And the story gets even better. Although ocean sediment cores have provided an array of proxies that can be used to track ice age climatic changes, cores of the ice itself have added an entirely new dimension. Just as the sediment cores do, they hold a number of proxies for ice age climate, but in addition they contain direct information about environments in the past. Entombed in the ice are air bubbles, samples of the ancient atmosphere that, in spite of their very small size, can be analyzed for many different constituents, even those present in trace amounts. Of special interest are the greenhouse gases that can trap the sun’s energy and raise the planet’s temperature. Measurements of these tiny time-capsule air bubbles have revealed that greenhouse gas concentrations in the atmosphere varied approximately in step with the cycles of glaciation and deglaciation. But deciding whether greenhouse gases are implicated as a cause of glacial-interglacial temperature variations or are simply a result requires very accurate timescales for both ice and ocean sediment cores, so that their respective records can be compared. Such accuracy is difficult to attain with conventional dating methods. But in recent years, an ingenious approach to this problem has been devised, based on the fact that the timing of the Earth’s orbital cycles is very precisely known. If they really are the root cause of climate cycles, they can be used as a kind of template to examine the changes observed in deep-sea sediments or glacial ice. All that’s required is to date one or more levels in the cores accurately and fix them relative to the orbital cycles—a procedure referred to as “tuning.” Analyses of this type show that the CO2 content of the atmosphere (based on ice-core measurements) and the Earth’s surface temperature (based on oxygen isotopes in ocean sediments) both changed in sync with the 100,000-year eccentricity cycle of the Earth’s orbit around the sun. These same analyses also show, as we saw earlier, that the volume of ice on the continents lags behind the temperature change—apparently by a few thousand years. The very close correspondence between temperature and CO2 variations suggests that somehow carbon dioxide in the atmosphere is regulated by changes in the eccentricity of the Earth’s orbit, and that it in turn regulates temperature. How this occurs is currently unknown. But these results once again confirm the Croll-Milankovitch theory—the regular orbital cycles act like a metronome, ticking out the rhythm of the planet’s climate cycles.
As should be obvious by now, examination of ice cores is an important part of research into the Pleistocene Ice Age. But it is a fairly recent endeavor. The story of ice coring, especially in the Greenland ice sheet, has been nicely told in a recent book titled The Two-Mile Time Machine by Richard Alley, a scientist at Pennsylvania State University who has been deeply involved in that effort. In the following, I trace the impact of ice-core science on our understanding of ice ages, drawing on both Alley’s book and other sources.
Coring a glacier, especially in the extreme climates of Greenland or the Antarctic, is no simple matter. Equipment has to be brought in, workers have to be housed and fed, and the cores, once collected, must be stored at temperatures well below freezing. If you hadn’t thought about it very seriously—or even if you had—you might question why anyone would want to go to such effort just to collect a bit of ice. Part of the answer is the pure curiosity of mankind; it’s like climbing a hill to see what’s on the other side. A feature of glaciers that must surely have piqued the curiosity of many who observed them over the years, and that certainly played a part in the desire to core into them, is their visible layering. Like the layers of sedimentary rocks, the layers of ice in a glacier record the passing of time, and like the pages of a diary, each layer contains clues about what happened in the past. The information is cryptic, but with the right tools it can often be deciphered.
Early attempts to recover stratigraphic (i.e., layer-by-layer) information from ice date back to 1957, when the International Council of Scientific Unions, a body that coordinates international activities in science, launched the International Geophysical Year (IGY) to promote geophysical research on a global scale. By its nature, geophysics is international, and under the aegis of the IGY, scientific projects were conducted quite literally from pole to pole. Sixty-seven different countries had official roles, and, in spite of its name, IGY lasted for eighteen months. An important focus was the polar regions, which at that time were still poorly known scientifically. Both the Arctic and Antarctic held much interest for geophysicists, because they are the home of the north and south magnetic poles, the aurora borealis, and, of course, the largest remaining ice sheets of the Pleistocene glaciation. But the early ice-sampling attempts were quite crude. Hand-dug pits were made in the layered ice at places like Byrd Station, a U.S. encampment on the Antarctic ice sheet at 80° south latitude. These surface holes, however, only penetrated through very young ice. They provided samples and data for the topmost part of the Antarctic glaciers, but could not capitalize on the third dimension, the great thickness of the ice. The U.S. Army Corps of Engineers already had a major laboratory devoted to “cold regions” research at the time of the IGY, and its chief scientist, a man named Henri Bader, pushed hard for scientific drilling of the ice. Although it didn’t happen immediately, within a decade after the end of the IGY, his organization had cored deep into both the Greenland and the Antarctic ice sheets.
