(The following article has been written by Surya Sankarasubramanian, a second year M.Sc. student from IIT Bombay)
The continent of Antarctica is covered with so much ice that if all of it were to melt, the sea level would rise by around 58 metres (Fretwell et al, 2013). We cannot imagine Antarctica as anything but the “white continent”. It is the world’s windiest, coldest and driest continent and as such is the most inhospitable continent on Earth. However, things were once different in Antarctica. It used to be a tropical paradise with creatures that could only live in hot and humid conditions. The ice sheets that removed them are in fact quite recent in geologic history.
For most of the past 250 million years, Antarctica was covered by vegetation. On his ill fated second expedition to the South Pole, Robert Scott and his team collected samples of some plant fossils. It is believed that his objective of collecting samples in the interest of scientific exploration of Antarctica may have contributed to his inability to reach safety before the onset of the Antarctic winter by increasing the burden that he and his team had to haul. These samples were later retrieved from his remains and were found to be Glossopteris fossils. This genus went extinct at the end of the Permian and was interpreted to grow in wet soil conditions as shown by McLoughlin (1993). Hence, geologists believed that Antarctica must have been covered by vegetation till at least 250 million years ago.
For quite some time it was argued that these fossils were of plants that had lived in the tropics but due to continental drift are now found in polar regions. However, with greater understanding of plate tectonics and palaeomagnetism, it is now believed that most of the forests grew in high latitudes. These trees must have grown in atmospheric conditions unlike our own. The climate was warmer. Even at high latitudes, temperatures were above freezing during the winter. Another result of being at high latitudes was the six-month long day and the six-month long night. Studies have showed hat trees can survive for six months without engaging in photosynthesis since they spend six months producing food to consume during the winter. Can you even imagine there being trees and dense forests at the south pole today?
So when did the ice come to be? Miller et al. (2005) suggest that there was ice on the Antarctic continent as early as the Late Cretaceous. Geological evidences suggest that the climate was very warm, even at high latitudes. However, it is possible that these glaciers originated near the middle of the continent but were too small to reach the coast. The Antarctic interior is not subjected to the moderating influence of the sea. Hence, it is possible that the centre of the continent had glaciers while the coast hosted lush green forests. The onset of winter in the middle of the continent in Antarctica is sudden and the transition from summer to winter is marked by violent snow blizzards. It is possible that Robert Scott did not take this factor into account and found himself stuck in one such blizzard on his journey from the pole to the coast. It trapped him in his tent where he and his team would die of exhaustion.
The presence of some small continental valley type glacier does not an ice sheet make. When did the ice sheets seem to take its current form as the feature that covers 98% of the Antarctic continent? According to Siegert & Florindo (2009) the Antarctic ice sheet first appeared in the start of the Oligocene. This is correlated with the major global drop in temperature that happened at the end of the Eocene. The end of the Eocene and the start of the Oligocene corresponds to a serious drop in the level of atmospheric carbon dioxide. According to the Raymo & Ruddiman (1992) this was because of the collision of the Indian Plate and the Eurasian Plate. This, they say, caused the Himalayas to rise. The rise of the Himalayas exposed a larger surface of the Earth to chemical weathering. This kind of weathering extracted high quantities of carbon dioxide from the atmosphere which reduced global temperatures.
Scientists were also exploring the possibility of the first appearance of large ice sheets on Antarctica being linked to climatic changes brought about by the split of Antarctica and South America and Antarctica and Australia. Kennett (1977) first suggested that the split of Antarctica and surrounding continents caused the creation of a cold proto-Antarctic Circumpolar Current that prevented warm low-latitude current from reaching Antarctica. This, according to Kennett (1977) created the ice sheets of Antarctica. However, DeConto & Pollard (2003) showed through numerical modelling that the main cause of the creation of glaciation was the decrease in carbon dioxide in the atmosphere and the opening of the Drake Passage between Antarctica and South America was at best a secondary cause.
The first ice sheets to form were small and highly dynamic. They would often melt completely and leave no ice cover behind. These kinds of ice sheets gave way to continent-wide ice caps by the Early Miocene. Climate change during the Earth's history in general and during the Cenozoic in particular has been affected by the periodic shifts in the eccentricity of the Earth's orbit and the obliquity of the Earth's axis of rotation. Zachos et al. (2001) correlated the increase in glacial extent at the Oligocene-Miocene boundary with a phase of low eccentricity and low-amplitude variability in the obliquity of the Earth's orbit. If the eccentricity were high, the orbit of the earth would be highly elliptical with the sun much closer to one end than to the other. In such a scenario, the season which occurs when the Earth is furthest from the Sun will be much longer in duration than the season which occurs when the Earth is much closer to the sun. However, a low eccentricity orbit results in a more uniform climate throughout the year. According to Wilson et al. (2009) the combination of low eccentricity and low amplitude of variability of in the obliquity of the Earth's orbit resulted in colder summers.
