The fossil treasures of Foulden Maar

Long-dead fish, flowers and insects are offering a window into our past

University of Otago scientist Daphne Lee’s new book Fossil Treasures of Foulden Maar is a glimpse into Otago’s paleontological site of international importance.

Foulden Maar is the site of interest for a commercial mining company, which has gone into receivership. The site is locked to the outside world and still has no legal protection.

But Lee's book offers a much more positive perspective, introducing us to her passion for the extraordinary spot, which was formed by a volcanic eruption some 23 million years ago, and which preserves countless rare and well-preserved fossils.

So why is Foulden Maar of such international significance?

“It's a very small site, it's only about a kilometer in diameter. It's kind of a circular crater and it's infilled with these layers of diatomite formed in a maar Lake 23 million years ago and the bottom of this little lake was anoxic," she says.

“So, anything that lived in the lake and then died and fell to the bottom or which eventually lived in the forest that grew after the lake was formed, is preserved on in these layers.

"Think of it as a book of about 120,000 pages. Every layer that was once the lake floor records the climate, and what was being blown or falling into the lake at the time.

“So, you can start at the bottom, we've cored it so we know what's at the bottom, and then you can work your way up, and basically have a glimpse into the past at year-by-year intervals. It's just incredible detail that you very rarely find anywhere in the world.”

Lakes that are preserved like this are very rare, she says.

“In the southern hemisphere there's probably a handful of them. This is the only one of its kind in New Zealand. And really the southern hemisphere hasn't got any other examples, certainly for the earliest Miocene.

"And the thing about the involvement of a volcano is that we can very precisely date the volcanic rocks and therefore the timing of the eruption and the age of the fossils and the lake.”

Life is vividly preserved in fossilized form, she says.  

“We've got beetles and ants, lace bugs and all sorts of other insects. When we first started research here there were only I think it was six fossil insects known from the whole fossil record of New Zealand,

“We've now got more than 260 different fossil insects, plus for spiders from Faulden Maar. And so, we went from virtually having no information on the past biodiversity of our insects, which are a really important part of the New Zealand forest ecosystem today to knowing an enormous amount about it.”

A volcanic explosion lead to these critters being preserved for the ages, she says.

“A very violent eruption where you've got hot magma rising up, hitting a water body of some kind which forms a crater that's about a kilometer across but hundreds of meters deep initially.

“The eruption cloud, volcanic rock, probably went up as an about 20 kilometers high, most of it fell back down into the crater, some of it formed a rim around the edge of the crater and that's important.

“So, you're left with a deep, small crater that very quickly would have filled with water from groundwater or rainwater.”

There were no streams carrying normal sediment like silt or into the lake, she says.

“The only thing that you get forming, sediments that build up in this lake are ones that were living in the lake, in this case mostly diatoms and little algae, that would have bloomed on the surface of a lake. And then every year they died and sank to the bottom.

“But the bottom part of the lake was completely oxygen free. So anoxic is the word. So, when the diatom sank to the bottom, or a leaf was blown in, or an insect was blown in or a fish died and sank down to the bottom, there's nothing there to cause decay.”

Consequently, they fall to the bottom, lie on the lake bed and are covered up by the next year's rain of diatoms coming down, she says.

This process goes without breaks on for more than 100,000 years.

“And so, you get this amazing stack, like pages of a book, with the fossils preserved, and no decay. So once they're down there, they're effectively pickled.”

As well as an important record of time and biodiversity the site is also a climate record, she says.

“We can look at the record of the climate in two different ways. One is by looking at the layers and the call that we took from the middle of the lake deposit, and also looking at the stomata and of the myriads of leaves, we've got tens of thousands of leaves.

“And we can work out what the climate was like from a whole lot of different proxies, it was probably at least eight degrees centigrade warmer, the mean annual temperature, than it is in southern New Zealand today.”

The site remains in private ownership..

“In New Zealand legislation there's no way of preserving the site. It's on private land, it's got a mining license over it, and so on. And if they want to destroy the whole site, there's actually nothing to stop them apart from public and scientific opinion.”

Lee remains hopeful Dunedin City Council can come to some arrangement with the current owner.

“The site is effectively locked. It's like having the Hokken Library or the Alexander Turnbull Library with the door chained shut and nobody's allowed to go in and open a book.”