RESULTS

Trends in isotopic values through the Campanian–Maastrichtian sections recovered in Holes 1183A and 1186A are quite similar (Figs. F3, F4; Table T2). In both holes, ¹⁸O values are 0.5–1.0 lower in upper Campanian–lower Maastrichtian samples than in the upper Maastrichtian samples and the rate of change is most rapid in the middle portion of the Maastrichtian. Further, in both holes ¹³C values are near 3_VPDB throughout most of the Campanian–Maastrichtian with an excursion near the Cretaceous/Tertiary (K/T) boundary. In Hole 1183A, ¹³C values exhibit a negative excursion of ~1.5 across ~10 m of section spanning the youngest Cretaceous and oldest Paleogene rocks, whereas in Hole 1186A, the oldest Paleogene sample has a ¹³C value ~1.5 lower than the youngest Cretaceous samples (Fig. F3). Aptian–Campanian powders (XRD residues) were also analyzed from Hole 1183A. In the middle Campanian (~1080–1100 meters below seafloor [mbsf]) ¹⁸O values are lower than in the Maastrichtian. Both ¹³C and ¹⁸O values are relatively high in Aptian–Albian samples (~1100–1140 mbsf), which are near the basalt/sediment contact. The one comparable sample from near the sediment/basalt contact in Hole 1186A (upper Aptian; 948.55 mbsf) also shows relatively high ¹³C and ¹⁸O values.

In younger samples from OJP, there is a general trend toward higher ¹⁸O values upsection consistent with the long-term cooling during the Cenozoic (Fig. F3). Samples from Holes 1183A and 1186A both show a prominent positive excursion in ¹³C in the Paleocene that seems to correspond to the late Paleocene excursion in the compilation of Zachos et al. (2001). This feature as well as the trend toward higher ¹⁸O values upsection was previously observed in Hole 807C (Corfield and Cartlidge, 1993). That study documented the Tertiary record on OJP in greater detail than provided by our analyses of the XRD residues and, thus, the Tertiary record is not discussed further here.

Features in the isotopic curves for the Campanian–Maastrichtian interval in Holes 1183A and 1186A seem to be supported at each of the other four sites (Fig. F4). Average ¹⁸O values for Campanian–Maastrichtian samples vary from –1.8_VPDB (Hole 807C) to 0.5_VPDB (Hole 288A) among the holes, but in each hole there is an upsection increase of 0.5–1.0 within the Maastrichtian. Further, ¹³C values in Holes 289, 317A, 807C, 1183A, and 1186A are near 3_VPDB across most of the Campanian–Maastrichtian interval, although in Hole 288A ¹³C values average 1.6_VPDB and appear to increase through the Maastrichtian. Finally, data for Holes 289 and 807C show an ~1.5 decrease in ¹³C just below the K/T boundary (Fig. F4), similar to the record in Hole 1183A.