By the middle of the Miocene, the ice sheet is thought to have become a persistent feature of the surface. Miller & Mabin (1998) reviewed the debate on whether or not the ice sheet that occupied Antarctica in the Miocene is the same as the one that occupies it today. They call the group that believes that “the Stabilists”. The opposing group believes that there have been drastic changes in the extent of the ice sheet due to the ice in it being wet and dynamic. This group is known as the “Dynamicists”.
The Stabilists point to some 4 to 15 Ma old unconsolidated, unweathered and uneroded ash beds within a few centimetres from the the ground surface at high elevations in the McMurdo Dry Valleys. They say that these sediments would have been brushed away by the precipitation that would occur if the climate were warm enough to allow the existence of a dynamic ice sheet. Hence, the climate must have been cold and dry. There are fossiliferous outcrops in the Transantarctic Mountains. These outcrops are collectively called the Sirius Group. The deposits in the Sirius Group are believed by the Dynamicists to have been deposited by moving glaciers . The lower limit of the ages of diatoms in the Sirius Group were found to be the Pliocene epoch. Hence, the Dynamicists claim that the ice would have been dynamic as recently as the Pliocene, since a hard and stable ice sheet would be incapable of moving the diatoms. It is more likely that since the Middle Miocene, the Antarctic ice sheet has been a patchwork of hard and stable ice and wet and dynamic ice.
Smellie et al. (2014) bypassed the uncertainties associated with dating diatoms by dating products of glaciovolcanic eruptions, i.e. those eruptions that occur under glaciers. These products were in the form of stratified volcanoclastic rocks. The type of association of these volcanoclastic rocks would reveal the kind of ice that the volcano erupted under. If the dated volcanoclastic sequence was associated with diamict, it would have been because the diamicts were concentrated there by melt-water and fluvial processes. This meant that the ice above the volcano was thin and prone to movement. On the other hand, if the volcanoclastic rocks were not associated with diamicts and did not exhibit any surfaces with signs of glacial erosion, they would have been produced by an eruption under cold, hard and stable ice. Hence, it was concluded that the middle Miocene did not see the entire ice sheet becoming hard and stable; at different points of time and in different places, there were patches of wet and dynamic ice within an overall body of ice that was below melting point except in a thin basal layer.
That the Antarctic ice is heterogeneous should not be surprising to us when considering that Antarctica is a large continent made of diverse landscapes. Antarctica has been permanently covered by ice since at least the Miocene. Some parts of it would have been covered by ice for even longer, possibly even since the Cretaceous period. Gradually, what was a greenhouse in the Mesozoic Era turned into the refrigerator of the present.
References :
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For most of the past 250 million years, Antarctica was covered by vegetation. On his ill fated second expedition to the South Pole, Robert Scott and his team collected samples of some plant fossils. It is believed that his objective of collecting samples in the interest of scientific exploration of Antarctica may have contributed to his inability to reach safety before the onset of the Antarctic winter by increasing the burden that he and his team had to haul. These samples were later retrieved from his remains and were found to be Glossopteris fossils. This genus went extinct at the end of the Permian and was interpreted to grow in wet soil conditions as shown by McLoughlin (1993). Hence, geologists believed that Antarctica must have been covered by vegetation till at least 250 million years ago.
For quite some time it was argued that these fossils were of plants that had lived in the tropics but due to continental drift are now found in polar regions. However, with greater understanding of plate tectonics and palaeomagnetism, it is now believed that most of the forests grew in high latitudes. These trees must have grown in atmospheric conditions unlike our own. The climate was warmer. Even at high latitudes, temperatures were above freezing during the winter. Another result of being at high latitudes was the six-month long day and the six-month long night. Studies have showed hat trees can survive for six months without engaging in photosynthesis since they spend six months producing food to consume during the winter. Can you even imagine there being trees and dense forests at the south pole today?
