2026 年 30 巻 p. 28-45
We examined biostratigraphic age inferences based on planktonic foraminifera and calcareous nannoplankton and documented the faunal associations of benthic foraminifera during the Quaternary in core PC311 taken from the top of a seamount (1,370 m water depth) in the Magellan Seamounts (western Pacific Ocean). Six and five biostratigraphic datums were recognized by planktonic foraminifera and calcareous nannofossils, respectively. In the lower part of the core, a planktonic foraminiferal datum occurred at the depth corresponding to 2.332 Ma, whereas the two datums of calcareous nannofossils were observed at 2.76 Ma and 2.78 Ma. As a result, we propose two age models. The diversity measures of benthic foraminifera declined gradually from ~1.3 Ma to the present. Stilostomellidae and Pleurostomelidae also decreased gradually after ~1.3 Ma and disappeared at ~0.7 Ma, as taxa went extinct across the Mid-Pleistocene Transition. In contrast, Pseudoparrella obtusa, a possible phytodetritus species, increased intermittently after ~1.3 Ma. In the lower part of the core, Globocassidulina subglobosa–Globocassidulina ryukyuensis plexus were abundant before ~1.5 Ma, whereas Globocassidulina obtusa became common after ~1.4 Ma. The faunal transition in core PC311 from the seamount top in the western Pacific Ocean appears to be related to the enhanced zonal gradient of sea surface temperature between the eastern and western Equatorial Pacific Ocean at ~1.3 Ma as well as the changes in global deep-water circulation including the persistent North Pacific Deep Water and/or the Upper Circumpolar Deep Water from the Southern Ocean-originated deep water.
Northern Hemisphere glaciation (NHG) marked an important global cooling event during the Cenozoic (Lyle et al., 2008; Fedorov et al., 2015). During this cooling trend, the meridional and equatorial gradients of sea surface temperature (SST) developed into an important climatic driver (e.g. Fedorov et al., 2015). In contrast, during the Late Neogene, the closing of the Indonesian Seaway and the Panama Isthmus was crucial for warming conditions in the western Pacific Ocean through the evolution of the Kuroshio Current (Lyle et al., 2008). These tectonic events led to the eventual development of the present-day Western Pacific Warm Pool (Lyle et al., 2008). The warmer SST condition in the eastern and western Equatorial Pacific Ocean during the Early Pliocene, known as “Permanent El Niño”, is markedly different from that of the modern ocean (Wara et al., 2005), whereas the Early-to-Middle Pleistocene transition represented an enhancement of the zonal SST gradient between the eastern and western Equatorial Pacific Ocean (e.g. Fedorov et al. 2015).
During the Early Pliocene, the temporal development of the Pacific Meridional Overturning Circulation (PMOC) (e.g. Motoi et al., 2005) was reported in the transient deep-water formation of the North Pacific Deep Water (NPDW) (Burls et al., 2017). Based on a neodymium isotope study of the carbonate fraction, Le Houedec et al. (2016) suggested that the NPDW was formed on the Ontong-Java Plateau (Ocean Drilling Program (ODP) Site 807) in the western Equatorial Pacific Ocean) during the Early Pliocene. During the same period, Feng et al. (2022) suggested the transient formation of NPDW based on the carbon isotope composition of benthic foraminifera at ODP Site 807. However, limited evidence of deep-water circulation at intermediate depths in pelagic settings has hampered our understanding of PMOC development.
The major extinction of deep-sea benthic foraminiferal taxa occurred during the Mid-Pleistocene Transition (MPT: ~1.25–0.70 Ma; Pisias and Moore, 1981) (e.g. Thomas, 2007; Hayward et al., 2012). Therefore, it is imperative to constrain the timings of taxon disappearances at various locations and depths. Furthermore, the diversification of Cassidulinidae has also occurred since the start of the Pleistocene (e.g. Nomura, 1983b). Thus, it is important to document the extinction/diversification processes of deep-sea benthic foraminifera at low-latitude bathyal depths in the western Pacific Ocean and consider possible relationships with global climate changes, such as the east-west SST gradient and global deep-water circulations.
