Allochthonous Diatoms in DSDP Site 436 on the Abyssal ... · 2. Materials and methods . 2.1. DSDP Site 436 Site 436 is located near the crest of the outer swell seaward of the Japan

27

JAMSTEC Rep. Res. Dev., Volume 14, March 2012, 27_38

— Original Paper —

Allochthonous Diatoms in DSDP Site 436 on the Abyssal Floor off Northeast Japan

Itaru Koizumi1* and Tatsuhiko Sakamoto2

During the interval from 4.9 to 4.1-3.7 Ma in the early Pliocene, extinct fossil freshwater diatom species Aulacoseira

praeislandica and near-shore marine fossil diatom species Koizumia tatsunokuchiensis are abundant at DSDP Site 436 on the abyssal

floor far east over the Japan Trench as sediment trap. The close correspondence of abundances of two characteristic diatom groups with

occurrence of coarse volcanic debris, increase of eolian material, and large numbers of fecal pellets in the early Pliocene sediment at

Site 436 suggests deposition by the settled water column after being transported to the area through the Kuroshio-Kuroshio Extension

system driven by winds and the atmosphere by the typhoons during a warming climate interval. The early Pliocene is considered to be

the warmest interval and a period of reduced latitudinal thermal gradients. Tropical cyclones (hurricanes and typhoons) and ocean

wind-driven circulations are simulated by modeling studies to have increased during the early Pliocene because of a vast pole-ward

expansion of the tropical warm pool that associated with expanded El Niño-like conditions. And the Pliocene coral records support that

stronger winds in the tropical Pacific act as a possible driving force of the western Pacific warm pool. Tropical storms in the early

Pliocene increased ocean vertical mixing and transported heat pole-wards. Atmosphere-ocean circulations are considered as efficient

media for the upwelling and transportation. The Pliocene extinct freshwater and near-shore marine diatoms at Site 436 may have been

derived from the continent or/and sea bottom by atmosphere-ocean dynamics along frontal boundaries moved northward off Japan

during the warm early Pliocene.

Keywords: DSDP Site 436, early Pliocene fossil freshwater diatoms, warm early Pliocene, ocean-atmosphere transport

Received 5 September 2011 ; Accepted 19 December 2011

1 Ocean Research Department, Visiting Researcher in 2000 (Emeritus Professor of Hokkaido University)

2 Institute of Biogeosciences (Biogeos), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

*Corresponding author:

Itaru Koizumi

Atsubetsu-kita 3-5-18-2, Atsubetsu-ku, Sapporo 004-0073, Japan

Tel. +81-11-893-4645

[email protected]

　Copyright by Japan Agency for Marine-Earth Science and Technology

41°N

140°E

40°

38°

39°

37°

36° 142° 144° 146°

HONSHU

SENDAI

438-439 584

436 435

440

441

434

1150

1151

200

1000

200

1000

2000

2000

4000

7000 6000

7000

6000

6000

J A P A N

7000

T R E N C H

PACIFIC OCEAN

Fig. 1. Japan Trench transect off northeast Japan. Numbers 434-436 ( ● ) indicate the drilling sites of DSDP Leg 56, 438-441 Leg 57, 584 Leg 87, and

1150-1151 ODP Leg 186. Contour lines are in meters.

1. Introduction

The Deep Sea Drilling Project (DSDP) cruised four times

off northeast Japan near the Japan Trench, and determined the

tectonic evolution in the forearc area between the Japanese Island

Arc and the Japan Trench, and also hydrographic variability in the

mixed water region between the Kuroshio Front and Oyashio

Front at latitude 38° to 41°N (Fig. 1).

Legs 56 and 57 in 1977 transected across the subduction

zone in the Japan Trench area at 7 sites. Sites 438 and 439 are

located on the continental terrace about 130-135 km off northeast

Japan, and Sites 435, 440, 441, 434 are on the inner trench slope,

and Site 436 on the ocean basin.

At DSDP Site 436, which constitutes the Pacific plate

reference section, the drilled sediments contained abundant

diatoms, much volcanic debris and fecal pellets (Shipboard

Scientific Party, 1980a). Those components included in the early

Pliocene sediments are quite different from those in the landward

wall (forearc) of the Japan Trench.

There is least possibility of transporting materials from

the dried-out lake and near-shore marine sediments including the

Pliocene extinct fossil freshwater diatoms and near-shore marine

diatoms to distal hemipelagic mud in offshore abyssal floor. The

fossil freshwater diatoms and fecal pellets in the early Pliocene

were not recovered from any other sites over the forearc area. And

volcanic vitric ash intimately mixed with terrigenous or biogenic

components suggest re-distribution and re-sedimentation. Site 436

also recorded the eolian signal that terrigenous materials were

transported by atmospheric transport from Asia (Schramm, 1989).

On the other hand, the warm Kuroshio Current, which flows

towards the northeast along the near-shore of southwestern Japan,

transports littoral-neritic diatoms into the Kuroshio Extension

since the last 150 kyr (Koizumi and Yamamoto, 2010) and the

Pliocene (Koizumi and Tanimura, 1985).

Surface water temperatures (SSTs) since 4.5 Ma were

estimated from diatoms at DSDP Sites 579 and 580 in the mid-

latitudes of northwest Pacific (Koizumi, 1985). They gradually

decreased in SSTs with higher pronounced peaks at 4.2-4.0, 3.7,

3.4 and 3.0 Ma similar to those at DSDP Site 436 during the

Pliocene section. Since 1990, PRISM (Pliocene Research,

Interpretation, and Synoptic Mapping) Project of the U.S.

