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Variations in the abundances of moderately volatile elements (MVE) are one of the most fundamental geochemical differences between the terrestrial planets. Whether these variations are the consequence of nebular processes, planetary volatilization, differentiation or late accretion is still unresolved. The element mercury is the most volatile of the MVE and is a strongly chalcophile element. It is one of the few elements that exhibit large mass-dependent (MDF) and mass-independent (MIF) isotopic fractionations for both odd (odd-MIF, Δ 199 Hg and Δ 201 Hg) and even (even-MIF, Δ 200 Hg) Hg isotopes in nature, which is traditionally used to trace Hg biogeochemical cycling in surface environments. However, the Hg isotopic composition of Earth and meteorites is not well constrained. Here, we present Hg isotopic data for terrestrial basaltic, trachytic and granitic igneous samples. These rocks are isotopically lighter ( δ 202 Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition ( δ 202 Hg = − 0.7 ± 0.5 ‰ ; 1 standard deviation). We show degassing during magma emplacement induces MIF that are consistent with kinetic fractionation in these samples. Also presented is a more complete dataset for chondritic (carbonaceous, ordinary and enstatite) meteorites, which are consistent with previous work for carbonaceous chondrites (positive odd-MIF) and ordinary chondrites (no MIF), and demonstrate that some enstatite chondrites exhibit positive odd-MIF, similar to carbonaceous chondrites. The terrestrial igneous rocks fall within the range of chondritic compositions for both MIF and MDF. Given the fact that planetary differentiation (core formation, evaporation) would contribute to Hg loss from the silicate portion of Earth and would likely fractionate Hg isotopes from chondritic compositions, we suggest that the budget of the mantle Hg is dominated by late accretion of chondritic materials to Earth, as also suggested for other volatile chalcophile elements (S, Se, Te). Considering the Hg isotopic signatures, materials with compositions similar to CO chondrites or ordinary chondrites are the most likely late accretion source candidates. Finally, eucrite meteorites, which are highly depleted in volatile elements, are isotopically heavier than chondrites and exhibit negative odd-MIF. The origin of volatile depletion in eucrites has been vigorously debated. We show that Δ 199 Hg versus Δ 201 Hg relationships point toward an equilibrium nuclear field shift effect, suggesting that volatile loss occurred during a magma ocean phase at the surface of the eucrite parent body, likely the asteroid 4-Vesta.

International audience; Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the Casingle bondO and Mgsingle bondO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the Casingle bondO difference between Grt and Cpx (ΔCasingle bondOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCasingle bondOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The Mgsingle bondO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.

International audience; Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9–91.2) and low spinel Cr# (8.4–19.3), together with moderate to high whole-rock Al2O3 contents (2.0–3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to

International audience; BLM is a multifunctional helicase that plays critical roles in maintaining genome stability. It processes distinct DNA substrates, but not nicked DNA, during many steps in DNA replication and repair. However, how BLM prepares itself for diverse functions remains elusive. Here, using a combined single-molecule approach, we find that a high abundance of BLMs can indeed unidirectionally unwind dsDNA from a nick when an external destabilizing force is applied. Strikingly, human replication protein A (hRPA) not only ensures that limited quantities of BLMs processively unwind nicked dsDNA under a reduced force but also permits the translocation of BLMs on both intact and nicked ssDNAs, resulting in a bidirectional unwinding mode. This activation necessitates BLM targeting on the nick and the presence of free hRPAs in solution whereas direct interactions between them are dispensable. Our findings present novel DNA unwinding activities of BLM that potentially facilitate its function switching in DNA repair.

