SEDIMENTATION AND ACCUMULATION RATES

Age-depth  models were developed using both biostratigraphic and paleomagnetic control points presented in the ³Biostratigraphy² and ³Paleomagnetism² sections. Typically, preference was given to paleomagnetic data because of their higher sampling resolution, then to nannofossils over foraminifers because of generally better stratigraphic resolution. Thus, some subjectivity was introduced in the construction of the model with the objective of providing an accurate general trend. A Stineman function was fit to all data points for a local smoothing effect (geometric weight on current point and 10% of data range). This smooth curve usually presented a reasonable average  when different age datums from the same core catcher sample were obtained.  If the smooth curve showed abrupt changes in slope as a result of the sampling error (sampling was usually every 10 m), apparent outliers were excluded from the model

The smoothed age-depth curve was sampled every 5 m to provide the final model for each site. For each depth point x (m), linear sedimentation rates (LSR) were computed for the 10 m window defined by the adjacent depth points:

LSR = (depth(x-5) - depth(x+5))/(age(x-5) - age(x+5)) [m/myr]

For the calculation of mass accumulation rates (MAR), dry density data (DD = dry mass/wet volume [g/cm^3]) and % carbonate data (%CARB) were also interpolated every 5 m. Total mass accumulation was then computed as

MAR(total) = DD * LSR/10 [g/cm^2/kyr]

and carbonate accumulation rate as

MAR(carbonate) = %CARB/100 * MAR(total)

The long-term trends in LSR and MAR are quite accurate and are not likely to change significantly based on future work. However, it should be kept in mind that LSR and MAR may change significantly (orders of magnitude) over short intervals (e.g., orbitally controlled glacial or monsoonal cycles) that are not resolved with shipboard sampling and analysis. Also, despite the use of a smoothed age-depth model, the preliminary nature and coarse sampling rate may cause apparent changes in rates that should not be over-interpreted.