Previously, it was thought that fine-grained carbonates, which constitute a major component of marine carbonate sediments, were derived primarily from either direct precipitation from sea water or from the breakdown of the skeletons of marine invertebrates and algae. But the present study showed that large volumes of carbonate crystals are precipitated inside the intestines of marine fish and are then excreted at very high rates.

Although the carbonates are produced in the fish’s guts, they are derived from calcium in the sea water they drink, rather than from their food. The individual crystals are usually less than 30 micrometres in diameter, but the volumes produced by individual fish are so immense that the carbonate has direct relevance to understanding marine carbonate budgets. Measurements made on fish in the Bahamas yielded conservative estimates for production in excess of six million kilograms of carbonate a year across the region, equivalent to an estimated 14 per cent of its total carbonate mud production.

‘Given how much carbonate these fish can produce, the findings also clearly have major implications for our understanding of different sources and sinks of carbonate sediment in the oceans, and some exciting implications for understanding where much of the mud in limestones and chalks may derive from,’ said the study’s lead author, Professor Chris Perry of Manchester Metropolitan University.

April 2011