Sahara Dust Sustains The Amazon Rainforest And More

Sahara Dust Sustains The Amazon Rainforest And More

The Sahara desert and the Amazon rainforest are so different and far apart one might not imagine that they share a vital ecological link.

A new paper in the Journal of Geophysical Research joins other papers extant proving that without the more than a hundred million tonnes of dust that blows in the wind across the Atlantic Ocean from the Sahara and there would be no great Amazon rainforest, the largest rainforest on Earth. In the dust that blows from Africa this paper focuses on the all important plant nutrient phosphorus. It is a tiny 1/ 10th of 1% of the Sahara dust but in total 22,000 tonnes of it falls upon the Amazon each year. This is an amount of vital phosphorus equal to what the rainforest rains wash away each year. It’s a remarkable perfect balance of nature.

This video illustrates just how vital dust in the wind is… for much of this blue planet “all we are is dust in the wind.”

For the first time, scientists led by a team from the University of Maryland have an accurate estimate of how much phosphorus makes this trans-Atlantic journey which is reported on in a new paper, accepted for publication Feb. 24, 2015 in the journal Geophysical Research Letters.

This phosphorus accounts for just 0.08% of the 27.7 million tons of Saharan dust that settles in the Amazon every year. The finding is part of a bigger research effort to understand the role of dust in the environment and its effects on local and global climate. Ten times that amount of dust blows off of the Sahara on its journey across the Atlantic. Many others have reported on the vital role of dust for the oceans.

bahama bank

The beautiful white sands of the Bahamas and the white carbonate banks that make up those islands depend on African dust that is blown to them on the wind every year. This ocean pasture detritus, marine carbonate, is one of the worlds great natural CO2 sequestration engines.

It’s been reported that as well as sustaining the Amazon rainforest that same Saharan dust is falls in the ocean waters of the Caribbean and as far north as Bermuda. The great white banks of marine carbonate rock that make up the Bahamas are similarly reported in recent studies to be derived from Saharan dust.

The dust that blows from land to the oceans proves to have yet another powerful mechanism through which in partnership with ocean pastures and their phyto-plankton. This natural process works to manage global CO2. In the ocean pastures around the Bahamas cyanobacteria/blue-green algae are stimulated to fix nitrogen which fuels plankton blooms and directly causes the precipitation and sinking of carbonate minerals sending vast quantities of CO2 to the seabed. One unexpected observation in this new paper is that cyano-bacteria need ten times the iron of more ordinary phyto-plankton in order to fix nitrogen. ( “The fertilization of the Bahamas by Saharan dust: A trigger for the geologic sinking of CO2 via carbonate precipitation?” was published in the Journal Geology June 2014)

“We know that dust is very important in many ways. It is an essential component of Earth system. Dust will affect climate and, at the same time, climate change will affect dust,” said lead author Hongbin Yu, an associate research scientist at the Earth System Science Interdisciplinary Center (ESSIC), a joint center of the University of Maryland and NASA’s Goddard Space Flight Center.

A large portion of the African dust blows from the Bodélé Depression in Chad. This ancient lake bed contains huge deposits of dead microorganisms that are loaded with phosphorus. Ocean pastures of the Atlantic and Amazonian soils, in turn, are short on phosphorus. The Amazon’s constant rains have long washed vital plant nutrients out of its soil leaving that soil barely able to grow plants Thus, the entire Amazon ecosystem depends on Saharan dust to replenish these losses.

CALIPSO Earth Observatory

CALIPSO Earth Observatory

Yu and his colleagues analyzed dust transport estimates based on data collected by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite between 2007 and 2013. The team focused on Saharan dust transported across the Atlantic Ocean to South America and beyond, to the Caribbean Sea, because it is the largest transport of dust on the planet. Indeed only this region and the region immediately down wind of the Arabian desert where dust blows into the Indian ocean are the only two sources of dust on the planet that have not seen dramatic decline due to global greening.

Great Green Walls In China and Growing Now In Africa Reduce The Loss of Topsoil, Dust In The Wind.


Africa’s Great Green Wall will help pastures sustain more livestock in the South Sahara but will also greatly reduce the amount of vital mineral dust feeding ocean pastures and the Amazon rainforest. click to enlarge

But the Southern Sahara is changing fast and nations in that part of the world are working to build an African Great Green Wall of restored grasslands as China’s Great Green Wall has succeeding in doing. Given the success of China at dramatically reducing top soil losses from wind erosion in Western China and Mongolia the African Great Green Wall will within a decade dramatically decrease the dust, soil, blowing away in the wind. This will be good news for the pastures on land in the south Sahara but terrible news for Atlantic ocean pastures and the Amazon rainforest.

The University of Maryland team estimated the phosphorus content of Saharan dust by studying samples from the Bodélé Depression and from ground stations on the Carribean island of Barbados and in Miami. They then used this estimate to calculate how much phosphorus gets deposited in the Amazon basin. Although the seven-year data record is too short to make conclusions about long-term trends, it is an important step toward understanding how dust and other wind-borne particles, or aerosols, behave as they move across the ocean.

“We need a record of measurements to understand whether or not there is a fairly robust, fairly consistent pattern to this aerosol transport,” said Chip Trepte, project scientist for CALIPSO at NASA’s Langley Research Center, who was not involved in the study.

Year by year, the pattern is highly variable. There was an 86 percent change between the highest amount of dust transported in 2007 and the lowest in 2011. Yu and his colleagues believe this variation is due to conditions in the Sahel, the long strip of semi-arid land on the southern border of the Sahara. Years of high rainfall in the Sahel were typically followed by low dust transport in the next year.

“This is a small world, and we’re all connected together,” Yu said.