Looking back in time – Lake Moawhitu

Rewi Newnham is a professor of physical geography at Victoria University of Wellington. He takes us through the process of gathering and analysing sediment cores from the bottom of a lake. Analysing ancient pollen has enabled Rewi to build a picture of Lake Moawhitu as it was before humans arrived in Aotearoa.

Jargon alert

Palynology is the study of dust. Palynologists analyse pollen, spores and other tiny particles found in the air or water or from sediments.

Questions for discussion

• How do sediment cores help scientists look back in time?

• What types of materials might palynologists find in sediment cores?

Transcript

Professor Rewi Newnham

Tēnā ra koutou katoa. Ko Rewi Newnham aho. Nō Te Tai Tokerau. E mahi ana au ki Te Whare Wānanga o Wikitōria.

My technical role is helping with palynology or pollen analysis, which is quite an important part of the whole Lakes380 project because it tells us about the history of the vegetation in the – not just in the catchment surrounding the lake but also some of the vegetation that’s growing in and at the margins of the lake.

The health of the lake – the mauri of the lake, if you like – is actually intricately, intimately related to what’s happening in the catchment. If we disturb the catchment, if we clear the forests for example, if we replace the trees with pasture and we put fertiliser on it, that material is going to end up in the lake.

A lake sediment core is a sample taken from the bottom of the lake that goes down through the sediments that have accumulated over time, and those sediments contain lots of material, including pollen. So the lake sediments build up over time rather like a book with pages and layers that we can turn and read, and they contain information in the form of pollen grains and other materials that have come into the lake that tell us about the environment in which that lake existed at the time.

First of all, we start with that lake sediment core and we have to slice it up a little bit, take samples or subsamples as we call them. And then we have to extract the pollen, look at it down the microscope and then the fun begins really because you’re presented with a whole world that existed in the past. It’s rather like walking through a time machine back through the forest or some other natural vegetation environment.

What has the pollen analysis – the palynology – of the Moawhitu sediment cores told us about that te ao tuatahi – the world before people arrived? The picture I have in my mind from the pollen work is of a closed canopy podocarp forest dominated by the likes of rimu, tōtara, mataī, miro – pretty close to the lakeshore. There would have been a narrow fringe of flax, probably wharariki. Raupō was there, some of the sedges in that narrow fringe around the lake. And then further away from the lake, we see evidence of beech forest. So we got a pretty good idea of what that environment looked like. It would have been a pretty special place.