2019 |
Bachari, M., D. Grosheny, S. Ferry, C. France-Lanord, and M. Hedi Negra. "The Cenomanian—Turonian Boundary Event (CTBE) in north-central Tunisia (Jebels Serj and Bargou) integrated into regional data (Algeria to Tunisia)." Cretaceous Research 94, no. 2 (2019): 108–125.
Résumé : Two new sections covering the Cenomanian—Turonian transition have been studied at the foot of the Kasserine platform in the northeastern Mellegue basin (Jebels Serj and Bargou). They show, in the uppermost part of the Cenomanian Fahdene Formation, a Pre-Bahloul unit overlain by the well-known Bahloul black shale. Combined foraminiferal and isotope data of the sections complement other published results: (1) extinction of rotaliporids atop of the Pre-Bahloul bed during the build-up of the Cenomanian—Turonian Boundary Event (CTBE) δ13C positive shift, (2) Heterohelix bloom during the deposition of the Bahloul black shale, (3) filament event during δ13C return to normal values, and (4) first occurrence of Helvetoglobotruncana helvetica during the transition to the marlstone of the Annaba Formation. The Bahloul black shale is here divided into three units, U1 to U3 which are compared with the other sections of the Mellegue basin.
The bulk of this paper is a comparison of the two new sections with revisited nearby sections of the Mellegue basin, as well as with those published from the south Tunisian Gafsa trough and Saharan Atlas of Algeria. The comparison suggests that the pre-Bahloul of the Mellegue basin is correlatable with the transgressive fine-grained limestone bed underlying the CTB black shale in the corridors between isolated, keep-up platform carbonates of the Ouled Nail in Algeria, as well as the fine-grained limestone bed overlying upper Cenomanian shallow-water deposits of southern Tunisia. Its sequence stratigraphic significance in the Mellegue basin should therefore be recognized as a deeper-water equivalent of a transgressive systems tract, instead of being interpreted as a lowstand or highstand as it is by other authors. If correct, the inception of the CTBE δ13C shift would correspond to a transgression over most of Algeria and Tunisia, as observed in the North American Western Interior.
Mots-clés : Cycla
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Balco, G., P. H. Blard, D. L. Shuster, J. O. H. Stone, and L. Zimmermann. "Cosmogenic and nucleogenic 21Ne in quartz in a 28-meter sandstone core from the McMurdo Dry Valleys, Antarctica." Quaternary Geochronology 52 (2019): 63–76.
Résumé : We measured concentrations of Ne isotopes in quartz in a 27.6-m sandstone core from a low-erosion-rate site at 2183m elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21Ne indicate a surface exposure age of at least 4.1 Ma and an erosion rate no higher than ca. 14 cm Myr−1 21Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape effects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is
not commonly observed in natural examples. We used a model for 21Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21Ne from nucleogenic 21Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below approximately 95 °C, near 160 Ma, which is consistent with existing apatite fission-track data and the 183 Ma emplacement of nearby Ferrar dolerite intrusions. Constraints on 21Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79 mb at 190 GeV for the cross-section for 21Ne production by fast muon interactions, but indicate that 21Ne production by negative muon capture is likely negligible.
Mots-clés : Cycla teer
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Bastian, L., N. Vigier, M. Revel, G. Yirgu, D. Ayalew, and R. Pik. "Chemical erosion rates in the upper Blue Nile Basin and related atmospheric CO2 consumption." Chemical Geology 518 (2019): 19–31.
Résumé : Silicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ7Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.
Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km2/year) and CO2 consumption rate (0.65 × 1012 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14–8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91–1.5 × 1012 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.
Mots-clés : Cycla
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Blard, P. H., M. Lupker, M. Rousseau, and J. Tesson. "Two MATLAB programs for computing paleo-elevations and burial ages from paired-cosmogenic nuclides." MethodsX 6 (2019): 1547–1556.
