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.

5. Results and Discussions

Distributions and fluxes of Mn in all three phases were calculated using the model equations and parameter values just described. Those results and their sensitivity to variations in parameter values away from central ones (Table 3) follow.

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Table 3. Central Values of Model Parameters Around Which Sensitivity Analyses Were Conducted

Value	Parameter
k = (360 days)	sorption rate of Mn onto fine particles
k = (120 days)	remobilization rate of Mn from fine particles
k = (60 days)	scavenging rate of fine by large particles
k = (6 days)	release rates of fine from large particles
w = 0.1 m/d	fine particle settling velocity
w = 100 m/d	large particle settling velocity
C = 15 µg/L	fine particulate concentration
C = 0.44 µg/L	macroaggregate concentration
E = 1.7 × 10 g/m²/d	sediment erosion rate
K = 0.8 cm²/s	vertical eddy diffusivity
K = 10 m²/s	horizontal eddy diffusivity
u = 0.2 cm/s	horizontal advection velocity westward
h = 240 m	Mn source height from seafloor
Q = 4.8 × 10 g/m/s	Mn source rate
= 0.1	benthic flux fraction

5.1. Water Column Transects

Model transects of Mn in dissolved and fine particulate phases at plume height (Figure 3, solid curve) have the general features of the observations (Figure 1). Maximum values of both model dissolved Mn (dMn = T) and total particulate Mn (pMn = T + T) are within a factor of 1.5 of the data, a result of the adjustment of the single parameter Q, the dMn source strength. Values for dMn peak on axis (x = 0, Figure 3), while those for pMn peak off axis; so do the data (Table 1). An off-axis maximum is consistent with the pMn data of Feely et al. [1992]. Both distributions decline sharply with distance off axis, though dMn declines more slowly and pMn declines more quickly than the observations suggest.

Figure 3. Model results under parameter conditions of Table 3 showing off-axis transects of (a) dissolved manganese (dMn = T) and (b) total particulate manganese (pMn = T + T) at heights of (solid) 240 m and (dashed) 120 m. (c) Model pMn flux (mg/cm/kyr) to the seafloor (solid curve) and measured weight percent of Mn in surficial sediments (squares, Table 2).

The rate of decrease of dMn off axis depends on all model variables, but dMn must be particularly sensitive to k and u, the advection speed. Accelerating the rate of conversion of dMn to fine particulate form by enlarging k could make model and measured dMn distributions (Figures 1a and 3a) more alike. This was not done because the value of k would no longer as well match the field measurements [Cowen and Li, 1991] and because Figure 1a represents just two samples of water column distributions that are likely to be highly variable about a long-term mean. Since the model is meant to address longer-term mean conditions, matching Mn distributions in sediments was given priority instead.

Transects at a height of 120 m (dashed curve, Figure 3) have a very much different appearance. Distributions are broader and maxima are farther off axis. The comparison of these with those at 240 m show the importance to the interpretation of data of securing samples at a fixed plume horizon. All model transects show upstream concentrations caused by horizontal diffusion, the model surrogate for tidal dispersion. The off-axis pMn distributions and the location of the maxima reflect, first, the slow uptake of dMn by capsuled bacteria and, past the distribution maxima, the declining availability of dMn for scavenging.

5.2. Regional Water-Column Distributions

A larger perspective on Mn cycling near the ridge is seen in the contoured fields for each Mn phase (Figure 4). The dMn concentrations of 2 nmol/L, several time background levels in the North Pacific (0.2-0.5 nmol/L [e.g., Martin and Knauer, 1982, 1985; Landing and Bruland, 1987], extends more than 200 km downstream. Overall the dMn plume is reminiscent of the He plume measured at the EPR [Lupton and Craig, 1981], but dMn, unlike He, is a nonconservative tracer and quantitative comparisons must be made cautiously. Unlike the He plume, the dMn plume shows slightly increasing concentrations at the seafloor, the result of remobilization of Mn in sediments and diffusion back into the water column. This feature of the distribution could be mistakenly interpreted as the consequence of a diffuse off-axis hydrothermal source, when it is the result of Mn deposition patterns and Mn diagenesis in the sediments. Diffuse off-axis hydrothermal sources of Mn cannot be ruled out, however.

Figure 4. Off-axis concentration distributions (T, T, T) of the three water column phases of hydrothermal Mn (nmol/L).

