Site 1092 was drilled to a TD of 188.5 mbsf, recovering sediments of Pleistocene to early Miocene age (Fig. F4). The sediments consist of pale brown-green to pure white nannofossil ooze with mixtures of diatom and foraminifer oozes and muds. Smear-slide analyses reveal that nannofossil abundance ranges from trace amounts to 98%, whereas foraminifer percentages vary between 0% and 70% (see "Site 1092 Smear Slides," p.59).
Nannofossils are the overall dominant lithologic (biogenic) component at this site. They alternate and are intermixed with foraminifers, diatoms, and mud, particularly in the upper 63 meters composite depth (mcd) of the sedimentary column. In this interval, foraminifer, foraminifer-bearing, diatom, and diatom-bearing varieties of nannofossil ooze alternate with ooze dominated by foraminifers and diatoms. Below 63 mcd, nannofossils are the predominant lithologic component and are replaced by diatoms or foraminifers in only a few intervals. The dominance of calcareous components is illustrated in Figure F5. The mud content of the entire sedimentary column is low, less than 20% based on smear-slide examination and X-ray diffraction (XRD) analyses (Fig. F5). Dropstones are present throughout the upper 63 mcd of the sedimentary column, although their exact placement within a core (in place or downhole contaminant?) is uncertain.
Recovery at this site was very good, reaching 96.6% in Hole 1092A, 85.6% in Hole 1092B, 91.1% in Hole 1092C, and 99.2% in Hole 1092D.
XRD measurements were performed on the noncarbonate fraction of 35 samples from Hole 1092A (Table T1, also in ASCII format in the TABLES directory). Opal content inferred from XRD measurements shows downhole fluctuations between 0 and 90 wt% (Fig. F5). Opal abundance was expressed as bulk sediment opal concentration using bulk carbonate concentrations measured in the same sample intervals (see "Geochemistry"). Bulk opal percentages are inversely correlated with bulk carbonate concentrations. In Quaternary and Pliocene sediments, bulk opal varies between 10 and 60 wt%, with a peak value of 80 wt% at the Pliocene/Pleistocene boundary (~33 mcd). Below a hiatus at ~65 mcd (early Pliocene), spanning a short time interval, and a series of hiatuses at the Miocene/Pliocene transition (~70-75 mcd), bulk opal abundances drop continuously to near 0 wt%. Proportions of the lithogenic fraction are below 15% in most intervals of the section. Higher lithogenic concentrations of up to 18%, associated with elevated values in magnetic susceptibility (see "Physical Properties"), appear below the Pliocene/Pleistocene boundary.
Quartz/feldspar values show maximal amplitudes and reach highest values in the Pliocene and Quaternary section; they remain constant at ~2.0 below the short hiatus at ~65 mcd. Clay minerals/(quartz+feldspar) values increase from 0.1 to 0.2 in the Miocene sediments below the hiatus. A pronounced downhole shift to values around 0.3 marks another hiatus (~176-177 mcd) that coincides with a sediment change from greenish white to reddish white nannofossil ooze and a further decrease of quartz/feldspar values toward 1.0. The long-term fluctuations of quartz/feldspar values and clay minerals/(quartz+feldspar) values, which are not discernible on smear slides, reveal a trend that is similar to the one observed at Site 1088, and probably reflect a downhole decrease in the grain size of lithogenic particles.
One lithostratigraphic unit was recognized and was subdivided into two subunits (IA and IB) (Figs. F4, F5) on the basis of visual color variations on Hole 1092A core photographs. The transition between these two subunits is gradual. In Hole 1092C, the boundary falls somewhere within Core 177-1092C-6H, which had only 10 cm recovery (used for biostratigraphy). Subunit IA extends, therefore, to the bottom of Core 177-1092C-5H and Subunit IB begins at the top of Core 177-1092C-7H.
Intervals: 177-1092A-1H through 5H (0-46.0 mbsf; 2.55-53.89 mcd); 177-1092B-1H through 6H-4, 20 cm (0-50.1 mbsf; 0.55-53.89 mcd); 177-1092C-1H through 5H (0-42.0 mbsf; 0-46.05 mcd); 177-1092D-1H through 2H-4, 74 cm (36.4-51.1 mbsf; 38.47-53.89 mcd)
Age: Pleistocene to late Pliocene
Subunit IA consists of alternations of foraminifer, nannofossil, and diatom ooze with various mixtures of all biogenic components plus admixtures of mud, particularly in the foraminifer- and diatom-rich sediment types. Dropstones and smaller sized ice-rafted debris (IRD) are scattered throughout but are concentrated in the pale tan to tan diatom/foraminifer-rich varieties that additionally contain 5%-10% (and in places more) mud.
Calcium carbonate is present throughout Subunit IA. Carbonate concentrations, based on coulometric analyses, vary between 17 and 95 wt% (Fig. F5). A good correlation exists between calcium carbonate concentrations estimated by smear-slide analyses and those inferred from coulometric titration (Fig. F5). The darker (i.e., pale tan to tan) diatom- and mud-rich sediment types presumably represent glacial intervals and contain ~55 wt% carbonate or less (by coulometry). Carbonate contents of 60 wt% or more occur in white to greenish white sediments that presumably represent interglacial intervals.
Intervals: 177-1092A-6H through 20H (46.0-188.5 mbsf; 53.89-210.84 mcd); 177-1092B-6H-4, 20 cm, through 18H (50.1-168.9 mbsf; 53.89-184.81 mcd); 177-1092C-7H through 18H (51.5-165.5 mbsf; 56.97-185.29 mcd); 177-1092D, 2H-4, 74 cm through Core 3H (51.1-64.9 mbsf; 53.89-68.92 mcd)
Age: late Pliocene to early Miocene in Hole 1092A; late Pliocene to middle Miocene in Holes 1092B and 1092C; late Pliocene in Hole 1092D, which ends at the Miocene/Pliocene transition
Subunit IB consists of nannofossil ooze, with variable amounts of foraminifers and diatoms down to ~100 mbsf in Hole 1092A (~112 mcd). Below this level, nannofossil ooze is the only major lithologic type; diatoms and foraminifers are minor admixtures. Mud is a minor component of these sediments. Rare dropstones occur in the upper portion of this subunit (e.g., at interval 177-1092D-3H-4, 8 cm). Many samples are completely free of terrigenous components, at least as determined by smear-slide analysis, particularly near the bases of the drill holes. Faint laminations, mottling, and rare burrows occur throughout this subunit.