20 Oct 2016

Otago leaf fossils help solve major climate mystery

From Nine To Noon, 9:37 am on 20 October 2016

As Antarctica melts, pushing sea levels higher, scientists in New Zealand and America have been looking at a similar phenomenon, 23 million years ago, using ancient leaves.

The study has proved for the first time, a  link between a CO2 spike and the ancient melt.

Beth Fox from the University of Waikato is one of the authors of the study.

“We knew the ice had melted, but we didn’t have any evidence that there was a change in the carbon dioxide at that time. So this study is the first evidence that we have that there was actually an increase of carbon dioxide associated with the ice melting”

Leaf fossils were collected from a lake bed just outside of Middlemarch in Otago. Scientists took a 180 metre core from the lake which was then split down the middle, revealing near-perfect layers of sediment, which helped the team to determine the age of the fossils.

It is rare for soft tissues such as leaves to be preserved as well as those found near Middlemarch. Scientists usually only have bones and shells to work with, which is what makes this discovery even more extraordinary.

The region was volcanically active 23 million years ago and individual eruptions would blow a crater, which would then fill with water, forming a lake, Beth says. The lakes tended to be narrow and deep and because there were no streams running into or out of it, the sediment at the bottom was never disturbed by moving currents.

“That means that there is no oxygen in the bottom waters. The lack of oxygen means there is nothing to decompose the organic matter. There’s nothing living down there to burrow down into the sediment.

“So that means that we get basically a preservation of annual layers just like tree rings or ice cores and nothing to burrow down to break those annual layers up and we also get preservation of the original soft tissues of the organisms that were living in the lake above the anoxic zone, that had died and sunk down into the sediments.”

The scientists also found that after the carbon dioxide increase and ice melt, carbon dioxide levels go back to the pre-existing levels after what Beth calls a geologically short period of time of 20,000 years. The ice continued to melt for another 100,000 to 250,000 years.

“The ice goes from about 125% of the present day Antarctic ice sheet to about 50% of the present day Antarctic ice sheet over that period of time. It’s a massive loss of ice, but the carbon dioxide levels do not stay high for the whole time, they actually go down and the ice appears to be able to keep on melting by itself presumably after it has been kick started by that carbon dioxide increase.”

While there is uncertainty around the carbon dioxide levels calculated in the fossils by Beth and her team, their research show the levels at around 425 parts per million (ppm) and then going up to about 750-1500ppm.

The carbon dioxide level in the atmosphere is currently at 400ppm.

“It is something that people have been looking at a lot lately, this idea of thresholds or tipping points in the climate, where once you go over one of these thresholds the climate will shift to a new state. And it’s not necessarily a gradual shift. And we don’t know where these thresholds are, but it looks like this may be a tipping point that we have captured in this carbon dioxide record.”

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