Geological Survey was organized to reconstruct environmental

conditions in the warm interval around 3.0 Ma, 3.29-2.97 Ma,

prior to the onset of Northern Hemisphere glaciation (Cronin

Allochtonous Diatoms in DSDP Site 436

28 JAMSTEC Rep. Res. Dev., Volume 14, March 2012, 27_38

and Dowsett, 1991). Evidence from the PRISM indicates mid-

Pliocene gobal mean temperatures 2-3℃ warmer than today

(Haywood and Valdes, 2004; Dowsett and Robinson, 2009).

However, the early Pliocene, approximately 5-3 Ma, was

approximately 4℃ much more warmer than today (Brierley

and Fedorov, 2010).

The Pliocene warm period (5-3 Ma) are widely studied as

an analogue of future global warmer climate than today. Change

in the tropical Pacific mean climatic state may influence the

amplitude of interannual or El Niño-Southern Oscillation (ENSO),

climate variability, which may in turn play a role in global

warming (Wara et al., 2005). The Pliocene meridional SST

gradient between the equator and the subtropical/mid-latitudes

was very weak, roughly 2℃ around 4 Ma (Brierley et al., 2009).

The early Pliocene had a vast warm water pool encompassing the

whole extent of the tropics rather than zonal uniformity along the

Equator implied by the permanent El Niño (Brierley and Fedorov,

2010 ). The interannual SSTs and precipitation using Philippine

fossil corals indicated that the characteristics of the Pliocene

ENSO were similar to those of recent ENSO events and

permanent El Niño conditions did not exist during the Pliocene

warm period (5-3 Ma) (Watanabe et al., 2011).

The extremely enhanced warm climate may contribute to

the increase of extinct freshwater diatom species in the early

Pliocene at DSDP Site 436. The increase of them could result

from greater weathering in the continent, greater injection by dust

storm activity, and increased transport efficiency by vigorous

winds.

This paper discusses the ocean-atmosphere transport,

particularly diatoms among others, to DSDP Site 436 on the

abyssal plain off northeast Japan due to expanded El Niño-like

conditions during the early Pliocene. Hopefully, this paper will

help the deep-sea cores recovered by DSDP, ODP and IODP be

used more effectively.

2. Materials and methods

2.1. DSDP Site 436 Site 436 is located near the crest of the outer swell

seaward of the Japan Trench in the water depth of 5240 m and

was drilled to a sub-bottom depth of 397.5 m (39°55.96Ń, 145°

33.4É). The sediments consist of vitric diatomaceous ooze in

upper 169.5 m up to Core 18 and diatomaceous vitric mud in the

interval from 169.5 m of Core 19 to 245.5 m of Core 26. Below

245.5-312 m of Cores 26 to 33, the sediment becomes increasing

in degree of lithification to be diatomaceous vitric mudstone (Fig.

2). Volcanic vitric ash is a major component of sediment and

makes several discrete ash layers as well as Site 584 drilled on the

upper slope of the Japan Trench off Sanriku (40°28.0Ń, 143°

57.1É; water depth 4078 m) (Shipboard Scientific Party, 1986).

The number of volcanic ash layers increase in the Pliocene section

at Site 436. The upper diatomaceous sediments at this site are

lithologically similar to those recovered at all other sites in the

forearc area off northeastern Japan. Diatoms are, therefore,

common to abundant in Cores 1 to 27 and become increasingly

less abundant in Cores 28 to 30. Fecal pellets of four

morphologically distinct A-D types and miscellaneous types were

also recovered between Cores 1 and 24 (Thompson and Whelan,

1980).

Bulk density of average 1.43 Mg/m3 in the upper 250 m

decreases to 1.25-1.35 Mg/m3 in the interval of 240-200 m of

Cores 26 to 22, corresponding to a zone of vitric mud (Carson and

Bruns, 1980). Shear strength determined by the Torvane also

suddenly decreased from about 55 kPa in 175 m of Core 19 to 12-

23 kPa in the interval of 200-180 m of Cores 22 to 20. Bulk

density measurements were discontinued below 213 m of Core 23

when cracking of the sediments indicated lack of cohesion.

2.2. Diatoms Diatoms are photosynthetic, single-celled algae that

inhabit in many aquatic and subaquatic environments. Diatoms

can be used, therefore, as indicators of such environmental

parameters as open ocean, littoral-brackish water, and various

freshwater. And displaced fossil diatoms suggest the stability, in

time and place, of the routes which they were transported. The

marine diatoms are useful in understanding biostratigraphy, of the

sediments, particularly of deep-sea sediment cores from middle–

to–high latitudes.

2.2.1. Preparation of samples and method of study Sample material was placed in an oven at 60℃ for 24 hr,

and 1.0 g of dried-up material was boiled in a 200 ml beaker with

about 20 ml of hydrogen peroxide solution (15%) for several

seconds and then left to stand for 24 hr after diluting with distilled

water. After pouring off the suspension, distilled water was added

up to 100 ml. After 10 sec, 0.5 ml of the suspension was taken by

a micropipette from the middle part of the water column in the

beaker, and was placed on a cover glass (18 × 18 mm in size). The

cover glass was dried on a hot plate at 60℃ and then mounted on

a slide glass using Pleurax.

