The past : a compass for future earth

Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called &#8216;biological carbon pump&#8217;, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the Adélie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the Adélie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the Adélie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene

Termite graveyards. Hidden geochemical patches?

International audienceEntombment, or the production of graveyards for the disposal of dead bodies, is not only a practice of human societies but is also observed in nature, including among small invertebrates such as termites. While the influence of termites on soil dynamics has largely been studied in comparing the specific properties of their mounds and protective sheeting with those of the surrounding soil, the properties of their graveyards have never been described before. Using incipient colonies of Macrotermes natalensis reared in a controlled environment, we showed that graveyard sheeting was characterized by a much higher C content in comparison with the reference soil and protective sheeting (4.7-fold increase). As a consequence, a slight increase in the C:N ratio was measured from 8 in the reference soil to 10 in graveyard sheeting. No changes in soil particle size fractions were measured. However, lower Fe and Al contents were measured in sheeting, and micrographs obtained from scanning electron microscopy revealed the presence of calcium carbonate, or calcium oxalate crystals, in sheeting, as well as the presence of organic substances and salt crystals covering termite corpses, most likely for controlling the spread of pathogens. The presence of calcium carbonates and/or calcium oxalate was explained by the very high Ca content within termite bodies. Therefore, this study shows that termite graveyards are likely to constitute unexplored patches of nutrients in soil

Seasonal variations, origin, and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps

International audienceThe Southern Ocean plays a pivotal role in the control of atmospheric CO2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace-limiting nutrient and dissolved silicon as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here we use δ30Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ; and Sub-Antarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ30Si composition reveal a range of insights, including that (i) the sinking rate of BSi exceeds 200 m d−1 in summer in the AZ yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, (ii) occasional vertical mixing events affect the δ30Si composition of exported BSi in both the SAZ and AZ, and (iii) the δ30Si signature of diatoms is well conserved through the water column despite strong BSi and particulate organic carbon (POC) attenuation at depth and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export

Signification environnementale de guano de salanganes et de chiroptères de Nouvelle-Calédonie. Premiers résultats

sera mis en ligne en texte intégral en septembre 2019International audienceGuano accumulated in caves by bats or birds contains a stratigraphic record of the environment which may supplement or supply conventionnal terrestrial sedimentary records like lacustrine/palustrine deposits or peat. Thus, guano study allows the understanding of recent or past climate and environment changes, which ability has expanded recently with the development of more sophisticated analytical tools, based on proxies that are indirect measures of past climates or environments preserved in natural archives. In New Caledonia, diurnal cave swiftlet Collocalia spodiopygia and nocturnal Chiroptera Micropterus robustior, both insectivores, share very often the same cavities. The comparison of the content of pollen and isotopes δ13C and δD in their respective guano, shows a great variability of constituents which reflects the sources of the feeding grounds of the eaten preys and their interaction with the ecosystem, more particularly the vegetation. The high richness of pollen taxa, identical to 50 % between the two guanos, underlines the respective food resource areas. The pollen content also shows seasonal variations probably in response to changes in food availability and in turn environmental change, if not climatic changes. This paper presents a preliminary set of results obtained on the 92cm-long core HAK1 retrieved in a guano mound taken under a cave swiftlet roost site in Hama Cave from Lifou (Loyalty Islands). The comparison between yearly mean daily rainfall, carbon (δ13C) and hydrogen (δD) isotopes compositions of high-molecular weight n-alkane (n-C29) from guano shows a good covariation, which is interpreted as a proxy of ENSO occurrences. Our work confirms that the guano piles are promising records for palaeoenvironmental change studies at high temporal resolution.L'intérêt est grandissant pour le potentiel présenté par les acccumulations cavernicoles de guano de chauves-souris et d'oiseaux en tant qu'enregistrements stratigraphiques continentaux, complémentaires voire alternatifs aux archives étudiées générale-ment (sédiments lacustres, palustres, tourbeux, etc.). Ces dépôts permettent des reconstructions de la variabilité climatique à haute résolution pour des échelles de temps récentes à plus anciennes, grâce à la calibration de nouveaux proxies susceptibles d'enregistrer ces variations. Ainsi la comparaison du contenu pollinique et des traceurs isotopiques δ13 C et δD des n-alcanes du guano de différents animaux insectivores, les salanganes diurnes (Collocalia spodiopygia) et les chiroptères nocturnes (Micropterus robustior) cohabitant dans de nombreuses grottes de Nouvelle-Calédonie, met en évidence une grande variabilité. Celle-ci est à l'image des cibles de nourrissage des proies appétées et de leur interaction avec les composantes de l'écosystème, en particulier les plantes. La grande richesse du contenu pollinique observée dans les deux types de guano, bien que présentant 50 % de taxons communs, souligne des aires de nourrissage préférentielles. Elle montre aussi des fluctuations interannuelles, marqueurs probables de changements environnementaux, sinon climatiques. Cet article présente des premiers résultats obtenus sur le sondage HAK1 prélévé dans un amas de guano de salanganes de la grotte Hama sur l'île de Lifou, l'une des îles Loyautés. La comparaison entre les données de géochimie isotopique moléculaire (isotopes du carbone δ13C et de l'hydrogène δD du n-alcane C29) et les données pluviométriques met en évidence une bonne covariation interprétée comme marqueur du signal climatique ENSO contemporain des dépôts de guano analysés. Ce travail préliminaire confirme le fort potentiel des amas de guano cavernicoles pour étudier à haute résolution les changements paléo-environnementaux

