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Anders E. Carlson - One of the best experts on this subject based on the ideXlab platform.
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PALEOCLIMATE | The Younger Dryas Climate Event
Encyclopedia of Quaternary Science, 2013Co-Authors: Anders E. CarlsonAbstract:The Younger Dryas is the last major abrupt climate change event of the last deglaciation occurring ~12 900–11 700 years ago. Large portions of the Northern Hemisphere cooled and much of the Southern Hemisphere warmed during the event in a bipolar seesaw pattern. While changes in net precipitation were more variable at higher latitudes, Northern Hemisphere subtropics and tropics were generally drier and the Southern Hemisphere subtropics wetter from southward migration of the Intertropical Convergence Zone. Many of the climate changes related to the Younger Dryas were likely a response to increased freshwater discharge to the North Atlantic and the attendant reduction in Atlantic meridional overturning strength. Although multiple freshwater forcing hypotheses have been proposed, the existing terrestrial and marine records indicate that the northward retreat of the southern margin of the Laurentide Ice Sheet from the Great Lakes caused a routing of freshwater from the western Canadian Plains from the Mississippi River to the St. Lawrence River, with the increased freshwater discharge to the North Atlantic slowing ocean circulation and ultimately causing the Younger Dryas.
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What Caused the Younger Dryas Cold Event
Geology, 2010Co-Authors: Anders E. CarlsonAbstract:The Younger Dryas Cold Event (ca. 12.9–11.6 ka) has long been viewed as the canonical abrupt climate event ([Fig. 1][1]). The North Atlantic region cooled during this interval with a weakening of Northern Hemisphere monsoon strength. The reduction in northward heat transport warmed the Southern
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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.
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Geochemical proxies of North American freshwater routing during the Younger Dryas cold event.
Proceedings of the National Academy of Sciences of the United States of America, 2007Co-Authors: Anders E. Carlson, Peter U. Clark, Brian A. Haley, Gary P. Klinkhammer, Kathleen Simmons, Edward J. Brook, Katrin J. MeissnerAbstract:The Younger Dryas cold interval represents a time when much of the Northern Hemisphere cooled from ≈12.9 to 11.5 kiloyears B.P. The cause of this event, which has long been viewed as the canonical example of abrupt climate change, was initially attributed to the routing of freshwater to the St. Lawrence River with an attendant reduction in Atlantic meridional overturning circulation. However, this mechanism has recently been questioned because current proxies and dating techniques have been unable to confirm that eastward routing with an increase in freshwater flux occurred during the Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca, and 87Sr/86Sr) measured in planktonic foraminifera at the mouth of the St. Lawrence estuary as tracers of freshwater sources to further evaluate this question. Our proxies, combined with planktonic δ18Oseawater and δ13C, confirm that routing of runoff from western Canada to the St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup = 106 m3·s−1). This base discharge increase is sufficient to have reduced Atlantic meridional overturning circulation and caused the Younger Dryas cold interval. In addition, our data indicate subsequent fluctuations in the freshwater flux to the St. Lawrence River of ≈0.06–0.12 Sverdrup, thus explaining the variability in the overturning circulation and climate during the Younger Dryas.
Bo Stromberg - One of the best experts on this subject based on the ideXlab platform.
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Younger Dryas deglaciation at mt billingen and clay varve dating of the Younger Dryas preboreal transition
Boreas, 2008Co-Authors: Bo StrombergAbstract:A clay varve chronology has been established for the Late Weichselian ice recession east of Mt. Billingen in Vastergotland, Sweden. In this area the Middle-Swedish end moraine zone was built up as a consequence of cold climate during the Younger Dryas stadial. A change-over from rapid to slow retreat as a result of climatic deterioration at the Allerod/Younger Dryas transition cannot be traced with certainty in the varve sequences, but it seems to have taken place just before 11,600 varve years BP. The following deglaciation was very slow for about 700 years — within the Middle-Swedish end moraine zone the annual ice-front retreat was only c. 10 m on average. A considerable time-lag is to be expected between the Younger Dryas climatic event and this period of slow retreat. The 700 years of slow retreat were succeeded by 200 years of more rapid recession, about 50–75 m annually, and then by a mainly rapid and uncomplicated retreat of the ice-front by 100–200 m/year or more, characterizing the next 1500 years of deglaciation in south and central Sweden. The change from about 50–75 m to 100–200 m of annual ice-front retreat may reflect the Younger Dryas/Preboreal transition. Clay-stratigraph-ically defined, the transition is dated at c. 10,740 varve years BP, with an error of +100 to -250 years. In the countings of ice layers in Greenland ice cores (GRIP and GISP-2) the end of the Younger Dryas climatic event is 800–900 years older. However, a climatic amelioration after the cold part of the Younger Dryas and in early Preboreal should rapidly be reflected by for example chemical components and dust in Greenland ice cores, and by increasing δ13C content in tree rings. On the other hand, the start of a rapid retreat of the inland ice margin can be delayed by several centuries. This can explain at least a part of the discrepancy between the time-scales.
