RESULTS

The oxygen-isotope record shows marked variations, ranging from -2.77 to -0.28 (Fig. 4). Oxygen-isotope values show three major features: (1) an irregular but consistent increase from -2.64 to -1.30, spanning across lithostratigraphic Unit IV and the lower part of Unit III, from the Chattian through the Aquitanian stages, (2) a phase of relatively constant values throughout the Burdigalian and Langhian stages, and (3) an abrupt increase at the boundary between lithostratigraphic Units III and II, from -1.30 to -0.52. These distinctive patterns in oxygen isotopes match lithostratigraphic Units II, III, and IV (see Fig. 4), raising the question on whether the oxygen-isotope values reflect in part diagenetic alteration. The bulk oxygen-isotope values at this site, comprised between -2.8 and -0.5, can be subdivided into two zones: a lower zone (510-650 mbsf) characterized by a large variability of ¹⁸O values ranging from -2.8 to -0.5, and an upper zone (250-510 mbsf) characterized by a minor variability of ¹⁸O from -2 to -0.5.Values comprised between -2.2 and -0.5 are typical values for planktonic organisms, suggesting that the ¹⁸O values of the upper zone primarily reflect the isotopic composition of planktonic foraminifers and nannoplankton.

Burial depth as well as CaCO₃ content are useful proxies to evaluate the extent of diagenetic modification to oxygen-isotope values. Frank et al. (1998) have shown that with increasing CaCO₃ content and burial depth, bulk-rock values become progressively more depleted in ¹⁸O. In order to evaluate the effect of diagenesis, oxygen-isotope values have been plotted against percentages of CaCO₃ (Fig. 5). Although, generally speaking, the samples from the lower zone at Site 999, characterized by the lower ¹⁸O values, coincide with high CaCO₃ content, there is not an unambiguous correlation between the two parameters (Fig. 5). The boundary between lithostratigraphic Units IV and III is found within the zone of light ¹⁸O values; below this stratigraphic boundary, the correlation between lower ¹⁸O values and high CaCO₃ seems more consistent.

This correlation would suggest that the ¹⁸O values might have been altered as a consequence of postdepositional precipitation of CaCO₃ in the pores, causing a modification to the depositional marine values when analyzed as bulk samples. However, these samples contain a large number of planktonic foraminifers, naturally characterized by lower ¹⁸O values than benthic foraminifers. The correlation between ¹⁸O and CaCO₃ content is reverted in Unit II, changing exactly at the boundary. At this shallow burial depth (<350 mbsf), no diagenetic modification to the ¹⁸O is expected, suggesting that there is a primary, depositional relationship between isotopic values and lithostratigraphic units.

In summary, the ¹⁸O values from the upper zone seem to have been unaffected by burial diagenesis, and therefore reflect primary values. The values for the lower zone, particularly from Unit IV have possibly been affected by burial diagenesis, which resulted in a depletion of the ¹⁸O values and must therefore be treated with caution when analyzed for paleoceanographic purposes.

The carbon-isotope record shows marked variations, from 0.47 to 2.44, and two major positive excursions can be recognized (Fig. 4). The first ¹³C shift is in the upper Oligocene within lithostratigraphic Unit IV and is a fairly abrupt increase from 0.92 to 2.03 found within ~23 m of sediment thickness. This shift shows a gradual and steplike return to lower values during the Aquitanian. The second ¹³C shift is in the upper lower Miocene within lithostratigraphic Unit III and is a gradual, stepwise increase over a thicker interval than the upper Oligocene increase initiated during the Burdigalian, from 0.94 to 1.84, returning to 1.31 and then rapidly increasing to 2.40 during the Langhian. The carbon values remain high for ~40 m and then start gradually returning to lower values throughout the Serravallian. The excursion terminates with a gradual but rapid return to 0.95 at the end of the Serravallian.

With respect to Site 999, the carbon- and oxygen-isotope data obtained from Site 1000 are characterized by higher time resolution, with an average of ~50 ka (Fig. 6). For clarity of data presentation, carbon- and oxygen-isotope data have been smoothed using a 5-point running average (Fig. 7).

The oxygen-isotope record (Fig. 6, Fig. 7) shows marked variations, ranging from -2.74 to -0.30, with a systematic trend toward heavier values upsection and the detailed character of lower and middle Miocene shifts. There are three first-order features that can be recognized in comparison with the record from Site 999: (1) an interval with high variability but without a marked trend during the Burdigalian, (2) a phase of progressive shift toward heavier values, from -2.34 to -0.93, peaking at -0.30, and (3) relatively constant values during the Serravallian with values ranging from -1.7 to -0.6. Compared to data from Site 999, the higher resolution ¹⁸O data at Site 1000 show in more detail the structure of the middle Miocene ¹⁸O increase; this, totaling 1.4, is found within several (three to four) quasi-cyclic steps.

Similar to Site 999, distinctive patterns in oxygen isotopes correspond to different lithostratigraphic subunits (Subunits IIB, IIA, ID, and IC) (see Fig. 3), raising the question on whether the oxygen-isotope values reflect, in part, diagenetic alteration. In order to evaluate the effect of diagenesis, especially in the deeper part of the record (590-696 mbsf), isotope values have been plotted against percentages of CaCO₃ (Fig. 8). At this site, in Subunit IIB, the highest variability of oxygen values corresponds to the highest variability in CaCO₃ values, suggesting that these values might have been altered.

The carbon-isotope record shows marked variations, from 0.52 to 2.18, and one major positive excursion can be recognized (Fig. 7). The positive ¹³C shift is a stepwise increase, initiated during the Burdigalian, with a gradual increase from 0.94 to 1.46, then rapidly increasing to 2.18 at the end of the Burdigalian, remaining high during the Langhian and reaching highest values in the early Serravallian. The ¹³C values start gradually returning to lower values throughout the Serravallian, reaching 0.7. Toward the end of the Serravallian there is a shift to higher values (up to 1.62), followed by an abrupt decrease from 1.62 to 0.61 at the end of the Serravallian.