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| Antarctica as it looks beneath the cover of ice sheets. (Image Courtesy : British Antarctic Survey and NASA) |
So when did the ice come to be? Miller et al. (2005) suggest that there was ice on the Antarctic continent as early as the Late Cretaceous. Geological evidences suggest that the climate was very warm, even at high latitudes. However, it is possible that these glaciers originated near the middle of the continent but were too small to reach the coast. The Antarctic interior is not subjected to the moderating influence of the sea. Hence, it is possible that the centre of the continent had glaciers while the coast hosted lush green forests. The onset of winter in the middle of the continent in Antarctica is sudden and the transition from summer to winter is marked by violent snow blizzards. It is possible that Robert Scott did not take this factor into account and found himself stuck in one such blizzard on his journey from the pole to the coast. It trapped him in his tent where he and his team would die of exhaustion.
The presence of some small continental valley type glacier does not an ice sheet make. When did the ice sheets seem to take its current form as the feature that covers 98% of the Antarctic continent? According to Siegert & Florindo (2009) the Antarctic ice sheet first appeared in the start of the Oligocene. This is correlated with the major global drop in temperature that happened at the end of the Eocene. The end of the Eocene and the start of the Oligocene corresponds to a serious drop in the level of atmospheric carbon dioxide. According to the Raymo & Ruddiman (1992) this was because of the collision of the Indian Plate and the Eurasian Plate. This, they say, caused the Himalayas to rise. The rise of the Himalayas exposed a larger surface of the Earth to chemical weathering. This kind of weathering extracted high quantities of carbon dioxide from the atmosphere which reduced global temperatures.
Scientists were also exploring the possibility of the first appearance of large ice sheets on Antarctica being linked to climatic changes brought about by the split of Antarctica and South America and Antarctica and Australia. Kennett (1977) first suggested that the split of Antarctica and surrounding continents caused the creation of a cold proto-Antarctic Circumpolar Current that prevented warm low-latitude current from reaching Antarctica. This, according to Kennett (1977) created the ice sheets of Antarctica. However, DeConto & Pollard (2003) showed through numerical modelling that the main cause of the creation of glaciation was the decrease in carbon dioxide in the atmosphere and the opening of the Drake Passage between Antarctica and South America was at best a secondary cause.
The first ice sheets to form were small and highly dynamic. They would often melt completely and leave no ice cover behind. These kinds of ice sheets gave way to continent-wide ice caps by the Early Miocene. Climate change during the Earth's history in general and during the Cenozoic in particular has been affected by the periodic shifts in the eccentricity of the Earth's orbit and the obliquity of the Earth's axis of rotation. Zachos et al. (2001) correlated the increase in glacial extent at the Oligocene-Miocene boundary with a phase of low eccentricity and low-amplitude variability in the obliquity of the Earth's orbit. If the eccentricity were high, the orbit of the earth would be highly elliptical with the sun much closer to one end than to the other. In such a scenario, the season which occurs when the Earth is furthest from the Sun will be much longer in duration than the season which occurs when the Earth is much closer to the sun. However, a low eccentricity orbit results in a more uniform climate throughout the year. According to Wilson et al. (2009) the combination of low eccentricity and low amplitude of variability of in the obliquity of the Earth's orbit resulted in colder summers.
By the middle of the Miocene, the ice sheet is thought to have become a persistent feature of the surface. Miller & Mabin (1998) reviewed the debate on whether or not the ice sheet that occupied Antarctica in the Miocene is the same as the one that occupies it today. They call the group that believes that “the Stabilists”. The opposing group believes that there have been drastic changes in the extent of the ice sheet due to the ice in it being wet and dynamic. This group is known as the “Dynamicists”.
The Stabilists point to some 4 to 15 Ma old unconsolidated, unweathered and uneroded ash beds within a few centimetres from the the ground surface at high elevations in the McMurdo Dry Valleys. They say that these sediments would have been brushed away by the precipitation that would occur if the climate were warm enough to allow the existence of a dynamic ice sheet. Hence, the climate must have been cold and dry. There are fossiliferous outcrops in the Transantarctic Mountains. These outcrops are collectively called the Sirius Group. The deposits in the Sirius Group are believed by the Dynamicists to have been deposited by moving glaciers . The lower limit of the ages of diatoms in the Sirius Group were found to be the Pliocene epoch. Hence, the Dynamicists claim that the ice would have been dynamic as recently as the Pliocene, since a hard and stable ice sheet would be incapable of moving the diatoms. It is more likely that since the Middle Miocene, the Antarctic ice sheet has been a patchwork of hard and stable ice and wet and dynamic ice.
