Carbon 14 and Lead Deposition
by R. Joel Duff*
Southern Illinois University, Carbondale, IL
From: PSCF 51 (March 1999): 6-7
The 11 September 1998 issue of Science included an article, "History of Atmospheric Lead Deposition Since 12,370 14C yr BP from a Peat Bog, Jura Mountains, Switzerland" (pp.1635ñ40, summarized on pp. 1622ñ3). This article reports the findings from the study of lead (Pb) and scandium (Sc) concentrations in a core from a Swiss peat bog. The lead concentrations were found to vary greatly over the length of the core and, based on previous studies,1 are attributable to the effects of atmospheric lead deposition alone. Interestingly, lead concentrations from a depth of 145 cm, dated at 3000 14C yr BP, to the top of the 650 cm core can be easily correlated with the record of lead production over the past 3000 years. At the 145 cm mark, the 206Pb/207Pb ratio begins to decrease and the amount of Pb relative to Sc no longer remains in the same proportions. This is indicative of the advent of mining and the subsequent release of lead into the atmosphere. Prior to this time, ratios of radiogenic lead and Pb/Sc match those from exposed soils from Europe and Northern Africa. Above 145 cm, changes in deposition rates and 206Pb/207Pb ratios follow patterns of the use of lead throughout history including the rise and fall of the Roman Empire, the medieval silver production in Germany, the Industrial Revolution, and, finally, to its highest levels with the introduction of gasoline containing lead.
14C dates of each of these events in the Swiss bog correlates well with the dates for these events as they are understood from other archeological and historical records. Hence, the lead concentrations in the core provide dependent confirmation of the validity of 14C dating over at least the past 3000 years. Further 14C dating of the core gives a date of 12,370 yr BP for the bottom of the core and the dates derived from 30 cm and below (above represents less compacted peat) suggest a uniform rate of peat production over the entire period. Lead concentrations increase, though the ratios of radiogenic and Pb/Sc do not change, at 220 cm dated at 5320 BP. This is thought to be the result of human occupation of the surrounding regions and deforestation resulting in increased release of dust into the atmosphere. Lower in the core, there are two notable peaks at ca 520 and 400 cm that were 14C dated at 10,590 and 8230 years BP, the former of which corresponds to the Younger Dryas period. These peaks are distinguished as being non-anthropogenic in origin because the 206Pb/207Pb and Pb/Sc ratios are reflective of the natural compositions of rocks that would have been exposed during these times of reduced vegetative cover and increased erosion.
Because presumably well-understood historical events are recorded in the top 145 cm of this 650 cm core, it seems natural to ask what happened during the deposition of more than 3/4 of the peat in this bog. Both 14C dates and peat production rates over 3000 years suggest at least 12,000 years of elapsed time. If the 12C/14C ratio had been much higher in the past, as some suggest, then the peat would have to be interpreted as having been deposited at much higher rates in the past. Though it is not impossible that these two could be correlated in some fashion, I would suggest that there are further complications that make this unlikely. For example, the timing of the origins of this bog is not clear. I would submit that no current model of a young earth would place the origins of this bog near the time of a global flood. Rather the bog would have originated at least several hundred years after the flood, due to the age and nature of the rock lying below the bog and the origins of the valley in which the bog is found. This would then force the production of over four meters of peat into a period of less than 1000 years, during a time postdating the supposed time of a much higher 12C/14C ratio. Further, the physical position of these bogs as isolated, raised mounds leaves virtually no opportunity to have materials transported into them from anywhere but the atmosphere. Examination of the entire core demonstrates that when the effects of anthropogenic sources of lead are eliminated, the amount of atmospheric influx of lead has been nearly constant throughout the entire core (except for two short periods around 520 and 400 cm). If the Sphagnum in the bog had simply accumulated at much faster rates in the past, I would not expect the amount of lead deposition in that Sphagnum to have remained at a nearly constant level. Lastly, the observed pattern of changes in lead concentrations in this core have been shown to be correlated with similar patterns in other bogs and lake sediment records in Europe and the Greenland GRIP ice core.
Note
1P. Steinmann and W. Shotyk, "Chemical composition, pH, and redox state of sulfur and iron in complete vertical porewater profiles from two Sphagnum peat bogs, Jura Mountains, Switzerland," Geochimica et Cosmochimica Acta 61:6 (1997): 1143ñ63; and óóó, "Geochemistry, mineralogy, and geochemical mass balance on major elements in two peat bog profiles (Jura Mountains, Switzerland)," Chemical Geology 138 (1997): 25ñ53.