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Observations of hydrothermally derived Mn in the water column and in sediments near ridge crests are numerous [see Rona, 1987]. Mn plumes extending many kilometers from ridge crests have been observed in the region of the East Pacific Rise [e.g., Klinkhammer and Hudson, 1986] and on the Juan de Fuca Ridge [e.g., Massoth et al., 1984; Cowen et al., 1986]. Plume dMn measured at high spatial resolution over the axial valley of the Juan de Fuca Ridge have recently been reported by Coale et al. .
Off-axis distributions of dissolved (dMn) and particulate (pMn) manganese away from the southern Juan de Fuca Ridge (JDFR) determined from water samples acquired in 1985 and 1986 are shown in Figures 1a and 1b. Here dMn is defined operationally and refers to Mn passing through a 0.2-Ám filter; Mn in colloidal form [e.g., Honeyman and Santschi, 1989] is included in this fraction. Particulate Mn is all Mn, as determined by X ray spectrometry [e.g., Feely et al., 1991], associated with particles retained by a 0.4-Ám filter; this particulate fraction is expected to be composed primarily of fine, slowly settling particles [McCave, 1975]. Samples were taken on a transect running westward and normal to the Cleft Segment of the JDFR at Vent 3 [Baker and Massoth, 1987] to a distance of 308 km (Table 1). Sample bottles were tripped at the depth of an isopycnal surface, as determined in real time by conductivity-temperature-depth measurements, corresponding to the density over the ridge at midplume depth.
|Station||Distance, km||Depth, m||dMn, nmol/L||pMn, nmol/L|
|Sample depths and distances from the ridge are indicated. Samples were taken along an isopycnal surface corresponding to the density over the ridge at plume depth.|
Figure 1. (a) Dissolved and (b) total particulate Mn as a function of distance normal to the Cleft segment of the JDFR ridge as measured (Table 1) in 1985 (solid curve) and 1986 (dashed curve). (c) Total particulate Mn as weight percent in surficial sediments (Table 2).
Concentrations of dMn near the ridge, 43-77 nmol/L (Table 1), are many times those of deep ocean background values in the North Pacific. Background dMn values range from 0.2-0.5 nmol/L based on deep (>2000 m) samples at stations away from continental margins taken by Martin and Knauer [1982, 1985] and Landing and Bruland . For pMn, reported background values range from 0.07-0.3 nmol/L, based on the same data sets plus measurements by Feely et al.  at 50░8N and 140░20W. On our off-axis transects (Figure 1b), pMn values exceed background values by a factor of 6 or more over the first 150 km away from the ridge (Table 1).
Both transects (Figures 1a and 1b) show Mn declining off axis, but dMn concentrations change more rapidly. At a distance of 43 km, dMn concentrations are only 1.6 nmol/L, a factor of 30 lower than the on-axis mean value. The pMn concentrations show evidence of having an off-axis maximum, a feature consistent with the pMn data of Feely et al. . While some differences in 1985 and 1986 data sets exist, the consistency of the basic patterns between years might suggest some stability in the discharge and dispersion processes. It would be incorrect, however, to assume that any of these transects represent data obtained along the centerline of an ideally shaped plume. Flow direction and speed are too variable [Cannon et al., 1991]. The transects more likely represent a sampling of recent and relict plume effluent, the boundaries of which are difficult to distinguish.
Water column Mn data (Figures 1a and 1b) likely reflect the results of several years of hydrothermal discharge as well as the effects of horizontal and vertical dispersion. The Mn in sediments flanking ridge crests, on the other hand, record the history of hydrothermal discharge and dispersion over much longer periods of time. Diagenetic processes must be added to the list of factors that influence off-axis distributions of Mn in sediments.
Sediment cores along a second transect normal to the southern JDFR (locations in Table 2) provide data on surficial sediment pMn (Figure 1c). Thin scrapings of the surface layer from box cores were taken for Mn analysis. Slight variations in this methodology, when they occurred, are noted in Table 2. Samples were analyzed by X ray spectrometry, as for water column pMn samples [Feely et al., 1991]. Results are expressed as weight percent of Mn in sediments.
The resulting distribution of pMn in surficial sediments (Figure 1c) shows a maximum concentration of 4.7% by weight, occurring off axis at a distance of 22-28 km (Table 2). Nearer the ridge the weight fractions plunge toward 1%. A more gradual decline occurs at greater distances from the ridge, with the weight fraction reaching an average value of 0.38% at 308 km. In comparison, Jones and Murray  found 3-4 % by weight of leachable Mn in the top 3 cm of sediment for cores located within ~30-40 km eastward of the northern JDFR. Data similar to that of Figure 1c to the east of the ridge (not shown) suggest some depositional symmetry about the ridge axis.
|Station||Ridge, km||Latitude/Longitude||Depth, m||% by weight|
|KC = Kasten core, SB = Soutar box core.|
|* Interval sampled 0-0.5 cm.|
|+ Interval sampled 0-1.0 cm.|
|++ Sampled at 2.5 cm from surface.|
The off-axis maximum in pMn weight percent in sediments is a feature observed at the East Pacific Rise (EPR) as well [Lyle, 1976]. Those data do not highly resolve at what distance the off-axis maximum occurs, but it is within 125 km of the ridge crest. Like our data (Table 2, Figure 1c), the pMn in sediments at the EPR decrease with distance off axis beyond the maximum in exponentiallike fashion. Weight fractions of Mn in sediments are much higher in that region, however. Dymond  indicates Mn concentrations in surficial sediments higher than 10% by weight. In both regions the weight fractions reflect the Mn depositional flux and the results of diagenetic remobilization of Mn, upward migration of dMn, and the rescavenging of dMn by particulates in the oxic surface layer of the sediment column.
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