Hydroclimate reconstruction during the last 1000 years inferred from the mineralogical and geochemical composition of a sediment core from Lake-Azuei (Haiti)

International audienceThis study aims to reconstruct the hydro-climatic variations over the last 1000 years in Haiti using mineralogical and geochemical composition of well dated lacustrine sediment core retrieved from Lake Azuei. The results show changes in sedimentological processes linked to environmental and climatic variations. The general pattern suggests a wetter Medieval Climate Anomaly (MCA), drier Little Ice Age (LIA), high climate variability during the MCA-LIA transition and more anthropogenic impacts that dominate natural climate during the Current Warm Period (CWP). The MCA period (~1000–1100 CE) thus appears marked by increase sedimentation rate supported by higher terrigenous input linked to erosive events particularly increases in precipitation. During the LIA, particularly from ~1450 to 1600 CE, there is a great variation toward a decrease of terrigenous input, which is related to a decrease on sedimentation rate and increase Mg-calcite precipitation, suggesting less precipitation and high evaporation respectively during dry climate conditions. The MCA-LIA transition (~1200–1400 CE) is characterized by variations between terrigenous input, Mg-calcite formation and organic matter deposition, which indicate succession of dry and humid conditions. The CWP (1800–2000 CE) shows a progressive increase on sedimentation rate and decrease of gray level, which indicate more organic matter sedimentation as consequence of anthropogenic activities in the surrounding basin of the lake. High-resolution gray level analysis, which reflects principally variations in terrigenous input, carbonate mineral formation and organic matter deposition, shows that the AMO, NAO, PDO and ENSO are the principal modes affecting the hydro-climatic changes in Haiti during the last millennium. In addition, temporal correlation of other Caribbean paleoclimate records with our geochemical and mineralogical data, suggests that trends observed in Lake Azuei were controlled by regional climate, likely associated with shifts in the position of the ITCZ

A new interlaboratory characterisation of silicon, rare earth elements and twenty-two other trace element concentrations in the natural river water certified reference material SLRS-6 (NRC-CNRC)

The natural river water reference material SLRS-6 (NRC-CNRC) is the newest batch of a quality control material routinely used in many international environmental laboratories. This work presents a nine-laboratory compilation of measurements of major and trace element concentrations and their related uncertainties, unavailable in the NRC-CNRC certificate (B, Cs, Li, Ga, Ge, Hf, Nb, P, Rb, Rh, Re, S, Sc, Se, Si, Sn, Th, Ti, Tl, W, Y, Y, Zr and REEs). Measurements were mostly made using inductively coupled plasma-mass spectrometry. The results are compared with equivalent data for the last batch of the material, SLRS-5, measured simultaneously with SLRS-6 in this study. In general, very low concentrations, close to the quantification limits, were found in the new batch. The Sr isotopic ratio is also reported

A New Interlaboratory Characterisation of Silicon, Rare Earth Elements and Twenty‐Two other Trace Element Mass Fractions in the Natural River Water Certified Reference Material SLRS ‐6 ( NRC ‐ CNRC )

International audienceThe natural river water reference material SLRS‐6 (NRC‐CNRC) is the newest batch of a quality control material routinely used in many international environmental laboratories. This work presents a nine‐laboratory compilation of measurements of major and trace element mass fractions and their related uncertainties, unavailable in the NRC‐CNRC certificate (B, Cs, Li, Ga, Ge, Hf, Nb, P, Rb, Rh, Re, S, Sc, Se, Si, Sn, Th, Ti, Tl, W, Y, Y, Zr and REEs). Measurements were mostly made using inductively coupled plasma‐mass spectrometry. The results are compared with equivalent data for the last batch of the material, SLRS‐5, measured simultaneously with SLRS‐6 in this study. In general, very low mass fractions, close to the quantification limits, were found in the new batch. The Sr isotopic ratio is also reported

Collaborative determination of trace element mass fractions and isotope ratios in AQUA-1 drinking water certified reference material

International audienceThe Isotrace CNRS workgroup in collaboration with National Research Council of Canada has characterized a number of trace element mass fractions and isotope ratios currently not certified in AQUA-1 natural drinking water reference material (NRC Canada). This survey further expands the use of this material as a tool for environmental quality control, method validation, and method development tool for the international community. Simultaneously, the SLRS-6 river water was analyzed as quality control and also in order to compare both water characteristics, which were sampled in the same area but having undergone different treatment. Mass fractions for B, Cs, Li, Ga, Ge, Hf, Nb, P, Rb, Rh, Re, S, Sc, Se, Si, Sn, Th, Ti, Tl, W, Y, Zr, REEs, and six isotopic ratios are proposed for Sr and Pb. Measurements were mostly performed using ICP-MS with various calibration approaches. The results are reported as consensus or indicative values depending on the number of available datasets, with their associated uncertainties