Smellie et al. (2014) bypassed the uncertainties associated with dating diatoms by dating products of glaciovolcanic eruptions, i.e. those eruptions that occur under glaciers. These products were in the form of stratified volcanoclastic rocks. The type of association of these volcanoclastic rocks would reveal the kind of ice that the volcano erupted under. If the dated volcanoclastic sequence was associated with diamict, it would have been because the diamicts were concentrated there by melt-water and fluvial processes. This meant that the ice above the volcano was thin and prone to movement. On the other hand, if the volcanoclastic rocks were not associated with diamicts and did not exhibit any surfaces with signs of glacial erosion, they would have been produced by an eruption under cold, hard and stable ice. Hence, it was concluded that the middle Miocene did not see the entire ice sheet becoming hard and stable; at different points of time and in different places, there were patches of wet and dynamic ice within an overall body of ice that was below melting point except in a thin basal layer.
That the Antarctic ice is heterogeneous should not be surprising to us when considering that Antarctica is a large continent made of diverse landscapes. Antarctica has been permanently covered by ice since at least the Miocene. Some parts of it would have been covered by ice for even longer, possibly even since the Cretaceous period. Gradually, what was a greenhouse in the Mesozoic Era turned into the refrigerator of the present.
References :
- DeConto, R. & Pollard, D. (2003), `Rapid cenozoic glaciation of Antarctica induced by declining atmospheric CO2', Nature 421, 245-249.
- Fretwell, P., Pritchard, H., Vaughan, D., Bamber, J., Barrand, N., Bell, R., Bianchi, C., Bingham, R., Blankenship, D., Casassa, G., Catania, G., Callens, D., Conway, H., Cook,A., Corr, H., Damaske, D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., Gim, Y Gogineni, P., Griggs, J., Hindmarsh, R., Holmlund, P., Holt, J., Jacobel, R., Jenkins, A., Jokat, W., Jordan, T., King, E., Kohler, J., Krabill, W., Riger-Kusk, M., Langley, K., Leitchenkov, G., Leuschen, C Luyendyk, B., Matsuoka, K., Mouginot, J., Nitsche, F., Nogi, Y Nost, O., Popov, S., Rignot, E., Rippin, D., Rivera, A., Roberts, J., Ross, N., Siegert, M., Smith, A., Steinhage, D., Studinger, M., Sun, B., Tinto, B., Welch, B., Wilson, D., Young, D., Xiangbin, C. & Zirizzotti, A. (2013), `Bedmap2: improved ice bed, surface and thickness datasets for Antarctica', The Cryosphere 7, 375-393.
- Kennett, J. P. (1977), `Cenozoic evolution of Antarctic glaciation, the circum-Antarctic ocean, and their impact on global paleoceanography', Journal of geophysical research 82(27), 3843-3860.
- McLoughlin, S. (1993), `Plant fossil distributions in some Australian Permian non-marine sediments', Sedimentary Geology 85, 601-619.
- Miller, K. G., Wright, J. D. & Browning, J. V. (2005), `Visions of ice sheets in a greenhouse world', Marine Geology 217(3), 215-231.
- Miller, M. & Mabin, M. C. (1998), `Antarctic Neogene landscapes-in the refrigerator or in the deep freeze?', GSA Today 8(4), 1-2.
- Osborne, C., Royer, D. & Beerling, D. (2004), `Adaptive role of leaf habit in extinct polar forests', International Forestry Review 6(2), 181-186.
- Raymo, M. & Ruddiman, W. F. (1992), `Tectonic forcing of late cenozoic climate', Nature 59(6391), 117-122.
- Siegert, M. & Florindo, F. (2009), Antarctic climate evolution, in F. Florindo & M. Siegert, eds, `Antarctic Climate Evolution', Elsevier, Amsterdam.
- Smellie, J. L., Rocchi, S., Wilch, T., Gemelli, M., Di Vincenzo, G., McIntosh, W., Dunbar, N., Panter, K. & Fargo, A. (2014), `Glaciovolcanic evidence for a polythermal Neogene east Antarctic ice sheet', Geology 42(1), 39-42.
- Wilson, G., Pekar, S., Naish, T., Passchier, S. & DeConto, R. (2009), From greenhouse to icehouse - the Eocene/Oligocene in Antarctica, in F. Florindo & M. Siegert, eds, `Antarctic Climate Evolution', Elsevier, Amsterdam.
- Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K. (2001), `Trends, rhythms, and aberrations in global climate 65 MA to present', Science 292(5517), 686-693.
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