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doi:10.2204/iodp.proc.302.202.2008

Initial composite record

During ACEX, overlapping recovery in the near-surface sediments (~30 mbsf) was achieved through drilling at three of the four sites visited during the expedition (Sites M0002–M0004). Only a core catcher sample was recovered from Site M0001A, and this site is not included in any of the integrated depth scales. The seafloor was captured in a single core, 302-M0004C-1H, and is confirmed by the lithologic character and microfossil abundances found in the sediment at the top of this core (see the “Sites M0001–M0004” chapter). Although Core 302-M0003A-1H was intended to sample material at the seafloor, clear stratigraphic ties indicate that the material in this core was recovered below 3.5 mbsf.

In the uppermost 18 mbsf, the published composite record relies on material from two holes (M0003A and M0004C) cored using the APC. Below 18 mbsf, a short interval from Core 302-M0004A-1H is incorporated before a tie is made to Core 302-M0002A-6X. Following this tie, the composite record incorporates only material from Hole M0002A until the base of the hole at 280.69 mbsf (Core 302-M0002A-62X). The remainder of the composite section is from Hole M0004A (Fig. F2).

Below Core 302-M0002A-6X, partial overlap in the mbsf scales existed between Cores 302-M0004C-6X through 9X and 302-M0002A-6X through 9X. Although tentative correlations using the physical and chemical properties of these sediments were made and the resulting offsets incorporated into the published mcd scale, the accuracy of these ties is diminished by the following factors:

  • Following excellent recovery in Cores 302-M0004C-1H through 4H, recovery in Cores 5X, 6X, and 7X was 15%, 61%, and 0%, respectively. This adds uncertainty to the true depths of Cores 302-M0004C-8X and 9X, which had 86% and 61% recovery, respectively.

  • Although recovery in Cores 302-M0002A-6X through 9X was good (>75%), these cores were severely affected by coring-induced disturbances. Of the 17.16 m recovered, visually disturbed intervals, tabulated when the cores were split (see Table T24 in the “Sites M0001–M0004” chapter), account for 11.18 m, roughly 65% of the recovered material.

  • Neither paleomagnetic inclination nor whole-core petrophysical measurements provide definitive ties between these cores, which is possibly related to the poor recovery and diminished core quality.

Despite these problems, the best efforts were made to tie these cores into the published composite depth scale through stratigraphic correlation. This resulted in a large offset (4.57 m) being applied to Core 302-M0002A-7X. This offset was gradually diminished through the subsequent cores from Hole M0002A and was gone by Core 302-M0002A-14X. Below Core 302-M0002A-14X, offsets in the published composite depth scale are solely defined by the amount of recovery versus penetration. Unless >100% recovery was acquired in the previous core, the tops of cores are assumed to be coeval with the top of the cored interval. However, when a core had >100% recovery the subsequent core was shifted down to the base of the recovered interval. In the absence of overlapping recovery, the method adopted below Core 302-M0002A-14X is a logical approach, keeping the composite depth scale as close to the original mbsf scale as possible.