David Bekaert

@article{Bekaert_etal2023,

title = {Last glacial maximum cooling of 9 textdegreeC in continental Europe from a 40 kyr-long noble gas paleothermometry record},

author = {D. V. Bekaert and P. H. Blard and Y. Raoult and R. Pik and R. Kipfer and A. M. Seltzer and E. Legrain and B. Marty},

doi = {10.1016/j.quascirev.2023.108123},

year  = {2023},

date = {2023-01-01},

journal = {Quaternary Science Reviews},

volume = {310},

pages = {108123},

abstract = {The Last Glacial Maximum (LGM; \^{a}`u26--18 kyr ago) is a time interval of great climatic interest characterized by substantial global cooling driven by radiative forcings and feedbacks associated with orbital changes, lower atmospheric CO2, and large ice sheets. However, reliable proxies of continental paleotemperatures are scarce and often qualitative, which has limited our understanding of the spatial structure of past climate changes. Here, we present a quantitative noble gas temperature (NGT) record of the last \^{a}`u40 kyr from the Albian aquifer in Eastern Paris Basin (France, \^{a}`u48textdegreeN). Our NGT data indicate that the mean annual surface temperature was \^{a}`u5 textdegreeC during the Marine Isotope Stage 3 (MIS3; \^{a}`u40--30 kyr ago), before cooling to \^{a}`u2 textdegreeC during the LGM, and warming to \^{a}`u11 textdegreeC in the Holocene, which closely matches modern ground surface temperatures in Eastern France. Combined with water stable isotope analyses, NGT data indicate $delta$D/NGT and $delta$18O/NGT transfer functions of +1.6 textpm 0.4texttenthousand/textdegreeC and +0.18 textpm 0.04texttenthousand/textdegreeC, respectively. Our noble-gas derived LGM cooling of \^{a}`u9 textdegreeC (relative to the Holocene) is consistent with previous studies of noble gas paleothermometry in European groundwaters but larger than the low-to-mid latitude estimate of 5.8 textpm 0.6 textdegreeC derived from a compilation of noble gas records, which supports the notion that continental LGM cooling was more extreme at higher latitudes. While an LGM cooling of \^{a}`u9 textdegreeC in Eastern France appears compatible with recent data assimilation studies, this value is greater than most estimates from current-generation climate model simulations of the LGM. Comparing our estimate for the temperature in Eastern France during MIS3 (6.4 textpm 0.5 textdegreeC) with GCM outputs presents a promising avenue to further evaluate climate model simulations and constrain European climate evolution over the last glacial cycle.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Piralla_etal2023,

title = {A unified chronology of dust formation in the early solar system},

author = {M. Piralla and J. Villeneuve and N. Schnuriger and D. V. Bekaert and Y. Marrocchi},

doi = {10.1016/j.icarus.2023.115427},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {Icarus},

volume = {394},

pages = {115427},

abstract = {The chronology of dust formation in the early solar system remains controversial. Chondrules are the most abundant high-temperature objects formed during the evolution of the circumsolar disk. Considering chondrule formation, absolute lead‑lead (Pb--Pb) ages and aluminum‑magnesium (26Al--26Mg) ages relative to calcium‑aluminum-rich inclusions (CAIs) provide inconsistent chronologies, with Pb--Pb ages showing early and protracted chondrule formation episodes whereas 26Al--26Mg ages suggest that chondrule production was delayed by \>1.5 Ma. Here, we develop a new method to precisely determine in situ 26Al--26Mg ages of spinelbearing chondrules, which are not affected by secondary asteroidal processes. Our data demonstrate that 26Al--26Mg chondrule formation ages are actually 1 Ma older than previously thought and extend over the entire lifetime of the disk. This shift in chondrule formation ages relative to CAIs, however, is not sufficient to reconcilethe Pb--Pb and 26Al--26Mg chronologies of chondrule and achondrite formation. Thus, either chondrules’Pb--Pb ages and volcanic achondrites’ 26Al--26Mg ages are incorrect or the age of CAIs should be reevaluated at 4,568.7 Ma to ensure consistency between chronometers. We favor the second hypothesis, given that (i) thecanonical age of CAIs was determined using only 4 specimens and (ii) older ages of 4,568.2 Ma have also been measured. We show that the adoption of 4,568.7 Ma as the new canonical age of CAIs and the use of our new spinel-derived 26Al--26Mg ages enable reconciling the Pb--Pb and 26Al--26Mg ages of chondrules and achondrites.This new chronology implies the existence of a 0.7--1 Ma gap between the formation of refractory inclusions and chondrules, and supports the homogeneous distribution of 26Al in the circumsolar disk.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Broadley_etal2022,