In this study, we examined biostratigraphic age inferences based on planktonic foraminifera and calcareous nannoplankton and documented the faunal associations of benthic foraminifera during the Quaternary in core PC311 which was collected from the bathyal depth (1,370 m) of the Magellan Seamounts (western Pacific Ocean). These results contribute to our understanding of paleoceanographic changes not only in the surface ocean, but also in the deep ocean of the low-latitude pelagic setting of the subtropical western Pacific Ocean.
Low-latitude regions in the western Equatorial Pacific Ocean are characterized by warm surface water, the North and South Equatorial Currents (NEC and SEC, respectively) and trade winds (e.g. Wyrtki, 1974; Li and Fedorov, 2022). The salinity front is formed between the NEC and North Pacific Gyre (NPG) owing to precipitation with the Intertropical Convergence Zone (ITCZ) in the western Pacific Ocean (e.g. Kao and Lagerloef, 2015) (Supplementary figure 1). The salinity front, which is usually positioned at ~15°N, is affected by seasonal variability and the El Niño-Southern Oscillation (Kao and Lagerloef, 2015). The chlorophyll-a concentration in the surface water of the study area is low because of the latitudinal distance from the equatorial wind-driven mixing zone (Supplementary figure 1). The abyssal depths in the western Pacific Ocean are largely bathed by Upper Circumpolar Deep Water (UCDW), which originates from the Southern Ocean (Reid, 1997). In contrast, the North Pacific Intermediate Water (e.g. Reid, 1965) has been strongly developed, particularly during glacial periods, originating from the high-latitude areas in the North Pacific Ocean, including the Bering Sea and Okhotsk Sea.
Piston core PC311 (598 cm in length) was collected from a seamount top (17°0.559ʹN, 152°1.907ʹE; 1,370 m water depth; Figure 1) of the Magellan Seamounts in the western Pacific Ocean. The core site is located at the boundary between the NEC and NPG. The lithology consists primarily of homogeneous and sometimes burrowed calcareous oozes without marked sedimentary structure. Samples were collected at 1-cm intervals, with the exception of the soupy top part of the core (5–0 cm). Core sediments in the 298–268 cm interval were disturbed during core handling. All samples were freeze-dried, and an aliquot of approximately 1–6 g of sediment was collected every 10 cm for foraminiferal analysis (62 samples). In addition, 44 samples (approximately 1 g) were collected separately from the lower part of the core at depths ranging from 598 cm to 235 cm for calcareous nannofossil analysis.
After being weighed, the 62 samples collected at ~10-cm intervals from 598.5–2.5 cm for foraminiferal analysis were soaked in warm water and wet-sieved using a 63-μm sieve. The residues were dried at 50°C, then re-weighed to calculate the %coarse fraction (>63 μm) and split into 1/2 to 1/32 aliquots. More than 200 benthic foraminiferal specimens were picked, identified and counted in the >105 μm fractions of 32 samples at approximately 20-cm intervals using a binocular microscope following Nomura (1995). Taxonomic assignments followed van Morkhoven et al. (1986), Jones (1994) and Kawagata (1999). The generic classification of Loeblich and Tappan (1987) was used and updated in certain instances, particularly for the uniserial taxa presented by Hayward et al. (2012) and for Cassidulinidae presented by Nomura (1983a, b, 1999).
We calculated the abundance of benthic foraminifera per unit weight of sediment using the specimen counts, number of splits, and weight of each sample. Expected diversity E(Sn) (expected number of species in samples rarefied to n individuals; n=50) was calculated for each sample in the statistical programming environment R (R Development Core Team, 2020) using the function from the Vegan community ecology package (Oksanen et al., 2019). The values of the Shannon-Wiener function (Hʹ) (this is the same as H(S) in Buzas and Gibson [1969]) and the Buzas-Gibson evenness (originally equitability in Buzas and Gibson [1969]) were calculated using Microsoft Excel.
Planktonic foraminiferaHalf of each washed residue aliquot from the 62 samples was used for the planktonic foraminiferal biostratigraphic study. Planktonic foraminiferal specimens larger than 125 μm were observed using a binocular microscope. A total of 10,000–40,000 specimens were observed for each sample.
