COMPOSITE DEPTH SCALE FOR SITE 1098

We examined several data sets initially to see which data could be most easily correlated from one hole to the next. We envisioned creating independent composite depth scales using different data sets in the hope that the scales would be very similar and that a best-fit composite depth scale could then be derived. Unfortunately, correlation of several of the data sets from one hole to the next was difficult and in some cases no convincing preliminary mcd scale could be produced. This was especially true for the color reflectance data (lightness [L*], the chromaticity parameters a* and b*, and the 500-nm wavelength signal) and the paleomagnetic inclination, though each of these data sets have a few depth intervals where anomalies can be correlated. The magnetic susceptibility records, on the other hand, contain anomalies that can clearly be correlated from hole to hole over nearly the entire cored interval (Figs. F2, F3, F4, F5). The gamma-ray attenuation (GRA) density (Figs. F6, F7, F8, F9), magnetic intensity (Figs. F10, F11), and the color reflectance parameter a* (Fig. F12) data provide several intervals that correlate very well, though not nearly as well overall as the susceptibility data. Hence, our preferred composite scale (Table T2) was generated using the susceptibility data. We then applied the mcd scale to the other data sets to confirm that their correlatable anomalies were indeed properly correlated.

Below, we outline the features (lithologic units and anomalies from susceptibility, GRA density, color reflectance, and paleomagnetic inclination and intensity) that we think should be aligned within any composite depth scale. Our preferred composite depth scale achieves this goal.

Two distinctive lithologic subunits (IA and IB) were identified by the Leg 178 sedimentologists (see the "Palmer Deep" chapter in Barker, Camerlenghi, Acton, et al., 1999). The contact between these subunits, as well as two other prominent lithologic contacts, were correlated and used as tie points in the composite depth scale. These contacts are described below, and their positions in each hole are presented in Table T4.

Subunit IA/IB Contact

Subunit IB, which contains more clastic material (ice-rafted debris and pebbles) than the overlying Subunit IA, was defined as occurring from Section 178-1098A-6H-3, 60 cm, to the base of Hole 1098A and from Section 178-1098C-5H-4, 50 cm, to the base of Hole 1098C (Barker, Camerlenghi, Acton, et al., 1999). The boundary between Subunits IA and IB is a zone less than ~20 cm wide, where laminated diatomaceous mud at the base of Subunit IA grades into silty clay with ice-rafted debris and pebbles of Subunit IB. The Subunit IA/IB contact can be correlated between cores with an accuracy of about ±15 cm.

Upper Contact of the Lower Laminated Interval

A laminated diatomaceous mud, ~340 cm thick, occurs at the base of Subunit IA. This lower laminated (LL) interval spans 345 cm in Hole 1098A (interval 178-1098A-6H-1, 15 cm, to 6H-3, 60 cm) and 330 cm in Hole 1098C (interval 178-1098C-5H-2, 25 cm, to 5H-4, 50 cm). In Hole 1098B, only 296 cm of the LL interval was recovered (interval 178-1098B-5H-5, 102 cm, to the bottom of the hole) during coring, which did not penetrate the lower portion of this interval. The upper boundary of the LL interval can be correlated between cores with an accuracy of about ±15 cm.

Homogeneous to Laminated Contact

The distinctive homogeneous to laminated (HL) contact between a laminated diatom ooze and underlying very homogeneous massive diatom ooze (a turbidite unit) occurs within the upper portion of Subunit IA in all three holes (Sections 178-1098A-4H-2, 80 cm [23.20 mbsf]; 178-1098B-3H-6, 40 cm [23.40 mbsf]; and 178-1098C-3H-5, 8 cm [24.28 mbsf]). The contact is sharp and can be easily identified in the core photographs. This contact (at 24.92 mcd), which can be correlated between cores with an accuracy of ±2 cm, provides a key constraint for the composite depth scale.

The magnetic susceptibility record was by far the easiest to correlate. Correlations established using the susceptibility record thus served as the basis for construction of the composite depth scale (Table T2) and the splice (Table T3). Characteristics of susceptibility data important in the correlation are described below.

