Age-depth models were constructed for each hole to translate depth measurements to age measurements. At all sites, age-depth models were constructed based on preliminary shipboard biostratigraphy and magnetostratigraphy. These models were then constrained using oxygen-isotope stratigraphy, where oxygen-isotope (18O) data was available, by comparing the 18O records from the sites with the standard 18O curve from Tiedemann et al. (1994). To infer the age at every sample depth, a constant sedimentation rate between dated depth points was assumed.
The shipboard age-depth model based on nannofossil horizons suggests a disagreement between the biostratigraphy and magnetostratigraphy between 90 and 120 mbsf (Shipboard Scientific Party, 1998c). Biostratigraphic data indicates that the core extends to beyond 1.0 Ma, yet there is no evidence for the Brunhes-Matayama polarity reversal, dated at 0.78 Ma, in the magnetic records (Berggren et al., 1995; Shipboard Scientific Party, 1998c).
The age-depth model for Hole 1076A was first constructed using the preliminary shipboard biostratigraphic data and subsequently revised using 18O records from the benthic foraminifer U. peregrina, which were compared with the standard 18O record from Tiedemann et al. (1994). The revised age-depth model, in fact, suggests that the upper 120 m of the core may not extend beyond 0.78 Ma and, therefore, that the shipboard biostratigraphic data may be incorrect.
Age-depth points established by correlation with the standard 18O curve of Tiedemann et al. (1994) are shown in Table T1, and age-depth points from shipboard data are provided in Table T2. The two age-depth models are plotted in Figure F4A. Comparison of the two models suggests that ages are older than biostratigraphic data in the interval from 19 to 94 mbsf, yet below this depth, ages are younger than the shipboard model. The good correlation between the standard 18O curve from Tiedemann et al. (1994) and the 18O curve generated for Hole 1076A, which also agrees with shipboard magnetostratigraphic data, suggests that the age-depth model can be assumed to be reliable until at least 94 mbsf (0.6 Ma), yet beyond this depth some caution should be taken in interpreting the results.
Because of the paraconformity in Hole 1076A at 120 mbsf, material was obtained from Hole 1077A to construct a record from the Congo Basin that extends to at least 1.2 Ma. The age-depth model for Hole 1077A was obtained from L.M. Dupont et al. (unpubl. data) and was generated using 18O data from the hole and oxygen isotope stratigraphy. The age-depth points used to constrain the model are shown in Table T3. This age-depth model is plotted in Figure F4B.
The age-depth model for Hole 1081A was initially constructed using shipboard biostratigraphic horizons and then constrained using oxygen isotope stratigraphy and comparison with the standard 18O curve of Tiedemann et al. (1994). The 18O records from the benthic foraminifer species U. peregrina and the planktonic species N. dutertrei were used for this comparison.
Shipboard records suggest that foraminifer abundance decreased greatly below 105 mbsf, yet the isotopic record only extends downcore to ~90 mbsf. Below this depth, shipboard ages and linear sedimentation rates were used to date the material. Additionally, there is evidence for some foraminifer-poor intervals in the upper 105 m of the record. Age-depth points are provided in Table T4, and age-depth points from the shipboard biostratigraphy data are given in Table T5. Both models are displayed in Figure F4C.