On the land, many hot springs are enriched in trace metals, and have laid down economically important deposits of gold, silver and mercury. Now, Peter Stoffers from Kiel University and co-workers from Canada and New Zealand have discovered the first mercury-producing hot springs under the sea, in water 200 m deep off the coast of New Zealand's North Island (Geology 27, 931–934; 1999).

The mercury-depositing vents are part of more than 20 in two groups discovered in the Whakatane graben, a fault-bounded depression on the sea floor. They are inhospitable, if colourful, places, reaching temperatures of 200 °C, laying down thick crusts of arsenic and sulphur, and producing liquid hydrocarbons from the ‘cracking’ of organic matter — many of the samples came up coated in an oily film that smelt of petroleum. The vents are inhabited by mats of bacteria that can metabolize sulphur.

The vents are also almost saturated with mercury. Much of it is present as cinnabar (HgS; the red crusts in the picture), but Stoffers et al. estimate that about 10% exists as free mercury, some of which may have condensed from a mercury vapour formed inside the boiling vents. The water pressure forces some of the mercury into pores in the rocks, silver droplets of which are released when the samples are brought to the surface.

The origin of the mercury is uncertain. The basement rocks beneath the vents are shales and graywackes (a form of sandstone). These rock types have been found to contain large mercury deposits which formed in the past and, as this result shows, formation may still be going on. Shales are rich in hydrocarbons, which would explain their presence in the vents. Mercury could also be entering the vents from nearby volcanoes. Stoffers et al. speculate that hydrothermal vents around the world could be adding sizeable amounts of mercury to the sea. That in some places this mercury could end up in fish and eventually humans is an obvious, although unquantified, concern.