To determine sedimentation rates, we must first generate an age-depth relationship. At a site with precisely determined paleomagnetic stratigraphy and with unambiguously identified chrons, accumulation rate uncertainties arise almost entirely from uncertainties in the ages and depths of reversal boundaries.
The chief uncertainty with shipboard biostratigraphic data arises from the fact that the samples that constrain the datums may be widely separated in depth. During many ODP legs, it has been necessary to reconstruct sedimentation rates using datums determined only in core catchers (i.e., within 9.5 m). The amount of uncertainty in each sedimentation-rate estimate derived in this way is related to the thickness interval over which it is averaged, divided by the combined uncertainty in the top and bottom controls. The second source of uncertainty in sedimentation rates is the age of the datums. Our aim is to use a primary set of datums, distributed <2 m.y. apart, and to determine these datums in all sites to within one section (1.5 m) or better. Where available, we have used paleomagnetic stratigraphy as our primary age control. Nannofossil datums serve as a very good backup to this stratigraphy. Other fossil groups, such as radiolarians, primarily serve as checks on stratigraphy until the ages of these biostratigraphic events are better calibrated using data from this leg.
Sedimentation rates (meters per million years) may be estimated from age-depth plots either by drawing best-fit lines through all the biostratigraphic and paleomagnetic data over successive depth intervals or by drawing straight-line segments. Inspection of the distribution of other datums about the lines suggests that it might be difficult to rigorously justify one approach vs. another, especially given the difficulty of evaluating the uncertainties in the accuracy of each datum. Age-depth plots give more direct evidence for or against rapid changes in sedimentation rates that might otherwise be artificially exaggerated by using best-fit lines through discrete intervals of data. All sedimentation rates were calculated using midpoints in the observed depth uncertainty range. In reality, each datum event has an age uncertainty that may vary from a few thousand years to a few hundred thousand years.
Bulk-sediment mass accumulation rates (MARs) (grams per square centimeters per thousand years) are calculated by multiplying linear sedimentation rate data and dry bulk density (DBD) data (in grams per cubic centimeters). The MAR of the bulk sediment was calculated for every depth where a bulk density value was measured by the onboard physical properties program. When elemental concentration data was available, the bulk MAR value was multiplied by the weight percent of an element to produce elemental MARs.