Fossil forests in the freezer
Stephen McLoughlin (Sweden)
South of the craggy limits of Patagonia, Africa and Tasmania, and beyond the piercing gales of the roaring forties and the furious fifties, lies Antarctica – the last great continent on Earth to be explored. Straddling the South Pole, it lies frozen in a winter that has lasted millions of years. Today, only a few plant species more robust than mosses eke out a harsh existence on its warmest fringes. The bitter cold and screaming katabatic winds (a katabatic wind is one that carries high density air down a slope under the force of gravity) that drain off the continental interior mean that few plants and animals can survive in Antarctica year-round.
However, this has not always been the case. Through much of deep time, it has not been the ‘white continent’ but a land of green forests and lush swamps. This forested landscape provided habitats for a wide range of terrestrial animals for most of the past 400 million years. The continent’s central location within the ancient southern supercontinent of Gondwana also meant that it held an important role in the exchange of plants and animals between the southern lands.
Little was known about Antarctica’s geology or fossil heritage until ‘the heroic era of exploration’ began to unlock the continent’s secrets in the 1800s. Some of the first explorers to realize that vegetation once clothed Antarctica’s landscape were the members of Captain Robert Scott’s team, who discovered coal and plant fossils on their ill-fated South Pole expedition in 1911. Scott’s team doctor, Edward Wilson, discovered the fossils on Mt Buckley, near the Beardmore Glacier, during their descent of the Transantarctic Mountains on the return journey from the Pole.
Although the plant remains were not particularly well preserved, such was the significance that the team placed on this discovery that they hauled the 16kg of fossils along with them on their impossible journey back towards McMurdo Sound. A search team found the bodies of Scott and two of his companions on the Ross Ice Shelf in the following spring, and beside them was the collection of fossilised Permian leaves. The fossils were later taken back to London where they were formally described by the eminent palaeobotanist, Sir A C Seward. These specimens reside today in the collections of the Natural History Museum.
Since the 1800s, Antarctica has been studied intensively and, today, several thousand researchers visit the continent annually. Plant fossils have now been recovered from rocks of Devonian to Pliocene age (400 to 3 million years). In some places, the remains have accumulated in such abundance that they form thick coal seams. According to the current treaty system, all territorial claims to the continent are held in abeyance. However, the main national players in Antarctic palaeontology have tended to confine their activities to specific regions. The UK and Argentina work mostly in the Antarctic Peninsula, the Nordic nations work mostly in Dronning Maud Land, while the USA and New Zealand focus their efforts in the Transantarctic Mountains and West Antarctica. Australia, whose territorial claims cover about a quarter of the entire continent, mainly targets East Antarctica, south of the Indian Ocean.
A fossil foray into the freezer
In the summer of 1994/95, Dr Andrew Drinnan (a colleague from the University of Melbourne) and I undertook an expedition to a remote part of East Antarctica to search for fossils from some of the continent’s ancient, lost forests. Our target was the Prince Charles Mountains – perhaps the last great mountain chain on Earth to be discovered, having been initially spotted from the air by a US pilot after the Second World War. These mountains have no permanent scientific base and can only be visited in the summer months by helicopter.
Most of their rocks are Precambrian granites and metamorphic rocks containing no fossils. However, one small part of the region (known as the Amery Oasis) contains downfaulted blocks of Permian (290 to 251 million year old) and Triassic (251 to 200 million year old) sedimentary rocks rich in plant fossils. Geologists had first reported fossils of this age in the region during the 1960s, but no thorough palaeontological study had ever been completed.
We stayed for two months in canvas polar tents and fibreglass ‘apple’ huts while collecting hundreds of kilograms of fossil plants and systematically documenting the distribution of about 70 coal seams near Beaver Lake in the Amery Oasis.
Some of the plant fossils were of dispersed leaves preserved as simple imprints in shales. Others were preserved in a form in which the original organic matter was compressed to form a black, carbonized film in the sediments. These fossils are more significant as it is possible to extract the original waxy cuticle from the original leaf surface, revealing features such as cell shapes and the distribution of stomata (pores on a leaf surface) that can be important for precise identification of species and interpretation of the palaeoclimate that prevailed at the time.
Still other fossils were preserved as siliceous permineralizations (petrifactions). These do not look particularly interesting on the surface – just grey blocks containing a jumble of twigs. However, the slabs reveal their true scientific importance when they are taken back to the laboratory, cut into thin-sections, and studied under a microscope.
These Antarctic remains are some of the best-preserved plant fossils on Earth. Some popular science books describe the process of permineralization as the atom-by-atom replacement of the plant’s tissues by minerals, but this is not strictly the case. True permineralization occurs where mineral ions impregnate the cell walls, bond to the cell surfaces and fill in the chambers of the original cells. In this sense, the process is one of entombment of the robust plant tissues rather than replacement. Indeed, if we dissolve the blocks of chert in hydrofluoric acid, the fossils drop out as a jumble of plant debris. If we were to let the debris dry we could even set fire to it!
