The Amazon is being fertilised by the remains of ancient fish from Africa.
The nutrient-rich material is being carried in millions of tonnes of dust blown across the Atlantic from the Sahara every year.
Scientists have long recognised the importance of this airborne train to the rainforest's health.
But now a UK team has been able to show that much of the essential phosphorous in the dust is derived from the bones and scales of fish and other organisms.
These are animals that lived in Megalake Chad, a massive body of water that covered north-central Africa thousands of years ago.
When they died, their remains sank into the muddy sediments, which today are exposed in what is one of the windiest places on Earth - the so-called Bodélé Depression.
Satellites regularly catch vast clouds of dust being whipped up in this region of Chad to be thrown across the ocean to South America.
The dust contains the apatite (phosphorus) mineral. Phosphorus is a nutrient essential for photosynthesis.
Scientists were unsure whether this apatite had been weathered out of rocks or perhaps had a biogenic source.
But by examining its crystalline structure, researchers have revealed its true origin.
"This is the first time that fish bone and scale phosphorus have been found in dusts," said Prof Karen Hudson-Edwards from Birkbeck, University of London.
"The finding is important because this type of phosphorus is more soluble and available to ecosystems like the Amazon than other types of phosphorus that come from rocks.
"The Bodélé fish phosphorus is like that found in fish bone meal that gardeners use as a fertiliser," she told BBC News.
The determination relied on work carried out at the Diamond Light Source in Oxfordshire.
This facility uses brilliant X-rays to probe the workings of matter on the smallest scales.
Its pictures could discern the delicate, tell-tale chemical signature of biogenic apatite in the Bodélé dusts.
The team's report in the journal Chemical Geology highlights the fact that this important source of phosphorus for the Amazon is finite.
The sediments of Megalake Chad will eventually be completely eroded by the winds blowing through the Sahara.
When that happens, it could have deleterious consequences for the rainforest, says co-worker Dr Caroline Peacock from Leeds University.
"A large part of the phosphorus that the Amazon receives currently is in this more useful soluble form. While the lake sediments remain - that's great. But when they're gone then the Amazon will have to make do with detrital (weathered rock) phosphorus, detrital apatite, which is that much harder to solubilise."
The team's aim is to go back to Chad to investigate precisely how long the important dusts can be sustained.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos