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Orbital obliquity cycles recorded in Kuroshio Current region, eastern Asia, around Plio–Pleistocene boundary
Quaternary Science Reviews 140 (2016) 67e74
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Orbital obliquity cycles recorded in Kuroshio Current region, eastern Asia, around PlioePleistocene boundary Hokuto Iwatani a, *, Yasuo Kondo b, Toshiaki Irizuki c, Masao Iwai b, Minoru Ikehara d a
School of Biological Sciences, The University of Hong Kong, Kadoorie Biological Sciences Building, Pokfulam Road, Hong Kong SAR, China Department of Natural Science, Kochi University, Kochi 780-8520, Japan c Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan d Center for Advanced Marine Core Research, Kochi University, B 200 Monobe, Nankoku, Kochi, 783-8502, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 19 March 2016 Accepted 23 March 2016
Global climate underwent a period of significant cooling at the PlioePleistocene Transition (~2.6 Ma). The influence of this change on the Kuroshio Current region in the Pacific Ocean, off eastern Asia, is not well known. In this study, we clarify temporal changes in the paleoenvironment under the influence of the Kuroshio Current during the late Pliocene and early Pleistocene using high-resolution faunal proxy records of fossil Ostracoda (Crustacea). The study unit is the Ananai Formation in the southeastern region of Shikoku, southwest Japan. The modern analog technique (MAT) is employed for the quantitative estimation of paleo-bottom water temperatures (PBWTs) and paleo-water depth (PWD) during the deposition of the formation. Ostracode MAT results show PBWT fluctuations during warmest and coldest months, with values of 16 Ce20 C and 12 Ce16 C, respectively, and a PWD of 70e140 m, reflecting sealevel oscillations. Moreover, the PBWT in the coldest month is 3 Ce4 C lower than present-day water temperatures at the same shallow water depths. Temporal changes in these paleoenvironmental variables based on MAT are in good agreement with global oxygen isotope records. Orbital obliquity cycles with 41-kyr periodicity are recorded for the first time in an onshore section in the Kuroshio Current region at the PlioePleistocene boundary interval. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Orbital obliquity cycle Kuroshio PlioePleistocene Ostracoda
1. Introduction The Kuroshio Current is one of the strongest warm currents and flows in the northwest Pacific Ocean, off southwest Japan. The current is characterized by high temperature, high salinity (Barkley, 1970), and low biological productivity (Qiu, 2001). At the same time, it advects a large amount of heat from the tropics to northern mid-latitudes (Sawada and Handa, 1998) and has been significantly influencing the marine climate and organisms in eastern Asia for millions of years (Gallagher et al., 2015). Despite the importance of determining the conditions of the paleo-Kuroshio Current, only a few studies have been conducted on PlioePleistocene formations deposited in the current's path (e.g., Iwatani et al., 2012). High-resolution quantitative proxy records from the Kuroshio Current region can provide essential insights for the synthetic understanding of western Pacific PlioePleistocene
* Corresponding author. E-mail address: [email protected] (H. Iwatani). http://dx.doi.org/10.1016/j.quascirev.2016.03.025 0277-3791/© 2016 Elsevier Ltd. All rights reserved.
climates. The modern global climate regime began to formulate around the PlioePleistocene boundary with the intensification of Northern Hemisphere glaciation (Pillans and Naish, 2004; Raymo et al., 2006) because of the restriction of low-latitude seaways and the influence of the seaways on oceanic heat transport (Ravelo et al., 2004). The climate was cooler compared with the present-day climate and characterized by a change in the large amplitude and short cycles of glacialeinterglacial intervals, derived from 41-kyr orbital obliquity cycles (Sosdian and Rosenthal, 2009). This changing climate from the late Pliocene to early Pleistocene was recorded in various studies. For example, the emergence of icerafted debris was observed at high latitudes (Maslin et al., 1995; Shackleton, 1997), and the presence of a cold-water calcareous nannofossil species drastically increased in North Pacific marginal seas in mid-to high-latitude regions (Sato et al., 2004). In the terrestrial sedimentary record, the deposition of desert loess in the Chinese Loess Plateau began from ~2.6 Ma, possibly connected with desert expansion resulting from the intensification of the East Asian winter monsoon (Ding et al., 2005). In seas adjacent to Japan, a
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branch of the Kuroshio Current repeatedly flowed into and intercepted the Japan Sea during the PlioePleistocene and rapid cooling apparently occurred at ~2.75 Ma in many areas around the Japan Sea (e.g., Yamada et al., 2005). This study aims to reconstruct bottom water conditions around the PlioePleistocene boundary in the Kuroshio Current region, eastern Asia, from fossil ostracode assemblages, which are an effective environmental indicator. Ostracodes used here were obtained from the Ananai Formation, southwest Japan, which is typical of shallow marine deposits in Pacific coastal areas (Fig. 1). 2. Studied sections and materials The PlioePleistocene Ananai Formation is sporadically distributed on the Muroto Peninsula, Shikoku, southwest Japan. The formation is mainly composed of poorly sorted siltstones and silty sandstones with intercalated conglomerate layers and shell beds (e.g., Iwai et al., 2006). According to Matsubara (2004), based on a compilation of biostratigraphic studies, the Ananai Formation is placed between calcareous nannofossil biozones CN12b and CN12d (Okada and Bukry, 1980) and assigned to planktic foraminifera zone N21 (Blow, 1969); the chronostratigraphic age is 2.78 or 2.73e1.97 Ma. A drill core of ~70-m length (recovery rate 96%) was obtained in 2006 from the Ananai Formation (ANA-1, latitude 33 260 N, longitude 133 570 E) in the southern part of the Muroto Peninsula (Fig. 1). Iwai et al. (2009) determined that a reversedenormal polarity transition occurred at 28-m depth in the core, which can be assigned to the GausseMatuyama polarity boundary (2.608 Ma; Lisiecki and Raymo, 2005). In the present study, we focused on horizons in the middle to upper part of the core (~20-m thick),
which exhibit especially clear sedimentary cycles, corresponding to the PlioePleistocene boundary. The study sequence is lithologically divided into five units (units 1e5 in the ascending order, Fig. 2). At the base of each unit, thin granule conglomerates that are rich in mollusks are intercalated with siltstones. According to Kondo et al. (2006), these lithological units can be correlated with transgressiveeregressive cycles based on mollusk assemblages. 3. Ostracode analysis More than 150 ostracode species were recorded in the study sequence. For fossil ostracode analyses, dried sediment samples (12e60 g) were weighed and disaggregated using sodium sulfate and naphtha methods (Maiya and Inoue, 1973). Residues were divided using a sample splitter, and 150e250 specimens were handpicked from residues coarser than 0.125 mm. Note that the number of specimens refers to the sum of left and right valves. One carapace was counted as two valves. We plotted the total ostracode abundance in the studied sediments on the geologic column (Fig. 2), expressed as the number of ostracodes per gram of dried sediment. 3.1. Modern analog technique To obtain firm estimates of paleo-bottom water environmental variables for the study site, we used the modern analog technique (MAT; Guiot, 1990). In previous studies, this method was successfully applied for the analysis of ostracode assemblages (Ikeya and Cronin, 1993; Cronin et al., 1994; Tanaka and Nomura, 2009; Iwatani et al., 2012). In this study, we employed the modern ostracode and modern climate datasets compiled by Iwatani et al. (2012). The ostracode dataset comprises 338 surface samples obtained from water depths of 0e986 m; the climate dataset was obtained from the Japan Oceanographic Data Center (J-DOSS, 2013), representing averages for 1874e2001. The climate dataset adopts three climate variables: bottom water temperature in the warmest month (BWTw), bottom water temperature in the coldest month (BWTc), and water depth. We used a squared-chord distance (SCD) as a dissimilarity coefficient value (Prentice, 1980), which was calculated using the following equation: 1/2 2 dij ¼ S(p1/2 ik pjk )
where dij is the dissimilarity coefficient value between two samples i and j and pik is the proportion of ostracode species k in sample i. SCD values were calculated using Visual Basic for Applications in Microsoft Excel 2013. SCD values ranged from 0 to 2 for 0.0 < pik < 1.0. In the present study, climate variables reconstructed for eight best modern analogs were averaged by a weighting inverse to the SCD (Nakagawa et al., 2002). The weight function w1 (x) is defined as follows: w1 (x) ¼ jxj1
3.2. Q-mode factor analysis
Fig. 1. Location of the study site, borehole ANA-1 in Ananai, southwest Japan. KC: Kuroshio Current, TWC: Tsushima Warm Current, and TsS: Tsushima Strait.
