Palynological assemblages from the majority of the samples of the section at Site 1148 contain abundant pollen and spores (mostly bisaccate pollen) as well as dinoflagellate cysts. These are accompanied by variable numbers of organic foraminiferal linings (microforams), and trace amounts of Tasmanites and the freshwater green algae Pediastrum (Table T1). The dinoflagellate assemblages generally show a mixture of neritic and oceanic taxa in the same sample. This indicates that horizontal transportation from a coastal/neritic environment played an important role during deposition of the assemblages. Vertical recycling at this site was generally not prevalent, as there is no evidence of assemblages containing uncharacteristic species. Virtually no pre-Paleogene taxa occur. Still, the fact that the early Miocene Sample 184-1148A-44X-CC contains late Oligocene nannofossil species indicates pronounced reworking. In addition, the evidence in lithologic Unit VI of episodic gravitational redeposition (Wang, Prell, Blum, et al., 2000) implies relatively short-period or local reworking. Thus, we assume that the palynological/dinoflagellate assemblages at Site 1148 may represent pericontemporaneous thanatocoenoses.
Among the 105 samples investigated, 27 contain no dinoflagellate cysts, 22 contain few, and the other 56 yielded moderate to abundant dinoflagellate cysts. Preservation of the dinoflagellate cysts is generally moderate to good. A total of 110 species/subspecies of 48 genera were recorded from the section studied. Dinoflagellate cyst assemblages from the whole section are dominated by chorate gonyaulacoid cysts. Two zones can be distinctly recognized in Table T1, the boundary between which lies at 448.05–473.1 mbsf. The lower part is sharply different from the upper part in cyst abundance, species diversity, and component taxa. On that basis, two assemblage zones and two subzones were recognized (Tables T1, T2). They will be discussed in ascending order.
This zone covers Cores 184-1148B-56X through Section 39X-CC and Sections 184-1148A-76X-6 through 52X-CC (844–473 mbsf) (Table T2).
With 93 species/subspecies in 41 genera recorded (see the "Appendix"), Zone A is defined by the highest occurrences (last abundant datum; LAD) of Enneadocysta arcuata, Enneadocysta multicornuta, Homotryblium plectilum, and Homotryblium tenuispinosum at its top. It is characterized by the co-occurrence of Cleistosphaeridium ancyreum, C. diversispinosum, Cleistosphaeridium placacanthum, Cordosphaeridium gracile, Cordosphaeridium inodes, H. plectilum, Hystrichokolpoma rigaudiae, Lingulodinium machaerophorum, Operculodinium centrocarpum, and Polysphaeridium zoharyi, which are all common and present almost continuously throughout the zone. Achomosphaera crassipellis, Apteodinium nanhaicum, Cordosphaeridium cantharellum, Cordosphaeridium exilimurum, Distatodinium ellipticum, E. arcuata, E. multicornuta, Hystrichokolpoma salacia, Lejeunecysta hyalina, Pentadinium laticinctum, Reticulatosphaera actinocoronata, and Selenopemphix nephroides occur intermittently in the zone, usually in small numbers. Those like Hystrichokolpoma cinctum, Thalassiphora patula, Thalassiphora pelagica, Wetzeliella articulata, Wetzeliella gochtii, and Wetzeliella symmetrica occur in one to several samples of Zone A. No species of Deflandrea was recorded.
Eighteen Zone A species, including C. inodes, Cribroperidinium tenuitabulatum, Enneadocysta pectiniformis, H. cinctum, Membranophoridium aspinatum, O. centrocarpum, P. laticinctum, T. pelagica, and W. symmetrica, were recorded more than 40 yr ago by Gerlach (1961) in her Oligocene dinoflagellate assemblages from northwest Germany. Some of these 18 species and other Zone A species were also found in contemporaneous dinoflagellate assemblages from different parts of the world (Table T3). For example, E. arcuata, E. pectiniformis, T. pelagica, and W. symmetrica were recorded in the Oligocene assemblage of the Labrador Sea (calibrated by nannofossil Zones NP21 through NP24; Head and Norris, 1989).
In a recent study on dinoflagellates from northwest Germany, Köthe (1990) recognized two dinoflagellate zones, Zones D14 and D15. Sixteen species including C. inodes, C. ancyreum, C. placacanthum, C. tenuitabulatum, E. arcuata, E. pectiniformis, H. tenuispinosum, and Phthanoperidinium amoenum occur in her Zone D14, which was calibrated by nannofossil Zones NP23–NP24 and foraminifer Zones P19–P21 as having a Rupelian age. In addition, the following six species occurred in the Köthe (1990) Zones D14 and D15: C. cantharellum, H. plectilum, M. aspinatum, P. laticinctum, T. pelagica, and W. symmetrica; Zone D15 was calibrated as having a Chattian age by the nannofossil Zone NP25 and foraminifer Zone P22. All of the 22 species from Zones D14 and D15 occur in our Zone A, which therefore may be correlated not only with the Oligocene assemblage of the Labrador Sea but also with Zones D14 and D15 of northwest Germany.
