Vor 6000 Jahren lag der größte Süßwasser-See der Erde in der Sahara. Lake Mega-Chad erstreckte sich im zentralen Südteil der heutigen Wüste über 360.000 Quadratkilometer und erreichte eine Tiefe von über 170 m. Lake Mega-Chad bildete sich vor etwa 15.000 Jahren und existierte bis vor etwa 5000 Jahren, als er abrupt schrumpfte. Damals verlagerte sich der westafrikanische Monsun-Regen nach Süden und konnte die Sahara nicht mehr erreichen. Vor etwa 3000 Jahren kam der Regen wieder zurück, und der Seespiegel stieg wieder an. Die lebenspendende Phase hielt jedoch nur wenige Jahrhunderte an, und die Gegend wurde schnell wieder zur Wüste. Nachzulesen in einer ausgezeichneten Publikation von Simon Armitage und Kollegen. Hier die dazugehörige Pressemitteilung der University of Royal Holloway London vom 29. Juni 2015 (via Science Daily):
Largest freshwater lake on Earth was reduced to desert dunes in just a few hundred years
Researchers from Royal Holloway, Birkbeck and Kings College, University of London used satellite images to map abandoned shore lines around Palaeolake Mega-Chad, and analysed sediments to calculate the age of these shore lines, producing a lake level history spanning the last 15,000 years. At its peak around 6,000 years ago, Palaeolake Mega-Chad was the largest freshwater lake on Earth, with an area of 360,000 km2. Now today’s Lake Chad is reduced to a fraction of that size, at only 355 km2. The drying of Lake Mega-Chad reveals a story of dramatic climate change in the southern Sahara, with a rapid change from a giant lake to desert dunes and dust, due to changes in rainfall from the West African Monsoon. The research, published in the journal Proceedings of the National Academy of Sciences confirms earlier suggestions that the climate change was abrupt, with the southern Sahara drying in just a few hundred years.
Part of the Palaeolake Mega-Chad basin that has dried completely is the Bodélé depression, which lies in remote northern Chad. The Bodélé depression is the World’s single greatest source of atmospheric dust, with dust being blown across the Atlantic to South America, where it is believed to be helping to maintain the fertility of tropical rainforests. However, the University of London team’s research shows that a small lake persisted in the Bodélé depression until about 1,000 years ago. This lake covered the parts of the Bodélé depression which currently produce most dust, limiting the dust potential until recent times.
„The Amazon tropical forest is like a giant hanging basket,“ explains Dr Simon Armitage from the Department of Geography at Royal Holloway. „In a hanging basket, daily watering quickly washes soluble nutrients out of the soil, and these need to be replaced using fertiliser if the plants are to survive. Similarly, heavy washout of soluble minerals from the Amazon basin means that an external source of nutrients must be maintaining soil fertility. As the World’s most vigorous dust source, the Bodélé depression has often been cited as a likely source of these nutrients, but our findings indicate that this can only be true for the last 1,000 years,“ he added.
Publikation: Simon J. Armitage, Charlie S. Bristow, and Nick A. Drake. West African Monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad. PNAS, June 2015 DOI: 10.1073/pnas.1417655112
