Sediment recovered from Site 1077 represented a continuous hemipelagic section spanning the entire Pleistocene. According to paleomagnetic and nannofossil evidence, drilling at Site 1077 did not penetrate the Pliocene/Pleistocene boundary; the oldest sediment recovered was between 1.67 and 1.77 Ma. Micropaleontological studies were carried out on core-catcher samples from Hole 1077A. Additional samples from within the cores were examined for calcareous nannofossil- and silicoflagellate-based biostratigraphy.
Abundance and preservation varies between the different microfossil groups. Benthic foraminifers are abundant and well preserved down to Core 175-1077A-13H, calcareous nannofossils down to Core 15H, and planktonic foraminifers down to Core 17H. Siliceous microfossils are abundant and well preserved throughout Hole 1077A.
Both planktonic and benthic foraminiferal assemblages display a major change at an approximate depth of 52 mbsf.
The nannofossil-derived stratigraphy is based on a high-resolution study of Hole 1077A. The overall abundance ranges from abundant to common from Core 175-1077A-1H through 14H and generally decreases from Core 15H to the base of Hole 1077A. Preservation follows the same pattern, with well-preserved specimens in the upper half of the section, to moderately and poorly preserved assemblages within the bottom half of the hole. Only a few samples examined are barren. All identified datums could, therefore, be constrained within a narrow depth range of ~4 m (Table 2).
The nannofossil biostratigraphy suggests that drilling at Site 1077 recovered a continuous stratigraphic record spanning the entire Pleistocene (Zones NN21b-NN19). Paleomagnetic data (see "Paleomagnetism" section, this chapter) indicate that Hole 1077A did not penetrate the Olduvai polarity Chron, the termination of which identifies the Pliocene/Pleistocene boundary (1.77 Ma). The onset of this chron (1.95 Ma) is synchronous with the Zone NN19/NN18 boundary, as shown by Berggren et al. (1995). Discoaster brouweri, whose last occurrence (LO) is used as the datum event for this boundary, is not recognized in samples from the bottom of Hole 1077A nor in additional core-catcher samples from the bottom part of Hole 1077B, thereby confirming the paleomagnetic-derived stratigraphy.
Sedimentation rates estimated from nannofossil datum events suggest that sediments representative of Zone NN21 (last 260 k.y.) and of the upper half of Zone NN19 (960-600 ka) accumulated at an average rate of 20 cm/k.y. This pattern is interrupted during Zone NN20 (460-260 ka) and the lower half of Zone NN19, where sedimentation rates are significantly lower (<5 cm/k.y.).
This dominance interval of Emiliania huxleyi is restricted to the top three cores of Hole 1077A (bottom of Zone NN21b, between Samples 175-1077A-3H-6, 70 cm, and 3H-CC).
This interval spans the lower half of isotope Stage 5 to the upper half of isotope Stage 8 (i.e., from 0.09 to 0.26 Ma). The Zone NN21a/NN20 boundary was reached in Hole 1077A at 58.32 mbsf (between Samples 175-1077A-7H-3, 130 cm, and 7H-5, 130 cm).
As already noted at Sites 1075 and 1076, sediments representative of this interval accumulated at a very low rate (~3 cm/k.y.) compared with younger sediments. The Zone NN20/NN19 boundary, which is defined by the LO of Pseudoemiliania lacunosa (0.46 Ma; i.e., within isotope Stage 12) was reached between Samples 175-1077A-7H-CC and 8H-3, 130 cm.
Five datum events were identified within this interval at Hole 1077A. The nannofossil chronology compares favorably with the position of the paleomagnetic chron boundaries (Fig. 8) for this interval. The depth-lag (~10 m) between the LO Small Gephyrocapsa acme event (0.96 Ma; Gartner, 1977) and the termination of the Jaramillo Chron (0.99 Ma) may be partly explained by the low sampling resolution in the vicinity of the nannofossil datum event.
The fauna at Site 1077 is not as strongly affected by dissolution as that at the other Congo Basin Sites 1075 and 1076. There are only two barren samples (Samples 175-1077A-19H-CC and 22H-CC) and one with poor preservation (Sample 175-1077A-18H-CC) at Hole 1077A.
The uppermost sample (175-1077A-1H-CC) is dominated by Orbulina universa with high abundances of Globigerinoides ruber (pink) and Globorotalia inflata (10%-30% of the population; Table 3). Other species present in significant abundance (5% or more of the total) include G. ruber (white), Globigerinoides sacculifer, Globorotalia crassaformis, Globigerinella siphonifera, and Neogloboquad-rina pachyderma (dextral).
Downcore faunal variations at Hole 1077A suggest a change in the hydrographic regime below Core 175-1077A-7H-CC. Abundant Globorotalia inflata has been correlated to the Angola-Benguela Front (ABF; Jansen et al., 1996); possibly, its abundance may be used to trace the position of the ABF downcore. The species is abundant in the upper 52 mbsf (Samples 175-1077A-1H-CC, 3H-CC, 5H-CC, and 6H-CC) and at 127.5 mbsf (Sample 175-1077A-14H-CC). The decrease in abundance at ~50 mbsf suggests that the position of the ABF changed; it may represent a stabilization of the ABF in its present position. A change in benthic fauna occurs at the same depth.
