Site 1090 was drilled to a total depth (TD) of 397 mbsf, recovering calcareous ooze and mud-bearing diatom and nannofossil ooze of Pleistocene to middle Eocene age (Fig. F3). A tephra layer and two chert layers were encountered. Carbonate and opal contents are highly variable, both ranging between 0 and ~80 wt%. The tephra layer was recovered in Cores 177-1090B-8H, 177-1090C-8H, 177-1090D-8H, and 177-1090E-7H. Above the tephra layer, a hiatus separates early Pliocene calcareous ooze from early Miocene mud-bearing diatom and nannofossil ooze (see "Biostratigraphy"). Chert layers in the top of Section 177-1090B-32X-1 are apparently associated with discontinuities in interstitial water profiles (see "Geochemistry").
Nannofossils, diatoms, and mud comprise the major lithologic components at the site as indicated by smear-slide analyses (see the "Core Descriptions" contents list). Nannofossils are the dominant calcareous particles in smear slides, and their abundance tracks carbonate percentages downhole. Foraminifers are present in low abundance (<20%) below 70 mbsf (early Miocene to middle Eocene), and increase uphole above ~70 mbsf (Pleistocene to early Pliocene) to maximum abundances of 80%. The percentage of mud varies from a few percent to ~60% throughout most of the cores, and rarely to nearly 100%. Diatom proportions range from a few percentages to a maximum of ~90%, with highest abundances between ~100 and ~350 mbsf. Sediments between ~220 and 350 mbsf with high diatom abundances and high opal percentages were deposited with higher sedimentation rates (see "Biostratigraphy") than the under- and overlying strata, and may indicate a period of high biologic productivity during the middle Eocene to early Oligocene.
Carbonate and nannofossil percentages covary with color reflectance (Fig. F4; see also "Physical Properties"). Smear-slide estimates of total calcareous particle, diatom, and mud abundance compare well with measurements of carbonate (by coulometry), opal (by X-ray diffraction [XRD]), and terrigenous sediment, indicating that the smear-slide data reliably characterize major downhole variations in lithology (Fig. F5).
Bioturbation is prevalent throughout the sedimentary succession and Planolites, Zoophycos, and Chondrites burrows are abundant (Figs. F6, F7).
Overall recovery at Site 1090 was 93.4% (see "Operations"). A complete splice covers approximately the upper 242 m of the section (see "Composite Depths"). Recovery averaged 92.3% below the splice in Hole 1090B (Fig. F3).
Intervals: 177-1090A-1H (0-6.87 mbsf; 0-6.87 mcd); 177-1090B-1H through 8H-4 (0-65.7 mbsf; 0.69.41 mcd); 177-1090C-1H through 8H-3 (0-62.1 mbsf; 0.5-69.91 mcd); 177-1090D-1H through 8H-1 (0-64.4 mbsf; 0.34-70.31 mcd); 177-1090E-1H through 7H-5 (0-61.0 mbsf; 3.38-69.55 mcd)
Age: Pleistocene to early Pliocene
This unit consists of Pliocene-Pleistocene meter-to-decimeter scale alternations between pale gray foraminifer nannofossil ooze and greenish gray mud- and diatom-bearing nannofossil ooze. Although the unit appears to have several hiatuses, it records lithologic variability on orbital time scales through at least the lower Jaramillo Subchron (see "Chronostratigraphy"). We have placed the lower boundary of Unit I at 70 mcd. This boundary marks a hiatus between lower Pliocene and lower Miocene strata, and appears 0.5 m above a tephra layer. The basal oozes (~69-70 mcd) of Unit I contain apparently reworked older components, indicated by mixed assemblages of Pliocene and Miocene microfossils (see "Biostratigraphy"). A few dispersed, and probably also reworked, manganese nodules are present as high as 2.5 m above the lower boundary of Unit I (Fig. F8).
Intervals: 177-1090B-8H-4 through 37X (65.7-339.6 mbsf; 69.41-351.59 mcd); 177-1090C-8H-3 through 8H-7 (62.1-69.3 mbsf; 69.91-77.11 mcd); 177-1090D-8H-1 through 24H (64.4-225.9 mbsf; 70.31-239.68 mcd); 177-1090E-7H-5 through 25H (61.0-236.7 mbsf; 69.55-258.14 mcd)
Age: early Miocene to early late Eocene
Unit II consists of mud-bearing diatom ooze, mud- and diatom-bearing nannofossil ooze, and chalk, between early Miocene and early late Eocene in age. The boundary between Units I and II corresponds to the Pliocene/Miocene hiatus at ~70 mcd and is associated with a lithologic change from the grayish foraminifer-bearing nannofossil ooze of Unit I to the pale reddish brown mud-rich nannofossil ooze of Unit II. Percentages of carbonate, though highly variable, are lower on average than in Units I and III. This variability is apparent in smear-slide estimates of nannofossil and diatom relative abundance, in color reflectance, and in opal abundance obtained from XRD analyses (Fig. F5).
