The Paleogene planktonic genera Morozovella and Acarinina have similar oxygen and carbon isotope values within samples, suggesting little or no relative depth stratification between these species. The most enriched
13C values occur in the upper Paleocene planktonic foraminifer Zone P4 (nannofossil Zone NP8) interval (Fig. F1). Here, planktonic
13C values range 5‰–6‰ VPDB. The Site 1257 data exhibit a decrease in
13C to near 3‰ in the upper Paleocene and upper Eocene. From planktonic foraminifer Zone P9 (nannofossil Zone NP13) upward, the low-resolution carbon isotope record exhibits little structure and no overall trend.
The low-resolution
18O record for the Paleogene (Fig. F2) has values from approximately –2.4‰ to –2.9‰ from planktonic foraminifer Zones P4–P9 (nannofossil Zones NP7–NP13). The oxygen isotope record of Morozovella species shows an overall increase of ~1‰ from Zone P9 to Zone P13, consistent with either a 1‰ increase in local water
18O or a 5°C decrease in upper ocean temperatures (Erez and Luz, 1983) over this interval.
The low-resolution Cretaceous
13C record (Fig. F3) exhibits no certain discernible trends. The Hole 1260B
18O record (Fig. F4) shows first an increase of values from approximately –4.3‰ to –4.5‰ low in the Cenomanian (Section 207-1260B-34R-2), to approximately –3.3‰ above (Section 37R-1). The lowest planktonic
18O values we obtained in the Cretaceous black shales were found in Sample 207-1260B-34R-2, 10–17 cm. Because of the extraordinarily low values obtained here, five analyses of the species H. delrioensis and Heterohelix globulosa were made in this sample. H. delrioensis has
18O ranging from –4.5‰ to –5.0‰ VPDB, with an average of –4.8‰. H. globulosa tests in the same sample have an average
18O of –4.7‰. The average for each species is more depleted than the lowest
18O values obtained by Wilson et al. (2002) from very well preserved Turonian samples taken previously on Demerara Rise. Data from DSDP Site 144 were interpreted by Wilson et al. (2002) as indicating upper ocean temperatures 3°–6°C higher than modern sea-surface temperatures. Given the same assumptions regarding water
18O values and pH, our Leg 207
18O data suggest even higher tropical temperatures in the mid-Cretaceous.