All diatoms were identified and counted until the number

I. Koizumi and T. Sakamoto

29JAMSTEC Rep. Res. Dev., Volume 14, March 2012, 27_38

5

10

15

20

30

25

Pliocene

late Miocene

early

late

Pleistocene


Diatom AgeLitho. Cores0

50

1 2 3 4

6 7

9 8

11 12 13 14

16 17 18 19

21 22 23 24

26 27 28 29

Neodenticula seminae

Thalassiosira

oestrupii s.l.

Proboscia curvirostris

Actinocyclus oculatus

Neodenticula koizum

ii

Neodenticula kamtschatica

0.5 ~ 3.5 4 ~ 15.5 16 ~ 49.5 50 <

Aulacoseira praeislandica

Koizumia tatsunokuchiensis

Abundance (%)of valves

Ranges and abundancesof characteristic diatoms

Vitric diatomaceous ooze

Diatomaceous vitric mud

D. v. m

udst.

＋

zones

Neodenticula seminae

Proboscia curvirostris

Actinocyclusoculatus

Sub-bottom Depth (m

)

100

150

Neodenticula koizumii

Neodenticula koizumii

Neodenticula kamtschatica

200

250

Thalassiosira oestrupii s.l.

Neodenticulakamtschatica

Fig. 2. Ranges and abundances of zonal marker diatom species and diatom zones, and characteristic diatom species at Site 436. Arrow indicates the

abundant occurrences of characteristic diatom species. White triangles on the range indicate the stratigraphically reliable positions of zonal boundaries.

The scale bar on the right side of figures equals 10 μm for all figures. D. v. mudst.: Diatomaceous vitric mudstone.

JAMSTEC Rep. Res. Dev., Volume 14, March 2012, 27_38 30

of individual valves totaled 200, excluding Bacteriastrum spp.

and Chaetoceros spp., using a lens combination of 10× wide-field

eyepieces and 70× oil immersion objective.

2.2.2. Diatom zonation The diatom zones and datum levels for zonal boundaries

with absolute ages of Koizumi et al., (2009) were used in this

study (Table 1, Fig. 2). The interval between Core 1 and 4 belongs

to the Neodenticula seminae Zone by the last occurrence of

Proboscia curvirostris (Jousé) Jordan & Priddle at 0.30 Ma. Cores

5 and 6 represent the P. curvirostris Zone by the last common

occurrence of Actinocyclus oculatus Janisch at 1.01-1.46 Ma.

Cores 7 to 10 are assigned to the A. oculatus Zone based on the

last occurrence of Neodenticula koizumii Akiba & Yanagisawa at

2.0 Ma. The partial range of N. koizumii interrupted by the last

common occurrence of Neodenticula kamtschatica (Zabelina)

Akiba & Yanagisawa at 2.61-2.68 Ma is assigned to the N.

koizumii Zone and are included to Cores 11 to 14.

According to the International Union for Quaternary

Research (INQUA) and International Commission on

Stratigraphy (ICS), the Pliocene/Pleistocene boundary was settled

to be 2.6 Ma at the Gauss-Matuyama polarity boundary (Head et

al., 2008). It was reported that the preglacial/glacial boundary

occurred between 2.50 Ma and 2.85 Ma at DSDP Site 579 (38°

37.6Ń, 153 ° 50.2É; water depth 5737 m) and Site 580 (41°

37.47Ń, 153°58.58É; water depth 5375 m) in the northwest

Pacific (Koizumi, 1985). The onset of significant Northern

Hemisphere glaciation at about 2.7 Ma occurs within the context

of progressive Cenozoic cooling (Koizumi, 1985; Barron, 1998;

Shimada et al., 2009), and also of a gradual increase in the mean

global ice volume during from 3.6 to 2.4 Ma (Ravelo et al., 2004).

The Pliocene/Pleistocene boundary at Site 436 is defined

by the last common occurrence of N. kamtschatica between Core

15 and 14 (Fig. 2). The concurrent range of N. koizumii and N.

kamtschatica by the first occurrence of N. koizumii in Core 21

define the N. koizumii-N. kamtschatica Zone in Cores 15 to 21.

The first occurrence of N. koizumii at 3.53 to 3.95 Ma

approximates to the early/late Pliocene boundary at 3.6 Ma. The

first occurrence of Thalassiosira oestrupii (Ostenfield) Proskina-

Lavrenko s.l. is in Core 25 and defines the base of the

Thalassiosra oestrupii s.l. Zone at 5.49 Ma. The Miocene/

Pliocene boundary at 5.3 Ma is defined in Core 25 based on the

age-depth plote of Site 436 (Fig. 3). The first occurrence of N.

kamtschatica at 7.4 Ma defines the base of the N. kamtschatica

Zone.

0

50

100

150

200

250

300

Sub-bottom Depth (m

)

0 1 2 3 4 5 6 7 Age (Ma)

L P. curvirostris

LC A. oculatus

L N. koizumii

LC N. kamtschatica

F N. koizumii

F T. oestrupii s.l.



Pleistocene late earlyPliocene late Miocene

Fig. 3. Sub-bottom depth (m) versus sediment age (Ma) curve and stratigarphic positions of characteristic diatoms at Site 436. Black arrow indicates the

abundant occurrences of Aulacoseira praeislandica and another Koizumii tatsunokuchiensis. L P. curvirostris: last occurrence of Proboscia curvirostris.

LC A. oculatus: last common or consistent occurrence of Actinocyclus oculatus. L N. koizumii: last occurrence of Neodenticula koizumii. LC N.

kamtschatica: last common or consistent occurrence of Neodenticula kamtschatica. F N. koizumii: first occurrence of Neodenticula koizumii. F T. oestrupii

s.l.: first occurrence of Thalassiosira oestrupii since loco.