In this study, the planktonic foraminiferal zonal scheme, biohorizons and taxonomy generally followed Wade et al. (2011; Supplementary document), who modified the zonal scheme of Berggren et al. (1995a, b) and compiled the astronomically tuned ages of planktonic foraminiferal biohorizons. In addition, the temporal changes of dominant coiling direction of Pulleniatina spp. have been used as a good stratigraphic marker for global correlation (Saito, 1976; Pearson and Penny, 2021). Pearson and Penny (2021) examined high-resolution coiling direction changes at IODP Sites U1483 and U1486 in the western tropical Pacific and the northwest Australian margin of the Indian Ocean, respectively. We referred to this framework as the reference succession of coiling direction change in this genus.
Calcareous nannoplanktonFor the 44 samples in the lower part of core PC311 (598–235 cm), calcareous nannofossil analysis for age estimation was conducted at Palynosurvey Co. Ltd. Sediment samples were processed following Takayama (1978). A small amount of the dried sediment was placed in a small beaker and suspended in 20 ml of water. The suspension was placed in a plastic straw tube after mixing for approximately 30 seconds and then placed on a cover glass (18 mm×23 mm). The suspension was then dried on a hotplate at ~40°C before being mounted on a microslide using Norland Optical Adhesive.
Two hundred nannofossils were identified and counted under a binocular phase-contrast microscope with a polarizing unit. Taxonomic assignments followed those of Aubry (1984), Perch-Nielsen (1985), Pujos (1987), and Young (1998). The genus Gephyrocapsa was first identified at the species-level, and then other forms (i.e., Gephyrocapsa spp.) were assigned to large (>6 μm), medium (3–6 μm), and small (<3 μm) size groups based on the coccolith long diameter, following Takayama and Sato (1987) and Matsuoka and Okada (1989). Neogene calcareous nannofossil datums were adopted as proposed by Okada and Burkry (1980), Takayama and Sato (1987), Sato and Takayama (1992), and Sato et al. (1998). In addition, revised ages for datums from the Pleistocene and Pliocene were adopted from Sato et al. (2009).
Biostratigraphic age inferenceA biohorizon may be present at any point between two samples. In addition, each sample is 1-cm thick. To account for this, we considered the sampling range for each data point, as we reported the depth of each biohorizon. For example, we recognized that the top horizon of a species lies between the base of the upper sample and the base of the lower sample. We converted these horizons into biostratigraphic ages based on Geologic Time Scale 2020 (Neogene: Raffi et al., 2020; Quaternary: Gibbard and Head, 2020). Within the disturbance interval (298–268 cm), we considered a datum with a 30-cm thickness to be within the error range.
Planktonic foraminifera were found in all 47 samples (Supplementary table 1). The preservation of foraminiferal tests was moderate to good (Figure 2). Figure 3 shows the stratigraphic occurrence of the index species in core PC311. The last occurrences of Menardella multicamerata and Globigerinoides extremus, Globigerinoidesella fistulosa, Globigerinoides obliquus, Globorotalia tosaensis and Globigerinoides ruber (pink form) were observed at 420.5 cm, 270.5 cm, 200.5 cm, 90.5 cm and 6.5 cm, respectively (Figure 3). On the other hand, the first occurrence of Globorotalia truncatulinoides was recognized at 570.5 cm (Figure 3). In addition, a change from dextral to sinistral coiling direction in Pulleniatina is observed at 380.5 cm (Figure 3), corresponding to W4 in Pearson and Penny (2021).
Based on the above results, the following datums for planktonic foraminifera are inferred. The first occurrence of G. truncatulinoides at 2.332 Ma was observed at 570.5 cm. The first appearance of this species is usually constrained to 1.93 Ma, whereas a diachrony on the first appearance of this species at 2.332 Ma was reported at ODP Site 806 (e.g. Lam et al., 2022). In the present study, we adopted the first appearance of this species as recognized by Lam et al. (2022). In addition, the last occurrences of G. obliquus (1.30 Ma), G. tosaensis (0.61 Ma) and G. ruber (pink form) (0.12 Ma) were observed at 200.5 cm, 90.5 cm and 6.5 cm, respectively. The last occurrence of G. fistulosa (1.88 Ma) was also observed at 270.5 cm, although we adopted this as the reference datum within the error range due to the core disturbance (298–268 cm). Furthermore, a change in the coiling direction of Pulleniatina at 2.03 Ma (Pearson and Penny, 2021) was observed at 385.5 cm (Supplementary figure 2). However, we judged that the last occurrence of M. multicamerata (2.98 Ma) at 420.5 cm was reworked compared with the other datums. In addition, the last occurrence of G. extremus (1.97 Ma) at 420.5 cm appeared less reliable than the stratigraphic position of W4 for the coiling direction of Pulleniatina.
