Have you ever wondered why some of the seashells you find are coloured black – like these oyster shells? Fresh empty shells, and the shells of the living animals, are not coloured like this. Oysters, for example, are usually a light creamy colour and sometimes have concentric bands of pink or purple – though sometimes you can find the flat shells with a browny-black horn-like outer covering. The dark colour does not, however, penetrate through the entire shell matrix.
The reason for this blackening of the shell lies deep down in the sediments of the beach. Very fine, poorly sorted sands can usually hold onto water as the tide goes out and also contain a lot of organic detritus. The sand grains provide an attachment surface for huge numbers of bacteria and diatoms; and support a rich fauna.
There is a sharp boundary between 5 to 15 centimetres down where the sand changes from yellow, through a band of grey, to black. Above the grey there is enough oxygen in the water to oxidise all the waste products of the vast community of micro-organisms (aerobic respiration). Below the grey boundary, the black sand has no oxygen. Here, some bacteria produce hydrogen sulphide that reacts with iron in the sand to give black iron sulphides (anaerobic respiration); it is these compounds that stain buried sea shells. As the black iron sulphides work their way upwards by animal actions, through the sediments that still contain oxygen, they are oxidised to ferric oxide which gives the yellow colour to sand.
Revised post that was first published 26 February 2009
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