Dr Marcus Vandergoes, of GNS Science, takes you on a tour of a sediment core from glacial and interglacial deposits near the Southern Alps. He explains the different environments that organic and inorganic layers represent and which layers are suitable for different dating methods. A layer of volcanic ash, dated to 27,000 years ago, provides a tie-in for relative dating.
DR MARCUS VANDERGOES
This core is actually from the eastern side of the Southern Alps between Lake Pūkakī and Lake Tekapo within the moraines of the last ice age. It’s primarily made up of wind-blown silts and sands, and it does have some organic material within that, so they represent different climate events.
So the darker layers here are highly organic rich layers – they represent primarily warmer, more stable environments. These lighter layers are inorganic made up of silts and clays, wind-blown often, and they represent colder phases. The large area here in the core of blue-grey silts and sand is an indication of a cold event within the core. We have this volcanic ash layer part way through that that we know erupted at around 27,000 years ago and that occurred right in the middle of the last glacial maximum as part of the last ice age, so we know that this big blue grey area of the core is representative of the last glacial maximum in New Zealand.
So these darker events going back through time are going to be older than 27,000 years. We can use the AMS dating on these organic bands. Some of them are rich in small plant fragments, and we can take very, very fine slices of this mud and get quite high-precision dates just from these small plant fragments. These other areas that are less abundant with organic material, we can do the pollen concentrate dating, so we can remove the pollen from the sediment by density separation and we can use that pollen in a pure form to get AMS ages from as well.
The other thing with this core – because it’s made up of a lot of inorganic silts and sands – is we can use OSL dating on it. The idea here is that this sediment contains small minerals like quartz and feldspar and they can be dated using this technique. This is particularly useful when we get back into the older parts of the core – and this part here could represent that age scale – is that we can use this OSL dating to date the core back up to about 100–150,000 years old.