RESULTS

Oxygen Isotopes and Age Model

The 18O record of the planktonic foraminifer G. ruber (pink) from Site 1077 exhibits the shape typical for Quaternary records (Fig. F2), resulting from changes in seawater 18O due to the buildup and retreat of glacial polar ice caps and changes in water temperature (e.g., Shackleton and Opdyke, 1973; Schneider et al., 1996). The Holocene peak at 1.25 mcd was fixed as age control point 7 ka. It corresponds to the Climatic Optimum and also reflects increased discharge of isotopically light Congo River water. Pastouret et al. (1978) and Schneider et al. (1994) had already observed this variation superimposed on the classical 18O record of planktonic foraminifers in this area. All age control points used are listed in Table T1, and the age model is plotted in Figure F3. Features in the 18O data set of ODP Site 1077 were aligned with the record from Site 677 (Panama Basin) (Shackleton et al., 1990). For depth intervals with low carbonate content (between 60 and 95 mcd), the record of magnetic susceptibility was successfully used to (1) align the isotope record and (2) assign age-depth intervals (Dupont et al., 2000). The magnetostratigraphy corresponds well with the isotope record (Matuyama/Brunhes boundary; Stage 19).

Sedimentation Rates

Sedimentation rates were calculated by linear interpolation between age control points (Figs. F2, F4). Lowest values (8-11 cm/k.y.) correspond to glacial Stages 6, 10, and 12; highest values ranged between 17 and 22 cm/k.y. within Stages 11, 9, 8, and during the past 120 k.y. In Figure F2, we compare Site 1077 sedimentation rates during the last 200 k.y. with those from a well-studied core (GeoB1008) (Schneider, 1991) retrieved from 3124 m water depth just south of our site and located within the Congo River plume (GeoB1008; 635S, 1019E; water depth = 3124 m). The shallower water depth at Site 1077 (2382 m) probably accounts for overall higher sedimentation rates. Discrepancies in the curve shapes (e.g., within Stages 5 and 6) may be attributed to local differences in sedimentation patterns and/or errors in age-depth alignment by correlation of individual 18O records to the SPECMAP standard stack.

Records of Biogenic Constituents

Organic Carbon

Only shipboard data are available for Site 1077 (Shipboard Scientific Party, 1998c). TOC values range from 1.9 to 4.7 wt% over the past 500 k.y. (Shipboard Scientific Party, 1998c). Higher resolution data do exist for core GeoB1008 (Schneider et al., 1997) and for nearby Site 1075 (447S, 104.5E; water depth = 2995 m) (Lin et al., in press). Both data sets are presented in Figure F2. Schneider et al. (1997) has shown that variations of TOC in the Congo Fan core GeoB1008 are mainly the result of changes in marine organic carbon (MOC) with values between 0.5 and 4 wt%. TOC concentrations at Site 1075 range from ~1.5 to 3.6 wt%. Here also, the organic matter appears to be mostly of marine origin (Shipboard Scientific Party, 1998b) and the relative contribution of terrestrial organic carbon is low (~0.5 wt%) (B. Jahn, pers. comm., 2000). It is evident that discrepancies in the timing of some peaks and valleys, especially during Stage 5, are due to (1) the very preliminary stratigraphy of Site 1075, which is based on shipboard nannofossil datum events (Shipboard Scientific Party, 1998b) and (2) differences in sampling strategy (every 5 cm for GeoB1008 and every ~1.5 m for Site 1075). However, the patterns are possibly comparable between both sites and higher percentages of MOC correspond to Stages 2 and 3 and to Substages 5.2, 5.4, 6.2, 6.4, and 6.6 (Fig. F2) (Schneider et al., 1994).

Biogenic Opal

Upper Quaternary Congo Fan sediments have high contents of biogenic silica (van der Gaast and Jansen, 1984; Schneider et al., 1997). In Figure F2, we plot two comparable records of biogenic silica for core GeoB1008 (Schneider, 1991) and Site 1075 (Lin et al., in press), both measured with automated wet leaching methods (Mller and Schneider, 1993, for GeoB1008; Mortlock and Froelich, 1989, for Site 1075). Values range from ~5 to 25 wt%. Although discrepancies in the timing of events over the last 200 k.y. are due to differences in sampling strategy and age model (see "Organic Carbon"), strong minima are observed in the Holocene and during the warmest period of the last interglacial (Substage 5.5 for core GeoB1008) and higher contents correspond to late Stage 3-early Stage 2, Substage 5.4, and within Stage 6.