Résumé : Methods based on cosmic-ray produced nuclides are key to improve our understanding of the Earth surface dynamic. Measuring multiple cosmogenic nuclides in the same rock sample has a great potential, but data interpretation requires rigorous and often complex mathematical treatments. In order to make progress on this topic, this paper presents two easy-to-use MATLAB© programs permitting to derive information from pairs of cosmogenic nuclides (26Al-10Be or 10Be-21Ne) measured in rock samples that have been exposed to cosmic rays in the past: “Paleoaltitude.m” and “Burial.m” Codes available here as supplementary material.
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“Paleoaltitude.m” computes paleoelevations from a sample whose burial age is known. This new paleoaltimetry method is presented in detail in Blard et al. [1]. The present article also develops the mathematical approach.
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Since the elevation of exposure may affect the accuracy of a burial age [1], the second MATLAB© script “Burial.m” is designed to compute burial ages from 26Al-10Be or 10Be-21Ne pairs, taking into account the position of a sample (elevation and latitude) during its preburial exposure history.
Mots-clés : Cycla teer
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Cadet, M., T. Sayavongkhamdy, V. Souksavatdy, T. Luangkhoth, P. Dillmann, C. Cloquet, J. Vernet, P. Piccardo, N. Chang, J. Edgar et al. "Lao's central role in Southeast Asian copper exchange networks: A multimethod study of bronzes from the Vilabouly Complex." Journal of Archaeological Science 109 (2019).
Résumé : The application of lead isotope-based provenance analysis in Southeast Asia over the last decade has strongly suggested a central role was played by the Lao PDR in regional copper production exchange networks for approximately 1500 years. The Vilabouly Complex, in central Lao’s Savannakhet Province, has revealed major copper mining and smelting sites dated to the regional Iron Age (c. 400 BC – AD 500) and possibly Bronze Age (c.1000 – 400 BC). Metallurgical practices at the Vilabouly Complex, and indeed for all of the Lao PDR, are unknown, and the propose of this paper is to provide a comprehensive analytical study of the Vilabouly Complex metal assemblage, including 60 copper-base artefacts of multiple typologies. Cut samples of these were subjected to morpho-stylistic, metallographic (OM), elemental (XRF, SEM-EDS) and lead isotope analyses (MC-ICP-MS) in order to reconstruct the range of forms, metalworking materials, techniques (alloying, casting and post-casting treatments) used at the Vilabouly Complex. The results revealed an assemblage composed of copper, bronze and leaded bronze alloys, with a majority consistent with the lead isotopic signature for the Vilabouly Complex copper. The consistent geochemical and technological signature of the majority of artefacts strongly corroborates the extensive onsite production evidence, and fits with the burgeoning regional copper-base metals database for copper metal demand being sated in large part by Lao PDR supply.
Mots-clés : Cycla
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Flahaut, J., J. L. Bishop, S. Silvestro, D. Tedesco, I. Daniel, and D. Loizeau. "The Italian Solfatara as an analog for Mars fumarolic alteration." American Mineralogist 104 (2019): 1565–1577.
Résumé : The first definitive evidence for continental vents on Mars is the in situ detection of amorphous silica-rich outcrops by the Mars Exploration Rover Spirit. These outcrops have been tentatively interpreted as the result of either acid sulfate leaching in fumarolic environments or direct precipitation from hot springs. Such environments represent prime targets for upcoming astrobiology missions but remain difficult to identify with certainty, especially from orbit. To contribute to the identification of fumaroles and hot spring deposits on Mars, we surveyed their characteristics at the analog site of the Solfatara volcanic crater in central Italy. Several techniques of mineral identification (VNIR spectroscopy, Raman spectroscopy, XRD) were used both in the field and in the laboratory on selected samples. The faulted crater walls showed evidence of acid leaching and alteration into the advanced argillic-alunitic facies, with colorful deposits containing alunite, jarosite, and/or hematite. Sublimates containing various Al and Fe hydroxyl-sulfates were observed around the active fumarole vents at 90 °C. One vent at 160 °C was characterized by different sublimates enriched in As and Hb sulfide species. Amorphous silica and alunite assemblages that are diagnostic of silicic alteration were also observed at the Fangaia mud pots inside the crater. A wide range of minerals was identified at the 665 m diameter Solfatara crater that is diagnostic of acid-steam heated alteration of a trachytic, porous bedrock. Importantly, this mineral diversity was captured at each site investigated with at least one of the techniques used, which lends confidence for the recognition of similar environments with the next-generation Mars rovers.
