cover interface is obtained through our geophysical data. In contrast, spreads more widely, pointing to biological production and consumption coupled to soil aeration, as well as a possible geological seep with diffusion processes controlled by clay/marls. The very distinct spatial variability observed for He and results from these different transport processes. A simple geochemical model is proposed to explain the geochemical signature of bubble gas, rich in and He, through exsolution of air dissolved at recharge, where radiogenic He originates from the granitic basement