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Search Results: 1 - 10 of 3054 matches for " Dahl-Jensen "
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Annual layering in the NGRIP ice core during the Eemian
A. Svensson,M. Bigler,E. Kettner,D. Dahl-Jensen
Climate of the Past (CP) & Discussions (CPD) , 2011, DOI: 10.5194/cp-7-1427-2011
Abstract: The Greenland NGRIP ice core continuously covers the period from present day back to 123 ka before present, which includes several thousand years of ice from the previous interglacial period, MIS 5e or the Eemian. In the glacial part of the core, annual layers can be identified from impurity records and visual stratigraphy, and stratigraphic layer counting has been performed back to 60 ka. In the deepest part of the core, however, the ice is close to the pressure melting point, the visual stratigraphy is dominated by crystal boundaries, and annual layering is not visible to the naked eye. In this study, we apply a newly developed setup for high-resolution ice core impurity analysis to produce continuous records of dust, sodium and ammonium concentrations as well as conductivity of melt water. We analyzed three 2.2 m sections of ice from the Eemian and the glacial inception. In all of the analyzed ice, annual layers can clearly be recognized, most prominently in the dust and conductivity profiles. Part of the samples is, however, contaminated in dust, most likely from drill liquid. It is interesting that the annual layering is preserved despite a very active crystal growth and grain boundary migration in the deep and warm NGRIP ice. Based on annual layer counting of the new records, we determine a mean annual layer thickness close to 11 mm for all three sections, which, to first order, confirms the modeled NGRIP time scale (ss09sea). The counting does, however, suggest a longer duration of the climatically warmest part of the NGRIP record (MIS5e) of up to 1 ka as compared to the model estimate. Our results suggest that stratigraphic layer counting is possible basically throughout the entire NGRIP ice core, provided sufficiently highly-resolved profiles become available.
Annual layering in the NGRIP ice core during the Eemian
A. Svensson,M. Bigler,E. Kettner,D. Dahl-Jensen
Climate of the Past Discussions , 2011, DOI: 10.5194/cpd-7-749-2011
Abstract: The Greenland NGRIP ice core continuously covers the period from present day back to 123 ka before present, which includes several thousand years of ice from the previous interglacial period, MIS 5e or the Eemian. In the glacial part of the core annual layers can be identified from impurity records and visual stratigraphy, and stratigraphic layer counting has been performed back to 60 ka. In the deepest part of the core, however, the ice is close to the pressure melting point, the visual stratigraphy is dominated by crystal boundaries, and annual layering is not visible to the naked eye. In this study, we apply a newly developed setup for high-resolution ice core impurity analysis to produce continuous records of dust, sodium and ammonium concentrations as well as conductivity of melt water. We analyzed three 2.2 m sections of ice from the Eemian and the glacial inception. In all of the analyzed ice, annual layers can clearly be recognized, most prominently in the dust and conductivity profiles. Part of the samples is, however, contaminated in dust, most likely from drill liquid. It is interesting that the annual layering is preserved despite a very active crystal growth and grain boundary migration in the deep and warm NGRIP ice. Based on annual layer counting of the new records, we determine a mean annual layer thickness close to 11 mm for all three sections, which, to first order, confirms the modeled NGRIP time scale (ss09sea). The counting does, however, suggest a longer duration of the climatically warmest part of the NGRIP record (MIS5e) of up to 1 ka as compared to the model estimate. Our results suggest that stratigraphic layer counting is possible basically throughout the entire NGRIP ice core provided sufficiently highly-resolved profiles become available.
Investigating the past and recent δ18O-accumulation relationship seen in Greenland ice cores
S. L. Buchardt, H. B. Clausen, B. M. Vinther,D. Dahl-Jensen
Climate of the Past (CP) & Discussions (CPD) , 2012,
Abstract: Decadal means of δ18O and accumulation rates from 52 ice core locations in Greenland are presented. The accumulation rates are derived from annual layers determined in the δ18O curve. Investigation of the δ18O-accumulation relationship across the ice divide reveals a significant Foehn effect with anticorrelation of δ18O and accumulation rate on the lee side of the divide in Southern Greenland, while no effect is seen in Central Greenland. Furthermore, the sensitivity of accumulation rate to changes in temperature is found to be smaller in Northern Greenland than in the central and southern parts. Four records in the data set contain sufficient recent data that the period of observed temperature rise from the 1990s and onwards can be investigated. All four records are from locations close to the ice divide in Northern Greenland and while three of them show increased temperatures, no conclusive statement can be made about the accumulation rate from these data.