The earliest drilling was done on a ships-of-opportunity basis, in places where camps or scientific stations already existed. It produced a lot of new information about the polar ice caps, but it also soon became apparent that a more coherent strategy was required, and that the greatest rewards would come from drilling where the longest possible undisturbed cores could be obtained, or where the accumulation rate of snow was especially favorable for obtaining high-resolution records. Several countries, both individually and in partnership, made ambitious plans for polar drilling, and by the mid 1990s, several deep-drilling projects had been completed on the Greenland and Antarctic ice sheets, and others were active. Particularly important for research on the Pleistocene Ice Age have been cores from central Greenland and Vostok Station in the Antarctic. In Greenland, under formidable weather conditions, teams from Europe and the United States drilled a pair of holes just thirty kilometers apart, almost dead center in the continent and at the very summit of the ice cap. The rationale for this seemingly double effort was that independent analysis of the cores would provide a cross-check on the reliability of the data—and also, two cores would provide twice as much ice for critical analyses. Both projects reached a depth of about three kilometers, where the ice is more than 110,000 years old. Except for the very oldest sections of the cores near the bottom of the holes, where flow over the underlying rocks of the Earth’s crust coupled with the great pressure of the overlying ice has distorted the layering, the agreement between the two drilling sites is amazingly good.
In January 1998, at the other end of the world in the Antarctic, ice that until very recently was the oldest yet drilled (more than 420,000 years old) was retrieved from a hole that reached 3,623 meters depth. Although ice cores have been collected at many sites in the Antarctic, including at the South Pole, this very long core from the Russian Vostok Antarctic Station has special importance, and has been heavily studied, because it reaches so far back into the Pleistocene Ice Age. It does so because annual snowfall is much less in the Antarctic than in Greenland, and a given thickness of ice therefore represents a far greater stretch of time than it does in Greenland. (In September 2003, a European consortium announced that ice they had recovered at another Antarctic site dates back to at least 750,000 years. Few data are yet available for this core at the time of writing.)
Ice drilling at Vostok (where the mean temperature is a chilly –55°C) actually began in the 1970s and was for a long time purely a Russian endeavor, but later became a joint Russia-France-U.S. project. Based on remote sensing measurements from the surface, the ice continues for more than 100 meters beyond the depth reached by drilling. However, a decision was made to stop, because below the ice lies a large
lake—Lake Vostok—that has been isolated from contact with the atmosphere for hundreds of thousands of years. Insulated from the frigid polar air above by three and a half kilometers of ice, and heated from below by the slow but steady escape of heat from the Earth, the lake remains unfrozen. It is thought that Lake Vostok may contain unique bacteria or other life that has long been isolated from the rest of the world’s biosphere, and biologists and chemists are anxious to devise a contamination-free sampling plan. The last thing they wanted was to plunge a dirty drilling string through the ice into its undisturbed waters.
The ice cores have yielded a massive amount of information about the Earth’s climate and environmental conditions through the few most recent glacial-interglacial cycles of the Pleistocene Ice Age, including data about the local temperatures at the drilling sites, the rates of snow accumulation, a record of distant volcanic eruptions (which are indicated by thin layers of very fine volcanic ash that is distributed globally through the atmosphere), and even the intensity of winds. The latter comes from analysis of the amount of ordinary dust and sea salt in the ice cores: the greater the wind intensity, the higher the content of these materials. And, of course, the ice also traps samples of the atmosphere as small bubbles, as we have already seen. The most recent part of the ice cores, especially for the time since the Industrial Revolution, also provides a very good record of how man’s activities have affected the global environment. Anything that forms gaseous molecules, or attaches itself to the very tiny particles that are transported around the globe by winds, can end up being deposited in polar ice. Mercury, lead, and freon are just a few examples of substances that have been measured in glaciers and can be traced directly to industrial processes.
Some of the properties measured in the long Vostok core are shown in figure 21. A remarkable feature of such graphs is that almost all of the properties that have been measured in the ice show patterns similar to those of oxygen isotopes in deep-sea sediments over the same time period. The Vostok core is long enough to show four complete glacial-interglacial cycles, and the approximately 100,000-year periodicity is readily apparent. As with the deep-sea sediments, setting up a reliable timescale for the variations observed in the ice cores is critical for their interpretation. Here glaciers have one great advantage over ocean sediments: snow accumulates relatively quickly, and annual layers are usually easily discernible. Even deep in a glacier, where high pressure and the flow of ice have thinned them, the layers can often be distinguished. If annual layers can be counted—and if you can be certain that none are missing—then the cores can be dated very precisely and all of the interesting environmental proxy indicators can be placed accurately on a timescale. Also, because of the very high resolution provided by the annual layers, even events of quite short duration can be identified accurately. But counting layers one by one is a tedious process—can you imagine counting tens of thousands of layers without making an error? Automation has helped. It turns out that the electrical conductivity of the ice changes subtly with the seasons, because different amounts of various trace compounds from the atmosphere are incorporated into snow in summer and winter. Long sections of core can be scanned quickly for changes in conductivity, and the wiggles of the output interpreted in terms of yearly cycles. But the human eye and brain have remained primary tools for constructing ice-core timescales. Where cross-checks are possible, layer counting has proved to be very accurate. For example, the dates of quite a few large volcanic eruptions are well known from historical records that stretch back several thousand years. Fine-grained volcanic ash and chemicals such as sulfur dioxide that are spewed out in these eruptions are quickly distributed through the atmosphere and deposited on the icecaps as discrete layers. Dating by layer counting generally places these marker horizons in the ice cores within a few years of their actual occurrences. Uncertainties get larger for older parts of the ice cores, but in the Greenland cores, where layer counting has been very successful, dating appears to be accurate to a few percent over the past 100,000 years—a remarkable accomplishment. In Antarctic cores, too, layer counting gives correct ages for events that can be dated independently in other localities—for example, a particularly rapid change in global temperature that shows up in proxy records of different types in different places. Such agreement provides a high degree of confidence in the method.
