Effect and Cause
A chance finding by a Swiss research team explains an event that happened centuries ago around Lake Geneva.
In the sixth century, Gregory of Tours, a chronicler of the Germanic people known as the Franks, told of an extraordinary event in what is now Switzerland, where the Rhône River spills into Lake Geneva. He wrote of a big rockfall in the year 563 AD in the vicinity of a place called Tauredunum. The debris plunged into the river, and a great mass of water ‘overwhelmed with a sudden and violent flood all that was on the banks as far as the city of Geneva’, more than 64 kilometres across the lake. The Geneva bridge was demolished, and several people inside the city walls were killed. Historians and scientists have long believed that Gregory and another chronicler, Marius of Avenches, who told a similar tale, were describing a tsunami that raced across the lake. But there has not been any direct evidence of it until now.
Researchers at the University of Geneva now say they have found that evidence in the form of a large deposit of sediment in the middle of the lake. In a study published in the journal Nature Geoscience, they also propose the sequence of events that caused the deadly surge. The researchers think that large boulders crashed down onto soft sediments which had accumulated at the river mouth because of the slowing of the river’s flow when it enters the lake. These sediments formed an underwater delta that had several canyon-like channels. When the falling rocks hit the delta, they destabilised the sediments and caused the canyons to collapse. It was this collapse that created the tsunami. The sediments from this collapse would have been propelled towards the lake’s centre.
Guy Simpson, a lecturer in the University of Geneva’s Department of Geology and Palaeontology, says the thick layer of sediment, which has the same curved shape as a lens, lies more than 305 metres down in the deepest part of the lake and was found largely by chance. Katrina Kremer, a University of Geneva doctoral student and the study’s lead author, had been conducting seismic soundings, searching for thin sediment layers that might be evidence of major floods that had taken place in prehistoric times, long before the event described by Gregory of Tours. ‘But we came across this enormous deposit,’ Simpson says. ‘We didn’t know straight away that it was the deposit that caused the sixth-century tsunami. But it was a jumbled mess of sediment. It was quite obvious that it was deposited rapidly.’
The researchers then took samples of the sediments and used carbon-dating techniques on remains of leaves and other organic matter they found to determine when the deposit formed. This narrowed the date to a period between the late fourth century and the early seventh century. Other than the rockfall, there is no record of any special event during that period, Simpson says.
The researchers estimated that the deposit, which is at least 9.6 kilometres long by 4.8 kilometres wide and averages about five metres thick, contains more than 248 million cubic metres of material. They ran multiple computer simulations showing that the collapse of that much sediment at the mouth of the Rhône would have caused a tsunami with an estimated height of 7.9 metres at Geneva, where it would have arrived in about 70 minutes. The rockfall itself may have been set off by a major earthquake, as some scientists have speculated.
Lake tsunamis, although unusual, are not unknown, says Richard Schweickert, an Emeritus Professor of Geology at the University of Nevada in Reno, United States. He cites evidence that the collapse of part of the shoreline of Lake Tahoe in northern California within the past 20,000 years caused a tsunami with wave heights of about 30 metres. There are two faults beneath the lake that could have caused an earthquake, he says, and the collapse of the Rhône delta sediments, as calculated by the Swiss researchers, would certainly have been capable of moving a large amount of material into the lake. He suggests that the findings could be corroborated by carefully mapping the shoreline to look for unusual deposits or erosion left behind by the giant waves.
Simpson says the Rhône delta sediments might collapse again, perhaps because of an earthquake or even their own weight. In the sixth century, Geneva was a small community, mostly behind walls on a hill, whereas today it is home to international organisations and about 200,000 people, many of whom live in low-lying areas near the water. Testing the stability of nearby slopes and creating more detailed models of how a tsunami could affect Geneva today would provide a more accurate assessment of whether this is something the lakeside city should be concerned about.
Most tsunamis occur in oceans and are generated by earthquakes. However, the study is a reminder that even a landlocked nation such as Switzerland is not immune to catastrophic waves.
Ancient People in the Sahara
On Oct. 13, 2000, Paul Sereno, a professor from the University of Chicago, guided a team of palaeontologists out of three broken Land Rovers, filled their water bottles and walked across the toffee-coloured Ténéré Desert. Ténéré, one of the most barren areas on Earth, is located on the southern flank of the Sahara. According to the turbaned Tuareg nomads, who have ruled this infertile domain for centuries, this California-sized ocean of sand and rock is a ‘desert within a desert’. In the Ténéré Desert, massive dunes might stretch a hundred miles, as far as the eye can see. In addition, 120-degree heat and inexorable winds can take almost all the water from a human body in less than a day.