We performed Q-mode factor analysis (CABFAC, Klovan and Imbrie, 1971) to reconstruct the evolution of the bottom water environment. Factor analysis is commonly used in micropaleontology to reveal and interpret common factors that affect assemblages. It was recently successfully applied in ostracode assemblage
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Fig. 2. Temporal changes in the total ostracode abundance and the varimax factor loadings (VFLs) for the first four fossil ostracode factors from the studied sequence. The curves of the paleo-bottom water temperatures (PBWTs) for the warmest month (PBWTw), PBWT for the coldest month (PBWTc), the seasonal difference in paleo-bottom water temperatures (PBWTd), and the paleo-water depth (PWD) are based on the modern analog technique. Solid lines show three-point moving-average values. The oxygen isotope curves and the marine isotope stages (MISs) after Lisiecki and Raymo (2005).
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et al., 2002; Irizuki et al., 2007; Yasuhara et al., 2008; analysis (Didie Iwatani et al., 2014). The analysis here was performed using the relative abundances of 89 taxa; they were selected when more than three specimens of each species were available per sample. In the present study, the analysis was performed using PAST 3.01 (Hammer et al., 2001).
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4. Results 4.1. Temporal changes in paleoclimate variables
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Based on MAT results, the minimum SCD value ranged from 0.50 to 0.90, with an average of 0.64. In the samples considered throughout the study sequence, the identified fossil ostracode assemblages were similar to those in modern samples, predominantly those collected from the Pacific coast off southwest Japan, Tsushima Strait, and the southern Japan Sea. The paleo-bottom water temperatures (PBWTs) for the warmest month (PBWTw) and PBWT for the coldest month (PBWTc) were 16 Ce20 C and 12 Ce16 C, respectively, abruptly decreasing at boundaries among units 2e3, 3e4, and 4e5 (Fig. 2). The seasonal difference in the PBWT (PBWTd) calculated as PBWTw PBWTc is 1 Ce5 C during the deposition of the Ananai Formation (Fig. 2). Note that the estimated paleo-water depth (PWD) ranged from 70 to 140 m and sharply increased in the lower parts of units 3 and 4 (Fig. 2). We compared paleoclimate values estimated through MAT with present-day values in Tosa Bay, the sea nearest to the study site (Fig. 1). The vertical profile of the BWTw during the PlioePleistocene was correlated well with present-day Tosa Bay values, although the BWTc was 3 Ce4 C lower compared with presentday values in shallow waters (Fig. 3). Overall, the PBWTd was higher than present-day seasonal variations (Fig. 3).
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Fig. 3. Comparison between paleoenvironmental variables in the Ananai Formation and present-day environmental variables for the study site. Solid gray circles denote paleo-bottom water temperatures with relation to paleo-water depth, based on MAT. Solid black circles are modern bottom water temperatures in Tosa Bay (latitude 33e34 N, longitude 133e134 E) with relation to water depth. Error bars on recent data are ±2.0 standard deviations. Data for recent environment variables were obtained from the Japan Oceanographic Data Center (J-DOSS, 2013).
4.2. Fossil ostracode assemblages Four varimax factors were calculated, which explained 83.3% of the total variance. Fig. 2 shows the first four varimax factor loadings (VFLs) for each sample. We considered the habitats of three taxa with the highest varimax scores for each factor to reconstruct the paleoenvironment. The first varimax factor explained 73.9% of the total variance and is characterized by Xestoleberis spp. (þ5.26), Neonesidea spp. (þ3.81), and Loxoconcha zamia (þ3.65). These species are typical intertidal and phytal taxa, which generally live on seaweeds and in nearshore environments with sandy bottoms (e.g., Hanai et al., 1977; Sato and Kamiya, 2007; Tsurumi and Kamiya, 2007). Thus, the first varimax
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Therefore, with the exception of A. munechikai, VFL 3 taxa appear to inhabit deeper environments compared with other dominant species. Temporal changes in VFL 3 were moderately correlated with those in the PBWTw and PWD (Fig. 4c, g). The fourth varimax factor explained 2.42% of the total variance and is characterized by Bicornucythere bisanensis (6.51), C. uchioi (3.06), and Aurila spp. (2.50). The most dominant species is B. bisanensis, a typical enclosed-bay species around Japan (e.g., Okubo, 1975; Ikeya and Shiozaki, 1993; Yasuhara et al., 2004; Yasuhara and Seto, 2006; Irizuki et al., 2008, 2009). Temporal changes in VFL 4 were not correlated with those in PBWTs and PWD (Fig. 4d, h).
factor represents the nearshore depositional environment. Temporal changes in VFL 1 were not correlated with those in PBWTs and PWD (Fig. 4a, e); therefore, other environmental factors such as water structure, salinity, and/or nutrients are reflected in VFL 1 changes. The second varimax factor explained 4.40% of the total variance and is characterized by Trachyleberis spp. (þ4.84), Neonesidea spp. (þ2.65), and L. tamakazura (þ2.38). According to ~o et al. (2013), Trachyleberis is restricted to shallow marine Branda environments in the mid-latitudes of the northwestern Pacific. Notably, L. tamakazura is distributed around the central parts of the Kuroshio Current and its A branch (Ozawa et al., 1995; Tanaka, 2008). Thus, the second varimax factor indicates the influence of the Kuroshio Current. Temporal changes in VFL 2 were not correlated with those in PBWTs and PWD (Fig. 4b, f). The third varimax factor explained 2.55% of the total variance and is characterized by Argilloecia spp. (4.37), Cytheropteron uchioi (4.14), and Aurila munechikai (3.29). The most dominant was Argilloecia spp., a typical mud-dweller in the lower sublittoral to upper bathyal zones of the Pacific, off southwest Japan, and in the East China Sea (e.g., Wang et al., 1988; Zhou, 1995). Moreover, C. uchioi has a broad distribution from the middle of the inner bay to the open sea (Yasuhara et al., 2005) and is dominant at ~100-m depth in the modern Tosa Bay (Zhou, 1995). Aurila munechikai was first observed at the Uranouchi Bay, Shikoku, southwest Japan (Ishizaki, 1968).
25 R² = 0.01
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The total ostracode abundance shows fluctuation trends similar to those of the PWD (Fig. 2), increasing in transgressive sequences (lower parts of units 3 and 4) and decreasing in regressive sequences (upper parts of units 3 and 4). It appears quite probable that the total ostracode abundance is controlled by the sedimentation rate. Transgressive sequences can be interpreted as a transgressive system tract (TST). TSTs are characterized by low sediment output and low sedimentation rate; ostracode fossils are concentrated in these intervals. In contrast, regressive sequences coincide with a highstand system tract (HST). HSTs are marked by high
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5. Discussion
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Fig. 4. The paleoenvironmental variables and varimax factor loadings (VFLs). aed: Relationships between of the paleo-bottom water temperatures (PBWTs) and VFLs. Solid black and gray circles are PBWT of the warmest month (PBWTw) and coldest month (PBWTc), respectively. eeh: Relationships between of the paleo-water depth (PWD) and VFLs. i: Relationships between of PBWTc and PBWTw. j, k: Relationships between of PWD and PBWTs. l: Relationships between of PWD and the seasonal difference in PBWT (PBWTd).