International audience; The incision history of the Three Rivers (Salween, Mekong, and Yangtze) region in the Southeast Tibetan Plateau has been linked to both tectonic and climatic controls. In this study, we report new apatite (U‐Th)/He and fission‐track thermochronology data from the >6,000‐m‐high Kawagebo massif, which forms the edge of the high plateau on the western flank of the steepened knickzone reach of the middle Mekong River valley. Thermal‐history modeling of a thermochronological age‐elevation profile shows rapid cooling since ~1.5 Ma and suggests a mean Quaternary exhumation rate of >1 km/Myr at the valley bottom. The amount of Quaternary exhumation is too high to be caused by fluvial incision alone and requires additional tectonic uplift. Comparing our data from the western flank of the Mekong River valley with published data from the eastern flank shows differential exhumation across the valley in the late Miocene, with the western flank undergoing more exhumation, but relatively uniform exhumation in the Quaternary. We relate rapid exhumation since the late Miocene on the western flank of the Mekong valley and the high topography of the Kawagebo massif to localized tectonic uplift associated with a restraining (left stepping) overstep between the still‐active right‐lateral Parlung and Zhongdian strike‐slip faults. The pattern of river steepness index across the knickzone also indicates that it results from locally focused uplift. Our results demonstrate the importance of detailed thermochronologic studies in this very active region to constrain the complex multiphase tectonic history before invoking any potential climatic forcing of river incision.

International audience; Through geological times, the main turnover of marine ostracod group occurred at the Permian-Triassic boundary (PTB) when the Palaeozoic world gave way to Meso-Cainozoic world. As quite all the groups, ostracods intensively suffered of the end Palaeozoic events but they have the particularity to pass the PTB and to be present in quite all marine environments. We have studied the ostracod assemblages from the best preserved marine Permian-Triassic sections through the world. These sections are, for most of them, located along the Palaeo-Tethys margins: South China Block and Tibet (R.P. China), Bükk Mountains (Hungary), Dolomites (Italy), Taurus Mountains (Turkey) and Elbourz Mountains (Iran). These areas present different environmental settings from very shallow water to deep shelf. The biostratigraphy is well constrained by the presence of conodont index. In deep environment from external shelf to slope, all the Permian ostracods disappear slightly before the PTB and are absent from the Early Triassic. The last representatives of palaeopsychrospheric forms occur in the Early Anisian. On the platforms, during the Late Permian, the ostracods are abundant and highly diversified everywhere along Peri-Tethyan margins. The typical Palaeozoic forms are mixed with the very first Meso-Cainozoic representatives. The specific extinction rates, in the latest Permian, vary from 74% to 100%. During the earliest Triassic, two different settings have to be considered: with or without microbialitic deposits. The development of microbialites gives favorable ecological conditions to ostracods. During the Griesbachian, the fauna is composed of both new comers and surviving Palaeozoic forms. The reduction of size is frequently observed. This interval is called "survival stage". During the Dienerian and the Smithian the fauna are very scarce and poor. These two substages represent the maximum of poverty for the ostracod fauna, may be due to the development of anoxia on the platforms. During the Spathian, occurs the beginning of the true recovery. The very last Palaeozoic representatives are found in the Earliest Anisian which is the radiation stage of ostracod fauna with typical new comers. The interval of Palaeozoic-Meso-Cainozoic ostracod turnover is quite long (about 15My).

International audience; We report a combined study of whole-rock major- and trace-element geochemistry, SmNd isotope composition, zircon U/Pb dating, and LuHf systematics of peralkaline and aluminous A-type granites from the Ririwai Ring Complex in north-central Nigeria. The Ririwai peralkaline and aluminous A-type granites are strongly ferroan, alkalic to alkali-calcic, and enriched in Hf, Zr, Ga, Rb, Y and REEs. They were emplaced between 176 ± 2.3 Ma and 169.6 ± 0.75 Ma. The peralkaline granites yield relatively higher ɛNd(t) (−2.3 to −1.2) and zircon ɛHf(t) values (−5.8 to −1.7) than the aluminous granites (ɛNd(t) = −3.6 to −3.3; zircon ɛHf(t) = −7.8 to −2.4). In addition, inherited zircons in the aluminous granites yield Pan-African (~590 Ma) ages and low ɛHf(t) values (−14.0). Taken together, these data suggest that the granites formed from extensive fractional crystallization of a transitional basaltic melt derived from an enriched OIB mantle source. The depletions of MgO, CaO, Ti2O, Sr and Ba in the granites indicate that Mg-rich olivine, calcic pyroxene, calcic amphibole, feldspars and FeTi oxides were the major fractionated phases during magma evolution. In addition, several types of evidence, e.g., moderately negative ɛNd(t) and ɛHf(t) values, and the presence of inherited zircons, imply that the parental melt was modified by assimilation of Pan-African upper crust into which the granites were emplaced. A transtensional regime generated prior to late Jurassic breakup of Gondwana, which led to reactivation of shear zones and opening of associated transcurrent faults, paved the way for emplacement of the A-type suite. The Sn–Nb–Zn mineralization of the Ririwai A-type suite was probably linked to complex magmatic evolutionary processes involving extensive fractional crystallization coupled with crustal assimilation and late-stage hydrothermal fluid activity.

