Lake Yoa
Encyclopedia
Lake Yoa is the most famous, and second largest of a series of Lakes in Borkou-Ennedi-Tibesti Region basin of northeastern Chad
. It is locaded in the Ounianga Kebir about 40km west of Ounianga Sarir. These lakes are remnants of a much larger lake which occupied this basin during the African Humid period which lasted from approximately 15,000 to 5,500 years before present
. There are currently 15 lakes in the basin with a total surface area of approximately 20 kilometres (12.4 mi) square.
due to a high rate of evaporation. In this case, even though the rate of evaporation from Lake Yoa is equivalent to a staggering 6 metres (19.7 ft) a year, the total lake depth is 25 metres (82 ft), unique physical factors combine to keep all of the lakes, except the central Lake Teli, fresh. First, water accumulated in an underground aquifer
during the wet millennia is supplied to the lakes. Second, wind blown sand separates the basin into ten lakes with Lake Teli occupying a lower position than the lakes around it. Third, thick mats of reeds cover the surface of the fresh water lakes where they slow evaporation, but are absent from the saline waters of Lake Teli. As a result, greater evaporation takes place from the surface of Lake Teli, keeping its water level low. This allows water from the adjacent lakes to flow through the permeable dune barriers into Lake Teli keeping their waters fresh.
, recovered a core
of sediment from the bottom of the lake. Because Lake Yoa has been in existence continuously since the Humid Period its waters have protected the sediments accumulated at the bottom from erosion and dispersal. After analysis of the pollen
preserved in the core, Kröpelin and his colleagues concluded that the shift from forest to desert in the area of Lake Yoa occurred gradually, with intervening periods of scrubland and grassland
before the onset of full desert conditions. This conclusion contrasts with work done by Peter deMenocal of Columbia University
and colleagues, who in 2000 drilled a core of ocean sediment off the western coast of Mauritania
. Because of the dust levels in this core deMenocal and his co-authors concluded that desert conditions came on quickly, over a period of a few centuries.
The discrepancies between these two accounts are not surprising given the differences in the data examined. The ocean core represents what is essentially a mass survey of the entire northern portion of the African land mass while the Lake Yoa data would survey more specifically conditions in the areas to the south and west of it, which supplied material to the prevailing trade wind
s of Holocene
time before they crossed Lake Yoa. It is possible both that, northern Africa became quickly and generally drier, and that, in specific areas the process of desertification
moved through a series of more gradual ecological transitions. (Ranson, 2008)
Chad
Chad , officially known as the Republic of Chad, is a landlocked country in Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic to the south, Cameroon and Nigeria to the southwest, and Niger to the west...
. It is locaded in the Ounianga Kebir about 40km west of Ounianga Sarir. These lakes are remnants of a much larger lake which occupied this basin during the African Humid period which lasted from approximately 15,000 to 5,500 years before present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
. There are currently 15 lakes in the basin with a total surface area of approximately 20 kilometres (12.4 mi) square.
Geography
These lakes form a hydrological system that is unique in the earth’s deserts. Ordinarily when water is exposed at the surface in highly arid environments it becomes salineSalinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
due to a high rate of evaporation. In this case, even though the rate of evaporation from Lake Yoa is equivalent to a staggering 6 metres (19.7 ft) a year, the total lake depth is 25 metres (82 ft), unique physical factors combine to keep all of the lakes, except the central Lake Teli, fresh. First, water accumulated in an underground aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
during the wet millennia is supplied to the lakes. Second, wind blown sand separates the basin into ten lakes with Lake Teli occupying a lower position than the lakes around it. Third, thick mats of reeds cover the surface of the fresh water lakes where they slow evaporation, but are absent from the saline waters of Lake Teli. As a result, greater evaporation takes place from the surface of Lake Teli, keeping its water level low. This allows water from the adjacent lakes to flow through the permeable dune barriers into Lake Teli keeping their waters fresh.
Climate change
Lake Yoa recently became of interest in the study of global climate trends when a team headed by Stefan Kröpelin, of the University of CologneUniversity of Cologne
The University of Cologne is one of the oldest universities in Europe and, with over 44,000 students, one of the largest universities in Germany. The university is part of the Deutsche Forschungsgemeinschaft, an association of Germany's leading research universities...
, recovered a core
Core sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...
of sediment from the bottom of the lake. Because Lake Yoa has been in existence continuously since the Humid Period its waters have protected the sediments accumulated at the bottom from erosion and dispersal. After analysis of the pollen
Pollen analysis
Analysis of the distribution of pollen grains of various species contained in surface layer deposits, especially peat bogs and lake sediments, from which a record of past climate may be inferred. Because the lake sediments accumulate over time, a core of the mud will show that the mud at the bottom...
preserved in the core, Kröpelin and his colleagues concluded that the shift from forest to desert in the area of Lake Yoa occurred gradually, with intervening periods of scrubland and grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
before the onset of full desert conditions. This conclusion contrasts with work done by Peter deMenocal of Columbia University
Columbia University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...
and colleagues, who in 2000 drilled a core of ocean sediment off the western coast of Mauritania
Mauritania
Mauritania is a country in the Maghreb and West Africa. It is bordered by the Atlantic Ocean in the west, by Western Sahara in the north, by Algeria in the northeast, by Mali in the east and southeast, and by Senegal in the southwest...
. Because of the dust levels in this core deMenocal and his co-authors concluded that desert conditions came on quickly, over a period of a few centuries.
The discrepancies between these two accounts are not surprising given the differences in the data examined. The ocean core represents what is essentially a mass survey of the entire northern portion of the African land mass while the Lake Yoa data would survey more specifically conditions in the areas to the south and west of it, which supplied material to the prevailing trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...
s of Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
time before they crossed Lake Yoa. It is possible both that, northern Africa became quickly and generally drier, and that, in specific areas the process of desertification
Desertification
Desertification is the degradation of land in drylands. Caused by a variety of factors, such as climate change and human activities, desertification is one of the most significant global environmental problems.-Definitions:...
moved through a series of more gradual ecological transitions. (Ranson, 2008)