Darwin’s Dust

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.

[2] http://www.nasa.gov/content/goddard/tracking-saharan-dust-across-the-atlantic/#.VFn0ImdKR8E

[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.

[4] http://www.bbc.co.uk/news/world-asia-27298939

Drinking water and the “miraculous” power of sand

In the developed world, we sometimes take drinking water for granted. Turn on the tap, or faucet, and out comes a seemingly endless supply of water that is safe to drink. Of course, the supply is not only used for drinking and cooking and, in London, < 5% is used for this purpose. ¹ The rest of this high quality water we use for personal washing and bathing; cleaning all manner of domestic items, from dishes to surfaces; we use it to wash cars and in high-velocity cleaning jets; and, often in quantity, to water our gardens and yards. Just as we have little interest where waste water goes (we know that it goes into a sewerage system of some kind and is therefore removed from our dwellings), most of us are not aware of how our drinking water is treated. We know it comes from the Water Company pipe system and originally from rainfall on to land, and we are also aware that supplies need to be maintained, so know that reservoirs may be constructed to hold water that might otherwise be lost as rivers flow to the sea. In some areas, we know that supplies come from boreholes into an aquifer within permeable rock strata.

Whether water comes from an aquifer or from surface run-off, it must first be treated to ensure that it is safe and does not have a bad taste. A number of treatment methods are used by Water Companies, but a common one is the use of beds of sand through which water is allowed to trickle and I will only discuss this one method here.  Slow sand filters take up a lot of space and a works may consist of more than ten beds, each of surface area > 1000 m². A bed consists of a concrete tank with porous bricks over its base, these being overlain with cobbles and then a thick layer of sand. Water is pumped into the tank above the level of the sand to a depth sufficient to create a head of pressure and it then percolates through the sand, cobbles and bricks, exiting from a pipe low down in the bed. Although slow sand filters may be used to “finish” water that has been pre-treated, they are effective at cleaning very dirty water and making it potable. During a visit to Japan, I visited the city of Ueda, where water of poor quality is drawn from a polluted river and fed to slow sand filter tanks that have masses of filamentous algae growing in them. Some of the organic matter is thus converted to large numbers of algal cells and these also charge the underlying sand with oxygen resulting from their photosynthesis. No-one would be tempted to drink the water in the bed, yet the quality of the filtrate is excellent, as I learned when trying a sample - it was crystal clear and sweet-tasting. So what is the “miracle” of the sand?

A slow sand filter bed under construction.

A filter bed at Ueda, Japan.

Beds of sand act as a physical filter for particles that cannot pass through the pores between grains, but how do the filter beds remove fine particles of organic matter, bacteria, and the like? A visit to a newly-laid slow sand filter gives us a clue to answering this question because water must pass continuously through the column of sand for days before the filter becomes effective. In this time, the sand grains become covered with microbial biofilm (directly analogous to the plaque we brush from our teeth) and this film is responsible for the removal of impurities, some of which become stuck on the matrix and some of which are taken up by the micro-organisms contained within the film (and which exude this sticky coating). Of course, pores must be kept open and this is achieved by a complex community of single-celled organisms that graze on the biofilm, while some of these organisms also capture fine particles from the water passing between sand grains. There are animals present too, with small worms of various kinds being abundant, feeding on organic matter and, in so doing, further ensuring that pores remain open. This complex community, that takes time to develop, keeps microbial activity in check and ensures that fresh adsorptive biofilms are developed continuously. It is a process that occurs within stable sand banks in natural water bodies and, in slow sand filters, we use it for our own purposes. In engineering a system for cleaning water for human use, we didn’t realise that we were adapting an ecosystem that has been in existence for hundreds of millions of years, a very considerable time before humans appeared.

One of the benefits of slow sand filters is that they can be used, on a small scale, by village communities, where water purification from polluted sources becomes possible, with obvious benefits for health. ² As with large-scale filters, the continuous process does require maintenance. If the water is rich in particles and nutrients, it is likely that a schmutzdecke (“dirty layer”) will form at the sand surface and this results in blocking and inefficiency. This is significant when a city needs a constant supply. There are a number of solutions and these include covering beds to reduce biological activity in the water column as, even with pre-treated water, there can be a considerable build up of algae and other matter in beds open to the atmosphere. Open beds are colonised by very large numbers of dancing midge larvae, many of which live within silk tubes on the surface of the sand. They graze over the surface and thus keep pores clean, transforming the organic matter into faecal pellets that are well-bound and therefore do not obstruct the functioning of the bed. The larvae thus play an important role in the functioning of the filter and may be found in densities approaching 50,000 per square meter; an additional benefit being that the silk of the tubes produced by many larvae is highly adsorptive and thus adds to the cleaning process. There comes a time when the bed, whether large or small, will need to have the surface of the sand scraped off and removed. Re-filling the bed with water then allows filtration to proceed, as the underlying sand is well-conditioned, and re-colonisation by midges begins within minutes.

The cleaned surface of a sand filter - the upper part shows the schmutzdecke that eventually reduced the efficient performance of the filter. The sand in the lower part is conditioned with a rich microbial community together with many other types of organism.

That water is cleaned by passage through beds of sand has been known since ancient times, so the engineers of the last few hundred years were following an established principle. In the first of the Great Plagues of Egypt we know that surface waters became undrinkable because something turned all water both red and toxic. I have argued that this redness resulted from the erosion of sediments that subsequently caused the widespread death and decomposition of aquatic organisms, and that this poisoned the usual sources of drinking water. ³ Interestingly, we read in Exodus Chapter 7 verse 24 that “....all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river.” ⁴ In a commentary, ⁵ we read further: “The Egyptians dug round about the river for water to drink, and it seems that the water obtained by this means was not bloody like that in the river...” and I suspect that they were digging down into sand deposits, through which water had passed and been purified by the microbial community coating the sand grains. It would not be regarded as a miracle, although it might seem to be such by some observers. Miracles usually have a rational explanation and we know just how “miraculous” water treatment using sand filters appears to be in providing us with the drinking water that is piped into our homes. If we don’t take the supply for granted, that is.

¹ Roger S Wotton and Helen Evans (2005) London’s Water Supplies. pp 135-143 in London’s Environment: Prospects for a Sustainable World City (ed. Julian Hunt). London, Imperial College Press

² http://www.youtube.com/watch?v=MR75pSCAeOc

³ http://www.opticon1826.com/article/view/119

⁴ http://www.kingjamesbibleonline.org/book.php?book=Exodus&chapter=7&verse=24

⁵ http://www.sacred-texts.com/bib/cmt/clarke/exo007.htm