middle Miocene boundary as inferred from radiolarian assemblages

Palaeogeography, Palaeoclimatology, Palaeoecology 487 (2017) 136–148

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Reconstruction of oceanic circulation patterns in the tropical Pacific across the early/middle Miocene boundary as inferred from radiolarian assemblages

MARK

Shin-ichi Kamikuria,⁎, Theodore C. Mooreb a b

Faculty of Education, Ibaraki University, Bunkyo 2-1-1, Mito, Ibaraki 310-8512, Japan Department of Earth and Environmental Sciences, University of Michigan, MI 48109-1005, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Mid-Miocene climatic optimum East-west thermocline gradient Upwelling Equatorial undercurrent Indo-Pacific seaway

Although there have been attempts to infer Neogene oceanic circulation patterns in the tropical Pacific on the basis of multiple marine proxies, oceanic circulation patterns across the early/middle Miocene boundary are still poorly understood despite paleoclimate having significantly changed during this interval. In this study, we reconstruct the changes in tropical oceanic circulation patterns in the Pacific across the early/middle Miocene boundary based on radiolarian assemblages obtained at Integrated Ocean Drilling Program Site U1335 in the eastern tropical Pacific. The radiolarian upwelling taxa increased during four intervals (18.4–18.1 Ma, 16.9–15.7 Ma, 15.2–14.6 Ma, 14.2–13.4 Ma). Radiolarian temperature index (RTI) was calculated using the relative abundances of the warm water (cluster B1) and cool water taxa (cluster B2) in order to evaluate relative temperature fluctuations of the surface water. The sea surface temperature index was relatively high from 16.8 to 16.0 Ma and gradually decreased from 16.0 to 14.6 Ma. Subsequently, the temperature index decreased stepwise at 13.9 and 13.4 Ma and became relatively low from 13.4 to 12.7 Ma. Comparison of the RTI record in the eastern tropical Pacific to the climatic index based on the benthic foraminiferal δ18O record shows that variability in the two records follows a roughly similar trend from the early to the middle Miocene. However, the sea surface temperature record does not indicate a sharp increase in warmth in the eastern tropical Pacific during the mid-Miocene climatic Optimum-1 (MMCO-1), as suggested by the benthic isotopic record. This is likely because of increased upwelling of cool, nutrient-rich water at this time. Beginning in the latest early Miocene (~ 17 Ma) radiolarian assemblages were dominated by different taxa in the eastern and western tropical Pacific. This pattern is interpreted as indicating a shallower thermocline in the east and a deeper thermocline in the west, based on the relative abundance of the upwelling taxa. There is an overall increasing trend in this difference since the latest early Miocene. We tie these events to the effective closure of the Indo-Pacific seaway and the development of a substantial western Pacific warm pool along with the development of a strong Equatorial Undercurrent.

1. Introduction Oceanic circulation patterns in the tropical Pacific changed significantly throughout the Neogene (Kennett et al., 1985; Moore et al., 2004; Kamikuri et al., 2009a; Nathan and Leckie, 2009; Rousselle et al., 2013). Kennett et al. (1985) reconstructed surface water circulation patterns in the tropical Pacific Ocean based on biogeographic patterns of planktonic foraminifera for three time periods: the early Miocene (22 and 16 Ma) and the late Miocene (8 Ma). In the early Miocene (22 and 16 Ma), distinct faunal assemblages were present in the western and eastern areas, and an east-west thermocline depth gradient was ⁎

Corresponding author. E-mail address: [email protected] (S.-i. Kamikuri).

http://dx.doi.org/10.1016/j.palaeo.2017.08.028 Received 22 June 2017; Received in revised form 18 August 2017; Accepted 18 August 2017 Available online 19 August 2017 0031-0182/ © 2017 Elsevier B.V. All rights reserved.

suggested to occur throughout the tropical Pacific Ocean, with a relatively shallower thermocline in the eastern tropical Pacific and a deeper thermocline in the west. The relatively sluggish equatorial circulation, the absence of the Equatorial Undercurrent (EUC), and a weak North Equatorial Countercurrent (NECC) allowed distinct paleoceanographic (biogeographic) differences to develop between the east and west (Kennett et al., 1985). By the late Miocene, the east-west differences in foraminiferal assemblages had largely disappeared across the entire tropical Pacific, and these changes were interpreted as reflecting the development of the EUC and ECC systems due to the effective closure of the Indonesian Seaway (Kennett et al., 1985). The closure of the

Palaeogeography, Palaeoclimatology, Palaeoecology 487 (2017) 136–148

S. Kamikuri, T.C. Moore

40°

Indonesian Seaway created a pile-up of surface waters in the western tropical Pacific and hence the development of the easterly flowing EUC, along with a deeper thermocline depth in the western tropical Pacific. The barriers also induced a strengthened ECC between the early Miocene (16 Ma) and the late Miocene (8 Ma) (Kennett et al., 1985). Subsequent studies with more precise age control suggested that major oceanic circulation changes in the tropical Pacific occurred in the earliest late Miocene (ca. 11.5–10.0 Ma) (Chaisson and Leckie, 1993; Jian et al., 2006; Li et al., 2006; Kamikuri et al., 2009a; Nathan and Leckie, 2009). The circulation patterns changed significantly from a state resembling El Niño (warm eastern tropical Pacific) in the late Miocene and early Pliocene to a state resembling La Niña (cool eastern tropical Pacific) starting in the late Pliocene. The deepening of the thermocline depth in the western tropical Pacific and the reduction of Eastern Pacific Warm Pool occurred at about 4.2 Ma (Cannariato and Ravelo, 1997; Jian et al., 2006; Li et al., 2006; Sato et al., 2008; Kamikuri et al., 2009a). However, changes in oceanic conditions from the early to middle Miocene are still poorly understood even though this is an important interval during which paleoclimate changed significantly. The lack of information about this period is attributable to the increased carbonate dissolution in the tropical Pacific obscuring the assemblage changes in carbonate microfossils, which were traditionally used as proxies to reconstruct oceanic circulation patterns (Keller, 1981; Barron et al., 1985; Lyle, 2003; Mitchell et al., 2003; Pälike et al., 2012). The purpose of this study is to reconstruct the changes in oceanic circulation patterns from 20 to 12.5 Ma across the early/middle Miocene boundary based on well-preserved radiolarian assemblages, using data from Integrated Ocean Drilling Program (IODP) Site U1335.

California Current

20° North Equatorial Current North Equatorial Countercurrent

0°

U1335 South Equatorial Current

20°S

150°W

120°

90°

Fig. 1. Location of IODP Leg 320 Site U1335 and modern oceanographic currents in the eastern tropical Pacific.

