The Sahara Desert is known to have undergone major, and abrupt, hydrological fluctuations and was vegetated at times in the past. During a wet phase in the Early Holocene known as the African Humid Period (AHP), the region currently occupied by the Sahara desert was vegetated, contained forests, grasslands, permanent lakes, and was occupied by human populations. When the AHP ended, the Sahara was transformed into a hyperarid desert. On orbital time scales, the large hydrological fluctuations in North Africa are linked to changes in the African monsoon, which is related to precession-forced variability in low-latitude summer insolation. The abrupt transitions between humid and arid conditions observed in marine and terrestrial paleoclimate records from North Africa, however, cannot be explained solely by gradual orbital forcing; thus, other nonlinear feedback processes are required to explain the abrupt climate responses to orbital forcing.
Every year, winds from the northeast sweep up hundreds of millions of tons of Saharan dust, depositing much of this sediment into the Atlantic Ocean, off the coast of West Africa. Layers of this dust, built up over hundreds of thousands of years, can serve as a geologic chronicle of North Africa’s climate history: Layers thick with dust may indicate arid periods, whereas those containing less dust may signal wetter eras. Scientists have analyzed sediment cores dug up from the ocean bottom off the coast of West Africa, for clues to the Sahara’s climate history. These cores contain layers of ancient sediment deposited over millions of years. Each layer can contain traces of Saharan dust as well as the remains of life forms, such as the tiny shells of plankton.
Layers of dust deposited off the west coast of Africa over the last 240,000 years were analysed. The researchers measured the concentrations of a rare isotope of thorium in each layer to determine how rapidly dust was accumulating on the seafloor. Thorium is produced at a constant rate in the ocean by very small amounts of radioactive uranium dissolved in seawater, and it quickly attaches itself to sinking sediments. As a result, scientists can use the concentration of thorium in the sediments to determine how quickly dust and other sediments were accumulating on the seafloor in the past. During times of slow accumulation, thorium is more concentrated, while at times of rapid accumulation, thorium is diluted. The pattern that emerged was very different from what others had found in the same sediment cores. Researchers found that some of the peaks of dust in the cores were due to increases in dust deposition in the ocean, but other peaks were simply because of carbonate dissolution and the fact that during ice ages, in this region of the ocean, the ocean was more acidic and corrosive to calcium carbonate. It might look like there is more dust deposited in the ocean, when in the face this is not the case.
Once the researchers removed this confounding effect, they found that what emerged was primarily a new “beat” in which the Sahara vacillated between wet and dry climates every 20,000 years, in step with the region’s monsoon activity and the periodic tilting of the Earth. Apparently, the increased solar flux in North Africa can intensify the region’s monsoon activity which makes the Sahara Desert wetter. However, when the planet’s axis swings toward an angle that reduces the amount of incoming summer sunlight, it produces a drier parched climate like what we see today. David McGee, an associate professor in MIT’s Department of Earth, Atmosphere and Planetary Sciences found that analysing this time series will help understand the history of Sahara desert and investigate what period could contribute to amenable conditions for humans to settle the Sahara desert and allowing them to cross this otherwise inhospitable region.
“We can now produce a record that sees through the biases of these older records, and so doing, tells a different story,” McGee says. “We’ve assumed that ice ages have been the key thing in making the Sahara dry versus wet. Now we show that it’s primarily these cyclic changes in Earth’s orbit that have driven wet versus dry periods. It seems like such an impenetrable, inhospitable landscape, and yet it’s come and gone many times, and shifted between grasslands and a much wetter environment, and back to dry climates, even over the last quarter million years.”
Detailed information can be obtained from the following journal :
Skonieczny, C., McGee, D., Winckler, G., Bory, A., Bradtmiller, L.I., Kinsley, C.W., Polissar, P.J., De Pol-Holz, R., Rossignol, L. and Malaizé, B., 2019. Monsoon-driven Saharan dust variability over the past 240,000 years. Science advances, 5(1), p.eaav1887.
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© 2018 Oceanographer Daneeja Mawren