Table 1. Occurrence of diatoms and diatom zones at Site 436 of Leg 56 off northeast Japan.

Xw: warm-water taxa. Xc: cold-water taxa. Xt: warm transitional taxa. Bold in Age B.P. indicates the datum level for zonal boundaries.

Xwt Taxa

Core 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Section 1 5 2 4 4 1 4 2 5 5 4 3 3 3 2 5 3 1 3 cc 5 1 4 1 1 1 1 cc 2 cc 1 cc cc cc 1 3 1 2 cc

Interval (1 cm) 15 15 50 7 20 60 4 74 50 110 90 100 100 20 20 20 10 39 7 7 49 91 61 23 39 90 70 9 20 5 40 2 14 10 41 41 30 19 4 Depth (mbsf) 0.2 6.2 10.0 22.7 31.7 37.1 41.4 48.2 52.5 62.6 70.4 78.5 8 8.0 96.7 104.7 109.2 115.6 122.4 135.0 140.6 147.5 151.4 155.6 160.2 169.9 188.0 189.2 199.3 209.2 215.5 217.4 219.7 227.5 236.8 245.9 248.9 255.3 266.2 278.0

Age B.P. (cal kyr) 0.02 0.05 0.08 0.18 0.26 0.30 0.46 0.71 0.87 1.241.24 1.41 1.59 1.81 2.002.00 2.14 2.21 2.32 2.44 2.652.65 2.76 2.90 2.98 3.06 3.16 3.35 3.72 3.743.74 4.11 4.48 4.71 4.78 4.86 5.15 5.495.49 5.82 5.93 6.17 6.57 7.00 Xc Xw

Xc Xc

Xw Xw

Xc

Xw Xw Xw Xw Xw Xc

Xc

Xc

Xt

Xw

Xw

Xc Xc Xc

Xw Xw Xw Xw

Xw

Xw Xc Xc

Xc Xw Xc Xw Xw

Xt

Xw

Xc Xc

Xc Xw

Xw Xc

Xc Xt

Xw

Xc

Actinocyclus curvatulus A. ellipticus A. ingens A. ochotensis A. oculatus A. octonarius

5 7

2 1

1 1

8

3 1 1

4

1

4

4

2

5 2

6

1 2

7

1 3 1

2 1

5

2 15

3

15 91

1

23

20

9 12 1

11

25 5

3

2 1 2 4

9

10 4

5

2

8 3

2

1

1

9

1

3 1

1

1 2

1

3 4 3 7 3

4 4

1 1 2

9 10

1 3

2

8

1

1 16 3

62

1

1 1

3 1 1

Actinoptychus senarius 1 1 2 3 1 2 7 3 1 5 9 7 1 1 3 3 1 8 10 11 15 5 8 22 16 14 11 4 11 29 24 12 13 Alveus marinus 2 3 5 5 3 3 1 1 3 3 1 2 2 3 2 1 1 2 1 2 1 1 2 1 2 1 2 5 2 2 1 5 1 1 1 Asterolampra marylandica 1 Asteromphalus actiloba A. robustus A. sp.

2 1 1 1 1 2

2 1 1 2

Aulacoseira praeislandica 1 1 1 1 1 9 5 17 25 23 29 Azpeitia africanus A. nodulifera A. perpolatus A. tabulatus A. vetustissimus

2 1

4 1

2 1 5

1

1 2

1 1 1

1 2 3 1 2

1 1 2 1 1

1

5

1

3

1

3 12

1

1

1

17

1

1

2

25

2

15 3 12 8

2 1

15

3

11 7 8

2

1 3

23 2

2

Bacterosira fragilis 16 12 6 6 1 3 1 5 2 1 Cocconeis californica C. costata C. pseudomarginatus C. scutellum C. vitrea

1 1

3 1

1 1

1

1

1 1

2

1 2

1 1 1 1

1 1 1 1

2

1 1

1

1

1 Coscinodiscus marginatus C. nitidus C. obsculus C. oculus-iridis C. pustulatus C. radiatus

3 1

1

16

3

5

1

4

10

7

1 2

4

8 4 11

1

2

2

1

6 4

1

3

2 2 2 2

1

25 10

1

14 16 9 38 32 40

1

43 10

1

7 33 31 13 1

111

1

13 46 10

2 2

24

2

39 75

Cyclotella chaetoceras C. striata 1 2 3 1 2 1

1 1 1 2

Delphineis surirella 1 2 1 1 4 2 1 2 4 2 Denticulopsis hustedtii D. hyalina

1 2 2 1 3 2 2 1

1 1 2 1 2

1 3

Diploneis bombus D. chersonensis D. smithi 1

1

1

1 1 2 1 2 1

Fragilariopsis doliolus 1 1 1 Goniothecium tenue 1 1 1 Grammatophoraspp. 1 1 2 2 Hemidiscus cuneiformis H. weissflogii

1 1 1 3 1 1 1 1 6 4 1 1 7 3 1 3 1 6 38 23 2

4 4 1

1 8 7 10 9 1

15 1 15 4 8 19 16

Koizumia tatsunokuchiensis 1 1 1 16 18 5 3 15 1 1 Neodenticula kamtschatica N. koizumii N. seminae 14 28 12 9 13