The fossiliferous chert from Beaver Lake represents a peat layer that became entirely entombed by silica. By studying the constituent fossils in the permineralized peat, we can understand the composition of the ancient, coal-forming swamp vegetation that formed it. This might have implications for understanding the variation in coal quality within seams elsewhere in Gondwana. To appreciate the importance of coals to the Southern Hemisphere economy, it is important to realise that Australia alone currently sells over AU$25 billion worth of coal annually, representing, by far, its largest source of export income.
What types of plants lived in these Antarctic, Permian swamps? Although we found a range of ferns, quillworts, conifers and horsetails in these deposits, the vegetation was overwhelmingly dominated by a group of now-extinct plants called glossopterids. Their fossil leaves are found in huge numbers in Permian rocks, not only in Antarctica, but also throughout all of the Southern Hemisphere and India. Indeed, this odd distribution was one of the crucial pieces of evidence used by Alfred Wegener to suggest that all of the Southern Hemisphere continents were once linked together into a single supercontinent.
Glossopterids were large trees. We found some logs up to 80cm in diameter, allowing us to estimate that they may have grown to 30m tall. They were also swamp specialists, but growing in a swamp has its problems – low nutrient levels, acidic waters and waterlogged soils, low in oxygen. Like some modern plants, glossopterids overcame the waterlogging problem by developing special roots with internal chambers that allowed oxygen to penetrate to the deepest parts of the root system.
The internal chambers make the roots look a little like an animal’s backbone, hence they were given the name Vertebraria by early palaeontologists. Glossopterid pollen grains and seeds bore thin wings to facilitate dispersal by the wind. The plants also developed elaborate structures to protect the seeds against desiccation and/or insects. With these many sophisticated adaptations, glosspterids came to dominate the Southern Hemisphere landscape of the Permian.
Survival at high latitudes
We know from palaeomagnetic evidence that the Prince Charles Mountains were located at around 65 to 70° south during the Permian – in much the same latitude as the area is located today. So, how could forests survive in Antarctica in this geological period when it is far too cold for forests to exist there today? It turns out that average global temperatures at the end of this geological period were somewhat higher than present and there was a more gentle climatic gradient between the equator and the poles than we see today. No continental ice sheets existed at the end of the Permian, allowing forests to grow right up to the poles.
However, at these high latitudes, plants would still have had to endure several months of winter darkness. How could they manage this when they would have needed sunlight for photosynthesis and would have still been losing energy through respiration? Glossopterid wood shows prominent growth rings – a characteristic that one would expect from plants living in a strongly seasonal environment. Each ring stops very abruptly, suggesting that the trees entirely closed down their growth during the polar winter. Their leaves are commonly found in dense seasonal mats, so glossopterids were deciduous like many modern high latitude trees. They simply shed their leaves and reduced their metabolism to a minimum to spare energy loss over the polar winter.
It is also likely that these polar trees were tall, conical and widely spaced, and their leaves hung vertically downwards to maximize collection of the low-angle sunlight during the polar summer. So, to modern eyes, they might have had the shape of a Christmas tree, but with broad, spoon shaped leaves that probably turned gold and red in the autumn. Although high-latitude plants suffer darkness each winter, they also benefit from several months of continuous sunlight each summer. Therefore, surprisingly, polar plants can produce as much wood as those in the tropics each year if the temperature and other environmental conditions are just right.
Almost all of the Antarctic glossopterid logs have numerous cavities dotted about their interiors. The several-centimetre-long holes were originally interpreted as resin canals like those of modern pine trees. However, they are not continuous tubes and are too broad to be resin canals. Some of them are randomly arranged and contain fine filaments. These were produced by wood-rotting fungi (like those which produce white pocket rot in modern timber trees).
Other stems had cavities neatly arranged along the outer margin of each growth ring. Microscopic examination revealed that these contained tiny pellets – the fossilized dung of very small insects. Mites and the larvae of cerambycid beetles produce similar cavities in modern wood. Therefore, the Permian glossopterid forests must have provided a veritable feast for these tiny creatures as some wood specimens contain hundreds of borings.
Comparison of the fossils with modern fungal-plant-insect interactions allowed us to develop a hypothesis about the ecology of the Antarctic Permian forests. We believe that the adult insects laid their eggs beneath the bark of the trees in the autumn. The eggs then hatched and the larvae fed directly on the timber, or on fungi that were attacking it. Some modern beetles actively disperse wood-rotting fungi between trees to allow their larvae to feed on it. In either case, the larvae survived within the wood, protected from the polar winter beneath the bark. The new adult insects emerged during the early spring, allowing the tree to close over the damaged area and produce a new growth ring in the following spring and summer. Occasionally, we even find the wings and scales of insects when we dissolve the surrounding sediments in acid.
The mother of all extinctions
The sedimentary rocks near Beaver Lake are especially significant because they preserve the boundary between the Palaeozoic and Mesozoic eras, which is marked by the greatest mass extinction event that the world has known. Perhaps, over 90% of all terrestrial and marine species disappeared. The causes of this extinction remain hotly debated, so any new evidence from the Prince Charles Mountains is crucial for shedding light on Earth’s greatest disaster.
