Showing papers by "Sigfus J Johnsen published in 2013"
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Dartmouth College1, Uppsala University2, Nagaoka University of Technology3, University of Copenhagen4, Heidelberg University5, Natural Resources Canada6, Oregon State University7, Centre national de la recherche scientifique8, Korean Ocean Research and Development Institute9, Swansea University10, University of Bern11, British Antarctic Survey12, University of Kansas13, National Institute of Polar Research14, University of Iceland15, Stockholm University16, Vrije Universiteit Brussel17, University of Colorado Boulder18, Alfred Wegener Institute for Polar and Marine Research19, University of Washington20, Arctic and Antarctic Research Institute21, Desert Research Institute22, Hokkaido University23, University of Grenoble24, University of California, San Diego25, Université libre de Bruxelles26, Utrecht University27, Oeschger Centre for Climate Change Research28, Max Planck Society29, Swiss Federal Institute for Forest, Snow and Landscape Research30, ETH Zurich31, United Arab Emirates University32, Paul Scherrer Institute33, École Polytechnique Fédérale de Lausanne34, University of East Anglia35, Geological Survey of Canada36
TL;DR: In this paper, the North Greenland Eemian Ice Drilling (NEEM) ice core was extracted from folded Greenland ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records.
Abstract: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
546 citations
01 Jan 2013
TL;DR: The new North Greenland Eemian Ice Drilling (‘NEEM’) ice core is presented and shows only a modest ice-sheet response to the strong warming in the early Eemians, which was probably driven by the decreasing summer insolation.
Abstract: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
451 citations
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19 Mar 2013339 citations
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TL;DR: In this paper, surface water vapor isotopic measurements were conducted from June to August 2010 at the NEEM (North Greenland Eemian Drilling Project) camp, NW Greenland (77.45 N, 51.05 W, 2484 m a.s.).
Abstract: We present here surface water vapor isotopic measurements conducted from June to August 2010 at the NEEM (North Greenland Eemian Drilling Project) camp, NW Greenland (77.45 N, 51.05 W, 2484 m a.s.l.). Mea- surements were conducted at 9 different heights from 0.1 m to 13.5 m above the snow surface using two different types of cavity-enhanced near-infrared absorption spectroscopy an- alyzers. For each instrument specific protocols were de- veloped for calibration and drift corrections. The inter- comparison of corrected results from different instruments reveals excellent reproducibility, stability, and precision with a standard deviations of 0.23 ‰ for 18 O and 1.4 ‰ for D. Diurnal and intraseasonal variations show strong rela- tionships between changes in local surface humidity and wa- ter vapor isotopic composition, and with local and synoptic weather conditions. This variability probably results from the interplay between local moisture fluxes, linked with firn-air exchanges, boundary layer dynamics, and large-scale mois- ture advection. Particularly remarkable are several episodes characterized by high (> 40 ‰) surface water vapor deu- terium excess. Air mass back-trajectory calculations from atmospheric analyses and water tagging in the LMDZiso (Laboratory of Meteorology Dynamics Zoom-isotopic) at- mospheric model reveal that these events are associated with predominant Arctic air mass origin. The analysis suggests that high deuterium excess levels are a result of strong ki- netic fractionation during evaporation at the sea-ice margin.
160 citations
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TL;DR: Using the new values for α, a Rayleigh distillation model shows significant changes in both magnitude and shape of an annual deuterium excess signal in Greenland.
Abstract: RATIONALE: The equilibrium fractionation factors govern the relative change in the isotopic composition during phase transitions of water. The commonly used results, which were published more than 40 years ago, are limited to a minimum temperature of –33°C. This limits the reliability in cold regions. With recent instrumental developments it is now possible to test the accuracy of the earlier results as well as extend the temperature range. METHODS: Novel measurements were made of the ice-vapor equilibrium fractionation factor α between 0°C and –40°C, from a unique experimental setup using both a Picarro cavity ringdown spectrometer and a TC/EA IRMS system. Using both systems allows for continuous monitoring of the equilibrium state of the system as well as testing for reproducibility. RESULTS: The results of the experiments show fractionation factors for δ 2 H and δ 18 O values, with a temperature dependency in accordance with theory for equilibrium fractionation. We obtain the following expressions for the temperature dependency of the fractionation coefficients: ln αδ2H ¼ 0:2133 � 203:10 T þ 48888 T 2 ln αδ 18 O ¼ 0:0831 � 49:192 T þ 8312:5 T 2 Compared with previous experimental work, a significantly larger α for δ 2 H is obtained while, for δ 18 O, α is larger for temperatures below –20°C and slightly lower for temperatures above this. CONCLUSIONS: Using the new values for α, a Rayleigh distillation model shows significant changes in both magnitude and shape of an annual deuterium excess signal in Greenland. This emphasizes the importance of a well-defined value of α for accurate studies of the processes in the hydrological cycle and underlines the significance of the differences between the results of this work and earlier work. Copyright © 2013 John Wiley & Sons, Ltd.
78 citations
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TL;DR: In this paper, a direct synchronization of Greenland and Antarctic ice cores at the Toba eruption based on matching of a pattern of bipolar volcanic spikes is proposed, which can be used to place palaeo-environmental records other than ice cores into a precise climatic context.
Abstract: . The Toba eruption that occurred some 74 ka ago in Sumatra, Indonesia, is among the largest volcanic events on Earth over the last 2 million years. Tephra from this eruption has been spread over vast areas in Asia, where it constitutes a major time marker close to the Marine Isotope Stage 4/5 boundary. As yet, no tephra associated with Toba has been identified in Greenland or Antarctic ice cores. Based on new accurate dating of Toba tephra and on accurately dated European stalagmites, the Toba event is known to occur between the onsets of Greenland interstadials (GI) 19 and 20. Furthermore, the existing linking of Greenland and Antarctic ice cores by gas records and by the bipolar seesaw hypothesis suggests that the Antarctic counterpart is situated between Antarctic Isotope Maxima (AIM) 19 and 20. In this work we suggest a direct synchronization of Greenland (NGRIP) and Antarctic (EDML) ice cores at the Toba eruption based on matching of a pattern of bipolar volcanic spikes. Annual layer counting between volcanic spikes in both cores allows for a unique match. We first demonstrate this bipolar matching technique at the already synchronized Laschamp geomagnetic excursion (41 ka BP) before we apply it to the suggested Toba interval. The Toba synchronization pattern covers some 2000 yr in GI-20 and AIM-19/20 and includes nine acidity peaks that are recognized in both ice cores. The suggested bipolar Toba synchronization has decadal precision. It thus allows a determination of the exact phasing of inter-hemispheric climate in a time interval of poorly constrained ice core records, and it allows for a discussion of the climatic impact of the Toba eruption in a global perspective. The bipolar linking gives no support for a long-term global cooling caused by the Toba eruption as Antarctica experiences a major warming shortly after the event. Furthermore, our bipolar match provides a way to place palaeo-environmental records other than ice cores into a precise climatic context.
73 citations
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18 Mar 201359 citations
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18 Mar 201327 citations
01 Apr 2013
2 citations