TY - JOUR A1 - Palkopoulou, Eleftheria A1 - Lipson, Mark A1 - Mallick, Swapan A1 - Nielsen, Svend A1 - Rohland, Nadin A1 - Baleka, Sina Isabelle A1 - Karpinski, Emil A1 - Ivancevici, Atma M. A1 - Thu-Hien To, A1 - Kortschak, Daniel A1 - Raison, Joy M. A1 - Qu, Zhipeng A1 - Chin, Tat-Jun A1 - Alt, Kurt W. A1 - Claesson, Stefan A1 - Dalen, Love A1 - MacPhee, Ross D. E. A1 - Meller, Harald A1 - Rocar, Alfred L. A1 - Ryder, Oliver A. A1 - Heiman, David A1 - Young, Sarah A1 - Breen, Matthew A1 - Williams, Christina A1 - Aken, Bronwen L. A1 - Ruffier, Magali A1 - Karlsson, Elinor A1 - Johnson, Jeremy A1 - Di Palma, Federica A1 - Alfoldi, Jessica A1 - Adelsoni, David L. A1 - Mailund, Thomas A1 - Munch, Kasper A1 - Lindblad-Toh, Kerstin A1 - Hofreiter, Michael A1 - Poinar, Hendrik A1 - Reich, David T1 - A comprehensive genomic history of extinct and living elephants JF - Proceedings of the National Academy of Sciences of the United States of America KW - paleogenomics KW - elephantid evolution KW - mammoth KW - admixture KW - species divergence Y1 - 2018 U6 - https://doi.org/10.1073/pnas.1720554115 SN - 0027-8424 VL - 115 IS - 11 SP - E2566 EP - E2574 PB - National Acad. of Sciences CY - Washington ER - TY - JOUR A1 - Casado, Mathieu A1 - Landais, Amaelle A1 - Picard, Ghislain A1 - Münch, Thomas A1 - Laepple, Thomas A1 - Stenni, Barbara A1 - Dreossi, Giuliano A1 - Ekaykin, Alexey A1 - Arnaud, Laurent A1 - Genthon, Christophe A1 - Touzeau, Alexandra A1 - Masson-Delmotte, Valerie A1 - Jouzel, Jean T1 - Archival processes of the water stable isotope signal in East Antarctic ice cores JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotope-enabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation-condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-1745-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 5 SP - 1745 EP - 1766 PB - Copernicus CY - Göttingen ER - TY - GEN A1 - Casado, Mathieu A1 - Landais, Amaelle A1 - Picard, Ghislain A1 - Münch, Thomas A1 - Laepple, Thomas A1 - Stenni, Barbara A1 - Dreossi, Giuliano A1 - Ekaykin, Alexey A1 - Arnaud, Laurent A1 - Genthon, Christophe A1 - Touzeau, Alexandra A1 - Masson-Delmotte, Valerie A1 - Jouzel, Jean T1 - Archival processes of the water stable isotope signal in East Antarctic ice cores T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotope-enabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation-condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 716 KW - dronning maud-land KW - shallow firn cores KW - near-surface snow KW - Dome C KW - Kohnen station KW - South Pole KW - climate varibility KW - Vostok station KW - deuterium content KW - GCM analysis Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-427054 SN - 1866-8372 IS - 716 ER - TY - JOUR A1 - Rehfeld, Kira A1 - Münch, Thomas A1 - Ho, Sze Ling A1 - Laepple, Thomas T1 - Global patterns of declining temperature variability from the Last Glacial Maximum to the Holocene JF - Nature : the international weekly journal of science N2 - Changes in climate variability are as important for society to address as are changes in mean climate(1). Contrasting temperature variability during the Last Glacial Maximum and the Holocene can provide insights into the relationship between the mean state of the climate and its variability(2,3). However, although glacial-interglacial changes in variability have been quantified for Greenland(2), a global view remains elusive. Here we use a network of marine and terrestrial temperature proxies to show that temperature variability decreased globally by a factor of four as the climate warmed by 3-8 degrees Celsius from the Last Glacial Maximum (around 21,000 years ago) to the Holocene epoch (the past 11,500 years). This decrease had a clear zonal pattern, with little change in the tropics (by a factor of only 1.6-2.8) and greater change in the mid-latitudes of both hemispheres (by a factor of 3.3-14). By contrast, Greenland ice-core records show a reduction in temperature variability by a factor of 73, suggesting influences beyond local temperature or a decoupling of atmospheric and global surface temperature variability for Greenland. The overall pattern of reduced variability can be explained by changes in the meridional temperature gradient, a mechanism that points to further decreases in temperature variability in a warmer future. Y1 - 2018 U6 - https://doi.org/10.1038/nature25454 SN - 0028-0836 SN - 1476-4687 VL - 554 IS - 7692 SP - 356 EP - 359 PB - Nature Publ. Group CY - London ER - TY - THES A1 - Münch, Thomas T1 - Interpretation of temperature signals from ice cores T1 - Interpretation von Temperatursignalen aus Eisbohrkernen BT - insights into the spatial and temporal variability of water isotopes in Antarctica BT - Einblicke in die räumliche und zeitliche Variabilität