For the older record (beyond the last 200 k.y.), the sample density of Site 1075 is too low and the age model is too preliminary to deduce accurate timing of changes in productivity based on TOC and opal fluctuations downcore. These preliminary data point to higher TOC values in late Stage 8-early Stage 7 and Stages 11 and 12. Obvious minima in opal weight percent correspond to Terminations 7/8 and 9/10.

Siliceous Organisms

Sediments of the Congo Fan area contain large amounts of siliceous microfossils. Only the dominant siliceous components are discussed below (in terms of accumulation rates, concentration, and species composition). Concentrations (per gram of dry sediment) of the most abundant microfossil groups counted are represented in Figure F5 and Table T2, and their accumulation rates are shown in Figure F4. The marine signal dominates at Site 1077, in agreement with previous studies (van Iperen et al., 1987; Jansen and van Iperen, 1991; Schneider et al., 1997). Abundances of marine diatoms are overwhelming (average = 5 107 valves/g); they compose 97% of the diatom assemblage (Table T3). The preservation state of the valves is moderate to good; lightly silicified species (e.g., vegetative cells and setae of Chaetoceros, Bacteriastrum elongatum/furcatum, and Skeletonema costatum) are present throughout. However, corroded valve edges were also observed (see also van Iperen et al., 1987). Silicoflagellates and radiolarians follow in second place, with abundances of 104-106 individuals/g dry sediment. For the three marine siliceous groups, concentrations tend to be higher (although highly variable) between 10 and 70 ka (especially for marine diatoms and silicoflagellates), during cooler conditions (Schneider et al., 1995) of Substages 5.2 and 5.4, and during glacial Stages 6 (especially for radiolarians), 8 and 9 (marine diatoms), and 10 and 12 (marine diatoms and silicoflagellates) (Fig. F5).

The continental signal is driven by freshwater diatoms with absolute abundances on the order of 106 valves/g (Fig. F5), an order of magnitude lower than marine diatoms. They originate from the drainage area of the Congo River. Chrysophycean cysts and phytoliths are present in almost all samples. Chrysophycean cysts are siliceous resting stages of chrysophyceaen algae, which are commonly found in lakes (Smol, 1988); phytoliths are discrete, solid bodies of opaline silica in epidermal cells of grasses (Alexander et al., 1997; Runge, 1999). Phytolith concentrations average 105 bodies/g dry sediment, and accumulation rates are ~107 bodies/cm/k.y. The ratio of phytoliths to marine diatoms (100) is low, 0.59, in agreement with the geographical distribution published by van Iperen et al. (1987). Phytolith maxima tend to coincide with glacial periods, whereas higher contributions of freshwater diatoms fall within interglacial stages (Fig. F5) and tend to occur during maxima in boreal summer insolation over Africa (Fig. F6). Both groups showed a significant difference in average concentration between glacial and interglacial times (Student's t test [Sokal and Rohlf, 1973]: T freshwater diatoms = 26.7, and T phytoliths = 4.8; with t(0.05; 128) = 2.626). Values for the PhFD index (Jansen and van Iperen, 1991) average 0.2 and range between ~0 (almost no phytoliths) and 0.8 (reduced influx of freshwater diatoms to the sediments); they are comparable to Jansen and van Iperen's (1991) PhFD indices for nearby cores T78-3 (511S, 758E) and T78-46 (650S, 1045E).

ARs were also calculated for each siliceous group. Although accumulation records are highly dependent on sedimentation rates (see discussion in Schneider et al., 1996), absolute abundances of each siliceous group are so high that they drive the AR pattern (cf. Figs. F5 and F4). ARs of marine diatoms range from 3.9 106 to 3.7 109 valves/cm/k.y. and are highest during Stage 2 and late Stage 3, Substages 5.2 and 5.4, Stage 8, and late Stage 9. Between 460 and 125 ka, silicoflagellate values oscillate around the mean (1.8 107 skeletons/cm/k.y.) and show an abrupt increase at ~100 ka with maxima at ~40, 60, and 85 ka (Fig. F4). Radiolarian AR values fluctuate between 1.6 106 and 4.2 107 tests/cm/k.y.

An interesting feature is that all siliceous organisms show both very low absolute abundances and accumulation rates at the Stage 5/6 boundary, a time of very low diatom diversity. This may point to a dissolution level also observed by Jansen and van Iperen (1991) in the Congo Fan area.

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