The pMn plume (Figure 4b) shows the effects of particle settling, particle resuspension at the seafloor, and the exchange of pMn with the other water column Mn reservoirs. Concentrations of 0.4 nmol/L extend to nearly 200 km in an ocean where background pMn values are 0.1-0.3 nmol/L [e.g., Martin and Knauer, 1982, 1985; Landing and Bruland, 1987]. The axis of the pMn plume slopes downward, the consequence primarily of large particle settling. Resuspension of particles from the seafloor raise concentrations between the seafloor and plume axis.

The distribution of macroaggregate Mn is, perhaps, the most interesting (Figure 4c). Concentration has a maximum value ~0.05 nmol/L in a region a few tens of kilometers off axis. Since Mn flux to the sediments is primarily in macroaggregate form, and that flux has a magnitude nearly equal to wC, Figure 4c suggests that hydrothermal Mn will be deposited close to the ridge, but deposition will be small on axis. The exact location of the off-axis maximum will depend on the values given model parameters, but a reasonable range enforces near-ridge deposition of Mn. The pattern of deposition is a consequence of the Mn having to pass through two phases before reaching the sediments, and the time scales of those phase conversions of Mn in comparison to the time scale of an off-axis distance divided by the advection velocity.

When sectioned vertically, as if taking a vertical cast, the profiles of dMn and pMn both show maxima at plume height and secondary maxima at the seafloor, just as do the observations [e.g., Cowen et al., 1986]. The vertical profiles of Mn in macroaggregate form have a much different shape. Vertical gradients are generally much slighter and so concentrations are likely to be more vertically uniform below plume height. If a vertical array of sediment traps could be moored below the plume, this model suggests that the hydrothermal flux of Mn into the traps should be nearly uniform. The prediction would be unwarranted very near the seafloor, however, because the model does not presently recognize the increased levels of turbulence in the benthic boundary layer. That increased vertical mixing should result in more near-bottom resuspension of particles, a process which is known to strongly effect near-bottom particle fluxes into traps [e.g., Walsh et al., 1988]. An actual flux measurement beneath a hydrothermal plume is made extremely difficult by the wafting three-dimensional nature of any real plume.

The weight fraction of Mn on particles, calculated from the quotient of the Mn concentration and the particle concentration, is distributed, for fine particles and macroaggregates, in the manner of Figures 4b and 4c. The reason is that model particle concentrations are taken to be spatially uniform, and so the quotient of T by C will have the off-axis distributional shape of T, where i = p or s. Weight fractions have maximum values of ~1.2% Mn for fine particles and ~0.6% Mn for macroaggregates, both occurring off axis. West of the ridge and 300 km downstream the particles bear only 0.1% Mn by weight. Near-axis data of Dymond and Roth [1988] on the Endeavor Ridge, for a sediment trap at a depth corresponding to the upper part of the plume (1950 m) but presumably away from the effects of resuspension, suggest Mn weight percent on trapped particles of 0.1%. A more easily sampled trap of particles and hydrothermal Mn is the sediment itself, the model results for which we now examine.

5.3. Vertical Mn Fluxes and Concentrations in Sediments

Fluxes into model "sediment traps" are given by the expression w T + wT. Since the first of these terms is very much larger than the second, the principal contribution to Mn flux is from macroaggregates. A horizontal transect of model Mn flux evaluated at the seafloor shows that the flux has a maximum value of -10 mg/cm/kyr, occurring approximately 25 km off axis while on axis and far downstream the flux is more nearly 1 mg/cm/kyr. The distribution resembles in shape that shown in Figure 3c, the model Mn flux to sediments (equation (11)). Those differ from Mn fluxes into sediment traps by the amounts of Mn remobilized in the sediments and diffused back into the water column as dMn and by the amount of Mn resuspended in particulate form. These rates are less, but for this model only slightly, than Mn vertical fluxes indicated by model "traps."

Mn fluxes of these magnitudes have been measured in other hydrothermal areas using actual sediment traps. Dymond and Roth [1988], for the Endeavor Ridge and at 2 km from an active vent field, found Mn fluxes at plume depth and below of 1.5-5 mg/cm/kyr. The larger values may reflect the results of particle resuspension [e.g., Walsh et al., 1988]. Fischer [1983] reported Mn fluxes 1.7 and 0.47 mg/cm/kyr at MANOP sites M and H, respectively, the first 25 km east of the EPR and the second much farther away. Martin et al. [1985] found Mn fluxes of 0.13-0.22 mg/cm/kyr at two deep northeastern Pacific stations which we presume to be far from hydrothermal influence.

Model Mn flux rates to the sediments and the measured weight percent of Mn in surficial sediment, though different quantities, was assumed to be similarly distributed. The likeness of the shape of the two distributions in Figure 3c was actually a condition achieved by model parameter adjustment, as will be discussed later. This approach leads to a relationship between the off-axis advection speed and the macroaggregate scavenging rate constant.

