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Gerrit Lohmann - One of the best experts on this subject based on the ideXlab platform.
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southern ocean origin for the resumption of atlantic thermohaline circulation during Deglaciation
Nature, 2003Co-Authors: Gregor Knorr, Gerrit LohmannAbstract:During the two most recent Deglaciations, the Southern Hemisphere warmed before Greenland1,2. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge3, which is generally assumed to reduce the formation of North Atlantic Deep Water4,5. Yet during Deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode6. Here we use a three-dimensional ocean circulation model7 to investigate the impact of Southern Ocean warming and the associated sea-ice retreat8 on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during Deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route9,10. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of Deglaciation, in agreement with the available proxy records.
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Southern Ocean Origin for Resumption of Atlantic Thermohalilne Circulation during Deglaciation
2003Co-Authors: Gregor Knorr, Gerrit LohmannAbstract:During the last two Deglaciations Southern Hemisphere warming preceded Greenland warming and the northern Atlantic has been exposed to meltwater discharge that is known to reduce North Atlantic deep water (NADW) formation. Yet, Deglaciation is accompanied by a transition from a weak glacial to a strong interglacial Atlantic thermohaline circulation (THC). Here we utilize a three-dimensional ocean circulation model to investigate the impact of Southern Ocean warming and associated sea ice retreat onto the Atlantic THC. We show that gradual warming in the Southern Ocean induces an abrupt resumption of interglacial Atlantic THC by increased mass transport via the warm and cold water route of the global oceanic conveyor belt circulation. This effect prevails over the destabilizing effect of deglacial meltwater input to the northern Atlantic. The mechanism provides a consistent picture of Southern and Northern Hemisphere climate change in agreement with proxy records during Deglaciation.
Derek Fabel - One of the best experts on this subject based on the ideXlab platform.
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Deglaciation of Fennoscandia
Quaternary Science Reviews, 2016Co-Authors: Arjen P. Stroeven, J. Kleman, Derek Fabel, Jonathan M. Harbor, Clas Hättestrand, Jakob Heyman, Ola Fredin, Bradley W. Goodfellow, John D. Jansen, Lars OlsenAbstract:Abstract To provide a new reconstruction of the Deglaciation of the Fennoscandian Ice Sheet, in the form of calendar-year time-slices, which are particularly useful for ice sheet modelling, we have compiled and synthesized published geomorphological data for eskers, ice-marginal formations, lineations, marginal meltwater channels, striae, ice-dammed lakes, and geochronological data from radiocarbon, varve, optically-stimulated luminescence, and cosmogenic nuclide dating. This is summarized as a Deglaciation map of the Fennoscandian Ice Sheet with isochrons marking every 1000 years between 22 and 13 cal kyr BP and every hundred years between 11.6 and final ice decay after 9.7 cal kyr BP. Deglaciation patterns vary across the Fennoscandian Ice Sheet domain, reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins. For example, the ice sheet margin in the high-precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice-marginal deposits in many fjords and coastal valleys. Retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern Sweden and rapid retreat of the calving ice margin in the Baltic Basin. Our reconstruction is consistent with much of the published research. However, the synthesis of a large amount of existing and new data support refined reconstructions in some areas. For example, the LGM extent of the ice sheet in northwestern Russia was located far east and it occurred at a later time than the rest of the ice sheet, at around 17–15 cal kyr BP. We also propose a slightly different chronology of moraine formation over southern Sweden based on improved correlations of moraine segments using new LiDAR data and tying the timing of moraine formation to Greenland ice core cold stages. Retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet, with the lowest rates on the high-elevation and maritime Norwegian margin. Retreat rates compared to the climatic information provided by the Greenland ice core record show a general correspondence between retreat rate and climatic forcing, although a close match between retreat rate and climate is unlikely because of other controls, such as topography and marine versus terrestrial margins. Overall, the time slice reconstructions of Fennoscandian Ice Sheet Deglaciation from 22 to 9.7 cal kyr BP provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the Fennoscandian Ice Sheet, and are an important resource for testing and refining ice sheet models.
