Deutsche Hydrographische Zeitschrift - German Journal of Hydrography 
152 
This corresponds to a maximum coverage of 572 
and 344 nodules per m 2 . Von Stackelberg [1997] 
reports a maximum coverage of 910 nodules per m 2 
close to the SEDIPERU site. 
Particle diameters in the upper 1 cm of the se 
diment vary between <20 and about 200 pm at both 
sites (nodule covered areas). The upper 8 to 10 cm 
of very soft sediment are called ‘semi-liquid layer’. 
The bottom boundary of the Peru Basin is a low- 
energetic boundary layer, i. e. there is no erosion 
due to natural bottom currents (Jankowski and 
Zielke [1995]). This is confirmed by geological and 
soil-mechanical investigations at both sites which 
gave no hints of erosional events. 
Near-bottom currents 
a) Long-term mooring D1 
At the DEA site, the mean velocity magnitude 
ranges between 2 and 4 cm/s with a clear increase 
towards the bottom. Rotor stalls were about 10% at 
15 mab, and about 20% between 30 and 200 mab 
(see Table 1). Average stall duration is 1 to 2 cycles 
(3 to 6 hours). The maximum stall interval was 
21 cycles (63 hours) at 200 mab. Between 15 and 
50 mab the flow direction is northwest, at 200 mab 
the mean current runs westward. 
The progressive vector diagram (PVD) in Fi 
gure 4 shows that the deep flow is characterized by 
2 to 5-month periods of relatively strong currents 
(>5 cm/s) with only few changes in the direction of 
flow. During these events, the speed significantly 
exceeds the total mean of 3 to 4 cm/s. For short pe 
riods of a few hours, current speed exceeded 
10 cm/s and even reached values up to 17 cm/s. 
Such phases of strong flow alternate with slow cur 
rent phases (<1-3 cm/s) of comparable duration but 
a great directional variability. A periodicity or sea 
sonal signal, however, is not discernible. 
Basic flow statistics, calculated from unfiltered 
data, are summarized in Table 2. For rotor stalls the 
D1 
west - east [km] 
Fig. 4: Progressive vector diagram of long-term moo 
ring D1. Time marks (x) are given every 30 days, 
the numbers at the tracks give the distance from 
bottom in metres. 
speed was asumed to equal zero. The small stabi 
lity factor SF 5) emphasizes the great variability of 
the local deep flow. This also holds true if diurnal 
and semi-diurnal tides are removed by means of a 
Gaussian low-pass filter. Bottom intensification of 
the mean kinetic energy k M 6) and (turbulent) eddy ki 
netic energy k E , as frequently seen in the deep-sea, 
occurred in the bottom 30 metres. Here, turbulent 
kinetic energy is produced by vertical shear of the 
mean flow (zero velocity at the bottom!). k E gene 
rally exceeds k M . 
The energy density spectra of the zonal and 
meridional velocity components show three typical 
periods of current variability (Klein [1993]): The 
inertial period (4.1 days), the diurnal tide (24.4 h), 
and the semi-diurnal tide (12 h). However, the tidal 
currents are very weak, i.e. they are smaller than 
the threshold velocity of the current meters, thus 
contributing to the high percentage of rotor stalls 
(see Table 2). 
51 SF= (vector mean/magnitude) x 100. Sfi=100 stands for a constant current direction. 
61 k M = 0.5 (U-+V 2 ). k R = 0.5 (ii'u'+v'v')