In geoscience research, radiocarbon dating of organic material is a common tool for dating landforms, deposits, and phases of development in past environments. In areas where organic material is scarce or where the age of the environment is beyond the limits of radiocarbon dating (about 50,000 years), optical luminescence dating of quartz and potassium feldspar (k-feldpar) sand grains is a viable alternative. Luminescence dating is inherently experimental given the varying nature of these sediments and we have been working towards a better understanding of the luminescence properties of sediments on Calvert Island.
Assigning ages to relict sedimentary landforms is a critical component for reconstructing the evolution of past landscapes. Radiocarbon (14C) dating is one commonly used method, but it is limited by the availability of organic material (not always adequately preserved, or found in stratigraphic context), potential contamination, and an upper age limit of about 50,000 years. Also, if radiocarbon ages are to be compared to calendar ages then detailed knowledge about the long-term production of 14C in the atmosphere is needed. Optical dating, on the other hand, is used to determine the time elapsed since last exposure of quartz or feldspar grains (the two most common minerals on Earth) to sunlight or heat (Lian and Roberts 2006). It is often used to date the time of formation and alteration of sedimentary landforms, which are important indicators of environmental change. Optical dating can also be used to date any fossil or artefact buried within a sedimentary landform and, if an object has been exposed to sufficient heat (e.g., fired pottery), then it may be dated directly. Optical dating has provided ages for samples as young as a few years to as old as several hundreds of thousands of years and, importantly, it gives ages in calendar years.
Optical dating is inherently experimental. This is because the luminescence characteristics of quartz and feldspar can vary between sites and this requires investigation of their luminescence properties so that reliable ages can be produced (Fig. 9). The goal of this component of our program is to develop suitable optical dating protocols that will allow for the development of a more rigorous chronology for post-glacial landscape evolution and human occupation on the central coast of BC. This study, led by Christina Neudorf and Olav Lian describes the luminescence characteristics of quartz and K-feldspar grains from coastal dune and beach deposits on Calvert Island.
Luminescence signals from Calvert Island quartz grains are “dim” and appear to lack the component that is most desirable for optical dating, and this makes their use as a chronometer impractical. K-feldspar signals, on the other hand, are sufficiently “bright” for optical dating. A single-aliquot regenerative-dose (SAR) protocol was developed for K-feldspar sand from dune and beach environments through a series of experiments that took into account so-called “anomalous fading” (a malign effect experienced by K-feldspar where the luminescence “clock” loses time even when not exposed to sunlight or heat) and “phototransfer” (where the high energy wavelengths present in direct sunlight effectively results in the “clock” gaining time when it should not). Laboratory testing (e.g., dose recovery tests) and comparison of calculated optical age values for various sites with associated radiocarbon ages confirmed the protocol to be applicable to providing reliable optical ages. A paper describing this research (Neudorf et al. 2015) is in press in the journal Quaternary Geochronology.