Processes in ice caves : and their significance for paleoenvironmental reconstructions

Unterirdisches Eis wird in den meisten alpinen Karstregionen dokumentiert und dies, obwohl die durchschnittliche Aussenlufttemperatur die Nullgradgrenze manchmal weit überschreitet. Trotz des historischen Interesses für diese "Eishöhlen", sind nur wenige Daten vorhanden um deren Entwicklung in einem ändernden Klimakontext vorauszusagen. Diese Doktorarbeit behandelt die Frage unter dem Aspekt der Thermodynamik. Eine Fallstudie aus der Eishöhle Monlési (Schweiz) unterstreicht die Hauptrolle der unterirdischen Luftzirkulationen für die Bildung und die Erhaltung des Höhleneises. Daraus folgt, dass die Massenbilanz einer Eishöhle hauptsächlich die winterlichen Aussenbedingungen widerspiegelt. Die Datierung tausend jährigen Eisablagerungen kündigt interessante paläoklimatische Beiträge an. The presence of subsurface ice is documented in numerous alpine karst regions even though the mean annual air temperature sometimes greatly exceeds 0°C. Despite the historical interest raised by these "ice caves", little data is available to predict their evolution in a changing climate context. This doctoral research project approaches the issue from a thermodynamics perspective. Data from a case study of Monlési ice cave in Switzerland emphasizes the major role of subsurface air circulations in the formation and the conservation of cave ice. Results lead to the conclusion that the cave ice mass balance reflects mainly the external winter atmospheric conditions. The dating of thousand-yearold ice deposits predicts interesting paleoclimatological contributions

Timing and causes of North African wet phases during the last glacial period and implications for modern human migration

We present the first speleothem-derived central North Africa rainfall record for the last glacial period. The record reveals three main wet periods at 65-61 ka, 52.5-50.5 ka and 37.5-33 ka that lead obliquity maxima and precession minima. We find additional minor wet episodes that are synchronous with Greenland interstadials. Our results demonstrate that sub-tropical hydrology is forced by both orbital cyclicity and North Atlantic moisture sources. The record shows that after the end of a Saharan wet phase around 70 ka ago, North Africa continued to intermittently receive substantially more rainfall than today, resulting in favourable environmental conditions for modern human expansion. The encounter and subsequent mixture of Neanderthals and modern humans – which, on genetic evidence, is considered to have occurred between 60 and 50 ka – occurred synchronously with the wet phase between 52.5 and 50.5 ka. Based on genetic evidence the dispersal of modern humans into Eurasia started less than 55 ka ago. This may have been initiated by dry conditions that prevailed in North Africa after 50.5 ka. The timing of a migration reversal of modern humans from Eurasia into North Africa is suggested to be coincident with the wet period between 37.5 and 33 ka

Ancient DNA from speleothems: opportunity or challenge?

Ancient environmental DNA retrieved from sedimentary records (sedaDNA) can complement fossil-morphological approaches for characterizing Quaternary biodiversity changes. PCR-based DNA metabarcoding is so far the most widely used method in environmental DNA studies, including sedaDNA. However, degradation of ancient DNA and potential contamination, together with the PCR amplification drawbacks, have to be carefully considered. Here we tested this approach on speleothems from an Alpine cave that, according to a previous palynomorphological study, have shown to contain abundant pollen grains. This offers a unique opportunity for comparing the two methods and, indirectly, trying to validate DNA-based results. The plant taxa identified by sedaDNA are fewer than those by pollen analysis, and success rate of PCR replicates is low. Despite extensive work performed following best practice for sedaDNA, our results are suboptimal and accompanied by a non-negligible uncertainty. Our preliminary data seem to indicate that paleoenvironmental DNA may be isolated from speleothems, but the intrinsic weakness of PCR-based metabarcoding poses a challenge to its exploitation. We suggest that newly developed methods such as hybridization capture, being free from PCR drawbacks and offering the opportunity to directly assess aDNA authenticity, may overcome these limitations, allowing a proper exploitation of speleothems as biological archive

Enhanced Mediterranean water cycle explains increased humidity during MIS 3 in North Africa