3. Oceanographic setting of the tropical Pacific In the modern ocean, the eastern tropical Pacific is one of the major upwelling areas and supports a significant proportion (18–56%) of the new biological production globally (Chavez and Barber, 1987). This high productivity not only raises the sedimentation rates of calcareous and siliceous ooze but also increases the amount of organic matter on the seafloor. Consequently, biological production in the eastern tropical Pacific plays a significant role in the global carbon cycle. The eastern tropical Pacific Ocean is situated between the subtropical gyres of the North and South Pacific. In this region, the eastern boundary currents (the California and Peru Currents) turn west and run parallel to the equator until they merge with the North and South Equatorial Currents (NEC and SEC, respectively) (Wyrtki, 1966, 1967; Fiedler and Talley, 2006; Kessler, 2006; Pennington et al., 2006) (Fig. 1). The NEC is formed by water from the California Current and flows from east to west roughly centered on 15°N in the tropical Pacific (Wyrtki, 1966; Kessler, 2006). The westward NEC bifurcates with part of the flow returning poleward at the start of the western boundary currents (the Kuroshio Current) and part turning equatorward to eventually feed the North Equatorial Countercurrent (NECC). The NECC flows eastward across the tropical Pacific roughly centered on 7°N between the NEC and SEC. The SEC is driven by the southeast trade winds in the South Pacific Ocean and flows westward roughly centered on 2°S (Kessler, 2006; Pennington et al., 2006). The southeasterly trade winds cross the equator, and with the change in sign of the Coriolis effect, they induce an Ekman divergence that transports surface waters away from the equator in both hemispheres and consequently produces strong equatorial upwelling (Kessler, 2006). The band of cool surface water, which occurs from about 3°S to 3°N and across the eastern and central tropical Pacific is called the equatorial “cold tongue” (Fiedler and Talley, 2006; Kessler, 2006; Pennington et al., 2006). In the equatorial cold tongue, surface nitrate and phosphate concentrations are generally high relative to those to the south and north of the equator. This upwelling also induces a shallower thermocline depth in the eastern Pacific than in the western Pacific (Jin, 1998). The Equatorial Undercurrent (EUC), which flows eastward under the SEC, is centered on the equator. The EUC rises along the sloping thermocline from west to east, bringing an admixture of cool water close to the surface, which adds to the biologically important mechanism of nutrient upwelling along the equator (Wijffels, 1993; Kessler, 2006; Pennington et al., 2006) and feeds into the upwelling off Peru (Toggweiler et al., 1991).

2. Paleoclimatic history through the early to middle Miocene interval The early/middle Miocene boundary represents a time of major climatic change during the Neogene (Woodruff and Savin, 1989; Wright et al., 1992; Flower and Kennett, 1994; Zachos et al., 2008). The early Miocene was characterized by a significantly warmer climate than at present, particularly at high latitudes. Based on benthic foraminiferal oxygen isotope data, a peak in deepwater temperatures was recorded at 17–15 Ma, at what is called the Mid-Miocene Climatic Optimum (MMCO) (Flower and Kennett, 1994; Zachos et al., 2008). During the MMCO, tropical/subtropical organisms extended their ranges to the mid-to-high latitudes (Wolfe, 1985; Yamanoi, 1992; Ogasawara et al., 2008; Yabe, 2008), and the sea surface temperatures (SSTs) based on oxygen isotope records, TEX86 and Uk'37, were 27–30 °C in the tropical Indian Ocean (Stewart et al., 2004), 28–30 °C in the tropical Atlantic (Zhang et al., 2013), and 27–29 °C in the eastern tropical Pacific (Rousselle et al., 2013). The MMCO corresponds with the mid-Neogene climatic optimum of Tsuchi (1987) and the climatic optimum 1 of Barron and Baldauf (1990). The mid-Neogene climatic optimum was subdivided into two intervals (lower and upper) by the Miocene isotope (Mi) 2 event (Irizuki et al., 1998). After the MMCO, a significant positive shift was recorded in benthic foraminiferal oxygen isotopes, which was called the Middle Miocene Climatic Transition (MMCT). This significant shift was associated with the growth of the East Antarctic Ice Sheet (Flower and Kennett, 1994; Zachos et al., 2008), abrupt major global cooling (Zhao et al., 2001; Billups and Schrag, 2002; Shevenell et al., 2004; Lear et al., 2010), global hiatuses (Keller and Barron, 1983; Woodruff and Savin, 1989; Kamikuri et al., 2004) and major turnovers in faunal and floral assemblages (Wei and Kennett, 1986; Barron, 2003; Thomas, 2007; Kamikuri et al., 2007, 2009b; Pound et al., 2012). These oceanographic/climatic changes were accompanied by the Monterey Carbon Excursion from the late early Miocene to middle Miocene, which is characterized by several distinct δ13C maxima related to large-scale organic carbon-rich sedimentation in upwelling areas (Vincent and Berger, 1985; Woodruff and Savin, 1991; Flower and Kennett, 1994). 137

Palaeogeography, Palaeoclimatology, Palaeoecology 487 (2017) 136–148

Lith. Unit

Epoch

Hole A Hole B

Depth (m)