30 pages; International audience; The transition from the Smithian substage to the Spathian substage of the Olenekian stage of the late Early Triassic was a critical time marked by a series of biological and environmental changes during the multimillion-year recovery interval following the end-Permian mass extinction. However, the Smithian/Spathian boundary (SSB) does not yet have an agreed definition, a shortcoming that complicates high-resolution analysis of events during the Smithian-Spathian transition. Here, we review key biostratigraphic (i.e., ammonoid and conodont) studies of the Smithian and Spathian substages in historically important regions (e.g., the Canadian Arctic for the Boreal realm, western North America for the eastern Panthalassic Ocean) and more recently re-studied locations (e.g., Pakistan and India in the southern Tethys, South China in the eastern Tethys) as well as the carbon isotope chemostratigraphy of 29 major Smithian-Spathian sections globally. Key ammonoid genera (e.g., Wasatchites, Anasibirites, Glyptophiceras and Xenoceltites of the late Smithian, and Bajarunia, Tirolites and Columbites of the early Spathian), conodont species (e.g., Scythogondolella milleri, Novispathodus waageni, and Borinella buurensis of the late Smithian, and ‘Triassospathodus’ hungaricus, Neogondolella aff. sweeti, and Icriospathodus spp. of the early Spathian), and carbonate carbon isotope excursions provide appropriate markers for constraining the SSB. Use of the first occurrence of the conodont Novispathodus pingdingshanensis as a potential marker of the SSB is also discussed. Based on correlations of biostratigraphic and carbon isotope data globally, we propose to revise previous placements of the SSB transition in some sections. Finally, we show that the Smithian Thermal Maximum (STM; herein named) was middle Smithian in age and not correlative with the SSB, as inferred in some earlier studies, and that the SSB coincided with a subsequent major global cooling event.

6 pages; International audience

International audience; Au cours de ces dernières années, les coupes les plus représentatives de la limite Permien-Trias en milieu marin ont été échantillonnées pour l'étude des ostracodes (Crustacea). Nous avons travaillé, entre autres, le GSSP de Meishan en Chine du Sud, la coupe de Bulla dans les Dolomies italiennes, la coupe du Cürük dağ dans le Taurus en Turquie, une coupe dans les Montagnes du Bükk en Hongrie et une coupe dans l'Elbourz en Iran, dans le but d'évaluer les taux d'extinction et de récupération chez ce groupe. Les taux d'extinctions spécifiques sont de 98 à 100% dans la majorité des coupes sauf en Hongrie. Les taux d'extinction génériques sont faibles de 30 à 39%. Après l'événement d'extinction en masse qui se produit toujours avant la limite Permien-Trias, la biodiversité des ostracodes est strictement contrôlée par l'environnement. Nous avons pu distinguer différentes phases pour la récupération post-crise des ostracodes :  Une phase de survivance (Griesbachien) avec des faunes mixtes paléozoïques et méso-cénozoïques  Un maximum de pauvreté (Dienérien-Smithien), avec une diversité et une abondance très faibles  Une phase de récupération-dispersion (Spathien) qui marque le début de la récupération  Une phase de radiation à l'Anisien Le niveau de biodiversité des ostracodes du Permien moyen ne sera pas atteint avant le Ladinien.

