Contents Updated: Monday, September 13, 1999
S.Gartner and co-workers in 1978 and 1979 sought the explanation of the extinctions in a sudden cooling. Their mechanism was that continental movements cut off the Arctic Ocean which effectively became a big inland sea or estuary with brackish or even fresh waters. Eventually the land barrier broke and the cold trapped water flowed into the more southerly oceans. The merely brackish water, though cold, would float on the warmer but saltier denser water of the oceans killing off lots of surface dwellers. Regrettably there is no sign that the Arctic Ocean was ever a freshwater lake. A sudden cooling could be the answer but a more convincing mechanism is needed.
Could volcanoes have triggered the cooling? Could they even have triggered an ice age? Louis Agassiz, the Swiss-American naturalist who studied under Cuvier, first discovered evidence of ice ages, or more explicitly The Ice Age (the most recent was originally thought to have been the only one) from his studies of the Jura mountains. Needless to say, orthodox geologists rejected Agassiz's idea for 25 years, preferring the Biblical flood as the explanation of the scratched rocks, erratics, and glacial tills he noted in the mountains. The earth covered in ice... Absurd! Now we know it was not absurd—volcanic eruptions can indeed cool the earth.
The year 1826 was long remembered as the year without a summer. In the previous year an enormous volcanic eruption had occurred in Java. The volcano, Tambora, blew up in a cataclysm bigger even than the more often quoted case of Krakatoa in 1883. According to H. and E. Strommel writing in the Scientific American, the eruption reduced the height of the volcano by 4200 feet, and blew out 25 cubic miles of matter, much of it into the atmosphere. In the vicinity of the eruption, the miasma blocked out the sun so thoroughly that the blackness was palpable. The explosion of Mount St. Helena in the North West USA bears witness to such a palpable blackness. An extended period of continuous vulcanism on this scale could severely change the climate, triggering global cooling.
But could it have destroyed the viability of the most successful animals the world had ever known? It is unlikely to have caused an ice age at the end of the Cretaceous. The earth's climate was not unstable then as it is now—it was not teetering on the edge of an ice age as it has been for the last few million years.
Plate tectonics cause ice ages. Through plate tectonics lighter rocks forming the continents drift on the heavier basaltic rocks below. From time to time a piece of a continent drifts over one of the poles and snow falls on its cool highlands. As time passes ice builds up until a vast ice sheet has been created. At the continental margins the glaciers calf into the sea as icebergs which drift into warmer waters, warming and melting as they progress. Melting abstracts heat from the surface water which is therefore cooled ultimately to 0 degrees Celsius. Salt water at 0 degrees Celsius is heavier than slightly warmer water, unlike pure water which has its maximum density at 4 degrees Celsius. The icy salt water therefore sinks to the depths, warmer water rises and the temperature of the whole ocean falls. On the shores of the polar continent the ocean freezes. When salt solutions freeze the ice which appears, apart from occluded salt (that trapped in tiny bubbles in the ice crystals), is pure, leaving a solution which is even saltier than before. The water gets denser, is obviously at 0 degrees Celsius and again sinks to the ocean's depths. Once the icy water is at the bottom of the ocean the sun cannot heat it—it only heats the surface. Thus the oceans continue to fall in temperature as long as a continental mass remains near one of the poles.
Emiliani has shown by isotopic methods that the temperature of the deep oceans has fallen monotonically from about 12 degrees Celsius to its present value of 2 degrees Celsius over the last 40 million years. The reason?—Antarctica is passing over the South Pole.
At the end of the Cretaceous period 65 million years ago, the surface temperature of the oceans was 25 degrees Celsius almost as far as the British Isles. By ten million years ago surface water at this temperature did not get beyond the tropics.
At the time of the dinosaur extinctions no continent was at a pole, though Antarctica and Australia were near the South Pole. There were no vast ice sheets locking up water and calving off bergs. The earth was warmer and climatically more stable than it is now.
