Chronology is the cornerstone of any archaeological interpretation because it brings into focus the social and cultural context of past human activity.
Archaeology relies on relative and absolute dating methods such as stratigraphy, typology, radiocarbon dating and dendrochronology.
Relative dating methods
Stratigraphic dating methods are based on the fact that layers and deposits on an archaeological site are formed in a chronological order and this usually means that lower layers represent earlier time frames than later ones. However cuts and pits that have been dug into the layers at different times will make straight-forward chronological interpretations problematic. Therefore the application of careful archaeological and excavation techniques and analyses have to be made before a stratigraphic chronological interpretation can be applied.
Typological dating methods are based on the premise that material culture changes gradually through time. Artifact types that have no practical function – such as decorative objects – are usually preferred for such analyses since they tend to change faster between epochs. For some periods relative dating methods will be superior to absolute methods because of their higher resolution.
Absolute dating methods
Radiocarbon dating is the most reliable and widely used scientific method for determining the age of a carbon-containing substance formed over the past 50,000 years. The method works from the observation that the chemical element, radiocarbon (14C), is absorbed and fixed into organisms during their lifespans, and decays at a constant rate over time after their biological death. The decay rate can be counted and an age can be determined for when the organism died, that is, when the organism stopped absorbing radiocarbon from the environment.
Carbon is incorporated in living organisms from the food they eat. Animals will use incorporated carbon in the production of structural proteins such as keratin (present in hair, horns, nails, claws, hooves, feathers, and beaks), collagen (present in bone and tooth dentine), and also in the formation of exoskeletons (e.g., reptile shells, shells in snails, bivalves and other molluscans). This is useful for archaeological research because, once formed, these structural proteins retain the isotopic record (14C, 13C, 12C, among other elements) of that moment in time, and many of these structures survive relatively well in archaeological contexts.
Articulated skeletal bones are among the most reliable category of samples for radiocarbon dating. Other samples of short-lived species, such as seeds, are also preferentially used, as long as the archaeological context is securely related to the human activity we want to date and study.
Radiocarbon dates may be reported as cal BCE (calibrated Before Common Era), BC. Another timescale may be used and is referred to as “cal BP” (Before Present, where Present is 1950 CE).
The consistent use of radiocarbon data combined with accurate stratigraphic data in archaeological contexts allows us to refine chronologies. This approach can provide solid evidence for when an activity occurred, and for how long, allowing more meaningful comparisons between past human populations.
Application within the ATLAS-project
The materials we work on within the ATLAS-program are all dated, and in most cases using several of the described techniques. Chronologies are usually the best way of putting a specific genetic signature into context (e.g. the genetic turn-over during the Neolithisation). This is also how we were able to stress the continuous contacts between Europe and Anatolia, in both directions and all through the Neoltithics. Of equal importance is the context. The date will tie samples to specific contexts, a grave on Björkö for instance belongs to the Viking age. Genetic analysis of the buried individual brought to light two X-chromosomes, suggesting person buried there was a woman.