title = {Origin of life-forming volatile elements in the inner Solar System},

author = {M. W. Broadley and D. V. Bekaert and L. Piani and E. F\"{u}ri and B. Marty},

doi = {10.1038/s41586-022-05276-x},

year  = {2022},

date = {2022-01-01},

journal = {Nature},

volume = {611},

pages = {245--255},

abstract = {Volatile elements such as hydrogen, carbon, nitrogen and oxygen are essential ingredients to build habitable worlds like Earth, but their origin and evolution on terrestrial planets remain highly debated. Here we discuss the processes that distributed these elements throughout the early Solar System and how they then became incorporated into planetary building blocks. Volatiles on Earth and the other terrestrial planets appear to have been heterogeneously sourced from different Solar System reservoirs. The sources of planetary volatiles and the timing at which they were accreted to growing planets probably play a crucial role in controlling planet habitability.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Almayrac_etal2022,

title = {The EXCITING experiment exploring the behavior of nitrogen and noble gases in interstellar ice analogs},

author = {M. G. Almayrac and D. V. Bekaert and M. W. Broadley and D. J. Byrne and L. Piani and B. Marty},

doi = {10.3847/PSJ/ac98b0},

year  = {2022},

date = {2022-01-01},

journal = {The Planetary Science Journal},

volume = {3},

pages = {252},

abstract = {Comets represent some of the most pristine bodies in our solar system and can provide a unique insight into the chemical makeup of the early solar system. Due to their icy volatile-rich nature, they may have played an important role in delivering volatile elements and organic material to the early Earth. Understanding how comets form can therefore provide a wealth of information on how the composition of volatile elements evolved in the solar system from the presolar molecular cloud up until the formation of the terrestrial planets. Because noble gases are chemically inert and have distinct condensation temperatures, they can be used to infer the temperatures of formation and thermal history of cometary ices. In this work, we present a new experimental setup called EXCITING to investigate the origin and formation conditions of cometary ices. By trapping nitrogen and noble gases in amorphous water ice, our experiment is designed to study the elemental and isotopic behavior of volatile elements in cometary ice analogs. We report new results of noble gas and nitrogen enrichment in cometary ice analogs and discuss the limitations of the experimental conditions in light of those supposed for comets. We show that forming ice analogs at \^{a}`u70 K best reproduce the noble gas and N2 abundances of comet 67P/Churyumov--Gerasimenko, considering a solar-like starting composition. This formation temperature is higher than previous estimates for cometary ices and suggests that the formation of cometary building blocks may have occurred in the protosolar nebula rather than in the colder molecular cloud.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Piralla_etal2020,

title = {Primordial water and dust of the Solar System: Insights from in situ oxygen measurements of CI chondrites},

author = {M. Piralla and Y. Marrocchi and M. J. Verdier-Paoletti and L. G. Vacher and J. Villeneuve and L. Piani and D. V. Bekaert and M. Gounelle},

