General palaeontology (Palaeobiochemistry) Biological activity and the Earth's surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record

Abstract The search for early Earth biological activity is hindered by the scarcity of the rock record. The very few exposed sedimentary rocks have all been affected by secondary processes such as metamorphism and weathering, which might have distorted morphological microfossils and biogenic minerals beyond recognition and have altered organic matter to kerogen. The search for biological activity in such rocks therefore relies entirely on chemical, molecular or isotopic indicators. A powerful tool used for this purpose is the stable isotope signature of elements related to life (C, N, S, Fe). It provides key informations not only on the metabolic pathways operating at the time of the sediment deposition, but more globally on the biogeochemical cycling of these elements and thus on the Earth's surface evolution. Here, we review the basis of stable isotope biogeochemistry for these isotopic systems. Rather than an exhaustive approach, we address some examples to illustrate how they can be used as biosignatures of early life and as proxies for its environment, while keeping in mind what their limitations are. We then focus on the covariations among these isotopic systems during the Archean time period to show that they convey important information both on the evolution of the redox state of the terrestrial surface reservoirs and on co-occurring ecosystems in the Archean. Résumé Apport des isotopes stables (C, N, S, Fe) à l'étude des interrelations entre activités biologiques et conditions physicochimiques de surface de la terre primitive. La recherche et la caractérisation des écosystèmes à la surface de la Terre primitive sont un défi, étant donné le faible degré de préservation des roches archéennes. Les quelques formations sédimentaires disponibles ont, en effet, été modifiées par de nombreux processus secondaires (métamorphisme, altération) qui excluent toute diagnose morphologique robuste des microfossiles et des minéraux associés. La recherche de traces de vie fossile et la caractérisation des environnements contemporains du dépôt reposent ainsi sur des indices chimiques dont les plus robustes sont les isotopes stables. Dans ce manuscrit, nous tenterons de résumer les bases de la biogéochimie des isotopes stables et nous illustrerons comment cette discipline peut permettre d'apporter des contraintes sur la vie primitive et son environnement. Quelques exemples choisis dans différents systèmes isotopiques pertinents pour l'étude de la vie (C, N, S, Fe) et pour l'étude des conditions d'oxydation de surface de la Terre primitiv

General palaeontology (Palaeobiochemistry) Biological activity and the Earth's surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record

Abstract The search for early Earth biological activity is hindered by the scarcity of the rock record. The very few exposed sedimentary rocks have all been affected by secondary processes such as metamorphism and weathering, which might have distorted morphological microfossils and biogenic minerals beyond recognition and have altered organic matter to kerogen. The search for biological activity in such rocks therefore relies entirely on chemical, molecular or isotopic indicators. A powerful tool used for this purpose is the stable isotope signature of elements related to life (C, N, S, Fe). It provides key informations not only on the metabolic pathways operating at the time of the sediment deposition, but more globally on the biogeochemical cycling of these elements and thus on the Earth's surface evolution. Here, we review the basis of stable isotope biogeochemistry for these isotopic systems. Rather than an exhaustive approach, we address some examples to illustrate how they can be used as biosignatures of early life and as proxies for its environment, while keeping in mind what their limitations are. We then focus on the covariations among these isotopic systems during the Archean time period to show that they convey important information both on the evolution of the redox state of the terrestrial surface reservoirs and on co-occurring ecosystems in the Archean. Résumé Apport des isotopes stables (C, N, S, Fe) à l'étude des interrelations entre activités biologiques et conditions physicochimiques de surface de la terre primitive. La recherche et la caractérisation des écosystèmes à la surface de la Terre primitive sont un défi, étant donné le faible degré de préservation des roches archéennes. Les quelques formations sédimentaires disponibles ont, en effet, été modifiées par de nombreux processus secondaires (métamorphisme, altération) qui excluent toute diagnose morphologique robuste des microfossiles et des minéraux associés. La recherche de traces de vie fossile et la caractérisation des environnements contemporains du dépôt reposent ainsi sur des indices chimiques dont les plus robustes sont les isotopes stables. Dans ce manuscrit, nous tenterons de résumer les bases de la biogéochimie des isotopes stables et nous illustrerons comment cette discipline peut permettre d'apporter des contraintes sur la vie primitive et son environnement. Quelques exemples choisis dans différents systèmes isotopiques pertinents pour l'étude de la vie (C, N, S, Fe) et pour l'étude des conditions d'oxydation de surface de la Terre primitiv

Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure fl ux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defi ned as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (inmost higher eukaryotes and some protists such as Dictyostelium ) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the fi eld understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation it is imperative to delete or knock down more than one autophagy-related gene. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways so not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field

Seasonality of recharge fluxes in southern Quebec from subsurface monitoring: processes controlling recharge in seasonally frozen permeable soils

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New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy)

International audienceA new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K-Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2-2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6-8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] 3), and Nb-Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11-15), low LREE/HFSE ratios (La/Nb=1-2) close to those of anorogenic lavas, and the Nb-Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K-Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene

Tephro-chronostratigraphy of the lacustrine interglacial record of Piànico, Italian Southern Alps: Identifying the volcanic sources using radiogenic isotopes and trace elements

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Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing

The goal of this study is to explain the origin of 234U–238U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3He/4He ratios, to gain insight regarding the evolution of groundwater in the region. (234U/238U) activity ratios, or (234U/238U)act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (234U/238U)act, 1.14 ± 0.01, was measured in Ca–HCO3-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234U–238U isotopic fractionation. The (234U/238U)act increases to 6.07 ± 0.14 in Na–HCO3–Cl-type groundwater. Preferential migration of 234U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (234U/238U)act and 3He/4He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (234U/238U)act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (14C ages of 6.6 kyrs), with (234U/238U)act of ≥6.07 and large amounts of radiogenic 4He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (234U/238U)act and the addition of excess 4He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4He and 234U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4He excesses measured in groundwater

Structural control on shallow hydrogeochemical processes at Caviahue-Copahue Volcanic Complex (CCVC), Argentina

International audienceThe Caviahue-Copahue Volcanic Complex (CCVC) hosts one of Argentina's most important geothermal systems. To provide new insights into origin, circulation, and residence time of fluids, the chemical and isotopic composition (3 He/ 4 He, δ 2 H-δ 18 O in H 2 O; δ 13 C-δ 18 O in CO 2 ; 87 Sr/ 86 Sr) of thermal waters was measured together with the 3 H and 14 C activities. Water samples were collected from hot springs (LM, TC, LMM, CB and AF) representing the five major thermal zones of the CCVC and assumed to be steam-heated meteoric waters, and a well condensate (COP-2). The LMM, CB, and AF chemical composition and 87 Sr/ 86 Sr ratios show that water chemistry is acquired locally from exchange with volcanic rocks (Sr, SiO 2 , among others) and from steam (H 2 S). Two surface geothermal manifestations (LM and TC), along with the well condensate, COP-2, contain a higher contribution of deeporiginating fluids, with 87 Sr/ 86 Sr indicating possible contribution from deep-seated granitoids or sediments from the underlying basement. Radiocarbon-based residence times indicate ages ranging between 13,540 and 17,520 yrs. BP, representing the minimum age for the geothermal reservoir waters. Tritium is mainly absent in hot spring waters except for LMM and CB where the activity is close to the detection limit. This indicates a minimum age older than 70 yrs. for the water circulating in the shallow circuit. This result suggests that shallow meteoric water have a more complex and/or deeper circuit, resulting in older residence times. Helium isotopes in the CCVC span a wide range, from a pure mantle-derived value, of 8.35Ra, to a more crustal radiogenic signature, of 4.6Ra. The spatial variation is explained by associating the geochemical data with the geological context, which includes the distribution of fault-fracture meshes and different sources of magmatic volatiles underlying the Copahue volcano. The first order control on helium isotope signatures observed in this study seems to be dominated by the degree of crustal assimilation of the magmatic sources, which is in turn controlled by the local arrays of faults