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doi:10.2204/iodp.proc.342.105.2014

Age-depth model and mass accumulation rates

Coring at Site U1404 recovered a 300 m thick sequence of Pleistocene to middle Eocene clay, with subordinate nannofossil ooze mainly in the Eocene/Oligocene boundary interval. Biostratigraphic and magnetostratigraphic datums from Hole U1404A (Table T15) were compiled to construct an age-depth model for this site (Fig. F15). A selected set of datums (Table T16) was used to calculate linear sedimentation rates (LSRs). Total mass accumulation rate (MAR), carbonate MAR (CAR), and noncarbonate MAR (nCAR) were calculated at 0.2 m.y. intervals using a preliminary shipboard splice rather than the sampling splice described in this volume (Table T17; Fig. F27).

Age-depth model

The main objective at Site U1404 was to recover an expanded record of upper Paleogene sediment. The expanded record recovered was somewhat younger than anticipated, comprising a 150 m thick sediment section through the lower Miocene and upper Oligocene bounded by upper and lower condensed intervals of Pliocene–Miocene and lower Oligocene–upper Eocene sediment. The lowermost 200 m of the site is a relatively expanded middle Eocene succession.

The age-depth model is tied to Pliocene–middle Miocene nannofossil datums in the upper 60 mbsf. Through the lower Miocene and upper Oligocene, nannofossils are the primary datums, but planktonic foraminifers provide tie points at critical levels. Paleomagnetic datums are the primary age control in the lower Oligocene and Eocene and are well constrained by nannofossils and radiolarians.

Linear sedimentation and mass accumulation rates

Middle Eocene LSRs are 1.5 cm/k.y., dropping to 0.2–0.3 cm/k.y. from the upper middle Eocene to upper Oligocene. LSRs increase again in the upper Oligocene (1.5 cm/k.y.) and peak at 8 cm/k.y. in the lowermost Miocene, gradually decreasing to 0.3 cm/k.y. through the lower Miocene to lower Pliocene.

Carbonate contents at Site U1404 are <10 wt% in the middle Eocene; thus, most of the MAR is driven by nCAR (Fig. F27). However, the pulsed pattern of carbonate sedimentation significantly alters the square wave pattern of MAR imparted by the LSR. In the middle Eocene, an antithetical relationship is apparent between CAR and nCAR. A general increase in MAR is primarily due to three pulses in nCAR, whereas MAR minima correspond to three pulses in CAR.

In the upper Eocene and lower Oligocene, MAR is 0.2–0.3 g/cm²/k.y. Pulses of higher carbonate content continue through the lowermost Oligocene and account for as much as 50% of MAR (Table T17). Above the lowermost Oligocene, noncarbonate components dominate mass accumulation, which peaks during the upper Oligocene (1.3 g/cm²/k.y.) and lowermost Miocene (4.5 g/cm²/k.y.).

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