Moreover, the presence of W. gochtii (ranging 32.8–26.6 Ma in the mid-latitudes of the Northern Hemisphere and 34–26? Ma in the low-latitude equatorial regions) (Williams et al., unpubl. data, [N1]) further supports an Oligocene age for Zone A. The co-occurrence of the following species gives further confidence for this age assignment: E. pectiniformis (36.5–29.3 Ma; Williams et al., unpubl. data [N1, N2]), D. ellipticum (41.4–26.3 Ma; Williams et al., unpubl. data [N2]), M. aspinatum (39.64–24.6 Ma; Williams et al., unpubl. data [N2]), Impagidinium dispertitum (41.3–24.6 Ma; Williams et al., unpubl. data [N2]), and P. laticinctum (50.15–8.55 Ma; Williams et al., unpubl. data [N2]).
Two subzones, the E. pectiniformis Subzone (Subzone A-1) and the C. gracile Subzone (Subzone A-2) were recognized based on general features and the occurrence of some key species (Fig. F2).
Cores 184-1148B-56X through Section 39X-CC and Sections 184-1148A-76X-6 through 60X-CC (844–531 mbsf) constitute Subzone A-1 (Fig. F2; Table T2). Its upper boundary with the overlying Subzone A-2 is defined by the LAD of E. pectiniformis. Distatodinium ellipticum, P. amoenum, and Xenicodinium conispinum have their ranges within the subzone. Homotryblium and Impagidinium are observed only rarely or sporadically in this subzone. The known range of E. pectiniformis (36.5–29.3 Ma; Williams et al., unpubl. data [N1, N2]) and P. amoenum (34–29 Ma; Williams et al., unpubl. data [N1, N2]) would constrain the age of Subzone A-1 to be early Oligocene (Rupelian). Xenicodinium conispinum, recorded in the lower Oligocene Boom Clay Formation of Belgium (Stover and Hardenbol, 1993) gives further support for this age assignment.
Sections 184-1148A-59X-4 through 52X-CC (526–473 mbsf) constitute Subzone A-2 (Fig. F2; Table T2). Directly overlying the LAD of E. pectiniformis, this subzone is marked by the simultaneous LADs of E. arcuata, E. multicornuta, H. plectilum, H. tenuispinosum, and H. cinctum as its top. Membranophoridium aspinatum occurs within this subzone. Many typical Paleogene species such as C. diversispinosum, C. gracile, C. inodes, D. ellipticum, E. arcuata, Heteraulacacysta campanula, W. articulata, W. gochtii, W. symmetrica, and T. pelagica have their LADs within Subzone A-2. Wetzeliella articulata and W. symmetrica are particularly abundant in one sample near the top of the subzone. Homotryblium and Impagidinium occur almost continuously throughout Subzone A-2; Homotryblium, in particular, may be abundant to very abundant (Table T2). As mentioned, C. cantharellum, H. plectilum, M. aspinatum, P. laticinctum, T. pelagica, and W. symmetrica were recorded in both the Chattian Zone D15 in northwest Germany (Köthe, 1990) and our Subzone A-2. Membranophoridium aspinatum, in particular, has a known range of 39.64–24.6 Ma (Williams et al., unpubl. data [N2]). Therefore, being part of the Oligocene Zone A and overlying the early Oligocene Subzone A-1, Subzone A-2 can be reasonably dated as late Oligocene.
Although C. diversispinosum has been reported to range from early Eocene (Ypresian) to early Oligocene (Rupelian), personal observation by Eaton et al. (2001) in material from the Grand Banks, offshore eastern Canada, showed that the species may be abundant in strata provisionally dated at least as young as late Oligocene. These authors expected a further upward extension of the range of this species. We find C. diversispinosum to be abundant throughout Subzone A-1 as well as Subzone A-2, with its LAD in Sample 184-1148A-54X-CC (i.e., close to the top of Subzone A-2). It often occurs together with C. ancyreum and C. placacanthum, and, although the occurrence of latter two species extends upward to above the lower Miocene interval, C. diversispinosum has its highest occurrence close to the top of Subzone A-2. These observations lend support for extending the range of C. diversispinosum to the late Oligocene.
This zone consists of Sections 184-1148A-48X-2 through 40X-1 (444–365 mbsf) (Tables T1, T2; Fig. F2).
