Drilling during ODP Leg 186 provided a high-quality suite of upper Cenozoic cores from the Japan Trench area. Sedimentary sequences containing a relatively continuous record of diatoms have been satisfactory for determining, at relatively high stratigraphic resolution, a biostratigraphic zonation. Based on the available taxonomic studies by Shiono (2000, 2001) and Shiono and Koizumi (2000, 2001), however, we have to examine whether diatom assemblages from the lower Pliocene are treated in conformity with the desirable biostratigraphic rules. Tables T5 and T6 document the stratigraphic occurrences of important taxa belonging to the genus Thalassiosira, including the sample depth in meters below seafloor. The stratigraphic ranges and events on the Pliocene and Quaternary key species in the T. trifulta group are shown in Figure F3, basically following the framework of Shiono and Koizumi (2001). New taxonomic revisions of Thalassiosira require a fresh look at the lower Pliocene.
The T. trifulta group consists of centric marine diatoms forming chainlike colonies that mainly range from the early Pliocene to the Holocene. The T. trifulta group is composed of three subgroups, the T. oestrupii subgroup, Thalassiosira frenguelliopsis subgroup, and Thalassiosira bipora subgroup; their taxonomic and biostratigraphic characteristics were vigorously examined by Shiono and Koizumi (2000, 2001). Based on Shiono and Koizumi's work on the T. trifulta group, taxonomic grouping of taxa and their stratigraphic ranges are summarized in Figure F3.
Thalassiosira praeoestrupii appears at 4.8-4.9 Ma and disappears at 3.2-3.4 Ma in the middle part of Zone NPD8. This is the oldest taxon in the T. oestrupii subgroup. Both the FOs of T. praeoestrupii f. juvenis and T. trifulta occur at the same horizon of the LO of T. praeoestrupii. A species representative of the subgroup is T. oestrupii, extending the range from 2.2 Ma to the Holocene. Unfortunately, the previously used FOs of both T. praeoestrupii and T. oestrupii are inaccurate.
A representative of the T. frenguelliopsis subgroup is T. frenguelliopsis, whose total range is restricted from 5.6-5.7 to 4.9-5.0 Ma, is compactly confined within only 700 k.y., and crosses over the Miocene/Pliocene epoch boundary in the Northern Hemisphere. Slightly below the epoch boundary, the FO of Thalassiosira tetraoestrupii is apparently recognizable at 5.4-5.5 Ma. In the T. frenguelliopsis subgroup, these are remarkable features detected from the northwest Pacific Ocean.
Near the epoch boundary, T. bipora f. prima, as the most primitive taxon of the T. bipora subgroup, first occurs at 5.1-5.3 Ma and becomes suddenly extinct after a short lifespan of <500 k.y. in the early Pliocene. It is a particularly characteristic event of the subgroup that at least four taxa appear simultaneously at 5.0-5.1 Ma, namely the FOs of T. bipora f. marginata, T. bipora f. minima, T. bipora s. str., and Thalassiosira centra coincide with each other immediately above the epoch boundary.
The lower Pliocene NPD7Bb Subzone corresponds to the upper subzone of NPD7B and is also named as T. oestrupii Subzone. However, diatom assemblages from the NPD7Bb Subzone neither include any specimens of T. oestrupii nor document the taxon's first appearance datum. The biostratigraphic unit of the T. oestrupii Subzone should be abandoned from the Pliocene zonal framework in the North Pacific diatom biochronostratigraphy, and we should replace both datums of the FO of T. oestrupii s. ampl. (5.49 Ma) and the FO of T. praeoestrupii (6.1 Ma) with a more reliable stratigraphic indicator as soon as possible.