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A 10Be-based reconstruction of the last Deglaciation in southern Sweden
Boreas, 2013Co-Authors: Johanna Anjar, Nicolaj K. Larsen, Lena Håkansson, Per Möller, Henriette Linge, Derek FabelAbstract:We present 23 cosmogenic surface exposure ages from 10 localities in southern Sweden. The new 10Be ages allow a direct correlation between the east and west coasts of southern Sweden, based on the same dating technique, and provide new information about the Deglaciation of the Fennoscandian Ice Sheet in the circum-Baltic area. In western Skane, southernmost Sweden, a single cosmogenic surface exposure sample gave an age of 16.8±1.0 ka, whereas two samples from the central part of Skane gave ages of 17.0±0.9 and 14.1±0.8 ka. Further northeast, in southern Smaland, two localities gave ages ranging from 15.2±0.8 to 16.9±0.9 ka (n=5) indicating a somewhat earlier Deglaciation of the area than has previously been suggested. Our third locality, in S Smaland, gave ages ranging from 10.2±0.5 to 18.4±1.6 ka (n=3), which are probably not representative of the timing of Deglaciation. In central Smaland one locality was dated to 14.5±0.8 ka (n=3), whereas our northernmost locality, situated in northern Smaland, was dated to 13.8±0.8 ka (n=3). Samples from the island of Gotland suggest Deglaciation before 13 ka ago. We combined the new 10Be ages with previously published Deglaciation ages to constrain the Deglaciation chronology of southern Sweden. The combined Deglaciation chronology suggests a rather steady Deglaciation in southern Sweden starting at c. 17.9 cal. ka BP in NW Skane and reaching northern Smaland, ∼200 km further north, c. 13.8 ka ago. Overall the new Deglaciation ages agree reasonably well with existing Deglaciation chronologies, but suggest a somewhat earlier Deglaciation in Smaland.
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Importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of Deglaciation
Quaternary Research, 2011Co-Authors: Arjen P. Stroeven, Marc W. Caffee, Derek Fabel, Jonathan M. Harbor, David Fink, Torbjørn DahlgrenAbstract:Deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of Deglaciation. In areas without good Deglaciation varve chronologies and/or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating, typically only general patterns and chronologies of Deglaciation have been deduced. However, mid-latitude ice sheets that had warm-based conditions close to their margins often produced distinctive Deglaciation landform assemblages, including eskers, deltas, meltwater channels and aligned lineation systems. Because these features were formed or significantly altered during the last glaciation, boulder or bedrock samples from them have the potential to yield reliable Deglaciation ages using terrestrial cosmogenic nuclides (TCN) for exposure age dating. Here we present the results of a methodological study designed to examine the consistency of TCN-based Deglaciation ages from a range of Deglaciation landforms at a site in northern Norway. The strong coherence between exposure ages across several landforms indicates great potential for using TCN techniques on features such as eskers, deltas and meltwater channels to enhance the temporal resolution of ice-sheet Deglaciation chronologies over a range of spatial scales.
Anders E. Carlson - One of the best experts on this subject based on the ideXlab platform.
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Final Deglaciation of the Scandinavian Ice Sheet and implications for the Holocene global sea-level budget
Earth and Planetary Science Letters, 2016Co-Authors: Joshua K. Cuzzone, Anders E. Carlson, David J. Ullman, Juha Pekka Lunkka, Vincent Rinterknecht, Peter U. Clark, Glenn A. Milne, Barbara Wohlfarth, Shaun A. Marcott, Marc W. CaffeeAbstract:Abstract The last Deglaciation of the Scandinavian Ice Sheet (SIS) from ∼ 21 , 000 to 13,000 yr ago is well-constrained by several hundred 10Be and 14C ages. The subsequent retreat history, however, is established primarily from minimum-limiting 14C ages and incomplete Baltic-Sea varve records, leaving a substantial fraction of final SIS retreat history poorly constrained. Here we develop a high-resolution chronology for the final Deglaciation of the SIS based on 79 10Be cosmogenic exposure dates sampled along three transects spanning southern to northern Sweden and Finland. Combining this new chronology with existing 10Be ages on Deglaciation since the Last Glacial Maximum shows that rates of SIS margin retreat were strongly influenced by deglacial millennial-scale climate variability and its effect on surface mass balance, with regional modulation of retreat associated with dynamical controls. Ice-volume estimates constrained by our new chronology suggest that the SIS contributed ∼ 8 m sea-level equivalent to global sea-level rise between ∼14.5 ka and 10 ka. Final Deglaciation was largely complete by ∼10.5 ka, with highest rates of sea-level rise occurring during the Bolling–Allerod, a 50% decrease during the Younger Dryas, and a rapid increase during the early Holocene. Combining our SIS volume estimates with estimated contributions from other remaining Northern Hemisphere ice sheets suggests that the Antarctic Ice Sheet (AIS) contributed 14.4 ± 5.9 m to global sea-level rise since ∼13 ka. This new constraint supports those studies that indicate that an ice volume of 15 m or more of equivalent sea-level rise was lost from the AIS during the last Deglaciation.
