From 11,000 to 5,000 years ago, the 3.5 million square miles of sand and dunes had pockets of water, where hippos cheerfully went about their business. But then, quite drastically, the climate changed, and Massachusetts Institute of Technology (MIT) and Columbia University scientists want to know why.
The research team used groundbreaking techniques to glean the rate at which dust settled in the area we now think of as the Sahara desert during the greener era known as the African Humid Period.
Using data collected by Columbia University and Woods Hole Oceanographic Institution, the team studied how quickly sand from the nearby region settled into an area on the northwest coast of Africa, where they took samples of deposits that settled as far back as 30,000 years ago.
First they employed thorium-230 normalization, a method that measures the presence of decayed uranium also known as thorium 230, in a specific sediment sample.
RD Mag describes the method thusly:
Over time, uranium decays to thorium-230, an insoluble chemical that sticks to any falling sediment as it sinks to the seafloor. The amount of uranium—and by extension, the production rate of thorium-230—in the world’s oceans is relatively constant. McGee [David McGee, an assistant professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences] measured the concentration of thorium-230 in each core sample to determine the accumulation rates of sediments through time.
Higher concentrations of thorium-230 indicate slow sedimentation rates.
Next it was necessary to determine how much of the deposits was carried by the wind – because eventually this will help to determine to what extent dust impacts/drives/changes/slows down climate change – knowledge that could inform us in the rapidly changing Holocene era.
Again, from RD Mag via MIT:
The researchers subtracted biological sediment from the samples by measuring calcium carbonate, opal and organic carbon, the primary remnants of living organisms. After subtracting this measurement from each sample layer, the researchers tackled the task of separating the remaining sediment into windblown dust and marine sediments—particles that circulate through the ocean, deposited on the seafloor by currents. McGee employed a second technique called grain-size endmember modeling, charting a distribution of grain sizes ranging from coarse grains of dust to fine grains of marine soil.
“We define these endmembers: A pure dust signal would look like this, and a pure marine sediment would look like this,” McGee says. “And then we see, OK, what combination of those extremes would give us this mixture that we see here?”
Based on this, they discovered that the Sahara today is five times dustier than it was 6,000 years ago. But the team needs to go further to understand the genesis of Sahara’s wetter days.
Image of camels and people in the Sahara desert, Shutterstock