S. Kamikuri, T.C. Moore

top of C5Cn.1n with an estimated age of 15.97 Ma (Gradstein et al., 2012). Hence, the early/middle Miocene boundary occurred at 184.045 m at Site U1335 (Fig. 3; Pälike et al., 2010). Investigations of radiolarian assemblages were generated in a 7.2 Myr.-long interval from the early to middle Miocene (269.62 m to 106.00 m; 19.8 Ma to 12.6 Ma). The average sampling spacing at the site is ~2.2 m (spacing in time = ~0.1 m.y.). A total of 74 sediment samples from the site were examined in this study. Samples were prepared following procedures similar to those described in Sanfilippo et al. (1985). Sediment samples were weighed, treated with 15% H2O2 to remove organic material, treated with 15% HCl solution to remove the calcareous fraction, washed, and sieved with 63 μm mesh. Residues were randomly settled on a slide (Moore, 1973) and covered with a 24 × 40 mm cover glass with Norland Optical Adhesive #61 as the mounting medium. All radiolarian skeletons on the slides were observed and counted under a light microscope at × 100 to × 250 magnification. More than 230 radiolarian taxa were encountered during this study (Supplementary information), and microphotographs of the most abundant taxa are presented in Kamikuri (submitted). The Shannon/ Wiener index (H′) describes the species richness and the relative proportion of each species within a sample (H′ = − ΣPi lnPi, where Pi is the proportion of each species) (Shannon and Weaver, 1949). Evenness (E) is the measure of the evenness of the species distribution within a sample and is affected by H′ and species number (E = eH′/Sp; where Sp is the number of species) (Buzas and Gibson, 1969). R-mode cluster analysis (using the program R; http://www.rproject.org/) was carried out on the 16 radiolarian taxa present with a relative abundance of ≥5% in one or more samples (Table 1; Fig. 3; Supplementary information). The 16 taxa made up 45–75% of the total assemblages. R-mode (species) and Q-mode (samples) cluster analyses were performed using the average linkage method with similarity measures based on a cosine theta coefficient. Before computation, we grouped Calocycletta robusta, C. virginis and C. caepa into a Calocycletta group in order to reduce data noiseness. Likewise, we grouped Cyrtocapsella tetrapera and C. cornuta into a Cyrtocapsella group; Stichocorys delmontensis and S. wolffii into a Stichocorys group; Didymocyrtis bassanii, D. prismatica, D. violina, D. tubaria, D. mammifera, and D. laticonus into a Didymocyrtis group; Carpocanium cingulata, C. bramlettei, Carpocanium sp. A, and Carpocanium sp. B into a Carpocanium group; Collosphaera brattstroemi, C. glebulenta, C. pyloma, C. reynoldsi, Collosphaera sp. A, and Collosphaera sp. B into a Collosphaera group; Dictyocoryne malagaense, Dictyocoryne sp. A and Dictyocoryne sp. B into a Dictyocoryne group; Siphonosphaera arkys, Siphonosphaera sp. A, Siphonosphaera sp. B, Siphonosphaera sp. C, Trisolenia combinata, T. megalactis, and Trisolenia sp. A into a Siphonosphaera/Trisolenia group; Tholospyris anthophora, T. kantiana, T. mammillaris into a Tholospyris group (Table 1; Supplementary information. Species labeled A. B, and C are illustrated in Supplementary information). The radiolarian assemblages live mainly in the near surface waters and mixed layer (Abelmann, 1992; Kling and Boltovskoy, 1995). In order to estimate the relative sea surface water temperature, paleoclimatic ratios based on fossil assemblages of calcareous nannofossils (Di Stefano and Incarbona, 2004; Villa and Persico, 2006), diatoms (Kanaya and Koizumi, 1966; Barron, 1992), planktonic foraminifera (Spezzaferri and Premoli Silva, 1991; Amore et al., 2004), and radiolarians (Nigrini, 1970; Yamauchi, 1986; Haslett, 1992) have been used. Haslett and Funnell (1996) introduced the Radiolarian Temperature Index (RTI) for use as a paleoclimatic ratio in the eastern tropical Pacific. The RTI uses two faunal end members, and is defined as: RTI = (Xw / (Xw + Xc)) × 100, where Xw is the frequency of warm water species and Xc is the frequency of cool water species. In this study, Xw and Xc are based on a combination of R-mode cluster analysis of the presented data, supported by literature information on the biogeographical affinity of taxa included in the various clusters. RTI ranges in value from 0 to 100 and becomes systematically larger from

Lithology

10H

11H 11H 12H 12H

middle

13H 13H 14H 14H

150

nannofossil ooze with radiolarians and foraminifers

15H 15H 16H

Sediment color from white to very pale brown

16H

18H 18H

Miocene

17H 17H

II

19H 19H

200

20H 20H 21H

22H 22H

early

21H

23H 23H

250

24H 24H 25H 25H 26H Nannofossil ooze

Nannofossil radiolarian ooze

Fig. 2. Lithologic summary from the lower to middle Miocene at Site U1335 in the eastern tropical Pacific (Pälike et al., 2010).

4. Material and methods During IODP Exp. 320/321, continuous sedimentary sequences were drilled at eight sites (Sites U1331 to U1338) in the low latitudes of the North Pacific (Pälike et al., 2010). All materials used in this study were obtained from Site U1335 (5°18.735′ N, 126°17.002′ W, water depth, 4327.5 m) (Fig. 1). The position of Site U1335 was estimated to have ranged in latitude from about 0° to 2°N during the early and middle Miocene (Pälike et al., 2010). The lower and middle Miocene consists of nannofossil ooze with well-preserved siliceous microfossils such as diatoms and radiolarians (Fig. 2). Increased carbonate dissolution was observed from 195 to 205 m (16.7 to 17.5 Ma). In this study we use the CCSF-A (composite core depth below seafloor) spliced sediment depth scale (Pälike et al., 2010). For construction of the age-depth model for the lower to middle Miocene sequence at Site U1335, we plotted paleomagnetic data and a few planktic foraminifer and calcareous nannoplankton events from intervals with poor paleomagnetic polarity records (Supplementary information; Pälike et al., 2010). The geomagnetic time scale was adopted from the Geologic Time Scale 2012 (Gradstein et al., 2012). For each sample collected at IODP Site U1335, the age was estimated using this age-depth model (Supplementary information). The early/ middle Miocene (Burdigalian/Langhian) boundary corresponds to the 138

23H 23H

250

24H 24H 25H 25H 26H

Polysolenia murrayana

22H 22H

Calocycletta group

21H

Age (Ma)

C5AA n

C5AC

Middle

C5AB

RN5

14

C5AD

15

C5B

b

Miocene

Carpocanium group

Zygocircus spp. Dictyocoryne group

21H

Didymocyrtis group

20H

II

c

Tetrapyle sp. A

20H

Collosphaera group

19H 19H

200

Siphonosphaera/Trisolenia group

18H 18H

Lophocyrtis aspera

17H 17H

Stichocorys group

16H

Stylodictya sp. A

15H 15H

13

a

Cyrtocapsella group

14H 14H

Spongodiscus resurgens

13H 13H

b

Eucyrtidium diaphanes

12H 12H

RN4

16

C5C

I

17 C5D

18 RN3

a

C5E

C6

Early

Tholospyris group

11H 11H

16H

20

Magnetic Chron

0

Radiolaria

Lithology

Hole A Hole B

ccsf-a Depth (m)

Relative abundance of dominant species (%)

10H

150

Interval

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19

RN2

Fig. 3. Relative abundance of dominant species of radiolarians from the early to middle Miocene at Site U1335. The biostratigraphic and magnetostratigraphic results at this site are adapted from Pälike et al. (2010). Dotted lines are interval boundaries based on Q-mode cluster analysis (see Fig. 4).

5.2. Q-mode cluster analysis

the high latitude region to the tropical region, showing a positive correlation with surface water temperature.

Q-mode cluster analysis divided the 74 samples into two clusters with five subclusters (Fig. 4). These clusters represent two intervals with five subintervals (Figs. 3, 4). Interval I (cluster X) is represented by 57 samples between U1335A25H-CC and U1335A-14H-3, 149–150 cm (269.62 to 142.69 m), and spans from 19.8 to 14.2 Ma. The subinterval Ia/b (clusters X1/2) boundary is located at 16.9 Ma between samples U1335B-19H-CC and U1335A-19H-2, 105–107 cm (197.12 and 194.26 m); subinterval Ib/c (clusters X1/3) boundary at 14.6 Ma between samples 1335A-15H-5, 105–107 cm and U1335A-15H-3, 149–150 cm (155.30 and 154.24 m). Interval II (cluster Y) consists of 17 samples from samples U1335A, 14H-2, 105–107 cm to U1335A, 10H–CC (140.74 to 106.00 m) and spans 14.2 Ma to 12.6 Ma. Subinterval IIa/b (clusters Y1/2) boundary is located at 13.4 Ma between samples 1335A-13H-3, 149–150 cm and U1335A-13H-2, 104–106 cm (131.14 and 129.19 m). Hence, a major faunal change in the middle Miocene (14.2 Ma), and three minor changes in the latest early Miocene (16.9 Ma) and the middle Miocene (14.6 Ma and 13.4 Ma) took place across the early/ middle Miocene boundary at Site U1335 in the eastern tropical Pacific.