The underlying assumptions here require some discussion. The weight percent of Mn in surficial sediments depends on the rates of both Mn and sediment accumulation. Mn in the surficial layer of sediments (equation (11)) is determined not only by the Mn weight percent of the depositing particles, but also on the enrichment of the surficial sediments by upward migration of Mn from deeper in the core. The evidence for this diagenetic enrichment will be given below. A second factor, that of off-axis changes in the net sediment accumulation rate caused by the dissolution of carbonates and silicates at the seafloor, also has the potential for enlarging the Mn mass fraction in accumulating sediment compared to that in settling particles.

In the absence of sufficient data to make quantitative the consequences of these several factors, we have been forced, for the present, to adopt off-axis invariance of the benthic flux fraction , the degree of Mn enrichment of the surficial sediments by diagenetic processes, and net sediment accumulation rates. Evidence bearing on the first comes from Roth and Dymond s [1989] compositional analysis of flux into sediment traps at the Juan de Fuca Ridge. They found that organic matter mass fraction was larger in traps near hydrothermal vents compared to those slightly (~2 km) away. Thus there may be an off-axis dependence to organic flux, which, in turn, could cause relative changes in the fraction () of Mn remobilized and released back into the water column from a thin surficial layer of sediments. It might also suggest, assuming uniformity over past time, that there should be off-axis differences in the degree of Mn remobilization at depth and enrichment near the surface of the sediment column. Patterns of off-axis organic material flux are presently not available.

Off-axis variations in sediment accumulation rates seem likely. Three cores (G. J. Massoth, unpublished data, 1987) taken at 15.6, 43 and 308 km west of the ridge show the weight percentage of CaCO measured at 2.5 cm depth in the cores to be 46, 10, and 0.3%, respectively. Water depths at core locations (Table 2) were 2635, 2970, and 3904 m. If carbonates were removed by dissolution in the deepest core, if they were initially as much as 46% by weight as was found in the near-axis core, and if carbonate dissolution is the only process altering sediment accumulation rate, the sediment accumulation rate at 308 km could be only one half those at 15 km off axis. Other rate altering processes, whether close to or at substantial distances from the ridge crest, require corresponding consideration. Reducing the net sedimentation rate would have the effect of increasing the weight fraction of Mn in the sediments for the same incident flux of Mn. If sediment accumulates more slowly the larger the distance off axis, the model Mn depositional flux curve (Figure 3c) will need to be made to decrease more quickly with off-axis distance than it now does so that the shapes of the Mn percent weight distributions from model and core data will agree. Decreasing rates of actual sediment accumulation off axis will cause hydrothermal Mn deposition to be more ridge oriented than even the present analysis suggests.

The difference in the magnitudes (but not shape) of the model and data Mn weight fractions is caused primarily, we believe, by diagenetic processes in the sediment column. The maximum measured weight percent of Mn in sediments (Figure 3c) is ~4.7%, but the model maximum value, taking a spatially invariant sedimentation flux of 1.6g/cm/kyr [Dymond and Roth, 1988], amounts to ~0.6%. The model particles bear only about 15% the amount of Mn as found in the surficial sediments. At depth (40 cm) in the cores taken 15.6, 30, 43, and 308 km from the ridge, the Mn weight percents are 0.2, 0.6, 0.6, and 0.1% (G. J. Massoth, unpublished data, 1987). These values are only 13-20% of those near the surface (2.5 cm) of the core. Reduction of Mn in an anoxic layer of sediments, dMn migration upward, and Mn reoxidation in the upper layer of sediments would cause such profiles. Pore water nutrient and Mn profiles from cores to the west of the ridge in Cascadia Basin [Jones and Murray, 1985] suggest reducing conditions at depth in the sediments and an oxic surface layer where dMn is rescavenged by particles.

The model benthic flux of dMn from the sediments back into the water column (equation (5)) will have a distributional shape similar to that in Figure 3c. Magnitudes are approximately 10% those of the depositional flux (Figure 3c); the maximum value of the flux is -18 nmol/cm/yr under the model conditions of Table 3. From two-point measurements at the top of MANOP Site M cores, Heggie et al. [1986] inferred benthic flux rates for Mn of 8-18 nmol/cm/yr. In the Guaymas Basin, where reducing conditions exist in sediments very near surface, Campbell et al. [1988] inferred flux rates of 130 ± 60 nmol/cm/yr using similar analyses. Since neighboring Cascadia Basin surface sediments are more oxidizing than reducing [Jones and Murray, 1985], the model benthic fluxes of dMn seem to have a reasonable magnitude range.