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Younger Dryas deglaciation at Mt. Billingen, and clay varve dating of the Younger Dryas/Preboreal transition
Boreas, 2008Co-Authors: Bo StrombergAbstract:A clay varve chronology has been established for the Late Weichselian ice recession east of Mt. Billingen in Vastergotland, Sweden. In this area the Middle-Swedish end moraine zone was built up as a consequence of cold climate during the Younger Dryas stadial. A change-over from rapid to slow retreat as a result of climatic deterioration at the Allerod/Younger Dryas transition cannot be traced with certainty in the varve sequences, but it seems to have taken place just before 11,600 varve years BP. The following deglaciation was very slow for about 700 years — within the Middle-Swedish end moraine zone the annual ice-front retreat was only c. 10 m on average. A considerable time-lag is to be expected between the Younger Dryas climatic event and this period of slow retreat. The 700 years of slow retreat were succeeded by 200 years of more rapid recession, about 50–75 m annually, and then by a mainly rapid and uncomplicated retreat of the ice-front by 100–200 m/year or more, characterizing the next 1500 years of deglaciation in south and central Sweden. The change from about 50–75 m to 100–200 m of annual ice-front retreat may reflect the Younger Dryas/Preboreal transition. Clay-stratigraph-ically defined, the transition is dated at c. 10,740 varve years BP, with an error of +100 to -250 years. In the countings of ice layers in Greenland ice cores (GRIP and GISP-2) the end of the Younger Dryas climatic event is 800–900 years older. However, a climatic amelioration after the cold part of the Younger Dryas and in early Preboreal should rapidly be reflected by for example chemical components and dust in Greenland ice cores, and by increasing δ13C content in tree rings. On the other hand, the start of a rapid retreat of the inland ice margin can be delayed by several centuries. This can explain at least a part of the discrepancy between the time-scales.
Martyn R. Drury - One of the best experts on this subject based on the ideXlab platform.
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The Younger Dryas impact hypothesis: a critical review
Quaternary Science Reviews, 2014Co-Authors: Annelies Van Hoesel, Wim Z. Hoek, G.m. Pennock, Martyn R. DruryAbstract:Abstract The Younger Dryas impact hypothesis suggests that multiple extraterrestrial airbursts or impacts resulted in the Younger Dryas cooling, extensive wildfires, megafaunal extinctions and changes in human population. After the hypothesis was first published in 2007, it gained much criticism, as the evidence presented was either not indicative of an extraterrestrial impact or not reproducible by other groups. Only three years after the hypothesis had been presented, a requiem paper was published. Despite this, the controversy continues. New evidence, both in favour and against the hypothesis, continues to be published. In this review we briefly summarize the earlier debate and critically analyse the most recent reported evidence, including magnetic microspherules, nanodiamonds, and iridium, shocked quartz, scoria-like objects and lechatelierite. The subsequent events proposed to be triggered by the impact event, as well as the nature of the event itself, are also briefly discussed. In addition we address the timing of the Younger Dryas impact, a topic which, despite its importance, has not gained much attention thus far. We show that there are three challenges related to the timing of the event: accurate age control for some of the sites that are reported to provide evidence for the impact, linking these sites to the onset of the Younger Dryas and, most importantly, an apparent age discrepancy of up to two centuries between different sites associated with the proposed impact event. We would like to stress that if the markers at different locations have been deposited at different points in time, they cannot be related to the same event. Although convincing evidence for the hypothesis that multiple synchronous impacts resulted in massive environmental changes at ∼12,900 yrs ago remains debatable, we conclude that some evidence used to support the Younger Dryas impact hypothesis cannot fully be explained at this point in time.
