SEDIMENTATION AND ACCUMULATION RATES

To determine sedimentation rates, one 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 of reversal boundaries.

Where biostratigraphic datums are used, the chief uncertainty arises from the fact that, with a limited amount of time for study, many datums are determined among widely separated samples. 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, which of course increases as the uncertainty in the datum ages increases. Our aim is to use a prime set of datums, distributed less than 2 m.y. apart, and to determine these datums in all sites to within one section (1.5 m) or better.

Sedimentation rates (in meters per million years) were estimated from age-depth plots by drawing best-fit lines through all the biostratigraphic or paleomagnetic data over successive depth intervals (i.e., by drawing straight-line segments 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. At a sedimentation rate of 20 m/m.y., datums spaced at 9.5-m intervals would only allow breaking the accumulation rates into roughly 4-m.y. increments, if we aim for an uncertainty better than ~20%.

Bulk sediment mass accumulation rates (MARs) (in grams per square centimeter per thousand years) are calculated from linear sedimentation segments and dry bulk density data (grams of dry sediment per wet volume; in grams per cubic centimeter) (see "Index Properties" in "Physical Properties"). Only those samples with both dry bulk density and nearby carbonate data (within 4 cm) were used to calculate MARs for the carbonate and noncarbonate fractions. Carbonate MARs were calculated by multiplying bulk sediment MARs by percent carbonate, whereas the noncarbonate MARs represent the difference between bulk and carbonate MARs. Ages were interpolated for all samples based on the linear sedimentation rate segments. Higher-frequency variations in percentage of carbonate obviously imply that higher-frequency variations in accumulation of either carbonate or noncarbonate are superimposed on relatively stable long-term accumulation rates. Variable preservation of the carbonate fraction may also play a role.

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