Shell growth bands

Trees aren’t the only organism with growth rings – did you know shells also put down growth bands?

Learn why Associate Professor Fiona Petchey is investigating shell growth bands as part of her research to improve radiocarbon dating methods.

Transcript

Associate Professor Fiona Petchey

Shells put down a range of different types of growth rings. They can put down daily bands of growth, monthly bands, weekly bands, but typically the most distinct ones are seasonal growth bands. You can go to a beach, pick up a pipi, and you can see these bands going back right to the centre of the shell – they are seasonal growth bands.

So in some parts of the world, what we find is that, in different seasons, you get major changes in the environment that certain shells are living in. In the tropics, you can get monsoon seasons where a lot of rain and a lot of terrestrial water gets input into these estuarine zones, and then you get other seasons where only marine water influences the shellfish. And the reason why this is problematic for dating them is because the ocean appears to be 500 years older than the terrestrial. So my thought was if we can go in and look at the seasonal bands in the shells and date those and see if we can see differences between each season, that might equate to fully marine – so 500 year offset – and fully terrestrial rainwater zero.

It’s quite difficult to date that small. We actually require quite a bit of material to get a date, so sometimes we have to cross multiple bands to be able to date.

So that is what I’m currently looking at is the ability to date the seasonal bands and see if we can actually refine the ages of these shells on the basis of that.

Acknowledgements

Pipi shell in hand, John Barkla, CC BY 4.0. Sourced from iNaturalistNZ.
4 Pipi shells in hand, Llyod Esler, CC BY-NC 4.0. Sourced from iNaturalistNZ.