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Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary
Proceedings of the National Academy of Sciences of the United States of America, 2012Co-Authors: Annelies Van Hoesel, Wim Z. Hoek, G.m. Pennock, F. Braadbaart, Johannes Van Der Plicht, Martyn R. DruryAbstract:The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact—after the other markers proved to be neither reproducible nor consistent with an impact—is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerod/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.
Annelies Van Hoesel - One of the best experts on this subject based on the ideXlab platform.
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A search for shocked quartz grains in the Allerød‐Younger Dryas boundary layer
Meteoritics & Planetary Science, 2015Co-Authors: Annelies Van Hoesel, Wim Z. Hoek, G.m. Pennock, Knut Kaiser, Oliver Plümper, Michał Jankowski, M. F. Hamers, Norbert Schlaak, Mathias Küster, Alexander V. AndronikovAbstract:The Younger Dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the Allerod interstadial resulted in the Younger Dryas cold period. So far, no reproducible, diagnostic evidence has, however, been reported. Quartz grains containing planar deformation features (known as shocked quartz grains), are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting. Although alleged shocked quartz grains have been reported at a possible Allerod-Younger Dryas boundary layer in Venezuela, the identification of shocked quartz in this layer is ambiguous. To test whether shocked quartz is indeed present in the proposed impact layer, we investigated the quartz fraction of multiple Allerod-Younger Dryas boundary layers from Europe and North America, where proposed impact markers have been reported. Grains were analyzed using a combination of light and electron microscopy techniques. All samples contained a variable amount of quartz grains with (sub)planar microstructures, often tectonic deformation lamellae. A total of one quartz grain containing planar deformation features was found in our samples. This shocked quartz grain comes from the Usselo palaeosol at Geldrop Aalsterhut, the Netherlands. Scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging, however, show that the planar deformation features in this grain are healed and thus likely to be older than the Allerod-Younger Dryas boundary. We suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the European sandbelt. The single shocked quartz grain at this moment thus cannot be used to support the Younger Dryas impact hypothesis.
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The Younger Dryas impact hypothesis: a critical review
Quaternary Science Reviews, 2014Co-Authors: Annelies Van Hoesel, Wim Z. Hoek, G.m. Pennock, Martyn R. DruryAbstract:Abstract The Younger Dryas impact hypothesis suggests that multiple extraterrestrial airbursts or impacts resulted in the Younger Dryas cooling, extensive wildfires, megafaunal extinctions and changes in human population. After the hypothesis was first published in 2007, it gained much criticism, as the evidence presented was either not indicative of an extraterrestrial impact or not reproducible by other groups. Only three years after the hypothesis had been presented, a requiem paper was published. Despite this, the controversy continues. New evidence, both in favour and against the hypothesis, continues to be published. In this review we briefly summarize the earlier debate and critically analyse the most recent reported evidence, including magnetic microspherules, nanodiamonds, and iridium, shocked quartz, scoria-like objects and lechatelierite. The subsequent events proposed to be triggered by the impact event, as well as the nature of the event itself, are also briefly discussed. In addition we address the timing of the Younger Dryas impact, a topic which, despite its importance, has not gained much attention thus far. We show that there are three challenges related to the timing of the event: accurate age control for some of the sites that are reported to provide evidence for the impact, linking these sites to the onset of the Younger Dryas and, most importantly, an apparent age discrepancy of up to two centuries between different sites associated with the proposed impact event. We would like to stress that if the markers at different locations have been deposited at different points in time, they cannot be related to the same event. Although convincing evidence for the hypothesis that multiple synchronous impacts resulted in massive environmental changes at ∼12,900 yrs ago remains debatable, we conclude that some evidence used to support the Younger Dryas impact hypothesis cannot fully be explained at this point in time.