5.4. Source Strength

The hydrothermal Mn source flux, Q, was determined by adjustment so that dMn and pMn values near the ridge would have magnitudes similar to those measured (Figures 1a and 1b). The resulting value of Q was 4.2 × 10 g/m/s (1.4 × 10 g/km/yr). Baker and Massoth [1986] estimated the flux of Mn averaged over a 10-km-long section of the southern JDFR at 11 × 10 g/m/s, a value nearly 3 times that suggested by this model. The differences in rates could be due to the shorter averaging time represented by the Baker and Massoth [1986] analysis, or to spatial differences in hydrothermal activity along the ridge crest. At the EPR, Lyle [1976] estimated a Mn flux rate of 5 × 10 g/km/yr, approximately 4 times that suggested by this model from JDFR data. The larger weight fraction of Mn in sediments at the EPR compared to the JDFR may make the difference meaningful.

5.5. Sensitivity Analysis

The sensitivity of results to changes in a number of parameters away from the values of Table 3 was examined. First k was given a value of (1080 days), so that k/k would be 3 rather than 1/3. Mn remobilization from capsuled bacteria then occurs over a period 3 times as long as the time scale for Mn scavenging. Exchange equilibrium tilts more toward the direction of pMn. The resulting plume distributions show less dMn at distance from the ridge; higher pMn concentrations to a distance of ~150 km; higher flux of Mn to the sediments over the initial 150 km; less pMn and lower flux of Mn to the seafloor beyond 150 km; concentrations reduced for dMn and slightly elevated for pMn near the seafloor. The available data for hydrothermal Mn at the JDFR cannot be used to differentiate between the two values (0.33 and 3.0) used for k/k.

A comparison was then made of distributions when T( = 1/k) equals 30, 60, and 180 days. The smaller value of T forces more rapid fine particle scavenging by macroaggregates while larger value indicates the opposite. For T = 30 days, dMn and pMn concentrations in the water column are about half of what they are for T = 60 days, the central T value of the analysis (Table 3). Any Mn taken up by capsules is more quickly removed to the sediments. Because the rate conversion of dMn to pMn was fixed as before, however, the rate of formation of pMn is unchanged. The deposition pattern is more sharply peaked than that of Figure 3c, but the off-axis location of the maximum in the distribution is nearly the same. When T = 180 days, remobilization of Mn from capsules (T = 120 days, Table 2) plays more of a role. In this case, pMn persists in the water column much longer, dMn stays elevated to a much larger distance, and the deposition pattern is far less peaked than that of Figure 3c. In order for the shape of the model Mn deposition pattern to resemble the shape of the measured Mn weight percent in sediments (Figure 3c) for this T, the off-axis advection speed would need to be very much less than 0.2 cm/s.

That relationship between T and u was quantified by comparing the misfit of the model Mn depositional flux distribution to the Mn weight percent data for sediments under various values of the two parameters. To understand how this was done, consider the differences in model deposition patterns when parameters are fixed as in Table 3, except K = 0, and u is allowed to vary (Figure 5a). The curves of Figure 5a represent Mn fluxes to the sediments, but flux scales are not given; the left-hand scale refers to the data only. As stated earlier in regard to Figure 3c, these fluxes should have the same distributional shape as the measured weight percent of Mn in the sediments (boxes, Figure 5a). To make quantitative the differences between the modeled and measured distributions, each flux curve was first rescaled so that the maximum flux had the same ordinate value as the maximum measured weight percent. A sum-of-squared-error measure of misfit was then evaluated. Only points at off-axis distances at and beyond that corresponding to the maximum ordinate data value were included in the sum, however. Otherwise, the difference errors for points at distances less than 20 km (Figure 5a) dominate the misfit sum, while the selection of "best fit" is more fairly based on the difference errors for points beyond 20 km (Figure 5c). It is reasonably clear from inspection (Figure 5a) that a u value of 0.15 cm/s results in a flux distribution having the essential features of the measured Mn weight percent distribution in the sediments, when the other parameters are fixed as they are. The sum-of-squared-error measure encompassing all data points, on the other hand, would result in a misfit measure that could not differentiate between the quality of the fit provided by any curve in Figure 5a.

Figure 5. (a) Distributions of model Mn flux to sediments (curves) for varying off-axis advection velocities when T = 60 days. Superimposed on model distributions are the measured weight percent of Mn in surficial sediments (squares) from Table 2. Flux distributions have been individually rescaled so that maximum has the same ordinate value as the data maximum; flux scales are consequently not shown. The degree of misfit between each distribution and the measured weight percents was calculated as the sum of squared errors, as described in the text. (b) The resulting sum-of-squared-error surfaces for a range of u and T values when k/k equals 0.33 and (c) when k/k equals 3.