doi = {10.1016/j.gca.2019.10.041},

year  = {2020},

date = {2020-01-01},

journal = {Geochimica et Cosmochimica Acta},

volume = {269},

pages = {451--464},

abstract = {As the chemical compositions of CI chondrites closely resemble that of the Suntextquoterights photosphere, their oxygen isotopic compositions represent a powerful tool to constrain the origin and dynamics of dust and water ice grains in the protoplanetarydisk. However, parent-body alteration processes make straightforward estimation of the primordial isotopic compositions of CI chondritic water and anhydrous minerals difficult. In this contribution, we used in situ SIMS measurements to determinethe oxygen isotope compositions of mechanically isolated olivine and carbonate grains from the CI chondrite Orgueil and carbonates in a polished section of the CI chondrite Ivuna. Most CI olivine grains have Earth-like O isotopic compositions(D17O ≈ 0texttenthousand) plotting at the intersection of the terrestrial fractionation line and the primitive chondrule minerals line. Ca-carbonates from Orgueil and Ivuna define a trend with d17O = (0.50 textpm 0.05) x d18O + (0.9 textpm 1.4) that differs from massindependent variations observed in secondary phases of other carbonaceous chondrites. These data show that CIs are chemically solar but isotopically terrestrial for oxygen isotopes. This supports models suggesting that primordial Solar System dust was 16O-poor (D17O ≈ 0texttenthousand) relative to the 16O-rich nebular gas. Based on results, mass balance calculations reveal that the pristine O isotopic compositions of carbonaceous chondrite matrices differ significantly from the CI composition, except for CR chondrites (calculated D17O values of CM, CO, CV and CR matrices being --3.97 textpm 1.19texttenthousand, --4.33 textpm 1.45texttenthousand, --7.95textpm 1.95texttenthousand, and --0.07 textpm 1.16texttenthousand, respectively). This confirms an open chondrule-matrix system with respect to oxygen isotopes where chondrule compositions reflect complex processes of chondrule precursor recycling and gas-melt interactions. As the Mg-Si-Fe chondrule budget is also partially controlled by gas-melt interactions, the complementary formation of chondrules and matrix from a single solar-like reservoir -if it exists- require that (i) this reservoir must have been in a closed system with the gas or (ii) the gas had a CI composition to satisfy the elemental mass balance.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Labidi_etal2020,

title = {Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen},

author = {J. Labidi and P. H. Barry and D. V. Bekaert and M. W. Broadley and B. Marty and T. Giunta and O. Warr and B. S. Sherwood and T. P. Fischer and G. Avice and A. Caracausi and C. J. Ballentine and S. A. Halldorsson and A. Stefansson and M. D. Kurz and I. E. Kohl and E. D. Young},

doi = {10.1038/s41586-020-2173-4},

year  = {2020},

date = {2020-01-01},

journal = {Nature},

volume = {580},

pages = {367--371},

abstract = {Nitrogen is the main constituent of the Earthtextquoterights atmosphere, but its provenance in the Earthtextquoterights mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earthtextquoterights accretion versus that subducted from the Earthtextquoterights surface is unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle $delta$15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative $delta$15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember $delta$15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased $delta$15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has $delta$15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume $delta$15N values may both be dominantly primordial features.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Broadley_etal2020_2,

title = {Noble gas variations in ureilites and their implications for ureilite parent body formation},

author = {M. W. Broadley and D. V. Bekaert and B. Marty and A. Yamaguchi and J. A. Barrat},

doi = {10.1016/j.gca.2019.11.032},

year  = {2020},

date = {2020-01-01},

journal = {Geochimica et Cosmochimica Acta},

volume = {270},

pages = {325--337},

abstract = {Ureilites are equilibrated carbon-rich olivine-pyroxene rocks from the partially melted mantle of a large (\>500 km diameter) heterogeneous parent body. Recently the ureilite parent body was interpreted as an incomplete mixture of material from two carbon-rich chondritic reservoirs, one (Mg-rich) with reduced iron, low $Delta$17O and low $delta$13C, and the other with oxidised iron, high $Delta$17O and high $delta$13C. Here we analyse noble gases (Ar, Kr and Xe) in six equilibrated (unbrecciated) ureilites from Northwest Africa (NWA 2236, NWA 7686, NWA 8049, NWA 8172, NWA 11032 and NWA 11368). We observe weak positive and negative correlations of $Delta$17O and Mg# with the elemental ratios of Ar/Xe and Kr/Xe, respectively, as well as a weak positive correlation of Mg# with the heavy isotopes of Xe. These correlations broadly support the idea of the two-component mixing hypothesis. Our analyses further suggest that the Mg-rich endmember was rich in Xe from presolar grains (HL-Xe) while the Mg-poorer component may have contained solar-derived noble gases. The observed correlations are less straightforward to reconcile with a recent model for the origin of the ureilite parent body, involving oxidation of metal by H2O from accreted ice with textquoteleftheavytextquoteright oxygen isotopes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2020_2,

