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Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
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 Posted: Thu Nov 01, 2007 9:17 am Post subject: The Younger Dryas Chapter |
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This is what I have so far in the grand paper about the glacial transition, isotopes, temperatures and humidity about the chapter Younger Dryas:
| Quote: | THE YOUNGER DRYAS
Dating the transition of the Allerod to the Younger Dryas
The most predominant feature of the Younger Dryas is thought to be a sudden return to glacial conditions with massive readvances of ice sheet and glaciers. We also note that there is quite some variance in the apparant start date of the Younger Dryas from 12,900 Cal years BP to 12,650 Cal years BP. The first boundary is likely the result of original carbon dated upper boundary 11,000 14C years BP, which converts to 12910 Years Cal BP. However there are several high resolution proxies available with counting techniques. Fig 3.1 shows the chronology of the three main ice cores.
Fig 3.1 NGRIP (North Greenland Ice Core Project members. 2004) GRIP (Johnsen et al 1997), GISP2 (Grootes and Stuiver. 1997.)
Note for NGRIP average values have been computed and the timescale is adjusted to the standard of 1950 being “present”. If we define the transition between intervals as halfway the transition of the data then the Younger Dryas then NGRIP and GRIP agree between 12,650 and 12,700 counted annual ice layers. GISP2 appears an outlier around 12,850 years ago. We compare this with chronology of the high resolution annual accumulation sediment layers (varve) counted of the Meerfelder maar sediment cores, (Bauer et al 2000, Lücke and Brauer 2004), central European lacustrine sediments (Lake Ammersee, von Grafenstein et al ., 1999; Lake Gosciaz, Goslar et al. , 1995). and see a high correlation between GRIP and NGRIP. We conclude that the beginning of the Younger Dryas is likely to be between 12,700 and 12,650 Cal years BP which would have been between 10,620 and 10,560 radiocarbon years ago. Hence when using the traditional 11,000 radiocarbon years boundary for the Younger Dryas then a considerable part of the last spike of the Allerod is also covered. We investigate if this may have contributed to the confusion about the real nature of the Younger Dryas.
Younger Dryas or Allerod features:
Pienitz et al 2000, conducted Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequence in the Yukon and find in interval I (c. 11 000–10 500 yr BP) that combined evidence from all stratigraphic markers suggests either deposition in a basin fed by glacial meltwaters from the receding Laurentide Ice Sheet or deposition during a cool and dry climatic episode corresponding to the Younger Dryas. However the indicated timeframe agrees with the original but incorrect YD interval of 11,000 – 10,600 radiocarbon years and hence is Bolling Allerod, preceding the Younger Dryas.
Osborn G, et al (1995) find an advance of the Reschreiter Glacier in Ecuador is interpreted to have culminated in the period ca. 11 to 10.6 14C ka BP. assuming that lake-level variations were caused by glacier advances and retreats. This calibrates to 12.9 to 12.7 Ka cal BP coinciding with the last spike of the Allerød
Denton, and Hendy (1994) date the advance of the Franz Josef Glacier in the Southern Alps of New Zealand at 11,050 C 14 years before present and assessed it to be Younger Dryas. However the modern calibration table’s yield a date of 12,940 Ca years BP dating this event in the Bølling Allerød instead.
Goodman et al (2001) find that following rapid deglaciation of unknown extent, an advance of the Quelccaya Ice Cap occurred between 13,090 and 12,800 cal yr B.P., and judge that to approximately with the onset of the Younger Dryas cooling in the North Atlantic region.
Lohne et al 2007 conclude from sea level variation and tectonic evidence that the so-called YD ice-sheet advance in western Norway started during the Allerød, possibly more than 600 years before the Allerød/YD transition
Conclusion:
It appears that the period between 12,900 and 12,650 may have been cold and wet on a large scale on the northern hemisphere, causing glacial re-advances. The original dating of the Younger Dryas boundary (11,000 radio carbon years) seem to have caused those to be attributed to the Younger Dryas but high precision dating based on annual layer counting, suggests that the last Allerod spike was about cold events.
