RESULTS

Oxygen Isotope Record

The oxygen isotope stratigraphy at Hole 1087A is based on the continuous record of C. wuellerstorfi, allowing the identification of MISs 1 through 13, which spans the last 500 k.y. The timescale calibration of the isotopic stages corresponds to the orbitally derived scales of Imbrie et al., (1984), Martinson et al. (1987), and Bassinot et al. (1994). The age-control points are listed in Table T2. Linear interpolation was applied between adjacent age control-points, assuming a constant sedimentation rate within the time interval. The average sedimentation rate is estimated at 2.7 cm/k.y. for the entire 13-m interval. In fact, sedimentation rates are irregular, with higher rates (3-4 cm/k.y.) during MISs 1, 2, 4, 5, 8, 10, 11, 13 and lower rates (1-1.9 cm/k.y.) during MISs 3, 7, 9, and 12.

The 18O record of Cibicides is given both as a function of depth (Fig. F1A) and of age (Fig. F1B). The lowest 18O values are measured during isotopic Event 5.5 (2.19) and MIS 11 (2.37), which characterize the warmest periods. The highest 18O values (4.27) are measured during MIS 12, which marks the coldest stage. At Site 704, which is near the Polar Front Zone in the South Atlantic (i.e., in the source area of AAIW), the coldest stage is also MIS 12, but there is no significant difference between the interglacial stages for the last 450 k.y. except the long warm period during MIS 11 (Hodell, 1993; Hodell et al., 2000). If one assumes that the G-IG salinity changes were negligible in the source area of AAIW and subtracts 1.6 and 1.2% for the maximum global ice effect during Terminations V and II, respectively (McManus et al., 1999), the maximum difference of 18O values between MIS 12 and 11 and MIS 6 and 5 will equal 0.3 and 0.6, which correspond to a maximum temperature change of ~1.5 and 2.5C in deep waters (using an 18O fractionation of -0.22/C). Otherwise, this G-IG temperature variation would be less if part of the 18O variation is caused by increased freshwater dilution during interglacials. The difference of 18O values between the LGM and the Holocene is 1.6, indicating that during Termination I the temperature increase was <2C in the deep waters at Site 1087.

The 18O records of the three species of planktonic foraminifers display similar variations, although the amplitudes of the difference between glacial and interglacial values (18O G-IG) differ for each species (Fig. F2). For the last four G-IG cycles, the 18O G-IG amplitude decreases from 2.1 in G. bulloides to 1.5 in G. inflata and to 1.2 in G. ruber. A comparison of the G. bulloides and G. inflata records shows in fact that the glacial values are very close for the two species, whereas the low interglacial values are much lower for G. bulloides than for G. inflata; this indicates that the temperature change in surface waters occurred mostly during interglacial stages.

As was the case for the benthic foraminifers, the highest 18O values of the planktonic foraminifers are found during MIS 12, which corresponds to the coldest stage of the last 500 k.y. (Fig. F2). The difference in 18O values between LGM and MIS 12 ranges from 1.2 (G. ruber) to 0.4 (G. bulloides), with G. inflata showing an intermediate value of 0.7. Subtracting the global ice effect difference of 0.4 between LGM and MIS 12 (McManus et al., 1999) translates into an MIS 12 cooling of surface waters compared to the LGM situation of ~4 and 1.5C, as recorded by G. ruber and G. inflata, respectively.

Carbon Isotope Record

The 13C record of C. wuellerstorfi shows a pattern different from the G-IG cycles but linked to large oscillations much higher than the 100-k.y. eccentricity period (Fig. F3). Two intervals of lower 13C values occur between 20-160 ka and 300-420 ka and are separated by an interval of higher 13C values during the period 160-300 ka. If the 13C values of benthic foraminifers are considered to be a proxy of ventilation and nutrient concentration at depth, the AAIW was well ventilated and nutrient poor prior to 420 ka and from 160 to 300 ka, whereas it was less ventilated and nutrient rich during both 20-160 ka and 300-420 ka intervals. Furthermore, 13C values tend to decrease at the IG-G transitions, a pattern particularly well expressed at the MIS 13/12, MIS 11/10, and MIS 7/6 transitions. This 13C decrease suggests that inputs of nutrients occurred during the beginning of cool conditions at the IG-G transitions and that the migration toward the equator of the Polar Front Zone was associated with a relatively lower rate of AAIW ventilation.

Similarly the planktonic foraminiferal 13C records do not display clear evidence of G-IG cyclicity (Fig. F4). Common to the three planktonic taxa is a large positive 13C oscillation spanning the time interval 260-425 ka; this increase is the highest (2.1) for G. bulloides, intermediate (1.2) for G. inflata, and the lowest (0.8) for G. ruber. In terms of productivity levels, this positive 13C excursion identifies a highly productive period that coincides with the global mid-Brunhes event (Jansen et al., 1986) known for its very high level of pelagic carbonate production during MISs 7, 9, and 11 (Hodell, 1993). A shorter period of 13C enrichment between 80 and 100 ka also suggests high productivity levels during the late MIS 5.

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