1. The interval with the largest uphole decrease in susceptibility from Hole 1098C (interval 178-1098C-5H-4, 124-138 cm) correlates with the similar interval within Hole 1098A (interval 178-1098A-6H-3, 130-142 cm) (Fig. F2). In this interval, the susceptibility decreases from 2040 to 50 in Hole 1098A, whereas it decreases from 2860 to 920 in Hole 1098C (all susceptibility values are given in raw meter values, which can be converted to SI volume susceptibility units by multiplying by ~0.7 × 10^-5). A second decrease occurs in Hole 1098C, about 20 cm further upcore. A slightly better correlation coefficient is obtained by correlating the Hole 1098A decrease with this higher decrease in Hole 1098C (correlation coefficient of 0.86 vs. 0.75 for our preferred correlation), but this misaligns the lithologic Subunit IA/IB contact by ~20 cm.
2. Susceptibility anomalies in the lower laminated intervals (43.19-43.50 mcd) are difficult to correlate with one another (Fig. F2). The susceptibility within this interval is ~10-50 times less than in the nonlaminated intervals. The largest anomalies are associated with dropstones (e.g., the two peaks at Section 178-1098A-6H-1, 48 cm, and 6H-1, 92 cm, occur precisely where two dropstones are present; see the core photo in "Site 1098 Core Descriptions" in Barker, Camerlenghi, Acton, et al., 1999).
3. Above 40 mcd, correlations between holes consistently give correlation coefficients >0.6 and often >0.8. Distinctive sets of anomalies occur throughout, which firmly establishes the accuracy of the composite depth scale (Figs. F2, F3, F4, F5). Difficulties within the 0- to 40-mcd interval arise mainly because some compression or expansion would be required to align every correlatable feature from one hole to another. An example where relative stretching within a core would be required occurs within the 34- to 40-mcd interval. Distinctive anomalies in the 34- to 36-mcd interval in Core 178-1098A-5H correlate precisely with those in Core 178-1098C-4H, but further downcore, at 36-40 mcd, other distinctive anomalies are ~4-12 cm deeper in Core 178-1098C-4H than in Core 178-1098A-5H.
4. Particularly good correlations occur at the HL contact (24.92 mcd). As with the GRA density and color reflectance parameter a* data, the susceptibility data show little variation directly below the contact and large variations above (Fig. F5).
5. The interval from ~16 to 22 mbsf is one of the more difficult to correlate (Figs. F4, F5). Above and below this interval, the correlation is clear; therefore the composite depth scale is well constrained, even within this interval.

1. Correlation coefficients are generally <0.4 for any correlation below ~30 mcd, with few exceptions (Fig. F6). One exception occurs at the very base of Holes 1098A and 1098C, where a gradual decrease in density occurs from the base of both holes to ~3 m uphole. This long-wavelength feature can probably only be correlated to within about ±30 cm (Fig. F9).
2. A distinctive sequence of anomalies, including a large positive density anomaly, at 28.5-29.6 mcd correlates well between Holes 1098A and 1098B (intervals 178-1098A-4H-4, 138 cm, to 4H-5, 98 cm, and 178-1098B-4H-1, 132 cm, to 4H-2, 92 cm).
3. The density within the very homogeneous massive diatom-ooze interval (24.9-28.0 mcd) in each hole is nearly constant (1.3-1.5 g/cm³), whereas in the overlying laminated diatom-ooze interval the values show much larger variation (1.3-2.0 g/cm³). The HL contact is thus associated with a change in the character of the density, which can be correlated well between holes (Fig. F8).
4. Correlation coefficients are <0.4 for any correlation from ~16 to 22 mcd.
5. The density records from ~2 to 16 mcd correlate well (correlation coefficients of 0.4-0.6) between all three holes.
6. The anomalies near the mudlines of Holes 1098A and 1098B correlate well, but their correlation with Hole 1098C is poor, probably owing to some coring disturbance (Fig. F7).

1. No unambiguous match can be made with either long- or short-wavelength anomalies from the L*, b*, or 500-nm data, except at the mudline. At the mudline, all three color reflectance parameters correlate very well between Holes 1098A and 1098B only. In general, the best correlation coefficients were <0.4. By smoothing the data, the correlations can be improved a little but overall they are quite poor.
2. Correlation of the color reflectance parameter a* data is mediocre but good enough to illustrate that the composite depth scale aligns distinctive anomalies from one hole to another. In general, the a* data give correlation coefficients of 0.3-0.6. Particularly good correlations are present at the HL contact and a few meters above and below. As with the GRA density data, the a* data show little variation below the contact and large variations above (Fig. F12).

