This weeks issue of "Science" (281:1635-1640 and 1622-1623) I thought had
an article on atmospheric lead deposition which I thought had some
interesting implications regarding Carbon 14 dating.
Briefly, a core from a Swiss peat bog was examined from preserved lead
deposits which presumably must reflect influxes of atmospheric lead.
What one sees over the 650 cm is that lead concentrations can be easily
correlated with the record of lead production over the past 5000 years.
Not only can one see the initial rise in lead when the use of lead first
occurred by humans but as advances in lead smelting occurred corresponding
increases occur in the amount of lead trapped in the peat bog. Especially
notable is how lead concentrations skyrocket during the height of the Roman
empire and then diminish until the industrial revolution and then shoot way
up with the introduction of leaded additives in gasoline.
What is most interesting is that all of these events take place in just the
top 200 cm of the core. 14C dating of the core gives a date of 12,370 yr
BP for the bottom of the core and dating from throughout the core suggest a
very even rate of peat deposition (buildup really) over the entire period.
The 220 cm portion where lead concentrations begin to rise is dated at
5320 BP. Lower in the core there are two notable peaks at ca 520 and 400
which were 14C dated at 10,590 and 8230 years the former of which
corresponds to the Younger Dryas period. These peaks are distinguished
as being non-anthropogenic because of the differing lead-206/lead-207
ratios and Pb/Sc ratios. This corresponds to times when ice-sheets were
moving back thus exposing a lot of surface resulting in high amounts of
dust being kicked up into the atmosphere.
If we assume that we have any knowledge of the history of lead use from
around 3000BP to present then it appears that the 14C dates are amazingly
concordant with those dates at which real historical changes in lead
productions are known to have occurred. Correct me if my logic is highly
flawed but these historical events appear to be recorded in only the top
220 cm at most and prior but that is only 1/3 of the core of which most
showed many times less atmospheric lead with only two periods of any
change. These peat bogs have little opportunity to have materials
transported into them from anywhere but the atmosphere and the bogs
themselves must have grown very slowly. If one accepts any portion of the
top of the column, which is well correlated with other bog and lack
sediment cores) then it suggest a much longer history than 4000 years. In
addition the position of the bog is in an area that was glaciated so the
origins of the bog are at least post-glacial and the core doesn't
necessarily reach the bottom of the core. Without suggesting that the
pattern of lead deposition is just coincidentally related to the pattern of
lead use through history and the 14C dates it appears that the 14C dating
is at least accurate over the past 3000 years. Some have suggested
changed concentrations of 14C over time to explain the apparent old dates
but this suggest all of this change occurred some time ago and even more
perplexing much have be associated with a radically increased rate of peat
deposition that slowed around 3000 BP and has been constant since.
Southern Illinois Unversity