1 5

9 8 4 30 11 47 70 51 21 1

112 66 2 70 66

2 1 72 66

9 64

15 15 34 25

27 28

35 24

24 4

72 6

27 15 21 25 32 3 3 3 2 1 1 1

Nitzschia constricta N. fossilis N. jouseae N. kolazekii N. miocenica N. cf. oceanica N. porteri N. reinholdii N. sciula N. suikoensis

2

2

1

7 1

3

3 7 5 2

2

18 5

1 12

12

11 10

9 9

5

8

4 1

9

4 3

2 4

3

6

5 4 9

8 5

1 2

6 2

1

2

2

3

1

1

5

3 1

8 5

5 6

2 2

5 8

2

3

6

13

4 1 5

14 1 2 1 3 5

1

1 4

2

5

2

2

1

Odontella aurita 11 6 1 12 1 2 1 12 2 1 3 1 2 1 Palaria sulcata 3 5 3 1 3 1 1 2 1 2 2 2 3 1 1 2 2 1 1 1 1 1 Planktoniella sol 2 1 1 1 1 2 1 Porosira glacilis 4 3 2 1 1 4 6 7 2 1 2 Proboscia curvirostris 3 4 2 3 2 1 Pseudopodosira elegans 1 2 6 Rhaponeis amphiceros 1 2 Rhizosolenia barboi R. bergonii R. hebetata R. praebergonii

2

9 6 9 3

1 1 3

1 2 3

1 1

1 6 3 2 1 13 4

2

1 1

4 4 1

5 4 12 1

2

3 3 3 1 1 2

2

Roperia tesselata 2 2 1 2 1 Rouxia californica R. peragalli

1 2 1

2 1

Stellamia stellaris 1 1 1 Stephanopyxis horidus S. turris 12 14 4 6 10 21 27 55 6 39 1 3 1 55 4 2 10

3 15

1 16 23 23 14 7 10 11 1 7 3 6 8 3 3 2 7 6 5 3 7

Synedra jouseana 4 Thalassionema nitzschioides 32 28 22 10 60 28 26 5 20 4 4 46 12 15 17 39 35 49 42 48 40 52 51 18 22 55 15 70 63 32 20 17 10 117 23 76 50 46 73 Thalassiosira antiqua T. borealis T. bramaputurae T. convexa T. decipiens T. eccentrica T. gravida T. hyalina T. jacksoni T. kryophila T. leptopus T. lineata T. miocenica T. nidulus T. nodulolineata T. nordenskioldii T. oestrupii s.l. T. opoosita T. pacifica T. praeconvexa T. temperi T. trifulta s. l. T. zabelinae T. sp.1

6 3 42 36

4

2 4

2 4 2 7

7 10

12 4

1

1

6 64 1

1 1 1

4 5

3

10 1 8

8 42 3

3 1

45

3

5

5

1 22

13

5 9

12

3

3 1 2 4 33 47 5 3

2 2

4 2

15 15

9 7 8

4 4

11 9

3

4 41 47 1

3 1

3 1

8 19

1 2 3

1

2 1

21 5

2

8 50

1 2

22

1 1

2

2 9

1

10

1

1

1

12

4

1

2 27

1

10

8

1

1 3

4 4

1

4

6

3

10 4 1

1 1

1 4

19 22

1

1

2

2 2

3

11

6

1

2

1

1 1 1

1

4

6

4

1

1 1

1

1 1 1 3

3 2 2

3 2

1 2 1

4 1 3 1

3

3

2 2

1 1

5

3

1

6

4 7

1 3 1 4

2 1

3 2

1 2 2

1 2 1 2 4 2

2

3

1

2

5 3

4

1 5

2

6 6

3 2

1

3 2

7

7

5

2

1

1

1 2 1

1

1

2

2

3

3

3

2

6 1

2

1 3

1 2 2 1 1

1 3

3 2

1

1

6

1 6 3

1

6 7

1 2

5 2

3 1

3

5

2

3

1

4

6

9 2

8 3 3

3 8 3 3

1

13 12

1

1

3

4

6

2

2

8

8

1

2

1

2

3

Total valves 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 Diatom zones Neodenticula seminae Proboscia curvirostris Actinocyclus oculatus Neodenticula koizumii Neodenticula koizumii-Neodenticula kamtschatica Thalassiosira oestrupii Neodenticula kamtschatica


2.2.3. Stratigraphic occurrences of characteristic fossil diatoms The two groups of characteristic diatom species occur

abundantly and continuously during the Thalassiosira oestrupii s.l.

Zone from Cores 24 to 22-21, indicating the early Pliocene (4.9 to

4.1-3.7 Ma). Marine fossils araphid diatom species Koizumia

tatsunokuchiensis (Koizumi) Yanagisawa occurs in Core 28 of the

middle part of the Neodenticula kamtschatica Zone, estimated to

be 6.2 Ma, and increases in abundance during the interval of the

Thalassiosira oestrupii s.l. Zone of early Pliocene (Cores 24 to 22

dating 4.9 to 4.1 Ma) (Table 1; Figs. 2 and 3). The last occurrence

of this species is in the Neodenticula koizumii-N. kamtschatica

Zone of the late Pliocene (Core 16) and is estimated to be 2.9 Ma.

This extinct fossil species occurs stratigraphically sporadic and

abundant, and lived fairly commonly in intra-bay sediments such

as near-shore shallow marine environments. And the species are

reported more commonly from on-land sequences than from

purely oceanic sediments (Koizumi, 1972, 1973; Yanagisawa,

1994).

