Highlights 2017

En datant par le ¹⁴C, les granules de vers de terre retrouvées dans une séquence de loess, une équipe pluridisciplinaire de chercheurs a permis de donner, pour la première fois, un cadre chronologique à toutes les phases de redoux de la dernière période glaciaire.

Alors que les événements de Dansgaard-Oeschger (succession de réchauffement (interstade)- refroidissement (stade) au cours de la dernière période glaciaire) ont été reconnus dans les carottes de glace et plusieurs carottes de sédiments marins, leur systématique présence et leur datation précise étaient encore à révéler sur le continent. C'est maintenant chose faite !  La séquence de loess de Nussloch (vallée du Rhin) était connue depuis plusieurs décennies comme l'enregistrement le plus complet de ces alternances climatiques du glaciaire, avec les phases de réchauffement caractérisées par des pseudo-sols, les gleys de toundra. Pendant les interstades, le développement des vers de terre s'est traduit par la présence d'excrétat carbonaté, les "granules". Tous les gleys ont été datés par datation ¹⁴C sur les granules. Tous les interstades enregistrés dans les glaces ont trouvé leur pendant sur le continent dans les gleys de Nussloch. Quelques horizons faiblement exprimés du Dernier Maximum Glaciaire se retrouvent même sans interstade contemporain au Groenland. Ces résultats révèlent la haute sensibilité des environnements loessiques aux changements climatiques glaciaires et une dynamique climatique plus complexe aux moyennes qu’aux hautes latitudes entre -27 000 et -20 000 ans. En perspective, cette étude ouvre les portes de reconsttitution des gradients paléoenvironnementaux régionaux, voire continentaux, propres à chaque fluctuation climatique glaciaire.

Toute la variabilité climatique du dernier glaciaire -  Moine

Abstract: The Arctic is among the fastest warming regions on Earth, but it is also one with limited spatial coverage of multi-decadal instrumental surface air temperature measurements. Consequently, atmospheric reanalyses are relatively unconstrained in this region, resulting in a large spread of estimated 30-year recent warming trends, which limits their use to investigate the mechanisms responsible for this trend.

Here, we present a surface temperature reconstruction over 1982-2011 at NEEM (51^{∘} W, 77^{∘} N), in North Greenland, based on the inversion of borehole temperature and inert gas isotope data. We find that NEEM has warmed by 2.7±0.33∘C over the past 30 years, from the long-term 1900-1970 average of -28.55±0.29^{∘}C. The warming trend is principally caused by an increase in downward longwave heat flux. Atmospheric reanalyses underestimate this trend by 17%, underlining the need for more in situ observations to validate reanalyses.

Authors : Anais J. Orsi, Kenji Kawamura, Valérie Masson-Delmotte, Xavier Fettweis, Jason E.Box, Dorthe Dahl-Jensen, Gary D. Clow, Amaelle Landais, Jeffrey P. Severinghaus

Reference :  Geophysical Research Letters, doi: 10.1002/2016gl072212, 2017

Storms and tsunamis, which may seriously endanger human society, are amongst the most devastating marine catastrophes that can occur in coastal areas. Many such events are known and have been reported for the Mediterranean. In a sediment core from the Mar Menor (SE Spain), we discovered eight coarse-grained layers which document marine incursions during periods of intense storm activity or tsunami events. Based on  radiocarbon dating, these extreme events occurred around 5250, 4000,  3600,  3010,  2300,  1350,  650,  and  80  years  cal BP. No comparable events have been observed during the 20thand 21st centuries. The results  indicate  little  likelihood  of a  tsunami  origin  for  these  coarse-grained  layers,  although historical tsunami events are recorded in this region. These periods  of  surge  events  seem  to  coincide  with  the  coldest periods  in  Europe  during  the late Holocene,  suggesting  a control by a  climatic mechanism  for periods of increased storm activity. Spectral analyses performed on the sand percentage revealed 4 major periodicities of 1228±327, 732±80, 562 ±58 and 319±16 years. Amongst the well -known proxies that have revealed a millennial-scale climate variability during the Holocene, the ice-rafted debris (IRD) indices in the North Atlantic developed by Bond et al. (1997, 2001) present a cyclicity of 1470±500 years, which matches the 1228±327-year periodicity evi-denced in the Mar Menor, considering the respective uncertainties in the periodicities. Thus,  an  in-phase  storm  activity  in  the  western  Mediterranean  is  found  with  the  coldest  periods  in  Europe  and with the North Atlantic thermohaline circulation. However, further investigations, such as additional coring and high-resolution coastal imagery, are needed to better constrain the main cause of these multiple events.

