Modern mid-ocean ridges and back-arc basins are sites of active hydrothermal vent activity. Deposits of sulphide and sulphate minerals are forming at these sites from vents where circulated, heated sea-water, up to 400°C, reacts with hot, recently cooled magma. This heated water dissolves and transports many elements including sulphur and economically important metals, such as copper, zinc and gold, carrying them to the sea floor, where they precipitate as sulphide and sulphate minerals. The vent sites are often marked by the occurrence of chimney structures termed 'black-smokers' where hot fluids carrying sulphide minerals form distinctive plumes.
Around these vent sites live communities of highly specialised animals. Tube worms, bivalves, gastropods and crustaceans live in a hostile environment of complete darkness, extreme pressure, and vent water temperatures that range from 10°C to 400°C;. All these creatures are dependant on bacteria which use hydrogen sulphide from vent water as a primary energy source.
Ancient analogues of these vent sites in the fossil record are often exploited for the sulphide minerals. Termed volcanogenic massive sulphide (VMS) deposits, they are important sources of copper, zinc, lead, silver and gold. Several ancient VMS deposits contain fossils, mostly tube fragments of uncertain affinity. Museum scientists are currently working on a unique Silurian-age (430 million years ago) fossil assemblage from the Yaman Kasy VMS deposit in the Urals of Russia (illustration above).
The deposit is a lens of sulphide minerals - here reconstructed
to its original Silurian form. Assemblages of fossils have been
found at three sites on the mound (see the link image to the right).
Evidence
for sea-floor venting during sulphide formation at Yaman Kasy
is the discovery of fragments of 'black smoker' chimneys
(illustration to the right) found during 1995 at Locality 3 shown on the reconstructed map.
The fossils discovered so far
have all been replaced by various sulphide minerals. Large tubes are
common fossils in the deposit. To some extent they resemble modern vestimentiferan
tube worms (as seen in the three images to the right).
Small ringed tubes (illustration to the right) are also common. These
may be fossils of polychaete worms, perhaps analogous to the alvinellids
found at modern hydrothermal vent sites.
Two other fossils
make up the assemblage found in the Yaman Kasy deposit - a large
cone-shaped monoplacophoran species and an inarticulate
brachiopod species (the two illustrations to the right). Both of these are new and await
further description. Interestingly, neither brachiopods nor monoplacophorans
have been found in modern vent communities.
Research on the Russian massive sulphide deposits and associated fossils is funded by a Museum interdisciplinary research fund project, led by Richard Herrington, Crispin Little and Noel Morris. The work continues jointly with Russian colleagues from Miass, Russia - lead by Victor Zaykov and Valeriy Maslennikov. They funded by the International Science Foundation and the Russian Academy of Sciences. NERC have recently approved funding for an isotopic study.
Further work is planned on other fossil sulphide deposits where fossils have
been reported (Red Dog, Alaska; New Brunswick, Canada).
Illustrations
This project is part of the Earth Materials, History and Processes research theme.
For further information contact:
Dr Crispin Little and
Dr Richard Herrington
The Natural History Museum
Cromwell Road
London SW7 5BD UK
Tel: +44 (0) 171 938 9259 Fax: +44 (0) 171 938 9268
Email: rmh@nhm.ac.uk
