Silicoflagellates belong to a small group of siliceous marine phytoplankton. Their siliceous skeleton is very susceptible to dissolution, and therefore their preservation is often hindered by diagenetic processes; moreover, their abundance is relatively low compared to that of other siliceous microfossils; both these reasons make their presence rare in the sedimentary record. Their paleogeographic distribution is often influenced by paleoceanography, driving adaptations of the paleocommunities to water masses and current systems that may control nutrient supply and temperature changes, giving origin to relatively well separated latitudinal bioprovinces. In recent years, a general biostratigraphic framework for temperate to warm waters at middle to low latitudes as been developed (Bukry, 1981b, 1983, 1985, 1995; Perch-Nielsen, 1985), whereas a different scheme is applicable to cold-water assemblages at high latitudes (Locker, 1995, and references therein). In general, the scheme used for Hole 1149A is that proposed by Bukry (1981b), but it differs in the identification of some datum levels because of the rarity or absence of some of the zonal markers used in Bukry's biozonation.
In the stratigraphic section recovered in Hole 1149A, four silicoflagellate biozones and three subzones have been recognized. Zones and particularly significant bioevents are described as follows in stratigraphic order.
Dictyocha aspera aspera, D. fibula, Dictyocha perlaevis perlaevis, Dictyocha brevispina brevispina, Dictyocha brevispina ausonia, Dictyocha stapedia aspinosa, Dictyocha pulchella, Dictyocha longa, Dictyocha calida calida, Distephanus speculum speculum, Distephanus crux s.l., Distephanus speculum minutus, D. boliviensis, Distephanus speculum pentagonus, Mesocena circulus, Mesocena elliptica, and Corbisema triachanta.
D. fibula s.l. and D. aspera, together with D. speculum speculum are the most common taxa in this interval. D. fibula, the zonal marker, is the dominant species in most of the assemblages.
Bukry (1983, 1995) subdivides this zone into three subzones, from bottom to top: D. neonautica, D. pulchella, and Dictyocha angulata Subzones. The D. neonautica Subzone, defined as the interval between the FO and last occurrence (LO) of the marker species, is missing in Hole 1149A; this is compatible with our record, since the upper Miocene part of the section is barren of siliceous microfossils. The scattered occurrence of D. pulchella allows the identification of the homonymous subzone, spanning from the LO of D. neonautica to the LO of the marker species in the original definition (Bukry, 1983). The LO of D. pulchella is present in Sample 185-1149A-8H-5, 126-132 cm (68.46 mbsf), just below the FO of D. stapedia stapedia, indicating the base of the homonymous zone. The D. angulata Subzone, defined as the interval of common occurrence of the marker species above the LO of D. pulchella and below the FO of D. stapedia stapedia (Bukry, 1983), is not identified in the studied material. Two hypotheses may justify this result: the D. angulata Subzone is very short (or condensed) in Hole 1149A and the sampling spacing (~1.5 m) did not allow its identification, or the subzone is missing because a hiatus occurs. This second hypothesis is not supported by the paleomagnetic data, which show a continuous and well-documented record of chronozones. The absence of D. angulata could also be ascribed to rarity of this marker species in our material. Unfortunately, Bukry (1982, 1983, 1984) has already pointed out that the upper boundary of the D. pulchella Subzone is troublesome, since D. pulchella has rarely been recorded also in the overlying D. stapedia stapedia Zone (Bukry, 1983).
In Hole 1149A then, only the D. pulchella Subzone is identified, and it corresponds to the upper part of the D. fibula Zone.
Two significant climatic bioevents occur in this zone: (1) from 75.11 mbsf the assemblage (dominated by Dictyocha species with the apical bar oriented along the minor axis [asperoid] in the interval below) is dominated by Dictyocha species with apical bar oriented along the major axis (fibuloid); this event is classically known as the asperoid/fibuloid reversal; (2) at ~75-70 mbsf we observe the decrease in abundance of Distephanus ssp., particularly the last occurrence of D. boliviensis, recorded in the upper part of the zone in Sample 185-1149A-8H-5, 126-132 cm (68.46 mbsf).
The presence of few Miocene taxa, such as M. elliptica and C. triachanta, is problematic. The presence of C. triachanta could be related to reworking, as suggested also by Bukry (1982) for samples from Deep Sea Drilling Project (DSDP) Site 503 from the same biozone. The same hypothesis is suggested for the presence of M. elliptica, a Miocene species, in Sample 185-1149A-11H-1, 136-142 cm (91.06 mbsf).