Figure 21.Data from the Antarctic ice cores at Vostok Station show that temperature changes (relative to the present) calculated from isotopic data and atmospheric CO2 content (in parts per million) from bubbles in the ice track each other very closely. Cold glacial periods had low CO2, whereas the warm interglacials had much higher values. The amount of dust in the ice, on the other hand, is highest during the cold periods, signifying generally windier conditions. These graphs are based on data from a paper by J.R. Petit et al. in the journal Nature, June 3, 1999.
The properties of the Vostok ice core shown in figure 21 illustrate some of the insights that have been gained into Pleistocene Ice Age climate from examination of polar ice. One, the close correspondence between temperature and the concentration of CO2 in the atmosphere, we have already discussed. Both seawater temperatures, deduced from proxy measurements in sediments, and the local temperature at the Vostok drilling site—also based on a proxy, the isotopic composition of hydrogen in the ice—are closely correlated with carbon dioxide. The amount of dust in the Vostok core, a proxy for windiness, shows a strong anti-correlation with other parameters. It is obvious from the graph that the windiest times of the past 400,000 years, when most dust was deposited on the ice, occurred at the height of the glacial periods, when the temperature in the Antarctic—and presumably also globally—was at a minimum. This is consistent with other evidence that at the times of maximum ice cover, the global climate was cold, arid, and windy. In such an environment, the extent of desert areas increased, providing more dust, and grasslands expanded at the expense of forests. Extensive loess deposits formed in some parts of the world. Yet another feature that is apparent in figure 21, especially in the temperature and CO2 plots, is the very sharp transition from cold to warm periods. This had already been noted in sediment cores, but it is even more striking in the Vostok data. Wally Broecker, an early investigator of the glaciation record in deep-sea cores, termed these quick shifts in temperature “rapid terminations,” implying a swift end to glacial episodes after long periods of cold. The best estimate of the actual temperature change at the Vostok site during these transitions is about 12°C. Note, too, that the warm periods do not last very long—in fact, the present warm period is considerably longer than any other in the ice-core record—and that during each glacial period, the temperature gradually cools to a minimum value just before the next “rapid termination,” giving the whole graph a saw-tooth appearance.
For completeness, there is one aspect of figure 21 that should be explained in more detail. It has to do with how measurements of the components of air bubbles correspond with other properties. Although both CO2 and the temperature proxy were measured in the same ice core, at a given depth in the ice, these parameters do not record conditions at the same time. It turns out that an adjustment has to be made to the air bubble data in order to bring it into correspondence with the other properties. The reason is quite simple. As anyone who has walked in a snowfall knows, fresh snow is very light, because it is mostly air. But as more and more snow accumulates, pressure increases on the underlying layers, and air is squeezed out. On a glacier, as long as there are still spaces around the snow crystals, air in the snow will continue to exchange with the atmosphere, moving in and out with the winds and with changes in atmospheric pressure. But eventually the pressure of the overlying layers causes the snow crystals to grow into a continuous mass of ice, trapping whatever air remains as sealed bubbles and preventing it from further exchange with the atmosphere. That instant, when there is no longer communication with surface air, is the critical one for scientists attempting to decipher the gas bubble data. Dependin
g on how quickly snow accumulates, this may happen at a depth in the glacier where the surrounding ice is anywhere from a few hundred to almost a thousand years old. In the latter case, a gas bubble analysis would refer to conditions that prevailed a thousand years after the temperature was recorded in ice at the same depth. Obviously, to make an accurate comparison of how various properties track one another, the timing must be known fairly accurately. Usually, it’s possible to estimate the rate of snow accumulation from the thickness of annual layers in the ice cores, so that the uncertainty in the age of the air that’s being analyzed can be minimized. Still, it should be realized that there may be small offsets in the records.
Up to this point, I have focused on the results from Greenland and Antarctic ice cores. That is where the remaining large ice caps are, and where the major drilling efforts have been mounted. But a few people have recognized that mountain glaciers, virtually all of them small remnants of much larger ice fields that existed at the height of the most recent glacial period, may also have interesting stories to tell. Because these smaller glaciers are widespread across the Earth, they also may hold clues about the global reach of the ice age climate. In particular, those that still exist at tropical or subtropical latitudes are often the only available storehouses of information about ice age climate away from the harsh environment of the polar regions.
Frozen Earth: The Once and Future Story of Ice Ages Page 21