antarktischer Isotopendaten N2 - Earth's climate varies continuously across space and time, but humankind has witnessed only a small snapshot of its entire history, and instrumentally documented it for a mere 200 years. Our knowledge of past climate changes is therefore almost exclusively based on indirect proxy data, i.e. on indicators which are sensitive to changes in climatic variables and stored in environmental archives. Extracting the data from these archives allows retrieval of the information from earlier times. Obtaining accurate proxy information is a key means to test model predictions of the past climate, and only after such validation can the models be used to reliably forecast future changes in our warming world. The polar ice sheets of Greenland and Antarctica are one major climate archive, which record information about local air temperatures by means of the isotopic composition of the water molecules embedded in the ice. However, this temperature proxy is, as any indirect climate data, not a perfect recorder of past climatic variations. Apart from local air temperatures, a multitude of other processes affect the mean and variability of the isotopic data, which hinders their direct interpretation in terms of climate variations. This applies especially to regions with little annual accumulation of snow, such as the Antarctic Plateau. While these areas in principle allow for the extraction of isotope records reaching far back in time, a strong corruption of the temperature signal originally encoded in the isotopic data of the snow is expected. This dissertation uses observational isotope data from Antarctica, focussing especially on the East Antarctic low-accumulation area around the Kohnen Station ice-core drilling site, together with statistical and physical methods, to improve our understanding of the spatial and temporal isotope variability across different scales, and thus to enhance the applicability of the proxy for estimating past temperature variability. The presented results lead to a quantitative explanation of the local-scale (1–500 m) spatial variability in the form of a statistical noise model, and reveal the main source of the temporal variability to be the mixture of a climatic seasonal cycle in temperature and the effect of diffusional smoothing acting on temporally uncorrelated noise. These findings put significant limits on the representativity of single isotope records in terms of local air temperature, and impact the interpretation of apparent cyclicalities in the records. Furthermore, to extend the analyses to larger scales, the timescale-dependency of observed Holocene isotope variability is studied. This offers a deeper understanding of the nature of the variations, and is crucial for unravelling the embedded true temperature variability over a wide range of timescales. N2 - Das Klima der Erde verändert sich stetig sowohl im Raum als auch in der Zeit, jedoch hat die Menschheit nur einen Bruchteil dieser Entwicklung direkt verfolgen können und erst seit 200 Jahren mit instrumentellen Beobachtungen aufgezeichnet. Unser Wissen bezüglich früherer Klimaveränderungen beruht daher fast ausschließlich auf indirekten Proxydaten, also Stellvertreterdaten, welche sensitiv auf Veränderungen in bestimmten Klimavariablen reagieren und in Klimaarchiven abgespeichert werden. Essentiell ist eine hohe Genauigkeit der erhaltenen Proxydaten. Sie erlaubt, Modellvorhersagen früherer Klimazustände quantitativ zu überprüfen und damit die Modelle zu validieren. Erst dann können mit Hilfe der Modelle verlässliche Aussagen über die anthropogen bedingten zukünftigen Klimaveränderungen getroffen werden. Die polaren Eisschilde von Grönland und Antarktika sind eines der wichtigsten Klimaarchive. Über die isotopische Zusammensetzung der im Eis eingelagerten Wassermoleküle zeichnen sie Veränderungen der lokalen Lufttemperatur auf. Jedoch stellen die Daten dieses Temperaturproxys keine perfekte Aufzeichnung früherer Klimaschwankungen dar – was im Übrigen für alle Proxydaten gilt –, da neben der Temperatur eine Fülle anderer Effekte Mittelwert und Varianz der Proxyschwankungen beeinflussen und damit die direkte Interpretation der Daten in Bezug auf klimatische Veränderungen beeinträchtigen. Insbesondere trifft dies auf Gebiete mit geringen jährlichen Schneefallmengen zu, wie z.B. das Polarplateau des antarktischen Kontinents. Diese Gebiete erlauben zwar prinzipiell die Gewinnung von Proxydatensätzen, die weit in die Vergangenheit zurückreichen, allerdings erwartet man im Allgemeinen auch eine starke Beeinträchtigung des ursprünglichen, in der isotopischen Zusammensetzung des Schnees eingeprägten Temperatursignals. Unter Verwendung von Beobachtungsdaten aus der Antarktis – hauptsächlich aus dem Niedrigakkumulationsgebiet