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Papers on Younger Dryas cold event

Posted by Ari Jokimäki on January 27, 2011

This is a list of papers on Younger Dryas cold event. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

North Atlantic Deep Water and climate variability during the Younger Dryas cold period – Elmore & Wright (2011) “The Younger Dryas, the last large millennial-scale climate oscillation (12.9–11.6 ka), has been widely attributed to a massive meltwater discharge event that disrupted ocean circulation and plunged the circum–North Atlantic back into a near-glacial state. Low-resolution deep-water reconstructions indicate lower North Atlantic Deep Water (NADW) production during the Younger Dryas, though the Δ14C record requires some deep-water production. Herein, we reconstruct deep-water mass variations using a southern Gardar Drift sediment core with an expanded Younger Dryas section. We show that southern-sourced water invaded the deep North Atlantic to start the Younger Dryas, but was replaced by NADW within 500 yr. Southern-sourced waters briefly reappeared at the end of the Younger Dryas. These deep-water reorganizations to start and end the Younger Dryas suggest that increased meltwater fluxes were limited temporally and focused on regions where deep-water convection occurred during the deglaciation.” Aurora C. Elmore and James D. Wright, Geology, v. 39 no. 2 p. 107-110, doi: 10.1130/G31376.1.

Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean – Murton et al. (2010) “The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.” Julian B. Murton, Mark D. Bateman, Scott R. Dallimore, James T. Teller & Zhirong Yang, Nature 464, 740-743 (1 April 2010) | doi:10.1038/nature08954. [full text]

Putting the Younger Dryas cold event into context – Broecker et al. (2010) “The Younger Dryas event is by far the best studied of the millennial-scale cold snaps of glacial time. Yet its origin remains a subject of debate. The long-held scenario that the Younger Dryas was a one-time outlier triggered by a flood of water stored in proglacial Lake Agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape. The recent suggestion that the Younger Dryas was triggered by the impact of a comet has not gained traction. Instead, evidence from Chinese stalagmites suggests that, rather than being a freak occurrence, the Younger Dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale.” Wallace S. Broecker, George H. Denton, R. Lawrence Edwards, Hai Cheng, Richard B. Alley, and Aaron E. Putnam, Quaternary Science Reviews, Volume 29, Issues 9-10, May 2010, Pages 1078-1081, doi:10.1016/j.quascirev.2010.02.019.

Precise dating of abrupt shifts in the Asian Monsoon during the last deglaciation based on stalagmite data from Yamen Cave, Guizhou Province, China – Yang et al. (2010) “Based on 33 U/Th dates and 1020 oxygen isotopic data from stalagmite Y1 from Yamen Cave, Guizhou Province, China, a record of the Asian Summer Monsoon (ASM) was established. The record covers the last deglaciation and the early Holocene (from 16.2 to 7.3 ka BP) with an average oxygen isotope resolution of 9 years. The main millennial-scale deglacial events first identified in Greenland (Greenland Interstadial Events: GIS 1e through GIS 1a) and later in China are clearly present in the Y1 record. By analogy to earlier work, we refer to these as Chinese Interstadials (CIS): CIS A.1e to CIS A.1a. The onset of these events in Y1 δ18O records are nominally dated at: 14750±50, 14100±60, 13870±80, 13370±80, and 12990±80 a BP. The end of CIS A.1a or the beginning of the Younger Dryas (YD) event is nominally at 12850±50 a BP and the end of the YD dates to 11500±40 a BP. The δ18O values shift by close to 3‰ during the transition into the Bølling-Allerød (BA, the onset of CIS A.1e) and at the end of the YD. Comparisons of Y1 to previously published early Holocene records show no significant phase differences. Thus, the East Asia Monsoon and the Indian Monsoon do not appear to have been out of phase during this interval. The Y1 record confirms earlier work that suggested that solar insolation and North Atlantic climate both affect the Asian Monsoon.” Yan Yang, DaoXian Yuan, Hai Cheng, MeiLiang Zhang, JiaMing Qin, YuShi Lin, XiaoYan Zhu and R. Lawrence Edwards, SCIENCE CHINA Earth Sciences, Volume 53, Number 5, 633-641, DOI: 10.1007/s11430-010-0025-z.

