(histogram at 116.51 mbsf) (Fig. F7). Muds in the bioturbated facies contain much more clay than in the laminated facies, in addition to the small admixture of sand.
Many parts of the recovered section show approximately orbital-scale cyclicity in texture, composition, and physical properties. Selected intervals will be described from the top down, from Quaternary to upper Miocene.
Marine isotope Stages 1-11 were identified in the upper 9 m of Holes 1095A and 1095D on the basis of lithologic description and the occurrence of the diatom Hemidiscus karstenii and the radiolarian Stylatractus universus. The lithologic cyclicity observed down to Stage 7 by Pudsey (2000) and O'Cofaigh et al. (in press) in piston cores from the other drifts, and by Lucchi et al (in press) in distal cores from Drift 7 continues in diatom percentage and silt/clay ratio down to Stage 11 (Fig. F5). Interglacials are browner in color, have lower magnetic susceptibility, and are diatom bearing, bioturbated, and sandy. Glacials are gray and almost barren of diatoms and have higher and more variable magnetic susceptibility. Glacial Stages 2-4 and the upper part of Stage 6 are laminated, but below Stage 7 both glacials and interglacials are bioturbated and contain a few percent unsorted sand (Fig. F5). Very little biogenic carbonate in the form of foraminifers or calcareous nannofossils is present at Site 1095. This contrasts with shallower sites on the drifts where carbonate contents reach 10%-15% in the upper part of Stages 5 and 7 (Pudsey, 2000).
This interval consists of alternating thin bioturbated units and thicker laminated units (Fig. F6). Bioturbated units contain 3%-20% of unsorted sand, whereas laminated units contain 0.6% or less of well-sorted very fine sand (possibly some turbidites were inadvertently sampled). Silt laminae are sharp based and graded and vary in thickness from <1 mm up to 5 cm (most are <1 cm). Thickness and abundance variations in silt laminae have not been studied in detail. There is a gradual downcore increase in diatom percentage, but detailed data on Core 178-1095A-10H show diatom percentage is not related to the alternation of sedimentary structures (Fig. F6). There is no obvious difference in diatom preservation between the bioturbated and laminated facies.
The grain-size data in Figure F6 show the bioturbated samples are sandy but deficient in coarse silt (histogram at 79.14 mbsf). Laminated samples, even from the fine-grained tops of turbidites, have a mode in the medium to coarse silt range (histogram at 79.81 mbsf). There is thus considerably more silt but less sand in the laminated facies, and the bioturbated facies cannot be derived simply from mixing of the laminated facies. This implies very marked changes in sediment supply.
The difference between the "obvious wiggles" and the results from spectral analysis of the data is discussed below.
This interval also consists of alternating bioturbated and laminated units, but without evident (to the human eye) regular periodicity in sediment type or physical properties (Fig. F7). Bioturbated units contain 2%-5% of unsorted sand, whereas laminated units contain 0%-0.2% of well-sorted very fine sand. Silt laminae vary in thickness from 2 mm to 1 cm; they have very sharp bases and sharp tops, that is, they do not grade smoothly into the overlying mud. Most are very dark gray, though very thin light gray silts occur from 110 to 110.4 and from 127.4 to 127.9 mbsf. There is a gradual downcore increase in diatom percentage, which is apparently unrelated to the alternation of sedimentary structures, with no obvious difference in diatom preservation between the facies.
Grain-size data show that muds in the laminated facies have a silt mode at ~6 (histogram at 116.51 mbsf) (Fig. F7). Muds in the bioturbated facies contain much more clay than in the laminated facies, in addition to the small admixture of sand.
This part of the section sees a return to cyclicity in biogenic content, with greener bioturbated units (containing up to 50% diatoms and 11% unsorted sand) alternating with grayer laminated units (7%-12% diatoms and 0%-0.2% sand) (Fig. F8). This cyclicity is well shown in color reflectance but is less clear in magnetic susceptibility. Note also that not all the sandy parts of the cycles are diatom rich (e.g., 239.7 mbsf). Grain-size analysis reveals a mode near the silt/clay boundary in the bioturbated samples and a weak mode in the medium silt range in the laminated samples (histograms in Fig. F8). One sample at 239.68 mbsf in a laminated (Facies L1) unit contains 11.3% of well-sorted very fine sand, exemplifying the difference between turbiditic and hemipelagic/ice-rafted facies.
The lithologic "cyclicity" is far from regular from 235 to 300 mbsf. Sandy, green, bioturbated intervals typically 0.3-1.0 m thick are interbedded with gray laminated intervals 0.5-5.0 m thick. Using the magnetostratigraphic timescale of Shipboard Scientific Party (1999), "cycle" length ranges from ~20 to 110 k.y.
