Biostratigraphic, paleomagnetic, and other data, if available, were used to construct age-depth models for all Leg 208 sites. The models are tabulated in the site chapters and plotted with all reliable age control points from which the models were derived. The depths used were mcd for each site (see "Composite Depth"). For the "Leg 208 Summary" chapter age plots, ~300 cyclostratigraphic tie points derived from the MST data were used to align the individual sites and thereby increase the precision of the age-depth models.
Linear sedimentation rates (LSRs) were calculated for 1-m.y. time intervals, and corrected LSRs were computed to compensate for the growth rates inherent in the mcd scale. Dry density data were used to convert corrected LSRs to total mass accumulation rates (MARs). Carbonate concentration data were used to estimate carbonate MARs. The results are briefly discussed in the site chapters and in the "Leg 208 Summary" chapter. The following is a description of the modeling principles employed and computations performed.
The timescales applied to the Leg 208 biostratigraphic and paleomagnetic datums are (1) the astronomically tuned timescale for the Neogene by Lourens et al. (in press) and H. Pälike et al. (pers. comm., 2003) for the late early–late Oligocene and (2) the geomagnetic polarity timescale of Cande and Kent (1995) for the Late Cretaceous to early Oligocene (top Chron C12n) time interval. Construction of the astronomically tuned timescale was based on tuning sedimentary successions from the western equatorial Atlantic Ocean (Ceara Rise, Leg 154), Mediterranean, and equatorial Pacific (Leg 199) (Lourens et al., in press) to the La2003 (Laskar et al., unpubl. data) full numerical solution. A seafloor anomaly profile from the Antarctic-Australian plate pair was employed to complete the polarity timescale for the interval between 13 and 23 Ma because of the lack of magnetostratigraphic records for ODP Leg 154 sites. The astronomical timescale resulted in a significantly younger age of 23.03 Ma for the O/M boundary than the 23.8 Ma age preferred in previous timescales (Berggren et al., 1995b).
The biohorizons used for Leg 208 are either directly tied to the new timescale via first-order calibrations, such as the standard low-latitude calcareous plankton zonations, or can be linked to it by recalibrating them to the associated polarity timescale. The ages for the biostratigraphic and magnetostratigraphic tie points are tabulated in "Magnetostratigraphy" in "Paleomagnetism" and in "Biostratigraphy."
The precision of the shipboard Leg 208 site-specific age-depth models is limited because of the generally low biostratigraphic sampling resolution (1.5–10 m) and the varying quality of the paleomagnetic results. We applied a procedure that resulted in conservative shipboard age models, satisfying as many constraints as possible without introducing artificial or speculative features. Construction of the age-depth curve for each site started with a plot of all control points from biostratigraphy and paleomagnetism. Age and depth uncertainties were represented by error bars. Obvious outliers and conflicting datums were then masked until the line connecting the remaining control points was contiguous (i.e., without age-depth inversions). Next, an interpolation curve was applied that passed through all control points with smooth inflections. Some tie points were depth shifted within their range of uncertainty to minimize the change of slope at the inflection points and to yield the smoothest age-depth curve possible.
Paleomagnetic data, if available and reasonably well calibrated with biostratigraphic data, were usually given first priority in constraining the age-depth model in case of conflicting datums, followed by calcareous nannofossils and planktonic foraminifers. In cases of conflicting microfossil datums, we also took into account the reliability of individual datums as global dating tools, the stratigraphic reliability of fossil groups or specific datums, abundance, preservation, and reworking, if indicated, and the uncertainties associated with the first or last occurrence of a datum (first occurrence datums are generally preferred).
In each site chapter, the age models are presented as plots of original datums with error bars overlain by the smooth age model and tabulated as age-depth series. For the age plots in Table T2, in the "Leg 208 Summary" chapter, ~300 cyclostratigraphic tie points derived from the MST data were used to align the individual sites and thereby increase the precision of the age model.
Once the final age models were defined, calculating and plotting LSRs and MARs was a process that involved a number of simple interpolations and computations. The only subjective part in that process was the selection of time intervals at which the smooth age model was sampled. Instead of calculating LSRs and MARs over intervals given by the original stratigraphic control points, we decided to choose regular and conservative time intervals. As a rule of thumb, a time interval should include at least three age control points (on average). It should be kept in mind that the time series is an arbitrary choice of an average sampling rate and does not reflect the resolution of the age model equally in all intervals. However, in conjunction with a smooth age model, it presents a realistic and conservative trend in LSRs and MARs.
First, we sampled the site-specific age models at 1-m.y. intervals to obtain interpolated depths in mcd for those ages. LSRs in meters per million years were then calculated as the first derivative of the age-depth series over the selected interval. The LSRs were subsequently corrected for the expansion of the recovered sediment sequence, as represented by the ratio of the mcd scale vs. the mbsf scale (see "Composite Depth"), dividing the LSR by the appropriate growth factor for each depth interval.
In preparation for computing the MARs, dry density obtained from MAD measurements (see "Physical Properties") and carbonate concentration data (see "Geochemistry") were plotted against mcd and outliers were removed. Ages were interpolated for each depth point using the age-depth model. Dry density and carbonate records were averaged over the regular 1-m.y. time intervals. This averaging step is important to avoid a commonly introduced bias that occurs when dry density data at relatively high age/depth resolution are multiplied by LSRs calculated at a much lower resolution. Such a bias gives the impression that we know the MAR variations at a high age/time and amplitude resolution, when, in fact, associated LSR variations overwhelm those trends and may also directly compensate for the dry density variations (higher dry density typically results in lower LSR). The generally low resolution of LSRs determines the resolution of MAR variations, even if the available dry density (or carbonate) record has a much higher resolution.
MARs were calculated using the equation:
Carbonate MARs were calculated by multiplying the bulk MAR by the fractional concentration of calcium carbonate.
Finally, a step plot of LSR, total MAR, and carbonate MAR summarizes the data in each site chapter. Although a smooth plot through the midpoints of the age intervals would convey the same information, we prefer the step plot because it illustrates clearly the selected age resolution that we consider to be reasonable and conservative.