During the passage of HMS
Beagle through the eastern Atlantic, Charles Darwin became intrigued by dust
that accumulated on parts of the ship:
On the 16th of January (1833),
when the Beagle was ten miles off the N.W. end of St. Jago [Santiago, Cape
Verde Islands], some very fine dust was found adhering to the under side of the
horizontal wind-vane at the mast-head; it appeared to have been filtered by the
gauze from the air, as the ship lay inclined to the wind. The wind had been for
twenty-four hours previously E.N.E., and hence, from the position of the ship,
the dust probably came from the coast of Africa. The atmosphere was so hazy
that the visible horizon was only one mile distant.. [1]
..Many scattered accounts have
appeared concerning the dust which has fallen in considerable quantities on
vessels on the African side of the Atlantic Ocean. It has appeared to me
desirable to collect these accounts, more especially since Professor
Ehrenberg’s remarkable discovery that the dust consists in considerable part of
Infusoria and Phytolitharia [the remains of the coverings of single-celled
organisms, the latter with siliceous coats].. [1]
..it is a very singular fact,
that out of the many forms [of the coverings of single-celled organisms] known
to Professor Ehrenberg as characteristic of Africa, and more especially of the
Sahara and Senegambian regions, none were found in the dust. From these facts
one might at first doubt whether the dust came from Africa; but considering
that it has invariably fallen with the wind between N.E. and S.E., that is,
directly from the coast of Africa; that the first commencement of the haze has
been seen to come on with these winds; that coarser particles have first
fallen; that the dust and hazy atmosphere is more common near the African coast
than further in the Atlantic; and lastly, that the months during which it falls coincide with those when the
harmattan blows from the continent, and when it is known that clouds of dust
and sand are raised by it, I think there can be no doubt that the dust which
falls in the Atlantic does come from Africa. [1]
We know from satellite images that considerable amounts of
dust are carried out far into the Atlantic, on to South America [2] and, with
changes in the jet stream and other meteorological events, to many other parts
of the Earth (see image below). It is intriguing to read in Darwin’s report
that the dust contains the identifiable remains of “Infusoria and Phytolitharia”. The northern
part of Africa, where these aquatic organisms originated, is now desert, but
there must clearly have been water present in earlier times and, indeed, a lake
covering 350,000 km2 existed in the region 6,000 years ago. [3] It
is likely that the remains of cells that Darwin, and others, reported from the
Atlantic were originally from this source and we now know that components of
the dust include residues from many other algae and animals, including fish,
and that these fall into the water, or are carried there by rains, and act as fertilisers. [3]
Anyone who gardens knows that fish bone meal is a good
source of phosphorus for growing plants and it is thus likely to play a similar
role when deposited into the oceans, or on to terrestrial landscapes. Recent research
by Karen Hudson-Edwards and colleagues [3] has shown that the mineral residues
of fish bones allow easier solution of phosphorus than many other bound organic
residues that are present in the dust being blown from the Sahara. There is, of
course, a finite amount of this material and, in time, it will form a smaller
and smaller fraction of that eroded from the region, as the strata
containing the organic remains begin to disappear.
Many people are familiar with the idea of fish, and other
animals, falling from the sky, having been sucked up by waterspouts from surface
waters, [4] but the rain-down of fossilised fish bone meal, and other
mineralised plant and animal material, is less well-known. It has been, and continues
to be, an important input of fertiliser to both aquatic and terrestrial
communities.
[1] Charles Darwin (1846) An account of the Fine Dust which
often falls on Vessels in the Atlantic Ocean. Quarterly Journal of the Geological Society 2: 26-30.
[3] Karen A. Hudson-Edwards, Charlie S. Bristow,
Giannantonio Cibin, Gary Mason and Caroline L. Peacock (2014) Solid-phase
phosphorus speciation in Saharan Bodélé
Depression dusts and source sediments. Chemical
Geology 384: 16-26.
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