Dating Palynological Samples
Palynologists must understand the temporal context of their samples, which means that they must be dated. A number of methods are available to palynologists for dating their samples of mud or peat. Most commonly used in typical palynological studies is a method known as radiocarbon dating, which takes advantage of the fact that once an organism dies and is removed from the direct influence of the atmosphere, it no longer absorbs additional carbon-14, a rare, radioactive isotope of this element. Therefore, the amount of carbon-14 decreases progressively as a sample of dead biomass ages, and this fact can be used to estimated the age of organic samples on the basis of the remaining quantity of carbon-14, and its ratio to stable carbon-12. The rate of radioactive decay of carbon-14 is determined by its half-life, which is about 5,700 years. Radiological dating using carbon-14 is useful for samples aged between about 150,000 and 40,000-50000 years. Younger samples can sometimes be dated on the basis of their content of lead-210, and older samples using other elemental isotopes having longer half-lives.
Some palynological studies have investigated sediment collected from an unusual type of lake, called meromictic, in which there is a permanent stratification of the water caused by a steep density gradient associated with a rapid changes in temperature or salt concentration. This circumstance prevents surface waters from mixing with deeper waters, which eventually become anoxic because the biological demand for oxygen exceeds its ability to diffuse into deeper waters. Because there is insufficient oxygen, animals cannot live in the sediment of meromictic lakes. Consequently, the seasonal stratigraphy of material deposition is not disturbed by benthic creatures, and meromictic lakes often have well-defined, annual sediment layers, called varves. These can be dated in carefully collected, frozen cores by directly counting backwards from the surface. Sometimes, a few radiocarbon dates are also measured in varved cores, to confirm the chronology, or to compensate for a poor collection of the youngest, surface layers. Although meromictic lakes are unusual and rare, palynologists seek them out enthusiastically, because of the great advantages that the varved cores have for dating and interpretation.
Sometimes, palynologists work in cooperation with archaeologists. In such cases, it may be possible to date sample locations through their physical association with cultural artifacts that have been dated by archaeologists, perhaps based on their known dates of occurrence elsewhere.
Sometimes it is not necessary to accurately know the absolute date of a sample—it may be enough to understand the relative age, that is, whether one sample is younger or older than another. Often, relative aging can be done on the basis of stratigraphic location, meaning that within any core of lake sediment or peat, older samples always occur deeper than younger samples.