Fossil freshwater diatom species Aulacoseira

praeislandica (Jousé) Simonsen continuously occurs during the

interval of the T. oestrupii s.l. Zone (Cores 24 to 21 dating 4.9 to

3.7 Ma) (Table 1; Figs. 2 and 3). This extinct freshwater species A.

praeislandica are predominantly included in the lower Pliocene

Iga Formation consisting of fluvial and lacustrine sediments,

which were deposited in ancient Lake Biwa of central Japan

(Tanaka et al., 1984). The descendant Aulacoseira islandica (O.

Müller) Simonsen is a cold-water euplanktonic species and has its

maximum development in less eutrophic still waters such as lakes

or slow-moving rivers.

3. Depth–age plot at Site 436 and sedimentary history in forearc area off northeast Japan

At Site 436, the sediment accumulation rates were

estimated based on the datum levels of zonal marker diatom

species (Fig. 3). The sedimentation rate increased from 30 m/m.y.

in the late Miocene to early Pliocene (7.0 to 3.7 Ma) to ~55 m/

m.y., approximately two times higher, through the late Pliocene

and early Pleistocene (3.7 to 1.3 Ma). Reduced sedimentation

rates at Site 436 imply reduced productivity over the site

compared with the high productivity of ODP Site 1150 (39°11Ń,

143°20É; water depth 2681 m) and Site 1151 (38°45Ń, 143°

20É; water depth 2178 m) where coastal upwelling was likely

stronger (Motoyama et al., 2004). Two Pliocene extinct fossil

diatoms of freshwater species Aulacoseira praeislandica and near-

shore marine species Koizumia tatsunokuchiensis are abundant

exclusively in the interval from 4.9 to 4.1-3.7 Ma in the early

Pliocene. The increased relative abundance of those Pliocene

fossil diatom species might reflect decreased overall relative

abundance of productivity-related diatoms. The accumulation of

bulk quartz and all minerals at this site increased by a considerable

amount during approximately 6-4 Ma (Schramm, 1989).

On the other hand, the lower Pliocene at Sites 438 (40°

37.79Ń, 143° 14.15É; water depth 1558 m), 1150, and 1151,

located on the continental slope off northeast Japan, are

characterized by generally increased sedimentation rate (Fig. 4).

0

1

2

3

4

5

6

7

8

9

Age (Ma)

Pleistocene

early late

Pliocene

late Miocene

0 20 0 20 40 60 80 100 0 20 40 60 Site 438 Site 1150 Site 1151

Sedimentation Rate (cm/k.y.)



Fig. 4. Comparison of sedimentation rates at Sites 438 (Barron, 1998), 1150 and 1151 (Motoyama et al., 2004), and stratigraphic horizons of occurrences

of Aulacoseira praeislandica and Koizumia tatsunokuchiensis at Site 436.



Site 438 contains a thick sequence of Pliocene and Miocene

diatomaceous sediments (Shipboard Scientific Party, 1980b). The

intervening 4.4-3.0 Ma interval of decreased accumulation rate at

Site 438 approximates to the interval of fastest sedimentation rate

at Detroit Seamount Site 883 (51°11.9Ń, 167°46.1É; water

depth 2396 m) and Site 882 (water depth 3255 m, about 90 km to

the south of Site 883) of ODP Leg 145 (Barron, 1998). At about

4.5 Ma, diatom accumulation rates increased in higher latitudes of

the northwest Pacific (Sites 882 and 883), while they declined in

the mid-latitudes of the northwest Pacific (Site 438) and northeast

Pacific (off-shore southern California). The sediments at Sites

1150 and 1151 are predominantly homogeneous diatomaceous silt

or clay with minor amounts of sand and ash layers (Sacks et al.,

2000). The lower Pliocene at Site 1150 is characterized by an

increased sedimentation rate and reaches a maximum of 116 cm/

k.y. at 4.3-4.2 Ma (Motoyama et al., 2004). During the interval

between 4.6 and 4.3 Ma, the accumulation rate interrupts and

declines to 40 cm/k.y. After 4.2 Ma, the accumulation rates

decrease to 9 cm/k.y. until 3 Ma (Fig. 4). At Site 1151, the

sedimentation rate increase during the Miocene/Pliocene

boundary to 4.8-4.6 Ma with the rate 65 cm/k.y. After 4.2 Ma of

the site, a hiatus occurs during the interval of 4.2-3.6 Ma.

4. Paleoclimate in the Pliocene at Site 436

Barron (1992) proposed the Twt ratio to interpret the

Pliocene paleoclimatic changing on the region of DSDP Site 580

in the northwest Pacific, because he aimed to investigate in more

detail the middle Pliocene, 3.2 to 2.6 Ma, where Koizumi (1985)

reported significant climatic warming around 3.0 Ma at Sites 579

and 580.

In applying the Twt ratio to Site 436, some modifications

are added. In Twt=(Xw+0.5Xt)/(Xc+Xt+Xw), Xw is the total

number of subtropical to tropical (warm-water) taxa. Xt is the total

number of the warm transitional taxa and Xc is the total number of

the subarctic to arctic (cold-water) taxa (Table 1). The Twt values

are expressed as a ratio of warm-water to cold-water taxa in the

counted 200 diatom valves for each sample (Table 2, Fig. 5).

