The low-latitude radiolarian biostratigraphy (Sanfilippo and Riedel in Saunders et al., 1985) and the code numbers of radiolarian zones (Sanfilippo and Nigrini, 1998) are applied for this study (Figs. F2, F3, F4, F5, F6, F7, F8). Zones RP17 (Cryptocarpium azyx Interval Zone; Sanfilippo and Riedel in Saunders et al., 1985), RP18 (Calocyclas bandyca Concurrent Range Zone; Sanfilippo and Riedel in Saunders et al., 1985), RP19 (Cryptocarpium ornatum Interval Zone; Maurrasse and Glass, 1976), and RP20 (Theocyrtis tuberosa Interval Zone; Riedel and Sanfilippo, 1978) are identified in this study. The zonal boundaries RP17/RP18, RP18/RP19, and RP19/RP20 are defined by the bottom morphotypic appearance (Bm) of C. bandyca, the top morphotypic appearance (Tm) of Thyrsocyrtis tetracantha, and the evolutionary transition from Lithocyclia aristotelis group to Lithocyclia angusta, respectively (see Sanfilippo and Nigrini, 1998).
The uppermost radiolarian zone in each of the examined sediment sequence from Holes 1218A, 1219A, and 1220A is identified to Zone RP20. Zones RP19 and RP18 are identified in the sequences of all three holes. Zone RP17 is the basal part of the examined sequences in Holes 1218A and 1220A.
Middle–upper Eocene Zones RP17, RP18, and RP19 are characterized by relatively larger species numbers than Oligocene Zone RP20 (Figs. F2, F3, F4). In general, T. tuberosa form A, L. aristotelis group, C. ornatum, and Periphaena triactis are common to abundant from Zones RP17 to RP19, whereas C. azyx and Thyrsocyrtis spp. are restricted in Zone RP17 and the lower part of Zone RP18. Lychnocanoma tripodium, Lophocyrtis aspera group, T. tuberosa group, and Stylosphaera coronata laevis occur from Zone RP17 to Zone RP20. L. tripodium is abundant to common in middle–upper Eocene Zones RP17, RP18, and RP19, whereas L. aspera group and T. tuberosa group are abundant in Oligocene Zone RP20.
The Bm of C. bandyca (Zone RP17/RP18 boundary), the Tm of T. tetracantha (Zone RP18/RP19 boundary), and the evolutionary transition from L. aristotelis group to L. angusta (Zone RP19/RP20 boundary) are identified between Samples 199-1218A-25X-3, 120–122 cm, and 25X-3, 75–77 cm (228.90–228.45 mbsf), Samples 24X-4, 121–123 cm, and 24X-4, 75–77 cm (220.41–219.95 mbsf), and Samples 23X-7, 6–8 cm, and 23X-6, 120–122 cm (214.56–214.20 mbsf), respectively (Fig. F2).
The radiolarian fauna from Zone RP17 to lower Zone RP18 is characterized by common to abundant C. azyx, T. tetracantha, L. tripodium group, T. tuberosa group, T. tuberosa form A, C. ornatum, Thyrsocyrtis lochites, P. triactis, L. aspera group, and Stylosphaera coronata laevis. This interval is noticeable by the presence of cosmopolitan species, known from equatorial to southern high-latitude areas such as Thyrsocyrtis spp., Eusyringium fistuligerum, L. aspera group, and T. tuberosa group. T. tuberosa group is abundant in the lowermost part of the examined sequence and decreases in the uppermost part of Zone RP17. In the upper part of Zone RP18, C. azyx, T. tetracantha, T. bromia form A, T. lochites, and S. coronata laevis decrease in abundance, whereas C. ornatum and L. aspera group increase.
In the middle–upper part of Zone RP19, T. tuberosa form A, L. tripodium group, L. aristotelis group, P. triactis, and C. ornatum decrease in abundance and T. tuberosa group becomes the dominant species. The radiolarian fauna in Zone RP20 is characterized by very abundant L. aspera group, Artophormis gracilis, and T. tuberosa group.
Zones RP18, RP19, and RP20 were distinguished in the analyzed interval from this hole (Fig. F3). The Tm of T. tetracantha (Zone RP18/RP19 boundary) is situated between Samples 199-1219A-17H-7, 130–132 cm, and 17H-7, 80–82 cm (157.65–157.15 mbsf), and the evolutionary transition from L. aristotelis group to L. angusta (Zone RP19/RP20 boundary) is located between Samples 199-1219A-17H-2, 80–82 cm, and 17H-2, 30–32 cm (150.80–150.30 mbsf).
The radiolarian fauna in Zone RP18 is marked by abundant to very abundant occurrences of C. ornatum, L. aspera group, and T. lochites with common occurrences of L. aristotelis group, P. triactis, T. tuberosa form A, and L. tripodium group.
In Zone RP19, several species that are abundant in Zone RP18 such as T. tuberosa form A, L. aristotelis group, P. triactis, C. ornatum, and L. tripodium group decrease, whereas T. tuberosa group increases in abundance. In Zone RP20, L. aspera group, T. tuberosa group, and A. gracilis become abundant.
In this hole, Zones RP17–RP20 were recognized in the analyzed interval (Fig. F4). Shipboard Scientific Party (2002c) reported the Zone RP17/RP18 boundary between Sections 199-1220A-9H-7, 45 cm, and 9H-6, 45 cm (85.45–83.95 mbsf). In this study, C. bandyca is present in Sample 199-1220A-9H-7, 25–27 cm (85.25 mbsf), but is absent from Sample 199-1220A-9H-7, 63–65 cm (85.63 mbsf). Therefore, the Zone RP17/RP18 boundary is located between Section 199-1220A-9H-7, 45 cm, and 9H-7, 25–27 cm (85.45–85.25 mbsf). The Tm of T. tetracantha (Zone RP18/RP19 boundary) is located between Samples 199-1220A-8H-6, 77–79 cm, and 8H-6, 25–27 cm (74.77–74.25 mbsf), and the evolutionary transition from L. aristotelis group to L. angusta (Zone RP19/RP20 boundary) is placed between Samples 8H-2, 75–77 cm, and 8H-2, 25–27 cm (68.75–68.25 mbsf).
From Zone RP17 to the middle part of Zone RP18, the radiolarian fauna is characterized by abundant T. lochites, L. aristotelis group, C. ornatum, T. tuberosa form A, P. triactis, S. coronata laevis, L. tripodium group, and L. aspera group. Although their presence persists into the overlying Zone RP19, common to abundant occurrences of T. lochites end within Zone RP18. T. bromia form A and Zealithapium mitra group also decrease in abundance at the same level.
In the middle–upper part of Zone RP19, abundances of L. aristotelis group, C. ornatum, and P. triactis decrease, whereas T. tuberosa group increases. In Zone RP20, L. aspera group and A. gracilis are abundant to very abundant. T. tuberosa group is one of the dominant species in the lower–middle part of Zone RP20 but exhibits a sharp decline in abundance in the upper part of Zone RP20.