5. Results 5.1. Radiolarian faunal composition The relative abundance of 16 radiolarian taxa having > 5% abundance in one or more samples is shown in Fig. 3. Taxa with abundance of 10% or more include the Stichocorys group, the Calocycletta group, the Tholospyris group, Stylodictya sp. A, Siphonosphaera/Trisolenia group, and Polysolenia murrayana. The Stichocorys group occurred abundantly from radiolarian Zone RN2 to the lower part of RN4, then decreased stepwise in the lower part of RN4 and RN5. The Calocycletta group increased abruptly in the lower part of RN4, synchronous with the first decrease of the Stichocorys group, but decreased stepwise in the lower part of RN5. The Tholospyris group occurred commonly from RN2 to the lower part of RN5, then decreased stepwise in the lower part of RN5, close to the second and third stepwise decrease in abundance of the Stichocorys group. The abundance of Stylodictya sp. A and Siphonosphaera/Trisolenia group increased in the lower part of RN5. Polysolenia murrayana shows four positive peaks in the middle part of RN3, the lower part of RN4, the lower part of RN5, and the middle part of RN5. Other common species are Lophocyrtis aspera, the Collosphaera group, Eucyrtidium diaphanes, Spongodiscus resurgens, the Cyrtocapsella group, Tetrapyle sp. A, the Dictyocoryne group, the Didymocyrtis group, Zygocircus spp., and the Carpocanium group.

5.3. R-mode cluster analysis The R-mode cluster analysis was carried out on the 16 taxa present with a relative abundance of ≥5% in one or more samples. The radiolarian assemblage was composed of three distinct clusters, A, B, and C, with a cosine theta coefficient of 0.4 used as the cutoff value (Fig. 5). Cluster A was dominated by Polysolenia murrayana (Fig. 5) with relative abundance ranging from 0% to 11% of the total assemblages 139

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Table 1 Species and grouping of species used in this study. See text for interpretation and references. Species/species groups Polysolenia murrayana Lophocyrtis aspera Stylodictya sp. A Spongodiscus resurgens Zygocircus spp. Tetrapyle sp. A Dictyocoryne group

Cyrtocapsella group Didymocyrtis group

Eucyrtidium diaphanes Calocycletta group

Tholospyris group

Stichocorys group Carpocanium group

Collosphaera group

Siphonosphaera/Trisolenia group

Grouped taxa

R mode species clusters

Regions of occurrence

Interpretation

Cluster A Cluster B1

Nutrient-rich waters Low latitude & cosmopolitan

Upwelling taxa Relatively cool water taxa

Cluster B2

Low latitude

Warm water taxa

Cluster C

Oligotrophic waters

Oligotrophic taxa

Dictyocoryne malagaense Dictyocoryne sp. A Dictyocoryne sp. B Cyrtocapsella tetrapera C. cornuta Didymocyrtis laticonus D. bassanii D. prismatica D. violina D. tubaria D. mammifera Calocycletta robusta C. virginis C. caepa Tholospyris anthophora T. kantiana T. mammillaris Stichocorys delmontensis S. wolffii Carpocanium cingulata C. bramlettei C. kinugasense Carpocanium sp. B Collosphaera brattstroemi C. glebulenta C. macropora C. pyloma C. reynoldsi Collosphaera sp. A Siphonosphaera arkys Siphonosphaera sp. A Siphonosphaera sp. B Siphonosphaera sp. C Trisolenia combinata T. megalactis Trisolenia sp. A

to subcluster B2 described below. Subcluster B2 is composed of five taxa: Eucyrtidium diaphanes, the Calocycletta group, the Stichocorys group, the Tholospyris group and the Carpocanium group. This subcluster was abundant (> 30%) from Interval Ia to the lower part of Interval Ib, gradually decreased from the upper part of Interval Ib to Interval IIa, and remained at < 10% in Interval IIb (Fig. 6). Previous studies have shown that these five taxa occurred in the low latitudes during the early and middle Miocene (Sancetta, 1978; Riedel and Sanfilippo, 1978; Nigrini and Lombari, 1984; Romine and Lombari, 1985; Lombari, 1985; Sanfilippo et al., 1985). Therefore, this assemblage is designated as warm water taxa. The variations in subcluster B2 inversely correlated with those in subcluster B1. Cluster C consists of the Collosphaera group and Siphonosphaera/ Trisolenia group, and gradually increased since the upper part of Interval Ia (~ 17 Ma) (Fig. 6). This assemblage was distributed in oligotrophic areas (Bjørklund and Goll, 1979; Swanberg, 1983; Casey, 1993).

and increases during four intervals (the middle part of Interval Ia, lower part of Interval Ib, upper part of Interval Ib, and Interval IIa) (Fig. 6). Cluster A is assigned to upwelling taxa, because P. murrayana is restricted to upwelling areas in the modern ocean (Molina-Cruz, 1977, 1984; Romine and Moore, 1981; Lombari and Boden, 1985; Caulet et al., 1992; Haslett, 2003). Cluster B consists of 13 taxa and can be subdivided into two subclusters (Fig. 5). Subcluster B1 contains eight taxa: Lophocyrtis aspera, Stylodictya sp. A (=Porodiscus sp. A of Nigrini and Moore, 1979), Spongodiscus resurgens, Zygocircus spp., the Dictyocoryne group, Tetrapyle sp. A, the Cyrtocapsella group, and the Didymocyrtis group. This subcluster makes up 19.8% to 31.4% of the total assemblages in Interval Ia. It decreased slightly in Interval Ib and increased again from Intervals Ic to IIb, reaching a maximum abundance of approximately 40% in Interval IIb (Fig. 6). This subcluster consists of both low latitude taxa and cosmopolitan taxa. Of the eight taxa, five taxa (the Dictyocoryne group, the Didymocyrtis group, Tetrapyle sp. A, Stylodictya sp. A, and Zygocircus spp.) occurred commonly in the low latitude ocean (Sancetta, 1978; Nigrini and Lombari, 1984; Lombari and Boden, 1985; Sanfilippo et al., 1985; Kamikuri et al., 2008), and three taxa (L. aspera, S. resurgens, and the Cyrtocapsella group) had distributions from the low to high latitude ocean (Nigrini and Lombari, 1984; Sanfilippo et al., 1985; Funayama, 1988; Sanfilippo and Caulet, 1998; Kamikuri et al., 2008). Therefore, this assemblage is represented by relatively cool water taxa compared

5.4. Species diversity: Shannon/Wiener index and evenness The Shannon/Wiener index (H′) ranged from 3.18 to 4.08 (mean, 3.64) over the range of samples, and the evenness of species distribution (E) ranged from 0.32 to 0.61 (mean, 0.47). The H′ shows a trend 140

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Fig. 4. Q-mode cluster analysis based on 16 radiolarian taxa present at ≥ 5% in one or more samples at Site U1335.