von Dronning Maud Land in Ostantarktika, in dem auch die Kohnen-Station liegt –, sowie durch Anwendung statistischer und physikalischer Methoden, trägt diese Dissertation zu einem besseren Verständnis der räumlichen und zeitlichen Variabilität der Isotopendaten über einen weiten Skalenbereich bei. Damit verbessert die vorliegende Arbeit die Anwendbarkeit dieses Temperaturproxys in Bezug auf die Rekonstruktion natürlicher Klimavariabilität. Im Speziellen wird aus den Beobachtungsdaten ein statistisches Modell abgeleitet, welches quantitativ die lokale räumliche (1–500 m-Skala) Variabilität erklärt; des Weiteren wird gezeigt, dass die zeitliche Variabilität hauptsächlich bedingt wird durch die Kombination zweier Effekte: einen klimatischen Jahreszyklus angetrieben durch den Jahresgang der Temperatur, und die Wirkung des Diffusionsprozesses auf einen zeitlich unkorrelierten Rauschterm. Diese Resultate führen zum einen zu einer wesentlich eingegrenzten Abschätzung der Repräsentativität einzelner, isotopenbasierter Proxyzeitreihen in Bezug auf lokale Temperaturveränderungen. Zum anderen beeinflussen sie erheblich die Interpretation scheinbarer Periodizitäten im Isotopensignal. Es wird darüber hinaus vermutet, dass die Gesamtstärke des Rauschens im Isotopensignal nicht nur durch die örtliche Akkumulationsrate bestimmt wird, sondern auch durch andere Parameter wie die lokale mittlere Windstärke und die räumliche und zeitliche Kohärenz der Niederschlagswichtung. Schließlich erlaubt die Erweiterung der Analyse auf größere räumliche und zeitliche Skalen die Untersuchung, inwieweit die Variabilität isotopenbasierter Proxyzeitreihen aus dem Holozän von der Zeitskala abhängt. Dadurch wird ein tieferes Verständnis der Proxyvariabilität erzielt, welches grundlegend dafür ist, die tatsächliche, in den Daten einzelner Zeitreihen verdeckt vorhandene Temperaturvariabilität, über einen weiten Zeitskalenbereich zu entschlüsseln. KW - climate physics KW - temperature variability KW - temperature proxy KW - proxy understanding KW - proxy uncertainty KW - stable isotopes KW - isotope variations KW - ice core KW - firn KW - noise KW - post-depositional KW - two-dimensional KW - Antarctica KW - Dronning Maud Land KW - Kohnen KW - Klimaphysik KW - Klimavariabilität KW - Temperaturproxy KW - Proxyverständnis KW - Proxyunsicherheit KW - stabile Isotope KW - Eisbohrkern KW - Antarktis KW - Dronning Maud Land KW - Kohnen Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-414963 ER - TY - JOUR A1 - Laepple, Thomas A1 - Münch, Thomas A1 - Casado, Mathieu A1 - Hoerhold, Maria A1 - Landais, Amaelle A1 - Kipfstuhl, Sepp T1 - On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Stable isotope ratios delta O-18 and delta D in polar ice provide a wealth of information about past climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured isotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are formed and stored in firn and ice. As delta O-18 and delta D in the snowfall are strongly correlated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given site. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of snow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in the delta O-18 and delta D profiles with typical wavelengths of similar to 20 cm. These observations are unexpected as the accumulation rates strongly differ between the sites, ranging from 20 to 80mmw.e.yr(-1) (similar to 6-21 cm of snow per year). Various mechanisms have been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with the similarity of the different profiles independent of the local accumulation conditions. Here, we systematically analyse the properties and origins of delta O-18 and delta D variations in high-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance between peaks) of similar to 20 cm by counting the isotopic maxima. Spectral analysis further shows a strong similarity between the sites but indicates no dominant periodic features. Furthermore, the appar-ent cycle length increases with depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal cycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of the isotopic variations. This in turn implies a similar distance between isotopic maxima in the firn profiles. Our results explain a large set of observations discussed in the literature, providing a simple explanation for the interpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results underline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small signal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-169-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 1 SP - 169 EP - 187 PB - Copernicus CY - Göttingen ER - TY - GEN A1 - Laepple, Thomas A1 - Münch, Thomas A1 - Casado, Mathieu A1 - Hoerhold, Maria A1 - Landais, Amaelle A1 - Kipfstuhl, Sepp T1 - On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - Stable isotope ratios delta O-18 and delta D in polar ice provide a wealth of information about past climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured isotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are formed and stored in firn and ice. As delta O-18 and delta D in the snowfall are strongly correlated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given site. