Paleodose and equivalent dose… what’s the difference?

In the scientific literature, OSL geochronologists commonly report an “equivalent dose” with units of Gray (also known as the De value) as well as an age for a sample. So, if we need to know the “paleodose” of a sample to calculate its age, what is this “De value” all about?

The equivalent dose (or De value) is obtained through experimentation in the laboratory and is simply our attempt at estimating the true paleodose. We say “estimating” because the accuracy of our De value depends entirely on the robustness of our laboratory procedures and the luminescence characteristics of our sample. This is one reason that independent age control is important to verify ages in any geochronologic approach.

A comparison between radiocarbon and luminescence (IRSL) ages from peat and beach sand, respectively on Calvert Island, BC, Canada. Photo credit: Christina Neudorf.

A comparison between radiocarbon and luminescence (IRSL) ages from peat and beach sand, respectively on Calvert Island, BC, Canada. Photo credit: Christina Neudorf.

There are several ways to measure the De value of a sample, but all methods involve measuring a samples’ response to radiation treatment in the lab. This requires measuring the luminescence intensity emitted by a sample after it is given a series of known laboratory doses.

Typical “dose response” or “regeneration curve”. The natural signal is shown as a small box on the y-axis. The De value is the corresponding value for the sample’s regeneration curve on the x-axis.

Typical “dose response” or “regeneration curve”. The natural signal is shown as a small box on the y-axis. The De value is the corresponding value for the sample’s regeneration curve on the x-axis.

OSL geochronologists plot a samples’ response to dose in a “dose response curve” or “regeneration curve”. We then measure the intensity of “natural signal” of the sample (i.e. the signal received in nature and obtained before the sample has received any kind of laboratory treatment). Where the natural signal falls on the dose response curve determines the value of the De. This is essentially a calibration method completed for each aliquot measurement. Multiple measurements of De are combined through statistical techniques to model the final paleodose value that is used in age calculation.

—Christina Neudorf

Quartz or Feldspar? Which mineral should I date?

Both quartz and feldspar (particularly potassium feldspar) are commonly used for luminescence dating. Which mineral is dated depends on its abundance in the sample, as well as its luminescence characteristics. Feldspar exists as a range of mineral species, but the one most commonly used in luminescence studies if potassium (K-) feldspar. All types of feldspar typically suffer from a phenomenon called “anomalous fading”, where the signal fades through geological time. The fading rate of each sample can be measured, however, and corrections can be made if the fading rate is not too high. Table 1 below summarizes advantages and disadvantages of both quartz and feldspar. Typically, for new sites, we don’t know which mineral will work better until we prepare and analyze the sample on a luminescence reader.

Table 1. Advantages and disadvantages of dating quartz and feldspar. Modified from  Lian (2007) .

Table 1. Advantages and disadvantages of dating quartz and feldspar. Modified from Lian (2007).

1. This applies to the most commonly used luminescence signals, blue stimulated luminescence (quartz) and infrared-stimulated luminescence (feldspar). More recent signals (e.g., violet stimulated or thermally-transferred luminescence signals) are being investigated as means of extending the dating range of luminescence dating techniques.

1. This applies to the most commonly used luminescence signals, blue stimulated luminescence (quartz) and infrared-stimulated luminescence (feldspar). More recent signals (e.g., violet stimulated or thermally-transferred luminescence signals) are being investigated as means of extending the dating range of luminescence dating techniques.