When sampling for luminescence dating, it is important to collect samples for water content measurement, as well as for optical and dose rate measurements. Water content (typically reported as a percentage of a mass of dry sediment) is determined for the sediments collected at the sampling site to help “guestimate” how moist a sample was over its entire burial history. (And yes, we do mean “guestimate”…)
So, why do we care?
Water absorbs radiation emitted from sediments at sample site. This means that if sediments were wet during their entire burial history, they will have lower environmental dose rates than those that were dry (all other factors being equal). So if we assume a sample has been dry, when in fact it was wet, our calculated age will be an underestimate.
Ironically, the moisture history of a sample is not only one of the largest controlling factors on its environmental dose rate, it is also the most difficult to determine. It is usually derived from measured sample water contents in the laboratory, as well as the climatic and geomorphic context of the site. Was the sample taken from a river terrace, alluvial fan, or some other landform that was inundated by a river in the past? Was the sample taken at a depth close to, or below the water table? Has the water table risen or dropped through time? All of these factors should be considered, and appropriate (i.e. large) error should be applied to any water content value to account for these uncertainties.
The influence that water content has on sample age, not only depends on the true age of the sample, but also the mineral and grain size fraction that was dated. Below are some examples.