So what do the rocks and fossils in this part of Antarctica tell us? As we trace through the strata from the uppermost Permian, glossopterids remain dominant and diverse but the thickness and abundance of coal seams declines. At the Permian-Triassic boundary, coal abruptly disappears and so too do the glossopterids – not only in Antarctica, but also across all the southern lands.
The sedimentary rocks of the lowermost Triassic have a very different complexion – red and green mudstones begin to appear. Higher in the Triassic, red-beds with desiccation cracks and calcrete nodules become ubiquitous. Fossil plants still occur in these strata but now they are sparse, small, ephemeral clubmoss species and new groups of seed-plants with very small leaves, thick cuticles and deeply sunken, gas-exchange pores, protected by overhanging hairs.
The combination of sedimentary features and plant characteristics suggests ecosystems that were adapted to semi-arid conditions. Indeed, red-beds occur throughout the world at this time, and the Early Triassic is known as the ‘Coal-Gap’ – perhaps the only interval in the past 350 million years when no coals were being deposited anywhere on Earth. The driving forces behind this calamitous climate change remain unclear but the evidence from Antarctica suggests that dramatic poleward expansion of aridity caused the ultimate demise of the glossopterids that had dominated the Gondwanan forests of the Permian for 50 million years.
The de-greening of Antarctica
Lush woodland returned to Antarctica in the Late Triassic and persisted with episodic changes throughout the Jurassic, Cretaceous and into the Cenozoic but, eventually, the advance of ice sheets doomed the mighty forests of Antarctica. So what happened? Why did the ice advance and obliterate the forests?
Antarctica and Gondwana were amalgamated into the megacontinent Pangea during the Mesozoic. The enormous scale of this megacontinent (reaching almost pole-to-pole) meant that cool polar ocean currents were deflected to mix with warm equatorial waters, creating oceans with relatively even, mild temperatures. This helped to maintain the ice-sheet-free greenhouse world of the Mesozoic. However, Pangea started to break up in the Jurassic and piece after piece began to separate throughout the Cretaceous.
Antarctica remained connected to South America and Australia until about 35 million years ago and, up to that time, retained a diverse flora. At the end of the Eocene, both Australia and South America pulled away northwards allowing the South Circumpolar Current to develop – an ocean current that maintains a continuously flowing pool of cold water around Antarctica. Once this cold-water current was established, it trapped Antarctica in a frigid grip. Ice caps began to develop on the highlands then rapidly linked up to form an extensive ice sheet. The ice has advanced and retreated many times with changes in the global climate and hardy plants such as the southern beech trees (Nothofagus) managed to linger on in isolated Antarctic refugia until as recently as five million years ago. However, intensified global cooling since that time has eliminated all woody plants and the ice sheet is now four kilometres thick at its centre.
The fossils we collected in Antarctica have now provided research material for over a decade and will continue to be studied for long into the future. Much research remains to be done, but steadily we are unraveling the lost history of life on this spectacular continent. The information we discover about coal resources, climate change, and mass-extinctions provides salient lessons for management of our planet into the future.
Is mining of coal permitted in Antarctica?
No. The Madrid Protocol, adopted in 1991 and which came into force in 1998, currently prohibits mining in Antarctica. It designates the continent as a natural reserve devoted to peace and science. The ban is for an indefinite period and there are strict rules for modifying the ban. However, the prohibition can be modified at any time if all parties agree, and the prohibition can be reconsidered in 2048 on the recommendation of a review conference and if three-quarters of the parties agree to the changes. It should also be noted that not all countries are signatories to the Madrid Protocol.
What is the origin of the word Gondwana?
Gondwana is the name of the ancient southern supercontinent that began to break up about 170mya. It once included all of the modern Southern Hemisphere continents plus Arabia and India. The name ‘Gondwanaland’ was first used by Austrian geologist, Eduard Suess, in the 1800s in reference to upper Palaeozoic and Mesozoic strata of central India, which share characteristics with similar-aged strata in other parts of the Southern Hemisphere. Several authors have pointed out that ‘Gondwanaland’ is tautological as it means ‘land of the Gonds land’ (the Gonds being a people of central India) and so the supercontinent should just be called Gondwana. Others disagree and say that Gondwanaland should be used to denote a greater collection of landmasses. The two sides can not agree so you will still see both terms used widely in the scientific literature.
What about fossils of animals in Antarctica?
A broad range of both marine and terrestrial animal fossils are also known from Antarctica. These include almost every major animal group, from the marine, sponge-like archaeocyathids of the Cambrian to large herbivorous dinosaurs of the Cretaceous and whales, giant penguins, marsupials and tiny insects of the Cenozoic. Much information is available about these discoveries on the web, so those interested should check out:
About the author
Stephen works at the Department of Paleobotany at the Swedish Museum of Natural History, Box 50007, 104 05 Stockholm, Sweden. He can be contacted at Steve.McLoughlin@nrm.se.
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