title = {The origin and fate of volatile elements on Earth revisited in light of noble gas data obtained from comet 67P/Churyumov-Gerasimenko},

author = {D. V. Bekaert and M. W. Broadley and B. Marty},

doi = {10.1038/s41598-020-62650-3},

year  = {2020},

date = {2020-01-01},

journal = {Scientific Reports},

volume = {10},

pages = {5796},

abstract = {The origin of terrestrial volatiles remains one of the most puzzling questions in planetary sciences. The timing and composition of chondritic and cometary deliveries to Earth has remained enigmatic due to the paucity of reliable measurements of cometary material. This work uses recently measured volatile elemental ratios and noble gas isotope data from comet 67P/Churyumov-Gerasimenko (67P/C-G), in combination with chondritic data from the literature, to reconstruct the composition of Earthtextquoterights ancient atmosphere. Comets are found to have contributed textasciitilde20% of atmospheric heavy noble gases (i.e., Kr and Xe) but limited amounts of other volatile elements (water, halogens and likely organic materials) to Earth. These cometary noble gases were likely mixed with chondritic ?\u{I} and not solar ?\u{I} sources to form the atmosphere. We show that an ancient atmosphere composed of chondritic and cometary volatiles is more enriched in Xe relative to the modern atmosphere, requiring that 8--12 times the present-day inventory of Xe was lost to space. This potentially resolves the long-standing mystery of Earthtextquoterights textquotelefttextquoteleftmissing xenontextquoterighttextquoteright, with regards to both Xe elemental depletion and isotopic fractionation in the atmosphere. The inferred Kr/H2O and Xe/H2O of the initial atmosphere suggest that Earthtextquoterights surface volatiles might not have been fully delivered by the late accretion of volatile-rich carbonaceous chondrites. Instead, textquotelefttextquoteleftdrytextquoterighttextquoteright materials akin to enstatite chondrites potentially constituted a significant source of chondritic volatiles now residing on the Earthtextquoterights surface. We outline the working hypotheses, implications and limitations of this model in the last section of this contribution},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2020,

title = {Xenon isotopes in Archean and Proterozoic insoluble organic matter: A robust indicator of syngenecity?},

author = {D. V. Bekaert and M. W. Broadley and F. Delarue and Z. Druzhinina and G. Paris and F. Robert and K. Sugitani and B. Marty},