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sediment supply and high sedimentation rate; thus, ostracode accumulations are diluted in these sequences. It is reasonable to suppose that granule conglomerate beds at boundaries between the each units correspond to sequence boundaries. Our results reveal clear cyclic changes in bottom water temperatures during the deposition of the study sequence. PBWTs decreased at boundaries among units 2e3, 3e4, and 4e5 (Fig. 2). When compared with the oxygen isotope curve of the LR04 stack (Lisiecki and Raymo, 2005), PBWTs exhibit similar fluctuation trends (Fig. 2). Therefore, bottom water environment conditions during the deposition of the study sequence potentially possess a deep connection with glacialeinterglacial climate cycles. No evidence exists for a large unconformity or hiatus in our study sequence. Thus, cooling events at boundaries among units 2e3, 3e4, and 4e5 correspond to marine isotope stages (MISs) G4, G2, and 104, respectively (Fig. 2), and the depositional age of the study sequence is estimated at ~2.7e2.6 Ma. Few authors quantitatively reconstructed sea-level variations during the PlioePleistocene. The amplitude of sea-level fluctuations at ~2.6 Ma is less than 100 m (Miller et al., 2005). Naish and Wilson (2009) estimated the amplitude of sea-level fluctuations during the PlioePleistocene using the LR04 stack reported by Lisiecki and Raymo (2005). According to their study, sea-level fluctuations between glacial and interglacial periods during 2.7e2.6 Ma were less than 60 m (Naish and Wilson, 2009). These are similar to the estimated fluctuations in the PWD in our study sequence. The basal granule conglomerate beds of units 3, 4, and 5 can be interpreted as sediments resulting from regressions during glaciation. Kondo (2005) and Kondo et al. (2006) reported the possibility that these lithological units can be correlated with transgressiveeregressive cycles based on changes of the sedimentary facies and mollusk assemblages. Our results showed clear transgressiveeregressive cycles derived from glacioeustatic cycles, in agreement with the indication by Kondo and his collaborator's studies. Around a downstream of the Kuroshio Current (the Kuroshio
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extension ), although the 19-kyr orbital cycle consistent with oxygen isotope records, the 41-kyr cycle is not observed (Venti and Billups, 2013). However, the 41-kyr cycle is clearly recorded in the present study sequence, as revealed by the fossil ostracode assemblage analysis. This study demonstrates for the first time that orbital obliquity cycles influenced the Kuroshio Current region during the PlioePleistocene transition. A cooling event at the boundary between units 4 and 5 corresponding to MIS 104 reconstruct the coldest PBWTs and shallowest PWD during the study interval. MIS 104 is correlated with the PlioePleistocene boundary and is associated with the first major incursion of the ice rafted debris in the North Atlantic (Shackleton, 1997). Regarding the terrestrial sedimentary record, the deposition of desert loess in the Chinese Loess Plateau began at the same time in relation with desert expansion, which was caused by the intensification of the East Asian winter monsoon and the weakening of the summer monsoon (Ding et al., 2005). Cooling of both the surface and bottom waters occurred at ~2.6 Ma in the Kuroshio Current region, southwest Japan, as demonstrated by the analysis of microfossils (Chiyonobu et al., 2012; Iwatani et al., 2012). Even in the South Hemisphere, a regression from just above the GausseMatuyama boundary recorded glacialeinterglacial climate cycles with 41-kyr periodicity and is prominent in shallow marine deposits in the Wanganui Basin, New Zealand (Pillans and Naish, 2004). Ikehara et al. (2010) measured the magnetic susceptibility using a multisensor logger at the same core as the present study. They observed a large positive peak at the boundary between units 4 and 5. According to Ikehara et al. (2010), the peak is attributed to the concentration of magnetic minerals, which is derived from an increased sediment output from a terrestrial area during the regression. Thus, it is quite likely that the cooling event at the boundary between units 4 and 5 was a distinct cooling event that occurred at the PlioePleistocene boundary. Temporal changes in the PBWTw and PBWTd are inversely correlated with those in the PWD (Figs. 4j, l: PBWTw vs PWD,
Fig. 5. Comparisons of the temporal changes of paleo-environmental variables and varimax factor loadings of the fossil planktonic foraminifera (VFLs-PF) in the Ananai (this study), Takanabe (Iwatani et al., 2012), and Yabuta (Cronin et al., 1994) Formations with oxygen isotope curve of the LR04 stack (Lisiecki and Raymo, 2005). Shaded area on the Takanabe Formation shows a period of the estimated subsidence (Iwatani et al., 2012). The geological ages of the Yabuta formation are based on the biostratigraphic data by Miwa et al. (2004). Gray and black lines of the paleo-bottom water temperatures (PBWTs) are PBWT of the coldest (PBWTc) and warmest month (PBWTw), respectively.
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R2 ¼ 0.63, P < 0.0001; PBWTd vs PWD, R2 ¼ 0.64, P < 0.0001). In general, deep/shallow water conditions correspond to low/high water temperatures and small/large seasonal differences in water temperatures. Conversely, temporal changes in the PBWTc are not inversely correlated with those in the PWD (Fig. 4k: PBWTc vs PWD, R2 ¼ 0.01, P < 0.0001). This finding suggests that changes in the PBWTc are not controlled by PWD fluctuations alone; other environmental factors such as the water structure are also reflected in PBWTc changes. In the parts of the sequence with low PBWTs values, high and low values of first and second VFLs (Fig. 2) suggest a nearshore depositional environment and a weakened influence by the Kuroshio Current, respectively. Moreover, compared with present-day water mass conditions, low PBWTc values are detected at shallow water depths in the study sequence (Fig. 3). Sediments deposited in these shallow water conditions are most likely formed under the influence of a weak Kuroshio Current and strong coastal waters relative to modern water conditions, characterized by a winter chill (J-DOSS, 2013). Therefore, changes in the PBWTc are linked with the fluctuations of the Kuroshio Current. This finding suggests that decreases in the PBWTc at boundaries between the different units were caused by the reductions of the Kuroshio Current's influence as a result of global cooling. In other marine strata in Pacific coastal areas, the weakening of the Kuroshio Current's influence was similarly recorded around the PlioePleistocene boundary. Planktonic foraminifera analysis suggested that the PlioePleistocene Takanabe Formation, Miyazaki Plain, southwest Japan (Fig. 1), clearly recorded a change from a marine climate under the influence of the axial part of the Kuroshio Current to that of the transitional water at around 2.6 Ma (Fig. 5, Iwatani et al., 2012). According to Iwatani et al. (2012), the water depth change at ~2.6 Ma in the Takanabe Formation suggests that the tectonic movement related to the rotation of the southern Kyushu Island occurred near this horizon. This subsidence is not recorded in the present study site; therefore, the local tectonic event was restricted to the Kyushu area. Moreover, the temporal changes of paleoenvironmental variables in the Takanabe Formation fluctuated over a large scale compared with those in the Ananai Formation, probably because of differences in PWDs between the two depositional paleoenvironments. Ananai sequences were formed in a shallower environment and closer to the coast compared with those of the Takanabe Formation. Therefore, the present study site appears to be more sensitive to paleoenvironmental changes. Ananai Formation outstandingly reflects hightime-resolution global climate changes. Around the Japan Sea side, in the PlioePleistocene Yabuta Formation, Noto Peninsula, central Japan (Fig. 1), eight glacial events recorded a 41-kyr obliquity cycle (Fig. 5, Cronin et al., 1994). According to Cronin et al. (1994), a progressive intensification of glacial events was recorded at ~2.5 Ma (around the glacial event 4), which corresponds to major changes in the global ice volume. The age was revised at ~2.6 Ma based on the recent biostratigraphic study by Miwa et al. (2004). Thus, the cooling event recorded in the Yabuta Formation occurred at around the PlioePleistocene boundary, the same as that recorded in the present study site. We compared the vertical PBWT profile of the Ananai Formation with the data on Takanabe (Iwatani et al., 2012) and Yabuta (Cronin et al., 1994) Formations. Similar values and trends were found between the Ananai and Takanabe Formations at the same water depths; that is, as the PWD decreases, seasonal differences in the water temperature increase (Fig. 6). It is possible that the Pacific Ocean off southwest Japan exhibited a similar water structure during the Plio-Pleistocene. On the other hand, in the Japan Sea side, the vertical PBWT profile of the Yabuta Formation clearly recorded cold conditions, even at a shallower water depth (Fig. 6).
