Geol 33 Environmental Geomorphology
J Bret Bennington
Quaternary Ice Ages and Dating Techniques
The idea that great ice sheets once covered large areas of Europe and North America took form in the late 1700's and early 1800's. Not surprisingly, it was the Swiss who took the lead in developing glaciology, studying the abundant modern glaciers in the Alps. In 1779, Saussure coined the term erratic for boulders of granite he observed lying over limestone bedrock in the Alps, although he believed those boulders to have been transported by water. In 1795, Hutton attributed transport of erratics to glaciers much larger than those presently existing. A succession of Swiss geologists took to championing the idea of a former Ice Age, culminating in the publications of Jean deCharpentier and Louis Agassiz. Interestingly, Agassiz set out to refute the idea of ice ages, only to return from the Alps utterly convinced of their reality. Agassiz gathered evidence of widespread glaciation from all over northern Europe and determined that most of the ice had not come from the Alps, but rather from Scandinavia, the only possible source of many of the erratics. After emmigrating to America and joining the faculty at Harvard, Agassiz was delighted to find the same evidence of widespread glaciation in New England.
During the past 2.4 million years, huge ice sheets have repeatedly built up in high northern latitudes in response to global climate change. At times, ice has covered up to 30% of the Earth's total land surface, affecting primarily northern North America, northern Europe, Siberia, and the world's mountainous regions. The southern hemisphere has been impacted much less due to the preponderance of ocean, although the Andes hosted extensive alpine glaciers and Antarctica remains glaciated to the present day.
The main centers of accumulation for the Pleistocene ice sheets have been the broad lowlands surrounding Hudson Bay in North America, central Scandinavia, and Siberia. Ice spread radially from these ice centers in all directions, including north toward the North Pole.
Episodes of glaciation can be reconstructed from several sources.
Glaciations in North America
Based on moraines and other glacial deposits, four major episodes of glacial advance have traditionally been recognized in North America, with interveening interglacial episodes of relative warming and ice retreat:
Wisconsinan - Late Pleistocene (130-10 k.a)
Illinoian - Late Middle Pleistocene
Kansan - Middle Pleistocene (.7-0.13 m.y.)
Nebraskan - Early Pleistocene (1.65-0.7 m.y.)
However, more recent work has documented evidence for from 8 to 12 separate glaciations, and the oxygen isotope record read from deep ocean cores indicates even more glacial events. The oldest till in North America has been dated to the Late Pleistocene, indicating that ice ages predate the Quaternary. (Indeed, the Pleistocene is not defined in reference to glaciations, rather it dates back to Lyell in the 1800's and is based on the percentage of modern species in marine strata in Italy).
Oxygen Isotope evidence for Ice Volume
Not all oxygen atoms are the same. In nature, two isotopes of oxygen exist - O18 and O16 - which differ from one another only in their atomic mass. Water contains a constant ratio of the the two oxygen isotopes, with O16 being much more abundant than O18 . When water evaporates, the light isotope, O16 , is more readily able to escape from the sea as vapor, so that evaporaton leaves the sea enriched in the heavier isotope. Because water vapor soon returns to sea as rainfall or runoff and mixes back in, the normal ratio of O16 to O18 is maintained. However, if the evaporating water falls as snow and becomes glacial ice, it does not return to the sea, leaving the oceans enriched in O18 . The more glacial ice that forms, the more enriched in the heavier oxygen isotope the oceans become.
Foraminifera are tiny one-celled animals that live throughout the oceans. Some forams live on the bottom of the ocean, others float around near the surface in the plankton. Forams construct tiny shells of calcium carbonate, using oxygen dissolved in the oceans. Their shells trap both oxygen isotopes, reflecting whatever ratio exists in the water, and preserve the isotope ratio in the shell. When forams die, their shells become buried and preserved in the layers of mud accumulation on the ocean floor. Water temperature also has an effect on the ratio of isotope utilization in shell carbonate, but this effect adds to the ratio change caused by ice volume. Thus, glaciations enrich seawater in heavy O, and the cold water temperatures caused by glaciations further enrich foram shell carbonate in heavy O, making oxygen isotope ratios excellent indicators of glacial and interglacial periods.
