9 Aug 2018

Micro-fossils, filing cabinets and past climate change

From Our Changing World, 9:06 pm on 9 August 2018

Deep in the bowels of the GNS Science campus in Lower Hutt, there is a room filled with filing cabinets. These cabinets, in turn, are filled with hundreds of glass slides preserving what looks like white dust.

It is the fruit of decades of collecting by New Zealand geologists: microscopic fossils, now being harnessed in a totally new way thanks to modern computer technology.

Palynologist Joe Prebble, of GNS Science, standing in front of a false-colour scanning electron microscope image of a pollen of the daisy family.

Palynologist Joe Prebble, of GNS Science, in front of the new pollen-based New Zealand climate record (left), and a false-colour scanning electron microscope image of a pollen of the daisy family (right). Pollen of this family first appears in the New Zealand fossil record about 25 million years ago. Photo: Margaret Low / GNS Science

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Geologists in New Zealand have been collecting microscopic fossils since the 1950s.

First, they broke down fossil-bearing rocks with acids. Then they studied the remaining organic matter through a microscope, noting down their findings and filing them away.
“We have this glorious old filing system,” explains Dr Joe Prebble as he opens one of the cabinets and flips through the glass slides. “Every time somebody looked at a fossil they had a system where they wrote a page of where the rock was from. Then, on the back side of the page, they wrote what fossils they found.”

Dr Prebble is a palynologist for GNS Science. “P for pollen,” he explains, although it could just as easily be ‘P for paleontology’, as most of the pollen he studies is millions of years old.

Pollen grain. Each plant species has a distinct and recognisable type of pollen grain - this one is very honeycombed.

Pollen grain. Each plant species has a distinct and recognisable type of pollen grain. Photo: GNS Science

One slide Joe Prebble shows me is full of fossils extracted from three million-year-old rock from Taranaki. Peering through the microscope in his office I can see a fragment of leaf cuticle, a fern spore and a dinoflagellate (a type of marine algae).

Compared to the toothy fossil mosasaur skull which sits outside Joe’s door, these fossils might not seem particularly exciting. But taken together, they do something no fossil bone can - they paint a picture of the wider ecosystem.

The dinoflagellate tells us we are in a shallow sea, while the fern spore and leaf cuticle suggest the sea is on the edge of a temperate forest. If we kept looking we’d find more and more tiny fossils, each adding more detail to a picture of prehistoric New Zealand.

Until recently, these pictures were the best thing to be gained out of the micro-fossil collection.
“But in the last 20 years ago we’ve been digitising,” says Joe Prebble. “It unlocks these thousands of samples in a way that wouldn’t be possible otherwise.”

Looking back in time

The publicly accessible online database allows scientists to look at the fossil collection on a much wider scale. Joe Prebble says they can now put together models which show how different types of plants have evolved and gone extinct over millions of years, based on abundances of fossil pollen and spores.

These models show how flowering plants first arrived in the Cretaceous period, and how the rising of the Southern Alps created new environments for a greater diversity of plant life.

Some of Dr Prebble’s most recent work with the database involves looking at pollen samples from the Miocene.

“In the Middle Miocene, around 15-16 million years ago, the landmass of New Zealand was quite a different shape. There weren’t Southern Alps yet. It was sitting quite a few degrees further south, somewhere about the latitude of the subantarctic Islands,” Dr Prebble says.

Despite the southerly position of the landmass, temperatures in the Middle Miocene were very warm, thanks to high levels of atmospheric carbon dioxide, ranging from 400 to 600 parts per million. The fossil pollen indicates New Zealand was covered in subtropical plants like those found in Northern Queensland or Papua New Guinea today.

About 15 million years ago New Zealand would have had the same sort of climate and tropical vegetation that New Guinea has today.

Photo: GNS Science

Over the next several million year the earth cooled, and the pollen database shows that was catastrophic for the subtropical plants of New Zealand.

“We lose a whole bunch of pollen types and tree species,” Dr Prebble says “As we’ve cooled over the last 15 million years many of those [subtropical] tree species have become extinct in New Zealand.”

Dr Prebble says part of the reason he’s so interested in studying the Miocene is that it is the most recent era in earth’s history when global carbon dioxide levels were as high as they are today. He says this can provide hints on the possible long-term effects of climate change on plant life.

Over all, the pollen has helped build a land-based climate record for New Zealand that goes back 34 million years.