Calcareous nannoplanktonCalcareous nannofossils were found in all 44 samples from the lower to middle part (597.5–230.5 cm) of core PC311 (Supplementary table 2). The preservation was generally moderate to good (Figure 4). Figure 3 shows the stratigraphic occurrences of the index species in core PC311. The last occurrences of Reticulofenestra cf. ampla, Discoaster tamalis, Discoaster surculus, Discoaster pentaradiatus and Discoaster brouweri were observed at 510.5 cm, 500.5 cm, 480.5 cm, 450.5 cm and 350.5 cm, respectively (Figure 3). On the other hand, the first occurrences of Gephyrocapsa caribbeanica, Gephyrocapsa oceanica and Gephyrocapsa (large) were observed at 270.5 cm, 260.5 cm and 240.5 cm, respectively (Figure 3).
From the above results, the datums for calcareous nannofossils are inferred as follows: The last occurrences of D. tamalis (2.76 Ma), D. surculus (2.53 Ma) and D. pentaradiatus (2.39 Ma) were observed at 500.5 cm, 480.5 cm and 450.5 cm, respectively. Although the last occurrence of D. brouweri at 1.99 Ma was difficult to specify owing to sporadic upward occurrences, we observed the last occurrence at 350.5 cm, based on its end of continuous occurrence. Determining the stratigraphic positions of the first occurrences of G. caribbeanica (1.763 Ma) and G. oceanica (1.706 Ma) was also problematic because of their continuous but sporadic occurrences. Thus, we do not recognize these datums, mainly because of core disturbance interval (298–268 cm). The first occurrence of Gyphyrocapsa (large) at 1.392 Ma was observed at 240.5 cm.
The last occurrence of D. brouweri and the first appearances of G. caribbeanica and G. oceanica were un clear (Figure 3). The unusual occurrences of these calcareous nannofossils in the core disturbance interval (298–268 cm) may represent possible contamination, and we excluded these datums from the discussion of age inferences.
Age models and sedimentation rateFigure 5 shows the age-depth relationship based on the stratigraphic horizons of the datums of planktonic foraminifera and calcareous nannofossils in core PC311. Compared to the stratigraphic horizons of the datums, several differences were observed between the two fossil groups. Planktonic foraminifera showed a relatively younger age at 575 cm, whereas the calcareous nannofossils showed relatively older ages in the lower part of the core (Figure 5A; Supplementary table 3).
We used the coiling direction of Pulleniatina in core PC311 as an independent time marker (Figure 3). The first occurrence of Gephyrocapsa (large) was consistent with this marker, whereas the last occurrences of D. pentaradiatus and D. surculus were inconsistent with it. However, because the coiling direction of Pulleniatina was not uniform between IODP Sites U1483 (Timor Sea) and U1486 (Bismark Sea) (Pearson and Penny, 2021), it was difficult to evaluate whether the last occurrence of D. tamalis was reliable or not using this metric. Therefore, the last occurrences of D. pentaradiatus and D. surculus were dismissed as unreliable datums, because of possible reworking. Instead, we used the first occurrence of Gephyrocapsa (large) and the last occurrence of D. tamalis.
We attempted to construct two age models based on the biostratigraphic datums of both planktonic foraminifera and calcareous nannofossils because these two fossil groups differ slightly in their stratigraphic positions in the lower part of the core (Figure 5B, C). The first age model adopts the first occurrence of G. truncatulinoides (2.332 Ma) at 570.5 cm as the core bottom age (Figure 5B). The second age model adopted the last appearance of D. tamalis (2.76 Ma) at 500.5 cm (Figure 5C). Based on the comparison of the coiling ratio of Pulleniatina and the faunal association of planktonic foraminiferal fauna, the first age model seems more plausible with less influence by the possible reworking of microfossils. In contrast, Khim et al. (2025) recently implied a possible diachrony for the first occurrence of G. truncatulinoides in core WP-GPC-202302, which was collected near PC311 core site. Thus, both age models should be argued further using additional datums compared to other age determinations in the surrounding areas.
