Deep-Sea Research, Vol. 32, No. 10, pp. 1281 to 1285, 1985. 0198--0149/85 $3.1Ji) + ().IX) Printed in Great Britain. © 1985 Pergamon Press Lld.

How bioturbation supports manganese nodules at the sediment-water interface

BRIAN SANDERSON*

(Received 8 November 1983; ill revised form 18 March 1984; acc~Tted 25 March 1984)

AbstractnTheoretical considerations suggest that oceanic manganese nodules arc supported at the sediment-water interface by bioturbation. Bioturbation can cause a horizontal Stokes transport from regions of high biological activity to regions of low biological activity. Less sediment motion might be expected beneath a nodule than in the surrounding sediment because a nodule is a barrier to the foraging activities of macrobenthic organisms and also because there is probably less food directly beneath the nodule than in the surrounding sediment. Sediment velocities, calculated from particle mixing coefficients of pelagic sediment, appear to be sufficient to prevent nodules from being buried.

I N T R O D U C T I O N

THE RATE at which oceanic manganese nodules grow in radius is typically O( 10 -3) times the rate of accumulation of associated sediment. Various processes have been proposed previously to explain why oceanic manganese nodules remain at the sediment-water interface. GLASaY (1977) suggested occasional overturning of nodules by bottom currents; however, nodules are not observed to be overturned at sufficiently frequent intervals (PIPER and FOWLER, 1980) for this mechanism to be generally valid.

PIPER and FOWLER (1980) proposed a process called wedging. This consists of an organism forcing a tunnel beneath the nodule as in Fig. la. The tunnel is formed by displacing material in situ (not by ejecting it from the tunnels mouth) and therefore lifts the nodule. If the tunnel is later filled in with sediment, of which some fraction comes from the surface, then the nodule will be held in its elevated position. However their theory, as stated, has one particular flaw. MENARD ( 1 9 7 6 ) observed that organisms can burrow under the nodule and deposit castings to one side. This would cause the nodule to subside. Clearly the net effects of all types of bioturbation should be considered in order to find how bioturbation can support nodules at the sediments surface.

The proposed alternative hypothesis retains the idea that organisms nudge or wedge under nodules thereby providing a lifting force. This must be the case for nodules at the surface of the sediment because benthic organisms can only collide with either the bottom or sides of such nodules. However there must also be material transported to the area beneath the nodule if it is to remain lifted. Assuming that bioturbation is not homogeneous but is less intense beneath the nodule than in the surrounding sediment then there will be a

* Department of Oceanography, The University of British Columbia, Vancouver. B.C., Canada V6T IW5. Present address: Department of Physics, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A 1B 3X7.

1281

1282 B. SANOI':RSON

A B

Fig. 1. Schematic of a one-dimensional field of turbulent motion. The amplitude of thc turbulent velocity is given by the length of the double-headed arrows.

mean Stokes transport of material to the area beneath the nodule. This mechanism requires no mean sediment velocity in the Eulerian sense. Further it is not necessary to consider particular types of organism activity since the one theory applies to the total field of sediment motion caused by all forms of bioturbation.

To physically illustrate how gradients in the magnitude of random movements can transport material, consider the one-dimensional field of motion illustrated in Fig. 1. The double-headed arrows indicate the mean amplitude of turbulent motion as a function of position. At any point the velocity is equally likely to be in either direction and its magnitude is independent of direction. Thus there is no mean velocity in the Eulerian sense. However, a particle originally at point A will be moved away from A more quickly than a particle originally at B will be moved from B. Considering many such particles it is clear that in the Lagrangian sense there will be a mean convergence of material towards B. Since there is no mean Eulerian motion this is a type of Stokes transport.

S T O K E S T R A N S P O R T IN AN I N H O M O G E N E O U S T U R B U L E N T

F I E L D OF M O T I O N

The mass transport past any fixed point does not depend solely on the mean velocity measured at that point, but depends also on other properties of the field of motion. For example, in the theory of surface gravity waves, the difference between the mean velocity at a given point (Eulerian mean velocity) and the mean velocity of a marked particle (known as the Lagrangian velocity) has long been recognized and referred to as the Stokes velocity, Us. LONGUET-HIGGINS (1969) gives a general discussion of the relation between Lagrangian and Eulerian mean velocities that is applicable in situations where the magnitude of particle motion over one period is small compared to the scale of gradients in the velocity field. This theory is inadequate for fields of motion caused by bioturbation, because such fields of motion will have a turbulent nature and in turbulent fields the scale of the particle displacement and that of gradients will be similar.