Half a meter below the upper boundary of Unit II, a tephra sequence was encountered in Cores 177-1090B-8H, 177-1090C-8H, 177-1090D-8H, and 177-1090E-7H, that ranges between 34 and 70 cm in thickness. Figure F8A shows the relative stratigraphic position of the tephra sequence in standard ODP mbsf in the four cores, and Figure F8B shows the position of the tephra sequence in mcd units after adjustment on the basis of gamma-ray attenuation (GRA) bulk density and magnetic susceptibility data. The tephra sequence commences with a sharp erosional contact to the underlying ooze and consists of several graded layers of vitric volcanic ash admixed with variable amounts of hemipelagic biogenic and terrigenous particles (Fig. F9). Sedimentary structures and textural features, described in detail in the barrel sheets, suggest reworking and redeposition of the tephra components by turbidity currents.
A number of distinctive intervals of diatom ooze are present within the early Miocene and Oligocene-Eocene sediments at Site 1090. A prominent horizon of white to pale green diatom ooze was recovered in interval 177-1090B-22X-6, 147 cm, to 22X-7, 25 cm (base not recovered), in disrupted form in interval 177-1090D-22H-1, 91 cm, to 22H-2, 48 cm, and intact in interval 177-1090E-21H-5, 121 cm, to 21H-6, 18 cm. This bed consists of dominantly white diatom ooze with irregular green streaks and is mixed with the overlying reddish brown diatom mud. In intervals 177-1090D-20H-1, 80-90 cm, and 20H-4, 90-130 cm, burrows are filled with a distinctive white diatom ooze dominated by large Coscinodiscus sp. (see "Biostratigraphy"). A further series of intermittently laminated diatom oozes is present in intervals 177-1090B-29X-2, 135 cm, to 29X-3, 42 cm; 29X-3, 137 cm, to 29X-4, 33 cm; 29X-5, 125-128 cm; 29X-6, 0-50 cm; 29X-6, 104-127 cm; 30X-6, 112-126 cm; and 32X-1, 6-24 cm. The diatom ooze in Section 177-1090B-32X-1 underlies a fragmented, dark green chert horizon that coincides with a major change in the interstitial water profile (see "Geochemistry"). The dominant diatom genus in many of these lower intervals is Pyxilla (see "Biostratigraphy").
Interval: 177-1090B-38X through 43X (339.6-397.5 mbsf; 351.59-409.49 mcd)
Age: early late Eocene to middle Eocene
This unit consists of middle Eocene mud-bearing nannofossil ooze, chalk, and two chert layers. Carbonate concentration shows high-amplitude fluctuations (0-80 wt%) in the upper part of this unit from ~340 to ~370 mbsf (~350-380 mcd), and higher but less variable abundances in the lower part of the unit from ~370 to 397 mbsf (~380-407 mcd). Calcareous microfossil abundance similarly rises downhole within Unit III and becomes the dominant lithologic component in the lower ~30 m of the hole. Diatom and opal abundances drop to near zero in this unit, and the highest mud abundance (from smear-slide estimates) in the hole is in the upper part of Unit III between ~340 and 397 mbsf (~350-407 mcd; Fig. F5). Clay abundance is also high relative to quartz and feldspar (see "X-ray Diffraction Results").
XRD measurements were made on the noncarbonate fraction of 81 samples (Table T1, also in ASCII format in the TABLES directory). Opal content of bulk sediment inferred from XRD measurements shows pronounced downhole fluctuations between 0 and 80 wt% (Fig. F5). Opal abundance was expressed as bulk sediment opal concentration (Fig. F4) using bulk carbonate concentrations (see "Geochemistry") at the same sample intervals to complete the sum of sedimentary components. Bulk opal contents in the carbonate-rich Pleistocene and Pliocene sediments of Unit I vary between 2 and 10 wt%. These values increase in early Miocene mud-rich sediments to average values of 20 wt%, with occasional peaks as high as 30 wt%. Highest average opal percentages and variability (between 15 and 65 wt%) are evident in the Oligocene and late Eocene sediments of Unit II and are associated with high variability in carbonate and terrigenous mud abundances. In the middle Eocene sediments of Unit III bulk opal concentrations drop to values <1 wt%.
Quartz/feldspar and clay minerals/(quartz+feldspar) values show long-term variations at Site 1090. Quartz/feldspar values >1.0 and clay minerals/(quartz+feldspar) values <0.2 characterize the terrigenous fraction of Pliocene-Pleistocene sediments. Directly below the Miocene/Pliocene hiatus, quartz/feldspar values decrease to 0.5 whereas clay mineral/(quartz+feldspar) values slightly increase. This Neogene trend, also seen at Site 1088, may be caused by a shift in terrigenous grain size from silt to clay dominance. Both mineralogies covary downhole, probably because of changes in weathering. Periods of enhanced chemical weathering in source areas may strengthen feldspar hydrolysis and dissolution and enrich terrigenous debris in both quartz and pedogenic clay minerals.
Highest abundance of clay minerals, probably smectite and mixed-layer clay minerals (as indicated by a broad XRD reflection around 12 Å), is noted in middle Eocene sediments and is associated with low opal content and authigenic zeolite (clinoptilolite). The latter can be seen in carbonate-free smear slides as fine silt-sized euhedral crystal lathes exhibiting low birefringence. The mineralogical assemblage of smectite and zeolite could originate from diagenesis combining silicate dissolution and silica reaction with ion-rich interstitial water solutions leading to the formation of cation-rich minerals.