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paleo-bottom water temperature (°C) Fig. 6. Vertical paleo-bottom water temperatures (PBWTs) profile of the Ananai (this study), Takanabe (Iwatani et al., 2012), and Yabuta (Cronin et al., 1994) Formations. Solid gray and black circles are PBWT of the coldest and warmest month of the Ananai Formation, respectively. Open gray and black circles are PBWT of the coldest and warmest month of the Takanabe Formation, respectively. Gray and black x-marks are PBWT of the coldest and warmest month of the Yabuta Formation, respectively.
At the same time (after ~2.7 Ma), calcareous nannofossil analysis reveals cool and eutrophic waters during the deposition of the PlioePleistocene Chikura Group, Boso Peninsula, central Japan (Fig. 1), which was caused by the southward movement of mixed water by a cold current (Oyashio Current) and a strengthening of the mixed water effect (Kameo et al., 2003). Thus, the weakening of the Kuroshio Current's influence around the PlioePleistocene boundary is widely recorded in southwest Japan. According to Kameo et al. (2003), the development of cool and eutrophic waters around central Japan is possibly related to the inflow of nutrientrich nearshore surface waters at coasts around the East Asian continent. The high nutrient content is derived from the weathering of the East Asian continent related to tectonic processes around the HimalayaneTibetan complex during the PlioePleistocene. Overall, a large seasonal difference in the water temperature is detected in the study sequence relative to recent water mass conditions (Fig. 3). Therefore, the Kuroshio Current's influence, which is characterized by suitable high temperature conditions, appears to be weak throughout the study sequence interval. The weakening of the Kuroshio Current's influence relative to modern conditions during the formation of the study sequence may have been a result of the strengthening effect of nearshore waters around the East Asian continent. The weakening of the Kuroshio Current's influence around the PlioePleistocene boundary in this region suggests that compared with present-day conditions, at that time, the current flowed at a greater distance from the Pacific coast off southwest Japan. 6. Conclusions The present study leads to four main conclusions. 1. More than 150 ostracode taxa are identified in the PlioePleistocene Ananai Formation (~2.6 Ma) on the Muroto Peninsula, Shikoku, southwest Japan.
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2. Results of the modern analog technique applied on fossil ostracode assemblages indicate a paleo-bottom water temperature during the warmest and coldest months, with values of 16 Ce20 C and 12 Ce16 C respectively, and a paleo-water depth between 70 and 140 m. 3. Fluctuations of the reconstructed paleo-bottom water temperatures correspond to 41-kyr orbital obliquity cycles. 4. Changes in paleo-bottom water temperature in the coldest month are linked to fluctuations in the Kuroshio Current intensity due to global cooling. 5. In comparison with present-day water mass conditions, a large seasonal difference in the water temperature is recorded in the study sequence, suggesting that the Kuroshio Current's influence, which is characterized by high temperatures, was weaker around the PlioePleistocene boundary. Acknowledgments We are grateful to Shigetaka Kita for discussions; Moriaki Yasuhara for important suggestions; Laura Wong and Hisayo Okahashi for continuous help in our study; two anonymous reviewers for their critical readings, and Henning A. Bauch for editing. This work was supported by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (No. 17540442 to T.I.) and the Faculty of Science Postdoctoral Fellowship of the University of Hong Kong (to H.I.). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2016.03.025. References Barkley, R.A., 1970. The Kuroshio current. Sci. J. 6, 54e60. Blow, W.H., 1969. Late middle Eocene to recent planktonic foraminiferal biostra€nnimann, P., Renz, H.H. (Eds.), Proc. 1st Intern. Conf. Planktonic tigraphy. In: Bro Microfossils, (Geneva, 1967). Leiden (E.J. Brill); Geneva, 1, 199-421. ~o, S.N., Yasuhara, M., Irizuki, T., Horne, D.J., 2013. The ostracod genus TraBranda chyleberis (Crustacea; Ostracoda) and its type species. Mar. Biodiv. 43, 363e405. Chiyonobu, S., Morimoto, J., Torii, M., Oda, M., 2012. Pliocene/Pleistocene boundary and paleoceanographic significance of the upper Miyazaki Group, southern Kyushu, Southwest Japan, based on calcareous nannofossil and planktic foraminiferal assemblages. J. Geol. Soc. Jpn. 118, 109e116 (in Japanese with English abstract). Cronin, T.M., Kitamura, A., Ikeya, N., Watanabe, M., Kamiya, T., 1994. Late Pliocene climate change 3.4e2.3 Ma: paleoceanographic record from the Yabuta formation, sea of Japan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 437e455. , C., Bauch, H.A., Helmke, J.P., 2002. Late Quaternary deep-sea ostracodes in the Didie polar and subpolar North Atlantic: paleoecological and paleoenvironmental implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 184, 195e212. Ding, Z.L., Derbyshire, E., Yang, S.L., Sun, J.M., Liu, T.S., 2005. Stepwise expansion of desert environment across northern China in the past 3.5 Ma and implications for monsoon evolution. Earth Planet. Sci. Lett. 237, 45e55. Gallagher, S.J., Kitamura, A., Iryu, Y., Itaki, T., Koizumi, I., Hoiles, P.W., 2015. The Pliocene to recent history of the Kuroshio and Tsushima currents: a multi-proxy approach. Prog. Earth Planet. Sci. 2, 23. Guiot, J., 1990. Methodology of the last climatic cycle reconstruction in France from pollen data. Palaeogeogr. Palaeoclimatol. Palaeoecol. 80, 49e69. Hammer, Ø., Harper, D.A.T., Ryan, P.D., 2001. PAST: paleontological statistics software package for education and data analysis. Palaeontol. Electron 4 (9) (March 2014). http://palaeo-electronica.org/2001_1/past/issue1_01.htm. Hanai, T., Ikeya, N., Ishizaki, K., Sekiguchi, Yajima, M., 1977. Checklist of Ostracoda from Japan and its Adjacent Seas. University of Tokyo Press, Tokyo, p. 119. Ikehara, M., Iwai, M., Kondo, Y., Kita, S., Hattori, N., 2010. Non-destructive Analyses of a Drilling Core ANA-1 from the Ananai Formation of the Tonohama Group on Muroto, Peninsula, Kochi. Sci. Rep. Kochi Univ. 59, 183e195. Ikeya, N., Cronin, T.M., 1993. Quantitative analysis of Ostracoda and water masses around Japan: application to Pliocene and Pleistocene paleoceanography. Micropaleontology 39, 263e281. Ikeya, N., Shiozaki, M., 1993. Characteristics of the inner bay ostracodes around the Japanese islands: the use of ostracodes to reconstruct paleoenvironments. Mem. Geol. Soc. Jpn. 39, 15e32 (in Japanese with English abstract). Irizuki, T., Kusumoto, M., Ishida, K., Tanaka, Y., 2007. Sea-level changes and water
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Orbital obliquity cycles recorded in Kuroshio Current region, eastern Asia, around PlioePleistocene boundary Hokuto Iwatani a, *, Yasuo Kondo b, Toshiaki Irizuki c, Masao Iwai b, Minoru Ikehara d a
School of Biological Sciences, The University of Hong Kong, Kadoorie Biological Sciences Building, Pokfulam Road, Hong Kong SAR, China Department of Natural Science, Kochi University, Kochi 780-8520, Japan c Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan d Center for Advanced Marine Core Research, Kochi University, B 200 Monobe, Nankoku, Kochi, 783-8502, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 19 March 2016 Accepted 23 March 2016
Global climate underwent a period of significant cooling at the PlioePleistocene Transition (~2.6 Ma). The influence of this change on the Kuroshio Current region in the Pacific Ocean, off eastern Asia, is not well known. In this study, we clarify temporal changes in the paleoenvironment under the influence of the Kuroshio Current during the late Pliocene and early Pleistocene using high-resolution faunal proxy records of fossil Ostracoda (Crustacea). The study unit is the Ananai Formation in the southeastern region of Shikoku, southwest Japan. The modern analog technique (MAT) is employed for the quantitative estimation of paleo-bottom water temperatures (PBWTs) and paleo-water depth (PWD) during the deposition of the formation. Ostracode MAT results show PBWT fluctuations during warmest and coldest months, with values of 16 Ce20 C and 12 Ce16 C, respectively, and a PWD of 70e140 m, reflecting sealevel oscillations. Moreover, the PBWT in the coldest month is 3 Ce4 C lower than present-day water temperatures at the same shallow water depths. Temporal changes in these paleoenvironmental variables based on MAT are in good agreement with global oxygen isotope records. Orbital obliquity cycles with 41-kyr periodicity are recorded for the first time in an onshore section in the Kuroshio Current region at the PlioePleistocene boundary interval. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Orbital obliquity cycle Kuroshio PlioePleistocene Ostracoda
1. Introduction The Kuroshio Current is one of the strongest warm currents and flows in the northwest Pacific Ocean, off southwest Japan. The current is characterized by high temperature, high salinity (Barkley, 1970), and low biological productivity (Qiu, 2001). At the same time, it advects a large amount of heat from the tropics to northern mid-latitudes (Sawada and Handa, 1998) and has been significantly influencing the marine climate and organisms in eastern Asia for millions of years (Gallagher et al., 2015). Despite the importance of determining the conditions of the paleo-Kuroshio Current, only a few studies have been conducted on PlioePleistocene formations deposited in the current's path (e.g., Iwatani et al., 2012). High-resolution quantitative proxy records from the Kuroshio Current region can provide essential insights for the synthetic understanding of western Pacific PlioePleistocene
* Corresponding author. E-mail address: [email protected] (H. Iwatani). http://dx.doi.org/10.1016/j.quascirev.2016.03.025 0277-3791/© 2016 Elsevier Ltd. All rights reserved.