Mots-clés : cycla
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Gou, L. F., Z. Jin, A. Galy, H. Sun, L. Deng, and Y. Xu. "Effects of cone combinations on accurate and precise Mg‐isotopic determination using multi‐collector inductively coupled plasma mass spectrometry." Rapid Communications in Mass Spectrometry 33 (2019): 351–360.
Résumé : Rationale
High‐precision determination of magnesium (Mg) isotopes can now be routinely achieved by multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The analytical sensitivity and instrumental mass discrimination behavior of this method are, however, sensitive to the types of sample and skimmer cones used in these measurements, so it is important that these parameters should be investigated.
Methods
Using the sample‐standard‐bracketing method in the wet‐plasma mode, four available combinations of sample and skimmer cones [Jet sample cone + H skimmer cone (Jet + H), standard sample cone + H skimmer cone (Standard + H), standard sample cone + X skimmer cone (Standard + X), and Jet sample cone + X skimmer cone (Jet + X)] were systematically investigated for peak shape, sensitivity, mass discrimination, accuracy, and precision in Mg‐isotopic ratio determination using a Neptune plus MC‐ICP mass spectrometer.
Results
The results showed that different cone combinations do not affect peak shapes but would significantly change the sensitivities for Mg‐isotopic determinations. Compared with using the Standard + H, the sensitivities of Mg‐isotopic determinations were enhanced by approximately a factor of 1.3, 1.4, and 1.9 by using the Standard + X, the Jet + H, and the Jet + X combinations, with the most stable mass discrimination behaviors obtained by the Jet + H. The instrumental mass fractionation slope for any combination of a modified cone geometry (i.e. Standard + X, Jet + X, and Jet + H) is 0.500, while it is 0.510 for the Standard + H. In addition, the mass discrimination behavior is related to Mg concentrations once the combination is set, indicating the necessity of concentration match during Mg‐isotopic determination.
Conclusions
The precision and accuracy of the Jet + H combination are better than those of the other combinations, and this is further supported by the validation of the Mg‐isotope data for four international reference materials: Cambridge‐1, NASS‐6, AGV‐2, and BHVO‐2. As the Jet + H combination also provides a high signal, this combination gives the most robust strategy for the highly precise and accurate determination of Mg isotopes.
Mots-clés : Cycla
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Gou, L. F., Z. Jin, P. A. E. Pogge von Strandmann, G. Li, Y. X. Qu, J. Xiao, L. Deng, and A. Galy. "Li isotopes in the middle Yellow River: Seasonal variability, sources and fractionation." Geochimica et Cosmochimica Acta 248 (2019): 88–108.
Résumé : To evaluate the roles of climate and hydrology in continental-scale silicate weathering, we applied Li isotopes to the Yellow River and systematically investigated seasonal Li flux, Li isotopic compositions and potential sources. We collected samples from the middle reaches of the Yellow River weekly over the full hydrological year of 2013. We find that the dissolved Li is mainly derived from silicates and evaporites in the arid to semi-arid Yellow River basin. Silicate weathering of loess during the monsoonal season dominates the Li flux in the middle reaches of the Yellow River, with a positive relationship between dissolved Li flux and physical erosion rate. Evaporite contribution for riverine Li was relatively constant in the middle reaches of the Yellow River but slightly increased after the storm event, with an average proportion of ∼25%, which might represent the proportion of evaporite contribution to global oceans. Seasonal variations in the riverine Li isotopic compositions are dominantly controlled by temperature with a fractionation gradient as −0.182‰ per °C over the full year with deviations likely driven by re-dissolution of suspended particulate matter, extreme hydrological events, and groundwater contribution. Temperature dependent δ7Li value variation of river water inputted into oceans indicates that Cenozoic climate cooling itself may be able to explain ∼2‰ of the 9‰ rise of Cenozoic seawater δ7Li value (Misra and Froelich, 2012). The seasonal variation in riverine Li isotopes highlights that erosion and weathering of loess may provide valuable clues on secular chemical weathering and seawater δ7Li variation spanning a range of time scales.
