A 359.5-mcd-thick (312.4 mbsf) upper Paleocene (~56 Ma) to Pleistocene pelagic sediment sequence was recovered at Site 1265. A total of 90 biostratigraphic and 24 magnetostratigraphic datums (Table T17) were used to construct an age-depth model for this site (Table T18; Fig. F48). Linear sedimentation rates (LSRs), total mass accumulation rates (MARs), and carbonate MARs were calculated at 1-m.y. intervals (see "Age Model and Mass Accumulation Rates" in the "Explanatory Notes" chapter).
The main objective of Site 1265 was to recover a complete and well-resolved upper Paleocene to lower Eocene section, and the site was chosen to yield this critical stratigraphic interval at a relatively shallow burial depth. The sediment section at Site 1265 therefore includes condensed intervals and unconformities, particularly in the middle Eocene (49–40 Ma; 227–221 mcd) and in the upper Miocene (8.9–5.3 Ma; 37–33 mcd). Biostratigraphic and magnetic reversal datums are generally in good agreement, although some planktonic foraminiferal datums are offset by as much as 2 m.y. from the ages provided by nannofossil and paleomagnetic datums. The age-depth model relies primarily on paleomagnetic and nannofossil data.
LSRs range between <1 and 23 m/m.y., and total MARs range from <0.1 to 3.1 g/cm2/k.y. Total MAR fluctuations essentially represent variations in carbonate MAR. LSRs and carbonate MARs show the highest values (>1.2 g/cm2/k.y.) in the upper Paleocene to lower Eocene. LSRs and MARs in the middle Eocene to Pleistocene record are low to moderate (<1.2 g/cm2/k.y.) and are punctuated by condensed intervals and unconformities.
Noncarbonate MARs are generally low (<0.2 g/cm2/k.y.), and the small fluctuations reported here are probably representative of the analytical uncertainty rather than environmental changes. The moderately high values (0.55 g/cm2/k.y.) in the P/E boundary interval (55 to 54 Ma) are an exception and are related to the carbonate-poor intervals of a few centimeters to several decimeters in thickness (see "Lithostratigraphy" and "Geochemistry"). These short-term carbonate dissolution events are smoothed out in the MAR record as a result of our 1-m.y. sampling of the age-depth model, dictated by the limited resolution of the shipboard age-depth control points and density and carbonate data.