Because the data were only collected every 5 cm and because the cryogenic magnetometer averages over about a 10-cm-long interval, the resolution of the magnetic intensity data is lower than for the susceptibility data. Even so, several features of these data are noteworthy:

1. Correlation coefficients are typically >0.5 for the upper 39 m but correlation is poor below this (Fig. F10).
2. An extremely good correlation (correlation coefficients >0.9) of a distinctive set of intensity anomalies occurs in the interval from 33.3 to 39.5 mcd (Fig. F11)
3. Above 24 mcd, the correlation of intensity data between Holes 1098B and 1098C is much better than between either of these holes and Hole 1098A (Fig. F10).

Correlation is poor over the entire cored interval, probably owing to coring deformation (shearing occurs on the outer part of the core as the piston corer is shot into the sediments) and measurement artifacts associated with the weakly magnetized sediments (Brachfeld et al., 2000). As with the intensity data, magnetic inclination correlation between the upper parts of Holes 1098B and 1098C is better than correlations obtained from either of these holes and Hole 1098A. However, none of the features in the inclination data are distinctive enough to help constrain the composite depth scale.

1. The large uphole decreases in susceptibility that occur in a narrow interval near the base of Holes 1098A and 1098C are aligned. Similarly, this scale aligns the largest GRA density anomaly, which occurs within the same interval.
2. The boundary positions between Subunits IA and IB recovered in Holes 1098A and 1098C agree to within 1 cm.
3. The upper boundary of the LL interval in Hole 1098A is 7 cm higher than in Hole 1098B and 15 cm higher than in Hole 1098C. Because the thickness of the LL interval varies between holes, improving the alignment of this boundary would misalign other features that are more distinctive.
4. An extremely good correlation (correlation coefficient >0.9) of magnetic intensity data in the interval from 33.3 to 39.5 mcd is obtained. Offsets >15 cm would degrade this good correlation significantly (correlation coefficients would decrease by ~0.2).
5. The HL contact correlates within ±2 cm in all three holes. Similarly, the distinctive anomalies in GRA density, color reflectance parameter a*, and susceptibility at this contact all provide confirmation that the composite depth scale is accurate and can be precisely constrained in this interval.
6. The composite depth scale requires overlap of 44 cm between Cores 178-1098B-4H and 5H. This overlap can be explained, perhaps entirely, by the 40-45 cm of disturbed sediment at the top of Core 5H.
7. Core 178-1098C-1H overlaps Core 2H by 14 cm. Again, the top 70 cm of Core 2H is disturbed, so this small overlap is not unexpected.
8. Core 178-1098C-1H is shifted up 10 cm from its mbsf depth. This shift can be explained by the 30 cm of coring disturbance that occurs at the top of this core.
9. The color reflectance (L*, a*, b*, and 500-nm signal), susceptibility, and density data sets from the mudlines in Holes 1098A and 1098B agree very well. Owing to coring disturbance of the mudline in Hole 1098C, which is visible in the core photo, the data sets from Hole 1098C correlate poorly with those from Holes 1098A or 1098B.
10. Overall, there are numerous characteristic anomalies in the magnetic susceptibility data that correlate well from one hole to another.

We have selected intervals for the splice using the susceptibility record and core photos exclusively (Table T3). Our splice avoids using core tops, voids, and data gaps when possible. We also avoided using data from Section 178-1098B-3H-3, because the susceptibility record for this section contained high-frequency noise not present in core sections above or below.

The splice was constructed from the susceptibility record and is not optimized for other data sets. It should, however, give representative composite records for the other data sets. In rare cases, the other data sets may have missing values within the spliced intervals, owing to the peculiarities of the instruments used during the cruise or the selection criteria used to cull data (e.g., interval 36.9-38.8 mcd for the spliced intensity record in Fig. F11). For example, an interval that may have been slightly disturbed by coring may have been considered unacceptable for paleomagnetic remanence measurements but acceptable for magnetic susceptibility, color reflectance, and other measurements. Another artifact that may occur in the composite records for the other data are discontinuities or jumps located at the splice tie points.

Spliced data sets for color reflectance parameter a*, GRA density, magnetic intensity, and susceptibility are included in Tables T5, T6, T7, and T8. Other data sets converted to the mcd scale can be retrieved from the ODP Janus database on the World Wide Web.

As is typically the case for composite depth scales (see references in "Introduction"), the total length of the cored interval is expanded by up to ~10% (Fig. F13). Sedimentation rates computed from the Site 1098 mcd scale will also be artificially high by up to 10%.