According to the first age model, sedimentation rates were higher in the lower part of the core (0.06–0.76 cm kyr−1 below ~270 cm) than in the upper part (0.17–0.43 cm kyr−1 above ~270 cm) (Figure 5B). Because the study site moved in the WNW direction with the Pacific Plate motion, the sedimentation rate change might be partially attributed to the tectonic movement of the study site. Given that the Pacific Plate moves by ~8 cm yr−1, the position of the core site drifted by ~152 km between ~1.9 Ma (~270 cm) and the present day. Therefore, it is difficult to consider that the core site has experienced the different water mass or different climatic zone of the present day due to the plate motion.
Several studies have suggested that the mean position of the ITCZ has migrated southward in the equatorial Pacific Ocean since the MPT (e.g. Seo et al., 2015; Wang et al., 2023). In particular, Wang et al. (2023) suggested that surface water salinity decreased from ~1.8 to 0.5 Ma based on the oxygen isotope record of seawater at ODP Site 871 (5.56°N) (Supplementary figure 3). They interpreted the cause of the long-term decline in surface water salinity to be increasing precipitation at ODP Site 871 due to the southward ITCZ displacement. Because the present-day collection site (~17°N) of core PC311 is located at the boundary (more oligotrophic) between the NEC and the NPG, the southward movement of the mean ITCZ position may affect the decrease of primary production. Because core PC311 primarily consists of biogenic carbonates such as calcareous nannofossils and foraminifera, the decreasing sedimentation rate above ~270 cm (~1.9 Ma) may be explained by the southward displacement of the mean ITCZ position, migrating from the equatorial wind-driven mixing zone, which is characterized by relatively high primary production.
Faunal associations of Quaternary benthic foraminifera in core PC311 Structure of the community and faunal associationFossil benthic foraminifera were observed in all 32 samples from core PC311 (Supplementary table 4). There was no apparent destruction or abrasion of tests of benthic foraminifera, even in taxa with fragile and thin test walls such as Epistominella exigua and Alabaminella weddellensis (Figures 6, 7, 8). Although we cannot solely dismiss possible transportation of the foraminiferal tests, these observations suggest that the benthic foraminiferal fauna was unlikely to be disturbed by strong transportation from other areas to the core site. The test conditions of benthic foraminifera also suggested no marked dissolution effect on the faunal data. In fact, because the %coarse fraction (>63 μm) is regarded as a proxy of carbonate corrosivity on the seafloor, it is generally stable throughout the core except for the lowermost part (Figure 9). This observation supports the absence of a marked change in carbonate corrosivity throughout core PC311.
The abundance of benthic foraminifera (i.e., number of occurrences per gram of sediment), ranging between 160 and 1421 individuals per unit weight (Figure 9), was high in the interval above 200 cm with frequent fluctuations compared to the interval below 200 cm. This pattern is generally similar to that of the %coarse fraction (Figure 9). Globocassidulina subglobosa, Globocassidulina obtusa, Pullenia spp. and Cibicidoides mundulus were common constituents, subordinated with Paracassidulina sulcata, E. exigua, A. weddellensis, Oridorsalis umbonatus, Gyroidinoides sp. A and Astrononion echolsi (Figures 6, 7, 8, 9, 10). It can be difficult to differentiate between Globocassidulina subglobosa and Globocassidulina ryukyuensis (Figure 6) in younger individuals because of their similar test morphologies. In this study, we regarded these two species as a combination, Globocassidulina subglobosa–Globocassidulina ryukyuensis plexus. Both rarefaction (E[S50]) and Shannon-Wiener (H′) are high in the interval below ~220 cm, particularly in ~570–540 cm (Figure 9). In contrast, both species diversity measures were relatively low above 200 cm, with a large temporal decline at 60 cm. Evenness of Buzas and Gibson (1969) decreased significantly in the interval above ~200 cm, which was close to the beginning of the relatively low species diversity indices (E[S50] and Shannon-Wiener [H′]). These decreasing patterns in the community structure across ~200 cm were generally the opposite of the increasing pattern in the abundance of benthic foraminifera (Figure 9).
