ALBERT

Description: 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.

Description: Methods of estimating manganese nodule grade and concentration were investigated using data from a well-explored east-central Pacific manganese nodule deposit. Bulk chemical analyses of 159 nodules recovered from 21 box cores show that the range in metal values between nodules from a single box core is commonly small but may be greater than the range in mean metal content of nodules from widely separated box cores. The metal exhibiting the greatest variability in the 21 box cores is Zn, followed in decreasing order by Cu, Mn, Co, Ni, and Fe. Approximately half of the box cores required analysis of 11 nodules or more to predict metal content within plus or minus 10 percent of the mean value.

Description: Study of the occurrence and physical and chemical character of manganese nodules collected in box cores in DOMES Sites A, B, and C indicates that virtually all nodules lie at the sediment-water interface. Buried nodules are rare. Nodules seem to be smaller and have smoother surfaces at Site A than at Sites B and C. Site C nodules are predominantly 2-6 cm in size and have smooth tops and granular bottoms. Nodules from all sites show non-uniform contents of Mn, Fe, Ni and Cu within individual box cores. The greatest variations were found in box cores from Sites A and C. Mn, Ni, and Cu are generally covariant whereas Fe content is antithetic to Mn content. Large variations in metal content are generally due to differences in oxide mineralogy but some variation is related to clay content. Large nodules tend to be relatively low in Mn, Ni, and Cu. Site B nodules have the highest mean values for Mn, Ni, and Cu and the lowest mean Fe content. Site C nodules and Site A nodules have the same Mn content, but Site C nodules are richer in Ni and poorer in Fe than Site A nodules.