Leaves long left alone have story to tell

Scientists studying leaves from a 23-million-year-old New Zealand forest have linked high levels of atmospheric carbon dioxide (CO₂) with increased plant growth and the hot climate of the time.

It has long been postulated, they say, that CO₂ was high at the time and that some plants could harvest it more efficiently for photosynthesis, but their findings, published in the journal Climate of the Past, are the first to show those things actually happened in tandem.

And they suggest this could indicate potential parallels with modern-day increases in CO₂ that could soon lead to a global “greening” effect in addition to rising sea levels and the other consequences of climate change.

“The amazing thing is that these leaves are basically mummified, so we have their original chemical compositions, and can see all their fine features under a microscope,” says lead author Tammo Reichgelt, from the University of Connecticut, US.

The deposit is located in a long-extinct volcanic crater at a site known as Foulden Maar on a farm near the southern city of Dunedin. About a kilometre across, it once held an isolated lake where successive layers of sediments built up from the surrounding environment.

Reichgelt and colleagues took samples from a 2009 drill core that penetrated 100 metres to near the bottom of the now-dry lakebed.

Larded in between whitish annual layers of silica-rich algae are alternating blackish layers of organic matter that fell in during other seasons. These include countless leaves from a subtropical evergreen forest.

It is rare for such fossils to retain their chemical compositions. This is the only known deposit in the Southern Hemisphere and is far better preserved than the few similar ones known from the north, the researchers say.