Bacteriogenic iron oxide precipitation at deep-sea hydrothermal vents: A 1.7 Gyr record
|Location||International Geological Congress,oslo 2008|
|Author||Little, Crispin۱; Thorseth, Ingunn۲; Grenne, Tor۳; Slack, John۴|
|Holding Date||28 September 2008|
Fe oxide deposits are commonly found at hydrothermal vent sites at mid-ocean ridge and back-arc sea floor spreading centres, seamounts associated with these spreading centres, and intra-plate seamounts, and can cover extensive areas of the seafloor. These deposits can be largely attributed to inorganic precipitation of hydrothermal or hydrogenous components from seawater, but they also contain micron-scale filamentous textures reflecting biogenic processes. Some filaments are cylindrical casts of Fe oxyhydroxide phases that formed around bacterial cells and are thus unquestionably biogenic. The filaments have distinctive morphologies that are very similar to the structures formed by modern neutrophilic Fe oxidizing bacteria. It is becoming increasingly apparent that Fe oxidizing bacteria play a significant role in the formation of Fe oxide deposits near marine hydrothermal vents and at least one species has been identified by molecular genetic studies. The presence of Fe oxide filaments in Fe oxide sediments is thus of great potential as a biomarker for Fe oxidizing bacteria in modern and ancient seafloor hydrothermal deposits. The ancient analogues of modern deep-sea hydrothermal Fe oxide deposits are iron formation and jasper (hematitic chert). A number of jaspers, ranging in age from late Palaeoproterozoic to late Eocene, contain abundant hematitic filamentous textures with a wide variety of morphologies. Some of these filaments are like the structures formed by modern Fe oxidizing bacteria and thus give us direct evidence for bacteriogenic Fe oxide precipitation at marine hydrothermal vent sites for at least the last 1.7 Gyr. The late Palaeoproterozoic occurrence is of particular interest, because in the "Canfield Ocean" model for the Palaeoproterozoic the deep oceans were sulphidic and hence could not have supported communities of microaerophilic Fe oxidizing bacteria at deep-sea vent sites. Our evidence suggests that the Canfield model needs to be reconsidered.