Using thermochronological data and thermokinematic modeling, we constrain the timing of late Miocene exhumation of the northern portion of the Gongga batholith, located in Southeast Tibet along the Xianshuihe Fault (XF). We show that rapid exhumation started in the north of the Gongga batholith at ∼9 Ma at a rate of ∼1.85 km/Myr and slowed down since ∼4 Ma. A magmatic pulse occurring during the early Pliocene (∼4 Ma) has overprinted the rapid Miocene exhumation phase in some parts of the batholith, which record mainly early Pliocene post-magmatic cooling. Slow exhumation since ∼4 Ma is consistent with the present-day lower relief observed in the centre of the batholith, which contrasts with the rugged high peaks located to the south. We propose that the northern segment of the XF, the Yalahe fault, which is not active at present, was active between 9 and 4 Ma, forming a restraining bend that focused exhumation south of it. Since ∼4 Ma, the Selaha and the Zheduotang faults form the present-day restraining bend south of which the highest part of the massif is located, including the Gongga Shan that rises more than 3000 m above the mean elevation of the plateau. In the north of the batholith, similar peaks have been removed since the Miocene by local relief reduction at high elevations. Considering that the onset of motion along the XF is contemporaneous with the onset of rapid exhumation recorded along the Kangding transect at ∼9 Ma and a total offset of ∼62 km documented for the XF, the average slip rate of the XF is ∼7 mm/yr since 9 Ma.

International audience; Water is often used as an internal standard in quantitative Raman spectroscopic measurements of dissolved species in aqueous solutions containing salts at varying temperatures. However, the effects of temperature and dissolved ions on the relative differential Raman scattering cross section (RSCS) of the OH stretching vibration band of water at elevated temperatures and salinities are not well defined quantitatively. In this study, the Raman spectra of NaCl solutions with different salinity (from 0 to 5 mol NaCl/kg·H2O) at 20 o C at atmospheric pressure and from 0 to 300 o C at 30 MPa were studied. The relative RSCS of the OH stretching vibration band of liquid water (σ(mNaCl, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa)) as a function of temperature (T, in o C) and salinity (mNaCl, in mol/kg·H2O) was established: σ(mNaCl, T, 30 MPa) /σ(Pure water, 20 o C, 30 MPa) = f (T, mNaCl) = a(T-20) + b where a = 0.000089 × mNaCl 1/2-0.001164; b = 0.0355 × mNaCl + 1; The RSCS of the OH stretching vibration band of water in pseudo back-scattering geometry decreases linearly with increasing temperature, but increases with the addition of dissolved NaCl within the whole temperature range. The enhancement factor of the RSCS by dissolved NaCl increases with temperature. Such effects of temperature and salinity should be considered in quantitative Raman spectroscopic study of species concentration in aqueous solution at high temperature when using water as internal standard.

International audience; The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800 m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16 Ma, AHe ages from 2 to 6 Ma, and one zircon fission track age is ~99 Ma. These data imply a tectonic quiet period sustained since at least 100 Ma with a slow denudation rate of ~0.03 km/Ma, interrupted at 7 to 5 Ma by exhumation at a rate between 0.59 and 0.99 km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

7 pages; International audience; We present the first study of micro-crustaceans (ostracods) associated with microbial crusts in the aftermath of the most devastating extinction, the end-Permian extinction (EPE). These post-extinction microbialites dominated shallow shelf marine environments and were traditionally considered as devoid of any associated fauna. We present a micro-palaeontological analysis of a large record from microbial and non-microbial settings following the EPE. This dataset documents the proliferation of ostracods strictly associated with microbialites. Based on the diet of extant ostracods and uniformitarianism, we propose that the abundant microbes in the mats served as an unlimited food supply. Photosynthetic cyanobacteria may also have locally provided oxygen under low oxygen conditions interpreted by others for the microbialites. Microbialites provided a specialised environment that may have acted as refuge for ostracods in the immediate aftermath of the EPE. The surviving faunas may have been progenitors for the starting of the latter radiation.

