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doi:10.2204/iodp.proc.342.112.2014

Age-depth model and mass accumulation rates

At Site U1411, we recovered a 255 m thick sequence of Pleistocene to upper Eocene clay and nannofossil clay. A relatively thin Pleistocene sequence overlies a lower Miocene to middle Oligocene succession, followed by an expanded lower Oligocene through upper Eocene succession. The Eocene–Oligocene boundary transition has sedimentation rates >1.5 cm/k.y. and as high as 5.0 cm/k.y. at the boundary.

Biostratigraphic datums and magnetostratigraphic datums from Hole U1411B (Table T3) were compiled to construct an age-depth model for this site (Fig. F19). A selected set of datums (Table T13) was used to create an age-depth correlation and calculate 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 T13; Fig. F20).

Age-depth model

The age-depth model is tied to Pleistocene nannofossil and paleomagnetic datums in the upper 10 m of Hole U1411A. Nannofossil and paleomagnetic datums also provide the primary tie points that span the transition from the condensed Miocene to the expanded Oligocene sequence. At ~92 mbsf, we infer a hiatus of ~1.5 m.y. based on nannofossil and foraminifer datums and the observation that the Chron C11r/C12n and C12n/C12r boundaries are not clearly identified in the shipboard paleomagnetic data and appear unusually condensed with respect to the background sedimentation rate. The presence of a hiatus at this interval is consistent with a 10 m broad zone of depressed NGR values in Cores 342-U1411B-10H and 11H and a sharp lithostratigraphic contact in Section 342-U1411B-10H-5. For the interval that spans the Eocene/Oligocene boundary, the age-depth model is tied to paleomagnetic datums in the upper part and planktonic foraminifer and nannofossil datums in the lower part. All three datum types agree very well through this interval.

Linear sedimentation rates

Hole U1411B comprises four distinct phases in LSR:

1. A Pleistocene interval with moderate LSR of 1.27 cm/k.y.;

2. A condensed Pleistocene to upper upper Oligocene interval with a very low LSR of 0.06 cm/k.y.;

3. A lower upper Oligocene to lower Oligocene interval of moderate to high LSR, averaging 1.53 cm/k.y.; and

4. Below the lower Oligocene hiatus noted above, a relatively expanded interval that extends from lowermost Oligocene to lower upper Eocene with an average LSR of 2.74 cm/k.y.

The maximum LSR of 5.02 cm/k.y., which is recorded across the Eocene/Oligocene boundary, may be an artifact of the uncertainties associated with biostratigraphic datum calibrations.

Mass accumulation rates

MARs at Site U1411 are predominantly composed of noncarbonate sedimentary components, primarily clay; biogenic silica is very scarce at this site. MAR increases to 1–1.5 g/cm²/k.y. in the upper Oligocene and rises to 3 g/cm²/k.y. immediately above a 1 m.y. hiatus in the lower Oligocene (Table T14). Carbonate content peaks in the lowermost Oligocene and briefly becomes the principal sedimentary constituent at Site U1411 from a mass accumulation perspective. MAR decreases briefly across the Eocene/Oligocene boundary, but then increases to ~4 g/cm²/k.y. during the upper Eocene.

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