To calculate Stokes velocities in a turbulent field of motion it will, therefore, be necessary to resort to a numerical simulation. Consider 100 bins standing side by side along a straight line. In each bin we deposit a very large number of particles so that the bins will not be emptied. Now let the top particle in a randomly chosen bin move a random distance to its left or right. Further let the magnitude of the random motion be modulated by a Gaussian function (Fig. 2A) so that particles moving from the center bin will generally move half as far as particles moving from bins near either end of the line. Moving only the top particle in a bin is equivalent to letting the mixed layer of the sediment be uniformly one particle deep. Reflecting conditions are imposed at each end of the line so that the total number of particles is conserved. If we consider many such modulated random movements of particles from randomly chosen boxes then this might reasonably model sediment motion in a turbulent flow that has no mean Eulerian motion but has a spacial gradient in the amplitude of turbulent motion. After 3 x 10 6 random movements then the distribu- tion of particles (relative to the initially uniform distribution) is as shown in Fig. 2B.

How bioturbation supports manganese nodules at the sediment-watcr interface 1283

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10 20 30 40 50 60 70 80 90 1 O0

BIN NUMBER

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10 20 30 40 50 60 70 80 90 100

BIN NUMBER

Fig. 2. (A) Shows the mean amplitude of turbulent displacements, d r (in units of the number of bins moved) plotted against bin number. (B) Shows the total transport of particles, N, into each bin

after 3 x l0 t' particle movements.

Clearly there is a Stokes velocity down the gradient in the turbulent velocity. The Stokes velocity increases as the gradient increases, thus we observe an increased number of particles where the Stokes velocity converges and a decreased number of particles where the Stokes velocity diverges.

Considering the transport of particles evident in Fig. 2B it is clear that in the central region the Stokes velocity is ---0.5 times the average magnitude of the turbulent velocity for the whole field. If a nodule were sitting in the center of Fig. 2B then it would be raised.

There may be many reasons for nonhomogeneity in the field of sediment motion in the vicinity of a nodule; the following are two examples. First, a nodule acts as a barrier to organism movement. If organisms move randomly in the horizontal directions then any preference, of the organisms, to be near a water-sediment interface would result in the organisms being found less frequently (and therefore causing less sediment motion) beneath the nodule than in the surrounding area. Surface deposit feeders would therefore be less likely to be found under the nodule than in the surrounding sediment. Second, in the deep-sea benthic community most of the food falls from above as particulate organic carbon. A nodule will therefore act as an umbrella to this rain of food from above. It follows that there will be less food and therefore less bioturbation beneath the nodule than in the surrounding sediment. Biological patch structure has been observed in the abyssal benthic community at scales ranging from kilometers to centimeters (BERNSTEIN et al., 1978). It would seem relatively straightforward, with a large ensemble of samples, to see if macrobenthic organisms occurred less frequently under nodules than in the surrounding sediment.

1284 B. SANDERSON

Let us now estimate how large the Stokes transport has to be to prevent the nodule from being buried. Consider a nodule with a circular profile of radius, R, when viewed from above. The volume of material transported underneath the nodule per unit time by the Stokes velocity is

2rtR [ U~-(R, z) I dz. 0

Assuming uniform bioturbation to a depth, D, below which there is no bioturbation, we see that the rate at which the nodule is lifted is

2D[ Us(R)] R

If the rate at which sediment accumulates on the surface of the ocean floor is H then if the nodule is not to be buried,

R H I U,(R)I ~> 2 D "

If D is typically 7 x 10 -: m and H is typically 4 × 10 -(' m y-J, then for a nodule of 5 × 10 -2 m radius to be maintained at the sediment surface the magnitude of the Stokes velocity must be at least 1.4 x 10 -~ m y-t.

The mean magnitude of sediment velocity can be estimated from particle reworking coefficients assuming that they can be defined as a product of a mixing length and the magnitude of the turbulent velocity. Such a definition is a direct analogy of that proposed by OBUKHOV (1941) and used by OKtJaO and EBBESMEYER (1976) for ocean eddy diffusivi- ties. Typically, the eddy reworking coefficient, K, is 2 × 10 -5 m 2 y-i for pelagic sediments (depths >4000 m) and the depth of the mixed sediment layer is 7 x 10 -: m (ALLER, 1982). Assuming that the magnitude of the horizontal component of turbulent sediment motion is the same as the vertical component then the resulting magnitude of horizontal turbulent velocities, u' , is K I D ~- 2.9 × 10 -4 m y-L. This is large enough to result in the above estimate for the Stokes velocity, even if the horizontal gradient in biological activity near the nodule is very small.