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition – Paquay et al. (2009) “High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabrean fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution. First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition.” François S. Paquay, Steven Goderis, Greg Ravizza, Frank Vanhaeck, Matthew Boyd, Todd A. Surovell, Vance T. Holliday, C. Vance Haynes Jr. and Philippe Claeys, PNAS December 22, 2009 vol. 106 no. 51 21505-21510, doi: 10.1073/pnas.0908874106. [full text]

Nanodiamonds in the Younger Dryas Boundary Sediment Layer – Kennett et al. (2009) “We report abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earth’s typical surficial processes but common to cosmic impacts. N-diamond concentrations range from ≈10 to 3700 parts per billion by weight, comparable to amounts found in known impact layers. These diamonds provide strong evidence for Earth’s collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.” D. J. Kennett, J. P. Kennett, A. West, C. Mercer, S. S. Que Hee, L. Bement, T. E. Bunch, M. Sellers and W. S. Wolbach, Science 2 January 2009: Vol. 323 no. 5910 p. 94, DOI: 10.1126/science.1162819. [full text]

An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period – Brauer et al. (2008) “The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition5. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679 yr BP, broadly coincident with other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate; we suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.” Achim Brauer, Gerald H. Haug, Peter Dulski, Daniel M. Sigman & Jörg F. W. Negendank, Nature Geoscience 1, 520 – 523 (2008), doi:10.1038/ngeo263. [full text]

Geochemical proxies of North American freshwater routing during the Younger Dryas cold event – Carlson et al. (2007) “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.” Anders E. Carlson, Peter U. Clark, Brian A. Haley, Gary P. Klinkhammer, Kathleen Simmons, Edward J. Brook, and Katrin J. Meissner, PNAS April 17, 2007 vol. 104 no. 16 6556-6561, doi: 10.1073/pnas.0611313104. [full text]

A deep-sea coral record of North Atlantic radiocarbon through the Younger Dryas: Evidence for intermediate water/deepwater reorganization – Eltgroth et al. (2006) “Our record of Younger Dryas intermediate-depth seawater Δ14C from North Atlantic deep-sea corals supports a link between abrupt climate change and intermediate ocean variability. Our data show that northern source intermediate water (∼1700 m) was partially replaced by 14C-depleted southern source water at the onset of the event, consistent with a reduction in the rate of North Atlantic Deep Water formation. This transition requires the existence of large, mobile gradients of Δ14C in the ocean during the Younger Dryas. The Δ14C water column profile from Keigwin (2004) provides direct evidence for the presence of one such gradient at the beginning of the Younger Dryas (∼12.9 ka), with a 100‰ offset between shallow (<∼2400 m) and deep water. Our early Younger Dryas data are consistent with this profile and also show a Δ14C inversion, with 35‰ more enriched water at ∼2400 m than at ∼1700 m. This feature is probably the result of mixing between relatively well 14C ventilated northern source water and more poorly 14C ventilated southern source intermediate water, which is slightly shallower. Over the rest of the Younger Dryas our intermediate water/deepwater coral Δ14C data gradually increase, while the atmosphere Δ14C drops. For a very brief interval at ∼12.0 ka and at the end of the Younger Dryas (11.5 ka), intermediate water Δ14C (∼1200 m) approached atmospheric Δ14C. These enriched Δ14C results suggest an enhanced initial Δ14C content of the water and demonstrate the presence of large lateral Δ14C gradients in the intermediate/deep ocean in addition to the sharp vertical shift at ∼2500 m. The transient Δ14C enrichment at ∼12.0 ka occurred in the middle of the Younger Dryas and demonstrates that there is at least one time when the intermediate/deep ocean underwent dramatic change but with much smaller effects in other paleoclimatic records.” Eltgroth, S. F., J. F. Adkins, L. F. Robinson, J. Southon, and M. Kashgarian (2006), Paleoceanography, 21, PA4207, doi:10.1029/2005PA001192. [full text]