L. Dezileau, A. Pérez-Ruzafa, P.  Blanchemanche, J.-P. Degeai, O. Raji, P. Martinez, C. Marcos,  U. von Grafenstein

Clim. Past, 12, 1389–1400.

As part of its ongoing project on repositories for high-activity, long-lived radioactive waste, a 2000 m deep borehole was drilled by the French Nuclear Waste Agency (ANDRA) in the layered structure of alternating aquifers and aquitards of the Eastern Paris Basin. Among the information retrieved from this borehole, the vertical distribution of chloride in porewaters showed that, in addition to vertical diffusion, lateral advection in the aquifers plays a major part in transporting chlorine away from the study area. Helium concentrations were also measured in porewaters along the borehole. Because the helium input function is different from that of chlorine, it represents an excellent alternative tracer to further constrain transport characteristics. We applied an advection–diffusion model to the helium profiles with the appropriate source term for ⁴He based on U–Th measured concentrations of uranium and thorium. ⁴⁰Ar/³⁶Ar data, which were available along the whole sequence, were also simulated. The modelled and measured ⁴He profiles were in good agreement, indicating that the transport parameters used for the chlorine simulations were robust. ⁴⁰Ar/³⁶Ar simulations also gave coherent results and confirmed that most of the radiogenic ⁴⁰Ar remained trapped in the rocks (primarily in clays and feldspars).

P. Jean-Baptiste, B. Lavielle, E. Fourré, T. Smith, M. Pagel

Geological Society Special Publications 443, (2016) 2017

Water stable isotopes in central Antarctic ice cores are critical to quantify past temperature changes. Accurate temperature reconstructions require to understand the processes controlling surface snow isotopic composition. Isotopic fractionation processes occurring in the atmosphere and controlling snowfall isotopic composition are well understood theoretically and implemented in atmospheric models. However, post-deposition processes are poorly documented and understood. To quantitatively interpret the isotopic composition of water archived in ice cores, it is thus essential to study the continuum between surface water vapour, precipitation, surface snow and buried snow.

Here, we target the isotopic composition of water vapour at Concordia Station, where the oldest EPICA Dome C ice cores have been retrieved. While snowfall and surface snow sampling is routinely performed, accurate measurements of surface water vapour are challenging in such cold and dry conditions. New developments in infrared spectroscopy enable now the measurement of isotopic composition in water vapour traces. Two infrared spectrometers have been deployed at Concordia, allowing continuous, in situ measurements for one month in December-January 2014-2015. Comparison of the results from infrared spectroscopy with laboratory measurements of discrete samples trapped using cryogenic sampling validates the relevance of the method to measure isotopic composition in dry conditions. We observe very large diurnal cycles in isotopic composition well correlated with temperature diurnal cycles. Identification of different behaviours of isotopic composition in the water vapour associated with turbulent or stratified regime indicates a strong impact of meteorological processes in local vapour/snow interaction. Even if the vapour isotopic composition seems to be, at least part of the time, at equilibrium with the local snow, the slope of δD against δ18O prevents us from identifying a unique origin leading to this isotopic composition.

Mathieu Casado,  Amaelle Landais  , Valérie Masson-Delmotte, Christophe Genthon,  Erik Kerstel, Samir Kassi, Laurent Arnaud, Ghislain Picard, Frederic Prie, Olivier Cattani, Hans-Christian Steen-Larsen, Etienne Vignon, Peter Cermak

Atmospheric Chemistry and Physics, Vol. 16, pages 8521-8538, 20163