 |
Earth Sciences Forum
This site is dedicated to the Earth Sciences. We are here for you to discuss issues regarding any aspect of the Earth sciences, at all levels of knowledge. Questions are welcomed, as are open scientific debates. Enjoy!!!
|
| View previous topic :: View next topic |
| Author |
Message |
Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
|
 Posted: Sun Jan 13, 2008 10:25 am Post subject: Exihibit #11 The Younger Dryas in Columba |
 |
|
This is so good. Incredible. In myspace I was accused of cherry picking, allegedly basing a not-cold YD only on the South Greenland lake cores of Bjorck et al and the Meerfelder maar (Lücke & Brauer 2004). These exhibits show otherwise anyway
How much evidence do you need?
Let's check out for instance some older publications:
Van’t Veer R., G.A. Islebe, H. Hooghiemstra, 2000; Climate change during the Younger Dryas chron in Northern South America: a test of the evidence. Quartenary Science Review Vol 19 (2000) pp 1821 - 1835
| Quote: | Abstract
New AMS and palynological data are presented from the Colombian Andes to assess vegetational and climatic change during the Lateglacial-Holocene transition, with special emphasis on the Younger Dryas (YD) chronozone. The new evidence is compared and discussed with other Colombian cores and with data from other countries in the region. The Lateglacial climatic reversal in Colombia, known as the El Abra stadial, has an estimated lower boundary of 11,200 (conventional) and 10,900 (interpolated) 14C yr BP, respectively.
Although the El Abra stadial is assumed to be the equivalent of the European YD, the present data suggest that the El Abra stadial is a biostratigraphical signal representing both the YD chronozone and the earliest Holocene (11,000}9000 14C yr BP). On the basis of new AMS dates and a re-evaluation of the pollen zones, we divided the Colombian El Abra biozone into two phases. From ca. 11,000 to ca. 10,500 14C yr BP there is a sharp increase of subparamo and paramo pollen, reflecting a relatively cool phase during the YD chronozone (zone Y1). After ca. 10,500 14C yr BP, a slight increase of arboreal pollen and the presence of Cactaceae (zone Z1) point toward a relatively milder but drier phase extending to ca. 9000 14C yr BP in the earliest Holocene. Our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern South America between 12,400 and 8800 14C yr BP.
We propose an environmental drought during the El Abra biozone, with subsequent erosion of the sediments deposited during that period, as a major factor explaining the poor presence of YD evidence in northern South America. We conclude that at hydrologically sensitive sites without a clear lithological change around 11,000-9000 14C yr BP, a hiatus during the YD chronozone can only be detected if other cores are available with bracketed time control.
We state that the start of the temperature decline in Central America and northern South America is related to the global YD cooling event. However, much work is still required to understand the duration and amplitude of the climatic reversal during the YD chronozone in this part of the globe. |
A few quotes in the text:
| Quote: | | After ca. 14,200 14C yr BP temperatures increased, indicated by a rapid increase of Andean forest taxa. |
Hey, that 14.200 converts to 16,950 Cal BP, yet another hint that the warming started in the mystery interval, preceding the Bolling Allerod.
| Quote: | Both pollen zones Y2 and Z1 represent the El Abra stadial' in Colombia: a biostratigraphical zone characterized by very low arboreal
pollen values (20}30%) and dominated by pollen of Poaceae and Asteraceae. Pollen zone Y2 is characterized by an increase in pollen of Poaceae; the upper boundary is interpolated at 10,500 14C BP |
We see that a lot all over the northern hemisphere, aridness grasses.
| Quote: | | However, interpretation of pollen spectra dominated by Poaceae (and Asteraceae) in terms of temperature is not always correct, since they can also reflect an increase in aridity (Markgraf, 1993a, b) |
That's what I figured, looking at the Meerfelder Maar where the weeds suggested a relatively mild climate.
| Quote: | We propose that the high values of Poaceae and Asteraceae
tubuliflorae after ca. 10,500 14C yr BP (zone Z1) point toward a decrease in e!ective precipitation rather than temperature. Evidence for this hypothesis is supported by high values of Cyperaceae, indicating a low lake level, and the presence of &drought indicators' like Cactaceae and Sapium in the El Abra-II-B3 record |
Isn't that what these threads are all about, proving that the Younger Dryas was about aridity rather than cooling? Because we have seen already that this is a basic error or misunderderstanding:
| Quote: | Many palaeo-data have become available, especially papers discussing the possible global character of the Younger Dryas chronozone
(11,000-10,000 14C yr BP, e.g. Kennett, 1990; Peteet, 1993, 1995; Bard and Kromer, 1995; Troelstra et al., 1995) |
If we define the Younger Dryas as the period between the major drop in isotopes, and CH4 concentration in the Greenland Ice cores, we should use the counted boundaries: http://earth.myfastforum.org/about80.html
The upper boundary is 12,675 +/- 25 (varve) years BP which would convert back to 10,580 radiocarbon years BP, isn't that cloe enought to that alleged inner boundary of the Younger Dryas between zones Y1 and Y2? Hence Y1 is no Younger Dryas at al, It's the alleged warm last Allerod spike. But:
| Quote: | | From ca. 11,000 to ca. 10,500 14C yr BP there is a sharp increase of subparamo and paramo pollen, reflecting a relatively cool phase during the YD chronozone (zone Y1). |
Hence, again that last Allerod Spike here misidentified as Y1 is also a true mystery interval.
I conclude that the records in Colombia reflect exactly what we are proposing.
_________________
Moderator of http://tech.groups.yahoo.com/group/climatesceptics/
"Don't worry about the world coming to an end today. It's already tomorrow in Australia."
Charley Brown |
|
| Back to top |
|
 |
Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
|
| Posted: Sun Jan 13, 2008 10:37 am Post subject: |
|
|
A second publication with other data seem to confirm all this:
Berrío, J.C., H. Hooghiemstra, R. Marchant, O Rangel 2002, Late Glacial and Holocene history of the dry forest area in the south Colombian Cauca Valley, Journal of Quarternary Science 17(7) pp 667-682
part of the conclusion:
| Quote: | Both cores record a series of discrete intervals, each with characteristic climatic and environmental conditions. The correspondence between both records is presented in Fig. 8. Dry forest has characterised the vegetation of the southern Cauca Valley since Late-glacial times. A change in the composition of dry forest, dated 10 520 14C yr BP, coincides with a shift to warmer and drier climatic conditions in central Colombia (Van ’t Veer et al., 2000) during the El Abra stadial, the equivalent of the Younger Dryas stadial of Europe (Van der Hammen and Hooghiemstra, 1995).
During the Holocene, vegetation change in the valley mainly reflects the dynamics between dry forest and grass-rich vegetation. On the slopes at higher elevation montane forest with characteristic taxa from the Andean forest belt (upper montane forest belt) is represented. Although the period from 7720 to 2880 14C yr BP is documented with less precision (owing to the hiatus at Quilichao, and very low accumulation rate at La Teta) we conclude that grass-dominated vegetation was abundant from 10 520 to approximately 9000 14C yr BP. |
Note that both publications are clear about the start of the warmer and drier period but that the end of El Abra is not identified. Apparantly that signal was stronger further north.
Note also that in the second publication there is no longer a Y1 phase. The chronological problems seemed to have been observed.
_________________
Moderator of http://tech.groups.yahoo.com/group/climatesceptics/
"Don't worry about the world coming to an end today. It's already tomorrow in Australia."
Charley Brown |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
|
FAQ
Search
Memberlist
Usergroups
Join! (free) 
Profile
Log in to check your private messages
Log in