Arctic freshwater forcing of the Younger Dryas cold reversal – Tarasov & Peltier (2005) “The last deglaciation was abruptly interrupted by a millennial-scale reversal to glacial conditions, the Younger Dryas cold event. This cold interval has been connected to a decrease in the rate of North Atlantic Deep Water formation and to a resulting weakening of the meridional overturning circulation owing to surface water freshening. In contrast, an earlier input of fresh water (meltwater pulse 1a), whose origin is disputed, apparently did not lead to a reduction of the meridional overturning circulation. Here we analyse an ensemble of simulations of the drainage chronology of the North American ice sheet in order to identify the geographical release points of freshwater forcing during deglaciation. According to the simulations with our calibrated glacial systems model, the North American ice sheet contributed about half the fresh water of meltwater pulse 1a. During the onset of the Younger Dryas, we find that the largest combined meltwater/iceberg discharge was directed into the Arctic Ocean. Given that the only drainage outlet from the Arctic Ocean was via the Fram Strait into the Greenland–Iceland–Norwegian seas, where North Atlantic Deep Water is formed today, we hypothesize that it was this Arctic freshwater flux that triggered the Younger Dryas cold reversal.” Lev Tarasov & W.R. Peltier, Nature 435, 662-665 (2 June 2005) | doi:10.1038/nature03617. [full text]

Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes – McManus et al. (2004) “The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies1, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning2. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.” J. F. McManus, R. Francois, J.-M. Gherardi, L. D. Keigwin & S. Brown-Leger, Nature 428, 834-837 (22 April 2004) | doi:10.1038/nature02494. [full text]

High concentration of atmospheric 14C during the Younger Dryas cold episode – Goslar et al. (2002) “THE various reservoirs of the global carbon cycle, with their very different residence times, are linked by a complex and evolving system of exchanges for which natural radiocarbon is the most robust tracer. Any change in the sizes of these reservoirs, or the exchange rates between them, could perturb the 14C/12C ratio of each other reservoir, and the smallest of them—the atmosphere— would be the most sensitive. In particular, high-resolution reconstructions of past atmospheric 14C/12C ratios may provide important clues to the mechanisms of abrupt climate change. Annually laminated lake sediments potentially provide an optimal record in this respect, as they preserve information about both past atmospheric 14C levels and climate changes, providing absolutely dated material beyond the range of tree-ring chronologies and, unlike corals, directly monitor 14C concentrations in atmospheric CO2. Here we report the relationship between atmospheric 14C concentration and climate changes during the Younger Dryas and early Holocene periods, derived from analyses of the annually laminated sediments of Lake Gosciaz, in central Poland. We find that atmospheric 14C concentrations during the Younger Dryas were abnormally high, which we interpret as a reduced ventilation rate of the deep ocean, most probably as a result of a decrease in intensity of the North Atlantic Deep Water formation.” Tomasz Goslar et al., Nature 377, 414 – 417 (05 October 2002); doi:10.1038/377414a0. [full text]

An Oceanic Cold Reversal During the Last Deglaciation – Stenni et al. (2001) “A detailed deuterium excess profile measured along the Dome C EPICA (European Project for Ice Coring in Antarctica) core reveals the timing and strength of the sea surface temperature changes at the source regions for Dome C precipitation. We infer that an Oceanic Cold Reversal took place in the southern Indian Ocean, 800 years after the Antarctic Cold Reversal. The temperature gradient between the oceanic moisture source and Antarctica is similar to the Dome C sodium profile during the deglaciation, illustrating the strong link between this gradient and the strength of the atmospheric circulation.” Barbara Stenni, Valerie Masson-Delmotte, Sigfus Johnsen, Jean Jouzel, Antonio Longinelli, Eric Monnin, Regine Röthlisberger and Enrico Selmo, Science 14 September 2001: Vol. 293 no. 5537 pp. 2074-2077, DOI: 10.1126/science.1059702. [full text]

The Younger Dryas cold interval as viewed from central Greenland – Alley (2000) “Greenland ice-core records provide an exceptionally clear picture of many aspects of abrupt climate changes, and particularly of those associated with the Younger Dryas event, as reviewed here. Well-preserved annual layers can be counted confidently, with only ≈1% errors for the age of the end of the Younger Dryas ≈11,500 years before present. Ice-flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty. Glaciochemical and particulate data record atmospheric-loading changes with little uncertainty introduced by changes in snow accumulation. Confident paleothermometry is provided by site-specific calibrations using ice-isotopic ratios, borehole temperatures, and gas-isotopic ratios. Near-simultaneous changes in ice-core paleoclimatic indicators of local, regional, and more-widespread climate conditions demonstrate that much of the Earth experienced abrupt climate changes synchronous with Greenland within thirty years or less. Post-Younger Dryas changes have not duplicated the size, extent and rapidity of these paleoclimatic changes.” Richard B. Alley, Quaternary Science Reviews, Volume 19, Issues 1-5, 1 January 2000, Pages 213-226, doi:10.1016/S0277-3791(99)00062-1. [full text]

Synchronous Radiocarbon and Climate Shifts During the Last Deglaciation – Hughen et al. (2000) “Radiocarbon data from the Cariaco Basin provide calibration of the carbon-14 time scale across the period of deglaciation (15,000 to 10,000 years ago) with resolution available previously only from Holocene tree rings. Reconstructed changes in atmospheric carbon-14 are larger than previously thought, with the largest change occurring simultaneously with the sudden climatic cooling of the Younger Dryas event. Carbon-14 and published beryllium-10 data together suggest that concurrent climate and carbon-14 changes were predominantly the result of abrupt shifts in deep ocean ventilation.” Konrad A. Hughen, John R. Southon, Scott J. Lehman and Jonathan T. Overpeck, Science 8 December 2000, Vol. 290 no. 5498 pp. 1951-1954, DOI: 10.1126/science.290.5498.1951. [full text]

Timing of the Antarctic cold reversal and the atmospheric CO2 increase with respect to the Younger Dryas Event – Blunier et al. (1997) “The transition from the Last Glacial to the Holocene is a key period for understanding the mechanisms of global climate change. Ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period. However, interactions between the two hemispheres can only be investigated if ice core records from Greenland and Antarctica can be synchronised accurately and reliably. The atmospheric methane concentration shows large and very fast changes during this period. These variations are well suited for a synchronisation of the age scales of ice cores from Greenland and Antarctica. Here we confirm the proposed lead of the Antarctic Cold Reversal on the Younger Dryas cold event. The Antarctic cooling precedes the Younger Dryas by at least 1.8 kyr. This suggests that northern and southern hemispheres were in anti‐phase during the Younger Dryas cold event. A further result of the synchronisation is that the long‐term glacial‐interglacial increase of atmospheric CO2 was not interrupted during the Younger Dryas event and that atmospheric CO2 changes are not necessarily dominated by changes in the North Atlantic circulation.” Blunier, T., J. Schwander, B. Stauffer, T. Stocker, A. Dällenbach, A. Indermühle, J. Tschumi, J. Chappellaz, D. Raynaud, and J.‐M. Barnola (1997), Geophys. Res. Lett., 24(21), 2683–2686, doi:10.1029/97GL02658.

