SLICS – Short Lived Isotope Counting Services
Principles of Lead-210 dating
To understand how lead-210 (210Pb) dating works, one must first understand “radioactive equilibria” vs. “radioactive disequilibria”. Uranium-238 (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 lead-210. Uranium-238 accounts for more than 99% of all uranium on the planet (natural abundance = 99.27%). Uranium-235 and Uranium-234 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 half-life 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 uranium-238, disequilibrium is highly probable since the 7th daughter product in the decay series is a gas which can escape, radon-222 (222Rn). In the case of lead-210 dating, it is the disequilibrium caused by radon-222 loss that makes an age approximation possible. Radon-222 is the immediate parent of lead-210, and its removal or addition to a given ecosystem will ultimately lead to an excess or depletion of lead-210 within another. Using its half-life and other environmental criteria, an age assessment can be made.
Lead-210, with a half-life 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 socio-geographic impacts in depositional systems. The optimal material for lead-210 is undisturbed sediments (e.g. low enery lake sediments) under-going a constant rate of deposition. More is discussed on this below.
In nature, the uranium-238 series is broken by the diffusion of radon-222 from minerals exposed at the earth’s surface. Radon-222 escapes into the atmosphere at a rate of ≈ 42 atoms per minute per square centimeter of land surface. With a half-life of 3.8 days, radon-222 decays through a series of very short half-life isotopes to lead-210. This process produces excess lead-210 in the atmosphere and subsequently the hydrosphere. The lead-210 is rapidly adsorbed onto or incorporated in particulate material within on-going depositional systems. This produces excess lead-210 over that lead-210 in equilibrium with ambient radium-226 already within the sediments. It is the measurement and interpretation of this excess lead-210 that provides for age assessment.
(For in-depth 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).
The simplest model, CF:CS (Robbins, 1978) assumes a constant flux of excess lead-210 from the atmosphere and a constant dry-mass sedimentation rate. Where these assumptions are satisfied the lead-210 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)e-km/r (2),
where C(0) is the excess lead-210, m is cumulative dry-mass, and k is the decay constant for lead-210 ( 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 lead-210 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 lead-210 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 lead-210, density measurements also. Ages are calculated by the following:
A is the accumulative residual excess lead-210 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:
The Constant Initial Concentration model assumes that the sediments have a constant initial excess lead-210 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 particle-by-particle 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 particle-by-particle 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 lead-210 with a half-life 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 time-interval divided by the total amount of the isotope that was introduced over the entire system (watershed plus basin) during the same time interval.