Decreasing. According to Figure 22.4 on p. 392 of Faure's _Principles
of Isotope Geology_ (2nd ed.) [1], the 14C/12C ratio was about 2%
higher in 1900 than it is today.
The recent decrease (since about 1900) is known as the Suess Effect.
It is believed to be due to the burning of fossil fuels, which release
a large amount of "dead" carbon into the atmosphere.
[1] Faure's reference for the data is: Lerman, J.C., et al., 1970.
"C14 in tree rings from different localities" in I.U. Olsson, Ed.,
_Radiocarbon Variations and Absolute Chronology_ pp. 275-299.
>> 2. Is there proof that the C14/C12 ratio has ever, in history, been
>> higher than it is today? (Published information indicates that
>> C14/C12 ratios fluctuate + or - 10 to 15% over time)
Yes, see above.
>> 3. Is there an established and verified trend of the C14/C12 ratio over
>> the last 6000 years? (i.e. has C14/C12 ratio been constant over this
>> time frame with fluctuations about the norm and does the norm have a
>> slope?)
Generally decreasing. See for example Figure 14.9 in Dickin's
_Radiogenic Isotope Geology_ ([2]). These fluctuations conform
quite closely to expected fluctations in production rate due to
fluctuations in the Earth's magnetic field. See for example
Figure 14.7 in Dickin ([3]).
[2] Dickin's reference for long-term 14C levels: Bruns, M., et al., 1983.
"The atmospheric 14C level in the 7th millenium BC." in _Physical
and Chemical Techniques in Archaeology_ vol. 8, pp. 511-516.
[3] Dickin's reference for 14C levels vs. expectations derived from
magnetic field changes: Bucha, V., and E. Neustupny, 1967.
"Changes in the Earth's magnetic field and and radiocarbon dating"
in _Nature_ vol. 215, pp. 261-263.
>> 4. What is the period if the C14/C12 fluctuation cycle and what does
>> it look like? (i.e how long does it take to go from high to low
>> to high concentration and does it follow a sinusoid or other pattern?)
It does not appear to follow a regular pattern (and given all of the
variables that can impact production rate, it would not be reasonably
expected to do so).
-- Chris (chris@stassen.com)