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doi:10.2204/iodp.proc.342.107.2014

Age-depth models and mass accumulation rates

Coring at Site U1406 recovered a 281 m thick sequence of Pleistocene to middle Paleocene nannofossil ooze with varying amounts of clay and biosiliceous material (mainly radiolarians). Sedimentation rates are relatively consistent at ~3 cm/k.y. for the lower Miocene to upper middle Eocene but are lower (~0.5 cm/k.y.) through the lower middle Eocene and the Paleocene. The lower Miocene to middle Eocene succession appears to be stratigraphically complete at the resolution of shipboard biostratigraphy, but a significant hiatus is apparent between the middle Eocene and uppermost Paleocene.

Biostratigraphic and magnetostratigraphic datums from Hole U1406A (Table T15) were compiled to construct an age-depth model for this site (Fig. F23). A selected set of datums (Table T16) 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 T17; Fig. F24).

Age-depth model

The age-depth model is tied to Pleistocene to lower Miocene nannofossil datums in the upper 10 mbsf. Through the lower Miocene and Oligocene, the paleomagnetic datums are used as the primary tie points for the age-depth correlation. Paleomagnetism and nannofossil datums agree well through this interval. A series of radiolarian and foraminifer datums in the Miocene suggest an alternative correlation and lower LSR, but these are at odds with a cluster of well-constrained nannofossil and paleomagnetism datums at ~17 Ma. The persistence of Chron C10r uphole to 146.99 mbsf indicates that there is a short hiatus in the upper Oligocene (~26–28 Ma). Below this level, the age model is tied to nannofossil and paleomagnetism datums until the lower middle Eocene. Nannofossil and radiolarian datums are the primary tie points for the lower Eocene and Paleocene and delimit a significant hiatus that spans the entire early Eocene, possibly including the Paleocene/Eocene boundary (48–56 Ma).

Linear sedimentation rates

Below the condensed Pleistocene–Miocene interval in the upper 10 mbsf, LSRs in Hole U1406A are highest in the lower Miocene and Oligocene/Miocene boundary interval (up to ~3.0 cm/k.y.), relatively high in the Oligocene to upper middle Eocene and upper Paleocene (0.7–0.8 cm/k.y.), and low in the lower middle Eocene (0.2 cm/k.y.).

Mass accumulation rates

MAR at Site U1406 increases in the upper Paleocene from 0.5 to 1 g/cm²/k.y., predominantly driven by changes in CAR. MAR is relatively low (0.3 g/cm²/k.y.) during the middle Eocene and increases to 0.8 g/cm²/k.y. in the upper middle Eocene. CAR and nCAR are roughly equivalent during this interval, as carbonate content during this interval is close to 50 wt%. MAR increases across the Eocene/Oligocene boundary interval, largely because of increasing rates of noncarbonate accumulation, but is accompanied by increasing carbonate accumulation. Following the hiatus in the upper Oligocene, MAR increases in a double peak spanning the Oligocene/Miocene boundary. In the upper Oligocene, MAR peaks at 2 g/cm²/k.y., decreases to 1 g/cm²/k.y. in the uppermost Oligocene, and then peaks at 1.6–2.0 g/cm²/k.y. in the lowermost Miocene. Following the double peak at the Oligocene/Miocene boundary, MAR decreases to 0.5 g/cm²/k.y. in the lower Miocene and then falls to <0.1 g/cm²/k.y. for the remainder of the sequence.

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