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Core distortion and misfit

The distribution of offsets over the cores from an entire hole is illustrated in Figure F10. In calculating the distribution of offsets, we interpolated the difference between tie points at 5 cm intervals over the entire depth of the hole, essentially assuming that stretching and compressing of one set of data to the other was evenly distributed between tie points. While this assumption might not be exactly physically correct, the fact that tie points were closely spaced where mismatches occurred minimizes any overall bias it might impose on the data. Hole U1304A contributed much of its section to the site composite (Fig. F7) and is typical of holes that dominate a composite. Hole U1304C represents the opposite extreme of poor agreement between the site composite and the compound core images. The results of the analyses of all of the holes (18) for five sites cored during Expedition 303 are displayed in Figure F11. For eight of the holes, 80%–90% of the features in the compound core images fall with 10 cm or so of the composite site image depths. For another eight holes, 65%–80% of the compound and composite image features are within 20 cm of each other. The two outliers in Figure F11 are Holes U1304C and U1308A. Both of these holes were cored during heavy seas, and their cores were not much used in making up the composite for the site. Less than 50% of their features are within 20 cm of the composite depths.

We did not attempt in this study to quantify or categorize the nature of the sources of core misfit seen in the data. Although it would be simple to assign depth differences solely to core disturbance, there were numerous instances where this was observably not the case. In particular, it was not unusual to see reasonably uniform layer thicknesses throughout most of a core offset by a single instance of a recognizable layer having a substantially different thickness from one core to the next. Thus, local variability plays an important role in the small differences we see in mcd in different holes. It remains for a follow-on study to determine if there is some regularity in the nature of this variability that might offer insights into the sedimentation process at Expedition 303 sites.