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Dinocyst biostratigraphy

The first occurrence of dinocyst in the studied sample set is located in the uppermost sample of the lithologic Subunit IVa Sample 67R-2, 57-61 cm (1045.4 mbsf, Fig. 1.5). Although dinocyst are still scarce in this sample and most of them are non-diagnostic long-ranging species, the presence of Xenascus gotchii (Fig. 1.5, 1.6; Pl. 1, Fig. 11) give some evidence for age determination. X. gotchii is known to appear Core Section cm Depth (mbsf) Core Section cm Depth (mbsf) 44R 6 060-062 8 during Senonian (Corradini, 1973), more precisely Coniacian-Santonian times according to (Kirsch, 1991). Thefollowing studied sample, Sample 66R- 3, 40-43 cm (1035.1 mbsf) at the base of the lithologic Unit III, yields the only occurrence of Cyclonephelium vannophorum (Fig. 1.6). Since, the last occurrence (LO) of C. vannoporum occurs in Coniacian strata (Williams and Bujak, 1985, p. 900, fig. 19) the presence of both X. gotchii and C.
vannophorum in the interval between 1045.4 mbsf and 1035.1 mbsf is here considered as indicative of Coniacian age (Fig. 1.6). This interpretation is consistent with the nannofossils (Watkins et al., 1998) that identified the zones CC13a to CC15b, Late Turonian to Early Santonian in age, between 1053 mbsf and 1029.3 mbsf and placed the Turonian / Coniacian boundary in the center of the CC13a zone at 1045 mbsf.
No dinocyst markers of the Santonian stage were identified during the present study. However, Moullade et al. (1998d) recognized the stage in the Core 65 (1033.7 – 1024.1 mbsf, Fig. 1.6) from the association of dinocyst Oboh-Ikuenobe et al. (1998), nannofossil (Watkins et al., 1998) and planktonic foraminifer (Kuhnt et al., 1998a).

Evolution of the Dinocyst assemblages, organofacies and pal ynofacies: a paleoenvironmental interpretation

Dinocyst assemblages from the studied 959D samples are characterized by a relative high abundance of Spiniferites spp. and Achomospharea spp. (Fig. 1.7), but successive rises and falls mark the history of the two genera. In the lower part of the studied interval (1045 – 1033.7 mbsf), Coniacian in age, Palaeohystrichopora spp. dominate and Dinogymnium spp. are also common. From the lower part of the Campanian (1011.91 mbsf) to the base of the Danian (893.8 mbsf) the taxa showing important peaks are; Circulodinium spp., Trichodinium castaneum, Spinidinium spp., Trithyrodinium spp., Kallosphaeridium spp. and Areoligera spp. In the upper part of the studied interval (from 886.8 mbsf to the top) dinocyst assemblages do not show clear peaks of dominance by a single taxon. The frequency distribution of taxa is more equitable in the assemblages.
Previous studies on the sedimentological, structural and paleoenvironmental evolution of the Côte d’Ivoire – Ghana transform margin suggest a first marked phase of deepening in the 959D site from the upper Coniacian to the lower Campanian (ca. 1035 to 1015 mbsf, Fig. 1.3) (Saint-Marc and N’Da, 1997; Basile et al., 1998; Wagner and Pletsch, 1999; Pletsch et al., 2001). The assemblages of benthic foraminifera from this interval suggest an extended oxygen minimum zone in the water column (Pletsch et al., 2001; Friedrich and Erbacher, 2006). Subsidence rates of the CIGMR crest and, northern and southern flanks are different, and reworking processes are considered almost constantly active (Wagner and Pletsch, 1999). After the high subsidence interval and low oxygen availability, the deepening rate was lower and almost constant during most of the Campanian-Maastrichtian interval. From the early Campanian upwards, the initial reliefs of the sea bottom fall deep through the subsidence (Fig. 1.3), which allowed the deep-water circulation (Saint-Marc and N’Da, 1997; Pletsch et al., 2001; Friedrich and Erbacher, 2006). Depth remained stable for the entire Paleogene (from ca. 890 mbsf to the top of the studied interval) (Pletsch et al., 2001).

Density of sampling and biostratigraphic resolution

Some interesting observations can be made form the comparison of sampling density, the stage boundaries position, the extension of intervals not assigned to any stage and the distance from the lower limit of stages proposed in each of the three studies herein considered to the lower limit of the stages proposed in the consensual biostratigraphy (Table 1.2, Fig. 1.6).
At a first sight it seems that the lower stratigraphic density of our samples prevents the identification of some stages, as the Santonian. Based on the co-occurrence of Isabellidium acuminatum and Unipontidinium grande (Pl. 1, Fig. 1-2) at 1025.37 mbsf (Fig. 1.6), Masure et al. (1998a) suggested a Santonian age for this level. Oboh-Ikuenobe et al. (1998) placed the lower Santonian boundary at 1031.7 mbsf because of the identification of FO of Dingodinium undulosum. The distance between the position of the lower Santonian boundary suggested from dinocyst biostratigraphy by Masure et al. (1998a) and Oboh-Ikuenobe et al. (1998) and that suggested from the combination of all micropaleontological data by Moullade et al. (1998d) result in 7.8 m and 2.0 m of error in the boundary placement, respectively. In the present study the dinocyst assemblage at 1025.37 mbsf was not studied, I. acuminatum was not observed in any of the studied slides and the FO of U. grande was identified at 1015.62 mbsf, within the interval dated as Campanian. The non-observation of these taxa implies that we cannot define the Santonian stage from our dinocyst dataset and that the error associated to the lower Santonian boundary rises to 40.9m in the present study (Table 1.2).