How about confusion about cold/warm dry arid events at the upper boundary of the Younger Dryas, the transition to the Preboreal:
In N America
Pienitz et al 2000, define an “interval II (c. 10 500–8100 yr BP)” and contend that: The early Holocene period showed initially high diatom-inferred salinity values (|20 g L-1), probably driven by low effective moisture and higher temperatures. However 10,500 yr BP calibrates to 12,590 cal years BP. Consequently their interval II includes most of the Younger Dryas.
Ager (2003) questions the nature of the Younger Dryas, seeing a invasion of Populus between 11 ka and 9,5 ka BP (12.9 – 10.7 ka cal BP)
Clark 2003 observes that the Sierra Nevada remained largely or entirely free of glacier ice, including during the Younger Dryas (YD) chronozone
Nordt et al (2002) Increased C4 production between 11,000 and 10,000 14C yr B.P. suggests that the Younger Dryas was a warm interval responding to the diversion of glacial meltwater away from the Mississippi River.
Willard et al 2007 Florida The Younger Dryas (12.9–11.6 ka) was characterized by two distinct phases: slightly drier than the peak Bølling/Allerød between 12.9 and 12.3 ka and much drier from 12.3 to 11.5 ka.
In S America
Paduanoa et al (2003); No evidence is found of a rapid cooling and warming coincident with the Younger Dryas at Lake Titicaca., Patagonia (Glasser et al 2004)
Latorre, et al (2002) Abrupt drying, evident in a dramatic decrease in grass abundance, occurred after 10.5 ka at all localities
Jacobi et al 2007 Northern Brazil isotope data show only a small rise in aridity during Younger Dryas event (13–11.5 ka.
In Europe
Atkinson et al 1987 is a rather renowned paper, frequently quoted to demonstrate extreme coldness in the UK. They do a detailed climate reconstruction in the UK during the last glacial transition and find clear climate oscillations in the range from mystery interval to the Holocene. Their accuracy of the carbon dating is very limited and many datings are extrapolated on a linear scale, while the carbon dating is far from linear however. Therefore no firm conclusions can be drawn about the temperature oscillations and interval boundaries
Meerfelder Maar pollen One of the most outstanding high resolution palynologic proxy covering the complete YD is a sediment core of the Meerfelder Maar Germany (Lücke and Brauer 2004),
fig 4 with original caption,
Its features during the YD are:
1. a strongly reduced pollen count, which depicts unfavourable conditions but arid is more unfavourable than cold.
2. Decline of forests at the expense of grassland suggesting arid conditions
3. Blooming of algae and duckweed suggesting lake low stand (arid) and relatively warm conditions.
4. Main meadow herbs common today as well in that area, no (sub)arctic species noted at any
time.
5. Genus Helianthenum and Sanguisorba officinalis both with limited winter hardiness (USDA zones 4-5) the palynologic data of the Meerfelder maar do not suggest cold rather than arid conditions during the Younger Dryas
Renssen and Van den Berghe (2003)Atmospheric model simulations with different extents of sea-ice are compared with reconstructed European mean annual temperatures derived from permafrost indicators. Moreover, the estimate for the Younger Dryas cooling conflicts with reconstructions based on marine proxy data.
Roos-Barraclough et al 2004 reconstruct a complete 14500-year-long effective precipitation proxy from two Swiss peat cores, showing a very arid Younger Dryas. Comparison with Leysin oxygen isotope record from Schwander et al. (2000) shows a good correlation from 13000 to 11000 cal. yr BP. They infer that temperature rather than precipitation dominated bog surface wetness during the cold, dry Younger Dryas period and the Lateglacial-Holocene transition. However the conclusion should have been that aridity and isotopes correlate, mimmicing the Greenland summit isotopes in that period.
