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doi:10.2204/iodp.proc.342.104.2014

Age-depth models and mass accumulation rates

Coring at Site U1403 recovered a 265 m thick sequence of Quaternary–upper Campanian clay, radiolarian clay, and nannofossil ooze with minor chert layers. Biostratigraphic datums and magnetostratigraphic datums from Hole U1403A and the base of Hole U1403B (Table T17) were compiled to construct an age-depth model for this site (Fig. F16). Selected datums (Table T18) were 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 T19; Fig. F24).

Age-depth model

The main objective at Site U1403 was to recover a complete and well-resolved upper Campanian to upper Eocene section, and the site was positioned to yield this critical stratigraphic interval at a relatively shallow burial depth. The sediment section in Hole U1403A is therefore relatively condensed through the lower Eocene to Pleistocene section (Fig. F16). The age-depth model is tied to Pleistocene nannofossil datums and Eocene paleomagnetic datums in the upper 65 mbsf. Through the middle and upper lower Eocene, paleomagnetic datums provide the primary tie points, supported by radiolarian datums in the upper 65–119 mbsf and nannofossil datums in the lower 119–140 mbsf. Radiolarian datums exhibit wide scatter through the Eocene, suggesting that they are either poorly calibrated to the GPTS or that they are diachronous in the North Atlantic. Below 140 mbsf, we relied primarily upon calcareous nannofossil datums to define the age-depth model, although radiolarians provide tie points in the lowermost Eocene. Nannofossil and planktonic foraminiferal datums are in good agreement in the lower Paleocene and Upper Cretaceous.

Linear sedimentation and mass accumulation rates

LSRs at Site U1403 show peaks between 1.0 and 1.6 cm/k.y. above background rates, which are generally <0.5 cm/k.y. MARs at Site U1403 show three distinct peaks, one each in the Maastrichtian, lower Eocene, and middle Eocene (Fig. F24). MAR peaks are driven by changes in LSR. The three MAR peaks are similar to each other in magnitude (1.55, 1.57, and 1.24 g/cm²/k.y., respectively); however, the contribution of carbonate to overall mass accumulation at Site U1403 progressively decreases toward the present, presumably in response to subsidence and superimposed changes in the local CCD. A smaller MAR maximum occurs in the lower Eocene that appears to be driven by changes in carbonate content. Accumulation of sediment between the MAR peaks occurs at background rates of 0.4 g/cm²/k.y.

Maastrichtian

In the Maastrichtian, MARs peak at 1.55 g/cm²/k.y. High carbonate content relative to the Paleocene indicates that much of this accumulation was driven by carbonate sedimentation, caused either by increased production, enhanced preservation, or a combination of both. The Maastrichtian MAR peak ends at ~68 Ma, corresponding to a decrease in LSR (Fig. F24).

Paleocene

In the Paleocene, MAR is at background levels of ~0.4 g/cm²/k.y. and generally dominated by noncarbonate components.

Lower Eocene

MAR sharply increase at the Paleocene/Eocene boundary to values ~1.6 g/cm²/k.y. This increase is driven by high rates of nCAR and may be related to climatic changes associated with the PETM (see “Lithostratigraphy” and “Geochemistry”). Lower Eocene MAR continues to increase to a maximum ~54 Ma as carbonate accumulation ramps up. Prior to the end of the peak, CAR accounts for 40% of the MAR. The lower Eocene MAR peak terminates at a hiatus at 53.5 Ma, identified by the co-occurrence of diachronous biostratigraphic datums (see “Age-depth model” and “Biostratigraphy”).

Middle Eocene

In the middle Eocene, MAR peaks at 1.24 g/cm²/k.y. at 46 Ma. The majority of the accumulation during this peak is contributed by noncarbonate components, as carbonate weight percent values fall to near 0 wt% in sediment younger than 46 Ma. MAR values step down between 45 Ma and 39 Ma and are stable at ~0.1 g/cm²/k.y. through the rest of the sequence.

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