Sedimentation rates are derived from the age-depth relationship of sediments recovered at each site drilled during Leg 207. The ages are based on biostratigraphic and magnetostratigraphic shipboard data.
Where biostratigraphic data are used, the main uncertainty arises from the fact that the samples that constrain the datums may be widely separated in depth because of the limited amount of time for shipboard study. For many ODP legs, it has been necessary to reconstruct sedimentation rates by using datums determined only in core catchers (i.e., within 9.5 m). The uncertainty of sedimentation rate estimates is thus related to the sediment thickness over which they are averaged divided by the combined uncertainty in the top and bottom controls. A second source of uncertainty in the calculation of average sedimentation rates is the age of the datums. Datums used during this leg are presented in "Zonal Schemes and Taxonomy" in "Biostratigraphy." Biostratigraphic data were augmented by paleomagnetic reversals recognized in shipboard measurements. The paleomagnetic record has been refined using shore-based measurements on minicores (see "Shore-Based Interpretations of Magnetostratigraphy" in "Paleomagnetism"), but these changes have not been integrated into sedimentation rate calculations.
Age-depth plots depict the distribution of age control points as a function of depth. Depending on the data used, uncertainty in age and/or depth may be depicted. Sedimentation rates (meters per million years) may be estimated from age-depth plots (see Fig. F9), either by fitting curves to some or all available biostratigraphic and paleomagnetic data over certain depth intervals or by connecting adjacent datums with straight line segments. Connecting datums is more likely to resolve short-term changes in sedimentation rates but also might exaggerate errors. Best-fit curves, on the other hand, might be more accurate but average out short-term variation. During Leg 207, we used the latter approach.
Age-depth plots indicate that at sites drilled during Leg 207 there are distinct changes in apparent sedimentation rates and sediment age (see Fig. F9) are present. Often, these breaks coincide with lithologic and/or color changes of the sediments recovered. These breaks may reflect nondeposition, condensed intervals, postdepositional erosion, or a combination of these. Therefore, most given sedimentation and accumulation rates are minimum estimates calculated for the lithologic segments in between these breaks. Bulk sediment mass accumulation rates (MARs) (grams per square centimeters per thousand years) are calculated by multiplying linear sedimentation rate (LSR) and dry bulk density data (in grams per cubic centimeters). The MAR of the bulk sediment was calculated for depths where a bulk density value was measured by the onboard physical properties program (see "Index Properties" in "Physical Properties").