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in water depths at or close to the CCD; in the DISCOL area this was between
3950 m and 4250 m 6 . Highest abundances were sampled at 4150 m and
only slightly lower values were found at 4000 m. However, at the shallower
depth a very tight “cobblestone” cover of small nodules (about 5 cm
diameter) was observed, while at the deeper location few but very large
nodules (about 15 centimetres in diameter) were scattered over the
sediment surface. The “cobblestone” cover resembles a hard ground
situation, providing a substrate for sessile organisms. This depth-related
trend combined with varying bathymetry results in a highly (but
systematically) variable environment on a local scale.
Conclusion: The observation of depth-related trends in manganese
nodule character and distribution provides a key to understanding a
complex pattern of marked environmental differences in the sediment
surface (“hard ground” vs. soft sediment).
2.2. Sediment composition, sedimentation rate, stratigraphy
Considerable variation of sediment composition in the Peru Basin
was observed in the topmost 30 cm. The main components, such as
carbonate, clay minerals and biogenic opal, differ laterally and vertically,
often by factors of several tens of percent. Influencing the composition, in
addition to water depth (carbonate dissolution), are sedimentation rate
(less than 0.1 - 2.3 cm/thousand years) and stratigraphy: thus, in areas of
locally high deposition (e.g. current-induced, behind obstacles) the topmost
sediment section may be of uniform composition due to its young age.
Other sites of low sedimentation showed a downcore increase in carbonate
of up to 20 percent in the upper 10 cm of the sequence (last glacial
carbonate maximum) 7 . We also encountered sites where carbonate ooze of
Tertiary age was exposed at the seafloor 8 .
Conclusion: Sediment composition is one parameter for defining the
substrate of benthic communities. Knowledge of its variations in the upper
30 cm of the sediment is also important for assessing the consequences of
mining activity (e.g., the character of the resuspended sediment plume) and
the mechanical properties of the sediment. Sediment age and
sedimentation rates provide an extremely helpful framework for
understanding observed variations in sediment composition and its
systematics (downcore zonation).
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