STUDIED CORES AND AGES

Cores 167-1017E-1H through 3H, which cover the top 24.9 m of the sediments, are dedicated for high-resolution study of late Quaternary paleoceanography of the California margin, and the top two of these are analyzed in this study. Recovery of the three cores was excellent, exceeding 100%. Immediately after cutting the cores into sections, the sections were tightly sealed and stored at -15°C for three months before sampling at Texas A&M University (TAMU). Correlation of gamma-ray attenuation porosity evaluator (GRAPE) and color profiles at Hole 1017E with those of other holes in conjunction with correlation of marker sand layers based on visual core descriptions (VCDs) was conducted to identify potential sediment loss at the core top and core gaps. Correlation with the other four holes suggests no significant sediment loss at the top of Core 167-1017E-1H. Details of core gap estimation and correction of meter below seafloor (mbsf) depths are described in Kennett et al. (Chap. 21, this volume).

Visual Core Description

Lithology of the studied cores were described based on visual inspection at TAMU, where cores were split and sampled, supplemented by smear-slide observation afterwards. The studied cores are composed of grayish olive (5Y 4/2 to 4/3), homogeneous hemipelagic clayey silt to silty clay with occasional intercalations of grayish olive (5Y 3/1) medium- to fine-grained sand layers (Fig. 2). Burrows are difficult to identify visually except open burrows of 0.5-1 cm in diameter, which are present in the upper 1.5 m. Carbonaceous plant remains occur at 2.3, 5.25, 10.4-11.2, 12.3-12.5, 13.4, and 13.9-14.0 mbsf, and shell fragments occur at 7.4 mbsf.

Sand layers are 1-3 cm thick, except one layer at 14.75 mbsf, which is 10 cm thick. Sand layers generally have sharp basal contacts, show normal size grading, and contain reworked microfossils and shell fragments, suggesting their turbidite origin. These sand layers are concentrated in two intervals. One is between 3.2 and 5.0 mbsf and the other is between 13.9 and 14.7 mbsf (Fig. 2), corresponding to marine isotope Stages (MIS) 2 and 4, respectively. Sand layers in MIS 2 generally contain bioclasts including foraminiferal skeletons. A sand layer at 5.07 mbsf is especially enriched in zircon, spinel, and other heavy minerals. On the other hand, a thick sand layer at 14.75 mbsf is characterized by abundant glauconite grains.

X-Ray Radiography

X-ray radiographs of the slab samples were taken using SOFTEX CMB-2 at voltage of 40 kV, current of 20 mA, and exposure time of 240 s to observe detailed sedimentary structures and fabrics. On X-ray radiographs, several types of burrows are recognized. Chondrites-like burrows that are ~1 mm in diameter, filled with less dense material (lighter color on X-ray radiographs), are heavily condensed to form a 1-10 cm thick zone (Fig. 3A). These Chondrites-like burrow zones are abundant in an interval between 0 and 1.0 mbsf, common in intervals between 1.0 and 2.0 mbsf and 4.0 and 5.3 mbsf, and also present at 11.6 mbsf (Fig. 2). Larger burrows that are as large as 1 cm in diameter and subparallel or oblique to stratification are also recognized occasionally (Fig. 3B) and schematically illustrated in the lithologic columns in Figure 2. These larger burrows occur in intervals between 0.0 and 2.8 mbsf, 7.9 and 10.3 mbsf, 11.6 and 12.5 mbsf, 13.4 and 13.6 mbsf, and 14.9 and 15.1 mbsf.

Other than these burrow intervals, clayey silt and silty clay show homogeneous to slightly mottled appearance on X-ray radiographs that may reflect intense bioturbation. These homogeneous to slightly mottled clayey silt to silty clay are further subdivided into three types based on their textures on X-ray radiographs. The first type includes those with abundant opaque filaments of less than 1 mm thick and as much as 3 cm long, which are oblique to stratification (Fig. 3C). Examination of the corresponding slab samples suggests that these filaments are composed of aggregates of framboidal pyrite. The matrix of this type generally shows transparent appearance on X-ray radiographs. The second type includes those with abundant disseminated submillimeter-size opaque particles, also considered to be framboidal pyrite (Fig. 3D). The matrix of this type exhibits a dusty appearance compared to the first type. The third type includes those without a significant amount of opaque particles or filaments, and its matrix is dusty compared to the first type (Fig. 3E). The first and second types alternate with the third type in decimeter scale below 1.5 mbsf.

The submillimeter-scale distinct parallel lamination characteristic of varves is not recognized on X-ray radiographs examined. However, millimeter-scale faint lamination is recognized at 3.2, 4.0, 5.2, 6.2, 7.4, 9.4, 10.0, 12.3, 14.1, and 14.6 mbsf (Fig. 3F), which occur within Type 3 homogeneous clayey silt to silty clay. In addition, sharp contacts of possible nondepositional surfaces are recognized at 2.55, 2.75, and 9.0 mbsf. These faint lamination and sharp contact surfaces are considered to be formed by winnowing of bottom current, as will be discussed later.

An age model for Hole 1017E has been constructed by Kennett et al. (Chap. 21, this volume) based on AMS ¹⁴C dates of mixed planktonic foraminifers in the upper part and correlation of ¹⁸O profile of Globigerina bulloides from Hole 1017E with the ¹⁸O curve of Martinson et al. (1987) in the lower part of the sequence. We adopted their age model for the part younger than 82 ka (Fig. 4). Sediment ages between the datums are estimated by linear interpolation. Linear sedimentation rates during the last 82 k.y. range from 2.7 to 31 cm/k.y. Low linear sedimentation rates of 6.6 cm/k.y. and 2.7-6.9 cm/k.y. correspond to stratigraphic intervals between 2.74 and 2.89 mbsf (12.2-14.4 ka) and 12.24 and 12.85 mbsf (59.0-70.8 ka), respectively. These estimated low linear sedimentation rates are suggested by sediment features. We found a possible erosional surface approximately at 2.75 mbsf, which may represent a small hiatus corresponding to the onset of the Younger Dryas, and a faintly laminated sandy interval between 12.4 and 12.2 mbsf, which may represent strong winnowing.