Browsing by Author "Spötl, C"

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    A palaeorainfall record from Central Pacific over the last millennia from speleothems: preliminary results
    (International Union For Quaternary Research (INQUA), 2014-07-07) Couchoud, I; Drysdale, RN; Hellstrom, JC; Hua, Q; Spötl, C; Etienne, S
    The South Pacific Convergence Zone (SPCZ) is a major feature of the South Pacific basin. Its position and activity fluctuate seasonally and at interannual and decadal timescales under the influence of ENSO and the IPO, driving changes in temperature and precipitation in the region. Zonal SPCZ events have also been shown to be associated to cyclogenesis in the Central Pacific, inducing serious hazards for the islands and populations of French Polynesia1. The reconstruction of past SPCZ position and intensity for pre-industrial periods could help to understand its dynamics and improve models aimed at predicting its evolution in a context of global change but long-term, highly resolved and chronologically well-constrained archives of palaeoclimate from the tropical South and Central Pacific are scarce. To address this issue, we collected speleothems from caves located on the uplifted atoll of Rurutu, in the Australes archipelago at the eastern part of the SPCZ. We present preliminary speleothem proxy data of regional palaeohydrology covering the last few millennia.
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    Partitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem
    (Elsevier, 2019-11-01) Drysdale, RN; Zanchetta, G; Baneschi, I; Guidi, M; Isola, I; Couchoud, I; Piccini, L; Greig, A; Wong, HKY; Woodhead, JD; Regattieri, E; Corrick, E; Paul, B; Spötl, C; Denson, E; Gordon, J; Jaillet, S; Dux, F; Hellstrom, JC
    The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’ subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with published cave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previous estimates at similar temperatures (∼8 °C). The DSr (0.100 ± 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (<1 μm a−1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 μm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation. © 2019 Elsevier Ltd
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    Reconstruction des paléoprécipitations sur les îles Australes (Pacifique central) au cours des derniers millénaires à partir de spéléothèmes : résultats préliminaires
    (Société Géologique de France (SGF), 2015-10-13) Couchoud, I; Drysdale, RN; Hellstrom, JC; Hua, Q; Spötl, C; Etienne, S
    La Zone de Convergence du Pacifique Sud (SPCZ) est une caractéristique majeure du bassin Pacifique Sud. Sa position et son intensité fluctuent de manière saisonnière et à des échelles interannuelles et décennales sous l’influence d’ENSO et de l’IPO, conduisant à des changements de température et de précipitations dans la région. L’association entre orientation zonale de la SPCZ et cyclogenèse dans le Pacifique central a été démontrée et induit de sérieux aléas pour les îles et populations de Polynésie française. La reconstruction des positions et intensités passées de la SPCZ pour la période pré-industrielle pourrait aider à comprendre sa dynamique et améliorer les modèles visant à prévoir son évolution dans un contexte de changement climatique global. Toutefois les archives climatiques de la zone tropicale du Pacifique Sud et du Pacifique central sont rares. Pour contribuer à combler cette lacune, nous avons collecté des spéléothèmes provenant de deux grottes localisées sur l’atoll soulevé de Rurutu, dans l’archipel des Australes (Polynésie française), dans la partie orientale de la SPCZ. Les spéléothèmes sont capables d’enregistrer les changements hydrologiques dans leur calcite sous forme de variations des rapports isotopiques de l’oxygène, de concentration en certains éléments traces ou encore de fines couches d’argiles déposées lors d’ennoiements. Nous présentons ici des résultats préliminaires d’analyse de ces proxies, interprétés en termes de paléohydrologie. Leurs enregistrements couvrent les derniers millénaires et sont calés chronologiquement par une combinaison de datations uranium-thorium et radiocarbone.