Because of a consistent occurrence of T. frenguelliopsis and a complete absence of T. bipora f. prima during the Pliocene interval in Hole 1150B, the stratigraphic span between Samples 186-1150B-11R-CC and 15R-CC (802.82 to 837.15 mbsf) can be compressed into the short age-assignment of 5.1-5.7 Ma (Table T5). T. bipora f. prima first occurs in Sample 186-1150B-10R-CC with an age estimate of 5.1-5.3 Ma, and this form shows a somewhat continuous occurrence upward to Sample 186-1150B-5R-CC. Both the FO of T. bipora (5.0-5.1 Ma) and the FO of T. praeoestrupii (4.8-4.9 Ma) are simultaneous with the FO of T. bipora f. prima (5.1-5.3 Ma) between Samples 186-1150B-10R-CC and 11R-CC. An age of 5.1-5.3 Ma is assigned to the stratigraphic interval just below Sample 186-1150B-10R-CC.
Although the FO of T. frenguelliopsis at 5.6-5.7 Ma or the FO of T. tetraoestrupii at 5.4-5.5 Ma should be an effective means to determine the Miocene/Pliocene boundary in the northwestern Pacific Ocean, neither of these events are recognized in Hole 1150B. It is indicated that the Miocene/Pliocene boundary probably lies somewhere at a deeper horizon than Sample 186-1150B-10R-CC (791.04 mbsf).
In Hole 1151A, the lowest sample (186-1151A-60R-CC) analyzed for the genus Thalassiosira contains such important taxa as T. frenguelliopsis, T. bipora f. prima, and T. bipora (Table T6). Their first occurrences are indirectly calibrated at 5.6-5.7, 5.1-5.3, and 5.0-5.1 Ma, respectively, so the age of Sample 186-1151A-60R-CC should be younger than 5.1 Ma.
The coincidence of the two events, the FO of T. tetraoestrupii (5.4-5.5 Ma) and the FO of T. praeoestrupii (4.8-4.9 Ma), is detected between Samples 186-1151A-57R-CC and 58R-CC, and the stratigraphic horizon immediately below Sample 186-1151A-57R-CC is estimated at 4.8-4.9 Ma based on the latter younger event. The interval between Sample 186-1151A-58R-CC and 60R-CC is well dated and has an age between 4.8 and 5.1 Ma. Because of the uncertainty of the FO datums among T. frenguelliopsis, T. tetraoestrupii, T. bipora f. prima, and T. bipora, we failed to determine the Miocene/Pliocene boundary, which is assuredly situated on a deeper horizon than Sample 186-1151A-60R-CC (640.76 mbsf).
The authors tried to detect the Miocene/Pliocene boundary derived from the diatom biostratigraphic zonation; they missed it after all because of their improper sample selection at both sites. It is practically impossible to divide the N. kamtschatica Zone (NPD7B) into the two subzones based upon the definition shown in Table T1 and Figure F2.
Koizumi (1985) introduced the Thalassiosira oestrupii Zone as an interval zone into the Neogene North Pacific diatom zonation. The zonal base is defined by the FO of T. oestrupii s. ampl., and the top is marked by the FO of N. koizumii (= Denticulopsis seminae var. fossilis). Yanagisawa and Akiba (1998) adopted the zone as a subzone representing the upper part of the N. kamtschatica Zone (NPD7B) and supplied the subzone with the code NPD7Bb in their framework. It has been described that diatom assemblages in the NPD7Bb Subzone are characterized by abundant N. kamtschatica with very rare T. oestrupii. The base event is dated at 5.49 Ma by Motoyama and Maruyama (1998) and at 5.5 Ma by Yanagisawa and Akiba (1998), respectively. The top event exhibits latitudinal dischronity in the North Pacific Ocean, which is calibrated at 3.53-3.95 Ma or 3.5-3.9 Ma (Motoyama and Maruyama, 1998; Yanagisawa and Akiba, 1998).
However, in the northwestern Pacific Ocean, the FO of T. oestrupii is recognized at 2.2 Ma. T. oestrupii f. vetus is inferred as an early form of T. oestrupii, and its FO is estimated at 2.4-2.5 Ma by Shiono and Koizumi (2001). The total range of T. oestrupii s. ampl., including the two taxa as mentioned above, is restricted to the upper Pliocene NPD9 Zone through the Quaternary NPD12 Zone, apparently. The name of T. oestrupii is unsuitable for the biostratigraphic unit corresponding to the NPD7Bb (= T. oestrupii) Subzone, because the nominative taxon T. oestrupii s. ampl. is lacking within this interval. In addition, the base of the subzone also should be replaced by another biohorizon.