The scales over which coherent sediment motions take place are also of critical importance. It would seem that if the scale of turbulent movements is much greater than the horizontal dimensions of the nodule then the associated gradient in the amplitude of turbulent velocity would be reduced since the nodule may not represent a significant obstruction to biological activity of such large scales. It follows that if there is a lower limit for the scale of turbulent displacements then by the previous arguments there should also be a similar nlinimum size for particles that can act as seeds upon which manganese can accumulate. HEATH (1982) shows that the rate of burial is constant for nodules with diameters ranging from 5 × 10 -3 m to 4 × 10- 2 m, which indicates that the sediment motion has energy distributed fairly evenly over these scales.

The existence of a Stokes transport depends critically upon the assumption that there is a gradient in the sediment motion. Such a gradient might be measureable by monitoring the rate of growth of horizontal variance of two ensembles of vertical line source patches of tracer in the sediment. If the patches belonging to one ensemble were beneath nodules and those of the other ensemble were away from nodules then the differing growth rates of variance will be attributable to spatial inhomogeneity of the velocity field. The time scale

How bioturbation supports manganese nodules at the sediment-water interface 1285

for the duration of the experiment would be of the order of L/I u' [ ~ 100 y, where L ~ 3 x 10 -2 m is the length scale for u'. Horizontal gradients of radio isotopes near nodules might also be used to infer horizontal variations in the sediment motion, although this would again require a very large ensemble of samples to smooth out temporal variability.

Sometimes manganese nodules are observed to have a patchy distribution on the ocean floor. This can be accounted for in terms of a Stokes transport. A group of nodules is expected to have less biological stirring of the sediment between the nodules than external to the group. This results in a gradient in the magnitude of the sediment velocities and causes horizontal convergence of the nodules due to Stokes transport. Essentially, each manganese nodule is at the center of a horizontal convergence zone and, when two such convergence zones get close enough to interact, then the nodules move towards each other.

Figure 2B indicates that there might be a trench surrounding the nodule. If the bottom currents are laminar then the sediment should be preferentially deposited in this trench, so it may be difficult to observe.

C O N C L U S I O N

The present paper suggests that convergence of sediment beneath nodules prevents them from being buried. This convergence results from a Stokes transport in an essentially random horizontal velocity field with a mean horizontal gradient of velocity amplitude so that generally smaller sediment velocities occur under the nodule. Such gradients of velocity might be caused by the nodules obstructing the foraging activity of benthic organisms or by the smaller amounts of food expected beneath the nodule.

AcknowledgementsmI thank Dr Akira Okubo for his penetrating review of an earlier version of this paper. He found errors that I had made in calculating the Stokes velocity,

I wish to thank Trish McKeen for first approaching me with this problem. Furthermore I am indebted to Dr P. Jumars (University of Washington), DrT. Pedersen, Dr S. Calvert, Dr P. LeBIond, Dr A. Lewis, Dr J. Parslow, Dr. T. Parsons, K. Thompson (all of the University of British Columbia), and Deborah Bray for helpful discussion and encouragement.

R E F E R E N C E S

ALLER R. C. (1982) The effects of Macrobenthos on chemical properties of marine sediment and overlying water. In: Animal sediment relations, P. L. McCALL and M. J. Z. TEVESZ, editors, Plenum Press, New York, pp. 53-102.

BERNSTEIN B. B., R. R. HESSLER, R. SMITH and P. A. JUMARS (1978) Spatial dispersion of benthic Foraminifera in the abyssal Central Pacific. Limnoiogy and Oceanography, 2,3,401--416.

GLASBY G. P. (1977) Why manganese nodules remain at the sediment-water interface. New Zealand Journal of Science, 20, 187-190.

LONGUET-HIGGINS M. S. (1969) On the transport of mass by time varying ocean currents. Deep-Sea Research, 16, 431-447.

MENARO H. W. (1976) Time, chance, and the origin of manganese nodules. American Scientist, 64, 519--529. OBUKHOV A. M. (1941) Energy distribution in the spectrum of turbulent flow. izvestia Akademii nauk SSSR,

Geography and Geophysics series, Nos 4-5, pp. 453--466. OKUBO A. and C. C. EBBESMEYER (1976) Determination of vorticity, divergence and deformation rates from

analysis of drogue observations. Deep-Sea Research, 23,453--466. PtPER D. Z. and B. FOWLER (1980) New constraint on the maintenance of Mn nodules at the sediment surface.

Nature, London, 286, 880-883.