Dr Marcus Vandergoes, of GNS Science, outlines the relative and absolute dating methods used on a sediment core from Ōkārito Pākihi in Westland. AMS radiocarbon dating of organic material is shown at the Rafter Radiocarbon Dating Laboratory of GNS. OSL dating of sediments is shown at the Luminescence Dating Facility of Victoria University of Wellington.
DR MARCUS VANDERGOES
Relative dating is just a sequence of events. If you look at sediment building up over time, you know that the sediment at the bottom is going to be older than the sediment at the top, so any environmental changes that have occurred at the top of a core are younger than what has happened at the bottom of the core.
Some of the different absolute dating methods are things like radiocarbon dating, but in particular AMS dating. We also use OSL dating, which is optically stimulated luminescence dating, and use the events in geological times such as volcanic eruptions that have a known age, and if we find evidence of those eruptions in our sediment cores, we can use those to help us date sediment sequences as well.
We use a number of different methods because sometimes you can only use one method at a different part of the core. Other times, we want to cross check that we have an actual age that we are relying on, so we can use different methods to compare against each other to make sure we know the age of an event that has occurred. We use AMS dating back to the last 50,000 years because that’s its age limit that it can be used in.
The main thing for radiocarbon dating and AMS dating is that it relies on using organic matter to get an age from. If you’ve got a part of the core that is primarily inorganic matter – silt, sands, clays – you have to look at a different technique and that is where we use OSL. Basically, OSL can only be used on inorganic material.
OSL for our situation is not as precise as AMS dating for instance, primarily because when we take a sample to use, we need to take a large amount of sediment to run one of these analyses on, and the larger amount means that you actually have to take a large representative of sediment in time to do that. So it has larger errors associated with the age – that age may actually incorporate several thousands of years – whereas the AMS age may only incorporate tens to a hundred years.
Dr Uwe Rieser & Ms Ningsheng Wang, Luminescence Dating Laboratory, Victoria University of Wellington
Dr Phil Shane, University of Auckland