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Or A Radioelement Calibration Problem Perhaps?

  1. Dec 18, 2008 #1
    'A Causality Problem For Milankovitch' is the title of this report, which challenges the insolation theory with reference to a cave in Nevada called Devils Hole. The penultimate termination of the ice age at 135 ka seems incompatible with the new data in 1992. The Devils Hole chronology was based on U-Th dating and then replicated with Uranium-Protactinium dating. But I suggest that it should be considered that orbital eccentricity affects the relative abundance of radioelements in the atmosphere and ocean. If this is so, then the discrepancy can be reduced or eliminated.

    Here is an wikipedia explanation for radiocarbon calibration:

    "A raw BP date cannot be used directly as a calendar date, because the level of atmospheric 14C has not been strictly constant during the span of time that can be radiocarbon dated. The level is affected by variations in the cosmic ray intensity which is in turn affected by variations in the earth's magnetosphere. In addition, there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rocks. Changes in the Earth's climate can affect the carbon flows between these reservoirs and the atmosphere, leading to changes in the atmosphere's 14C fraction."

    If orbital eccentricity affects the magnetosphere, then the calibration for atmospheric radioelements would need to be adjusted. This could potentially solve a lot of problems with dating anomalies.

    This article (Dec 18, 2008) highlights my viewpoint with regard to carbon-dating. How long before questions are asked about the accuracy of U-Th dating?
     
    Last edited: Dec 18, 2008
  2. jcsd
  3. Dec 18, 2008 #2
    Actually it appears that your article is just explaining the dating differences as I did in the other thread. However calibrating the 14C is now limited to maximum of 26,000 Calendar years and:

    Apparantly yet another record is adding more robustness to the existing calibration tables and is to extend it to 30,000 years.

    However, in between all that useful information we find a sudden dissonant:

    Most definitely not, take for instance for instance this listing compared to the start of the warm Bolling Allerod interval at 14,500 Cal years BP:

    Refs should be in here

    See that there is a stubborn difference of some 2500 years between all these warming signs and the isotope thermometer of the Greenland ice cores?:

    [​IMG]
    Alley 2000

    then
    That's a different ballgame, It's fine to challenge anything but there are several dating methods for the longer range. And again, the robustness of each method is the ability to get consistent results for the same sample using several techniques. So far the only outlier we seem to see are the greenland ice cores
     
    Last edited: Dec 18, 2008
  4. Dec 18, 2008 #3

    CRGreathouse

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    I don;t think there are likely to be the same kinds of problems. With 14C dating, we have to guess how much 14C there was originally, because we can't tell daughter 12C from native 12C. With U-Th dating we can see the daughter products since they don't really occur otherwise, so there isn't a problem of that sort. The main problem there is the precision of measurement of U and Th levels, which need to be much more precise to get good time resolution (since the decay rate is so much slower, naturally).
     
  5. Dec 18, 2008 #4
    Exactly, that is... almost, the most likely daughters of 14C are 14N and a negatron (ß-)

    http://www.cq.rm.cnr.it/c-14.html
     
  6. Dec 19, 2008 #5

    Many thanks to both of you for your helpful comments. I'm new to this subject, so I'll need time to digest it all.

    Is it the case that cosmogenic isotope dating is potentially subject to calibration adjustment due to a varying magnetosphere with orbital eccentricity? The Greenland ice core data is from oxygen-18, which isn't affected by cosmic ray flux. All the examples you give above, Andre, are either 14C or 36Cl (I think). I propose that the 18O isotope data is a measure of the strength of the Pacific Warm Current, which is why it indicates the 100-kyr cycle. This is why there is a discrepancy in the two types of isotope proxy data.
     
    Last edited: Dec 19, 2008
  7. Dec 19, 2008 #6
    You're most welcome, and thanks for your responsiveness. It may be clear now that one needs a thorough base knowledge just about everything before one could do some philosophizing about what has happened in the late Pleistocene.

    If so it is masked by many unclear processes. If one plots the delta14C data the result would be a rather randomly looking plot with unexpected spikes. The ratio of 14CO2 in atmospheric CO2 is mainly a function of total concentration of CO2, cosmic rays intensity (production), ocean overturning (fractination 14CO2 prefers to be dissolved in water over 12CO2) and also a little photosynthesis, Plants prefer 12CO2. So it's a complex function and it's rather difficult to single out causes of changes.


    Careful, to have a better understanding in the variation in water isotopes, it's better to take a course hydrography first.

    and also study how the researchers came to their conclusions:

    Jouzel, J. et al 1997. Validity of the Temperature Reconstruction from Water Isotopes in Ice Cores; Journal of Geophysical Research Vol 102, No C12 pp 26,471-26,487, November 30
     
    Last edited by a moderator: Dec 20, 2008
  8. Dec 19, 2008 #7
    I agree with this. Okay, so I need to do some homework first. I'm on the case..
     
  9. Dec 23, 2008 #8
    I've thought about this for a few days and agree with the authors that Imbrie and Imbrie's coral terrace dating is short by around 10 ka. This would put the dates of high sea levels in correlation with High Southern Hemisphere Summer Insolation and low eccentricity. I suggest that there is less sea level rise with high Northern Hemisphere insolation due to the fact that the melting of the ice cap wouldn't affect sea level. A higher percentage of land to sea ice in Antarctica is the reason for an increase in sea level compared to insolation in the North Polar Regions.

    My earlier considerations of inaccurate isotope calibration doesn't apply in this instance.
     
  10. Dec 23, 2008 #9
    There are still more problems around the dating of the last interglacial, the Eemian aka Sangamonian.

    Esat et al 1999

    and
    Andrews et al 2007

    but also similar problems earlier Henderson 2006


    (*) MIS= Maritime Isotope Stage, commonly used counted periods of general high and low isotope ratio's (δ18O) in the oceanic sediment cores, we are now in MIS1, MIS2 was the last glacial maximum.
     
    Last edited: Dec 23, 2008
  11. Dec 30, 2008 #10
    It seems incredible to me, as a newcomer to the science of paleoclimatology, that phrases such as 'sub-oribital' are being used. The Red Sea sea-level curve shows rapid oscillations over relatively short time spans. But should this really be a surprise when the magnitude and significance of the millennial cycle has been known about for over a decade, evidenced by the Greenland ice cores?

    I have come to the conclusion that there are three forcings which need to be considered as a single system:

    • the 100-kyr cycle
    • the bipolar seesaw
    • the millennial cycle
    A complex series of climate change results from the interaction of these three forcings and it is imperative that any isotope proxy data be judged with this in mind. Imbrie and Imbrie's initial concept of insolation is just a part of the bipolar seesaw, which is itself affected by the strength of the 100-kyr cycle and the millennial cycle. Any questions and speculations also need to be made with this in mind. The article questioning 'A Causality Problem for Milankovitch' now seems written in a language which is out-of-date. The first line makes me smile in it's childish simplicity:

    "The Milankovitch theory postulates that changes in incident solar radiation (insolation) in the Northern Hemisphere during the summer months is responsible for driving the Earth's ice ages."
     
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