Review of Survey activities 2010: Comprehensive Nuclear-Test-Ban Treaty – a peace-keeping initiative with scientific impact
Larsen, Tine B.,Voss, Peter H.,Dahl-Jensen, Trine,Gregersen, S?ren
Geological Survey of Denmark and Greenland Bulletin , 2011,
Abstract:
Review of Survey activities 2012: Seismic activity in Denmark: detection level and recent felt earthquakes
Dahl-Jensen, T.,Voss, P.H.,Larsen, T.B.,Gregersen, S.
Geological Survey of Denmark and Greenland Bulletin , 2013,
Abstract:
Review of Survey activities 2003: Exploring for extended continental shelf claims off Greenland and the Faroe Islands – geological perspectives
Marcussen, Christian,Christiansen, Flemming G.,Dahl-Jensen, Trine,Heinesen, Martin
Geological Survey of Denmark and Greenland Bulletin , 2004,
Abstract:
Review of Survey Activities 2005: Earthquake seismology in Greenland – improved data with multiple applications
Larsen, Tine B.,Dahl-Jensen, Trine,Voss, Peter,J?rgensen, Thomas M?ller
Geological Survey of Denmark and Greenland Bulletin , 2006,
Abstract:
Review of Survey activities 2009: Greenland ice sheet monitoring network (GLISN): a seismological approach
Dahl-Jensen, Trine,Larsen, Tine B.,Voss, Peter H.,GLISN group
Geological Survey of Denmark and Greenland Bulletin , 2010,
Abstract:
Investigating the past and recent δ18O-accumulation relationship seen in Greenland ice cores
S. L. Buchardt,H. B. Clausen,B. M. Vinther,D. Dahl-Jensen
Climate of the Past (CP) & Discussions (CPD) , 2012, DOI: 10.5194/cp-8-2053-2012
Abstract: Decadal means of δ18O and accumulation rates from 52 ice core locations in Greenland are presented. The accumulation rates are derived from annual layers determined in the δ18O curve. Investigation of the δ18O-accumulation relationship across the ice divide reveals a significant Foehn effect with anticorrelation of δ18O and accumulation rate on the lee side of the divide in Southern Greenland, while no effect is seen in Central Greenland. Furthermore, the sensitivity of accumulation rate to changes in temperature is found to be smaller in Northern Greenland than in the central and southern parts. Four records in the data set contain sufficient recent data that the period of observed temperature rise from the 1990s and onwards can be investigated. All four records are from locations close to the ice divide in Northern Greenland and while three of them show increased temperatures, no conclusive statement can be made about the accumulation rate from these data.
A 60 000 year Greenland stratigraphic ice core chronology
K. K. Andersen,M. Bigler,H. B. Clausen,D. Dahl-Jensen
Climate of the Past Discussions , 2007,
Abstract: The Greenland Ice Core Chronology 2005 (GICC05) is a time scale based on annual layer counting of high-resolution records from Greenland ice cores. Whereas the Holocene part of the time scale is based on various records from the DYE-3, the GRIP, and the NorthGRIP ice cores, the glacial part is solely based on NorthGRIP records. Here we present an 18 kyr extension of the time scale such that GICC05 continuously covers the past 60 kyr. The new section of the time scale places the onset of Greenland Interstadial 12 (GI-12) at 46.9±1.0 kyr b2k (before year AD 2000), the North Atlantic Ash Zone 2 layer in GI-15 at 55.4±1.2 kyr b2k, and the onset of GI-17 at 59.4±1.3 kyr b2k. The error estimates are derived from the accumulated number of uncertain annual layers and can be regarded as 1σ uncertainties. In the 40–60 kyr interval the new time scale has a discrepancy with the Meese-Sowers GISP2 time scale of up to 2.4 kyr, whereas GICC05 compares well to the dating of the Hulu Cave record with absolute age differences of less than 800 years throughout the 60 kyr period. The new time scale is generally in close agreement with other independently dated records and reference horizons, such as the Laschamp geomagnetic excursion and the Kleegruben speleothem record from the Austrian Alps, suggesting high accuracy of both event durations and absolute age estimates.
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