climates. The modern global climate regime began to formulate around the PlioePleistocene boundary with the intensification of Northern Hemisphere glaciation (Pillans and Naish, 2004; Raymo et al., 2006) because of the restriction of low-latitude seaways and the influence of the seaways on oceanic heat transport (Ravelo et al., 2004). The climate was cooler compared with the present-day climate and characterized by a change in the large amplitude and short cycles of glacialeinterglacial intervals, derived from 41-kyr orbital obliquity cycles (Sosdian and Rosenthal, 2009). This changing climate from the late Pliocene to early Pleistocene was recorded in various studies. For example, the emergence of icerafted debris was observed at high latitudes (Maslin et al., 1995; Shackleton, 1997), and the presence of a cold-water calcareous nannofossil species drastically increased in North Pacific marginal seas in mid-to high-latitude regions (Sato et al., 2004). In the terrestrial sedimentary record, the deposition of desert loess in the Chinese Loess Plateau began from ~2.6 Ma, possibly connected with desert expansion resulting from the intensification of the East Asian winter monsoon (Ding et al., 2005). In seas adjacent to Japan, a
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branch of the Kuroshio Current repeatedly flowed into and intercepted the Japan Sea during the PlioePleistocene and rapid cooling apparently occurred at ~2.75 Ma in many areas around the Japan Sea (e.g., Yamada et al., 2005). This study aims to reconstruct bottom water conditions around the PlioePleistocene boundary in the Kuroshio Current region, eastern Asia, from fossil ostracode assemblages, which are an effective environmental indicator. Ostracodes used here were obtained from the Ananai Formation, southwest Japan, which is typical of shallow marine deposits in Pacific coastal areas (Fig. 1). 2. Studied sections and materials The PlioePleistocene Ananai Formation is sporadically distributed on the Muroto Peninsula, Shikoku, southwest Japan. The formation is mainly composed of poorly sorted siltstones and silty sandstones with intercalated conglomerate layers and shell beds (e.g., Iwai et al., 2006). According to Matsubara (2004), based on a compilation of biostratigraphic studies, the Ananai Formation is placed between calcareous nannofossil biozones CN12b and CN12d (Okada and Bukry, 1980) and assigned to planktic foraminifera zone N21 (Blow, 1969); the chronostratigraphic age is 2.78 or 2.73e1.97 Ma. A drill core of ~70-m length (recovery rate 96%) was obtained in 2006 from the Ananai Formation (ANA-1, latitude 33 260 N, longitude 133 570 E) in the southern part of the Muroto Peninsula (Fig. 1). Iwai et al. (2009) determined that a reversedenormal polarity transition occurred at 28-m depth in the core, which can be assigned to the GausseMatuyama polarity boundary (2.608 Ma; Lisiecki and Raymo, 2005). In the present study, we focused on horizons in the middle to upper part of the core (~20-m thick),
which exhibit especially clear sedimentary cycles, corresponding to the PlioePleistocene boundary. The study sequence is lithologically divided into five units (units 1e5 in the ascending order, Fig. 2). At the base of each unit, thin granule conglomerates that are rich in mollusks are intercalated with siltstones. According to Kondo et al. (2006), these lithological units can be correlated with transgressiveeregressive cycles based on mollusk assemblages. 3. Ostracode analysis More than 150 ostracode species were recorded in the study sequence. For fossil ostracode analyses, dried sediment samples (12e60 g) were weighed and disaggregated using sodium sulfate and naphtha methods (Maiya and Inoue, 1973). Residues were divided using a sample splitter, and 150e250 specimens were handpicked from residues coarser than 0.125 mm. Note that the number of specimens refers to the sum of left and right valves. One carapace was counted as two valves. We plotted the total ostracode abundance in the studied sediments on the geologic column (Fig. 2), expressed as the number of ostracodes per gram of dried sediment. 3.1. Modern analog technique To obtain firm estimates of paleo-bottom water environmental variables for the study site, we used the modern analog technique (MAT; Guiot, 1990). In previous studies, this method was successfully applied for the analysis of ostracode assemblages (Ikeya and Cronin, 1993; Cronin et al., 1994; Tanaka and Nomura, 2009; Iwatani et al., 2012). In this study, we employed the modern ostracode and modern climate datasets compiled by Iwatani et al. (2012). The ostracode dataset comprises 338 surface samples obtained from water depths of 0e986 m; the climate dataset was obtained from the Japan Oceanographic Data Center (J-DOSS, 2013), representing averages for 1874e2001. The climate dataset adopts three climate variables: bottom water temperature in the warmest month (BWTw), bottom water temperature in the coldest month (BWTc), and water depth. We used a squared-chord distance (SCD) as a dissimilarity coefficient value (Prentice, 1980), which was calculated using the following equation: 1/2 2 dij ¼ S(p1/2 ik pjk )
where dij is the dissimilarity coefficient value between two samples i and j and pik is the proportion of ostracode species k in sample i. SCD values were calculated using Visual Basic for Applications in Microsoft Excel 2013. SCD values ranged from 0 to 2 for 0.0 < pik < 1.0. In the present study, climate variables reconstructed for eight best modern analogs were averaged by a weighting inverse to the SCD (Nakagawa et al., 2002). The weight function w1 (x) is defined as follows: w1 (x) ¼ jxj1
3.2. Q-mode factor analysis
Fig. 1. Location of the study site, borehole ANA-1 in Ananai, southwest Japan. KC: Kuroshio Current, TWC: Tsushima Warm Current, and TsS: Tsushima Strait.
We performed Q-mode factor analysis (CABFAC, Klovan and Imbrie, 1971) to reconstruct the evolution of the bottom water environment. Factor analysis is commonly used in micropaleontology to reveal and interpret common factors that affect assemblages. It was recently successfully applied in ostracode assemblage
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Fig. 2. Temporal changes in the total ostracode abundance and the varimax factor loadings (VFLs) for the first four fossil ostracode factors from the studied sequence. The curves of the paleo-bottom water temperatures (PBWTs) for the warmest month (PBWTw), PBWT for the coldest month (PBWTc), the seasonal difference in paleo-bottom water temperatures (PBWTd), and the paleo-water depth (PWD) are based on the modern analog technique. Solid lines show three-point moving-average values. The oxygen isotope curves and the marine isotope stages (MISs) after Lisiecki and Raymo (2005).
