

SLICS – Short Lived Isotope Counting Services Principles of Lead210 dating To understand how lead210 (210Pb) dating works, one must first understand “radioactive equilibria” vs. “radioactive disequilibria”. Uranium238 (238U) is a good isotope to use in describing this phenomenon. It not only has a long decay series, but is ultimately the origin of lead210. Uranium238 accounts for more than 99% of all uranium on the planet (natural abundance = 99.27%). Uranium235 and Uranium234 account for the very small remainder. It is distributed throughout the earths crust in small quantities but is also found highly concentrated in certain minerals (e.g. uraninite) and phosphate rock deposits. It has a halflife of 4.47 billion years and is ubiquitous to the biosphere. The 238U decays series includes 15 isotopes, ultimately ending with 206Pb. It is said to be in “equilibrium” when each successive isotope within the decay series has the same activity. This will only occur when each successive isotope is bound, quantitatively, to the decay series production. This criteria is readily achieved in a closed system (e.g. rock matrix). “Disequilibrium” occurs when any one of the isotopes within the decay series is lost (or added), in whole or part. For uranium238, disequilibrium is highly probable since the 7th daughter product in the decay series is a gas which can escape, radon222 (222Rn). In the case of lead210 dating, it is the disequilibrium caused by radon222 loss that makes an age approximation possible. Radon222 is the immediate parent of lead210, and its removal or addition to a given ecosystem will ultimately lead to an excess or depletion of lead210 within another. Using its halflife and other environmental criteria, an age assessment can be made. Lead210, with a halflife of 22.3 years, is an ideal chronometer for most ecosystem studies where changes have occurred within the last century. Its capacity to provide age information, especially with regards to sedimentation rate, in recent depositional environments provides an excellent tool for assessing recent natural and sociogeographic impacts in depositional systems. The optimal material for lead210 is undisturbed sediments (e.g. low enery lake sediments) undergoing a constant rate of deposition. More is discussed on this below. In nature, the uranium238 series is broken by the diffusion of radon222 from minerals exposed at the earth’s surface. Radon222 escapes into the atmosphere at a rate of ≈ 42 atoms per minute per square centimeter of land surface. With a halflife of 3.8 days, radon222 decays through a series of very short halflife isotopes to lead210. This process produces excess lead210 in the atmosphere and subsequently the hydrosphere. The lead210 is rapidly adsorbed onto or incorporated in particulate material within ongoing depositional systems. This produces excess lead210 over that lead210 in equilibrium with ambient radium226 already within the sediments. It is the measurement and interpretation of this excess lead210 that provides for age assessment.
(For indepth critique and descriptions of these models see Robbins (1978) and Oldfield and Appleby (1984). The selection of which model to used is subjective and should be based on criteria defined by the sedimentary system being studied. Other mathematical models have been described, but these are computationally intense requiring a large amount of computer time (Carroll and others, 1995). CF:CS (Constant Flux : Constant Sedimentation) The simplest model, CF:CS (Robbins, 1978) assumes a constant flux of excess lead210 from the atmosphere and a constant drymass sedimentation rate. Where these assumptions are satisfied the lead210 concentration will vary exponentially in accumulating sediment. The rate of accumulation is obtained from the following equation and solved for r, the dry mass accumulation rate. C = C(0)ekm/r (2), where C(0) is the excess lead210, m is cumulative drymass, and k is the decay constant for lead210 ( 0.03114). The resulting profile will be linear with the slope –k/r. The sedimentation rate can be determined graphically from the mean slope determined by a best or least squares fit. The Constant Rate of Supply model assumes a constant lead210 flux but permits the sediment supply to vary. Intuitively, this model seems to apply to most sedimentary systems where the sediment supply may vary in response to climatic or anthropogenic changes. An example of where this model appears to be applicable in the Everglades of south Florida. A core taken in the environmentally impacted area of the northern Everglades, shows a change in sediment accumulation brought on by increase flux of nutrients. As the lead210 flux has remained constant over the last 100 years (Dephino and others, 1994), the CRS model of analysis appears to be valid. This model requires, in addition to the determination the excess lead210, density measurements also. Ages are calculated by the following: A is the accumulative residual excess lead210 beneath sediments of depth (x) or cumulative dry mass (m) ) and ρ = dm/dx is the dry weight/wet volume ratio, it is then shown that the age of the sediment of depth (x) is given by the equation:
CIC (Constant Initial Concentration) The Constant Initial Concentration model assumes that the sediments have a constant initial excess lead210 concentration. In this model the age is calculated by the following: T = 1/k ln(C(0)/C) (5) This model requires the following condition to met:
In the sedimentary setting of the terrestrial and near shore environments, the requirements of the CIC model are extremely rare. However, in systems such as the deep ocean where the nuances and system variations are muted the CIC model may be applicable. In the application of any of these models, it is assumed that there is no mixing or disturbance of the particlebyparticle settling of the sediment. Analyzing cores by radiography provides information on the sedimentation processes. The presence of undisturbed lamination or bedding is good evidence of particlebyparticle sedimentation. If mixing is seen, there are procedures in dealing with the material to back out useful age information (Robbins, 1986). Two parameters derived from the data are very informative are the isotopic inventory and the focusing factor (FF). Inventory is defined as the total amount of the isotopes in a sediment deposit. For lead210 with a halflife of 22.3 years, a flux of one dpm/cm2/yr would produce an inventory of ~32 dpm/cm2. Sites greatly exceeding the theoretical value as determine by the flux, suggests the sediment picked up material during transport. When this is suspected the other derivative parameter is important. The Focusing Factor (FF) is defined as the ratio of the total amount an isotope deposited during a timeinterval divided by the total amount of the isotope that was introduced over the entire system (watershed plus basin) during the same time interval.
