AGE MODEL FOR SITE 1006

We attempted to correlate the Site 1006 oxygen isotope record to the Site 806 profile as provided by Berger et al. (1993) to facilitate low-latitude Atlantic-Pacific comparisons. The stable isotope record of Site 806 was tuned by using the obliquity cycles apparent in the record (Berger et al., 1993). Site 806 is located in an area where equatorial upwelling influences are minimal; therefore, sea-surface temperatures were not affected by upward transport of cool, deeper waters.

The oxygen isotope and aragonite profiles of Hole 1006A were first used for identifying the oxygen isotopic stages (Fig. 2). Hole 1006B provided a means to fill in apparent gaps in the oxygen isotope record or condensed sections for those portions of the record that the Leg 166 Shipboard Scientific Party (1997) indicated were missing in Hole 1006A. Gaps were identified based on correlations between the magnetic susceptibility profiles of Holes 1006A and 1006B. We did not use the shipboard version of the meters composite depth (mcd) scale directly because it was obvious at the time of construction that potential errors were made and that in most places Hole 1006A was more complete than originally thought.

High-amplitude ¹⁸O variations are apparent throughout the entire record of Hole 1006A and the amplitude seems to increase within the top 17 m of sediment. U/Th-series dating and nannofossil biostratigraphy were used to constrain the assignment of stable oxygen isotopic stages, keeping in mind the position of the core breaks. The influence of the core breaks on the completeness of the record at this time is unknown. Absolute dates based on the U/Th-series were provided by Henderson et al. (Chap. 3, this volume). The three given dates indicate the position of oxygen isotopic Stages 1, 5, and 9. This gives us an immediate problem because isotopic Stage 7 seems to be missing because of the first core break (Fig. 2). Sediments from Hole 1006B were used in an attempt to fill the gap and an interval was found, evidently representing isotopic Stage 7, that was not present in Hole 1006A. Unfortunately it is not entirely clear whether the isotopic Stage 7 to 8 transition in Hole 1006B is complete because the interval used from Hole 1006B was also terminated by a core break; therefore, part of the record might still be missing. There is no distinct overlap with Hole 1006A.

The nannofossil biostratigraphy was used to further constrain the assignment of isotopic stages. Nannofossil datum levels were provided by T. Sato (unpubl. data). The last occurrence (LO) of Pseudoemiliania lacunosa indicates the position of isotopic Stage 12 in the top of Core 166-1006A-3H. Isotopic Stages 10 and 11 in the lower half of Core 166-1006A-2H and isotopic Stages 13, 14, 15, 16, and 17 in Core 166-1006A-3H were apparent. The second and third core breaks straddle the transitions between isotopic Stages 12 and 11 and Stages 18 and 17, respectively. However, inspection of the magnetic susceptibility record of the Hole 1006B shows that little or no sediment is missing at these boundaries. Core 166-1006A-4H contains isotopic Stages 18, 19, 21, and 22, the latter indicated by the LO of Reticulofenestra asanoi. The mid-Pleistocene revolution, as defined by Berger et al. (1993), can now readily be seen in the Hole 1006A ¹⁸O record at ~33 meters below seafloor (mbsf) (Fig. 2).

The lower part of the Hole 1006A record is well constrained by three excellent nannofossil events. The base of Gephyrocapsa parallela occurs in the middle of Core 166-1006A-5H and indicates the position of isotopic Stage 30. Recognition of this datum shows that the core break between Cores 166-1006A-4H and 5H has not significantly affected the completeness of the record, as all isotopic stages from 22 to 32 could be clearly recognized. The next deeper nannofossil event, the base of R. asanoi, occurs at the lower end of Core 166-1006A-5H, and the LO of large Gephyrocapsa, which indicates isotopic Stage 37, lies within Core 166-1006A-6H. Inspection of Hole 1006A shows that little sediment is missing across the core break between Cores 166-1006A-5H and 6H.

After identification of most of the isotopic stages, the Hole 1006A depth scale was ready for conversion to a time scale. Before doing so, the interval from Hole 1006B representing isotopic Stage 7 was patched into Hole 1006A to create a new depth scale (mcd) for Site 1006 (Fig. 3; Table 3). This was the only portion of the Hole 1006B record we have used, because it appeared that not much of the record in Hole 1006A had disappeared across the other core breaks. The stratigraphic tie points used to correlate to Site 806 are indicated in Table 4. A plot of linear sedimentation rates based on this age model is presented in Figure 4.