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Diachronous retreat of the Greenland ice sheet during the last Deglaciation
Quaternary Science Reviews, 2016Co-Authors: G. Sinclair, Anders E. Carlson, Glenn A. Milne, Alan C. Mix, Benoit S. Lecavalier, A. Mathias, Christo Buizert, Robert M. DecontoAbstract:Abstract The last Deglaciation is the most recent interval of large-scale climate change that drove the Greenland ice sheet from continental shelf to within its present extent. Here, we use a database of 645 published 10Be ages from Greenland to document the spatial and temporal patterns of retreat of the Greenland ice sheet during the last Deglaciation. Following initial retreat of its marine margins, most land-based Deglaciation occurred in Greenland following the end of the Younger Dryas cold period (12.9–11.7 ka). However, Deglaciation in east Greenland peaked significantly earlier (13.0–11.5 ka) than that in south Greenland (11.0–10 ka) or west Greenland (10.5–7.0 ka). The terrestrial Deglaciation of east and south Greenland coincide with adjacent ocean warming. 14C ages and a recent ice-sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west Greenland, showing Deglaciation occurring later than observed in 10Be ages. This model-data misfit likely reflects the absence of realistic ice-ocean interactions. We suggest that oceanic changes may have played an important role in driving the spatial-temporal ice-retreat pattern evident in the 10Be data.
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southern laurentide ice sheet retreat synchronous with rising boreal summer insolation
Geology, 2015Co-Authors: Anders E. Carlson, Allegra N. Legrande, Faron S. Anslow, David J. Ullman, Marc W. Caffee, Angus K Moore, Kent M SyversonAbstract:Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive Deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible Deglaciation triggers. Using boulder 10 Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO 2 supports the Milankovic hypothesis of orbital forcing of Deglaciations.
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Why there was not a Younger Dryas-like event during the Penultimate Deglaciation
Quaternary Science Reviews, 2008Co-Authors: Anders E. CarlsonAbstract:Abstract The Younger Dryas cold event is a relatively unique feature of the last Deglaciation when compared to previous Deglaciations, suggesting a unique trigger rather than the commonly held forcing mechanism of North American freshwater routing to the North Atlantic. Here, I compare the last (T-I) and penultimate (T-II) Deglaciations and provide new support for the argument that the lack of a Younger Dryas-like event during T-II is due to the rapidity of Northern Hemisphere ice sheet retreat under greater boreal summer insolation forcing. Faster ice retreat suppressed Atlantic meridional overturning circulation (AMOC) until near the end of T-II, while during T-I AMOC increased relatively early. During T-I, the eastward routing of freshwater that caused the Younger Dryas happened after AMOC resumption, whereas during T-II this routing occurred prior to the resumption of AMOC. Thus the increased flux of freshwater to the North Atlantic during T-II had little effect on AMOC, explaining the lack of a Younger Dryas-like climate oscillation during this Deglaciation.
Gregor Knorr - One of the best experts on this subject based on the ideXlab platform.
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southern ocean origin for the resumption of atlantic thermohaline circulation during Deglaciation
Nature, 2003Co-Authors: Gregor Knorr, Gerrit LohmannAbstract:During the two most recent Deglaciations, the Southern Hemisphere warmed before Greenland1,2. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge3, which is generally assumed to reduce the formation of North Atlantic Deep Water4,5. Yet during Deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode6. Here we use a three-dimensional ocean circulation model7 to investigate the impact of Southern Ocean warming and the associated sea-ice retreat8 on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during Deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route9,10. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of Deglaciation, in agreement with the available proxy records.
