13C record of G. bulloides at Site 975 is shown as a function of depth (Fig. 3A) and as a function of age (Fig. 3B). Detailed 13C-age records with the location of sapropels are also represented in Figure 4A-C.
The 13C record of G. bulloides at Site 975 is shown as a function of depth (Fig. 3A) and as a function of age (Fig. 3B). Detailed 13C-age records with the location of sapropels are also represented in Figure 4A-C.
The carbon isotope variability throughout the Pleistocene, although somewhat noisy, exhibits positive and negative peaks that can be roughly correlated with the oxygen isotope stages (Fig. 3A, B). The highest 13C values are generally coeval with the highest 18O values of the glacial stages, and the lowest 13C values are coeval with the lowest 18O values of the interglacial stages. This means that surface productivity was higher during glacial times than during interglacial times, a conclusion which has been already verified in the global ocean as well as in the Tyrrhenian Basin (Vergnaud-Grazzini et al., 1990). However, looking in more detail, the covariation of 18O and 13C values at Site 975 is not fully verified for the entire Quaternary, meaning that a local Mediterranean overprint is superimposed on the global signal.
Four intervals may be distinguished by their mode and amplitude of 13C variations; their age boundaries match well with those defined from the oxygen isotope curve.
The 13C values oscillate between -0.15
and -1.35 throughout the Holocene-upper Pleistocene period, giving a maximum amplitude of 1.2 for the 13C variations (Fig. 4A). The 13C values measured in the sapropels are low (-1.55 to -1.17), except in sapropel 506 where the value is moderately low (-0.5).
Before 0.42 Ma and down to 0.9 Ma, the maximum amplitude of 13C variations reached 1.6, the 13C values displaying regular oscillations between 0.1 and -1.5 (Fig. 4B). In this evolution, most of the lowest 13C values are attributed to the sapropel occurrences.
During the lower Pleistocene, the amplitude of 13C variations decreased to about 1.3, and the maximum and minimum 13C values increased progressively by about 1.0 throughout the whole period (Fig. 4C). This increase, as well as the increasing trend in the total carbonate content (Comas, Zahn, Klaus, et al., 1996), indicates a major change toward a more productive regime in the surface waters. A similar behavior was observed at ODP Site 653 in the Tyrrhenian Basin for the same time interval (Vergnaud-Grazzini et al., 1990). The 13C values recorded in the sapropels within this time interval correspond mostly to the lowest values of the oscillations.
The amplitude of the 13C oscillations remained at the level of 1.3 in the lowermost Pleistocene, but it decreased slightly below the Pliocene/Pleistocene boundary where the maximum and minimum 13C values became stable at 0.5 and -0.4, respectively (Fig. 4C).
