Wednesday, September 23

Roughly 90 million years ago, a rainforest grew near the South Pole

Once upon a time, there was
a swampy rainforest near the bottom of the world.

Buried sediment extracted
from the seafloor off West Antarctica contains ancient pollen, fossilized roots
and other chemical evidence of a diverse forest that flourished millions of
years ago, less than a thousand kilometers from the South Pole.

The sediment offers the southernmost glimpse yet into just how warm Earth was during the mid-Cretaceous Period, between 92 million and 83 million years ago. By analyzing traces of vegetation in the sediment, researchers reconstructed climate conditions at the site. Average annual temperatures in the forest were about 13° Celsius, with summertime temperatures reaching as high as 20° or 25° C, the team reports in the April 2 Nature.

The mid-Cretaceous is known to have been one of the
warmest periods on Earth in the last 140 million years, based on analyses of
fossils and sediment collected from the seafloor closer to the equator. Atmospheric
carbon dioxide levels are thought to have been at least 1,000 parts per
million. (Today’s atmospheric carbon dioxide levels average around
407 ppm, the highest in the last 800,000 years.)

But for a forest to thrive so far south, even more
potent greenhouse conditions must have existed than previously thought, with atmospheric
carbon dioxide levels between 1,120 and 1,680 ppm, says marine geologist Johann
Klages.

“It shows us the extreme potency of carbon dioxide — what
carbon dioxide can really do,” says Klages, of the Alfred Wegener Institute in
Bremerhaven, Germany. “Even without light for four months, [Antarctica] could
still have a temperate climate.”

The team retrieved the 30-meter-long core from within
the Amundsen Sea Embayment, where today fast-melting
Thwaites and Pine Island glaciers drain into the sea (SN: 1/7/20). Even before analyzing the
core, Klages says, the researchers knew it was special: The bottom three meters
of sediment, corresponding in time to the mid-Cretaceous Period, showed traces
of roots.

“We’ve seen many cores from Antarctica, but we’ve never
seen anything like that,” Klages says.

researchers with a sediment core
Researchers led by Johann Klages (right) analyzed seafloor sediment from off the coast of West Antarctica. Analyses of one sediment core revealed a dense network of roots as well as pollen, spores and other chemical traces of freshwater plants.Thomas Ronge/Alfred Wegener Institute

The pollen in the core suggested that this soggy, ancient
forest was home to conifers, ferns and flowering shrubs, as well as mats of bacteria (SN: 5/16/11). Sediment analyses showed
no traces of salt, suggesting it was a freshwater swamp.

The forest data are also strong evidence that
Antarctica was largely ice-free during the mid-Cretaceous, Klages says. High
carbon dioxide alone wouldn’t have been enough to keep temperatures balmy so
close to the pole. If a bright white ice sheet were present, for example, it
would have reflected much of the incoming sunlight back into space, keeping the
land cold. But vegetative cover has the opposite effect, absorbing more solar
heat and amplifying greenhouse warming.

The study represents “an unambiguous record of not
just warmer conditions, but a diverse forest flora” at the South Pole, says Julia
Wellner, a geologist at the University of Houston.

“This paper is a great reminder that, just because
there [is] a continent sitting at the South Pole, [that] doesn’t mean it
necessarily has to have ice everywhere, or even be particularly cold,” Wellner
says.

As for what significance, if any, this finding might
have to modern climate change, and Antarctica’s melting glaciers, Wellner notes
that it is difficult to make direct parallels. Today’s atmospheric carbon
dioxide levels are well below mid-Cretaceous levels, but climbing. And continental
landmasses have moved over millions of years — pushed and pulled by Earth’s
shifting tectonic plates — leading, in part, to ocean and atmospheric
circulation patterns that differ from those of the deep past.

The study does highlight the powerful role of different
feedbacks, such as the presence or absence of ice cover, to the overall climate,
Wellner says. What role such feedbacks might play in the future is not yet
clear. The current, existing Antarctic ice sheet, for example, could
theoretically temper runaway greenhouse warming, even as carbon dioxide
continues to accumulate in the atmosphere.

Klages agrees. “Ice present
on the planet is a big gift,” he says. “And [we] should do everything we can to
keep it.”