DISCUSSION

The marked variations in oxygen isotopes recognized in the records of Sites 999 and 1000 can be traced between the two sites, although they show a different temporal resolution. Both Sites 999 and 1000 record clearly the middle Miocene ¹⁸O increase from ~15.5 to 13 Ma (Fig. 9).

A number of features emerge from a comparison of the ¹⁸O record from Sites 999 and 1000 with globally recognized Miocene oxygen-isotope events (as defined by Miller et al., 1991; Wright et al., 1992; Miller, Mountain, et al., 1996, 1998; Abreu and Haddad, in press). At Site 1000, the ¹⁸O record contains the Miocene oxygen-isotope events Mi2, Mi3, Mi4, and Mi5 (as defined by Miller et al., 1991; Wright et al., 1992). At Site 999, in addition to the events seen at Site 1000, the oxygen-isotope record also contains events Mi1, Mi1a, and Mi1aa.

One of the more interesting features of this record is that at Site 1000 the long-term ¹⁸O increase between ~15.5 and 13 Ma shows superimposed internal quasi-cyclic structures, which appears to reflect a periodicity of 400 k.y. or 1 m.y., as described by Miller et al. (1998), reflecting the orbital eccentricity.

Because the sites are located in the tropical ocean, variations in sea-surface temperature are expected to be minor and are, therefore, unlikely to cause the observed long-term trend of ~1.4 between ~15.5 and 13 Ma, which is interpreted to reflect mainly changes in Antarctic ice volume.

The marked variations in ¹³C recognized in the Site 999 and 1000 records can be clearly correlated between the two sites (Fig. 10). The two most prominent features include the ¹³C increase near the Oligocene/Miocene boundary (see Zachos et al., 1997), observable at Site 999, from ~24.5 to 23 Ma, and, observable at both sites, the well-known interval of high ¹³C values between ~17 and 13.5 Ma (the Monterey Excursion) (Vincent and Berger, 1985). In addition, the ¹³C record from Sites 999 and 1000 shows all of the seven ¹³C maxima (CM1 through CM6) (Woodruff and Savin, 1991; Flower and Kennett, 1993b) found within the Monterey Carbon Isotopic Excursion. The ¹³C maxima occur with quasi-cyclic periods of ~400 k.y.

The Miocene was a time of unusually high accumulation of organic matter around the Pacific, Atlantic, and Mediterranean (e.g., Monterey Formation, California: Vincent and Berger, 1985; Ingle, 1981; Florida shelf: Compton et al., 1990; Tellaro Formation, Sicily: Mutti et al., in press). Woodruff and Savin (1991) suggested that the seven ¹³C maxima found within the Monterey Carbon Isotopic Excursion correspond to episodes of accumulation of especially large amounts of organic matter around the world. The enhanced burial of organic matter during intervals of high ¹³C values may have been a major factor in reducing atmospheric CO₂ levels. This is further suggested by the association of ¹³C maxima with the ¹⁸O maxima (Woodruff and Savin, 1991) at Sites 999 and 1000.

The data from this study confirm that the increase in carbon-isotope values at 17 Ma leads the first major cooling step in Antarctica at ~15.5 Ma (e.g., Vincent and Berger, 1985; DePaolo and Finger, 1991). Even though the first trend toward increased ¹³C values is associated with an immediate response in oxygen isotopes at Site 1000, suggesting modest changes in ice volume, an additional 1.5 m.y. is needed until full development of the Monterey Formation is reached and major ice-sheet growth in Antarctica is evident as indicated by the oxygen- and carbon-isotope records of Sites 999 and 1000, and as also described elsewhere (Wright et al., 1991; Woodruff and Savin, 1991). During this major intensification of ice-sheet growth between ~15.5 and 14.5 Ma, the quasi-cyclic waxing and waning of Antarctica ice sheets is paralleled by positive excursions in carbon isotopes.