|Register to reply||
Isotope issues on palaeo climate and carbon dating
|Share this thread:|
Jun5-04, 03:58 PM
Last week some climate issues have been brought to the attention. So Nereid asked me about palaeo climate. A good incentive to have a look into that area again. I have made some pertinent statements and I think it may be time again to explain why.
Basically I think that we over estimate our knowledge about palaeclimate and palaeo temperatures. To show that I would like to compare the history of carbon dating to the history of climate and isotope proxies, to pint out some remarkable differences in very similar processes
This for instance, is this true?
Vostok ice core data
What makes it that we put a temperature scale on the Y-axis of the lower graph? The simple assumption that isotopes are temperature. Okay some corrections account for local effects but the main idea is that the ratio of heavy and light atoms is mainly dependant on temperature. In this case here the temperature scale is an assumption based upon the ratio of normal hydrogen (H) and heavy hydrogen atoms (Deuterium - D), in the ice of the Vostok ice cores. The ratio is also indicated as “dD” or also “d2H”. So most of the time you find this graph with “dD”on the Y-axis. At least that’s correct.
Now here the carbon dating comes in, since carbon dating is also playing with isotopes, but what a problems it encountered. So let me explain first and then compare it to palaeo thermometry.
Carbon dating was developed in the 1950, here is how it basically works. Just heavy radioactive 14C atoms that are produced in the air and that decay again with a half time of some 5700 years (normal –light- carbon is 12C). Live tissue has the same heavy 14C – 12C carbon ratio as the air, while dead tissue starts to loose the 14C, and the 14C – 12C ratio decreases. this can be used measuring it’s age.
In the early days, it was as simple as that. Just have a radioactivity geiger counter, count the ticks, assume that it is decaying 14C that you count. Do the math and there you are, the age of the thing. Well, in fact, the calculation brought anything but the correct age because numerous complications have been neglected. The large errors were mostly attributed to sample contamination but this is something that really doesn’t happen that often.
So what are the real problems exactly?
- radioactive noise
- Variation of the concentration of carbon dioxide in the air, depending on the activity of sources and sinks with it’s own fractionation characterises, producing or reducing carbon dioxide concentration depleted of heavy isotopes or even enriched with heavy isotopes.
- Variation of radiogenic production of heavy 14C. The more cosmic activity, the more 14C
- The fractionation of heavy atoms during the growing of that particular tissue. Photo synthetic processes, predating and digestive processes, air breathing, or underwater elements etc. But these fractionation processes are equally dependant on temperature and relative humidity as the dD in Vostok ice cores.
These problems have been exposed numerous times by erratic results so they have been addressed in the recent past, finally resulting in more or less adequate solutions.
radioactive noise: counting 14C nowadays is done with a mass spectrometer, not with radioactivity anymore, just like all the other isotope counting procedures, a tremendous improvement.
Variation of carbon dioxide and all the processes that change the 14C concentration in the air are now adjusted by a calibration table that has been build by comparing carbon age with annual layer counting, annual layers of tree rings (dendrochronology) ice cores layers and lake sedimentation layers (varves). Differences up to 2500 years are normal in the ice age period.
Variation due fractionation processes with all its variables are dealt with by also counting the stable moderate heavy 13C atoms. As the 13C-12C ratio tells something about the fractionation of heavier 14C atoms, we can adjust the assumption of the original 14C ratio, just by multiplying 13C fractionation differences by 2. The 13C ratio (d13C) in a specific time frame could be derived from the CO2 in the ice cores, but I don’t think that we are that far yet, so for the moment I think we still assume a constant d13C for the CO2 in the air.
Since carbon dating is a very important instrument for basic dating we have put a lot of effort in correcting for all the errors, we could think of. Finally, we have a system with a reasonable degree of accuracy and trustworthiness.
Isotope Paleo thermometry is still in the 1950’s of the carbon dating. Isotope ratios = temperature, it’s as simple as that. We assume a basic ratio of the source like standard sea water (SMOW) and we assume basic fractionation processes like evaporating and condensing. We are not triggered to look into more detail of these processes, since the ice cores seem to reflect exactly what we think we know about palaeo climates of the ice age (unaware of many circular reasonings). So obviously there seem to be no errors that would trigger us to investigate the problems of variation of isotope ratios of sources and variations and complications of fractionation processes. So why make it more complicated? Unfortunately Occams Razor does not work that way and we have miles to go before we sleep. The bottom graph is wrong, we do not see temperatures, we see isotopes as a result of many processes, nothing more, no climate.
Earth sciences news on Phys.org
|Register to reply|
|Carbon Dating||Advanced Physics Homework||0|
|Carbon-14 isotope question||Biology||4|