25 pages; International audience; One of the most complete Permian-Triassic boundary sections located in the Bükk Mountains (Hungary) was sampled for ostracod study. Seventy−six species are recognized, belonging to twenty genera. Fifteen new species are described and figured: Acratia? jeanvannieri Forel sp. nov., Acratia nagyvisnyoensis Forel sp. nov., Bairdia anisongae Forel sp. nov., Bairdia davehornei Forel sp. nov., Callicythere? balvanyseptentrioensis Forel sp. nov., Cytherellina? magyarorsza− gensis Forel sp. nov., Eumiraculum desmaresae Forel sp. nov., Hollinella fengqinglaii Crasquin sp. nov., Hungarella gerennavarensis Crasquin sp. nov., Langdaia bullabalvanyensis Crasquin sp. nov., Liuzhinia venninae Forel sp. nov., Liuzhinia bankutensis Forel sp. nov., Microcheilinella egerensis Forel sp. nov., Reviya praecurukensis Forel sp. nov., Shemonaella? olempskaella Forel sp. nov. One species is renamed: Bairdia baudini Crasquin nom. nov. Comparison of the Bálvány North section with the Meishan section (Zhejiang Province, South China), Global Boundary Stratotype Sec− tion and Point (GSSP) of the Permian-Triassic Boundary (PTB), reveals discrepancies linked to the environmental set− ting and particularly to bathymetry. The stratigraphical distribution of all the species is given and diversity variations are discussed. The Bálvány North section exhibits the lowest extinction rate of all PTB sections studied for ostracods analysis associated with a high level of endemism.

International audience; Ocean acidification in modern oceans is linked to rapid increase in atmospheric CO 2 , raising concern about marine diversity, food security and ecosystem services. Proxy evidence for acidification during past crises may help predict future change, but three issues limit confidence of comparisons between modern and ancient ocean acidification, illustrated from the end-Permian extinction, 252 million years ago: (1) problems with evidence for ocean acidification preserved in sedimentary rocks, where proposed marine dissolution surfaces may be subaerial. Sedimentary evidence that the extinction was partly due to ocean acidification is therefore inconclusive; (2) Fossils of marine animals potentially affected by ocean acidification are imperfect records of past conditions; selective extinction of hypercalcifying organisms is uncertain evidence for acidification; (3) The current high rates of acidification may not reflect past rates, which cannot be measured directly, and whose temporal resolution decreases in older rocks. Thus large increases in CO 2 in the past may have occurred over a long enough time to have allowed assimilation into the oceans, and acidification may not have stressed ocean biota to the present extent. Although we acknowledge the very likely occurrence of past ocean acidification, obtaining support presents a continuing challenge for the Earth science community. OPEN ACCESS Geosciences 2012, 2 222

The Global Stratotype Section and Point (GSSP) of the Permian–Triassic boundary, located in Meishan, Zhejiang Province, South China, was sampled bed-by-bed for ostracods. Ninety-eight species belonging to forty-three genera are recognized. Twenty-five new species are described and figured: Bairdia bassoni Crasquin sp. nov., B. broutini Crasquin sp. nov., B. deweveri Crasquin sp. nov., B. fangnianqiaoi Crasquin sp. nov., B. gaelleae Crasquin sp. nov., B. limatusformis Forel sp. nov., B. paussi Crasquin sp. nov., B. pierrevalentini Crasquin sp. nov., B. wushunbaoi Crasquin sp. nov., Baschkirina ballei Crasquin sp. nov., B. huzhouensis Forel sp. nov., Basslerella annesophieae Crasquin sp. nov., Hollinella martensiformis Crasquin sp. nov., Kempfina taihuensis Forel sp. nov., Knightina hongfui Crasquin sp. nov., Liuzhinia praeantalyaensis Forel sp. nov., Microcheilinella rectodorsata Forel sp. nov., M. shicheni Crasquin sp. nov., M.? multinodosa Forel sp. nov., Orthobairdia lemairei Crasquin sp. nov., Paraparc...

Ostracods (Crustacea) are benthic inhabitants well known for their consistent qualities as paleoenvironment markers. In particular, they are reliable indicators of water oxygenation level: filter feeders are more common in poor oxygen conditions, contrasting with deposit feeders, which are abundant in well-oxygenated settings. In the Permian/Triassic (P/Tr) boundary transition in the Great Bank of Guizhou, ostracod species are dominated by deposit feeders, showing well-oxygenated conditions from the latest Permian, through the extinction level into the earliest Triassic. These results are consistent with ostracod faunas from northwest Guangxi Province. However, these two examples are in contrast with coeval ostracods from Sichuan, which show lower-than-normal oxygen levels in the earliest Triassic. The Great Bank of Guizhou forms an isolated platform in the large Nanpanjiang Basin on the south side of the South China Block; northwest Guangxi is nearby, in a marginal setting: both faced the Panthalassa Oce...

International audience