RESULTS

Strontium isotope ratios of bulk samples from Holes 1183A and 1186A (Table T1) are plotted vs. depth (Figs. F1, F2) and converted to ages by plotting Sr isotope values on the LOWESS curve (McArthur et al., 2001) (Fig. F3). Age-depth curves generated using these age estimates are compared with nannofossil biostratigraphic datums for Holes 1183A and 1186A in Figures F4 and F5, respectively.

Strontium isotope values in Hole 1183A separate into three distinct stratigraphic groupings (Table T1; Figs. F1, F4):

  1. Between ~1130 and ~1112 meters below seafloor (mbsf), 87Sr/86Sr values suggest a late early Aptian (118.6 Ma) to early Albian (109.9 Ma) age; from ~1130 to ~1118 mbsf values decrease from 0.707377 to 0.707302 and then increase to 0.707361 at 1112.36 mbsf;
  2. Between 1111 mbsf and 1091 mbsf 87Sr/86Sr values lie between 0.707436 and 0.707454 and suggest an early (85.2 Ma) to late (84.5 Ma) Santonian age;
  3. Between 1079 and 985 mbsf, values increase from 0.707640 to 0.707830, consistent with a late Campanian (74.4 Ma) to latest Maastrichtian (65.0 Ma) age.

Three anomalously high 87Sr/86Sr values were measured in Samples 192-1183A-50R-1, 133-136 cm (87Sr/86Sr = 0.707744), 50R-2, 37-40 cm (0.707945), and 54R-1, 50-51 cm (0.707752) (Fig. F1; these values are not included in other figures).

Strontium isotope values in Hole 1186A correspond closely to the youngest stratigraphic group recognized in Hole 1183A. Values increase linearly from 0.707617 at 930 mbsf to 0.707845 at 832 mbsf, and, as in Hole 1183A, the highest sample(s) have slightly lower Sr isotope values than a subjacent sample (Table T1; Figs. F2, F5). In both cases the apparent latest Maastrichtian decrease is small and results from higher values in only one or two samples; however, latest Maastrichtian decreases have also been reported in Nelson et al. (1991), Martin and Macdougall (1991), Vonhof and Smit (1997), McArthur et al. (1998), and MacLeod et al. (2003). Predicted ages from 87Sr/86Sr for the studied section in Hole 1186A range from late Campanian (75.4 Ma) to latest Maastrichtian (65.0 Ma).

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