U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

A Model for the Deposition of Hydrothermal Manganese Near Ridge Crests

J. W. Lavelle

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington

J. P. Cowen

Department of Oceanography, University of Hawaii, Honolulu, Hawaii

G. J. Massoth

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington

Journal of Geophysical Research, 97(C5), 7413-7427 (1992)
Copyright ©1992 by the American Geophysical Union. Further electronic distribution is not allowed.

4. Values Assigned Model Parameters

4.1. Exchange Rate Coefficients

The sorption and remobilization rates, k, and k, fix the exchange of Mn between dissolved and fine particulate reservoirs. Cowen and Li [1991] suggest that only a small fraction of the fine particles, specifically the metal scavenging capsuled bacteria, are responsible for the bulk of the dMn scavenging.

The rate k was measured by Cowen et al. [1990] using the radioisotope Mn and water samples taken in the vicinity of the JDFR at ridge crest depth. Uptake of dMn was seen to occur predominately with a time scale of the order of 1 year, but faster time scales were also evident depending on the population of capsuled bacteria. In fact, Cowen and Li s [1991] data suggest that rate coefficients increase with plume age and perhaps the scavenging efficiency of each capsule increases as well, so k may be other than a constant. Such age and efficiency dependence cannot be encompassed at this stage of model development. Instead k is here assumed to be constant in space and time (360 days).

No direct determinations of the remobilization rate of Mn from capsules, k, have yet been made. Consequently, Mn data from deep ocean stations removed from hydrothermal influence are here used to suggest its magnitude. If the similarity of the scavenging-remobilization process between hydrothermal and nonhydrothermal regions can be assumed, and if dMn and pMn are nearly in exchange equilibrium at stations well away from hydrothermal influence, the ratio pMn/dMn (~T/T) at those distal stations may serve to approximate the ratio k/k. T differs from pMn in that pMn includes macroaggregate Mn, but macroaggregate Mn should have little effect on the ratio k/k because of the relatively low concentration of macroaggregates compared to fine particles. Of course, strict equilibrium cannot be achieved when particles are undergoing removal to the seafloor by settling, but partitioning data presently provide the best available means of narrowing the range of possible values for k.

Only measurements taken at depths greater than 1500 m are considered here. Landing and Bruland [1987], for a station at 15N and 160W, present dMn and pMn data that lead to a average value for k/k of 0.33. Individual values ranged from 0.17 to 0.66, with the lowest and highest values occurring at 3000 m and near the seafloor, respectively. For a station "~2000 km west of central Mexico" and at 1500 m, Martin and Knauer [1982, p. 1214] report dMn and pMn concentrations that lead to a k/k average of 0.29. At 28N and 155W, Martin and Knauer s [1985] data for 2000 m suggest k/k = 0.13. Samples at 1500-1600 m depth near the JDFR show dMn ranging from 0.7 to 1.6 nmol/L and pMn ranging from 0.2 to 0.5 nmol/L [Cowen et al., 1990; Cowen and Li, 1991]. These dMn and pMn values are both slightly above the background ranges that were inferred from data at more distant stations, as earlier described, and they lead to k/k ratios ranging from 0.2 to 0.57. Thus even at these depths on the JDFR there may normally be some hydrothermal effect. Episodic influence of hydrothermal activity at 1500 m is certain [Baker et al., 1989]. In comparison, for points between 500 and 1000 m above the bottom over the mid-Atlantic ridge, a depth interval which is presumed to be out of hydrothermal influence, Klinkhammer et al. [1985] found Mn concentrations that result in an average k/k ratio of 0.26. All these k/k estimates are based, of course, on the assumption that chemical exchange equilibrium prevails.

Taken together, these estimates suggest that the time scale for remobilization of Mn from fine particles is about 1/3 the length of the time scale for dMn scavenging. On the other hand, if particles other than metal scavenging bacteria play an important role in Mn scavenging in these background regions, the k/k ratios in hydrothermal regions may be somewhat different than 1/3. In the analysis to follow, we will use separate values of 1/3 and 3 for k/k in order to examine the model dependence over a range of remobilization rates. Since k is chosen to be (360 days), k then has values of (120 days) and (1080 days), respectively, for the two stated values of the k/k ratio.

