Browsing by Author "Couchoud, I"
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- ItemCharacterising the expression of sub-millennial scale climate events in western Europe during the early last glacial period using multi-proxy speleothem records(Australasian Quaternary Association Inc., 2022-12-06) Corrick, E; Drysdale, RN; Hellstrom, JC; Couchoud, I; Wong, HKY; Didier, C; Hai, C; Jaillet, S; Tocino, SPast abrupt climate changes act as critical analogues for understanding how the climate system may respond to future abrupt changes. One of the best examples of naturally occurring abrupt climate change is the series of millennial-scale Dansgaard-Oeschger (D-O) events that took place during the last glacial period (115,000 – 11,500 years ago). D-O events are clearly recorded in ice-cores from Greenland, with coincident climate changes detected in marine and terrestrial records spanning a range of climate zones. Greenland ice cores also record shorter-lived ‘sub-millennial’ scale events that occur within the main D-O event sequence, particularly during the early last glacial period. To what extent these sub-millennial events were expressed outside of Greenland is currently poorly understood. Here we characterise the response to sub-millennial scale climate changes in western Europe using five multi-proxy (δ18O, δ13C, Mg and Sr) speleothem records from Saint-Marcel and Orgnac Caves, France, that collectively span 127 – 87 kyr BP. The replicated speleothem records clearly preserve both millennial D-O events and sub-millennial events, demonstrating the strong coupling between the climate of south-east France and the North Atlantic across both millennial and sub-millennial timescales. Interestingly, the multiproxy record reveals a decoupling between broad temperature (indicated by δ13C) and precipitation changes (indicated by δ18O) during some of these sub-millennial scale events. This suggests that climate teleconnections operating during sub-millennial events were in some ways different to those during the stronger millennial-scale D-O events.
- ItemMagnesium in subaqueous speleothems as a potential palaeotemperature proxy(Springer Nature, 2020-10-06) Drysdale, RN; Couchoud, I; Zanchetta, G; Isola, I; Regattieri, E; Hellstrom, JC; Govin, A; Tzedakis, PC; Ireland, T; Corrick, E; Grieg, A; Wong, HKY; Piccini, L; Holden, PJ; Woodhead, JDFew palaeoclimate archives beyond the polar regions preserve continuous and datable palaeotemperature proxy time series over multiple glacial-interglacial cycles. This hampers efforts to develop a more coherent picture of global patterns of past temperatures. Here we show that Mg concentrations in a subaqueous speleothem from an Italian cave track regional sea-surface temperatures over the last 350,000 years. The Mg shows higher values during warm climate intervals and converse patterns during cold climate stages. In contrast to previous studies, this implicates temperature, not rainfall, as the principal driver of Mg variability. The depositional setting of the speleothem gives rise to Mg partition coefficients that are more temperature dependent than other calcites, enabling the effect of temperature change on Mg partitioning to greatly exceed the effects of changes in source-water Mg/Ca. Subaqueous speleothems from similar deep-cave environments should be capable of providing palaeotemperature information over multiple glacial-interglacial cycles. © 2020 The Author(s)
- ItemA 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, SThe 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.
- ItemPartitioning 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, JCThe 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
- ItemRadiometrically dated speleothem records of Terminations IV and V and linkages to the North Atlantic(International Union for Quaternary Research (INQUA), 2019-07-28) Pollard, T; Drysdale, RN; Woodhead, JD; Edwards, RL; Hellstrom, JC; Cheng, H; Dux, F; Daëron, M; Li, XL; Wong, HKY; Couchoud, I; Regattieri, E; Zanchetta, G; Isola, IPaleoclimate archives tied to accurate and precise chronologies are crucial for developing a dynamical understanding of the causes and nature of Quaternary glacial terminations [1]. While numerous well-dated archives provide good chronological control through Terminations I and II, there is presently a lack of well-dated records spanning earlier terminations. A notable exception to this is a suite of remarkably well-dated Chinese speleothems that have been used to date the commencement of each termination over the past 640 kyr [2,3]. This has been achieved by correlating weak Asian Monsoon anomalies as captured by speleothem oxygen isotope signals with terminal Heinrich events in the North Atlantic. While this approach is well-suited to testing orbital hypothesis by comparing the timing of termination commencement with insolation metrics, it only provides precise age control at the beginning of each termination. This precludes assessment of the progression of climatic changes over the full course of the termination, and the timing at which full interglacial conditions are reached. Here we present a composite speleothem record spanning glacial terminations IV and V from the Antro del Corchia cave system located on the western coast of central Italy. This record is anchored to a uranium-thorium based chronology and contains numerous proxies representing both local and regional climate, including Δ47-based temperature data derived from a pool carbonate formed under very slow degassing conditions. By taking advantage of established links between speleothem proxies from this cave site and marine proxies from the North Atlantic, we fix the marine sediment data to a radiometric age scale. This allows us to constrain the timing of ocean circulation and SST changes occurring in the North Atlantic throughout the duration of Terminations IV and V, and compare these with terrestrial temperatures in central Italy. © The Authors.
- ItemReconstruction 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, SLa 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.