ALBERT

Beschreibung: The environmental conditions and processes that result in the origin and growth of marine manganese nodules have been of interest to both science and industry for more than 100 years, yet they are still poorly understood. The best way to investigate the significant internal features of whole nodules is to study their polished cross sections by the methods of ore microscopy. In this report, basic principles of ore microscopy are reviewed and methods of making good sections from difficult materials are described. The use of "map" photographs and mineral identification by X-ray analysis of microsamples are recommended.

Beschreibung: Recent study of many box cores and deep sea photographs from DOMES Sites A, B, and C throws new light on the mode or occurrence of manganese nodules in the Horn Region o f the Pacific. Most bos cores from which the supernatant water has been carefully removed show nodules resting on a relatively firm mud surface, and deep imbedding is rare except for very large nodules; yet many bottom photographs from sites nearby show nodules well-imbedded in what appears to be soft muddy sediment. Observations of nodules still covered by sea water in box cores and experiments designed to preserve the natural relationships between nodules and sediment may explain this paradox. We conclude that at least the upper part of the original sediment, which we call the sediment-water interface "boundary layer", collapses when de-watered to form a wet mud only a fraction the thickness of the material on the sea bed. The relatively firm mud upon which nodules rest in many box cores is thought to underlie the boundary layer at a fairly abrupt contact, but the boundary layer proper probably grades upward, becoming progressively more water-rich, to a diffuse contact with sea water. Small nodules tend to be suspended in the boundary layer and grow largely or totally surrounded by it. As nodules grow, they may reach a size which permits anchoring in the firmer mud below by growth o f oxide dendrites downward, with the result that morphology becomes more asymmetrical as overall growth proceeds. Nodules grow by adding the crystalline manganese oxides todorokite and birnessite where they are in contact with sediment or sediment rich water and iron-rich amorphous oxides where they are in contact with the sea water above. Nodules which show alternating Mn-rich and Fe-rich internal layers may owe this pattern to long-continued sedentary growth during which periods of exposure to sea water alternated with periods during which the upper nodule surface was covered by the boundary layer. Similar nodule structures may also result from periodic overturn of nodules, and many nodules probably experience both kinds of events. The long-standing problem of concentration of manganese nodules at the present deep sea sediment-water interface may be partly explained by the unusual properties of the boundary layer. Although it appears to be both fragile and mobile, it also seems to have a cohesiveness or strength which permits it to hold nodules in suspension in spite of the relatively high density or most nodules. The presence o f organic material (mucoprotein) may contribute to the special properties of the boundary layer. There is photographic evidence that the boundary layer may hide small nodules from view in camera surveys, but the layer is so mobile that even bottom organisms can remove it. The use o f some kind of simple "cleaning" device during camera surveys may produce photographs which give a more accurate record o f nodule distribution than is presently possible.

Beschreibung: Manganese nodules collected off Baja California have layers (zones) which are texturally and compositionally distinct. Massive and mottled zones consist mostly of cryptocrystalline todorokite and birnessite containing relatively high concentrations of Mn, Ni and Cu. Compact, columnar and laminated zones consist mostly of more iron- rich opaque amorphous material. Within these two groups the individual zones can be distinguished in part by their clay content. These results indicate that systematic research on nodule chemistry, mineralogy, growth rates and beneficiation should include careful analysis of individual layers within the nodules.