Height 0.5

0.4

0.3

0.2

0.1

0.0 sample no. 41 36 42 35 25 34 32 33 27 26 28 39 40 37 38 31 29 30 52 53 51 54 43 44 45 64 69 57 59 46 63 49 65 66 58 55 56 62 60 61 50 68 47 48 71 72 67 73 70 74 22 23 24 19 18 20 21 13 14 17 15 16 2 12 4 5 1 3 6 8 9 10 7 11

X1

X

X2

X3

Y1

Y Y2

6. Discussion

similar to the E (Fig. 7). Relatively low species diversity (H′ and E) was evident during Interval Ia in the early Miocene due to the high abundance of the Stichocorys group, which was the dominant taxon (Fig. 3). Species diversity increased from Intervals Ib to IIb from the latest early Miocene, as the Stichocorys group decreased in abundance.

6.1. Radiolarian temperature index across the early/middle Miocene boundary in the eastern tropical Pacific In order to evaluate relative temperature fluctuations of the surface water, the RTI was calculated using the relative abundances of the warm water and cool water taxa from the early to middle Miocene at Site U1335 in the eastern tropical Pacific (Fig. 8; Table 1). According to literature on the biogeographic distribution of radiolarians, cluster B1

141

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Fig. 5. R-mode cluster analysis of radiolarian taxa across the early/middle Miocene boundary at Site U1335.

Height 0.4

0.5

0.3

0.2

0.1

0.0

A

Polysolenia murrayana Lophocyrtis aspera Stylodictya sp. A Spongodiscus resurgens Zygocircus spp.

B1

Dictyocoryne group Tetrapyle sp. A

B

Cyrtocapsella group Didymocyrtis group Eucyrtidium diaphanes Calocycletta group Stichocorys group

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Tholospyris group Carpocanium group Siphonosphaera/Trisolenia group

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a 14.2 Ma

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14.6 Ma u-3

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16.9 Ma

a u-1

r

Burdigalian

early Miocene

faunal changes

(%)

r

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Fig. 6. Stratigraphic distribution of species clusters across the early/middle Miocene boundary at Site U1335.

C5C M4

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r n r n r n r n r n r C5B C5AD C5AC C5AB C5AA C5A M5 M6 M7 M8 RN4 RN5

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upper part of Stage Ib from 16.0 to 14.6 Ma. Subsequently, the RTI decreased stepwise at 13.7 and 13.4 Ma and reached minimum values during the interval from 13.4 to 12.6 Ma. Comparison of the RTI record to the climatic index derived from the benthic foraminiferal δ18O record (global ice volume plus bottom-water temperatures, Fig. 8d) (Tian et al., 2014; Holbourn et al., 2015) in the eastern tropical Pacific shows that variability in the two records have a roughly similar trend from the

was considered to be a relatively cool water indicator, while cluster B2 was considered to be a relatively warm water indicator (e.g., Sancetta, 1978; Nigrini and Lombari, 1984; Sanfilippo et al., 1985; Sanfilippo and Caulet, 1998). The RTI fluctuated from 9.6 to 74.2 (avg. 52.1) at Site U1335 (Fig. 8). The RTI showed relatively high values during the lower part of Stage Ib from 16.9 to 16.0 Ma and gradually decreased through the 142

Palaeogeography, Palaeoclimatology, Palaeoecology 487 (2017) 136–148

0.4

0.5

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Evenness

Radiolaria

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Interval

Lithology

Hole A Hole B

ccsf-a Depth (m)

S. Kamikuri, T.C. Moore

Fig. 7. Shannon/Wiener index (H′) and evenness (E) from the early to middle Miocene at Site U1335.

10H

11H 11H

b

13H 13H 14H 14H

150

15H 15H

13

II

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RN5

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17H

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may have been the result of better opal preservation as well as the result of this site being directly under the equator from 17.5 to 17.0 Ma (Piela et al., 2012).

early to middle Miocene.

6.2. Changes of equatorial upwelling across the early/middle Miocene boundary 6.3. Faunal differences/similarities between eastern and western areas in the tropical Pacific

The eastern tropical Pacific is one of the largest areas of upwelling in the modern ocean (Wyrtki, 1966, 1967; Fiedler and Talley, 2006; Kessler, 2006; Pennington et al., 2006). The uppermost layer is subject to more rapid temperature changes associated with changes in climate and strength of upwelling. From the early to middle Miocene at Site U1335, the relative abundance of radiolarian upwelling taxa (Cluster A) increased in four intervals (18.4 to 18.1 Ma, 16.9 to 15.7 Ma, 15.2 to 14.6 Ma, and 14.2 to 13.4 Ma). Here, we designate these intervals as u-1 to u-4 from the oldest to more recent (Figs. 8, 9). Equatorial upwelling returns nutrients to the sea surface from the deeper layer, resulting in greater biological productivity. High abundances of biogenic opal and barium are used as proxies for increased biological productivity (Fig. 9). Data for biogenic opal and Ba from the early to middle Miocene at Site U1337 in the eastern tropical Pacific reported by Shackford et al. (2012) shows a general correspondence between increased upwelling taxa in the present study and high biogenic opal and Ba content, except for the interval from 17.5 to 17.0 Ma. During the interval from 17.5 to 16.7 Ma, increased carbonate dissolution was identified based on shipboard observations (Fig. 2; Pälike et al., 2010) and is referred to as the “carbonate famine” (Lyle, 2003; Pälike et al., 2012; Piela et al., 2012). Keller (1981) also reported the carbonate famine and noted that a relatively higher abundance of siliceous microfossils (diatoms and radiolarians) occurred simultaneously in the latest early Miocene in the tropical Pacific an event that may have been caused by upwelling of cool nutrient-rich deep water. However, radiolarian fauna are not characterized by upwelling taxa in this interval, despite it being opal-rich. The increased biogenic opal