Stilostomellidae and Pleurostomellidae generally decreased upward, with several fluctuations in the interval above ~200 cm (Figure 10) and disappeared at ~60 cm. These taxa became extinct across the MPT (e.g. Hayward et al., 2010, 2012). Our results also imply the extinction of these taxa across the Early/Middle Pleistocene, which is similar to the results of previous studies (e.g. Hayward et al., 2012). These extinct taxa may explain the general decreasing trend in species diversity in the upper part of the core (Figures 9, 11). Alternatively, Abditodentrix pseudothalmanni increased throughout the 200–100 cm interval (Figure 10). In addition, Stilostomellidae temporally decreased between ~380–200 cm (centered at approximately 300 cm), whereas A. echolsi temporally increased throughout this interval. Thus, the faunal transition of benthic foraminifera was observed at ~200 cm and temporally at ~60 cm in core PC311, based on the diversity measures (Figure 9). In addition, temporal and gradual changes in the faunal association occurred in ~380–200 cm (centered at ~300 cm) (Figure 11).
Over ~2 million years, an increase of A. echolsi with a temporal decrease of Stilostomellidae was discernible in the middle part of the core (~380 cm and 200 cm). Because A. echolsi seems to be a species related to the Southern Ocean-originated deep water (Nomura, 1995), the common occurrence of A. echolsi may be explained by the greater influence of the Southern Ocean-originated deep water (i.e., UCDW in the Pacific Ocean) at the shallower (lower bathyal) depth of our study site. Although the formation of the NPDW intensified during the Early Pliocene, it gradually weakened over time (e.g. Burls et al., 2017). The neodymium isotope of bulk carbonates and carbon stable isotope data of benthic foraminifera from ODP Site 807 (2807 m water depth at present) imply a gradual weakening of NPDW formation toward ~2 Ma (Figure 11; Le Houedec et al., 2016; Feng et al., 2022). The termination of the NPDW formation and the greater influence of the UCDW on our study site may have affected the faunal changes of benthic foraminifera in core PC311 at the lower bathyal depth of the western Pacific Ocean.
Deep-sea benthic foraminifera in pelagic settings are thought to strongly depend on the food supply from the ocean surface and various species adopt different trophic conditions (Gooday, 1994; Jorissen et al., 2007). The relative abundance of C. mundulus, an oligotrophic species (e.g. Altenbach et al., 1999), increased above ~180 cm (Figure 10). This is consistent with our interpretation of decreasing primary production in the upper part of the core presumably due to the southward migration of the mean ITCZ position, similar to the decreasing sedimentation rate. Although the abundance of benthic foraminifera per unit weight increased in the same interval, this can be interpreted by the expense of the decreasing matrix (i.e., fine fraction, such as calcareous nannoplankton).
Epistominella exigua was continuously common in the interval below ~180 cm, whereas A. weddellenesis increased in ~180–70 cm and Pseudoparrella obtusa was occasionally dominant in the interval above ~180 cm, particularly at ~70 cm and the core-top (Figure 10). Epistominella exigua and A. weddellensis are phytodetritus species that can adapt to an episodic food supply from the ocean surface (e.g. Gooday, 2003). Although there is little ecological information on P. obtusa, the test morphology of this species is similar to that of the phytodetritus species, which is characterized by the small test and thin walls with relatively large comma-shaped apertures with teeth (Figure 7). These alternations in common taxa could be explained by the replacement of ecological niches after major extinction across the MPT.
Throughout the Quaternary, SST in the western Equatorial Pacific Ocean has changed little, whereas SST in the eastern Equatorial Pacific Ocean has declined since the Pliocene owing to the enhancement of equatorial upwelling by trade winds (e.g. Fedorov et al., 2015). The faunal transitions in core PC311 at ~1.3 Ma was roughly coincident with the enhancement on the zonal SST gradient between the eastern and western Equatorial Pacific Ocean (e.g. Fedorov et al., 2015; Figure 11). The enhanced zonal SST gradient may stimulate interannual variations in food supply from surface water in the Equatorial Pacific Ocean (e.g. Fedorov et al., 2015). Consequently, such an enhanced interannual variation in food supply may explain the increased episodic food supply from the ocean surface to the benthic foraminiferal fauna on the seafloor.
