ANCIENT ANTARCTICA—A JOURNEY FROM FORESTS TO ICE

When you think of Antarctica, what pictures come to your mind? Ice, penguins, frozen ocean? While this is what Antarctica looks like now, hidden in its rocks and ice are clues that Antarctica has not always been a freezing, white land. Fossils of plants and animals tell us that, millions of years ago, Antarctica was warm and covered in forests. Dolphins swam in the sea and crocodiles wallowed in the shallows! So what happened to turn this green world into the icy continent it is today? In this article, we go on a journey back through time, exploring ancient Antarctica and discovering what caused ice and snow to creep over the land.

still high in sky.Where are you?It might sound like a tropical beach, but a long, long time ago, as much as million years ago, palm trees similar to those that grow in modern Indonesia lined the coast of Antarctica, our southernmost continent and site of the South Pole.Inland, the forests were full of beech trees, like modern New Zealand or Patagonia [ ].The plants in Antarctica survived for months of the year in darkness during the long polar winter, and months of sunlight during summer.Antarctica today looks very di erent, covered in huge amounts of ice with some rocks, but no vegetation.So how did a lush, green land change into a frozen continent?And how do scientists find out what Antarctica looked like in the past?
When explorers first started to investigate the rocks of Antarctica they found clues of a warmer past.An expedition in the early s, led by Robert Falcon Scott, collected rock samples of -million-year-old fossil leaves as they attempted to be the first people to reach the South Pole.Fossils are ancient remains of life that can be preserved in rocks (Figure ).Unfortunately, due to the harsh weather, they died trying to return to their boats at the coast.The important rocks with the ancient leaf fossils were found alongside their bodies.Throughout the s, geologists (scientists who study the Earth) discovered more and more plant and animal fossils in rocks and outcrops scattered OUTCROP An exposed area of rock on the land.
around the small patches of Antarctica that are not covered in ice.These discoveries showed that plant life once thrived on the continent, but our understanding of how and when the climate changed there  did not start to be pieced together until analysis of the seafloor near Antarctica, in the s.

HOW DO WE KNOW WHAT ANTARCTICA LOOKED LIKE IN THE PAST?
We can get information about the past climate by using drill rigs to drill sediment cores, which we can study in laboratories.Sediment cores base, and they leave this material behind in rubble-like layers known as diamicts.Fossils in these sediments-like plant pollen, remains of

DIAMICTS
Rubble-like sediments of rocks, sand and mud deposited under an ice sheet.
ocean plankton, or chemical compounds that once formed the waxy coating on leaves-can be used to build a more detailed picture of the environment, telling us about which plants and animals were present, what the temperatures were, or how wet or dry the climate was.

WHEN DID ANTARCTICA BECOME ICE COVERED?
When scientists started to collect sediment cores from around Antarctica, they noticed a big change about million years ago (Figure ).Before this, the sediments and fossils suggested a mostly ice-free, warm landscape, with a wide variety of plant life.But million years ago, evidence of ice started to appear.Dropstones, DROPSTONES Rocks carried by ice bergs, which drop out of the ice berg and fall to the seafloor when it melts.small rocks sitting in the seafloor mud, indicated the presence of icebergs floating in the water above.Changes in fossils showed a cooling climate and plant life became more tundra-like, similar to Arctic landscapes of northern Canada or Russia today (Figure ).In some cores close to the continent, diamicts started to appear.All this was evidence for the first appearance of large, continent-wide ice  WHY DID ANTARCTICA GET COLD?
The Antarctic climate continued to slowly cool over the last million years, eventually becoming the icy, plant-free place we know today [ ].This cooling was caused by decreasing levels of carbon dioxide (CO ) in the atmosphere.Earth's carbon is split between separate parts of the environment-the atmosphere, the oceans, the biosphere (vegetation), and sediments.Fifty million years ago, more carbon was stored in the atmosphere, causing the warm, greenhouse-like temperatures that led forests to grow in Antarctica.Since that time, several important environmental processes have played a role in transferring carbon from the atmosphere into sediments at the bottom of the ocean, cooling the global climate.
The surface of the Earth is cracked into huge pieces, called tectonic plates.Over time, tectonic plates move, changing ocean currents,

TECTONIC PLATES
Massive slabs of Earth's outer crust that move slowly and fit together like a giant puzzle covering the planet.
building mountains in places like the Himalaya, and changing the shapes of continents.Movement of tectonic plates around Antarctica pushed away Australia and South America, which isolated Antarctica and led to the development of the Southern Ocean and the Antarctic Circumpolar Current, which is a fast flowing ocean current going all the way around Antarctica.This very big, windy area of ocean around Antarctica takes a huge amount of carbon out of the atmosphere, partly because CO dissolves in seawater, and partly due to little ocean creatures that use carbon to grow and build shells.When these organisms die, their shells sink into the sediment at the bottom at the ocean, locking carbon away.Other natural processes can remove CO from the atmosphere and lock it away, too.For example, rocks containing certain minerals can react with CO as they are eroded, removing CO from the atmosphere and transporting it through rivers into the ocean and deep ocean sediments.

