Four holes were drilled at Site 1083 with a maximum penetration of 202.75 mbsf (see Fig. 1). Sediments from Site 1083 form one lithostratigraphic unit composed of moderately bioturbated clayey nannofossil ooze. This unit has been subdivided into two Subunits, IA and IB, based on the changing abundance of diatoms. Subunit IA extends to Core 15 at Holes 1083A, 1083B, and 1083D and with the exception of few 1- to 1.5-m-thick diatom-rich intervals, contains diatoms in rare to frequent abundance. In Subunit IB, diatoms become consistently common. The stratigraphic variation in diatom abundance and the definition of the subunits is comparable to those at Sites 1081 and 1082 (see "Lithostratigraphy" sections, "Site 1081" and "Site 1082" chapters, this volume). A major characteristic of sediments from Site 1083 is the repeated occurrence of dark–light color cycles throughout the drilled sedimentary succession. As at Site 1082, the lighter colored cycles are more calcium carbonate–rich compared to the adjacent more clay-rich, darker colored cycles. Clay-rich intervals are generally 60 to 150 cm thick and occur approximately every 2 to 5 m.
Subunit IA is composed of moderately bioturbated intervals of pale yellow (5Y 7/3) and pale olive (5Y 6/3) foraminifer-rich clayey nannofossil ooze. Dark layers of olive-gray (5Y 3/2 to 5Y 4/2) and olive (5Y 4/3) diatom-bearing or diatom-rich nannofossil clay are present throughout this subunit. Similar to Site 1082 (see "Litho-stratigraphy" section, "Site 1082" chapter, this volume), sediments in Subunit IA have high carbonate and organic carbon contents, which average 68 and 2.3 wt%, respectively (see "Organic Geochemistry" section, this chapter). The contact between Subunits IA and IB is relatively sharp and occurs below a dark nannofossil clay layer in Cores 175-1083A-15H-6, 175-1083B-15H-3, and 175-1083D-15H-4. The boundary is marked by an increase in diatom abundance from diatom-bearing and sporadically diatom-rich to consistently diatom-rich.
Subunit IB is composed of intervals of pale yellow (5Y 7/4), pale olive (5Y 6/3) diatom- and foraminifer-rich clayey nannofossil ooze. Olive (5Y 4/3), and olive-gray (5Y 4/2) nannofossil-rich clay and nannofossil clay are present as 30- to 100-cm-thick intervals and are characterized by lower abundances of diatoms and foraminifers. Compared to Subunit IA, abundances of diatoms in the nannofossil ooze are relatively constant. Concentrations of calcium carbonate and organic carbon average 48 and 2.8 wt.%, respectively, which is slightly lower than their average concentrations in Subunit IA.
Smear-slide analyses indicate that the detrital component of the sediments in the two subunits consists of clay with rare silt-sized, angular and subangular, mono- and polycrystalline quartz, and feldspar grains. Muscovite and biotite are present in trace amounts. Grain sizes of identifiable detrital components are relatively constant throughout the two lithostratigraphic subunits. Authigenic minerals, such as framboidal pyrite and dolomite rhombs, are rare or present in trace amounts only. Carbonate minerals are generally rare to frequent in abundance. Carbonate minerals are common in the smear slides taken at Sections 175-1083A-15H-2, 50 cm, and 22H-6, 80 cm, and are associated with nannofossil ooze.
Smear-slide analyses of sediment from Subunit IA reveal abundant to very abundant nannofossils, abundant to common foraminifer fragments, rare siliceous sponge spicules, and trace amounts of radiolarians and silicoflagellates. Diatom abundances vary from common to barren. The relative abundance of the biogenic components changes frequently within any given core. Individual intervals are between 30 and 250 cm thick. The intercalated dark olive-brown and black clay intervals have distinctly lower abundances of biogenic components and occasionally show higher abundances of silt-sized mono- and polycrystalline quartz grains. Smear slides from the darkest layers contain common amorphous brown aggregates of organic matter. In Subunit IA, dark layers have consistently higher abundances of diatoms compared with the adjacent lighter colored intervals. In Subunit IB, abundances of diatoms are lower within the dark layers than within the adjacent lighter colored intervals. Furthermore, nannofossils and foraminifer fragments are, in general, less abundant in Subunit IA than in Subunit IB. These relationships suggest varying dilution of the biogenic component by clastic detritus and possible coupling among diatom productivity, abundance of organic matter, and the dissolution of calcareous microfossils.
Color data were measured every 4 cm for Holes 1083A, 1083B, and 1083D. At Site 1083, total reflectance values range between 30% and 65% (Fig. 2). Total reflectance records for the three holes are very similar, except for the upper 50 mbsf of Hole 1083A where core recovery was poor. Variations in total reflectance generally reflect the relative proportions of calcium carbonate to clay. There is a similarity between the top 70 mbsf at Site 1083 and the top 100 mbsf at Site 1082 (see "Lithostratigraphy" section, "Site 1082" chapter, this volume) in the total reflectance data. Using the age model (see "Biostratigraphy and Sedimentation Rates" and "Paleomagnetism" sections, this chapter), high total reflectance values between 25 and 50 mbsf at Hole 1083D correspond to marine oxygen-isotope Stages 9 and 11; this is also observed at Site 1082 (see "Lithostratigraphy" section, "Site 1082" chapter, this volume). Marine oxygen-isotope Stage 19 can also be identified from the peak in the total reflectance values at 68 mbsf (Holes 1083A, 1083B, and 1083D) and is supported by the identification of the Brunhes/Matuyama magnetic reversal at this stratigraphic position (see "Paleomagnetism" section, this chapter). The total reflectance between 150 and 200 mbsf at all three holes exhibits a pronounced cyclicity (see Fig. 2). The age of this interval is constrained by the identification of the Olduvai (1.99 Ma) and Gauss (2.6 Ma) magnetic reversals (see "Paleomagnetism" section, this chapter) and suggests that the cyclicity in the total reflectance record is on the order of 40 k.y. These preliminary results suggest that total reflectance may be used as a high-resolution stratigraphic tool to reconstruct glacial/interglacial cycles.