Introduction
In the GH78-1 area manganese nodules were obtained from 37 stations among 39 total ones. Here we report the preliminary results of the observation done mainly on-board the ship. Types of manganese nodule are first described follow- ing the classification established in the previous cruise (Moritani et al. 1977), and then the pattern of nodule distribution is discussed in relation to the topo- graphy, substrate stratigraphy and surface sediment types. In addition, a short description on the obtained and observed rocks from several stations are included in this chapter.
Method of nodule study
Bottom sampling was done at stations over a distance of usually 1° or about 110 km apart, and in some area at those additionally placed in the middle of usual grids, or spaced more closely in case of the detailed survey. For regular sampling, three types of sampler were used: Okean (Ocean)-70 grab which can take a bottom portion of about 0.5 m² square area and 0.35 m depth, double- spade box corer with a catch area of 0.16 m² and 0.4 m depth, and the freefall photograb with a catch area of about 0.116 m². The freefall photograb sampler was dropped usually in two sets at each sampling site by the wire-lined samplers of Okean-70 grab or double-spade box corer, in a very close distance each other from a few hundred to a few thousand meters. Nodule samples obtained were described and studied on board for 1) obser- vation of occurrence and morphology in and outside samplers, type classification, size classification, measurement of weight and calculation of abundance (kg/m2); 2) photographing whole nodules on plates with respective catch square areas of each sampler, and typical samples on plate with 5 cm grid scale; and 3) obser- vation of internal structures of the nodules on cut section. Determination of mineral composition by X-ray diffractometer and chemical analysis were carried out in the onshore laboratory after the cruise as described in Chapter XV (FaKioglu et al., in this report) and in Chapter XIV (NaKao et al., in this report). The results are summarized in the sample lists (Table IX-2) and photograph log (Fig. IX-3). The relation of manganese nodules to the geological environ- ments was examined by referring to other data of related studies, such as topography, sedimentology, and acoustic survey.
Occurrence of manganese nodules
Similarly in the previous GH76-1 and GH77-1 areas, it was confirmed that
Here, each size class roughly corresponds in maximum diameter; small- < 4 cm, medium-4-6 cm, large- > 6 cm, Morphological symbols indicate; S-spheroidal, E-ellipsoidal, D-discoidal, P-poly or intergrown, I-irregular, V-variable, F-faceted, B-biological, T-tabular or flattened. within the sediment column of 35 cm to 40 cm deep almost in all cases manga- nese nodules of the GH78-1 area occurred on the sea bottom surface or exactly speaking in the water-sediment interface though their state was more or less different as more exposed or buried. However, exceptionally at St. 1057-2 a clear horizon of buried manganese nodules was recognized at a depth of 25 cm below the bottom surface.
Types of manganese nodules
The morphological types were determined according to the classification schema established in the previous GH76-1 cruise as shown in Table IX-1. This is based both on the surface structure, smooth(s) and rough(r) or botryoidal(b) and on the shapes, such as spheroidal(S), ellipsoidal(E), discoidal(D), inter- grown or polylobate(P), irregular(1) and variable(V) forms, and each type is represented by the combination of the initials of both terms as Sr or Ss. Nine types such as Sr, SPr, SEr, Db, Ss/SPs, DPs, ISs, IDPs and Vs were originally recognized in the GH76-1 area, and these were applicable as a whole for the GH77-1 area without modification though there were some slight variations. As theoretically thought, there are expected variations or gradations of such nodule types both in surface structure and shape. In the GH78-1 area, the intermediate type of surface structure, smooth to rough(s r), was newly distinguished, and for shape type original Ss/SPs was separated as Ss and SPs. Altogether, as shown in Table IX-2, 17 types were identified in this area. They are for rough surface type group Sr, SPr, SEr, Db and Vr, for intermediate type group Ss·r, SPs r, Ds r and IDPs r, and for smooth type group Ss, SPs, Ds, DPs, ISs, IDs, IDPs and Vs. Variation in shapes relates to the inner structures or nucleus shape of manganese nodules. As nuclei there were recognized sedimentary rocks,
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The abundance in the parentheses shows the probably incorrect values due to the grab sampler. M: Many small fragments including broken ones. volcanic rocks, clayey materials, shark's teeth, fragments of broken nodule etc. Shape types are not likely to have close relation to nodule mineralogy and chemistry. On the contrary, the surface structure types are important regarding nodule mineralogy and chemistry. The surface structure relates to nodule occur- rence in the water-sediment interface as smooth(s) type in more exposed state and rough(r) type in more buried state. This seems to show the condition of nodule growth as precipitation of metal components from sea water in exposed state and precipitation from interstitial water in buried state respectively, re- flecting on the difference in mineralogy and chemistry of nodules as discussed in Chapters XIV and XV in this report. Generally speaking, the nodule types of the GH78-1 area have similarity to those of the GH77-1 and then of GH76-1 areas. But a characteristic trend can be pointed out in that larger size nodules of Ss or SPs types are more pre- dominant in the GH78-1 area, though this trend was already seen in the GH77-1 area, as compared with smaller size nodules of Ss/SPs type in GH76-1 area. Particularly, larger size Ss type with remarkable sphericity like cannon ball shape is recognized in some stations of the south-western mountainous part of the GH78-1 aera, such as Sts. 1036, 1037, 1067 and 1070. Also, it is interesting that the above mentioned buried nodules of St. 1057-2 are of s type group similar to that on the surface, suggesting possibility of burial after their growth on the bottom surface and then stop of growth after burial.
Distribution of manganese nodules
Two generalized maps were made to show the trend of the regional distribu- tion of manganese nodules and its relations to the geological environment.
