DISCUSSION: TEMPORAL FLUCTUATIONS
IN THE COMPOSITION OF SILICEOUS MICROFOSSIL ASSEMBLAGES

Marine Signal

A total of 154 diatom taxa were identified and counted. The most common taxa (28, with an overall average contribution of >1%) are summarized in Table T3. Marine diatoms dominate the assemblage in terms of relative contribution (~97% marine vs. ~3% freshwater), as well as concentrations per gram, and accumulation rates (see "Sedimentation Rates", and "Siliceous Organisms", both in "Results").

In the sediments of the Congo Fan, van Iperen et al. (1987) defined six marine diatom groups in surface sediments that reflect properties of the overlying water masses. Essentially, we keep their groupings with slight modifications based on our own experience with temporal and spatial distribution of diatom species in sediment traps and surface sediments in the equatorial and tropical Atlantic (summary in Romero et al., 1999a), including additional observations of Pokras and Molfino (1986). The composition of the resulting seven groups and their preferred environmental conditions is given in Table T3, their distribution downcore is illustrated in Figure F7, and their concentration per gram dry sediment is summarized in Table T4.

In accordance with the results of van Iperen et al. (1987), the marine diatom assemblage is dominated by two neritic, high-nutrient indicators, T. nitzschioides var. nitzschioides and resting spores of the genus Chaetoceros, and by C. litoralis. The latter can be considered a plume-related species in the Congo and the Niger area (van Iperen et al., 1987; Pokras, 1991). It is possible that in these previous studies, C. litoralis was misidentified as C. striata (van Iperen, pers. comm, 1999). Although little is known about the ecological preferences of C. litoralis, the species has been recurrently found in neritic environments with highly variable salinities (C. Lange, unpubl. data). Fluctuations in relative abundance and accumulation rates of this species at Site 1077 suggest two long-term periods of lowered salinity and enhanced river discharge, between 220 and 325 ka (moderate) and in the last 125 ka, including two strong pulses at 10-50 ka and 75-125 ka.

The occurrence of Chaetoceros spp. resting spores and remains of vegetative cells in the Congo area can be attributed to seasonal variability of coastal upwelling and nutrient input from the river outflow (the Congo effect) and probably also reflects advection from the shelf (see discussion in Jansen and van Iperen, 1991). They are common members of the diatom assemblage (~20%), and their abundance pattern shows high variability, especially during the past 250 k.y. Maxima in the relative abundance are seen during interglacial stages (Fig. F7).

We consider the record of T. nitzschioides var. nitzschioides as indicative of increased nutrient supply by the coastal upwelling process, in agreement with Jansen and van Iperen (1991), rather than of river influence (Pokras and Molfino, 1986). At Site 1077, a long-term trend is evident with a clear dominance (>50%) during 460-125 ka, dropping to low levels at Termination II (Fig. F7). On shorter time scales, abundance peaks are generally associated with glacial periods.

The presence of the littoral group (composed of benthic, tycopelagic, epiphytic species, etc.) is constant but rare (~1%); moderate peaks occurred during interglacials. A significant (5% confidence level) difference in average concentration between glacial and interglacial times was found (Student's t test; Sokal and Rohlf, 1973): TT = 4.669, theoretic t-value = 2.626. The warm-water group was always present in low numbers (highest peak in Stage 11) as an indication that our study area has always been under the influence of warm and saline waters of the SECC. Episodic maxima of the oceanic temperate assemblage may be associated with cold water intrusions of the BC.

It is interesting to note two time intervals with prominent peaks in the relative abundance of a cold water silicoflagellate, Dictyocha speculum (at ~25 and 40-50 ka), and of the oceanic temperate diatom assemblage (at 40-70 and 225 ka) (Fig. F8). In particular, the period 15-65 ka, and to a lesser extent 80-100 ka, coincides with high diatom accumulation rates and lower sea-surface temperatures (Schneider et al., 1995) in the Congo Fan area (Fig. F8). These observations correlate well with inferences made by Jansen et al. (1996) of northward movements of the ABF. Although the peaks may not represent BCC waters per se (Jansen et al., 1996), we suggest that these times are characterized by enhanced productivity caused by frontal mixing of cold, nutrient-rich waters from the south (Fig. F8) and river-induced upwelling from the coast (high abundances of Chaetoceros spp. and C. litoralis) (Fig. F7). We do not know if the 225-ka peak can also be related to an earlier northerly position of the ABF.