During the interval from 4.9 to 4.1-3.7 Ma of the early

Pliocene at Site 436, the Twt ratio become higher with increase of

Azpeitia nodulifera (A. Schmidt) Fryxell & Sims (Xw) and

Thalassionema nitzschioides (Grunow) Mereschkowsky (Xt)

suggesting the warming climate. Near-shore marine fossil

Koizumia tatsunokuchiensis and freshwater diatom species

occurred limitedly in this interval (Fig. 5).

The much warmer interval then warming at 3.0 Ma in the

late Pliocene was also observed around 4.2-4.0, 3.7 and 3.4 Ma at

Sites 579 and 580 in the northwestern Pacific (Koizumi, 1985). At

about 4.5 Ma, a prolonged period of high-latitude warming,

nominated as event C in the late Neogene diatom sedimentation

of the North Pacific by Barron (1998), started and sustained least

1 m.y. with the increased sedimentation rate by primary (diatom)

production. Evidence from the PRISM indicates middle Pliocene

mean temperatures 2-3℃ warmer than today (Haywood and

Valdes, 2004; Dowsett and Robinson, 2009). The early Pliocene

was approximately 4℃much more warmer than today (Brierly

and Fedorov, 2010). The early Pliocene, 4.9 to 4.1-3.7 Ma, is the

warmest interval during the Pliocene (Fig. 5) and a period of

reduced latitudinal thermal gradients from the Equator to mid-

latitudes, indicating a pole-ward expansion of the ocean warm

pool (Brierley et al., 2009). D

epth

(mbs

f)

Age

B.P

. (ca

l kyr

)

Xw

0.5X

t

Sub

tota

l

Xc

Xt

Xw

Sub

tota

l

Twt=

(Xw

+0.5Xt)/

(Xc+Xt+Xw

)

0.2 0.02 10 17.5 27.5 104 35 10 149 0.18 6.2 0.05 7 17.5 24.5 114 35 7 156 0.16

10.0 0.08 10 15.0 25.0 129 30 10 169 0.15 22.7 0.18 14 8.5 22.5 126 17 14 157 0.14 31.7 0.26 6 38.0 44.0 92 76 6 174 0.25 37.1 0.30 14 19.5 33.5 97 39 14 150 0.22 41.4 0.46 11 17.0 28.0 101 34 11 146 0.19 48.2 0.71 14 3.5 17.5 86 7 14 107 0.16 52.5 0.87 20 11.5 31.5 120 23 20 163 0.19 62.6 1.24 13 3.5 16.5 124 7 13 144 0.11 70.4 1.41 6 2.0 8.0 180 4 6 190 0.04 78.5 1.59 22 29.0 51.0 108 58 22 188 0.27 88.0 1.81 14 11.5 25.5 139 23 14 176 0.14 96.7 2.00 37 10.5 47.5 80 21 37 138 0.34

104.7 2.14 25 18.0 43.0 121 36 25 182 0.24 109.2 2.21 29 30.5 59.5 93 61 29 183 0.33 115.6 2.32 25 20.5 45.5 113 41 25 179 0.25 122.4 2.44 28 27.0 55.0 84 54 28 166 0.33 135.0 2.65 15 22.0 37.0 119 44 15 178 0.21 140.6 2.76 17 24.5 41.5 94 49 17 160 0.26 147.5 2.90 22 21.5 43.5 101 43 22 155 0.28 151.4 2.98 24 27.0 51.0 77 54 24 155 0.33 155.6 3.06 50 26.5 76.5 78 53 50 181 0.42 160.2 3.16 54 9.0 63.0 98 18 54 170 0.37 169.9 3.35 38 11.5 49.5 98 23 38 159 0.31 188.0 3.72 35 28.5 63.5 74 57 35 166 0.38 189.2 3.74 15 8.5 23.5 124 17 15 156 0.15 199.3 4.11 40 35.0 75.0 44 70 40 154 0.49 209.2 4.48 42 32.0 74.0 29 64 42 135 0.55 215.5 4.71 27 16.0 43.0 63 32 27 122 0.35 217.4 4.78 52 10.5 62.5 57 21 52 130 0.48 219.7 4.86 45 9.5 54.5 46 19 45 110 0.50 227.5 5.15 53 6.0 59.0 116 12 53 181 0.33 236.8 5.49 53 58.5 11.5 16 117 53 186 0.06 245.9 5.82 87 12.5 99.5 50 25 87 162 0.61 248.9 5.93 48 38.0 86.0 17 76 48 141 0.61 255.3 6.17 21 25.0 46.0 87 50 21 158 0.29 266.2 6.57 66 23.0 89.0 41 46 66 153 0.58 278.0 7.00 25 36.5 61.5 79 73 25 177 0.35

Table 2. Downcore Twt ratio and values of taxa for Twt at Site 436.

Bold in age B.P. (cal kyr) indicates the datum level for zonal bounaries.



5. Discussion

5.1. DSDP Site 436 At DSDP Site 436, a close correspondence was

recognized among abundances of Pliocene extinct fossil diatoms

of freshwater species and near-shore marine species, abundances

of coarse volcanic debris, increases of quartz and minerals, and

large numbers of fecal pellets in the early Pliocene sediment (Fig.

6).

Ash is sometimes intimately mixed with terrigenous or

biogenic components, suggesting re-distribution and re-

sedimentation. In the northwestern Pacific, the eolian

sedimentation shows variations of components in response to

changes in the climatic and tectonic factors that control erosion

and atmosphere-ocean transport. Clay minerals gradually

increased in relative abundance through the Pliocene at Site 436.