Mots-clés : Cycla
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Lanceleur, L., E. Tessier, M. Bueno, R. Pienitz, F. Bouchard, C. Cloquet, and D. Amouroux. "Cycling and atmospheric exchanges of selenium in Canadian subarctic thermokarst ponds." Biogeochemistry (2019).
Résumé : The levels and speciation of dissolved, particulate and gaseous Se have been measured in five thermokarst ponds in a sporadic and discontinuous permafrost region of northern Que´bec (Canada) during summer oxygen stratification. Evolution of Se concentrations with depth was investigated in sediment cores collected in three different ponds. The potential for inorganic Se transformation in natural pond waters was investigated by experimental incubations of isotopic species-specific tracers of selenite and selenate. Experimental and monitoring observation revealed that high dissolved carbon concentration, suspended particle matter concentration, heterotrophic activity and periphytic biofilms have a significant role in the formation of gaseous selenium species. In sediment, Se is mainly associated with organic matter in the concentration range 0.14–1.6 mg kg-1. Despite low outgassing rates of volatile Se toward the atmosphere (1–97 ng Se m-2 day-1), the large surface occupied by ponds in northern Canada may lead to important exchange rates (ca. 1 ton year-1) at the global scale. Detailed measurements of Se speciation, including volatile compounds, and its reactivity toward heterotrophic activity in selected thaw ponds from the Canadian Subarctic provides new insight to better constrain the biogeochemical pathways leading to Se removal from the water column via atmospheric gas exchange and sediment accumulation.
Mots-clés : cycla
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Lee, H., V. Galy, X. Feng, C. Ponton, A. Galy, C. France-Lanord, and S. J. Feakins. "Sustained wood burial in the Bengal Fan over the last 19 My." Proceedings of the National Academy of Sciences 116, no. 45 (2019): 22518–22525.
Résumé : The Ganges–Brahmaputra (G-B) River system transports over a billion tons of sediment every year from the Himalayan Mountains to the Bay of Bengal and has built the world’s largest active sedimentary deposit, the Bengal Fan. High sedimentation rates drive exceptional organic matter preservation that represents a long-term sink for atmospheric CO2. While much attention has been paid to organic-rich fine sediments, coarse sediments have generally been overlooked as a locus of organic carbon (OC) burial. However, International Ocean Discovery Program Expedition 354 recently discovered abundant woody debris (millimeter- to centimeter-sized fragments) preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the Bengal Fan (∼8°N, ∼3,700-m water depth) with recovery spanning 19 My. Analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically. In the last 5 My, export included C4 plants, implying that coarse woody, lowland export continued after C4 grassland expansion, albeit in reduced amounts. Substantial export of coarse woody debris in the last 1 My included one wood-rich deposit (∼0.05 Ma) that encompassed coniferous wood transported from the headwaters. In coarse layers, we found on average 0.16 weight % OC, which is half the typical biospheric OC content of sediments exported by the modern G-B Rivers. Wood burial estimates are hampered by poor drilling recovery of sands. However, high-magnitude, low-frequency wood export events are shown to be a key mechanism for C burial in turbidites.
Mots-clés : Cycla
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Animators : Christophe Cloquet & Guillaume Paris
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