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doi:10.2204/iodp.proc.303306.104.2006

Composite section

Cores were initially depth-shifted on the basis of magnetic susceptibility data collected with the “Fast Track” magnetic susceptibility core logger (MSCL) soon after recovery. The correlation was refined once density, natural gamma radiation (NGR), and color reflectance data were available from the multisensor track (MST) and archive multisensor track. Magnetic susceptibility and NGR proved most useful for correlating between holes at Site U1304. Features in the magnetic susceptibility and NGR profiles are well aligned between Holes U1304A and U1304B, as these cores were not significantly stretched, squeezed, or disturbed during the coring process (e.g., Fig. F20). The offsets and composite depths are listed in Table T20. Good weather conditions during the early occupation of Site U1304 led to excellent recovery in both Holes U1304A and U1304B. Weather deteriorated as coring in Hole U1304C began, and the condition of the recovered cores was generally poor. Because of the highly disturbed nature of Cores 303-U1304C-2H, 3H, and 7H, it was not possible to correlate this core to the other holes. Ship heave also disrupted some of the cores from Hole U1304D, resulting in no correlation for Core 303-U1304D-4H. In the deeper sections of Hole U1304D, core quality was variable and some sections are well correlated with Holes U1304A and U1304B, whereas others are clearly disturbed.

The sections of core used for the splice are identified in Table T21. The spliced composite section mainly consists of sections from Holes U1304A and U1304B. We generally avoided using sections from Holes U1304C and U1304D because of core disturbance. Two short segments from Cores 303-U1304D-18H and 19H were needed in the splice to fill core breaks in Holes U1304A and U1304B. The cores from Site U1304 provide a continuous stratigraphic sequence to ~258.1 mcd with a single potential break in an 8 m thick diatom mat at the base of Cores 303-U1304A-19H, 303-U1304B-19H, and 303-U1304D-14H and the tops of Cores 303-U1304A-20H, 303-U1304B-20H, and 303-U1304D-15H (Figs. F20, F21). These cores were appended to one another at ~199.3 mcd because partial strokes in Cores 303-U1304A-19H and 303-U1304D-14H failed to cover the break between Cores 303-U1304B-19H and 20H.

A growth factor (GF) of 1.12 is calculated by linear regression for all holes at Site U1304, indicating a 12% increase in mcd relative to mbsf (Fig. F22). We used this value of GF to calculate corrected meters composite depth (cmcd) presented in Table T20 to aid in the calculation of mass accumulation rates.

We calculated sedimentation rates using paleomagnetic and biostratigraphic datums. Dinocyst datums were not used because they represent minimum ages. Linear regression provides a mean sedimentation rate of 17.8 cm/k.y. for the last 0.78 m.y. (i.e., Brunhes Chronozone) (Fig. F23). Sedimentation rates decrease to 12.2 cm/k.y. for the interval from 0.78 to 1.77 Ma, representing the period from the Brunhes/Matuyama boundary to the top of the Olduvai Subchronozone. If only paleomagnetic datums are used, the interval sedimentation rates are more variable during the upper Matuyama (0.78–1.77 Ma), ranging from 10.6 to 13.9 cm/k.y. (Table T22).

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