BEST PRACTICES FOR COLLECTION OF LUMINESCENCE SAMPLES AND ESTIMATING WATER CONTENT FOR DOSE-RATE DETERMINATION: WHY SAMPLE SELECTION, COLLECTION TECHNIQUE AND WATER CONTENT MATTERS

Use of geochronologic techniques has become a cornerstone of Quaternary geology, archaeological research and paleoenvironmental reconstruction. Optical and infrared stimulated luminescence (OSL and IRSL) dating are an important part of the toolkit for geologists and archaeologists, as luminescence dating is widely applicable to diverse depositional environments and archaeological settings. However, the accuracy and precision of OSL and IRSL dating results are directly related to the type and quality of the material sampled and in the field sample collection methods. We present essential background information on how to properly collect samples and the types of materials that are suitable for luminescence dating.

In addition to the collection of sediment in a light-proof container (for equivalent dose calculation, D_E), additional samples are needed for determining the radioactivity of the surrounding sediment (environmental dose rate, DR) and water content, which attenuates the DR. While measurement of the DR is equally important as the D_E (Lumin. Age = D_E/DR), sample collection for these measurements is commonly overlooked.

Most uncertainty in DR calculation is related to the water content of the sample. Generally dependent on weather conditions, sediment-air interface (outcrop drying effects), compaction/ mineral precipitation, and local water table fluctuations, in-situ moisture content samples may under or over represent average soil moisture content through geologic time. We describe how grain-size characteristics for sand to clay-dominated samples can be used in a pedotransfer function model to estimate soil hydraulic data and water retention parameters. In conjunction with mean annual water state determination, this approach allows for estimation of water content variation over changing sediment saturation conditions.