An age-depth model for the Quaternary section of Holes 1168A, 1170A, 1171A, and 1172A (Fig. F3) was constructed from biomagnetostratigraphic, benthic oxygen isotope (BOI), and reflectance stratigraphy datums as reported by Stickley et al. (this volume). The line represents the visual best-fit age model. Vertical error bars represent uncertainty in the datum depth intervals because of sample spacing. The biomagnetostratigraphic datums used are listed in Table T5, along with their age, depth, and interval. The reader is referred to Stickley et al. (this volume) for the BOI and reflectance stratigraphy datums as well as a description of materials and methods for these stratigraphies.
The age depth model for Hole 1168A is based on 10 nannofossil datums, one foraminifer datum, the FO of Globorotalia truncatulinoides (Table T5), and benthic oxygen isotope stratigraphy (see Stickley et al., this volume). There is good agreement between the nannofossil datums and the BOI datums down to 0.9 Ma (bottom of the BOI datums), with linear sedimentation rates (LSRs) averaging 0.9 cm/k.y. A hiatus defined by the LOs of R. asanoi, H. sellii, and C. macintyrei occurs at 10.8 mbsf and lasts ~0.4 m.y. Sedimentation rates in the lower Pleistocene are high at 2.8 cm/k.y. before decreasing across the Pliocene/Pleistocene boundary to ~0.7 cm/k.y. The Pliocene/Pleistocene boundary (1.77 Ma) is placed at ~15 mbsf (Fig. F3) based on linear extrapolation.
Site 1168 is the only Leg 189 site in which the FO of G. truncatulinoides falls within the best-fit age depth model. In Holes 1170A, 1171A, and 1172A, the FO of G. truncatulinoides falls well off the best-fit line. This disparity could be a result of the large sampling resolution (~9 m) of the currently available datums, or it may be the result of a poorly calibrated age for this event in the southwest Pacific Ocean, as a similar disparity was noted at ODP Leg 181 (Carter, McCave, Richter, Carter, et al., 1999) and Leg 182 sites (Brunner et al., 2002).
Hole 1170A displays a pattern of sedimentation (Fig. F3) similar to Site 1168. The age-depth model is based on seven nannofossil datums and is constrained by one radiolarian datum, two diatom datums, one magnetostratigraphic datum (Table T5), and BOI datums (Stickley et al., this volume). However, the nannofossil and the BOI datums only agree to ~12 mbsf. Beyond that, the nannofossil datums diverge significantly from the BOI datums, which indicates fairly linear sedimentation to ~26 mbsf. The core recovered in this interval does show significant disturbance (Fig. F3) (Shipboard Scientific Party, 2001d), which may explain the disparity between the stratigraphies. The authors prefer the sedimentation pattern defined by nannofossils, as it shows the most similarity to Site 1171, where the stratigraphy is much less in doubt.
Sedimentation rates average ~2 cm/k.y. down to ~26 mbsf in Hole 1170A. They decrease to 0.2 cm/k.y. for ~0.3 m.y. and then increase to ~2 cm/k.y. in the lower Pleistocene sediments. The sedimentation pattern across the Pliocene/Pleistocene boundary is constrained by nannofossil, diatom, and magnetostratigraphic datums. The Pliocene/Pleistocene boundary (1.77 Ma) is located at ~33 mbsf (Fig. F3) based on linear extrapolation.
The Hole 1171A age model is based on seven nannofossil, six diatom, and five magnetostratigraphic datums (Table T5) as well as light reflectance stratigraphy (see Stickley et al., this volume). There is excellent agreement between the nannofossil datums and the light reflectance stratigraphy down to ~10 mbsf, where the light reflectance datums end. The sedimentation pattern is similar to that seen in Hole 1170A, with an average LSR in the upper Pleistocene of ~1.9 cm/k.y. The LSR decreases to ~0.7 cm/k.y. from 1.26 Ma until the end of the Pleistocene. The Pliocene/Pleistocene boundary is located at 24.0 mbsf based on the termination of Chron C2n (Table T5).
Hole 1172A shows a similar sedimentation pattern to Hole 1168A. There is good agreement between the nannofossil stratigraphy and the BOI datums down to ~11 mbsf, where the BOI stratigraphy ends, with the exception of the LO of R. asanoi. The LO of R. asanoi falls well off the age-depth plot and is not included in the calculations of LSRs. Under the light microscope the specimens show no obvious evidence of being reworked, and the cores through this interval are not highly disturbed. Linear sedimentation rates average ~1.5 cm/k.y. down to ~20 mbsf. This interval is further confined by the radiolarian LO of Stylatractus universus and the onset of Chron C1n (Table T5). A hiatus from 1.26 to 1.59 Ma is identified at 21.71 mbsf and is similar to the hiatus noted in Hole 1168A. The nannofossil datums indicate an increased LSR (~2.5 cm/k.y.) in the lower Pleistocene sediments and across the Pliocene/Pleistocene boundary (Fig. F3), similar to the patterns seen in Holes 1168A and 1170A. There is disagreement with the magnetostratigraphic datums in the lower Pleistocene–upper Pliocene sediments, with the magnetostratigraphy placing the Pliocene/Pleistocene boundary at 17.1 mbsf, well above the nannofossil datums. The disagreement may be caused by core disturbance (Fig. F3) (Shipboard Scientific Party, 2001f). The magnetostratigraphy of Site 1172 also shows disagreement with nannofossil datums in the Miocene (McGonigal and Wei, this volume), and possible causes for this disagreement are discussed by Stickley et al. (this volume).