The presence of D. calida calida and D. stapedia aspinosa, both common taxa of the overlying D. stapedia stapedia Zone, confirms Bukry's observation (1983, 1984) and supports an extended range for these two species, at least in the Pacific Ocean.
D. fibula, D. perlaevis perlaevis, D. stapedia aspinosa, D. calida calida, D. stapedia stapedia, Dictyocha perlaevis flexatella, D. perlaevis ornata, D. perlaevis delicata, Dictyocha calida ampliata, Dictyocha lingii, D. speculum speculum, D. crux s.l., D. speculum minutus, D. speculum pentagonus, M. circulus, M. quadrangula, C. triachanta (Pls. P1, P2).
D. stapedia stapedia and D. calida ampliata are the most common taxa in this biozone; D. calida calida and D. fibula are also present along most of the interval, whereas other taxa such as D. speculum speculum and D. perlaevis perlaevis gradually decrease and eventually disappear at the top of the zone. Several subspecies of Dictyocha perlaevis were observed in this interval (D. perlaevis flexatella, D. perlaevis ornata, and D. perlaevis delicata), some of them allowing the identification (from bottom to top) of the D. perlaevis ornata Subzone (Bukry, 1981b) and the D. perlaevis delicata Subzone (Bukry, 1976).
In general, the fossil assemblage characterizing this zone has a lower specific diversity with respect to the underlying zone. Most of the species belonging to the genus Distephanus have their last occurrence at the top of the zone.
The presence of D. crux s.l. and C. triachanta in this zone are problematic and could be related to reworking. In particular, the presence of C. triachanta is also recorded in the D. stapedia stapedia at DSDP Site 503A at ~51 mbsf (Bukry, 1982), suggesting reworking of older sediments.
The presence of D. lingii, a taxon usually reported from younger sediments, could be explained by an extended range of this species, as also suggested by Bukry (1983), who reported this taxon in the D. stapedia stapedia Zone from the bottom of the D. perlaevis ornata Subzone.
D. speculum speculum, M. circulus, D. fibula, D. crux s.l., D. perlaevis perlaevis, Distephanus speculum minutus, D. speculum pentagonus, D. stapedia aspinosa, C. triachanta, D. calida calida, D. stapedia stapedia, D. perlaevis flexatella, D. perlaevis ornata, and D. calida ampliata.
The marker species D. perlaevis ornata is rare in the assemblage. Noteworthy are the presence of D. perlaevis flexatella, which is usually rare, and limited to the lower D. perlaevis ornata Subzone, and the scattered occurrence of D. speculum speculum above Sample 185-1149A-8H-3, 127-133 cm (65.47 mbsf).
D. speculum speculum, D. fibula, D. crux s.l., D. perlaevis perlaevis, D. speculum pentagonus, D. stapedia aspinosa, D. calida calida, D. stapedia stapedia, D. perlaevis ornata, D. calida ampliata, D. lingii, M. quadrangula, and D. perlaevis delicata.
The marker species D. perlaevis delicata, together with M. quadrangula, are present in low abundances in this subzone.
D. fibula, D. speculum pentagonus, D. calida calida, D. stapedia stapedia, D. calida ampliata, D. lingii, M. quadrangula, Dictyocha aculeata subaculeata, Distephanus octangulatus, and D. aculeata aculeata.
In the original definition, this zone is described as the acme interval of the marker M. quadrangula (Bukry and Foster, 1973). This sharp signal has not been recorded in the studied material, where M. quadrangula shows a relatively high abundance only in Sample 185-1149A-5H-3, 131-137 cm. Other samples studied from Core 185-1149A-5H are barren of silicoflagellates, and this could obliterate the acme signal in the studied section.
Since the acme end of M. quadrangula is not clearly recognizable at Site 1149, in this work the top of the zone is here recognized with the LO of D. lingii, which correlates to the acme end of M. quadrangula (Perch-Nielsen, 1985). In addition, D. lingii is the most common taxon in this zone.
Within this zone, the specific diversity and total abundance of silicoflagellates drop dramatically. The extreme rareness of the genus Distephanus also characterizes this zone.
D. aculeata aculeata occurs in high abundance in the upper part of this subzone; this datum is in agreement with other findings in the same time interval (Perch-Nielsen, 1985) and suggests an extended range of this taxon.
D. fibula, D. calida calida, D. aculeata subaculeata, and D. aculeata aculeata.
D. aculeata aculeata dominates the assemblage; in general, other Dictyocha spp., with a non-aculeatid morphology, dramatically decline in this interval. Specimen size is smaller than in the underlying sediments.