Deep sea drilling
Ideally, geologists and climatologists would like to find a continuous record of climate change going back in time as far as possible without interruption. The best place to find such a record is in the deep ocean, where mud and forams accumulated continuously, undisturbed by waves, into layer after layer of sedimentary deposits.
To access the deep sea sedimentary record, geologists use a specially equiped ship that can place a hollow drill miles down on the ocean floor and drill and remove a core sample of sediment. Currently, the ship used is the JOIDES resolution - a large, floating drill platform and laboratory complex. Cores from the seafloor are examined layer by layer and samples of the foram shells from each layer are analyzed using a mass spectrometer to determine their oxygen isotope ratio. From this data a curve can be constructed showing the changing volume of glacial ice through time.
Ice core drilling
Much like sedimentary layers, glacial ice builds up slowly as each years‚ snowfall is compacted into a thin layer of ice. Drilling down through a glacier reveals a layercake of yearly ice laminations. Large continental glaciers such as those over Greenland and Antarctica are more than 2 miles thick at their centers, containing hundreds of thousands of years of ice layers. Drilling a deep core from this ice allows scientists to directly measure the oxygen isotope ratios that were present in the snow that formed the ice at different times. Again, from this data a curve can be constructed showing the changing volume of glacial ice through time. This curve can be matched to the curve generated using deep sea forams. If they agree (and they do) it confirms that the isotope ratios are showing global changes in climate.
Dating of Glacial Deposits
How can the age of glacial deposits be determined? The law of superposition is of some use if drift deposits of several glacial episodes can be found in stratigraphic succession in one place. Two tills separated by sediments bearing evidence for a warm interglacial episode can be dated relative to one-another, however, absolute dates are more satisfying. Generally, biostratigraphy does not work because glacial drift rarely contains fossils and because there is not much species turnover through the Pleistocene. Here are some techniques that have been applied to dating glacial deposits:
1. Radiocarbon Dating -For very young organic material such as wood, shell, bone, the ratio of carbon 14 to carbon 12 can be used. Carbon dating is only useful for determining ages between 0 and 80,000 years because the half life of C14 is very short:
Carbon 14 - Nitrogen 14 5,730 years
Radioactive Carbon 14 is produced in the upper atmosphere by cosmic rays that bombard the nitrogen and oxygen gas nuclei producing neutrons that collide with nitrogen atoms causing them to transform to atoms of C14. This has been going on for long enough that the ratio of C14 to the other isotopes of carbon, C13 and C12, has reached a steady state. Because carbon dioxide is removed from the atmosphere or from the water and incorporated into living tissue by all organisms, either as carbohydrates, proteins, or shell, all organisms acquire and maintain the normal isotopic ratio for carbon.
However, as soon as an organism dies, it ceases to recycle its supply of organic carbon and the C-14 ratio begins to decrease as it decays over time, providing a radiometeric clock.
Very young minerals and glasses containing Uranium 238 can be dated using fission tracks. For approximately every 2 million atoms of U238 that decay by emitting an alpha particle, one atom decays by fission, meaning its nucleus flies apart in two halves. Each half moves away from the other in opposite directions, stripping electons off of the atoms they collide with. This leaves a distinct flaw in the mineral or glass that can be made visible by etching. By counting the number of fission tracks in an area of sample an estimate can be made of the number of total decay events which equals the number of daughter atoms produced. The remaining parent isotope of U238 can be measured by standard methods using a mass spectrometer, or the sample can be bombarded by neutrons to drive the remaining Uranium 238 atoms to decay and the total number of fission tracks can be counted. This technique is relatively inexpensive and has been applied to dating pottery samples as young as 700 years.