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Southern Ocean Origin for Resumption of Atlantic Thermohalilne Circulation during Deglaciation
2003Co-Authors: Gregor Knorr, Gerrit LohmannAbstract:During the last two Deglaciations Southern Hemisphere warming preceded Greenland warming and the northern Atlantic has been exposed to meltwater discharge that is known to reduce North Atlantic deep water (NADW) formation. Yet, Deglaciation is accompanied by a transition from a weak glacial to a strong interglacial Atlantic thermohaline circulation (THC). Here we utilize a three-dimensional ocean circulation model to investigate the impact of Southern Ocean warming and associated sea ice retreat onto the Atlantic THC. We show that gradual warming in the Southern Ocean induces an abrupt resumption of interglacial Atlantic THC by increased mass transport via the warm and cold water route of the global oceanic conveyor belt circulation. This effect prevails over the destabilizing effect of deglacial meltwater input to the northern Atlantic. The mechanism provides a consistent picture of Southern and Northern Hemisphere climate change in agreement with proxy records during Deglaciation.
Frank Bassinot - One of the best experts on this subject based on the ideXlab platform.
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Antarctic Intermediate Water penetration into the Northern Indian Ocean during the last Deglaciation
Earth and Planetary Science Letters, 2018Co-Authors: Christophe Colin, Laure Meynadier, Shiming Wan, Nejib Kallel, Sophie Sepulcre, Arnaud Dapoigny, Frank BassinotAbstract:The two-stage increase in atmospheric carbon dioxide (CO2), and the associated decrease in radiocarbon (14C) during the last Deglaciation, are thought to have been linked to enhanced Southern Ocean upwelling and the rapid release of sequestered 14C-depleted CO2. Antarctic Intermediate Water (AAIW), originating from the Southern Ocean, reflects variations in the Southern Ocean and, crucially, mirrors the chemical signature of upwelling deep water. However, the penetration of AAIW into the Northern Indian Ocean and its relationship with deglacial climate changes have not been thoroughly elucidated to date. Here, we present the neodymium isotopic composition () of mixed planktonic foraminifera from core MD77-176 from an intermediate depth in the Northern Indian Ocean to reconstruct the past evolution of intermediate water during Deglaciation. The record in the Northern Indian Ocean displays two pulse-like shifts towards more radiogenic Southern Ocean values during the Deglaciation, and these shifts coincide with excursions in and records in the Pacific and Atlantic Oceans. These results suggest invasion of AAIW into the Northern Hemisphere oceans associated with enhanced Southern Ocean ventilation during Deglaciation. Our new record strongly supports the close linkage of AAIW propagation and atmospheric CO2 rise through Southern Ocean ventilation during Deglaciation.
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Antarctic Intermediate Water penetration into the Northern Indian Ocean during the last Deglaciation
Earth and Planetary Science Letters, 2018Co-Authors: Christophe Colin, Laure Meynadier, Shiming Wan, Nejib Kallel, Sophie Sepulcre, Arnaud Dapoigny, Frank BassinotAbstract:Abstract The two-stage increase in atmospheric carbon dioxide (CO2), and the associated decrease in radiocarbon (14C) during the last Deglaciation, are thought to have been linked to enhanced Southern Ocean upwelling and the rapid release of sequestered 14C-depleted CO2. Antarctic Intermediate Water (AAIW), originating from the Southern Ocean, reflects variations in the Southern Ocean and, crucially, mirrors the chemical signature of upwelling deep water. However, the penetration of AAIW into the Northern Indian Ocean and its relationship with deglacial climate changes have not been thoroughly elucidated to date. Here, we present the neodymium isotopic composition ( e Nd ) of mixed planktonic foraminifera from core MD77-176 from an intermediate depth in the Northern Indian Ocean to reconstruct the past evolution of intermediate water during Deglaciation. The e Nd record in the Northern Indian Ocean displays two pulse-like shifts towards more radiogenic Southern Ocean values during the Deglaciation, and these shifts coincide with excursions in Δ 14 C and e Nd records in the Pacific and Atlantic Oceans. These results suggest invasion of AAIW into the Northern Hemisphere oceans associated with enhanced Southern Ocean ventilation during Deglaciation. Our new e Nd record strongly supports the close linkage of AAIW propagation and atmospheric CO2 rise through Southern Ocean ventilation during Deglaciation.