The paleomagnetic stratigraphy obtained from continuous core recovery in Hole 1149A (103% average recovery rate in Unit I) (Plank, Ludden, Escutia, et al., 2000) allowed silicoflagellate events to be directly calibrated to the geomagnetic timescale (Berggren et al., 1995) for the first time. The boundary between the D. fibula and the D. stapedia stapedia Zones falls in Subchron C2An.1n, in the upper middle Pliocene, in agreement with data presented by Rio et al. (1989). The boundary between the D. perlaevis ornata and the D. perlaevis delicata Subzones falls in Subchron C1r.2r.2r and results as a good proxy for the Pliocene/Pleistocene boundary (Fig. F2). The base of the M. quadrangula Zone is correlated to Subchron C1r.2r.1r (lower Pleistocene).
Several authors in the past have stressed that silicoflagellates are very sensitive to temperature variations (Gemeinhardt, 1934; Mandra, 1969; Ciesielsky and Weaver, 1974; Van Valkenburg, 1980; Bukry and Monechi, 1985). At present, assemblages dominated by the genus Distephanus correlate with low surface temperature, whereas the genus Dictyocha dominates in warm to temperate waters (Mandra, 1969; Mandra and Mandra, 1971, 1972; Ciesielsky and Weaver, 1974; Ciesielsky, 1975; McCartney, 1993). The Dictyocha/Distephanus ratio is then used as temperature proxy in extant and fossil material. Moreover, in older sediments, the ratio between two Dictyocha morphotypes (asperoid/fibuloid) could be used as paleotemperature proxy, with dominant fibuloid morphologies correlating to warmer conditions. We record this reversal in lower Pliocene sediments, confirming the observation of Bukry (1981b, 1995) that this bioevent can no longer be used as a proxy for the Miocene/Pliocene boundary (Martini, 1971); in fact it is diachronous and it only has climatic significance, indicating a major climatic change toward warmer climates.
At Site 1149 the asperoid/fibuloid reversal occurs in Sample 185-1149A-9H-3, 141-147 cm (75.11 mbsf), and it is closely followed by the change in the Distephanus/Dictyocha ratio above (Distephanus decreases abruptly at 70.11 mbsf). The LO of D. boliviensis, a well-known cool-water marker, is recorded in Sample 185-1149A-8H-5, 126-132 cm (68.46 mbsf). All these events, pointing to a warming of the surface waters between 75 and 68 mbsf (Subchron C2An.1n; lower-middle Pliocene), occur in the upper part of the D. fibula Zone (D. pulchella Subzone). The close succession of these events, pointing to increasingly unstable water conditions (asperoid/fibuloid reversal and Distephanus crisis) followed by a sharp warming (disappearance of D. boliviensis) occur in a relatively short time interval (middle Pliocene; 3.3-2.8 Ma). This same time interval has recently been indicated as the final stage of the closure of the Central American Seaway (Tsuchi, 1997), culminating in the final emergence of the Isthmus of Panama. This paleotectonic event marked a dramatic change in the hydrodynamic conditions of both the Pacific and Atlantic Oceans, decreasing the east-west flow and improving the circulation of gyrelike currents. According to Tsuchi (1997), this resulted in an increased strength of the warm Kuroshio Current and led to the present-day hydrodynamic regime. The Kuroshio Current moved its flow closer to the eastern edge of the Asian continent, thus warming both the open ocean waters and the nearshore areas, as it is also recorded by the so-called Sagara molluscan fauna from the sediments of the Pacific coast of Japan (Tsuchi, 1997).
Other biostratigraphic and paleoclimatic signals recorded in our material are the beginning of the acme of M. quadrangula occurring in Sample 185-1149A-5H-3, 131-137 cm (37.01 mbsf), which correlates to a well-known warm-water event (Martini, 1971; Bukry and Foster, 1973; Burkle, 1977; Bukry, 1979).
The beginning of the acme of D. aculeata is present in Sample 185-1149A-4H-3, 134-141 cm (27.54 mbsf), and correlates to the sharp decline of other (fibuloid) dictyochid morphotypes; small-sized aculeatid silicoflagellate skeletons that record adverse environmental conditions (Perch-Nielsen, 1985; McCartney, 1993) are present upsection from this level.
We have also recorded the abundance of aberrant morphologies in the silicoflagellate assemblages; this datum has been regarded as an interesting palaeoecological proxy in recent studies on extant laboratory cultured assemblages (Van Valkenburg and Norris, 1970; McCartney and Loper, 1989; McCartney, 1993). However, our preliminary data do not show a sharp correlation with silicoflagellate blooms or with low volcanic glass in the sediments (Fig. F2, Table T1), even if, in general, high abundances of volcanic glass correlate with low silicoflagellate abundances and absence of aberrant morphologies.