Values for the sum of the squared error were evaluated in this way for a range of u and T values and for two values of the k/k ratio. The resulting error values were contoured in the u, T plane (Figure 5b and 5c). For both values of k/k, the error surface shows a trough, indicating that nearly equivalent deposition distributions can be obtained for a number of u and T pairs. T and u are also anticorrelated. If T is small, i.e., rapid macroaggregate scavenging of fine particles, the observed distribution of Mn in sediments is consistent only with a larger (e.g., 0.25 cm/s) off-axis component of advection. For a smaller velocity component (e.g., 0.1 cm/s), rapid (T = 20 days) macroaggregate scavenging would cause the Mn to be deposited too close to the ridge. Extending the deposition pattern off axis to resemble the data for small u (e.g., 0.1 cm/s) requires that the macroaggregate scavenging proceed at a slower rate (e.g., T = 100 days, Figure 5b). This general picture is not much changed if k/k = 3.0 (Figure 5c). Since it seems reasonable to think that T has a time scale of the order of weeks to months, based on currently available reports, the results (Figures 5b and 5c) make it seem likely that u is in the range 0.1 to 0.3 cm/s.

5.6. Residence Times

The residence time of hydrothermal Mn in the water column has been studied with the model under the given parameter values (Table 3). The time dependent solution of equations (1)-(3), with horizontal transport terms set to zero, was examined for a unit impulse discharge of dMn at plume height. The constants u and K can be set to zero because, in calculating residence times, the horizontal location of the tracer is immaterial. Water column concentrations of dMn, pMn, macroaggregate Mn, and Mn deposited in the sediment were calculated in time over a period of 1400 days. Water column concentrations were integrated with height and all results expressed as fractions of the original amount of Mn discharged. The consequent time histories (Figure 6) show the passage of Mn through all the three water column stages and into the sediments. The term residence time can be defined in many different ways. In simple systems undergoing exponential loss, the time for the concentration to be reduced to 1/e (37%) its initial value can be called the residence time. The same definition is used here: residence time will mean that time at which only 37% of the initial amount of Mn remains in the water column.

Figure 6. Percent of initial discharge in each of four Mn reservoirs as a function of time. Parameter settings are as in Table 3, but with (a) k/k = 0.33 and (b) k/k = 3.

The time histories (Figure 6a) show the slow conversion of dMn to other Mn phases. After 100 days, more than 80% of the Mn remains in dissolved form, while fine particulate Mn accounts for only -10% of Mn stock. Because of this slow exchange, it is to be expected that near the ridge Mn will be seen primarily in dissolved form. By 700 days, dMn is reduced to 37% of the initial concentration and by 750 days, the residence time, 63% of the initial Mn discharge has been deposited in the sediments. Note, however, that even after 1400 days more than 10% of the stock remains as dMn. If the off-axis advection is 0.2 cm/s (Table 3), for example, dMn levels should exceed background dMn levels several fold even beyond 240 km from the ridge (Figure 4a).

These results are somewhat changed when the k/k is set to 3 (Figure 6b). Remobilization of Mn from capsule bacteria is less favored, so the dMn decreases more quickly. Now at 700 days, little more than 20% of the Mn stock is in dMn form. The residence time of Mn is shortened to ~500 days.

Figures 6a and 6b show too that the hydrothermal fine particulate Mn stock never exceeds more than -15% of the total hydrothermal Mn stock. Macroaggregate Mn is never more than 0.5%, yet effectively all Mn passes through this stage to be deposited in the sediments. Macroaggregate concentrations mirror the distribution of Mn flux to the sediment (Figure 4c) under exchange of time and space axes possible in the case of steady advection.

Interestingly, under these model conditions, by 1400 days (240 km when u = 0.2 cm/s) more than 80% of the Mn is deposited in the sediments. This means that much of the hydrothermal Mn is unavailable to the oceans over larger scales. Still dMn concentrations are many times background beyond that distance (Figure 4a). Under actual conditions, the fraction deposited may not be quite as high as depicted in Figure 6. Off axis, the bottom falls away from ridge crest depths and longer settling distance may have the effect of lengthening depositional time scales. On the other hand, the sediment distribution pattern of Figure 1c will still have to be accommodated, so the effect of sloping bathymetry may only just force quickened scavenging or larger settling velocities. The consequences of variable depth on the results has not yet been studied. With some of the processes underlying the model, e.g., macroaggregate scavenging and release, now so poorly understood, it does not seem warranted at this point.

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