U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Though it is well documented that hydrothermal Mn finds its way into sediments at relatively short distances from active ridge crests [e.g., Bostrom and Peterson, 1969; Dymond, 1981], the process by which this apparent rapid deposition occurs has been rarely studied [Lyle et al., 1986]. One step in the deposition process appears to involve capsuled bacteria. Cowen et al. [1986] documented the importance of metal-depositing capsuled bacteria in the uptake of dissolved Mn in hydrothermal plumes. They found that a large fraction of the Mn in the plume could be borne on the extracellular capsules found on some bacteria. Still, the uptake of Mn by capsuled bacteria cannot provide the whole answer because capsuled bacteria are too small (1-2 µm) to settle rapidly. A second step in the transport process is needed to accelerate the transport of these particles to the seafloor. That step, we propose, involves macroaggregates, large rapidly settling particles [e.g., Fowler and Knauer, 1986; Alldredge and Silver, 1988]. Large particles have been recognized for some time to dominate the vertical flux of particles in the ocean [e.g., McCave, 1975], and they have been implicated in the vertical transport of fine particles and particle-borne chemical constituents [e.g., Bishop et al., 1977; Honjo, 1980; Bacon et al., 1985; Fowler and Knauer, 1986; Nozaki et al., 1987]. Deep ocean transit of material from surface to seafloor can occur in the matter of weeks as a result [e.g., Deuser et al., 1981]. Consequently, it seems reasonable to suggest that macroaggregates scavenge Mn-bearing capsuled bacteria from hydrothermal plumes and deposit them on the seafloor. In this paper a model of the deposition process for hydrothermal Mn is developed that incorporates this double scavenging scheme. The consequences of the model for Mn water column distributions, fluxes, and deposition patterns are explored, and results are compared with Mn distributions measured on transects running normal to the southern Juan de Fuca Ridge at and below hydrothermal plume depth.
Return to Abstract or go to next section