Continental Signal

The presence of freshwater diatoms and phytoliths in marine sediments from the Atlantic Ocean is attributed to three different transport mechanisms: eolian, fluvial, and transport by turbidity currents (Pokras, 1991; Pokras and Mix, 1985; van Iperen et al., 1987; Gasse et al., 1989; Stabell, 1986; Treppke, 1996). The interpretation of these siliceous remains in the water column and sediments raises the problem of identifying their source area and transporting agents. In the equatorial and tropical regions between 20N and 10S and west of 2E, eolian transport with direct settling over the open ocean is assumed to be the main transport mechanism of freshwater diatoms and phytoliths (Pokras, 1991; Pokras and Mix, 1985; Stabell, 1986; Gasse et al., 1989; Romero et al., 1999b). In contrast, in nearshore areas influenced by river discharge (e.g., Niger, Congo, and Amazonas), fluvial transport is responsible for the deposition of freshwater diatoms, phytoliths, and other continental remains (Melia, 1984; Gasse et al., 1989; van Iperen et al., 1987; Jansen and van Iperen, 1991). Site 1077 is strongly influenced by Congo River discharge (Fig. F1); it lies outside of the turbidite fan area (van Weering and van Iperen, 1984; van Iperen et al., 1987; Pufahl et al., 1998). Although eolian influence at our study site cannot be completely ruled out as a possible mechanism of deposition of continental remains, wind contribution is probably minor when compared to the riverine input. The continental signal recorded by freshwater diatoms is dominated by the genus Aulacoseira (mostly Aulacoseira granulata and Aulacoseira islandica), which represents an average of 64% of the freshwater assemblage (Table T3). Other freshwater species in sediments of Site 1077 include Cyclotella meneghiniana, Luticola mutica, and Stephanodiscus astrea. Their abundances may be underrepresented, because Aulacoseira-rich sediments from Site 1077 may also be a consequence of differential dissolution during transport within the river waters and discharge into the ocean, where dissolution increases as a result of low dissolved silica contents and high ionic concentration (van Bennekom and Berger, 1984; Hurd, 1983). However, Aulacoseira species are commonly found in African water bodies, especially in the Congo River, where the genus makes up 80% of the total diatom population (Gasse et al., 1989). We believe that Aulacoseira values (and total freshwater loads) at Site 1077 are useful for reconstructing temporal fluctuations in the intensity of Congo River discharge. They reflect humid periods with increased rainfall and river discharge fostered by an intensified monsoon during times of maximum insolation in the Northern Hemisphere (Fig. F6) (Schneider et al., 1997; Gingele et al., 1998).

The continental signal derived from chrysophycean cysts is probably related to both humidity on land and changes in the Congo drainage area and inland water bodies. An interesting long-term contrasting trend is evident when comparing freshwater diatom and chrysophycean cyst records, with chrysophycean cysts being more abundant between 460 and 125 ka (except for two freshwater diatom maximas at ~170 and ~370 ka) and freshwater diatoms dominating during the last 122 k.y. (Fig. F6). Because chrysophycean cysts are of limnic origin, we speculate that their higher values prior to Termination II may be attributed to a larger number of lakes drained by the Congo. Apparently, pre-Pleistocene times were characterized by lakes and swamps covering a large area around the Congo River (Beadle, 1974). Changes in vegetation were also recorded. A drop in the proportion of plants of the family Cyperaceae at and around Termination II was reported by Dupont et al. (1999). This family includes aquatic plants adapted to life in marshy and freshwater environments.

Our third continental proxy, the contribution of phytoliths, does not show marked long-term changes. Phytoliths were almost always present in much lower numbers than freshwater diatoms and chrysophycean cysts; highest relative abundances were observed during Stages 6 and 7 (Fig. F6). On the other hand, the PhFD index shows a spiky pattern with a moderate long-term trend pointing to more arid conditions on land prior to ~122 ka and more humid conditions thereafter.

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