The differences and similarities between faunas in the eastern and western areas of the tropical Pacific represent an important piece of information for reconstructing the establishment of the modern eastwest gradient in thermocline depth across the tropical Pacific (Kennett et al., 1985; Kamikuri et al., 2009a; Nathan and Leckie, 2009). Romine and Lombari (1985) presented a time-series of abundance data on radiolarian assemblages during the Miocene at DSDP Site 289 in the western tropical Pacific. We use the difference/similarity of radiolarian assemblages from this site compared to assemblages at Site U1335 in the eastern tropical Pacific to reveal changes in the east-west thermocline gradient during the early to middle Miocene (Fig. 8b). In the early Miocene, large abundances of two radiolarian species, the Stichocorys group and the Calocycletta group, were identified from 20 to 17 Ma in the eastern and western tropical Pacific. In the latest early Miocene, the abrupt decrease of the Stichocorys group and a sharp increase of the Calocycletta group occurred at about 17 Ma in the eastern area (Fig. 3). In contrast for the western area, there was an abrupt increase of the Stichocorys group and decrease of the Calocycletta group at about 17 Ma along with the increase of Phorticium pylonium and Stylochlamydium asteriscus at about 15 Ma (Romine and Lombari, 1985). These data indicate that the similarity of radiolarian assemblages in the eastern and western tropical areas was relatively high in the early Miocene from 20 to 17 Ma but became relatively low since the latest early Miocene (~ 17 Ma). Consequently, the tropical Pacific since the latest early Miocene was dominated by radiolarian assemblages that exhibited definite east-west faunal provinces. Here we develop a climatic curve 143

Palaeogeography, Palaeoclimatology, Palaeoecology 487 (2017) 136–148

Interval NH3

0 0

δ13C (‰)

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b

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u-3 Site 575

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(b)

u-4

r n r n r n r n r C5AD C5AC C5AB C5AA C5A

warming

Site U1335

80 Upwelling

0

Species diversity Re-w ratio 0.3 0.3 0.6 1.0 Site 289

Chron

This study RTI

r n C5B

15

middle Miocene

14

Langhian

13

Serravallian

Age (Ma) Epoch Stage

S. Kamikuri, T.C. Moore

CM2

CM1

Mi1b

(c)

a

Mi1ab

(a)

Fig. 8. Comparison of climatic changes and radiolarian records. (a) diatom warm-water species at DSDP Sites 574 and 575 (Barron, 1985), (b) Re-w ratio at DSDP Site 289 based on data from Romine and Lombari (1985), (c) silica switch and global hiatuses (Keller and Barron, 1983), (d) the oxygen and carbon isotope stratigraphy (Tian et al., 2014; Holbourn et al., 2015). The so-called mid-Miocene climatic optimum is divided into two intervals (MMCO-1 and MMCO-2) by the Mi2 event.

During this interval, the radiolarian assemblages were dominated by the Stichocorys group in the eastern and western areas (Sites U1335 and 289) across the tropical Pacific (Fig. 3; Romine and Lombari, 1985), and the Re-w ratio was relatively similar (average 0.8) except for around 18.5 Ma (Fig. 8). The relatively high similarity in radiolarian assemblages across the tropical Pacific indicates that the thermocline was relatively deeper in the eastern area during this stage and that the east-west gradient of the thermocline was relatively weak in the tropical Pacific (Fig. 10). During this stage, equatorial upwelling was weak and biological productivity was low in the eastern area due to the deeper thermocline (Figs. 8 and 10). However, lowered similarity in the radiolarian assemblages at around 18.5 Ma suggests that the east-west gradient of the thermocline was temporarily relatively steep in the tropical Pacific. The shallower thermocline in the eastern area led to the strengthened equatorial upwelling of cool nutrient-rich water and relatively high biological productivity (u-1 event) at around 18.5 Ma (Fig. 8). This cooling of the surface water coincided in timing with the oxygen isotope Mi1ab event reported by Pekar and DeConto (2006) and the global hiatus NH1b event of Keller and Barron (1983), which has been associated with increased ice volume on the Antarctic continent (Fig. 8). Hence, the strengthened equatorial upwelling and high bioproductivity in the eastern tropical Pacific seems to occur during a temporary global cool period during the early Miocene.

constructed based on this east-west provincialism (Fig. 8). Dominance of western fauna is represented as its proportion of the total eastern (E) and western (W) fauna (W / W + E = Re-w ratio) (E, the Calocycletta group; W, the Stichocorys group, P. pylonium, and S. asteriscus). In the eastern area (Site U1335), the Re-w ratio decreased at about 17 Ma and showed an overall trend toward stabilization since the latest early Miocene. In the western area (Site 289), the Re-w ratio increased at about 17 Ma and showed an overall trend toward stabilization since the latest early Miocene (Romine and Lombari, 1985). Hence, substantial faunal differences between the eastern and western areas have developed since the latest early Miocene (~ 17 Ma). In addition, there are three major rebounds, which are associated with end of u-2, u-3 and u-4 events within that trend (Fig. 8). The faunal assemblages were similar between the eastern and western areas during end of upwelling intervals. This might mean faunas in the eastern Pacific were sourced from the western Pacific during the end of the u-3 and u-4 upwelling intervals. Such radiolarian faunal differences are presumed to be related to water-mass properties. 6.4. Changes in surface water mass structure across the early/middle Miocene boundary in the tropical Pacific We have reconstructed the changes in surface water mass structure based on the radiolarian assemblages across the early/middle Miocene in the tropical Pacific (Fig. 10), and summarize their variability through time as follows:

2) 16.9 Ma to 12.6 Ma (uppermost sample): Intervals Ib to IIb In the tropical Pacific, the radiolarian assemblages were dominated by different taxa in the eastern and western areas based on the Re-w ratio at ~16.9 Ma, in the latest early Miocene (Fig. 8). We interpret

1) 19.8 Ma (lowest sample) to 16.9 Ma: Interval Ia

144

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0

20

(%)

Shackford et al. (2012) Interval

P. foram. Radiolaria

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Site 1335 Cluster A

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BaSO4 (wt.%) 60 0

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BaSO4

2.0 0

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2.0

2 MAR (g/cm /k.y.)

0.05

b II u-4

a c

u-3

b I

u-2

u-1

r

n C6 M2 RN2

19

a

CaCO3 dissolution event

r n C5D M3 RN3 n C5E

r

Burdigalian

early Miocene

r

17

18

This study

C5C M4

n

16

r n r n r n r n r n r C5B C5AD C5AC C5AB C5AA C5A M5 M6 M7 M8 RN4 RN5

15

middle Miocene

14

Langhian

13

Serravallian

Age (Ma) Epoch Stage

S. Kamikuri, T.C. Moore

20 Fig. 9. Stratigraphic distribution of cluster A (upwelling taxa) at Site U1335 (this study), and fluctuations of biogenic opal and Ba (original data smoothed by a moving average) at Site U1337 in the eastern tropical Pacific (Shackford et al., 2012).