In summary, the faunal transition of bathyal benthic foraminifera in core PC311 can be attributed to changes in deep-water circulation, including the influence of the NPDW/UCDW and the enhancement of the zonal SST gradient between the eastern and western Equatorial Pacific Oceans. These paleoceanographic circumstances may also provide important clues for understanding the long-term transition of present-day deep-sea benthic foraminifera in the low-latitude pelagic region of the western Pacific Ocean.
Diversification of Cassidulinidae during the QuaternaryAnother feature of the benthic foraminiferal faunas in core PC311 is the common occurrence of various species of Cassidulinidae beginning at ~2.4 Ma or ~2.8 Ma, depending on the first and second age models (Figure 5B, C). First, the Globocassidulina subglobosa–Globocassidulina ryukyuensis plexus was abundant in ~500–460 cm (~2.2 Ma or ~2.8–2.5 Ma based on the first or second age model). Then, Paracassiduline sulcata was abundant in ~460–380 cm (~2.0 Ma or ~2.2 Ma in the first or second age model). Finally, Globocassidulina obtusa was common in the interval of ~220–180 cm (~1.4–1.1 Ma) and temporally at ~100 cm (Figure 11). The Pliocene–Pleistocene was a time of diversification for Cassidulinidae (Nomura, 1983b). The onset of the abundance of the G. subglobosa–G. ryukyuensis plexus with common P. sulcata from either ~2.4 Ma or ~2.8 Ma corresponds to the timing slightly after the NHG (Figure 11). In the North Pacific Ocean, the transient formation of the NPDW weakened after the Early Pliocene (Figure 11; Le Houedec et al., 2016; Feng et al., 2022). Thus, the occurrence of Cassidulinidae appears to be related to deep-water circulation changes in the North Pacific Ocean.
In addition, G. subglobosa prefers deep-sea environments with high organic carbon content (Miao and Thunnel, 1993) or more pronounced seasonal food pulse (Gooday, 1994; Suhr et al., 2003; Eberwein and Mackensen, 2006; Suhr and Pond, 2006; Gooday et al., 2008). There are few ecological reports on G. obtusa, but this species seems to be recognized in other studies as Globocassidulina crassa. Globocassidulina crassa has been reported from bathyal depths in the Ross Sea, where its habitat is shallower than that of G. subglobosa (e.g. Fillon, 1974). Such an occurrence is consistent with our inference regarding the possible influence of the UCDW in ~380–200 cm, if the Southern Ocean-originated deep water continuously influenced at our study site. Globocassidulina crassa also responds to seasonal phytoplankton blooms (Suhr and Pond, 2006). Gastaldello et al. (2024) regarded this species as one of “phytodetritus exploiting taxa”, with G. subglobosa in the western Equatorial Pacific Ocean. Given that G. obtusa as sampled from core PC311 has ecological characteristics similar to those of G. crassa, the common occurrence of G. obtusa is consistent with an intensified episodic food supply and an enhanced zonal SST gradient in the Equatorial Pacific Ocean (Figure 11). Thus, the diversification of Cassidulinidae in the lower bathyal zone of the low-latitude region of the western Pacific Ocean during the Early Pleistocene seems to have been affected by trophic changes, particularly the enhanced episodic food supply from the surface ocean in addition to the influence of the UCDW from the Southern Ocean-originated deep water.
We studied fossil calcareous nannoplankton and planktonic and benthic foraminifera during the Quaternary in core PC311 from the seamount top of the Magellan Seamounts (western Pacific Ocean). Our conclusions are as follows.
(1) Six and five datums were recognized by planktonic foraminifera and calcareous nannofossils, respectively. In the lower part of the core, a planktonic foraminiferal datum occurred at 2.332 Ma at 570.5 cm, whereas two datums of calcareous nannofossils were determined at 2.76 Ma at 500.5 cm. Based on these datums, we proposed two age models that differ below ~400 cm. The first age model is consistent with the coiling direction of Pulleniatina and the faunal association of planktonic foraminifera, although a precise age model should be evaluated further using more robust datums.
