Fossilised fish bones from Africa are fertilising the Amazon forest
A team of researchers from the UK has determined the source of phosphorus in the dust which blows from Africa to the Amazon region across the Atlantic Ocean. Phosphorus is a very important nutrient in the process of photosynthesis in the ocean and terrestrial ecosystems such as the Amazon, both vital for the process of carbon sequestration. The source has now been identified as the bones and scales of fish that used to inhabit the mega lake once located in the Sahara region, but which has since dried up.
The study, undertaken by a team of researchers from Birkbeck (University of London), the Diamond Light Source (a not-for-profit science facility) and the University of Leeds, was published in Chemical Geology on September 25.
The source of the dust is the Bodélé Depression, cited by the paper as “the world’s greatest source of dust”. It is the lowest point in Chad in central Africa and is located in the north of the country. The depression was a part of the largest of the inland seas, the ancient or paleolake Mega-Chad. Mega-Chad was said to be larger than Lake Caspian, the largest enclosed inland body of water today.
The samples were collected as part of the Bodélé Dust Experiment in 2005, a multidisciplinary project to study the dust from the depression, its climatic impact and composition. On analysis, the dust was found to contain considerable amounts of plant nutrients, including phosphorus, but it was previously unknown whether the source of the phosphorus in the dust was biogenic, or inorganic, as from rocks.
Fish bones, commonly used as soil fertiliser, are formed from poorly crystalline apatite, a phosphate mineral. Phosphorus is much more soluble in this form than in that available from inorganic sources, and is thus more readily available for use by plants. This makes dust from the Bodélé invaluable to the Amazon in terms of the nutrients it provides. However, the paper mentions that the Bodélé should be treated as a transient source of mineral dust, since the amount of fossilised fish bones and scales available is finite.
According to Karen Hudson-Edwards from Birkbeck, University of London, who led the study, it is unknown how long the dust will provide phosphorus to global ecosystems. “No one knows the true thickness of the Bodélé deposits. We are currently trying to get the funding to return to Chad to do the fieldwork necessary to answer this question,” she said.
On the question of the impact on the Amazon forests once this source runs out, she says, “When the fish source of phosphorus from the Bodélé finishes, the Amazon will have to rely on inputs of phosphorus from rock sources, which are less soluble and less bioavailable than fish phosphorus.”
Research: Solid-phase phosphorus speciation in Saharan Bodélé Depression dusts and source sediments
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