The impact of glacial lake runoff on the goldthwait and champlain seas: The relationship between glacial lake agassiz runoff and the younger dryas – Rodrigues & Vilks (1994) “The freshwater flux to the North Atlantic Ocean during the last deglaciation included runoff (meltwater plus precipitation) from glacial Lake St Lawrence (ca. 11.6 ka BP), from glacial Lake Agassiz (10.9–9.9 and 9.5–8.0 ka BP) and from glacial Lake Barlow-Ojibway (9.5–8.0 ka BP). Runoff from the glacial lakes does not appear to have mixed with the deep water of the Goldthwait Sea in the Gulf of St Lawrence and was part of the surface outflow to the North Atlantic Ocean. Radiocarbon-dated invertebrate faunal assemblages show that the major impact of Lake Agassiz runoff to the North Atlantic Ocean during the 10.9 to 9.9 ka BP interval occurred after 10.5 ka BP, resulting in the freshening of the Champlain Sea. The Younger Dryas cold episode, on the other hand, began about 11.0 ka BP. The discordance between the major impact of Lake Agassiz runoff on the Champlain Sea and the beginning of the cold episode indicates that Lake Agassiz runoff did not trigger the Younger Dryas cooling. However, the runoff from glacial Lake Agassiz may have sustained the cold climate during the latter part of the Younger Dryas cold episode.” Cyril G. Rodrigues and Gustavs Vilks, Quaternary Science Reviews, Volume 13, Issues 9-10, 1994, Pages 923-944, doi:10.1016/0277-3791(94)90009-4.

Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event – Alley et al. (1993) “THE warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate, the best-known of which is the Younger Dryas event. Despite much study of the causes of this event and the mechanisms by which it ended, many questions remain unresolved. Oxygen isotope data from Greenland ice cores suggest that the Younger Dryas ended abruptly, over a period of about 50 years; dust concentrations in these cores show an even more rapid transition (20 years). This extremely short timescale places severe constraints on the mechanisms underlying the transition. But dust concentrations can reflect subtle changes in atmospheric circulation, which need not be associated with a large change in climate. Here we present results from a new Greenland ice core (GISP2) showing that snow accumulation doubled rapidly from the Younger Dryas event to the subsequent Preboreal interval, possibly in one to three years. We also find that the accumulation-rate change from the Oldest Dryas to the Bø11ing/Allerød warm period was large and abrupt. The extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system.” R. B. Alley, D. A. Meese, C. A. Shuman, A. J. Gow, K. C. Taylor, P. M. Grootes, J. W. C. White, M. Ram, E. D. Waddington, P. A. Mayewski & G. A. Zielinski, Nature 362, 527 – 529 (08 April 1993); doi:10.1038/362527a0. [full text]

A Large Drop in Atmospheric 14C/12C and Reduced Melting in the Younger Dryas, Documented with 230Th Ages of Corals – Edwards et al. (1993) “Paired carbon-14 (14C) and thorium-230(230Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14C and 230Th ages suggests that the atmospheric 14C/12C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14C/12C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.” R. Lawrence Edwards, J. Warren Beck, G. S. Burr, D. J. Donahue, J. M. A. Chappell, A. L. Bloom, E. R. M. Druffel and F. W. Taylor, Science 14 May 1993: Vol. 260 no. 5110 pp. 962-968, DOI: 10.1126/science.260.5110.962.

The younger dryas cold spell—a quest for causes – Berger (1990) “The deep-sea record shows evidence of abrupt climatic change centered on the last deglaciation (14-8 ka), and resulting in a severe cold spell between 11,000 and 10,000 years ago, known as the Younger Dryas period. The origin of this climate catastrophe is not known. Three types of possible causes must be considered: (1) running the system’s positive feedback loop in reverse (albedo, CO2, ocean circulation), (2) disturbance from internal threshold feedback (collapse of ice sheets), and (3) system-external forcing (volcanism, solar output, supernova, cosmic dust). A shutdown of the Nordic heat pump due to excessive meltwater input is one possible cause for the cold spell. This pump is driven by North Atlantic Deep-Water (NADW) production and results in advection of warm water to the Norwegian-Greenland Sea. However, NADW shutdown or slowdown occurred both before and after the Younger Dryas, while warming proceeded rapidly. Consequently, other heat pumping mechanisms may have been more important during deglaciation (e.g., import of warm surface or intermediate waters and export of cold surface water and floating ice). It was the interference with those mechanisms, then, which aided in the Big Freeze. In addition, a short-term reduction in pCO2 during the Younger Dryas appears indicated. The search for a specific cause for the Younger Dryas cold spell may be futile. In a chaotic system near its point of bifurcation, small disturbances can result in large effects, from positive feedback amplification. Every link in the feedback loop is both cause and effect. The possibility of external influence cannot be discounted: there is evidence both for increased volcanic activity and extraterrestrial disturbance at the time.” W.H. Berger, Global and Planetary Change, Volume 3, Issue 3, December 1990, Pages 219-237, doi:10.1016/0921-8181(90)90018-8.

