Saharan dust: the Amazon’s fertiliser

A dust storm is swept from the Sahara across the Atlantic in June 2009. Images from NASA's MODS instrument. Image: NASA Earth Observatory

A dust storm is swept from the Sahara across the Atlantic in June 2009. Images from NASA’s MODS instrument. Image: NASA Earth Observatory

Most PhD students try to perfect a way of describing their research in a few sentences. These ‘elevator pitches’ (cringe) come in useful when meeting new people at seminars, conferences or the pub. My own little snippet is this: dust from the Sahara fertilises the Amazon rainforest.

In the early eighties Carl Jordan was one of the first scientists to recognise that the Amazon relies on the atmosphere as a source of nutrients such as phosphorus. Surprisingly for somewhere that seems so lush, the Amazon is actually deficient in nutrients, and phosphorus in particular.

Phosphorus is easily washed out of the Amazon ecosystem due to shallow soils and frequent rains, but is vital for photosynthesis. Jordan noticed that the rate of nutrient loss of the forest was never more than the nutrient input from the atmosphere. This meant that unless the forest was decreasing in size, its nutrient cycles were limited by the amount obtained from the atmosphere, rather than the soil.

Around the same time, Joseph Prospero reported observations of dust that seemed to be coming from the Sahara in Cayenne, French Guiana. Unlike the black, summer-time aerosols associated with biomass burning in South America, the dust was observed in the winter and was beige in colour. Prospero also noted that the peaks in his measurements of dust collected at Cayenne corresponded with large dust outbreaks measured in Dakar, on the other side of the Atlantic, a few days earlier. Could the atmospheric inputs of dust to the Amazon therefore be reliant on dust that has travelled over two thousand miles?

A decade later, Robert Swap confirmed that this was the case. Using more detailed observations of the dust, in combination with observations from the AVHRR satellite instrument and meteorological data, Swap estimated that thirteen million tonnes of dust from the Sahara was deposited into the Amazon basin each year. Swap’s findings were extraordinary: one ecosystem was dependent on another thousands of miles away.

But Swap’s results still left questions unanswered. His estimate for the amount of dust deposited corresponded to around only a third of the nutrients needed by the ecosystem. Recent advances in remote sensing of aerosols have allowed scientists to examine this discrepancy.

In 2005, Yoram Kaufman used measurements of how much light was absorbed or scattered by dust from the Sahara to obtain a new estimate of its mass. Remarkably, Kaufman’s analysis of data from NASA’s MODIS instrument suggested that fifty million tonnes of dust is carried each year from the Sahara to South America. This is four times Swap’s estimate, and possibly fills the gap in the Amazon nutrient budget.

Part of the reason I think that dust fertilising the Amazon makes for such an interesting ‘elevator pitch’ is that it highlights the interconnections between aspects of the Earth system. How will the Amazon be affected if wind or precipitation patterns change over the Sahara? These interconnections cross fields of study as well as oceans – to understand the role of dust in fertilising the Amazon rainforest requires exciting work from diverse fields including ecology, atmospheric physics and geochemistry. 

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Siân Williams

I am a PhD student in the Space & Atmospheric Physics Group and Grantham Institute for Climate Change at Imperial College London. My research involves looking at the links between atmospheric dust and vegetation. I enjoy organising the monthly ClimateSnack meetings at Imperial.

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