Estimates for the rate of fine particle scavenging by macroaggregates, k, are just beginning to appear in the literature. Nozaki et al. [1987] used two Th profiles from the western Pacific to infer k values of (30 days)and (154 days), though Lavelle et al. [1991] claim, by reanalyzing the same data, that k for that data is more likely (320-370 days). In a deep estuary with particle concentrations much higher than the deep ocean, k has been estimated to be (2-6 days) [Lavelle et al., 1991].

In the Panama Basin, Asper [1987] measured size, number density, and settling velocity of macroaggregates, all factors in determining the k scavenging rate. Consider a unit volume of water in which the number density of macroaggregates is N. Let the macroaggregates have a common settling velocity of w and a common cross-sectional impact diameter of d. During any time interval dt, each of N macroaggregates sweeps a volume of water equal to (/4)d wdt. If the mass concentration of fine particles in the same unit volume of water is C, and if the fine particles encountered by each macroaggregate are scavenged with a retention efficiency (0 1), then the rate of fine particle removal by macroaggregate scavenging would be


The term in brackets represents the scavenging rate k. The rate depends on d, N, and w, which Asper [1987] measured, and the scavenging efficiency .

Asper [1987] found macroaggregate size classes with diameters 2 and 3.4 mm contributed 89 % of the large particle flux at his Panama Basin site. The smaller size class (2 mm) settled at an average speed of 36 m/d and had number density of 1.4 particles per liter. The larger size class (3.4 mm) settled at an average speed of 26 m/d and had a number density of 0.58 particles per liter. If = 1, these data and equation (12) result in k values of (6 days) and (5 days), respectively. For smaller values of , k is made smaller and the scavenging time scale T( = 1/k) is lengthened proportionately. If were 0.2, a k value of approximately (30 days) would result, and if were -0.02, a value for k closer to (360 days) would result. This uncertainty in assigning a value to is exacerbated by the likely site-to-site variability of the other factors determining k. In the absence of appropriate data, no certain value for k can be assigned for the JDFR region, but it does seem reasonable to assume that T has a time scale of weeks to months. Here we have chosen to examine the consequences of k ranging from (20 days) to (180 days), using a k of (60 days) around which to center the analysis.

The final exchange rate, k, determines the time scale for release of fine particles from macroaggregates. The release process is not well defined, but k must include the effects of total or partial large-particle disaggregation or other mechanisms of fine particle loss from macroaggregates. The Western Pacific data of Nozaki et al. [1987] suggest that an appropriate time scale, T( = 1/k), is ~6 days [Lavelle et al., 1991]. Apparently, the scavenging of fine particles occurs more slowly than does fine particle release. The T adopted here is 6 days. This time is long compared to the transit time from a height of 240 m (model source height) to the seafloor if macroaggregates settle at a speed as high as 100 m/d [e.g., Alldredge and Silver, 1988]. Consequently, in this model environment, the release to the water column below plume depth of capsuled bacteria by macroaggregates can occur, but the process is not favored.

4.2. Physical Environment

Observations of currents within the central ridge valley and adjacent regions of the southern JDFR show a spatially complicated and time variable pattern of flow [Cannon et al., 1991]. Tidal, inertial, and atmospherically induced 4-day oscillations dominate the spectra of currents. Mean currents from observations exceeding 6 months in duration for locations below 2100 m (~ ridge crest depth) range from 0.3 to 1.9 cm/s, but the directions are principally northward and southward along the ridge rather than off axis. Occasional divergent off-axis subtidal flow of duration exceeding several weeks is observed.

The evidence on longer term off-axis mean flow is more circumstantial. Reid s [1981] diagrams of phosphate and silica concentrations on an isopycnal surface at approximately 2600 m depth suggest westward mean flow near the JDFR. The patterns of Mn in sediments (Figure 1c), as they do on the EPR [Klinkhammer and Hudson, 1986], suggest long-term off-axis flow as well. Stommel s [1982] model of baroclinic flow caused by hydrothermal heat flux for the EPR suggests a westward mean flow with speeds of -0.2 cm/s. J. Lupton (personal communication, 1991) suggests that a helium plume extends to the southwest of the southern JDFR. Differences in these suggestions and the results of Cannon et al. [1991] may be related to the shorter averaging time and more limited spatial scales of the current meter observations.