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Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary
Proceedings of the National Academy of Sciences of the United States of America, 2012Co-Authors: Annelies Van Hoesel, Wim Z. Hoek, G.m. Pennock, F. Braadbaart, Johannes Van Der Plicht, Martyn R. DruryAbstract:The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact—after the other markers proved to be neither reproducible nor consistent with an impact—is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerod/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.
Svante Björck - One of the best experts on this subject based on the ideXlab platform.
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PALEOCLIMATE RECONSTRUCTION | Younger Dryas Oscillation, Global Evidence
Encyclopedia of Quaternary Science, 2013Co-Authors: Svante BjörckAbstract:The research history of the Younger Dryas oscillation is briefly described. Different Younger Dryas archives are presented, and the methods by which the event is dated are discussed. Possible mechanisms behind the oscillation, its presence in Northern and Southern Hemisphere records, and its possible role within a bipolar seesaw climatic pattern are discussed.
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tree rings and ice cores reveal c 14 calibration uncertainties during the Younger Dryas
Nature Geoscience, 2008Co-Authors: Raimund Muscheler, Bernd Kromer, Anders Svensson, M. Friedrich, Klaus Felix Kaiser, Svante Björck, John SouthonAbstract:The Younger Dryas interval during the Last Glacial Termination was an abrupt return to glacial-like conditions punctuating the transition to a warmer, interglacial climate. Despite recent advances in the layer counting of ice-core records of the termination, the timing and length of the Younger Dryas remain controversial. Also, a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval, recorded primarily in the Cariaco Basin, has been difficult to reconcile with simulations of the Younger Dryas carbon cycle. Here we discuss a radiocarbon chronology from a tree-ring record covering the Late Glacial period that has not been absolutely dated. We correlate the chronology to ice-core timescales using the common cosmic production signal in tree-ring 14C and ice-core 10Be concentrations. The results of this correlation suggest that the Cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the Younger Dryas climate reversal in the Cariaco basin. This bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval. Our tree-ring-based radiocarbon record is easily reconciled with simulated production rates and carbon-cycle changes associated with reduced ocean ventilation during the Younger Dryas. Attaching a ‘floating’ tree-ring chronology to ice core records that cover the abrupt Younger Dryas cold interval during the last glacial termination provides a better estimate of the onset and duration of the radiocarbon anomaly. The chronology suggests that marine records may be biased by changes in the concentration of radiocarbon in the ocean, which may affect the accuracy of a popular radiocarbon calibration program during this interval.
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Younger Dryas oscillation, global evidence
2007Co-Authors: Svante BjörckAbstract:The research history of the Younger Dryas oscillation is briefly described. Different Younger Dryas archives are presented and the methods by which the event is dated are discussed. Possible mechanisms behind the oscillation, its presence in northern and southern Hemisphere records and its possible role within a bi-polar seesaw climatic pattern are discussed.
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Variations of atmospheric 14C concentrations over the Allerød-Younger Dryas transition
Climate Dynamics, 1999Co-Authors: Tomasz Goslar, Svante Björck, Barbara Wohlfarth, Göran Possnert, Jonas BjörckAbstract:Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS 14 C-dated records, Lake Gościąz, and a floating Late Weichselian glacial varve chronology at the Allerod-Younger Dryas transition allowed us to assess in detail atmospheric Δ 14 C changes between late Allerod and early Preboreal. The combined data set shows a drastic rise in Δ 14 C during the first 200 years or so of Younger Dryas and the two following about 500 year- long 1 4 C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO 2 variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ 14 C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ 14 C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ 14 C signal in the middle of Younger Dryas remains an open question.
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Variations of atmospheric 14 C concentrations over the Alleroød-Younger Dryas transition
Climate Dynamics, 1999Co-Authors: Tomasz Goslar, Svante Björck, Barbara Wohlfarth, Göran Possnert, Jonas BjörckAbstract:Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS 14C-dated records, Lake Gościąz, and a floating Late Weichselian glacial varve chronology at the Allerod-Younger Dryas transition allowed us to assess in detail atmospheric Δ14C changes between late Allerod and early Preboreal. The combined data set shows a drastic rise in Δ14C during the first 200 years or so of Younger Dryas and the two following about 500 year-long 14C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO2 variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ14C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ14C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ14C signal in the middle of Younger Dryas remains an open question.