doi = {10.1016/j.precamres.2019.105505},

year  = {2020},

date = {2020-01-01},

journal = {Precambrian Research},

volume = {336},

pages = {105505},

abstract = {Insoluble organic materials (kerogens) isolated from ancient sedimentary rocks provide unique insights into the evolution of early life. However, establishing whether these kerogens are indeed syngenetic with the deposition of associated sedimentary host rocks, or contain contribution from episodes of secondary deposition, is not straightforward. Novel geochemical criterions are therefore required to test the syngenetic origin of Archean organic materials. On the one hand, the occurrence of mass-independent fractionation of sulphur isotopes (MIF-S) provides a tool to test the Archean origin of ancient sedimentary rocks. Determining the isotope composition of sulphur within kerogens whilst limiting the contribution from associated minerals (e.g., nano-pyrites) is however challenging. On the other hand, the Xe isotope composition of the Archean atmosphere has been shown to present enrichments in the light isotopes relative to its modern composition, together with a mono-isotopic deficit in 129Xe. Given that the isotopic composition of atmospheric Xe evolved through time by mass dependent fractionation (MDF) until textasciitilde2.5 to 2.0 Ga, the degree of MDF of Xe isotopes trapped in kerogens could provide a time stamp for the last chemical equilibration between organic matter and the atmosphere. However, the extent to which geological processes could affect the signature of Xe trapped in ancient kerogen remains unclear. In this contribution, we present new Ar, Kr and Xe isotopic data for four kerogens isolated from 3.4 to 1.8 Gy-old cherts and confirm that Xe isotopes from the Archean atmosphere can be retained within kerogens. However, new Xe-derived model ages are lower than expected from the ages of host rocks, indicating that initially trapped Xe components were at least partially lost and/or mixed together with some Xe carried out by younger generations of organic materials, therefore complicating the Xe-based dating method. Whilst non-null $Delta$33S values and 129Xe deficits relative to modern atmosphere constitute reliable imprints from the Archean atmosphere, using Xe isotopes to provide information on the syngenetic origin of ancient organic matter appears to be a promising -- but not unequivocal -- tool that calls for further analytical development.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Marty_etal2019,

title = {Geochemical evidence for high volatile fluxes from the mantle at the end of the Archaean},

author = {B. Marty and D. V. Bekaert and M. W. Broadley},

doi = {10.1038/s41586-019-1745-7},

year  = {2019},

date = {2019-01-01},

journal = {Nature},

volume = {575},

number = {4865},

pages = {485--488},

abstract = {The exchange of volatile species---water, carbon dioxide, nitrogen and halogens---between the mantle and the surface of the Earth has been a key driver of environmental changes throughout Earthtextquoterights history. Degassing of the mantle requires partial melting and is therefore linked to mantle convection, whose regime and vigour in the Earthtextquoterights distant past remain poorly constrained1,2. Here we present direct geochemical constraints on the flux of volatiles from the mantle. Atmospheric xenon has a monoisotopic excess of 129Xe, produced by the decay of extinct 129I. This excess was mainly acquired during Earthtextquoterights formation and early evolution3, but mantle degassing has also contributed 129Xe to the atmosphere through geological time. Atmospheric xenon trapped in samples from the Archaean eon shows a slight depletion of 129Xe relative to the modern composition4,5, which tends to disappear in more recent samples5,6. To reconcile this deficit in the Archaean atmosphere by mantle degassing would require the degassing rate of Earth at the end of the Archaean to be at least one order of magnitude higher than today. We demonstrate that such an intense activity could not have occurred within a plate tectonics regime. The most likely scenario is a relatively short (about 300 million years) burst of mantle activity at the end of the Archaean (around 2.5 billion years ago). This lends credence to models advocating a magmatic origin for drastic environmental changes during the Neoarchaean era, such as the Great Oxidation Event.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Marty_etal2018,

title = {Salinity of the Archaean oceans from analysis of fluid inclusions in quartz},

author = {B. Marty and G. Avice and D. V. Bekaert and M. W. Broadley},

doi = {10.1016/j.crte.2017.12.002},

year  = {2018},

date = {2018-01-01},

journal = {Comptes Rendus Geoscience},

volume = {350},

pages = {154--163},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Marrocchi_etal2018,