T. bipora is easily identified based on the characteristic two central porelike structures (occluded areola and central fultoportula) and occupies the solid situation of evolutionary speciation in the T. bipora subgroup. The FO of T. bipora at 5.0-5.1 Ma can be correlated with the lower part of the NPD7Bb Subzone in the early Pliocene. T. bipora f. prima is inferred as an preexisting taxon of T. bipora and first appears at 5.1-5.3 Ma, immediately above the Miocene/Pliocene boundary. But it is almost impossible to classify the two taxa with each other by means of the LM observations. We are able to take a realistic view of T. bipora sensu amplificato including T. bipora, T. bipora f. prima, and other forms and to indicate the FO of T. bipora sensu amplificato as a tentative biodatum, which is at ~5.0-5.3 Ma in the earliest Pliocene.
According to Shiono and Koizumi (2001), T. tetraoestrupii has a long range extending from the early Pliocene to the Quaternary, and its FO can be correlated with the base of the NPD7Bb Subzone, which is dated at 5.4-5.5 Ma. Unfortunately, T. tetraoestrupii occurs too rarely through the interval representing Subzone NPD7Bb to be nominated as representative of the subzone. The FO of T. frenguelliopsis is also very close to the base of the NPD7Bb Subzone, which is calibrated at 5.6-5.7 Ma (Shiono and Koizumi, 2001). It is actually difficult, however, to distinguish T. frenguelliopsis in LM observations from T. praeoestrupii because there are many morphologic similarities between two species.
Although stratigraphic distributions of T. tetraoestrupii and T. frenguelliopsis are evidently observed from Holes 1150B and 1151A (Tables T5, T6), further research into potential candidates for representation needs to be done for the North Pacific Ocean. Dumont et al. (1986) reported that the FO of their T. praeoestrupii was tied to the top of paleomagnetic event C3An.1n (or 5.9 Ma) at Santa Cruz in California and also that those typical specimens occurred below the FO of T. oestrupii s. ampl. in upper Miocene sediments. It is highly probable that their T. oestrupii s. ampl. includes specimens of both our T. tetraoestrupii and T. frenguelliopsis. Shiono and Koizumi (2000) perceived that T. tetraoestrupii and T. frenguelliopsis may have been confused with T. oestrupii in previous studies because their FOs are close to the FO of T. oestrupii previously described.
Although T. bipora is very rare in relative abundance, it shows successive occurrences during the Pliocene through Quaternary in the northwest Pacific Ocean (Shiono and Koizumi, 2001). Similarly, the appearance of T. praeoestrupii ordinarily typifies the early Pliocene interval, but its abundance is also very rare. Because of the presence of the characteristic trifultate fultoportula, it is easy to identify T. praeoestrupii in the SEM observations. The FO of T. praeoestrupii is recognized near the base of the NPD7Bb Subzone, which is well dated at 4.8-4.9 Ma, considerably younger than the Miocene/Pliocene boundary. The LO is estimated at 3.2-3.4 Ma from the middle part of the NPD8 Zone above the FO of N. koizumii (3.53-3.95 Ma).
We would like to modify the T. oestrupii Subzone toward the new T. bipora Subzone, the base of which is definable at 5.0-5.3 Ma by the FO of T. bipora sensu amplificato including T. bipora, T. bipora f. prima, and other forms. The authors did not succeed in the establishment of precise biostratigraphy across the Miocene/Pliocene boundary in Holes 1150B and 1151A, and therefore they refrain from making a new proposal based on T. bipora sensu amplificato. The problem subzone still remains to be solved and is beyond our biostratigraphic study on Sites 1150 and 1151.