Diatom warm-water species also slightly increased in abundance during the MMCO-1 from 16.9 to 16.0 Ma but remained low during the MMCO-2 at DSDP Sites 574 and 575 in the eastern tropical Pacific (Fig. 8; Barron, 1985). However, the MMCO in other areas was also characterized by apparent warmth (Flower and Kennett, 1994; Zachos et al., 2008; Foster et al., 2012). During this comparatively warm period the radiolarian fauna from the eastern tropical Pacific reflected the influence of upwelling of relatively cool water masses from deeper depths. The equatorial upwelling induced relatively high biological productivity during the MMCO except for a brief cooling at ~ 15.5 Ma, which is associated with the Mi2 oxygen isotope event. Previous studies also suggested high bioproductivity during the MMCO in the eastern tropical Pacific based on opal content (diatoms and radiolarian shells) and sediment accumulation rates (Keller, 1981; Keller and Barron, 1983; Moore et al., 2004; Piela et al., 2012; Shackford et al., 2012). In general, upwelling/high bioproductivity in the eastern tropical Pacific has occurred during periods of global cooling at least since the middle Eocene (Lyle et al., 1988; Moore et al., 2008; Kamikuri et al., 2005, 2013; Funakawa et al., 2006). However, during the MMCO in the eastern tropical Pacific, the upwelling/high bioproductivity increased during the warm periods MMCO-1 and -2, and decreased in intervening cool period represented by Mi2. Herold et al. (2011, 2012) suggested that intensification of the cold tongue is associated with stronger upwelling into the surface water of the tropical Pacific during the MMCO based on the Community Climate System Model 3 (CCSM3), although it is difficult to explain the cause.

these changes as demonstrating the development of a shallower thermocline in the east and a deeper thermocline in the west reaching across the tropical Pacific. This interpretation is supported by changes in the relative abundance of the upwelling taxa that show an overall increase in the eastern tropical Pacific since the latest early Miocene (Figs. 8, 10). The mid-Miocene Climatic Optimum (17–15 Ma; MMCO) is characterized by global warmth and relatively high CO2 and is thought to have been associated with a significant retreat of the Antarctic Ice Sheet (Foster et al., 2012). The MMCO is recorded as taking place from 16.9 to 14.7 Ma based on the oxygen isotopes of benthic foraminiferal shells at Site U1337 in the eastern tropical Pacific (Tian et al., 2014; Holbourn et al., 2015) and was interrupted by a brief cooling event associated with Antarctic glaciation, called the Mi2 event (Miller et al., 1991; Billups and Schrag, 2002; Pekar and DeConto, 2006). The latter event allows us to separate a first extreme warm period from 16.9 to 16.0 Ma, here called the mid-Miocene Climatic Optimum 1 (MMCO-1), from a second moderate warm period from 15.6 to 14.7 Ma, here called the mid-Miocene Climatic Optimum 2 (MMCO-2) (Fig. 8). In the eastern tropical Pacific, the RTI index indicates that surface water during the latest early Miocene from 16.9 to 16.0 Ma (lower part of Interval Ib) became slightly warmer than during Interval Ia, followed by a period from 16.0 to 14.6 Ma (upper part of Stage Ib) during the earliest middle Miocene that showed slight cooling (moderate warmth) to surface water RTI values similar to those observed during Interval Ia (Fig. 8). 145

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0

IS 80

MMCT

middle Miocene

13

15

NECC EC

NECC EC

EUC

warming

14

W.

CAS

NECC

E.

EC

EUC

EUC

Interval

(iii) ~11.6-9.6 Ma RTI

strong E-W thermocline gradient

Upwelling

Age (Ma) Epoch

S. Kamikuri, T.C. Moore

b

La Niña-like

II proto-warm pool; more restricted IS

a c

(ii) ~17.0-11.6 Ma IS W.

NECC EC

MMCO-2

NECC

CAS

NECC EC

EUC

E.

EC

EUC

moderate E-W thermocline gradient

b

partial closure?

16

I

weak warm pool?; moderately restricted IS and CAS?

MMCO-1

18

early Miocene

17 (i) ~20.0-17.0 Ma IS W.

CAS EC

EC

EC

EC

EC

E.

a

no/weak E-W thermocline gradient

pre-closure

19

no warm pool?; open Indo-Pacific Seaway and Central American Seaway

20 Fig. 10. Evolution of the oceanic circulation in the tropical Pacific across the early/middle Miocene boundary. (i) The relatively high similarity in radiolarian assemblages across the tropical Pacific indicates that the east-west gradient in the thermocline depth was weak. (ii) The relatively low similarity in radiolarian assemblages across the tropical Pacific indicates that the east-west gradient in the thermocline depth was moderate. An overall strengthened upwelling occurred in the eastern tropical Pacific since the latest early Miocene. These changes were interpreted as indicating the initiation of the EUC. (iii) The development of the western Pacific warm pool, the EUC, and strengthened east-west thermocline depth gradient occurred in the late Miocene (Kamikuri et al., 2009a; Nathan and Leckie, 2009). The western surface fauna of radiolarians increased during this period in the eastern Tropical Pacific, which has been interpreted as indicating a better developed countercurrent (NECC) (Kamikuri et al., 2009a).

With the development of a thick pool of warmer waters in the western Pacific and the initiation of a strong EUC we have a possible mechanism to enhance the nutrient richness of upwelled waters in the eastern equatorial Pacific even during climatically warmer times (Fig. 10).

6.5. Development of the Equatorial Undercurrent Today the Equatorial Undercurrent (EUC) is driven along the thermocline by pressure differences between the thick pile of waters in the western Pacific warm pool and the cooler waters of the eastern tropical Pacific. Its eastward flow is confined to the equator by the Coriolis effect that acts to correct any deviation to the north or south. As it approaches the shallower thermocline in the eastern Pacific, mixing associated with the counter flow of the SEC toward the west tends to enhance the delivery of nutrients to upwelled waters associated with the Ekman divergence (Fiedler and Talley, 2006), as well as providing nutrient-rich waters to the upwelling off Peru (Toggweiler et al., 1991). There is no clear evidence on exactly when this counter current began to play an important role in the productivity of the eastern equatorial Pacific. However, we suggest that the blocking of the Indo-Pacific gateway 15–17 Ma (Nishimura and Suparka, 1997) was the key to the development of the large western Pacific pool of very warm waters and the resulting establishment of a deep thermocline in the western Pacific, a strong EUC and a substantial difference in the radiolarian faunas of the western and eastern tropical Pacific. Prior to this tectonic blockage, the trade wind-driven circulation of the Indo-Pacific was essentially one system. Any development of a warm pool at its western terminus would have resulted in a much steeper overall east to west gradient in the thermocline (Fig. 10).

