RESULTS

All sediments collected consist mainly of unlithified, weakly to strongly bioturbated foraminifer wackestone or packstone as well as unlithified mudstone of yellowish gray to light olive-gray color. Shell fragments and also complete shells of gastropods and pteropods are found in coarse layers. The mud fraction (<0.03 mm) mainly consists of calcareous nannofossils, foraminifers, bryozoans, and sponge and tunicate spicules (Heck et al., 1999). Physical property data do not show important variations within and among the cores and are therefore not shown here.

Nannofossils constrain the age of the sediments. In the three cores examined for calcareous nannofossils (GC-10, GC-06, and GC-03), at least 12 genera were identified and grouped according to size into 33 different categories (Table T2). Florisphaera profunda (40%–70%) is by far the most abundant species in all samples, followed by Emiliania huxleyi (15%–40%) and Gephyrocapsa (5%–24%). Umbilicosphaera is present in nearly all samples but only in minor quantities (1%–4%). Gladiolithus flabellatus and Thorosphaera flabellata were found in almost all samples (0%–7%).

E. huxleyi is present in all samples and at all core depths. This places the recovered sediments in the E. huxleyi nannofossil Zone NN21 and therefore sets an upper age of ~270–290 ka (Brown, 1998). In the deepest part (370 to ~300 cm) of the longest core (GC-10; site CS-03), Gephyrocapsa is more abundant than E. huxleyi. This dominance reversal from Gephyrocapsa to E. huxleyi (Fig. F2) may represent the start of the E. huxleyi acme Zone in the upper part of Zone NN21. To confirm this hypothesis, more fossil counts (>400 per smear slide) would be required; however, the clear trend based on the 100 counts per slide indicate that this reversal zone correlates as suggested in our age model. The E. huxleyi acme zone starts at ~85 ka in tropical and subtropical waters (Thierstein et al., 1977). Because the dominance reversal is only identified in the longest core GC-10, the other two cores can be assigned a maximum age of the start of the acme Zone at ~85 ka. Not enough nannofossils were counted to observe significant size/core-depth relationships among different categories of the same genus (i.e., small Gephyrocapsa <2 µm, large Gephyrocapsa >5 µm).

IC content in the cores ranges from 79 to 93 wt% with a mean of 87 wt% (Table T3). A relatively small amount of TOC was measured in the collected sediments (0–1.7 wt%; mean = 0.4 wt%), which is typical for high-carbonate sediments in general. Only two cores have higher than 1 wt% TOC concentrations at the following sample depths: GC-08 at 112, 120, and 127 cm and GC-09 at 111, 169, 195, and 264 cm (Fig. F3). No correlation with any other measured parameter in the core has been found. IC content is usually lower in darker-colored lithologies (Fig. F4).

Based on the nannofossil data, three cores (GC-03, GC-06, and GC-10) seem to be younger than marine isotope Stage (MIS) 8, which is known to coincide with the start of Zone NN21 (E. huxleyi Zone). Based on the dominance of E. huxleyi, cores GC-03 and GC-06 likely correlate with the E. huxleyi acme Zone (Thierstein et al., 1977) and, consequently, are likely to be younger than MIS 5b and 5a. According to the tentative nannofossil dates, MIS 5b and 5a should be located at ~3 m in core GC-10. This must be confirmed with further stable oxygen isotope data.

The ¹⁸O values vary between –1.2‰ and 0.6‰. Because ¹⁸O data are from bulk sediment, different isotope effects are superimposed on the measured signal and therefore this signal is relatively "noisy" as compared to foraminifer species-specific ¹⁸O data. However, several maxima in the downhole trends can be recognized (Fig. F3). For instance, in the longest core (GC-10), three local minima can be detected near 60 cm, 170 cm, and the base of the core at 360 cm (Fig. F4). Using the correlation shown in Figure F4, the base of the core is interpreted to be the beginning of MIS 6 (130–140 ka) and the two minima higher in the core represent stages 4 and 2, respectively. This postulated chronology for core GC-10 inferred from ¹⁸O isotopes is in accordance with ¹⁴C ages of the same core (M. Page, pers. comm., 2003).