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doi:10.2204/iodp.proc.342.106.2014 Age-depth model and mass accumulation ratesCoring at Site U1405 recovered a 307 m thick sequence of Pleistocene to upper Oligocene clay, biosiliceous nannofossil ooze, and nannofossil ooze. Biostratigraphic and magnetostratigraphic datums from Hole U1405A (Table T15) were compiled to construct an age-depth model for this site (Fig. F17). A selected set of datums (Table T16) was used to create an age-depth correlation and 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. F30). Age-depth modelThe main objective at Site U1405 was to recover an expanded record of upper Paleogene sediment. An expanded record through the Oligocene–Miocene transition was recovered, comprising a ~100 m thick sediment section with LSRs from 4 to 10 cm/k.y. The age-depth model is tied to Pleistocene to lower Miocene nannofossil datums in the upper 30 mbsf. Through the lower Miocene and upper Oligocene, a well-integrated suite of biostratigraphic and paleomagnetic datums identifies four hiatuses within the Miocene separated by four intervals of relatively high LSRs from 4 to 6 cm/k.y. Paleomagnetic and nannofossil datums identify a maximum LSR of 10.4 cm/k.y. across the Oligocene/Miocene boundary. Below 190 mbsf, the age model is based on three widely spaced nannofossil datums, which indicate that LSR decreases progressively downhole through the upper Oligocene. Linear sedimentation ratesBelow the condensed Pleistocene–Miocene interval in the upper 30 mbsf, LSRs in Hole U1405A are relatively high to very high (2–10.4 cm/k.y.) through the lower Miocene and lower Oligocene. Mass accumulation ratesMARs at Site U1405 are driven predominantly by noncarbonate components, as indicated by low carbonate contents of generally <40 wt%. A double peak in MAR spans the uppermost Oligocene (4 g/cm2/k.y.) to Oligocene/Miocene boundary (6 g/cm2/k.y). These MARs are among the highest encountered during Expedition 342. In the lower Miocene sequence, MAR is ~2 g/cm2/k.y. |
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