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Rarity effect and the recognition of age markers

All dinocyst events identified by Masure et al. (1998a) and by the present study do not coincide exactly. The dinocyst events herein identified but not identified by Masure et al. (1998a), are the FO of Areoligera spp. and Cerodinium spp. at 996.16 mbsf; the FO of Palaeocystodinium lidiae at 981.18 mbsf; the FO of Carpatella cornuta at 901.9 mbsf; the FO of Adnatosphaeridium multispinosum at 842.48 mbsf and; the occurrence of Areoligera gippingensis at 828.7 mbsf. Obviously, the analysis by Masure et al. (1998a) also identified of some dinocyst events that have not been identified in the present study, like the FO of Glaphyrocysta group, Alterbidinium varium and Cerodinium leptodermum at 952.74 mbsf and the FO of Damassadinium californicum at 901.9 mbsf. When considering individually both studies these differences in the identification and placement of the dinoflagellate events would result in a difference of about 15 meters in the placing of the lower boundary of the Campanian and of about 14 meters for the lower boundary of the Thanetian.

Taxonomic revisions and instability of stage boundaries

Frequency distribution of the abundance of dinocyst considered as biostratigraphical markers in Oboh-Ikuenobe et al. (1998) and/or in the present study. Data from Masure et al. (1998a) are not considered for this comparison because their publication contains exclusively prensence / absence data.
Frequency distribution of the abundance of dinocyst considered as biostratigraphical markers in Oboh-Ikuenobe et al. (1998) and/or in the present study. Data from Masure et al. (1998a) are not considered for this comparison because their publication contains exclusively prensence / absence data.
The present study also evidences an example of personal observational bias. We identified Sample 56R-4, 60-62 cm (942.5 mbsf) the FO of Turbiosphaera sp. (Pl. 1, Fig. 9, 12-14). Specimens here identified as Turbiosphaera are similar to that identified by Oboh-Ikuenobe et al. (1998) as Cordosphaeridium fibrospinosum (illustrated in Pl. 3, Fig. 12, Oboh-Ikuenobe et al., 1998). Turbiosphaera differs from Cordosphaeridium in having a more elongate body, wide and flat processes of which the apical and antapical processes are larger than the others and by having a paracingulum represented by a shelf-like projection (Stover and Evitt, 1978). From my point of view, the specimen illustrated by (Oboh-Ikuenobe et al., 1998, Pl. 3, Fig. 12) as C. fibrospinosum possesses an antapical process larger than the others, which is a Turbiosphaera character, and processes are not expanded distally, which is a character for identifying Cordosphaeridium. So the specimens should all be considered as Turbiosphaera. Problems in indentifying the different fibrous dinocyst (as Turbiosphaera, Cordosphaeridium, Damassadinium and Fibrocysta) are well known (e.g. Soncini, 1990). These problems are illustrated by the fact that the type species of the genus Turbiosphaera (T. filosa, Archangelsky, 1969) was originally described as Cordosphaeridium filosum (Wilson, 1967). For these reason, it is advised not to use problematic morphotypes (such as Cordosphaeridium and Turbiosphaera) as biostratigraphic markers. Fortunately, within the whole pool of considered dinocyst events problems on confidence of taxonomic identification are very rare and, in general terms, results obtained by different operators are reproducible (Mertens et al., 2009).

Influence of paleoenvironmental factors on the dinocyst stratigraphic record

Paleoenvironmental control on the dinocyst taxa relative frequency can impact the reconstruction of their stratigraphic distribution. The comparison of dinocyst distribution (Fig. 1.7) with the paleoenvironmental reconstructions proposed by previous studies shows a good fit see Wagner and Pletsch, 1999; Pletsch et al., 2001; Friedrich and Erbacher, 2006. Modifications of environmental conditions have already been proposed for explaining variations in the distribution of dinocyst taxa along sedimentary sequences. For example, see surface temperature variations have been proposed as triggers of dinocyst frequency variations along the K/T boundaries from different regions (e.g. Brinkhuis et al., 1998; Vellekoop et al., 2015). They found that some of the chronobiostratigraphic dinocyst markers (as Palynodinium grallator, Pierceites pentagonus, Phelodinium maginficum or Senegalinium bicavatum) are influenced by SST variations. Improving our knowledge about how paleoenvironmental variation control dinocyst distribution is fundamental to realize the full potential of dinocyst as biostratigraphic markers.

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