INORGANIC sediment suspended in the sea is unstable as single mineral particles and occurs as flocculated aggregates with settling speeds many times greater than that of the constituent grains. This has been demonstrated both through observations of natural samples1–5 and through laboratory experiments6,7. In near-shore waters with high concentrations of terrestrially derived mineral matter the transport and deposition of fine sediment are greatly affected by flocculation. The exact mechanism of floc formation in the sea is not well understood. The concentration, mineralogy and grain size of the suspended particles and the salinity, turbulence and temperature of the water are commonly regarded as controlling factors8,9. Flocculated aggregates composed largely of organic matter have been observed in the sea10,11 and organic matter may be an important ingredient in the flocculation of inorganic sediment.