Insert Fig8Roos with original caption
Vescovi et al 2007 In the Southern Alps find 12,700–12,500 cal yr BP, opening of the forests and an expansion of herbs,
Scotland Though a small glacier appears to have developed northeast of The Storr during the subsequent Loch Lomond (Younger Dryas) Stade of c. 12.9–11.5 cal. ka BP, there is no evidence that the area of the Storr landslip was reoccupied by glacier ice at this time
(Ballantyne, 1990).
Asia
Shakun et al 2007 (Jemen) Indian Ocean rainfall decreased continuously and gradually through the Allerød and Younger Dryas. The Holocene began abruptly with increased precipitation at 11.4 ka.
China Monsoonal variations since the last glaciation have also been recognized in the loess profile. An et al. (1993) and Zhou et al. (1998) have associated climatic changes that occurred during the Younger Dryas with a drier–wetter–drier sequence in the Loess Plateau. Also Maher and Hu 2006 record a short arid interval at - 12.5 to 11.5 ka BP (the Younger Dryas) and subsequent summer monsoon intensification as with the northwest African/southwest Asian monsoon records on the West Chinese loess plateaus.
in Mediterranean / AFRICA
Zielhofer et al 2004: After 12.4 ka cal. BP (Younger Dryas) increased fluvial activity led to the sedimentation of coarser overbank deposits (Figure 6) and rhythmically laminated channel fills Nevertheless, the Younger Dryas in northern Tunisia is considered to be induced by an aridification of the Mediterranean climate, because increased geomorphic activity in semi-arid Mediterranean river systems corresponds with drier conditions and not with changes in temperature (Rohdenburg, 1977; 1983; Giessner, 1990; White et al., 1996). Other palaeoclimatic findings indicate a clear aridification within the central Mediterranean region during the Younger Dryas as well (e.g., Chondrogianni et al., 1996; Watts et al., 1996; Allen et al., 1999; Ramrath et al., 2000). The results of the Medjerda valley support a shorter than usual (12.4 to 11.8 ka cal. BP) Younger Dryas event in northern Tunisia. This may be connected to the fact that the interpretation in this study is based on sedimentological and pedological archives, which predominantly reconstruct geomorphic processes. On the other hand, the Ghardimaou Basin possibly did not represent a 'climatically sensitive zone' during the Younger Dryas as shown in palynological records of the adjacent Kasserine region in central Tunisia (Medus and Laval, 1997). However, only 600 years appear to have been effective geomorphologically. There are only few records of late-Quaternary environments in northwestern Africa, which are of a comparable temporal resolution to that of the mid-Medjerda river.
Detailed studies by Lamb et al. (1995) in the mountainous Mediterranean zone of Morocco (lacustrine archive), by Swezey et al. (1999) in the Chott region of southern Tunisia (dunes and sebkha archives) and from Sebkha Mellala in Saharan Algeria (Gasse et al., 1990) show an aridification during the Younger Dryas. The 6180 curve of Sebkha Mellala (Figure 3) indicates a brief (but extreme) dry event during the Younger Dryas lasting less than 500 years (12.6 to 12.1 ka cal. BP). This period matches well with our phase of enhanced fluvial activity in the Medjerda river system
Umer et al 2007, Ethiopia Pollen core: Younger Dryas interval is represented by a small increase in Artemisia and reduced Cyperaceae, indicating aridity. Just after the start of the Holocene (11,200 cal BP), increased moisture and temperature
In New Zealand
Vandergoes et al 2003 observe local vegetation changes around the time of the Younger Dryas chronozone (11–10 ka 14C BP) but those are not necessarily a result of regional climate cooling.
Conclusion
The Younger Dryas shows little if any glacial readvance. Also temperature drops are debatable. It can be observed though that with some exceptions the world was a much more arid place than the previous Allerød and the following Preboreal transition. |
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