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Water Stable Isotopes in Alpine Ice Cores As Proxies for Temperature and Atmospheric Circulation”

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Water Stable Isotopes in Alpine Ice Cores As Proxies for Temperature and Atmospheric Circulation” Water stable isotopes in Alpine ice cores as proxies for temperature and atmospheric circulation Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakultät der Universität Bern vorgelegt von Isabella Mariani aus Italien Leiterin der Arbeit: Prof. Dr. M. Schwikowski Departement für Chemie und Biochemie der Universität Bern Von der Philosophisch – naturwissenschaftlichen Fakultät angenommen. Der Dekan: Bern, 16. Dezember 2013 Prof. Dr. S. Decurtins 2 Summary Water stable isotope ratios and net snow accumulation in ice cores are commonly interpreted as temperature or precipitation proxies. However, only in a few cases a direct calibration with instrumental data has been attempted. Taking advantage of the dense network of observations in the European Alpine region, in this PhD study the relationship of the annually and seasonally resolved proxy data from two highly-resolved ice cores was rigorously tested, comparing the proxies with local temperature and precipitation. The analysis focused on the most recent decades, where the highest amount and quality of meteorological data are available and the ice core dating uncertainty is minimal. The two ice cores were from the Fiescherhorn glacier in the Northern Alps (3900 m a.s.l.) and Grenzgletscher in the Southern Alps (4200 m a.s.l.). Due to the orographic barrier, the two flanks of the Alpine chain are affected by distinct patterns of precipitation. The different location of the two glaciers therefore offered a unique opportunity to test if such a specific setting is reflected in the proxy data. On a seasonal scale a high fraction of δ18 O variability was explained by the seasonal cycle of temperature (~60% for the ice cores, ~70% for the nearby stations of the Global Network of Isotopes in Precipitation, GNIP). When the seasonality was removed, the correlations decreased for all sites indicating that factors other than temperature such as changing moisture sources and/or precipitation regimes affected the isotopic signal on this timescale. Post- depositional phenomena may additionally have modified the ice core data. On an annual scale the high variability of precipitation, especially at high-altitude sites, might considerably bias the isotopic signal toward the season with more precipitation. The annual δ18 O/temperature relationship was significant at the Fiescherhorn, whereas for Grenzgletscher this was the case only when weighting the temperature with precipitation. The fact that at Grenzgletscher the annual mean of δ18 O is only representative for temperature during precipitation was attributed to a strong interannual variability of precipitation distribution. For such a glacier site, only a precipitation weighted temperature can be reconstructed. In both cases the fraction of interannual temperature variability explained was ~20%, comparable to the values obtained from the GNIP stations data. Even for glacier sites in close proximity (only 60 km) within the same mountain range, distinct local precipitation patterns can result in variations how the temperature proxy signal is preserved. Thus, a careful individual calibration of the local δ18 O/T relation is essential for every ice core site. Consistent with previous studies, an altitude effect for the δ18 O of -0.17‰/100 m was observed, considering an extended elevation range i.e., combining data of the two ice core sites and four GNIP stations. 3 Net accumulation on the glacier was significantly correlated with precipitation for Grenzgletscher during the entire period of investigation, whereas for Fiescherhorn this was the case only for the less recent period (1961-1977). Local phenomena, probably related to wind, seem to partly disturb the Fiescherhorn accumulation record. In addition the net accumulation is more sensitive to post-depositional processes and to initial layer assignment than intrinsic core parameters like the δ18 O. Spatial correlation analysis showed the two glaciers to be influenced by different precipitation regimes, with the Grenzgletscher reflecting the characteristic precipitation regime south of the Alps and the Fiescherhorn accumulation showing a pattern more closely linked to northern Alpine stations. A further study involving seasonal δ18 O and deuterium excess from three GNIP stations and the Fiescherhorn ice core was conducted. The aim was to analyze the spatial coherence of the signal in terms of altitudinal changes since all the sites are less than 20 km distant and are assumed to be affected by the same precipitation regimes. The δ18 O was well correlated between all the sites and with temperature and showed an altitude effect slightly higher in summer compared to the other seasons, explainable with partial re-enrichment at the lower altitude sites. For the ice core, the absence of subannual stratigraphic markers introduces an additional uncertainty in the attribution of the winter and summer signal and may in part explain the low correlations obtained. The deuterium excess, characterized by high frequency fluctuations, showed no correlation between the ice core signal and the lower elevation sites and tends to increase nonlinearly with altitude. For this parameter a weak seasonal cycle was observed, with lower values in spring and higher in fall that might be explained in terms of in- cloud and sub-cloud kinetic processes. No homogeneous relationship with the meteorological parameters was found. The common increasing trend of summer deuterium excess over 1993- 2011 was explained with the increasing frequency of South-Westerly weather situations, carrying moisture from the high-deuterium excess Mediterranean basin. In winter, the medium altitude site Grimsel Hospiz showed significant anticorrelation between the deuterium excess and the NAO, suggesting for the first time a connection between the second order parameter and the atmospheric circulation over the Alpine region. Part of this project was dedicated to the setup and characterization of a Wavelength-Scanned Cavity Ring Down Spectrometer (Picarro L2130-i). This laser technique is now widely used in isotope hydrology because of the advantage of measuring both the δ18 O and the δD with high precision and easier operation compared to other methods. The instrument was characterized through the evaluation of memory effect and drift. The analytical uncertainty was quantified as 0.1‰ for the δ18 O and 0.5‰ for the δD. Three internal laboratory standards 4 were prepared to represent the Alpine isotopic range and were calibrated against International Atomic Energy Agency (IAEA) reference materials. The Grenzgletscher samples were measured allowing for a comparison with previous measurement performed with another technique, Isotopic Ratio Mass Spectrometry. Results showed good agreement within the analytical uncertainty. Although it was not straightforward to interpret the isotopic signal in the two ice cores at seasonal and interannual timescale, major changes in temperature, precipitation or atmospheric circulation patterns might nevertheless be accessible on longer timescales. Future work will be directed toward detailed interpretation of the 350 years information contained in the Fiescherhorn ice core and the even longer record from Colle Gnifetti. 5 6 Contents Summary................................................................................................................................... 3 1 Introduction ...................................................................................................................... 9 1.1 Climate change and paleoclimate research ................................................................ 9 1.2 Aim of this study...................................................................................................... 13 References ............................................................................................................................ 15 2 Ice core proxies, regional setting and study sites......................................................... 19 2.1 Water stable isotopes................................................................................................ 19 2.1.1 Physical principles and equilibrium fractionation............................................ 20 2.1.2 δ notation.......................................................................................................... 23 2.1.3 Rayleigh fractionation...................................................................................... 24 2.1.4 Equilibrium and kinetic fractionation .............................................................. 25 2.1.5 Global Meteoric Water Line (GMWL) and deuterium excess......................... 26 2.1.6 δ18 O-temperature relationship .......................................................................... 28 2.1.7 Post-depositional processes on glaciers ........................................................... 30 2.2 Precipitation reconstruction...................................................................................... 32 2.3 Climate of the Alpine region.................................................................................... 34 2.3.1 Intraseasonal precipitation variability.............................................................. 35 2.3.2 Interannual variability: the North Atlantic Oscillation .................................... 37 2.3.3 Climate change in the Alps in the last two centuries ....................................... 41 2.3.4 Water stable isotope research in the Alpine area ............................................
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