7. Conclusions The changes in oceanic circulation patterns of the tropical Pacific were reconstructed from 20 to 12.5 Ma across the early/middle Miocene boundary based on well-preserved radiolarian assemblages, using data from IODP Site U1335. 1) R-mode cluster analysis was carried out on the 16 taxa having a relative abundance of ≥ 5% in one or more samples. This reveled four distinct clusters, A (upwelling taxa), B1 (cool-water taxa), B2 (warm-water taxa) and C (oligotrophic taxa); 2) Q-mode cluster analysis divided the 74 samples into two clusters with five subclusters. This allows us to identify a major faunal change that occurred in the middle Miocene (14.2 Ma), and three minor changes that occurred in the latest early Miocene (16.9 Ma) and the middle Miocene (14.6 Ma and 13.4 Ma); 3) Radiolarian temperature index (RTI) was calculated using the relative abundances of the warm water (cluster B1) and cool water 146

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4)

5)

6)

7)

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The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling. Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 537–555. Foster, G.L., Lear, C.H., Rae, J.W.B., 2012. The evolution of pCO2, ice volume and climate during the middle Miocene. Earth Planet. Sci. Lett. 314-344, 243–254. Funakawa, S., Nishi, H., Moore, T.C., Nigrini, C.A., 2006. Radiolarian faunal turnover and paleoceanographic change around Eocene/Oligocene boundary in the central equatorial Pacific, ODP Leg 199, Holes 1218A, 1219A and 1220A. Palaeogeogr. Palaeoclimatol. Palaeoecol. 230, 180–203. Funayama, M., 1988. Miocene radiolarian stratigraphy of the Suzu area, northwestern part of the Noto Peninsula, Japan. In: The Contributions of the Institute of Geology and Paleontology, Tohoku University. 91. pp. 15–41 (in Japanease with English abstract and description of new species). Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M., 2012. The Geologic Time Scale 2012. Elsevier, Boston, U.S.A. http://dx.doi.org/10.1016/B978-0-444-59425-9. 00004-4. Haslett, S.K., 1992. Early Pleistocene glacial-interglacial radiolarian assemblages from the eastern equatoriall Pacific. J. Plankton Res. 14, 1553–1563. Haslett, S.K., 2003. Upwelling-related distribution patterns of radiolarians in Holocene surface sediments of the eastern equatorial Pacific. Rev. Esp. Micropaleontol. 35, 365–381. Haslett, S., Funnell, B.M., 1996. Sea-surface temperature variation and palaeo-upwelling throughout the Plio-Pleistocene Olduvai subchron of the eastern equatoriall Pacific: an analysis of radiolarian data from ODP sites 677, 847, 850 and 851. In: Moguilevsky, A., Whatley, R. (Eds.), Microfossils and Oceanic Environments. University of Wales, Aberystwyth-Press, Aberystwyth, United Kingdom, pp. 155–164. Herold, N., Huber, M., Müller, R.D., 2011. Modeling the Miocene climatic optimum. Part I: land and atmosphere. J. Clim. 24, 6353–6372. Herold, N., Huber, M., Müller, R.D., Seton, M., 2012. Modeling the Miocene climatic optimum: ocean circulation. Paleoceanography 27, PA1209. http://dx.doi.org/10. 1029/2010PA002041. Holbourn, A., Kuhnt, W., Kochhann, K.G.D., Andersen, N., Sebastian Meier, K.J., 2015. Global perturbation of the carbon cycle at the onset of the Miocene climatic optimum. Geology 43, 123–126. Irizuki, T., Ishizaki, K., Takahashi, M., Usami, M., 1998. Ostracode faunal changes after the mid-Neogene climatic optimum elucidated in the middle Miocene Kobana Formation, central Japan. Paleontol. Res. 2, 30–46. Jian, Z., Yu, Y., Li, B., Wang, J., Zhang, X., Zhou, Z., 2006. Phased evolution of the southnorth hydrographic gradient in the South China Sea since the middle Miocene. Palaeogeogr. Palaeoclimatol. Palaeoecol. 230, 251–263. Jin, F.-F., 1998. A simple model for the Pacific Cold Tongue and ENSO. J. Atmos. Sci. 55, 2458–2469. Kamikuri, S., 2017. Radiolarian assemblages from the early to middle Miocene in the eastern tropical Pacific. In: News of Osaka Micropaleontologists, (submitted). Kamikuri, S., Nishi, H., Motoyama, I., Saito, S., 2004. Middle Miocene to Pleistocene radiolarian biostratigraphy in the Northwest Pacific, Ocean Drilling Program Leg 186. Island Arc 13, 191–226. Kamikuri, S., Nishi, H., Moore Jr., T.C., Nigrini, C., Motoyama, I., 2005. Radiolarian faunal turnover across the Oligocene/Miocene boundary in the central eastern Pacific. Mar. Micropaleontol. 57, 74–96. Kamikuri, S., Nishi, H., Motoyama, I., 2007. Effects of late Neogene climatic cooling on North Pacific radiolarian assemblages and oceanographic conditions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 249, 370–392. Kamikuri, S., Motoyama, I., Nishimura, A., 2008. Radiolarian assemblages in surface sediments along 175°E in the Pacific Ocean. Mar. Micropaleontol. 69, 151–172. Kamikuri, S., Motoyama, I., Nishi, H., Iwai, M., 2009a. Evolution of Eastern Pacific warm pool and upwelling processes since the middle Miocene based on analysis of radiolarian assemblages: response to Indonesian and Central American seaways. Palaeogeogr. Palaeoclimatol. Palaeoecol. 280, 469–479. Kamikuri, S., Motoyama, I., Nishi, H., Iwai, M., 2009b. Neogene radiolarian biostratigraphy and faunal evolution rates in the eastern equatorial Pacific ODP sites 845 and 1241. Acta Palaeontol. Pol. 54, 713–742. Kamikuri, S., Moore Jr., T.C., Lyle, M., Ogane, K., Suzuki, N., 2013. Early and middle Eocene radiolarian assemblages in the eastern equatorial Pacific Ocean (IODP Leg 320 Site U1331): faunal changes and implications for paleoceanography. Mar. Micropaleontol. 98, 1–13. Kanaya, T., Koizumi, I., 1966. Interpretation of diatom thanatocoenoses from the North Pacific applied to a study of core V20-130 (studies of a deep-sea core V20-120, part IV). Sci. Rep. Tohoku Univ. 37, 89–130. Keller, G., 1981. 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taxa (cluster B2) in order to evaluate relative temperature fluctuations of the surface water. Comparison of the RTI record to the benthic foraminiferal δ18O record showed roughly similar patterns from the early to the middle Miocene; Faunal differences between the eastern and western tropical areas developed since the latest early Miocene (17 Ma). The changes are interpreted as indicating the establishment of a shallower thermocline in the east and a deeper thermocline in the west across the tropical Pacific, resulting in a steeper west to east gradient in the thermocline depth. Upwelling strengthened at four intervals (18.4–18.1 Ma, 16.9–15.7 Ma, 15.2–14.6 Ma, 14.2–13.4 Ma) from the early to middle Miocene in the eastern tropical Pacific; The initial development of a steep semi-permanent E-W thermocline gradient in the latest early Miocene has led to the upwelling of nutrient-rich water of the intermediate-deep layer during the warm MMCO; We tie these events to the effective closure of the Indo-Pacific seaway and the development of a substantial western Pacific warm pool along with the development of a strong EUC.

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