A drastic change in component taxa, cyst abundance, and species diversity distinguishes Zone B from Zone A. The LADs of typical Paleogene species such as E. arcuata and H. cinctum in Sample 184-1148A-52X-CC marks the top of Zone A. Zone B is characterized by the first abundant datums (FADs) of typical Neogene or Miocene species such as Hystrichosphaeropsis obscura and Melitasphaeridium choanophorum. In addition, the following species also have their FADs in Zone B: Achomosphaera callosa, Cerebrocysta satchelliae, Membranilarnacia? picena, Operculodinium israelianum, Operculodinium piaseckii, Schematophora speciosa, and Spiniferites ramosus subsp. angustus. The ranges of Cleistosphaeridium ancyreum, C. placacanthum, H. rigaudiae, P. laticinctum, Pentadinium taenigerum, and P. zoharyi extend from Zone A into Zone B.
At the boundary between Zones A and B the number of cysts drops from 137 (Sample 184-1148A-52X-CC; 473 mbsf) at the top of Zone A to only 16 specimens (Section 48X-2; 444 mbsf) at the base of Zone B. Indeed, cyst abundance over the entire Zone A is generally much higher (54–7790 cysts, usually >100) than that for Zone B (0–81 cysts, usually <30). Species diversity drops from 93 species/subspecies of 41 genera in Zone A to 54 species/subspecies of 32 genera in Zone B (Table T1; also see "Appendix"). A striking feature of Zone B is the steady presence of Impagidinium, no matter how low the cyst abundance in the individual assemblage. The upper part of Zone B, corresponding to the interval of lithologic Unit IV, has particularly low cyst abundance and species diversity, with only 21 species of 14 genera recorded.
Williams et al. (unpubl. data [N2]) tabulated an age range of 18.93–7.34 Ma for H. obscura. Heilmann-Clausen and Costa (1990) thought this species, well known from the Miocene in various parts of the world, to be an index fossil defining the base of their standard European dinoflagellate Zone D17 (early Miocene). However, they recorded the FAD of this species in association with Tuberculodinium vancampoae (index fossil of Zone D16, latest Oligocene–earliest Miocene) from a transitional horizon in northwest Germany. Stover and Hardenbol (1993) reported H. obscura from the Rupelian (lower Oligocene) Boom Clay Formation of Belgium. Biffi and Manum (1988) defined the Oligocene/Miocene boundary in the Marche region of central Italy by the last occurrence of Deflandrea phosphoritica (and three other less common species) coupled with the earliest occurrence of H. obscura. De Verteuil and Norris (1996) recorded from the United States Mid-Atlantic coastal margin the FAD of H. obscura within their DN1 Chiropteridium galea Interval Zone (late Oligocene–early Miocene; calibrated with the top of nannofossil Zone NP25–NN1 to the lower NN2 and with the top of foraminifer Zone P22 to the lower N4). Based on these records, H. obscura is a typical Miocene species, but its earliest occurrence may extend into the (latest) Oligocene.
Melitasphaeridium choanophorum, a typical Miocene indicator that ranges from 23.9 to 3.75 Ma (Williams et al., unpubl. data [N2]), has been recorded from Miocene strata of northwest Germany (Gerlach, 1961) and from early Miocene offshore eastern Canada (Williams and Bujak, 1977).
Tuberculodinium vancampoae was originally reported only from the Miocene (Benedek, 1986), and the FAD of this species was thought to be early Miocene (Williams and Bujak, 1977; Williams et al., 1993). However, its range was extended to latest Oligocene by de Verteuil and Norris (1996). This is compatible with its occurrence in Zone D16 of northwest Germany and the top sample of our Subzone A-2. Yet, this species is generally recognized as a Neogene indicator.
The earliest occurrence of M.? picena is taken as defining the base of the early Miocene Zone DM1, calibrated by nannofossil Zone NN1 and foraminifer Zone N4, in the Marche Region of central Italy (Biffi and Manum, 1988).
Cerebrocysta satchelliae was first described in the DN2 Sumatradinium soucouyantiae Interval Zone of the United States Mid-Atlantic coastal margin (de Verteuil and Norris, 1996), which is middle lower Miocene–upper lower Miocene, its LAD coinciding with the upper boundary of nannofossil Zone NN2 and located within foraminifer Zone N5. Schematophora speciosa, although described originally from the ?lower Eocene of Australia (Deflandre and Cookson, 1955), has a range of Eocene–Miocene (Williams et al., 1998).
Operculodinium longispinigerum and O. piaseckii were recorded mostly in the lower lower Miocene sediments from different parts of the world (de Verteuil and Norris, 1996). The P. zoharyi Assemblage Zone (Zone B in short) thus can be confidently dated as early Miocene on the basis of the records discussed above.
An interval of ~25–30 m (473–449 or 473–444 mbsf) between Zones A and B is barren of dinoflagellates and other palynomorphs. Nannofossil and foraminifer records indicate a hiatus at 473 mbsf, the period between the lowermost Zone NP25/P22 and Zone NN2/N4 (27–24 Ma) missing (Wang, Prell, Blum, et al., 2000). This is evidence for significant environmental shift during the transition from latest Paleogene to earliest Neogene.