READING THE PAGES OF ANTARCTICA'S ANCIENT HISTORY
The climate of Antarctica has generally cooled over the last million years but, on shorter timescales, there has been lots of variation.Short cycles of cooling and warming, known as glacials and interglacials, GLACIALS Periods of time with cold climates, like the last Ice Age.

INTERGLACIALS
Periods of time with warm climates, in between glacials.
have happened consistently over millions of years.You might know our most recent glacial, , years ago, as the last Ice Age.These cycles are caused by changes in the orbit of the Earth, which changes the amount of sun and heat reaching the polar regions.This happens in a periodic way, with glacials occurring every , or , years depending on Earth's exact orbit.These cycles also happened many millions of years ago, as shown by repeating changes in layers of sediment in drill cores.Cores in the Ross Sea o shore Antarctica often show a layer of diamict followed by sands and mudstones with occasional dropstones.This represents a change in the environment from an ice sheet (the diamict) to a floating ice shelf.When the ice shelf melts and only the ocean sits above the core site, the sediments become full of fossils of tiny plants and animals that were living in the ocean.This collection of sediment layers, as shown in the sediment core in Figure , is usually repeated over and over again in these cores, as the ice sheet grew and shrank through glacial and interglacial cycles.Scientists read these sediment layers like pages of a book, telling the story of how the Antarctic ice sheet has changed in the past.

ANTARCTICA IN THE FUTURE
Paleoclimatologists build a picture of the environment of the past to help us to understand what sort of changes may happen to our planet with current and future climate change.In Antarctica, paleoclimatologists often focus on times when CO and temperatures were higher than they are today, as this gives us guidelines of about what Antarctica might look like if we reach similar temperatures or CO values in the future.As CO levels rise, the world of our future will become more and more like worlds further back in the past.While it is unlikely we will be sitting under a palm tree on a beach near the South Pole anytime soon, retreating ice and a warmer climate may eventually cause the return of a green Antarctica.

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figure
Figure

Figure
FigurePaleoclimatologists study the past climate in Antarctica using three key methods.Ice cores contain layers of ice built up from annual snow fall.Sediment cores are made of layers of sand, mud, gravel, and fossils which have been deposited at the bottom of lakes and the ocean.The sort of sediment in each layer can be linked to the environment at the core site during the time that layer was deposited.Rock outcrops are an exposed area of rocks on land.

FigureFigure
Figure and Golledge.This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY).The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.No use, distribution or reproduction is permitted which does not comply with these terms.YOUNG REVIEWERS ADDY, AGE: I am in th grade and really enjoy volunteering at my local museum.I really like helping animals and I hope to become an environmental lawyer when I get older.My favorite class in school is my dance class (I am very flexible), and I love to travel.LEAF, AGE: I am in third grade, and my favorite subject is art and science.I love observing changes in the world.I like to work as a Young Reviewer as I can observe many more changes using scientist's equipment.In my spare time, I like hiking, swimming, and riding bikes with my friends.be an engineer.I have really enjoyed trying and struggling to learn how to engineer and code things.I am dyslexic.I really love math (especially algebra and geometry).I like to travel.I bike miles a week most weeks.AUTHORS BELLA DUNCAN Bella is a Research Fellow in the Antarctic Research Centre, Victoria University of Wellington, New Zealand.She has been studying the past climate of Antarctica for the last years, starting during her Ph.D. and continuing now in her postdoctoral research.She uses molecular fossils, the chemical remnants of life, to investigate what the Antarctic environment used to look like in the past.At the start of her Ph.D., she was lucky enough to go to Antarctica, where she found ancient leaf fossils preserved in rocks amongst the ice.This inspired her journey to becoming an Antarctic climate scientist!*bella.duncan@vuw.ac.nzSIMONE GIOVANARDI Simone is an illustrator and a recently graduated researcher studying extinct penguin fossils.Passionate about Paleoart (art which shows ancient life or environments) and illustrations for communicating science, Simone also works for videogames.NICHOLAS R. GOLLEDGE Nicholas is a Professor of Glaciology at Victoria University of Wellington, New Zealand, and has been working in Antarctica o and on for the last years.Before moving to New Zealand, he spent years as a geologist at the British Geological Survey in Edinburgh, UK.Nick has written or co-authored over a hundred scientific papers, and recently served as one of the lead authors for the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.He leads a number of research projects in NZ and collaborates with scientists around the world.
"Cenozoic history of Antarctic glaciation and climate from onshore and o shore studies," in Antarctic Climate Evolution, eds F. Florindo, M. Siegert, L.