Four holes were drilled at Site 1089 to a TD of 264.9 mbsf (280 meters composite depth [mcd]), recovering Pleistocene and upper Pliocene nannofossil ooze and diatom mud (Fig. F4). Nannofossil abundance varies from 0% to 90% (see "Site 1089 Smear Slides"). Mud content varies from a few percent to ~80%. Foraminifer abundance ranges from 0% to 10% and is fairly constant downhole to about 220 mbsf (235 mcd) where it goes to zero. Diatoms range from a few percent to a maximum of 50% and are generally present in the 20%-30% range. The coarse sample resolution does not yield any systematic downhole trend in components (see Figs. F4, F5; Table T1, also in ASCII format in the TABLES directory; and "Site 1089 Smear Slides"). Carbonate values range from 0 to 60 wt% (Figs. F4, F5A; see "Geochemistry"). Variations in carbonate and nannofossil percentages agree well with the blue-band reflectance record (see "Physical Properties").
Recovery was generally good (Fig. F4), and in Holes 1089B and 1089C less than 3% of the recovered material was disturbed during coring (Tables T2, T3). A complete splice covers approximately the upper 94 m of the section (see "Composite Depths"). The splice does not extend deep into the core because of several intervals of disturbed sediment. Small normal and reverse faults with offsets of as much as 8 cm were common (Fig. F6), but it is not clear whether they were pre-existing fractures or caused by coring disturbance.
Two principal lithologies were recovered: nannofossil ooze and diatom mud. Several subordinate mixed lithologies were also recovered. Nannofossil ooze is pale to very pale gray. Rare, nearly pure nannofossil ooze (>80% nannofossils) is present only in the upper nine cores (80 mbsf). Diatom- and mud-bearing nannofossil ooze is present in Cores 177-1089B-2H (4.8 mbsf, 10.5 mcd) through 26H (240 mbsf, 255 mcd) and is absent in the lower part of the core. Diatom mud is present beginning with Core 177-1089B-3H and is common further downhole. Diatom mud is dark in color, with dark gray, dark greenish gray, dark olive, and olive green dominating. Mud-bearing diatom ooze is present but not common. It is brown, dark gray, and dark olive gray. Pure diatom ooze is present only in Core 177-1089B-18H. Nannofossil-bearing mud diatom ooze is also present. One lithostratigraphic unit was identified.
Intervals: 177-1089A-1H through 23H (0-216.3 mbsf, 0-229.3 mcd); 177-1089B-1H through 29H (0-264.9 mbsf, 0-280 mcd); 177-1089C-1H through 21H (0-194.4 mbsf, 0-205.6 mcd); 177-1089D-1H through 13H (0-118.0 mbsf, 0-125.5 mcd)
Age: late Pliocene to Pleistocene
Unit I consists of alternating beds of nannofossil-rich and nannofossil-poor sediment, such as diatom- and mud-bearing nannofossil ooze and diatom mud. Most contacts are gradational, but when sharp contacts are observed they are equally likely to have light (nannofossil-rich) or dark (nannofossil-poor) sediment below the contact. It is not clear if the contacts represent a depositional unconformity or whether they were caused by faulting. Thin minor beds include a brown pelloidal nannofossil-bearing diatom mud in interval 177-1089A-1H, 39-41 cm, and a tan silt lamina in interval 177-1089B-16H-5, 113 cm (Fig. F7).
Bioturbation is prevalent throughout and Planolites ichnofossils are abundant. Thin purple bands commonly surround burrows. They give the appearance of color laminae and are probably Liesegang banding (Fig. F8). In addition to the purple Liesegang laminae, several green layers, harder than the surrounding sediment, are present (Fig. F9). These color bands may be related to a slight degree of carbonate recrystallization, with purple bands potentially reflecting rhodochrosite formation (Mn-carbonate) and green bands reflecting siderite formation (Fe-carbonate). Many of the burrows contain preserved pyrite molds and disseminated pyrite is relatively common (Fig. F6). Small (millimeters in diameter) silt pods were observed throughout and contain ~80% angular quartz with minor biosilica (sponge spicules and diatoms) (Figs. F6, F7). Macroscopically similar features have been observed in sediment cores recovered during several previous ODP legs (e.g., Leg 112), and may originate from a shelf environment or may have been carried to their present position within sponges (Martini and Locker, 1990).