The Younger Dryas Cool Episode in the Gulf of Mexico – Flower & Kennett (1990) “Data are presented from Orca Basin piston core EN32-PC4 in the Gulf of Mexico that confirm the existence of surface water cooling during the Younger Dry as chronozone (11–10 ka). Late glacial planktonic foraminiferal species made a reappearance between 11.4 and 9.8 ka, an episode also marked by distinctly higher oxygen isotopic values derived from the planktonic foraminifer Globigerinoides ruber. The presence of the Younger Dryas event in the gulf at 27°N demonstrates that surface water cooling extended to mid-latitude regions in the North Atlantic. The cool surface water interval is bracketed by rapid shifts in δ18O related to changes in the influx of meltwater to the Gulf of Mexico. A chronology based upon seven accelerator radiocarbon dates indicates that cooling commenced over a ∼500 year period and ended in less than 200 years. These results are among the first deep-sea sediment data documenting the climatic transitions bracketing the Younger Dryas with a rapidity observed in ice core records. A rapid decrease in δ18O values measured in the white form of Gs. ruber at 10.2 ka is explained by significant meltwater influx into the gulf and rapid increase in sea surface temperatures. Surprisingly, a similar decrease is not observed in the pink form of Gs. ruber, a summer surface water dweller in the gulf. This discrepancy may be explained by continued meltwater influx throughout the Younger Dryas during the summers only, such that there was no change in the δ18O of the pink form at the end of the episode. An additional possibility is that warming at the end of the Younger Dryas raised year-average temperatures and summer temperatures remained constant. The coincidence of rapid shifts in δ18O with the Younger Dryas strongly suggests a dynamic causal relationship and therefore supports a model for the cause of the Younger Dryas cooling based on changes in the routing of Laurentide glacial meltwater.” Flower, B. P., and J. P. Kennett (1990), Paleoceanography, 5(6), 949–961, doi:10.1029/PA005i006p00949.

Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode – Broecker et al. (1989) “ROOTH proposed that the Younger Dryas cold episode, which chilled the North Atlantic region from 11,000 to 10,000 yr BP, was initiated by a diversion of meltwater from the Mississippi drainage to the St Lawrence drainage system. The link between these events is postulated to be a turnoff, during the Younger Dryas cold episode, of the North Atlantic’s conveyor-belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the North Atlantic region. This turnoff is attributed to a reduction in surface-water salinity, and hence also in density, of the waters in the region where North Atlantic Deep Water (NADW) now forms. Here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from Orca Basin core EN32-PC4 which reveal a significant reduction in meltwater flow through the Mississippi River to the Gulf of Mexico from about 11,200 to 10,000 radiocarbon years ago. This finding is consistent with the record for Lake Agassiz which indicates that the meltwater from the southwestern margin of the Laurentide Ice Sheet was diverted to the northern Atlantic Ocean through the St Lawrence valley during the interval from ~11,000 to 10,000 years before present (yr BP).” Wallace S. Broecker, James P. Kennett, Benjamin P. Flower, James T. Teller, Sue Trumbore, Georges Bonani & Willy Wolfli, Nature 341, 318 – 321 (28 September 1989), doi:10.1038/341318a0.

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