+<0.1,  ◇ 0 1-1.0'  △ 1.0-5.0,  △ 5.0-10,  ③ 10-20,  ● ≥20 kg/m² Fig. IX-1 shows the distribution of both abundance and types of all nodules, together with that of surface sediment types. Fig. IX-2 represents a simplified distribution of nodule abundance in way of isopleth (iso-abundance line). The local variation in nodule types and abundance in a limited station area is generally small, judging from the data by Okean-70 grab or double spade box corer, and simultaneously used two freefall photograb samplers. About nodule abundance though some larger difference in value are shown at several stations, such as Sts. 1039, 1045, 1048. 1050, 1051, 1053, 1057 and 1064, all the smallest grade values are thought due to mechanically imperfect catch of the bottom probably caused by the existence of large rock fragment or rock bed, and also to obtained rock samples themselves in stead of manganese nodules. Thus as far as nodule abundance is concerned, the grade values are similar in one station area, being within nearest two grades. However, as for type variation there are only two cases in which both s type and s r type exist together in one station area at Sts. 1044 and 1061. As already mentioned, the vertical distribution of manganese nodules in the sediment column of 35 cm and 40 cm depth was confirmed in the obtained samples by Okean-70 grab and double- spade box corer respectively. Almost in all cases nodules were found on the bottom sediment surface, or in the water-sediment interface. One exception is St. 1057-2 where a horison of buried nodules occurred at the depth of 25 cm below the bottom surface. But, the buried nodules were s type similar to those on the sediment surface. As shown in regional distribution map of manganese nodule abundance (Fig. IX-2), marked area of highest grade value above 20.0 kg/m 2 is in the south- west mountainous area and some local ones are in the northeastern seamount
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# area and in two parts of central to southeastern deep sea basin. Also, broad zone of higher grade values of 10-20 kg/m² extends with branches along the latitudes around 10°N and 11°N of the central part of the deep sea basin, while the lower grade value areas lie, one in the northwestern abyssal plain and the others in the southeastern deep sea basin to the southward from around the latitude 10°N. This regional distribution features of nodule abundance shows good harmony with regional topography. The highest grade abundance areas seem to be confined to the mountainous or ragged topography with thinner acoustic trasparent layer of Type A (Unit I) particularly in the southwestern mountainous area, and the higher grade abundance areas to the repeated hills and troughs area of the central deep sea basin with rather thin acoustic transparent layer of Type A (Unit I). The zonal area of the above mentioned higher grade abundance represents the westward extension of the highest grade abundance zone in the adjacent GH77-1 area. Similarly, the lower grade abundance area in the southeastern deep sea basin to the southward from around the latitude 10°N corresponds to the north- westward extension of general topographic depression to the northwest of Magellan Rise in the adjacent GH77-1 area. Here the thick acoustic turbidites layer of Type C (Unit I) presumably derived from the Magellan Rise, buries the depression. The other lower grade abundance area remarkably coinsides with the flat topography of the northeastern abyssal plain. In this case, however, the acoustic layer of Type A (Unit I) is rather thinner. As shown in Fig. IX-1, the types of surface sediment show roughly zonal distribution. From calcareous ooze or calcareous-siliceous ooze at topographic highs in the southwestern mountainous area, towards northeast broad zone of siliceous clay and then wider zone of deep sea clay in the central and south- eastern deep sea basin are developed. And there are distributed locally a zeolitic mud zone in the deep sea clay zone in the northwestern abyssal plain, a siliceous ooze zone between siliceous clay and deep sea clay zones in the southeastern deep sea basin. On this zonal distribution it is difficult to point out clear re- lations between surface sediment types and nodule types or abundance, except that calcareous ooze and siliceous ooze zones coincide to part of none or scarce nodule abundance as generally recognized in the GH77-1 or GH76-1 areas. However, the highest grade abundance value of 49.2 kg/m² was found on calcareous-siliceous ooze at St. 1070, though we have not enough data to inter- prete this unusual association. The nodule types, especially of r and s surface structure type groups rather show close relation with distribution of the nodule abundance. The distribution of r type nodules is more or less restricted to the lower grade abundance area less than 10.0 kg/m² in the southeastern deep sea basins to southward from around Iatitude 10°N, and in the northern area including the northwestern abyssal plain. In conclusion, both the nodule abundance and, to some extent, types especially on surface structure seem to have relation to the sedimentation rate as the total effects of topographic condition, and bottom current regime, resulting abundant nodule distribution in lower sedimentation rate areas. Fig. IX-2 — 129 — (4) (6) (7) (8)
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FG 85-1
(12) — 138 — (14) i — 142 —
(17)
(22) (26) (27)
(28)
(30) (32) — 159 — (34) (35) — ! 6 l — — 163 — — 164 — (40)
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(42)
(44)
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Description of obtained and observed rocks
Rock samples were obtained at the bottom sampling by Okean-70 grab, double spade box corer and freefall grab samplers. Also, existence of rocks was re- cognized at observation of sea bottom by deep sea camera. Stations and obtained rock samples are as follows: St. 1039, upper slope of a hill, brecciated basalt; St. 1053, base of a hill, a large number of basalt frag- ments; St. 1069, slope base of hill, basalt fragment; St. 1040, irregular surface of top of a small hill, chert fragments. Also some exist as relatively large size nuclei of manganese nodules: St. 1045, slope base of a hill, basalt; St. 1048, upper slope of a hill, basalt, St. 1051, irregular surface of top of a hill, con- solidated sediments; St. 1073, slope of a hill, phosphorite. Besides, observation of sea bottom by deep sea camera revealed occurrence of rock fragments at Sts. 1039 and 1059, and outcrop of rock bed at St. 1071.