title = {Origin and abundance of water in carbonaceous asteroids},

author = {Y. Marrocchi and D. V. Bekaert and L. Piani},

doi = {10.1016/j.epsl.2017.10.060},

year  = {2018},

date = {2018-01-01},

journal = {Earth and Planetary Science Letters},

volume = {482},

pages = {23--32},

abstract = {The origin and abundance of water accreted by carbonaceous asteroids remains underconstrained, but would provide important information on the dynamic of the protoplanetary disk. Here we report the in situoxygen isotopic compositions of aqueously formed fayalite grains in the Kaba and Mokoia CV chondrites. CV chondrite bulk, matrix and fayalite O-isotopic compositions define the mass-independent continuous trend ($delta$17O =0.84 textpm0.03 texttimes$delta$18O −4.25 textpm0.1), which shows that the main process controlling the O-isotopic composition of the CV chondrite parent body is related to isotopic exchange between 16O-rich anhydrous silicates and 17O-and 18O-rich fluid. Similar isotopic behaviors observed in CM, CR and CO chondrites demonstrate the ubiquitous nature of O-isotopic exchange as the main physical process in establishing the O-isotopic features of carbonaceous chondrites, regardless of their alteration degree. Based on these results, we developed a new approach to estimate the abundance of water accreted by carbonaceous chondrites (quantified by the water/rock ratio) with CM (0.3--0.4) �WCR (0.1--0.4) �WCV (0.1--0.2) \>CO (0.01--0.10). The low water/rock ratios and the O-isotopic characteristics of secondary minerals in carbonaceous chondrites indicate they (i) formed in the main asteroid belt and (ii)accreted a locally derived (inner Solar System) water formed near the snowline by condensation from the gas phase. Such results imply low influx of D-and 17O-and 18O-rich water ice grains from the outer part of the Solar System. The latter is likely due to the presence of a Jupiter-induced gap in the protoplanetary disk that limited the inward drift of outer Solar System material at the exception of particles with size lower than 150$mu$m such as presolar grains. Among carbonaceous chondrites, CV chondrites show O-isotopic features suggesting potential contribution of 17--18O-rich water that may be related to their older accretion relative to other hydrated carbonaceous chondrites.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Boucher_etal2018,

title = {Spatial analysis of the atmospheric helium isotopic composition: Geochemical and environmental implications},

author = {C. Boucher and T. Lan and J. Mabry and D. V. Bekaert and P. G. Burnard and B. Marty},

doi = {10.1016/j.gca.2018.06.010},

year  = {2018},

date = {2018-01-01},

journal = {Geochimica et Cosmochimica Acta},

volume = {237},

pages = {120--130},

abstract = {Spatial variations in the atmospheric helium isotopic composition (RA = 3He/4Heair = 1.39 texttimes 10−6) might be induced by localized and/or regional inputs of 3He and/or 4He into the air. It has been suggested that inputs of 4He from hydrocarbon exploitation may generate latitudinal variations in atmospheric 3He/4He. In order to test the possibility of such global variations, we performed high precision analyses of the helium isotopic composition (3He/4He) of sixteen air samples collected in 500 cc metal bottles around the world (79textdegreeN--75textdegreeS) between 2013 and 2015. In most cases, the 3He/4He of these air samples are indistinguishable from that of air sampled around Nancy (France) within ≤2texttenthousand (95% confidence interval). Only two air samples, collected at Dome C (Antarctica) and at Tokyo (Japan), exhibit statistically higher 3He/4He ratios interpreted as potential 3He excesses of (2.0 textpm 1.4)texttenthousand and (1.7 textpm 1.5)texttenthousand (95% confidence interval), respectively. Excesses such as these suggest that potential helium isotopic variations in air are likely lower than 4texttenthousand and might be generated temporarily by regional and/or local phenomenon (e.g. auroral precipitation, stratospheric to tropospheric exchanges). Thus, this study supports the use of the atmospheric helium isotopic ratio as an inter-laboratory He standard.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2018_4,

title = {High-temperature ionization-induced synthesis of biologically relevant molecules in the protosolar nebula},

author = {D. V. Bekaert and S. Derenne and L. Tissandier and Y. Marrocchi and S. Charnoz and C. Anquetil and B. Marty},