Gypsum was observed in smear slides from Core 177-1089B-7H (roughly 55-60 mbsf). It may also be present as high in the hole as Core 177-1089B-5H, although the mineral was less well developed and therefore more difficult to identify. The presence of gypsum in other deep-sea sediments has been attributed to authigenic formation under reducing conditions, where calcium derived from the dissolution of biogenic carbonate reacts with sulfate diffusing downward from seawater (e.g., Briskin and Schreiber, 1978; Siesser and Rogers, 1976). Interstitial water shows a decrease in sulfate from 24 mM at the surface to 0 at this level, and calcium reaches its lowest values (see "Geochemistry").
Only two (>1 cm) dropstones were recorded: a 1.5-cm-long tonalite(?) in interval 177-1089A-20H-5, 20 cm, and a 1-cm-long rounded quartzite in interval 177-1089B-8H-2, 75 cm.
Soft sediment deformation is common below 80 mbsf and as deep as 250 mbsf (Fig. F10). The deformed sediments consist of sharply dipping beds with clear color contacts (Fig. F11), and contorted beds (Fig. F12). Although microfaulting is present throughout the cores, it is most abundant in the deformed intervals. A layer exhibiting graded bedding is present at 243.3 mbsf within or at the very top of a deformed sedimentary interval (Fig. F13).
The deformed intervals are difficult to correlate between the four holes, even though the holes are separated by relatively short distances on the seafloor (a maximum of 30 m; Fig. F10B). Holes 1089A, 1089B, and 1089C can be correlated above 70 mbsf and below 160 mbsf. No further sedimentary deformation is observed down to 240 mbsf in Core 177-1089B-27H; however, Hole 1089B is the only hole to reach this depth. Between deformed intervals, the sediment succession reveals horizontal layering and appears to be undeformed. It is possible that the soft-sediment deformation is of minor extent and therefore not necessarily correlatable between holes, although the thickness of the deformed intervals (up to 15 m) and possible correlations between deformed intervals among several holes (Fig. F10A) argue for a larger scale phenomenon.
X-ray diffraction (XRD) measurements were conducted on the noncarbonate fraction of 57 samples (Fig. F5). Opal contents in the noncarbonate fraction vary between 10 and 24 wt% with some lower values, especially below 195 mbsf. Opal fluctuates independently from carbonate variations of the bulk-sediment fraction and, thus, probably cannot be related to glacial-interglacial cyclicity.
The lithogenic fraction consists of quartz, feldspar, clay minerals, and pyrite. Amphibole and clinopyroxene appear in trace amounts below significant XRD detection. All components show marked downhole variations. However, there seems to be no coherent pattern with respect to carbonate cycles or opal fluctuations, at least at this coarse resolution. Even mutual correlation among the lithogenic components is poor.
One important trend, however, is the increase of the illite/(kaolinite+chlorite) value (10 Å/7 Å) below 230 mbsf, which coincides with a change in lithology from gray carbonate-bearing sediments to predominantly greenish diatom-bearing and diatom muds and the absence of nannofossil ooze. Higher illite/(kaolinite+chlorite) values are also evident in two samples of the greenish layers that were intercalated in the overlying carbonate-bearing sediments (e.g., Fig. F8). The significance of these mineralogical changes has to be confirmed through further shore-based XRD analysis of clay minerals.
Site 1089 is located on a gently sloping drift deposit. The alternating dark and light beds may document variations in relative input of biogenic and terrigenous sediments in response to glacial-interglacial changes, with decreased terrigenous input and/or enhanced biogenic carbonate production and preservation during interglacial stages. These well-preserved lithologic cycles offer an excellent opportunity to examine paleoceanographic environmental changes on both Milankovitch and millennial time-scale variability. The fine grain size and scarcity of dropstones suggest that most of the terrigenous sediment is wind blown and/or derived from bottom currents. Clay-mineral and grain-size analyses will provide better clues of terrigenous sediment provenance and modes of sediment transport. Soft-sediment deformation processes, recorded in sediments below 130 mcd, were probably caused by sediment slumping that was triggered by high sedimentation rates, by undercutting bottom currents, or by a combination of both.