The naturally lead isotope 210Pb may be used to date aquatic sediments and peat bogs.
The method is suitable for dating approximately 100 years back.
DHI offers dating of sediment cores by means of the 210Pb method and may assist with sampling in both marine and freshwater systems. The sediment samples must remain totally undisturbed during sampling to obtain the best possible dating. Subsequently, they are cut into slices of approximately 1 cm and dried, whereupon the content of dry matter and possibly the loss on ignition are determined.
The result of the dating is delivered in the form of a report stating the age of the sediment, the rate of sedimentation, the mixing depth and the mixing intensity. In addition, the results of the sensitivity analysis are reported.
The sediment dating may be supplemented with estimations of the content of alga pigments in the depths required. Thus, it will be possible not only to describe the sediment age, but also to give a picture of the dominant mixture of plankton alga at the same time.
Sediment studies may also be supplemented with determinations of e.g. pools of phosphorus at selected depths.
Decay of uranium in the earth's crust releases the gas radon. This gas produces 210Pb by decay in the atmosphere. The lead isotope enters the earth’s crust or the water through precipitation.
In the water phase 210Pb is adsorbed to particulate matter and together they are deposited in the sediment.
210Pb decays with a half-life of about 22 years. The remaining amount of 210Pb at a certain depth will reveal the age of the given sediment layer.
This dating method covers the past period of 75-100 years due to the half-life of about 22 years.
Scope of application
Dating of sediment is used to calculate sedimentation rate and accumulation rates for different substances. A number of studies have focused on the accumulation of heavy metals during the past 100 years.
If the concentration of substances at different depths is compared with the corresponding ages, the accumulation rates of these substances at different times may be determined from the sediment accumulation rate. This gives a better picture of the historical development in a given lake or marine area.
In addition to the mentioned rates, the dating may also give a picture of the stability of the sediments and thus tell whether the chronology of the layers is intact or whether the activity of the benthic fauna has changed the chronology by bioturbation. Local changes of current conditions may also cause changes of the chronology of the layers.
The 210Pb dating may be supplemented with dating of the caesium 137Cs isotope. This isotope was released in huge quantities in connection with the atmospheric nuclear explosions in 1963 and later by the Tjernobyl accident in 1986.
Furthermore, the 210Pb method may be used to carry out a sensitivity analysis to estimate the suitability of the sediment to repeated monitoring of, for instance of changes in the flux of contaminants in the sediments.