The one-dimensional nature and off-axis orientation of the data confine our present analysis to the component of mean flow in the off-axis direction only. Should long-term mean flow actually be directed other than westward in the southern JDFR region, any suggestions of the magnitude of off-axis advection derived here must apply only to the ridge-normal component of that flow. The Mn sediment data and limitations on scavenging time scales restrict the range of possible current speeds allowed that component, however.

Vertical eddy diffusivity, K, is given a value of 8 10 cm/s, though values range from 10 to 3-4 10 cm/s in the deep ocean away from boundaries, regions where velocity shears are low [e.g., Garrett, 1979; Gargett, 1984]. For our purpose, K is taken to be vertically uniform. At the seafloor, values for K 10 to 50 times those are more likely appropriate [e.g., Garrett, 1979], but the effects of increased benthic boundary layer turbulence and bathymetry on Mn distributions are both left for later study. Horizontal diffusivity, K, is given a value of 10 m/s [e.g., Okubo, 1971]. This diffusivity accounts for the effects on Mn distributions of unresolved currents including tidal flow, and it permits some upstream presence of Mn as indicated, for example, by observations of pMn across the width of the axial valley.

4.3. Particle Settling Velocities, Concentrations, and Fluxes

The settling velocity of fine particles, w, is set to 0.1 m/d in these model realizations. This corresponds to the Stokes settling speed of, for example, 1.9 m diameter particles with specific gravity 2.6 or 3.2 m particles with a specific gravity of 1.55. Particles in this size range have been documented in hydrothermal plumes of the southern JDFR [Walker and Baker, 1988]. Furthermore, Cowen et al. [1986] note that capsuled bacteria from the plumes have diameters of 1-2 m. The settling velocity of macroaggregates, w, is understandably very much larger. The location closest to the JDFR at which macroaggregate settling speed has been inferred is some 1300 km to the west-northwest at Ocean Station Papa [Honjo, 1985; Asper, 1986]. Those data suggest that w is as much as 175 m/d, but the temporal resolution of the data makes the confidence interval for that estimate large. In other regions the settling speed of macroaggregates are found to have considerable range [e.g., Alldredge and Silver, 1988; Alldredge and Gotschalk, 1988], with 100 m/d being considered a typical value. For purpose of our analysis, w is set to that value.

Particulate concentrations prove useful in evaluating Mn mass fractions. Concentrations out of the plume and away from the seafloor, the background concentration of particles, average 15 g/L [Cowen et al., 1986; Cowen et al., 1990]. Within the plume, concentrations are 2-3 times this [Cowen et al., 1986]. Only a small fraction of this particulate concentration is due to macroaggregates. Macroaggregate concentration can be estimated from settling flux rates and the macroaggregate settling velocity. The reason is that large particles should comprise the bulk of the vertical flux [e.g., McCave, 1975], so the settling flux is nearly equal to w C. At the Endeavor segment of the JDFR, Dymond and Roth [1988] trapped particles over a period of nearly a year; for sample points away from the effects of resuspension, those flux data average 1.6 g/cm/kyr. Duncan [1968] measured a sediment accumulation rate of 3 cm/kyr in a core west of the JDFR. This rate is comparable to that of Dymond and Roth [1988] assuming a sediment porosity of 0.8 and a sediment density of 2.6 g/cm. Dividing flux by w(100 m/d) leads to a C value of 0.44 g/L. This is less than 1/30th the concentration of fine particles, but this is only a crude estimate for C that awaits better definition by direct field measurement. Model concentrations C and C are taken to be spatially uniform.

The rate of erosion of sediment, E, was given a magnitude of 1.7 10 g/m/d, a flux rate about 4% that of the settling flux. This rate is comparable to fine sediment erosion rates in shallower environments under conditions of low bed stress [Lavelle et al., 1984]. E, along with vertical diffusivity and settling velocity, determines gradients of T and T at the seafloor (equations (9) and (10)).

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