Mike Hettwer, a photographer on the team, was attracted by the amazing scenery and walked to several dunes to take photos of the landscape. When he reached the first slope of a dune, he was shocked by the fact that the dunes were scattered with many bones. He photographed these bones with his digital camera and hurried back to the Land Rover. ‘I found some bones,’ Hettwer said to the other group members. ‘To my great surprise, they do not belong to dinosaurs. They are human bones.’
One day in the spring of 2005, Paul Sereno got in touch with Elena Garcea, a prestigious archaeologist at the University of Cassino in Italy, asking her to return to the site with him. After spending 30 years researching the history of the Nile in Sudan and the mountains in the Libyan Desert, Garcea was well acquainted with the life of ancient people in the Sahara. But she did not know Sereno before this exploration. His claim of having found so many skeletons in the Ténéré Desert seemed unreliable to some archaeologists, one of whom considered Sereno just a ‘moonlighting palaeontologist’. However, Garcea was sufficiently intrigued by his perspective to accept his invitation.
In the following three weeks, Sereno and Garcea, along with five excavators, five Tuareg guides and five soldiers from Niger’s army, sketched a detailed map of the site, which was dubbed Gobero after the Tuareg name for the area. After that, they excavated eight tombs and found twenty artefacts from the Kiffian and Tenerian civilisations. From these artefacts, it was evident that Kiffian fishermen caught not only small fish but also some huge ones: the remains of Nile perch, a fierce fish weighing about 300 pounds, along with those of alligators and hippos, were found near the dunes.
Sereno went back with some essential bones and artefacts and planned the next trip to the Sahara. Meanwhile, he carefully pulled out the teeth of skeletons and sent them to a research laboratory for radiocarbon dating. The results indicated that the smaller ‘sleeping’ skeletons might date back about 6,000 years, well within the Tenerian period, while the larger, tightly bundled burials were approximately 9,000 years old, just in the heyday of the Kiffian era. The scientists could now distinguish one culture from the other.
In the fall of 2006, the researchers returned to Gobero with a larger excavation team and six extra specialists. Garcea hoped to excavate about 80 more burials. At the site, Chris Stojanowski, a bioarchaeologist at Arizona State University, found clues in the bones. Judging from the bones, the Kiffian appeared to be peaceful and hardworking. ‘No injuries to heads or forearms indicate that they did not fight too much,’ he said. ‘And they had strong bodies.’ Pointing to a long, narrow femur, he continued, ‘From this muscle attachment, we could infer the huge leg muscles, which means this individual lived a strenuous lifestyle and ate much protein. Both of these inferences coincide with the lifestyle of people living by fishing.’ To create a striking contrast, he displayed the femur of a Tenerian male. Its ridge was scarcely visible, indicating a less laborious lifestyle, which might be expected of a herder.
Stojanowski concluded that the Tenerians were herders, which was consistent with other scholars’ dominant view of the lifestyle in the Sahara 6,000 years ago, when the dry climate favoured herding rather than hunting. But Sereno raised a confusing point: if the Tenerians were herders, where were the herds? Despite thousands of animal bones excavated in Gobero, only three cow skeletons were found, and none belonging to goats or sheep were found. ‘It is common for herding people not to kill cattle, particularly in a cemetery,’ Elena Garcea remarked. ‘Even modern pastoralists such as Niger’s Wodaabe are reluctant to slaughter animals in their herd.’ Sereno suggested, ‘Perhaps the Tenerians in Gobero were a transitional group that still relied greatly on hunting and fishing and had not adopted herding completely.’
History of Refrigeration
A Refrigeration is a process of removing heat, which means cooling an area or a substance below the environmental temperature. Mechanical refrigeration makes use of the evaporation of a liquid refrigerant, which goes through a cycle so that it can be reused. The main cycles include vapour-compression, absorption, steam-jet or steam-ejector, and air. The term ‘refrigerator’ was first introduced by Thomas Moore, a Maryland farmer, in 1803, but it was in the 20th century that the appliance we know today first appeared.
B People used to find various ways to preserve their food before the advent of mechanical refrigeration systems. Some preferred using ice or snow for cooling, which meant that diets consisted of very little fresh food, fruit or vegetables, but mostly bread, cheese and salted meat. Milk and cheese were very difficult to keep fresh, so such foods were usually stored in a cellar or window box. In spite of those measures, they could not avoid rapid spoilage. Later, people discovered that adding chemicals such as sodium nitrate or potassium nitrate to water could lead to a lower temperature. In 1550, when this technique was first recorded, people used it to cool wine, and the term ‘to refrigerate’ was also recorded. Cooling drinks grew very popular in Europe by 1600, particularly in Spain, France and Italy. People used a new technique: rotating long-necked bottles in water containing dissolved saltpetre. The solution was intended to create very low temperatures and even make ice. By the end of the 17th century, iced drinks, including frozen juices and liquors, had become extremely fashionable in France.