0
water depth (m)
et al., 2002; Irizuki et al., 2007; Yasuhara et al., 2008; analysis (Didie Iwatani et al., 2014). The analysis here was performed using the relative abundances of 89 taxa; they were selected when more than three specimens of each species were available per sample. In the present study, the analysis was performed using PAST 3.01 (Hammer et al., 2001).
warmest month
0
coldest month
seasonal difference
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4. Results 4.1. Temporal changes in paleoclimate variables
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temperature (°C)
Based on MAT results, the minimum SCD value ranged from 0.50 to 0.90, with an average of 0.64. In the samples considered throughout the study sequence, the identified fossil ostracode assemblages were similar to those in modern samples, predominantly those collected from the Pacific coast off southwest Japan, Tsushima Strait, and the southern Japan Sea. The paleo-bottom water temperatures (PBWTs) for the warmest month (PBWTw) and PBWT for the coldest month (PBWTc) were 16 Ce20 C and 12 Ce16 C, respectively, abruptly decreasing at boundaries among units 2e3, 3e4, and 4e5 (Fig. 2). The seasonal difference in the PBWT (PBWTd) calculated as PBWTw PBWTc is 1 Ce5 C during the deposition of the Ananai Formation (Fig. 2). Note that the estimated paleo-water depth (PWD) ranged from 70 to 140 m and sharply increased in the lower parts of units 3 and 4 (Fig. 2). We compared paleoclimate values estimated through MAT with present-day values in Tosa Bay, the sea nearest to the study site (Fig. 1). The vertical profile of the BWTw during the PlioePleistocene was correlated well with present-day Tosa Bay values, although the BWTc was 3 Ce4 C lower compared with presentday values in shallow waters (Fig. 3). Overall, the PBWTd was higher than present-day seasonal variations (Fig. 3).
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temperature (°C)
Fig. 3. Comparison between paleoenvironmental variables in the Ananai Formation and present-day environmental variables for the study site. Solid gray circles denote paleo-bottom water temperatures with relation to paleo-water depth, based on MAT. Solid black circles are modern bottom water temperatures in Tosa Bay (latitude 33e34 N, longitude 133e134 E) with relation to water depth. Error bars on recent data are ±2.0 standard deviations. Data for recent environment variables were obtained from the Japan Oceanographic Data Center (J-DOSS, 2013).
4.2. Fossil ostracode assemblages Four varimax factors were calculated, which explained 83.3% of the total variance. Fig. 2 shows the first four varimax factor loadings (VFLs) for each sample. We considered the habitats of three taxa with the highest varimax scores for each factor to reconstruct the paleoenvironment. The first varimax factor explained 73.9% of the total variance and is characterized by Xestoleberis spp. (þ5.26), Neonesidea spp. (þ3.81), and Loxoconcha zamia (þ3.65). These species are typical intertidal and phytal taxa, which generally live on seaweeds and in nearshore environments with sandy bottoms (e.g., Hanai et al., 1977; Sato and Kamiya, 2007; Tsurumi and Kamiya, 2007). Thus, the first varimax
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Therefore, with the exception of A. munechikai, VFL 3 taxa appear to inhabit deeper environments compared with other dominant species. Temporal changes in VFL 3 were moderately correlated with those in the PBWTw and PWD (Fig. 4c, g). The fourth varimax factor explained 2.42% of the total variance and is characterized by Bicornucythere bisanensis (6.51), C. uchioi (3.06), and Aurila spp. (2.50). The most dominant species is B. bisanensis, a typical enclosed-bay species around Japan (e.g., Okubo, 1975; Ikeya and Shiozaki, 1993; Yasuhara et al., 2004; Yasuhara and Seto, 2006; Irizuki et al., 2008, 2009). Temporal changes in VFL 4 were not correlated with those in PBWTs and PWD (Fig. 4d, h).
factor represents the nearshore depositional environment. Temporal changes in VFL 1 were not correlated with those in PBWTs and PWD (Fig. 4a, e); therefore, other environmental factors such as water structure, salinity, and/or nutrients are reflected in VFL 1 changes. The second varimax factor explained 4.40% of the total variance and is characterized by Trachyleberis spp. (þ4.84), Neonesidea spp. (þ2.65), and L. tamakazura (þ2.38). According to ~o et al. (2013), Trachyleberis is restricted to shallow marine Branda environments in the mid-latitudes of the northwestern Pacific. Notably, L. tamakazura is distributed around the central parts of the Kuroshio Current and its A branch (Ozawa et al., 1995; Tanaka, 2008). Thus, the second varimax factor indicates the influence of the Kuroshio Current. Temporal changes in VFL 2 were not correlated with those in PBWTs and PWD (Fig. 4b, f). The third varimax factor explained 2.55% of the total variance and is characterized by Argilloecia spp. (4.37), Cytheropteron uchioi (4.14), and Aurila munechikai (3.29). The most dominant was Argilloecia spp., a typical mud-dweller in the lower sublittoral to upper bathyal zones of the Pacific, off southwest Japan, and in the East China Sea (e.g., Wang et al., 1988; Zhou, 1995). Moreover, C. uchioi has a broad distribution from the middle of the inner bay to the open sea (Yasuhara et al., 2005) and is dominant at ~100-m depth in the modern Tosa Bay (Zhou, 1995). Aurila munechikai was first observed at the Uranouchi Bay, Shikoku, southwest Japan (Ishizaki, 1968).
25 R² = 0.01
19 16 13 R² = 0.03 0.2
0.4
0.6
0.8
16 13 10
1.0
R² = 0.04 0.0
0.2
VFL 1 150
e PWD (m)
PWD (m)
110 90
0.2
0.4
0.6
0.0
150
f
0.2
110 90
0.8
50
1.0
0.0
0.2
0.4
0.6
PWD (m)
13.5
0.8
1.0
R² = 0.28 11.5 15.0 16.0 17.0 18.0 19.0 20.0
50
PBWTw (°C)
R² = 0.01 0.0
0.2
0.4
150
0.0
0.2
0.4
0.6
0.8
h
110 90
50 0.0
1.0
R² < 0.01 0.2
0.4
PBWTw (°C)
0.6
0.8
1.0
VFL 4 150
k
l
130
110 90
50
1.0
70 R² = 0.37
110 90 70
70 R² = 0.63
0.8
130
130
15.0 16.0 17.0 18.0 19.0 20.0
0.6
VFL 4
90
150
j
90 70
13
VFL 3
110
12.5
16
10
1.0
110
50
130
14.5
0.8
70 R² = 0.09
150
15.5
0.6
g
VFL 2
i
0.4
130
VFL 1 16.5
R² = 0.03
R² = 0.03
19
VFL 3
70 R² = 0.05 0.0
13 10
1.0
130
70
PBWTc (°C)
0.8
16
VFL 2
130
50
0.6
PWD (m)
150
0.4
R² = 0.36
19
d
22
PWD (m)
0.0
R² = 0.19
19
25
c
22
PWD (m)
10
22
PBWTs (°C)
PBWTs (°C)
22
25
b
PWD (m)
a
The total ostracode abundance shows fluctuation trends similar to those of the PWD (Fig. 2), increasing in transgressive sequences (lower parts of units 3 and 4) and decreasing in regressive sequences (upper parts of units 3 and 4). It appears quite probable that the total ostracode abundance is controlled by the sedimentation rate. Transgressive sequences can be interpreted as a transgressive system tract (TST). TSTs are characterized by low sediment output and low sedimentation rate; ostracode fossils are concentrated in these intervals. In contrast, regressive sequences coincide with a highstand system tract (HST). HSTs are marked by high
PBWTs (°C)
25
5. Discussion
PBWTs (°C)
70
R² = 0.02 11.5 12.5 13.5 14.5 15.5 16.5
PBWTc (°C)
50
R² = 0.64 0.0
1.0
2.0
3.0
4.0
5.0
PBWTd (°C)
Fig. 4. The paleoenvironmental variables and varimax factor loadings (VFLs). aed: Relationships between of the paleo-bottom water temperatures (PBWTs) and VFLs. Solid black and gray circles are PBWT of the warmest month (PBWTw) and coldest month (PBWTc), respectively. eeh: Relationships between of the paleo-water depth (PWD) and VFLs. i: Relationships between of PBWTc and PBWTw. j, k: Relationships between of PWD and PBWTs. l: Relationships between of PWD and the seasonal difference in PBWT (PBWTd).