doi = {10.3847/1538-4357/aabe7a},

year  = {2018},

date = {2018-01-01},

journal = {Astrophysical Journal},

volume = {859},

number = {142},

abstract = {Biologically relevant molecules (hereafter biomolecules) have been commonly observed in extraterrestrial samples, but the mechanisms accounting for their synthesis in space are not well understood. While electron-driven production of organic solids from gas mixtures reminiscent of the photosphere of the protosolar nebula (PSN; i.e., dominated by CO--N2--H2) successfully reproduced key specific features of the chondritic insoluble organic matter (e.g., elementary and isotopic signatures of chondritic noble gases), the molecular diversity of organic materials has never been investigated. Here, we report that a large range of biomolecules detected in meteorites and comets can be synthesized under conditions typical of the irradiated gas phase of the PSN at temperatures=800 K. Our results suggest that organic materials---including biomolecules---produced within the photosphere would have been widely dispersed in the protoplanetary disk through turbulent diffusion, providing a mechanism for the distribution of organic meteoritic precursors prior to any thermal/photoprocessing and subsequent modification bysecondary parent body processes. Using a numerical model of dust transport in a turbulent disk, we propose that organic materials produced in the photosphere of the disk would likely be associated with small dust particles, which are coupled to the motion of gas within the disk and therefore preferentially lofted into the upper layers of the disk where organosynthesis occurs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2018_3,

title = {Archean kerogen as a new tracer of atmospheric evolution: Implications for dating the widespread nature of early life},

author = {D. V. Bekaert and M. W. Broadley and F. Delarue and G. Avice and F. Robert and B. Marty},

doi = {10.1126/sciadv.aar2091},

year  = {2018},

date = {2018-01-01},

journal = {Science Advances},

volume = {4},

number = {2},

pages = {1--8},

abstract = {Understanding the composition of the Archean atmosphere is vital for unraveling the origin of volatiles and the environmental conditions that led to the development of life. The isotopic composition of xenon in the Archean atmosphere has evolved through time by mass-dependent fractionation from a precursor comprising cometary and solar/chondritic contributions (referred to as U-Xe). Evaluating the composition of the Archean atmosphere is challenging because limited amounts of atmospheric gas are trapped within minerals during their formation. We show that organic matter, known to be efficient at preserving large quantities of noble gases, can be used as a new archive of atmospheric noble gases. Xe isotopes in a kerogen isolated from the 3.0--billion-year--old Farrel Quartzite (Pilbara Craton, Western Australia) are mass fractionated by 9.8 textpm 2.1 per mil (texttenthousand) (2$sigma$) per atomic mass unit, in line with a progressive evolution toward modern atmospheric values. Archean atmospheric Xe signatures in kerogens open a new avenue for following the evolution of atmospheric composition through time. The degree of mass fractionation of Xe isotopes relative to the modern atmosphere can provide a time stamp for dating Archean kerogens and therefore narrowing the time window for the diversification of early life during the Archean eon.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2018_2,

title = {Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016},

author = {D. V. Bekaert and G. Avice and B. Marty},

doi = {10.1111/maps.13069},

year  = {2018},

date = {2018-01-01},

journal = {Meteoritics \& Planetary Science},

pages = {1--14},

abstract = {Abstract--Lunar basalt 15016 ( 3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm-sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO2 laser. No magmatic/primordial component could be identified ; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained 0.2%, 0.02%, 0.002%, and 0.02% of the total amount of cosmogenic 21Ne, 38Ar, 83Kr, and 126Xe, respectively, produced over the basalttextquoterights 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/ minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for 3He and 21Ne, and to a lesser extent for 36Ar (Signer et al. 1977), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Avice_etal2018,

title = {Noble gases and nitrogen in Tissint reveal the composition of the Mars atmosphere},

author = {G. Avice and D. V. Bekaert and H. Chennaoui Aoudjehane and B. Marty},

doi = {10.7185/geochemlet.1802},

year  = {2018},

date = {2018-01-01},

journal = {Geochemical Perspectives Letters},

volume = {6},

pages = {11--16},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Marty_etal2017,

title = {Xenon isotopes in 67P/Churyumov- Gerasimenko show that comets contributed to Earthtextquoterights atmosphere},