We report a new fluid inclusion dataset from northeastern Libyan speleothem SC-06-01, which is the largest speleothem fluid inclusion dataset for North Africa to date. The stalagmite was sampled in Susah Cave, a low-altitude coastal site, in Cyrenaica, on the northern slope of the Jebel Al-Akhdar. Speleothem fluid inclusions from the latest Marine Isotope Stage (MIS) 4 and throughout MIS 3 (∼67 to ∼30 kyr BP) confirm the hypothesis that past humid periods in this region reflect westerly rainfall advected through the Atlantic storm track. However, most of this moisture was sourced from the western Mediterranean, with little direct admixture of water evaporated from the Atlantic. Moreover, we identify a second moisture source likely associated with enhanced convective rainfall within the eastern Mediterranean. The relative importance of the western and eastern moisture sources seems to differ between the humid phases recorded in SC-06-01. During humid phases forced by precession, fluid inclusions record compositions consistent with both sources, but the 52.5–50.5 kyr interval forced by obliquity reveals only a western source. This is a key result, showing that although the amount of atmospheric moisture advections changes, the structure of the atmospheric circulation over the Mediterranean does not fundamentally change during orbital cycles. Consequently, an arid belt must have been retained between the Intertropical Convergence Zone and the midlatitude winter storm corridor during MIS 3 pluvials

Caracterización de carbonatos criogénicos en una cueva helada del Pirineo

Se aportan datos micromorfológicos, isotópicos y cronológicos de carbonatos criogénicos CCC) de la cueva helada Sarrios-6, situada a 2780 m s.n.m. en el macizo de Monte Perdido (Pirineo central). Es el primer estudio de este tipo de espeleotemas en la Peninsula Ibérica. En una masa de hielo aparecen cristales romboédricos de calcita de tamaño milimétrico constituidos por un núcleo interno de cristales esqueléticos rodeados por un crecimiento externo de color pardo-rojizo. Indican una fase rápida inicial de precipitación de calcita y otra posterior más lenta. Los dos tipos de calcita presentan composición isotópica diferente (núcleo: valor medio de δ13C = 4,8‰VPDB, valor medio de δ18O = -20,8‰VPDB; crecimiento externo: valor medio de δ13C = 5,3‰ VPDB, valor medio de δ18O = -21,3‰ VPDB). La datación de una semilla incluida en la masa de hielo indica que la formación de la CCC tuvo lugar durante la Anomalía Climática MedievalWe provide micromorphological, isotopic and chronological data on cryogenic cave carbonates (CCC) from Sarrios-6 ice cave (2780 m a.s.l.) in the Monte Perdido Massif (central Pyrenees). It is the first report of such speleothems on the Iberian Peninsula.Millimeter-sized white skeletal calcite rhombohedrons overgrown by brown rhombohedral crystals are present within a perennial ice body. The morphology of two carbonate generations suggests an early stage of fast carbonate precipitation followed by a second phase formed at a slower precipitation rate. The two generations show distinct isotopic compositions (skeletal cores:mean δ13C = 4.8‰,mean δ18O = -20.8‰; overgrowths: mean δ13C = 5.3‰, mean δ18O = -21.3‰). A preliminary radiocarbon date of a seed found in the same ice layer suggests that the precipitation of CCC likely occurred during the Medieval Climate Anomal

Interpreting historical, botanical, and geological evidence to aid preparations for future floods

River flooding is among the most destructive of natural hazards globally, causing widespread loss of life, damage to infrastructure and economic deprivation. Societies are currently under increasing threat from such floods, predominantly from increasing exposure of people and assets in flood-prone areas, but also as a result of changes in flood magnitude, frequency, and timing. Accurate flood hazard and risk assessment are therefore crucial for the sustainable development of societies worldwide. With a paucity of hydrological measurements, evidence from the field offers the only insight into truly extreme events and their variability in space and time. Historical, botanical, and geological archives have increasingly been recognized as valuable sources of extreme flood event information. These different archives are here reviewed with a particular focus on the recording mechanisms of flood information, the historical development of the methodological approaches and the type of information that those archives can provide. These studies provide a wealthy dataset of hundreds of historical and palaeoflood series, whose analysis reveals a noticeable dominance of records in Europe. After describing the diversity of flood information provided by this dataset, we identify how these records have improved and could further improve flood hazard assessments and, thereby, flood management and mitigation plans

Evaluating model outputs using integrated global speleothem records of climate change since the last glacial

Although quantitative isotopic data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to use the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally-distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotopic values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotopic data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on 18O values, the optimum period for the modern observational baseline, and the selection of an appropriate time-window for creating means of the isotope data for palaeo time slices

The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ 18O, δ 13C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at https://doi.org/10.17864/1947.147