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sediment supply and high sedimentation rate; thus, ostracode accumulations are diluted in these sequences. It is reasonable to suppose that granule conglomerate beds at boundaries between the each units correspond to sequence boundaries. Our results reveal clear cyclic changes in bottom water temperatures during the deposition of the study sequence. PBWTs decreased at boundaries among units 2e3, 3e4, and 4e5 (Fig. 2). When compared with the oxygen isotope curve of the LR04 stack (Lisiecki and Raymo, 2005), PBWTs exhibit similar fluctuation trends (Fig. 2). Therefore, bottom water environment conditions during the deposition of the study sequence potentially possess a deep connection with glacialeinterglacial climate cycles. No evidence exists for a large unconformity or hiatus in our study sequence. Thus, cooling events at boundaries among units 2e3, 3e4, and 4e5 correspond to marine isotope stages (MISs) G4, G2, and 104, respectively (Fig. 2), and the depositional age of the study sequence is estimated at ~2.7e2.6 Ma. Few authors quantitatively reconstructed sea-level variations during the PlioePleistocene. The amplitude of sea-level fluctuations at ~2.6 Ma is less than 100 m (Miller et al., 2005). Naish and Wilson (2009) estimated the amplitude of sea-level fluctuations during the PlioePleistocene using the LR04 stack reported by Lisiecki and Raymo (2005). According to their study, sea-level fluctuations between glacial and interglacial periods during 2.7e2.6 Ma were less than 60 m (Naish and Wilson, 2009). These are similar to the estimated fluctuations in the PWD in our study sequence. The basal granule conglomerate beds of units 3, 4, and 5 can be interpreted as sediments resulting from regressions during glaciation. Kondo (2005) and Kondo et al. (2006) reported the possibility that these lithological units can be correlated with transgressiveeregressive cycles based on changes of the sedimentary facies and mollusk assemblages. Our results showed clear transgressiveeregressive cycles derived from glacioeustatic cycles, in agreement with the indication by Kondo and his collaborator's studies. Around a downstream of the Kuroshio Current (the Kuroshio
71
extension ), although the 19-kyr orbital cycle consistent with oxygen isotope records, the 41-kyr cycle is not observed (Venti and Billups, 2013). However, the 41-kyr cycle is clearly recorded in the present study sequence, as revealed by the fossil ostracode assemblage analysis. This study demonstrates for the first time that orbital obliquity cycles influenced the Kuroshio Current region during the PlioePleistocene transition. A cooling event at the boundary between units 4 and 5 corresponding to MIS 104 reconstruct the coldest PBWTs and shallowest PWD during the study interval. MIS 104 is correlated with the PlioePleistocene boundary and is associated with the first major incursion of the ice rafted debris in the North Atlantic (Shackleton, 1997). Regarding the terrestrial sedimentary record, the deposition of desert loess in the Chinese Loess Plateau began at the same time in relation with desert expansion, which was caused by the intensification of the East Asian winter monsoon and the weakening of the summer monsoon (Ding et al., 2005). Cooling of both the surface and bottom waters occurred at ~2.6 Ma in the Kuroshio Current region, southwest Japan, as demonstrated by the analysis of microfossils (Chiyonobu et al., 2012; Iwatani et al., 2012). Even in the South Hemisphere, a regression from just above the GausseMatuyama boundary recorded glacialeinterglacial climate cycles with 41-kyr periodicity and is prominent in shallow marine deposits in the Wanganui Basin, New Zealand (Pillans and Naish, 2004). Ikehara et al. (2010) measured the magnetic susceptibility using a multisensor logger at the same core as the present study. They observed a large positive peak at the boundary between units 4 and 5. According to Ikehara et al. (2010), the peak is attributed to the concentration of magnetic minerals, which is derived from an increased sediment output from a terrestrial area during the regression. Thus, it is quite likely that the cooling event at the boundary between units 4 and 5 was a distinct cooling event that occurred at the PlioePleistocene boundary. Temporal changes in the PBWTw and PBWTd are inversely correlated with those in the PWD (Figs. 4j, l: PBWTw vs PWD,
Fig. 5. Comparisons of the temporal changes of paleo-environmental variables and varimax factor loadings of the fossil planktonic foraminifera (VFLs-PF) in the Ananai (this study), Takanabe (Iwatani et al., 2012), and Yabuta (Cronin et al., 1994) Formations with oxygen isotope curve of the LR04 stack (Lisiecki and Raymo, 2005). Shaded area on the Takanabe Formation shows a period of the estimated subsidence (Iwatani et al., 2012). The geological ages of the Yabuta formation are based on the biostratigraphic data by Miwa et al. (2004). Gray and black lines of the paleo-bottom water temperatures (PBWTs) are PBWT of the coldest (PBWTc) and warmest month (PBWTw), respectively.
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R2 ¼ 0.63, P < 0.0001; PBWTd vs PWD, R2 ¼ 0.64, P < 0.0001). In general, deep/shallow water conditions correspond to low/high water temperatures and small/large seasonal differences in water temperatures. Conversely, temporal changes in the PBWTc are not inversely correlated with those in the PWD (Fig. 4k: PBWTc vs PWD, R2 ¼ 0.01, P < 0.0001). This finding suggests that changes in the PBWTc are not controlled by PWD fluctuations alone; other environmental factors such as the water structure are also reflected in PBWTc changes. In the parts of the sequence with low PBWTs values, high and low values of first and second VFLs (Fig. 2) suggest a nearshore depositional environment and a weakened influence by the Kuroshio Current, respectively. Moreover, compared with present-day water mass conditions, low PBWTc values are detected at shallow water depths in the study sequence (Fig. 3). Sediments deposited in these shallow water conditions are most likely formed under the influence of a weak Kuroshio Current and strong coastal waters relative to modern water conditions, characterized by a winter chill (J-DOSS, 2013). Therefore, changes in the PBWTc are linked with the fluctuations of the Kuroshio Current. This finding suggests that decreases in the PBWTc at boundaries between the different units were caused by the reductions of the Kuroshio Current's influence as a result of global cooling. In other marine strata in Pacific coastal areas, the weakening of the Kuroshio Current's influence was similarly recorded around the PlioePleistocene boundary. Planktonic foraminifera analysis suggested that the PlioePleistocene Takanabe Formation, Miyazaki Plain, southwest Japan (Fig. 1), clearly recorded a change from a marine climate under the influence of the axial part of the Kuroshio Current to that of the transitional water at around 2.6 Ma (Fig. 5, Iwatani et al., 2012). According to Iwatani et al. (2012), the water depth change at ~2.6 Ma in the Takanabe Formation suggests that the tectonic movement related to the rotation of the southern Kyushu Island occurred near this horizon. This subsidence is not recorded in the present study site; therefore, the local tectonic event was restricted to the Kyushu area. Moreover, the temporal changes of paleoenvironmental variables in the Takanabe Formation fluctuated over a large scale compared with those in the Ananai Formation, probably because of differences in PWDs between the two depositional paleoenvironments. Ananai sequences were formed in a shallower environment and closer to the coast compared with those of the Takanabe Formation. Therefore, the present study site appears to be more sensitive to paleoenvironmental changes. Ananai Formation outstandingly reflects hightime-resolution global climate changes. Around the Japan Sea side, in the PlioePleistocene Yabuta Formation, Noto Peninsula, central Japan (Fig. 1), eight glacial events recorded a 41-kyr obliquity cycle (Fig. 5, Cronin et al., 1994). According to Cronin et al. (1994), a progressive intensification of glacial events was recorded at ~2.5 Ma (around the glacial event 4), which corresponds to major changes in the global ice volume. The age was revised at ~2.6 Ma based on the recent biostratigraphic study by Miwa et al. (2004). Thus, the cooling event recorded in the Yabuta Formation occurred at around the PlioePleistocene boundary, the same as that recorded in the present study site. We compared the vertical PBWT profile of the Ananai Formation with the data on Takanabe (Iwatani et al., 2012) and Yabuta (Cronin et al., 1994) Formations. Similar values and trends were found between the Ananai and Takanabe Formations at the same water depths; that is, as the PWD decreases, seasonal differences in the water temperature increase (Fig. 6). It is possible that the Pacific Ocean off southwest Japan exhibited a similar water structure during the Plio-Pleistocene. On the other hand, in the Japan Sea side, the vertical PBWT profile of the Yabuta Formation clearly recorded cold conditions, even at a shallower water depth (Fig. 6).