author = {B. Marty and K. Altwegg and Balsigern H. and A. Bar-Nun and D. V. Bekaert and J. J. Berthelier and A. Bieler and C. Briois and U. Calmonte and M. Combi and J. De Keyser and B. Fiethe and S. A. Fuselier and S. Gasc and T. I. Gombosi and K. C. Hansen and M. H\"{a}ssig and A. J\"{a}ckel and E. Kopp and A. Korth and L. Le Roy and U. Mall and O. Mousis and T. Owen and H. R`eme and M. Rubin and T. S\'{e}mon and C. Y. Tzou and J. H. Waite and P. Wurz},

doi = {10.1126/science.aal3496},

year  = {2017},

date = {2017-01-01},

journal = {Science},

number = {356},

pages = {1069--1072},

abstract = {The origin of cometary matter and the potential contribution of comets to inner-planet atmospheres are long-standing problems. During a series of dedicated low-altitude orbits, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta spacecraft analyzed the isotopes of xenon in the coma of comet 67P/Churyumov-Gerasimenko. The xenon isotopic composition shows deficits in heavy xenon isotopes and matches that of a primordial atmospheric component. The present-day Earth atmosphere contains 22 textpm 5% cometary xenon, in addition to chondritic (or solar) xenon.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bekaert_etal2017,

title = {Stepwise heating of lunar anorthosites 60025, 60215, 65315 possibly reveals an indigenous noble gas component on the Moon},

author = {D. V. Bekaert and G. Avice and B. Marty and B. Henderson and M. S. Gudipati},

doi = {10.1016/j.gca.2017.08.041},

year  = {2017},

date = {2017-01-01},

journal = {Geochimica et Cosmochimica Acta},

volume = {218},

pages = {114--131},

abstract = {Despite extensive effort during the last four decades, no clear signature of a lunar indigenous noble gas component has been found. In order to further investigate the possible occurrence of indigenous volatiles in the Moon, we have reanalyzed the noble gas and nitrogen isotopic compositions in three anorthosite samples. Lunar anorthosites 60025, 60215 and 65315 have the lowest exposure duration (textasciitilde2 Ma) among Apollo samples and consequently contain only limited cosmogenic (e.g.124,126Xe) and solar wind (SW) noble gases. Furthermore, anorthosites have negligible contributions of fissiogenic Xe isotopes because of their very low Pu and U contents. As observed in previous studies (Lightner and Marti, 1974; Leich and Niemeyer, 1975), lunar anorthosite Xe presents an isotopic composition very close to that of terrestrial atmospheric Xe, previously attributed to textquotelefttextquoteleftanomalous adsorptiontextquoterighttextquoteright of terrestrial Xe after sample return. The presumed atmospheric Xe contamination can only be removed by heating the samples at medium to high temperatures under vacuum, and is therefore different from common adsorption. To test this hypothesis, we monitored the adsorption of Xe onto lunar anorthositic powder using infrared reflectance spectroscopy. A clear shift in the anorthosite IR absorbance peaks is detected when comparing the IR absorbance spectra of the lunar anorthositic powder before and after exposure to a neutral Xe-rich atmosphere. This observation accounts for the chemical bonding (chemisorption) of Xe onto anorthosite, which is stronger than the common physical bonding (physisorption) and could account for the anomalous adsorption of Xe onto lunar samples. Our high precision Xe isotope analyses show slight mass fractionation patterns across 128--136 Xe isotopes with systematic deficits in the heavy Xe isotopes (mostly 136 Xe and marginally 134 Xe) that have not previously been observed. This composition could be the result of mixing between an irreversibly adsorbed terrestrial contaminant that is mostly released at high temperature and an additional signature. Solar Wind (SW) Xe contents, estimated from SW-Ne and SW-Ar concentrations and SW-Ne/Ar/Xe elemental ratios, do not support SW as the additional contribution. Using a x2 test, the latter is best accounted for by cometary Xe as measured in the coma of Comet 67P/Churyumov-Gerasimenko (Marty et al., 2017) or by the primordial U-Xe composition inferred to be the precursor of atmospheric Xe (Pepin, 1994; Avice et al., 2017). It could have been contributed to the lunar budget by volatile-rich bodies that participated to the building of the terrestrial atmosphere inventory (Marty et al., 2017).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}