4. Amino Acid Racemization - This technique has been applied to shell material that retains some protein material. All organic molecules, including amino acids found in proteins, have a left-handed stereochemistry. However, over time, some organic molecules will spontaneously reorganize to a right-handed form, in a process analogous to radioactive decay. The relative rate of racemization can be estimated from laboratory studies and applied to an assay of the percentage of left- vs. right-handed molecules remaining in the sample to yield an age estimate. Because the racemization rate is dependent on temperature and pH conditions, results obtained are only as good as are the assumptions regarding the thermal and pH history of the sample.
5. Thermoluminescence Dating - TL is emitted from a mineral when it is heated. The decay of radioisotopes in the surrounding matrix produces free electrons that become trapped in defects in the crystal lattice of the mineral. Heating opens these defects, releasing TL. The longer the mineral has remained unheated, the more electrons become trapped, the greater the TL on heating. The method is calibrated by exposing the sample or a similar material to artificial radiation to induce TL, and by measuring the amount of radioactive U, Th, and K in the sample matrix. Materials that have been heated, such as pottery or sediments beneath a fire, or that have been exposed to sunlight, reset to zero and their TL records the time elapsed since they were heated.
6. Obsidian Hydration Dating - Volcanic glass reacts with water in the air or surrounding soil and forms a hydration rind that increases in thickness over time. Rind thickness can be measured from thin section under a microscope. Because the formation rate of the rind is temperature dependent, volcanic glass samples of similar composition and known age must be used to calibrate the observations made on samples of unknown age.
The Record of Climate Change in the Recent Past
Paleoclimate data show that the Earth‚s climate began to slowly cool at the end of the Eocene Stage, about 40 million years ago. The Eocene was unusually warm, with hippopatamus living in England and alligators living north of the arctic circle. By about 5 million years ago, cooling began to accelerate, leading to the modern Ice Age beginning at about 2 million years ago.
The oxygen isotope record shows climate for the past 1.6 million years fluctuating between relatively cold glacial periods, when ice sheets grow, and warm interglacial periods, when ice sheets shrink. Although only four glaciations are evident from the record of moraines and glacial features preserved in the landscape, the oxygen isotope record shows more than 60 episodes of glacial advance over the last 2 million years.
At first, glacial and interglacial periods appear to have been of equal duration, each lasting about 50,000 years, with only a moderate difference between warm and cold climate states. This changed about 1 million years ago when glacial episodes became fewer and longer (about 100,000 years) with much shorter interglacial warm periods. Also, the contrast between glacial and interglacial periods became more intense, with larger growth of ice sheets followed by more extensive melting during interglacials.
The most recent glacial episode (called the Wisconson Glaciation in North America and the Wurm Glaciation in Europe) began about 115,000 years ago. From a climate very similar to that of today, perhaps slightly warmer, temperatures declined by 21,000 years ago to an average of 12°F to 18°F cooler. High latitude temperatures were at least 27°F cooler and huge ice sheets covered northern North America, western Europe, the British Isles, and ice caps covered most mountain ranges. Albany, New York was buried beneath more than a mile of ice in a sheet that extended south to the center of Long Island. At about 17,000 to 18,000 years ago the Laurentide Ice Sheet reached its maximum southern extent in North America and began depositing the terminal moraines of Long Island.
Although it took tens of thousands of years for the great ice sheets of the Wisconson Glaciation to form, they melted away in only a few thousand years. The retreat of the ice sheet from the center of Long Island northward can be followed in a series of recessional moraines developed throughout New York State and Canada.
A brief warm period, the Allerod, at 11,500 bp was followed by a short glacial readvance, the Younger Dryas, which ended abruptly with rapid warming around 10,00 years bp. By 10,000 years bp the ice had retreated into northern Canada. By 8000 years bp North America was relatively ice free.
