THE OXYGEN ISOTOPE RECORD AT SITE 975

The oxygen isotope stratigraphy from the Holocene to the upper Pliocene at Site 975 is based on the continuous record of G. bulloides allowing the identification of Isotopic Stages 1 to 69. The time-scale calibration of the oxygen isotopic stages corresponds to the orbitally derived time scales of Shackleton et al. (1990), Bassinot et al. (1994), Tiedemann et al. (1994), and Lourens et al. (1996). The age control points of the isotopic events used for this calibration are listed in Table 3. Linear interpolation was applied between two adjacent age control points, assuming a constant sedimentation rate within this time interval. The chronologies deduced from the oxygen isotope stratigraphy and from the nannofossil biostratigraphy (de Kaenel et al., Chap. 13, this volume) were carefully compared to correlate the two time scales.

The ¹⁸O record of G. bulloides is shown as a function of depth (Fig. 1A) and as a function of age (Fig. 1B). Four major intervals can be recognized, based on the amplitude of the isotopic oscillations between glacial and interglacial stages and on the frequency of the glacial periodicity. The detailed ¹⁸O-age records and the location of the sapropels are reported in Figure 2A-C. Table 1 gives the age estimations and location of sapropels for the oxygen isotope curve.

Interval I, isotopic stages 1 to 11, corresponds to the Holocene-upper Pleistocene period and has a very high ¹⁸O amplitude (average 2.9). All isotopic stages and substages are recognized, with the shape of the peaks similar to those of the classical standard curve of Prell et al. (1986). The average maximum and minimum ¹⁸O values reach 3.5 and 0.6, respectively. The periodicity of the isotopic oscillations is typically that of eccentricity (i.e., 100 k.y. cycle), as is also evidenced by the data from Sites 976 and 977 (von Grafenstein et al., Chap. 37, this volume).

Nine sapropels are present in this interval. Although sapropels 501, 502, 503, 504, and 505 were not sampled, their age was estimated from their depth position, and they are tentatively located on the oxygen isotope curve (Fig. 2A; Table 1). The sapropels of interval I occur in isotopic levels 5.5, 6.5, 7.1, 7.5, 9.3, and 11.3 (i.e., during warmer climatic episodes).

In interval II, between isotopic stages 12 and 22, the amplitude of ¹⁸O variations decreases to 1.9; it includes the middle Pleistocene and the upper part of the lower Pleistocene. Again, all the isotopic stages are well recorded at Site 975. The average maximum and minimum ¹⁸O values reach 2.5 and 0.6, respectively. The periodicity of the isotopic oscillations remains that of the eccentricity controlled 100 m.y. cycles in this interval.

The nine sapropels present in this interval have all been sampled. They occur in isotopic levels 12, 13.11, 13.3, 15.1, 15.3, 15.5, 16.3 and 18.3 (Fig. 2B; Table 1). These sapropels were deposited during warmer climatic episodes, mostly during interglacial stages. Two exceptions, sapropels 510 and 517, were deposited during warmer episodes of glacial stages 12 and 16.

In interval III, between isotopic stages 23 and 60, the amplitude of ¹⁸O variations decreases slightly to 1.6; this is because of a slight decrease of the maximum and minimum ¹⁸O values, which average 2.1 and 0.5, respectively. The periodicity of the glacial-interglacial cycles becomes that of the obliquity at 40 m.y.

Fifteen sapropels are present in this interval. They are located in isotopic stages 23, 25, 27, 29, 31, 33, 35, 37, 41, 45, and 51 on the isotopic curve (i.e., again during warm interglacial episodes Fig. 2C; Table 1).

The upper limit of interval IV corresponds to isotopic stage 61; isotopic stage 69 was identified at the base of the sampled section. The Pliocene/Pleistocene boundary is placed at 1.83 Ma, and thus is included within interval IV. This interval is characterized by the small amplitude (1.0) of the ¹⁸O variations, which is caused mostly by a weak decrease by 0.3 of the maximum ¹⁸O values (averaging 1.8) and by a weak increase by 0.3 of the minimum ¹⁸O values (averaging 0.8).

The three sapropels within this interval (534, 535, and 536) correspond to the deepest layers at Site 975. They were not sampled, but their age estimations deduced from their depth position indicate that they were deposited at the onset of Pleistocene during the interglacial isotopic stages 61 and 65 (Fig. 2C; Table 1).