C People’s demand for ice soon became strong. Consumers’ soaring demand for fresh food, especially green vegetables, resulted in reform in people’s dieting habits between 1830 and the American Civil War, accelerated by a drastic expansion of urban areas and rapid improvement in the population’s economic circumstances. With the growth of cities and towns, the distance between the consumer and the source of food increased. In 1799, as a commercial product, ice was first transported from Canal Street in New York City to Charleston, South Carolina. Unfortunately, this transportation was not successful because when the ship reached its destination, little ice was left. Frederick Tudor and Nathaniel Wyeth, two New England businessmen, grasped the great potential opportunities for the ice business and managed to improve the storage method of ice during shipment. Tudor, the acknowledged ‘Ice King’ at that time, concentrated his efforts on bringing ice to tropical areas. To guarantee that the ice arrived at its destination safely, he tried many insulating materials and successfully constructed ice containers, which reduced ice loss from 66 per cent to less than 8 per cent. Wyeth invented an economical and speedy method to cut ice into uniform blocks, which had a tremendous positive influence on the ice industry. He also improved processing techniques for storing, transporting and distributing ice with less waste.
D When people realised that ice transported from a distance was not as clean as previously thought and gradually caused many health problems, it became more important to seek clean natural sources of ice. To make matters worse, by the 1890s, water pollution and sewage dumping made clean ice even less available. The adverse effect first appeared in the brewing industry and then spread seriously to meat-packing and dairy industries. As a result, clean mechanical refrigeration was urgently needed.
E Many inventors with creative ideas took part in the development of refrigeration, and each version was built on previous discoveries. Dr William Cullen began studying the evaporation of liquids under vacuum conditions in 1720. He demonstrated the first known artificial refrigeration at the University of Glasgow in 1748 by allowing ethyl ether to boil in a partial vacuum. American inventor Oliver Evans designed the first refrigeration machine using vapour rather than liquid in 1805. Although his conception was not put into practice, the mechanism was adopted by the American physician John Gorrie, who made a similar cooling machine in 1842 to reduce the temperature of patients with yellow fever in a Florida hospital. In 1851, Gorrie obtained the first US patent for mechanical refrigeration. In 1820, Michael Faraday first liquefied ammonia to cause cooling. In 1859, Ferdinand Carré of France invented the first ammonia-water cooling machine. In 1873, Carl von Linde designed the first practical and portable compressor refrigerator in Munich, and in 1876 he abandoned the methyl ether system and began using an ammonia cycle. In 1894, Linde created a new method, known as the Linde technique, for liquefying large amounts of air. Nearly a decade later, this mechanical refrigerating method was adopted by the meat-packing industry in Chicago.
F From 1840, cars with refrigerating systems were used to deliver and distribute milk and butter. By 1860, most seafood and dairy products were transported with refrigerated systems. In 1867, refrigerated railway cars were patented by J. B. Sutherland of Detroit, Michigan, who invented insulated cars by installing ice bunkers at the ends of the cars. Air came in from the top, passed through the bunkers and circulated through the cars by gravity, while different quantities of hanging flaps controlled different air temperatures. Depending on the cargo, such as meat or fruit, different car designs came into existence. In 1867, the first refrigerated car to carry fresh fruit was manufactured by Parker Earle of Illinois, who shipped strawberries on the Illinois Central Railroad. Each chest was freighted with 100 pounds of ice and 200 quarts of strawberries. In 1949, the trucking industry began to be equipped with a refrigeration system using a roof-mounted cooling device invented by Fred Jones.
G From the late 1800s to 1929, refrigerators employed toxic gases—methyl chloride, ammonia and sulphur dioxide—as refrigerants. In the 1920s, a large number of fatal accidents took place because of methyl chloride leaking from refrigerators. Therefore, American companies began to seek safer methods of refrigeration. Frigidaire discovered a new class of synthetic refrigerants called halocarbons, or CFCs (chlorofluorocarbons), in 1928. This research led to the discovery of chlorofluorocarbons (Freon), which quickly became the prevailing material in compressor refrigerators. Freon was safer for people in the vicinity, but in 1973 it was discovered to have detrimental effects on the ozone layer. After that, new improvements were made, and hydrofluorocarbons, with no known harmful effects on the ozone layer, were used in cooling systems. Nowadays, CFCs are no longer used and have been made illegal in several places, making refrigeration far safer than before.