COLLOIDAL particles in sea water may be important in the marine chemistry of elements such as carbon and iron1,2. Non-living, submicrometre particles (0.4–1.0 µm) have recently been found to be abundant (106–107 particles ml–1) in the north Pacific3 and off Nova Scotia4, but smaller colloidal particles in sea water remain largely uncharacterized. Here we report that marine colloids <120nm in size are at least three orders of magnitude more abundant than larger submicrometre particles. Moreover, the distribution with depth of these small colloids differs markedly from that reported for their larger counterparts3. This vertical stratification suggests that small colloidal particles in sea water are reactive. Examination by transmission electron microscopy and energy-dispersive X-ray spectroscopy indicates that the colloids are largely organic, although trace metals (Fe, Al, Co) may also be present. The existence of such colloids may contribute to the discrepancy between standard and new high-temperature methods for measuring dissolved organic carbon1.
