The Nature of Sulfide Weathering in the Submarine Environment: Evidence From the Southern Explorer Ridge.
Abstract
Extinct hydrothermal chimneys discovered in July, 2002, during a NOAA expedition to Southern Explorer Ridge in the Northeast Pacific Ocean, provide new insights into submarine sulfide weathering. The chimneys, which occur largely in an inactive vent field referred to as Ochre Gardens (extinct chimneys were also sampled in the Magic Mountain area), range up to 5m in height, and are distinguished by a thick (up to 1 to 2cm), black coating of Mn-oxides. Some of the taller chimneys still provide habitats for a variety of fauna, and, in one case, mats of filamentous bacteria were observed. Despite similar external appearance, the chimneys vary enormously in their internal composition. At one extreme, the interiors of the chimneys consist entirely of a yellow ochrous powder composed dominantly of ferric hydroxides. In the other compositional extreme, the Mn-coating passes sequentially into a thin (less than 5mm) zone of ferric hydroxide, a 5mm-thick sulfate zone (barite and anhydrite), and a highly porous pyritic core (up to 1m in diameter) containing variable sphalerite and minor disseminated chalcopyrite and barite. In the latter chimneys, pipe-like hydrothermal conduits are evident in the pyritic core by an inward zonation from chalcopyrite to barite and sphalerite, and in some cases to anhydrite. Some of the anhydrite-cored conduits are incompletely filled, and the anhydrite coated by a thin film of filamentous silica. Ferric hydroxide also commonly coats the anhydrite, or where anhydrite is not present, the sphalerite. In one sample, goethite crystals grown on sphalerite are terminated by semi-spherical (100 micron diameter), cauliflower-like structures. Between the two compositional extremes are chimneys in which the pyritic core has been extensively dissolved and disaggregated to form a pyritic sand. We propose that the extinct chimneys described here reflect varying degrees of weathering of pyritic sulfides by seawater, and that this weathering proceeds largely from the inside out due to prior formation of an impervious Mn-coating. The weathering is facilitated by former hydrothermal conduits in the chimneys that remain open to the surface or are reopened due to dissolution of anhydrite by cool hydrothermal water or seawater. These conduits expose the pyritic cores of the chimneys to seawater, which causes them to dissolve and disaggregate, thereby increasing surface area. This in turn accelerates the subsequent seawater oxidation of the sulfides to ferric hydroxides. The formation of goethite crystals with unusual structures, coatings of filamentous silica and the presence of bacterial mats on extinct chimneys suggest further that bacterial activity may also play a role in sulfide weathering.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T11C1269L
- Keywords:
-
- 1045 Low-temperature geochemistry;
- 1050 Marine geochemistry (4835;
- 4850);
- 3035 Midocean ridge processes;
- 4805 Biogeochemical cycles (1615);
- 4885 Weathering