64 per cent of the sun's incident radiation at present gets through to the earth's surface. An increase in the reflectivity of the earth's atmosphere would cut the proportion below 64 per cent and the whole earth would become colder. Minute ice crystals in the upper atmosphere—the ones that cause halos round the sun and moon, mock suns and moons and so on—are highly reflective. By returning incident radiation to space, only a small increase in them could seriously reduce the sun's radiation getting to the surface.
Fred Hoyle shows that ice crystals of this sort form at temperatures below -40 degrees Celsius. But heat from the lower atmosphere normally prevents water vapor high in the air from reaching such low temperatures. The water vapor releases the heat as latent heat of condensation when it forms rain droplets. Rainfall of 25 inches (63 cm) supplies enough latent heat to prevent ice crystals from forming in the upper atmosphere but less rain would not provide enough heat to suppress crystal formation. Today the only extended regions where there is less than 25 inches of rain and this condition is met are the polar regions—exactly where the earth is cold enough for glaciation to occur. The average rainfall on the earth is about 30 inches (75cm), only 5 inches (12 cm) above the critical level. If it were to fall below 25 inches then ice crystals would form almost everywhere and a worldwide ice age would have arrived.
The cooling of the oceans as Antarctica passes over the South Pole has brought the risk of a freeze up within the bounds of possibility. Before about seven million years ago it was impossible. The earth's heat store is the ocean and the heat stored in the warm surface waters until then easily caused sufficient evaporation to put plenty of latent heat into the upper atmosphere. With warm oceans there is no chance that ice crystal formation could occur in the skies and therefore heat from the sun gets to the surface of the oceans, keeping it warm. It is a positive feedback system. But once ocean temperatures drop so much that the threshold of ice formation is crossed a new feedback system starts to operate. Ice crystals in the upper atmosphere spread to lower latitudes cutting down the sunlight and cooling the oceans even more. Evaporation reduces further still and the ice crystals become more permanent. The land cools quickly and snow gradually builds up to form an icecap. When it is sufficiently large, glaciers start to calf into the sea, again cooling it and reducing the earth's store of heat. There is less evaporation, less latent heat transfer, more ice crystals form, less sunlight penetrates—a new feedback system locked into ice age has arrived.
Today the oceans of the world have a heat store of about ten years of sunlight. If the light of the sun were cut down for ten years it could switch on an ice age. Fine dust in the upper atmosphere could do it. It would mimic the action of the ice crystals, reflecting solar heat away from the surface of the earth. Fine dust of diameter less than a thousandth of a millimeter will stay in the stratosphere for long periods, certainly for a year but possibly for a decade or more depending upon the amount, its height and its size, and may be carried for hundreds or thousands of miles. The explosion of Thera in the Aegean Sea in 1500 BC carried dust to Egypt causing some of the plagues of Pharaoh at the time of Joseph. Very fine particles could stay aloft for a very long time indeed and, though they are not in themselves as effective as reflectors of heat as coarser ash or ice crystals, they can act as nuclei for ice crystals to condense upon.
The greatest volcanic explosion in the last two million years was the eruption of Toba in Sumatra 73,000 years ago. It was a hundred times bigger than the eruption of Krakatoa, throwing 500 cubic miles (2000 km^3) of dust into the air and creating a crater 25 miles (40 km) in diameter. Changes in the pollen in European sediments from this time show a marked cooling followed by a period of erratic weather and, after a delay of a thousand years, an ice age. This enormous eruption eventually triggered an ice age—but it failed to cause a mass extinction!
What then of Late Cretaceous times? Although a world wide drop in temperature did occur, there is no evidence of even a short ice age when the dinosaurs became extinct. As far as we know there were no icecaps, no calving glaciers and the oceans were warm. A Cretaceous freeze up did not occur because no highlands or landlocked seas were sufficiently near to the poles. Furthermore the heat capacity of the oceans was greater than it is now because little water was locked up as ice and sea levels were higher. The higher temperature and greater heat store in the oceans provided a greater safety margin over the possibility of an ice age starting. Instead of only ten years, 50 years or more of darkness would have been needed to trigger an ice age. Only in the last seven million years has the threshold of climatic instability been crossed which permitted the recent ice ages.