Only one lithologic unit was identified at Site 1147 (Fig. F5). Unit I is composed of clay with quartz and smaller amounts of nannofossils. A few intervals display a notable increase in lightness, apparent by visual observation and in the lightness L* index of the spectrophotometric data (Fig. F5). The increases in lightness correspond to increases in nannofossil abundance as observed in smear slides. These lighter intervals are also characterized by an increase in the occurrence of green clay layers, higher foraminifer content, and intense bioturbation. An ~3-cm-thick reddish brown oxidized sediment layer, with a dusky brown manganese precipitation zone at the base, is observed at the top of Cores 184-1147A-1H, 184-1147B-1H, and 184-1147C-1H. This interval probably represents the modern seafloor.
Slightly lighter intervals with higher carbonate content are observed, but these are less pronounced than at Site 1146, and many of them are observed only in the spectrophotometric data. Visible light-colored layers are compiled in Table T5. Typical features of the light layers include abundant foraminifers visible on the core surface, common green clay layers (typically 1-3 cm thick), well-defined bioturbation, and slightly yellowish gray patches or mottles, which represent traces of bioturbation. Smear-slide estimates indicate calcareous nannofossil contents of >10%, compared to around 0%-5% in the background clay.
Green layers and irregular green mottles occur frequently and are well developed in the upper part of the hole, reaching a maximum frequency above ~40 mcd. From this maximum, the green layers decrease in frequency downhole, being rare at the base of the drilled section (Fig. F5). The layers are characterized by a clay that is stiffer than the dominant clay lithology of the section, and they have a lower water content and higher P-wave velocity.
Bioturbation is intense throughout the cored interval. The sediment is generally completely homogenized, and individual burrows are observed only in exceptional cases. Pyrite-filled burrows are the exception to this pattern. They occur at a frequency of one to two burrows per section in the upper part of the unit and increase in frequency downhole (Fig. F5). Large pyrite-filled burrows reach several centimeters in length and as much as 2 cm in diameter. Pyrite-filled burrows first appear at ~48 mcd, and no "iron sulfides" are found above ~38 mcd. Pyrite-filled burrows are probably formed by the diagenetic replacement of organic matter-rich burrow fills.
Well-preserved distinct volcanic ash layers are missing at Site 1147. However, we observed occurrences of volcanic ash as burrow fill (finely dispersed within the sediment), as isolated pumice clasts, and as thin layers (Table T6). The maximum ash layer thickness is ~1 cm. The poorly preserved ash layering is probably a result of the intensive bioturbation observed in all three holes at Site 1147.