0 50
paleo-water depth (m)
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100 150 200 250
Yabuta F. win sum
300
Ananai F. win sum Takanabe F. win sum
350 400
0
2.5
5
7.5
10
12.5
15
17.5
20
paleo-bottom water temperature (°C) Fig. 6. Vertical paleo-bottom water temperatures (PBWTs) profile of the Ananai (this study), Takanabe (Iwatani et al., 2012), and Yabuta (Cronin et al., 1994) Formations. Solid gray and black circles are PBWT of the coldest and warmest month of the Ananai Formation, respectively. Open gray and black circles are PBWT of the coldest and warmest month of the Takanabe Formation, respectively. Gray and black x-marks are PBWT of the coldest and warmest month of the Yabuta Formation, respectively.
At the same time (after ~2.7 Ma), calcareous nannofossil analysis reveals cool and eutrophic waters during the deposition of the PlioePleistocene Chikura Group, Boso Peninsula, central Japan (Fig. 1), which was caused by the southward movement of mixed water by a cold current (Oyashio Current) and a strengthening of the mixed water effect (Kameo et al., 2003). Thus, the weakening of the Kuroshio Current's influence around the PlioePleistocene boundary is widely recorded in southwest Japan. According to Kameo et al. (2003), the development of cool and eutrophic waters around central Japan is possibly related to the inflow of nutrientrich nearshore surface waters at coasts around the East Asian continent. The high nutrient content is derived from the weathering of the East Asian continent related to tectonic processes around the HimalayaneTibetan complex during the PlioePleistocene. Overall, a large seasonal difference in the water temperature is detected in the study sequence relative to recent water mass conditions (Fig. 3). Therefore, the Kuroshio Current's influence, which is characterized by suitable high temperature conditions, appears to be weak throughout the study sequence interval. The weakening of the Kuroshio Current's influence relative to modern conditions during the formation of the study sequence may have been a result of the strengthening effect of nearshore waters around the East Asian continent. The weakening of the Kuroshio Current's influence around the PlioePleistocene boundary in this region suggests that compared with present-day conditions, at that time, the current flowed at a greater distance from the Pacific coast off southwest Japan. 6. Conclusions The present study leads to four main conclusions. 1. More than 150 ostracode taxa are identified in the PlioePleistocene Ananai Formation (~2.6 Ma) on the Muroto Peninsula, Shikoku, southwest Japan.
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2. Results of the modern analog technique applied on fossil ostracode assemblages indicate a paleo-bottom water temperature during the warmest and coldest months, with values of 16 Ce20 C and 12 Ce16 C respectively, and a paleo-water depth between 70 and 140 m. 3. Fluctuations of the reconstructed paleo-bottom water temperatures correspond to 41-kyr orbital obliquity cycles. 4. Changes in paleo-bottom water temperature in the coldest month are linked to fluctuations in the Kuroshio Current intensity due to global cooling. 5. In comparison with present-day water mass conditions, a large seasonal difference in the water temperature is recorded in the study sequence, suggesting that the Kuroshio Current's influence, which is characterized by high temperatures, was weaker around the PlioePleistocene boundary. Acknowledgments We are grateful to Shigetaka Kita for discussions; Moriaki Yasuhara for important suggestions; Laura Wong and Hisayo Okahashi for continuous help in our study; two anonymous reviewers for their critical readings, and Henning A. Bauch for editing. This work was supported by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (No. 17540442 to T.I.) and the Faculty of Science Postdoctoral Fellowship of the University of Hong Kong (to H.I.). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2016.03.025. References Barkley, R.A., 1970. The Kuroshio current. Sci. J. 6, 54e60. Blow, W.H., 1969. Late middle Eocene to recent planktonic foraminiferal biostra€nnimann, P., Renz, H.H. (Eds.), Proc. 1st Intern. Conf. Planktonic tigraphy. In: Bro Microfossils, (Geneva, 1967). Leiden (E.J. Brill); Geneva, 1, 199-421. ~o, S.N., Yasuhara, M., Irizuki, T., Horne, D.J., 2013. The ostracod genus TraBranda chyleberis (Crustacea; Ostracoda) and its type species. Mar. Biodiv. 43, 363e405. Chiyonobu, S., Morimoto, J., Torii, M., Oda, M., 2012. Pliocene/Pleistocene boundary and paleoceanographic significance of the upper Miyazaki Group, southern Kyushu, Southwest Japan, based on calcareous nannofossil and planktic foraminiferal assemblages. J. Geol. Soc. Jpn. 118, 109e116 (in Japanese with English abstract). Cronin, T.M., Kitamura, A., Ikeya, N., Watanabe, M., Kamiya, T., 1994. Late Pliocene climate change 3.4e2.3 Ma: paleoceanographic record from the Yabuta formation, sea of Japan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 437e455. , C., Bauch, H.A., Helmke, J.P., 2002. Late Quaternary deep-sea ostracodes in the Didie polar and subpolar North Atlantic: paleoecological and paleoenvironmental implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 184, 195e212. Ding, Z.L., Derbyshire, E., Yang, S.L., Sun, J.M., Liu, T.S., 2005. Stepwise expansion of desert environment across northern China in the past 3.5 Ma and implications for monsoon evolution. Earth Planet. Sci. Lett. 237, 45e55. Gallagher, S.J., Kitamura, A., Iryu, Y., Itaki, T., Koizumi, I., Hoiles, P.W., 2015. The Pliocene to recent history of the Kuroshio and Tsushima currents: a multi-proxy approach. Prog. Earth Planet. Sci. 2, 23. Guiot, J., 1990. Methodology of the last climatic cycle reconstruction in France from pollen data. Palaeogeogr. Palaeoclimatol. Palaeoecol. 80, 49e69. Hammer, Ø., Harper, D.A.T., Ryan, P.D., 2001. PAST: paleontological statistics software package for education and data analysis. Palaeontol. Electron 4 (9) (March 2014). http://palaeo-electronica.org/2001_1/past/issue1_01.htm. Hanai, T., Ikeya, N., Ishizaki, K., Sekiguchi, Yajima, M., 1977. Checklist of Ostracoda from Japan and its Adjacent Seas. University of Tokyo Press, Tokyo, p. 119. Ikehara, M., Iwai, M., Kondo, Y., Kita, S., Hattori, N., 2010. Non-destructive Analyses of a Drilling Core ANA-1 from the Ananai Formation of the Tonohama Group on Muroto, Peninsula, Kochi. Sci. Rep. Kochi Univ. 59, 183e195. Ikeya, N., Cronin, T.M., 1993. Quantitative analysis of Ostracoda and water masses around Japan: application to Pliocene and Pleistocene paleoceanography. Micropaleontology 39, 263e281. Ikeya, N., Shiozaki, M., 1993. Characteristics of the inner bay ostracodes around the Japanese islands: the use of ostracodes to reconstruct paleoenvironments. Mem. Geol. Soc. Jpn. 39, 15e32 (in Japanese with English abstract). Irizuki, T., Kusumoto, M., Ishida, K., Tanaka, Y., 2007. Sea-level changes and water
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