For the last 9,000 years of the modern interglacial climate has been relatively stable, with fluctuations in average temperature of less than 6° F. There have been four cycles of high to low temperature, with the lows referred to as little ice ages and the highs called little climatic optima. The most recent, the Little Ice Age, lasted from about 1450 to 1850 AD. Prior to The Little Ice Age warm temperatures had allowed the Vikings to colonize Greenland (which actually was green, at least in areas along the coast) and Iceland, growing crops in both places. Following 1200, the cooling climate forced colonies in Greenland to be abandoned and wheat growing in Iceland had to be given up in favor of sheep farming. Similarly, in England growing grain became impossible above 600 ft elevation and severe winters forced the river Thames to freeze more than 2 dozen times (it has not frozen again since 1815). In the Alps, glaciers advanced far down their valleys - as evidenced by moraine deposits and paintings from that period. The high mountains of Ethiopia were blanketed in snow.
The current trend of climate change
If the cycles of the last 10,000 years continue we can expect, on average for a little ice age to occur every 2000-3000 years. If the cycles are symmetrical (meaning you spend half the time going into one and half the time coming out of one) then we should expect global temperatures to climb for 1000 to 1500 years following the middle of the last little ice age. So, global climate should warm slowly at least until the year 2600, although it probably won‚t do so steadily and smoothly.
Significant Geomorphic Features Related to Deglaciation
During and following the peak of the Wisconsinin glacial advance, cool, moist air masses flowing south from the glacial front caused increased levels of precipitation across the southwestern US. In the basin and range, this additional rainfall caused many of the basins to fill with large, freshwater lakes. These pluvial lakes appear to be characteristic of times of glacial advance. During interglacials such as at present, the arid climate of the region dries up the lakes. In Utah, Lake Bonneville once covered 50,000 km2 and was as much as 300 m deep in places. Today the saline remnants of Lake Bonneville are seen as the Great Salt Lake, Sevier Lake, and Utah Lake.
During the retreat of continental glaciers meltwater is prone to ponding between the ice front and moraines or in topographic basins created by isostatic depression. These proglacial lakes have at times been very large and have left extensive flat lake-bottom deposits and abandoned shorelines in many regions of the northern US.
Lakes Missoula and Columbia - The two greatest western lakes, Missoula and Columbia, formed south of the melting Cordilleran ice sheet beginning at about 15,000 BP. The largest, Lake Missoula, was about the size of Lake Ontario and was contained between the ice front to the northeast and rising topography to the southwest. Between Lake Missoula and Lake Columbia to the west was a lobe of glacial ice that developed along a n-s trending valley. This ice lobe formed a natural dam, behind which the waters of Lake Missoula steadily rose. When the lake became deep enough, the lobe of glacial ice floated free from its bed and the water of Lake Missoula drained out catastrophically, draining the lake in a matter of weeks.
Evidence for this scenario occurring at least 40 times is seen in a broad region of flood-carved topography known as the channeled scablands. Features of this area include massive sandbars and ripples, and deep, wide channels (now dry) with islands of undisturbed land.
Glacial Lake Albany and "Lake Long Island"
Locally, two important glacial lakes are worth mentioning. During recession of the Laurentide ice sheet from New York State, the Hudson valley lowlands in the region of Albany hosted a large proglacial lake - Lake Albany. Lake bottom sediments on the valley floor and beach terraces testify to the former presence of a large lake. On the North Shore of Long Island the northern side of the moraine is mantled in places with stratified lake deposits such as clays and sand layers typical of Gilbert deltas. "Lake Long Island" would have occupied the depression beneath Long Island Sound, and was likely ponded between the ice front to the north and the morainal deposits of Long Island to the south.
Marine Flooding During Eustatic Sea-level Rise
As deglaciation continued, sea level began to rise again, flooding back onto the coastal plain. However, the land, although liberated from its burdon of ice, had not yet been able to isostatically rebound and so was below sea level in many areas. The initial flooding of regions within eastern North America was greatest around 12,000 to 10,000 BP, and then gradually the sea retreated as isostatic rebound raised the continental margin above sea level again. Large areas of Maine were submerged for almost 2000 years, and the sea encroached up the St. Lawrence Valley, flooding Lake Champlain and extending west to Lake Ontario. Recently, a fossil whale from the Champlain Sea was discovered near Burlington, in northern Vermont.