O. universa and N. pachyderma (dextral) are abundant in nearly all samples downcore. O. universa is found in very high abundances in the Benguela Current region (Bé and Tolderlund, 1971). N. pachyderma is an indicator of upwelling in the tropics, along with abundant G. bulloides and N. dutertrei (Mix and Morey, 1996). High abundances of G. ruber (pink and white) in the tropics are associated with warm tropical conditions (Mix and Morey, 1996). G. bulloides is abundant downcore in Samples 175-1077A-5H-CC, 12H-CC, 13H-CC, 15H-CC, and 21H-CC and indicates high productivity associated with upwelling. G. crassaformis is a major component of the assemblage at the shallower water site (1076) but is not significant here, with the exception of a single excursion in Sample 175-1077A-3H-CC.
The benthic foraminifers are well preserved and abundant in the upper 13 core catchers of Hole 1077A (Samples 175-1077A-1H-CC through 13H-CC), except for Sample 9H-CC, which contains very few specimens. Farther downcore, the abundance is significantly lower, and the lowermost four core catchers (Samples 175-1077A-19H-CC through 22H-CC) contain only a few benthic foraminifers or are completely barren.
As at Sites 1075 and 1076, the diversity of benthic foraminifers deteriorates downcore as a result of lower absolute abundance. This results in major difficulties when interpreting the variation in relative abundance of the various species. In the upper 50 mbsf of Hole 1077A, Bulimina aculeata, Cassidulina laevigata, Melonis barleeanum, Oridorsalis umbonatus, and Uvigerina hispidocostata are the dominant species (Table 4; Fig. 9). Below 50 mbsf, the species B. aculeata disappears and is replaced by Bulimina exilis and Melonis pompilioides as dominant species. The contribution of Cassidulina laevigata, Melonis barleeanum, Oridorsalis umbonatus, and Uvigerina hispidocostata is less pronounced. Other species are present in low relative abundances and/or are present only in a few samples.
Radiolarians are present in all of the core-catcher samples from Hole 1077A (Table 5). Radiolarian abundance is common to abundant, and preservation is good. No apparent reworking has been identified.
The radiolarian fauna indicates a Quaternary age for Hole 1077A. The absence of Axoprunum angelinum suggests that the uppermost cores (175-1077A-1H-CC through 7H-CC) are within either the Pleistocene Collosphaera tuberosa Zone or the Pleistocene to Holo-cene Buccinosphaera invaginata Zone of Moore (1995). A finer zonal resolution could not be achieved because of the absence of B. invaginata.
Although the diagnostic species Anthocyrtidium angulare is absent throughout the core, Samples 175-1077A-8H-CC through 18H-CC are assigned to the Pleistocene A. angelinum Zone or Amphirhopalum ypsilon Zone of Moore (1995) based on the presence of A. angelinum and the absence of Lamprocyrtis neoheteroporos. The diagnostic species C. tuberosa, used to recognize the A. angelinum and A. ypsilon Zones, is absent from Samples 8H-CC through 18H-CC. A LO of L. neoheteroporos is found in Sample 175-1077A-19H-CC, indicating an age older than 1.07 Ma for Samples 19H-CC through 22H-CC. The single occurrence of A. angulare in Sample 16H-CC can place the upper boundary of the A. angulare Zone somewhere between Samples 15H-CC and 16H-CC. The LO of L. neoheteroporos is more reliable and useful for correlating Holes 1075A, 1076A, and 1077A, and the upper boundary of the A. angulare Zone is tentatively placed between Samples 18H-CC and 19H-CC.
Diatoms are the dominant microfossil group at Hole 1077A. Species counts and identification were carried out on smear-slides. In addition, opaline phytoliths and silicoflagellates were also counted without distinction of species or morphotypes. In general, diatoms are abundant and well preserved throughout Hole 1077A, except for in Sample 175-1077A-14H-CC (Table 6). Examination of the core-catcher samples indicates a Pleistocene age for this hole. Samples 175-1077A-1H-CC through 12H-CC are assigned to the Pseudo- eunotia doliolus Zone, and Samples 175-1077A-15H-CC through 21H-CC to the Nitzschia rheinholdii Zone. A diatom biostratigraphic marker species is lacking from Samples 175-1077A-13H-CC, 14H-CC, and 22H-CC. The silicoflagellate Mesocena quadrangula is, however, present in Samples 13H-CC (LO, 0.8 Ma) through 16H-CC (first common occurrence at 1.0 Ma; Locker, 1996) and may be used as a biostratigraphic marker instead. This is in agreement with paleo-magnetic data for Hole 1077A (see "Paleomagnetism" section, this chapter).
The diatom flora is dominated by upwelling-indicator species (>50% of total diatom assemblage Thalassionema nitzschioides var. nitzschioides and Chaetoceros resting spores and setae; see Table 6; Fig. 10), accompanied by freshwater taxa (e.g., Aulacoseira granu-lata, A. islandica, and Cyclotella spp.) and neritic species (e.g., Actinoptychus senarius). Two distinct pulses of the coastal species Skeletonema costatum are observed in Samples 175-1077A-4H-CC (~32%) and 8H-CC (~21%), respectively; a single peak of Thalassiosira spp. (mainly T. eccentrica and T. oestrupii) is seen in Sample 4H-CC. In general, diatom assemblages characterize Hole 1077A as a coastal upwelling site with variable freshwater input (Fig. 10).
As at Holes 1075A and 1076A, the presence of freshwater diatoms at Hole 1077A is attributed to supply by the Congo River, and high abundances may be interpreted as signals for humid intervals on the African continent (e.g., Jansen et al., 1989). The average contribution of the freshwater assemblage is somewhat higher here (7.2%) than at Sites 1075 and 1076. The abundance pattern is similar at Site 1076, with two large peaks (~30%) in the upper 50 mbsf at Hole 1077A and a third peak at ~100 mbsf. Opaline phytoliths are rare (Table 6).