Two-toned dust plumes blew northward off the coast of Libya on October 26, 2007
This image comes from the photography blog The Big Picture from The Boston Globe. Each week’s post contains wonderful captures around a particular topic. The week of January 14 was “Earth, observed.” The dust blowing over the Atlantic brings to mind Darwin’s 1845 paper, “An account of the FINE DUST which often falls on Vessels in the ATLANTIC OCEAN,” from the Quarterly Journal of the Geological Society of London (see this paper here on Darwin Online). Darwin writes:
On the 16th of January (1833), when the Beagle was ten miles off the N.W. end of St. Jago, 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. During our stay of three weeks at St. Jago (to February 8th) the wind was N.E., as is always the case during this time of the year; the atmosphere was often hazy, and very fine dust was almost constantly falling, so that the astronomical instruments were roughened and a little injured. The dust collected on the Beagle was excessively fine-grained, and of a reddish brown colour; it does not effervesce with acids; it easily fuses under the blowpipe into a black or gray bead.
From the several recorded accounts it appears that the quantity of dust which falls on vessels in the open Atlantic is considerable, and that the atmosphere is often rendered quite hazy; but nearer to the African coast the quantity is still more considerable. Vessels have several times run on shore owing to the haziness of the air: and Horsburgh recommends all vessels, for this reason, to avoid the passage between the Cape Verd Archipelago and the main-land. Roussin also, during his survey, was thus much impeded. Lieut. Arlett found the water so discoloured, that the track left by his ship was visible for a long time; and he attributes this in part to the fine sand blown from the deserts, “with which everything on board soon becomes perfectly caked.”
Professor Ehrenberg has examined the dust collected by Lieut. James and myself; and he finds that it is in considerable part composed of Infusoria, including no less than sixty-seven different forms. These consist of 32 species of siliceous-shielded Polygastrica;3 of 34 forms of Phytolitharia, or the siliceous tissues of plants; and of one Polythalamia. The little packet of dust collected by myself would not have filled a quarter of a tea-spoon, yet it contains seventeen forms.
In 2007, several microbiologists published in Environmental Microbiology an article titled “Life in Darwin’s dust: intercontinental transport and survival of microbes in the nineteenth century.” The abstract:
Charles Darwin, like others before him, collected aeolian dust over the Atlantic Ocean and sent it to Christian Gottfried Ehrenberg in Berlin. Ehrenberg’s collection is now housed in the Museum of Natural History and contains specimens that were gathered at the onset of the Industrial Revolution. Geochemical analyses of this resource indicated that dust collected over the Atlantic in 1838 originated from the Western Sahara, while molecular-microbiological methods demonstrated the presence of many viable microbes. Older samples sent to Ehrenberg from Barbados almost two centuries ago also contained numbers of cultivable bacteria and fungi. Many diverse ascomycetes, and eubacteria were found. Scanning electron microscopy and cultivation suggested that Bacillus megaterium, a common soil bacterium, was attached to historic sand grains, and it was inoculated onto dry sand along with a non-spore-forming control, the Gram-negative soil bacterium Rhizobium sp. NGR234. On sand B. megaterium quickly developed spores, which survived for extended periods and even though the numbers of NGR234 steadily declined, they were still considerable after months of incubation. Thus, microbes that adhere to Saharan dust can live for centuries and easily survive transport across the Atlantic.
Darwin relied on finding the means of dispersal of many organisms because, if all life on earth is related through common ancestry, some organisms had to have found ways to travel to new locations (single centers of creations versus the multiple centers of creation that some naturalists, like Louis Agassiz, postulated in order to stay true to scripture). Whether floating as seeds may do, hitchhiking on the feet or in the bowels of birds, or transporting via logs or other flotsam, or even on trains and cars, life finds a way (yes, Malcolm) to new places.