CHRONOSTRATIGRAPHIC FRAMEWORK

A detailed chronological framework for the sediments studied has been developed by means of micropaleontological analysis, nannoplankton data (Comas, Zahn, Klaus, et al., 1996) paleomagnetic data, and oxygen-isotope stratigraphy available from Site 975 (Pierre et al., Chap. 38, this volume) and Site 976 (von Grafenstein et al., Chap 37, this volume). The paleomagnetic data show that all the samples belong to the Brunhes Chron (0-0.78 Ma).

As shown by several authors (Cita et al., 1977; Vergnaud-Grazzini et al., 1977; Capotondi et al., 1994) the paleoclimatic curve can approximate in good detail the oxygen-isotope records for the time interval considered. A comparison between the shape of the 18O curve with the fluctuations of the paleoclimatic curve indicates that they yield similar trends. Positive and negative peaks in the planktonic foraminifer record correspond to interglacial and glacial isotope stages. Based on these considerations, we have tuned the ages of the sediments to the oxygen-isotope stages of Martinson et al. (1987).

Samples from Hole 974B represent the longest record with poor time constraints. On-board data of calcareous nannoplankton record the first occurrence (FO) of E. huxleyi (NN21A; 0.26 Ma) at the bottom of the first core (161-974B-1H-CC; 6.5 mbsf) and the last appearance datum (LAD) of P. lacunosa (NN20; 0.46 Ma) at about 11.61 mbsf. A somewhat speculative interpretation of the stratigraphic framework, which is based on a comparison with ODP Site 652, in the Tyrrhenian Sea (Vergnaud-Grazzini et al., 1990), suggests that our last sample belongs to oxygen-isotope stage 15 (0.565-0.62 Ma; according to Imbrie et al., 1984).

The age of the sediments studied from Site 975 appear well constrained by the first occurrence (FO) of the E. huxleyi (0.26 Ma) that has been recognized at a depth of about 21.67 m (Comas, Zahn, Klaus, et al., 1996), which means very close to our last sample (22.4 mcd). Further constraints in the age of the uppermost six isotope stages have been obtained by comparison with the data of a previously studied core (Bal84-2) recovered in the same area Table 3; Capotondi et al., 1987).

At this site, the paleomagnetic data exhibit a short interval of reversed polarity between 7.77 and 8.35 mcd. As discussed in the paragraph about the NRM of the sediments, the stratigraphic position of this interval suggests that it represents the Blake Event, which has been observed at several sites around the world and has been dated in the Mediterranean Sea between 117 and 123 ka (Tucholka et al.,1987).

A closely spaced sampling of up to 4 samples per meter was performed at Site 976. The 18O profile exhibits a good agreement with the record observed at Site 659 in the Equatorial Atlantic and suggest that our sampling represents oxygen-isotope stages 1-6 (von Grafenstein et al. Chap. 37, this volume). The paleoclimatic curve obtained for this site is in agreement with this interpretation, so the sampling spans about 150 k.y. For the last glacial time interval, a detailed time control was obtained on the basis of the ecozones method described by Pujol and Vergnaud-Grazzini (1989). The identified peaks in frequency of N. pachyderma (P1, P2, P3, P4), G. quinqueloba (Q1, Q2), and G. bulloides (B1, B0) have been correlated with the events dated by these authors, and they represent eight well-defined time-control points (see Table 4) for the Last Glacial Maximum (LGM)-Holocene interval.

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