(2) Both rarefaction and Shannon-Wiener (Hʹ) declined gradually from ~220 cm (~1.3 Ma). These decreasing patterns in community structure were generally the opposite of the increasing pattern in the abundance of benthic foraminifera across ~200 cm. Stilostomellidae and Pleurostomellidae decreased gradually after ~1.3 Ma and practically disappeared at ~0.7 Ma. These taxa became extinct during the Mid-Pleistocene Transition, whereas the earlier taxonomic loss of these taxa may explain the general decline in species diversity measures in core PC311. This timing roughly coincides with the enhanced zonal gradient of sea surface temperature between the eastern and western Equatorial Pacific Ocean.
(3) Stilostomellidae and Pleurostomellidae generally decreased upward with several fluctuations. In contrast, Pseudoparrella obtusa, a possible phytodetritus species, increased intermittently after ~1.3 Ma. In addition, the Globocassidulina subglobosa–Globocassidulina ryukyuensis plexus was abundant prior to ~1.5 Ma, whereas Globocassidulina obtusa became more common after ~1.4 Ma. The temporal decrease in Stilostomellidae and increase in Astronion echolsi may indicate the influence of the Upper Circumpolar Deep Water from the Southern Ocean-originated deep water on the shallower (lower bathyal) depth at our core site, rather than North Pacific Deep Water, across ~2 Ma. Thus, such a faunal transition can be explained by changes in global deep-water circulation and the enhanced east-west zonal gradient of sea surface temperature in the Equatorial Pacific Ocean.
We thank the shipboard crew of R/V Onnuri (Korean Institute of Ocean Science and Technology) for collecting a piston core PC311. We appreciate Akira Tsujimoto (Simane University) for his help taking SEM and light micrographs of planktonic foraminifera. We are indebted to Shungo Kawagata (Yokohama National University) for his suggestions regarding the taxonomy of benthic foraminifera. We also thank the associate editor (Kazuhiko Fujita) and reviewers (Bruce Hayward, Shun Chiyonobu and an anonymous reviewer) for their constructive comments to improve the manuscript. This work was supported by the Korea Institute of Ocean Science and Technology (KIOST) research program (PEA0182), Korea Institute of Marine Science & Technology (KIMST) funded by the Ministry of Oceans and Fisheries (RS-2023-00256330), and a part of the project titled ‘Selection of prospective mining area for Co-rich ferromanganese crust in western Pacific seamounts: 3-D resource estimation and environmental impact evaluation’, funded by the Ministry of Oceans and Fisheries (No. 20220509).
Supplementary figure 1. Temperature, salinity, and chlorophyll-a concentration in the surface water around our study area taken from Acker and Leptoukh (2007); Supplementary figure 2. Correlations of the dextral coiling ratios of Pulleniatina spp. between IODP Site U1486 (Pearson and Penny, 2021) and core PC311 (this study) with planktonic foraminiferal (PF) and calcareous nannofossil (CN) biohorizons. Error bars in the coiling plot represent 95% confidence interval; Supplementary figure 3. Stratigraphic variations of sea surface temperature and sea surface salinity at ODP Site 871 (Wang et al., 2023); Supplemental document. Faunal References of planktonic foraminifera; Supplementary table 1. Occurrences of planktonic foraminifera in core 311; Supplementary table 2. Occurrences of calcareous nannofossils in core PC311; Supplementary table 3. Biohorizons of planktonic foraminifera and calcareous nannofossils in core PC311; Supplementary table 4. Occurrences of benthic foraminifera in core PC311™.
HT was responsible for conducting the faunal analysis of benthic foraminifera of core PC311 and for writing the manuscript. HH and SH were also responsible for conducting the analyses of planktonic foraminifera and calcareous nannofossils, respectively, and for biostratigraphic age inferences. RN assisted HT with the faunal analysis of benthic foraminifera. CMY was in charge of taking sediment core PC311 during the expedition. BKK designed the research, contributed to writing the manuscript, and managed the discussion for consensus. All authors approved the submission of the manuscript.