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of snow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in the delta O-18 and delta D profiles with typical wavelengths of similar to 20 cm. These observations are unexpected as the accumulation rates strongly differ between the sites, ranging from 20 to 80mmw.e.yr(-1) (similar to 6-21 cm of snow per year). Various mechanisms have been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with the similarity of the different profiles independent of the local accumulation conditions. Here, we systematically analyse the properties and origins of delta O-18 and delta D variations in high-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance between peaks) of similar to 20 cm by counting the isotopic maxima. Spectral analysis further shows a strong similarity between the sites but indicates no dominant periodic features. Furthermore, the appar-ent cycle length increases with depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal cycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of the isotopic variations. This in turn implies a similar distance between isotopic maxima in the firn profiles. Our results explain a large set of observations discussed in the literature, providing a simple explanation for the interpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results underline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small signal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 932 KW - Dronning-Maud-Land KW - ice-core KW - climate variability KW - surface snow KW - stable-isotopes KW - water-isotopes KW - oxygen-isotope KW - south-pole KW - athmospheric circulation KW - mathematical-analysis Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-446052 SN - 1866-8372 IS - 932 SP - 169 EP - 187 ER - TY - JOUR A1 - Münch, Thomas A1 - Laepple, Thomas T1 - What climate signal is contained in decadal- to centennial-scale isotope variations from Antarctic ice cores? JF - Climate of the past : CP N2 - Ice-core-based records of isotopic composition are a proxy for past temperatures and can thus provide information on polar climate variability over a large range of timescales. However, individual isotope records are affected by a multitude of processes that may mask the true temperature variability. The relative magnitude of climate and non-climate contributions is expected to vary as a function of timescale, and thus it is crucial to determine those temporal scales on which the actual signal dominates the noise. At present, there are no reliable estimates of this timescale dependence of the signal-to-noise ratio (SNR). Here, we present a simple method that applies spectral analyses to stable-isotope data from multiple cores to estimate the SNR, and the signal and noise variability, as a function of timescale. The method builds on separating the contributions from a common signal and from local variations and includes a correction for the effects of diffusion and time uncertainty. We apply our approach to firn-core arrays from Dronning Maud Land (DML) in East Antarctica and from the West Antarctic Ice Sheet (WAIS). For DML and decadal to multi-centennial timescales, we find an increase in the SNR by nearly 1 order of magnitude (similar to 0.2 at decadal and similar to 1.0 at multi-centennial scales). The estimated spectrum of climate variability also shows increasing variability towards longer timescales, contrary to what is traditionally inferred from single records in this region. In contrast, the inferred variability spectrum for WAIS stays close to constant over decadal to centennial timescales, and the results even suggest a decrease in SNR over this range of timescales. We speculate that these differences between DML and WAIS are related to differences in the spatial and temporal scales of the isotope signal, highlighting the potentially more homogeneous atmospheric conditions on the Antarctic Plateau in contrast to the marine-influenced conditions on WAIS. In general, our approach provides a methodological basis for separating local proxy variability from coherent climate variations, which is applicable to a large set of palaeoclimate records. Y1 - 2018 U6 - https://doi.org/10.5194/cp-14-2053-2018 SN - 1814-9324 SN - 1814-9332 VL - 14 IS - 12 SP - 2053 EP - 2070 PB - Copernicus Gesellschaft mbH CY - Göttingen ER -