25 September 2014 Last updated at 10:34
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Ancient African fish dust nourishes
Amazon
By Jonathan AmosScience correspondent, BBC News
A Nile perch is seen eroding out of the sediments in the Bodélé Depression
Continue reading the main story
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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.
Satellites regularly picture vast duststorms picking up in the Bodélé Depression
"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