The fecal pellets in Site 436 sediment are composed of

fine silts and clays admixed with biogenic silica debris, all

identical to the surrounding hemipelagic sediments. The earliest

occurrence of pellets is in Core 24 dating 4.9 Ma from the early

Pliocene (Thomson and Whelan, 1980). The increase in

abundance and diversity of pellets are closely similar to the

increase in abundance of coarse volcanic debris in the upper part

of Core 24 (Fig. 6). The abundance and variety of pellet types are

low and the distribution is patchy upwards through Core 17. The

fecal pellets in the early Pliocene are not recovered from any other

sites over the forearc area.

Because of the large proportion of detrital mineral matter

in the fecal pellets, burrowing sediment-eaters such as polychaetes

0 0.2 0.4 0.6 0.8 1.0 Twt=(Xw+0.5Xt)/(Xc+Xt+Xw)

0

1

2

3

4

5

6

7

Aul

acos

eira

pra

eisl

andi

caK

oizu

mia

tats

unok

uchi

ensi

s

Age(Ma)

Pliocene

early

late

late Miocene

Pleistocene

cold warm Climate

150

200

250

17

18

19

20

21

22

23

24

25

26

27

Diatomaceous vitric mud

V.d.o

Present Common Abundant

N. koi–N

. kam

T. oestrupii s.l.

N. kam

early

late

Pliocene

late

Miocene

Freshw

ater

diatom

s

Koizumia

tatsunokuchiensis

Volcanic ash Misc Type

D

Sub-bottom Depth (m

)

Lithology

Cores

Recovery

Diatom

Zones

Age

Fig. 5. Plots of Aulacoseira praeislandica and Koizumia

tatsunokuchiensis against paleoclimatic ratio (Twt) at Site 436. Xw:

fredquency of subtropical-tropical taxa. Xt: frequency of warm-water

transitional taxa. Xc: frequency of subarctic-arctic taxa. Vertical line

indicates the average value.

Fig. 6. Abundance and comparison of freshwater diatoms, fossil near-

shore marine diatom Koizumia tatsunokuchiensis, volcanic ash, and

fecal pellets at Site 436. Abundances of diatoms are Present (1-3),

Common (4-15), and Abundant (16-49) in 200 diatom valves.

Abundances of volcanic ash and fecal pellet types (Thompson and

Whelan, 1980): Present (1-2), Common (3-10), and Abundance (11-25)

in 10 cc samples. Type D and Miscellaneous types are the morphology

of fecal pellet types. Type D is described as irregularly ellipsoidal in

outline, about 2.5 times as long as broad; color gray, friable texture,

composed of silt and biogenic silica tests. Miscellaneous types are small,

cylindrical, composed of fine silt and rare diatom fragments. V.d.o:

Vitric diatomaceous ooze. Misc: Miscellaneous. N. koi-N.kam:

Neodenticula koizumii-Neodenticula kamtschatica. T. oestrupii s.l.:

Thalassiosira oestrupii since locö. N. kam: Neodenticula kamtschatica.



or holothurians are assumed to be the producer (Shipboard

Scientific Party, 1980a). And also the greatest frequency of pellets

occurs between peaks of volcanic ash layers indicate that floods of

volcanic debris disturbed the benthic ecology (Fig. 6).

5.2. Sedimentation rates The sedimentation rates during the early Pliocene

generally increased all over the forearc-trench region. The tectonic

events over the Japanese Island Arcs caused differential

subsidence of the forearc sedimentary basins under the mid-slope

terrace and deep-sea terrace (Niitsuma, 2004). The early Pliocene

marks beginning of the increase in primary (diatom) productivity

at 4.5 Ma named as event C in diatom mass accumulation rates

occurred after 10 Ma in the North Pacific (Barron, 1998). The

oxygen isotope curve by benthic foraminifera Cibicidoides spp. at

Sites 588-590 in the southwest Pacific suggest that 4.5 Ma marked

the onset of a period of sustained high latitude warming that lasted

at least 1 m.y. (Kennett, 1986).

The results from eastern equatorial Pacific ODP transect

Sites 848, 849, and 853 by using the Mg/Ca ratio of Globorotalia

tumida (Brady) which has a depth habit of about 100 m show that

the subsurface temperature rapidly cooled during the interval from

4.8 to 4.0 Ma in the early Pliocene and continued to be cool to the

present, indicating that the thermocline shoaled since Pliocene

(Ford et al., 2010). Rapid subsurface cooling between 4.8 and 4.0

Ma is generally related to the uplift of Panama Isthmus, but is

likely related to thermocline waters sourced from subtropical

surface waters in the northwest Pacific (Ravelo et al., 2004;

Brierley and Fedorov, 2010).

5.3. Tropical cyclones and ocean wind-driven circulation The multiple proxy studies of SST across the equatorial

Pacific, using Mg/Ca andδ18O analyses of foraminifera, alkenone

and faunal assemblages, show that the east-west gradient was

greatly reduced during the Pliocene warm period and the ocean

warm pool expanded into a pole-ward (Wara et al., 2005; Ravelo

et al., 2006; Dowsett and Robinson, 2009).

The simulation of atmospheric circulation in the early

Pliocene showed weaker Hadley and Walker cells (Brierley et al.,

2009). In the early Pliocene, the strengthening of the mid-latitudes

jet stream is associated with the increased meridional SST

gradient and thermal wind balance (Brierley and Fedorov, 2010).

The strongest typhoons in simulated activities occur east of the

Philippines and Japan in the warm early Pliocene (Fedorov et al.,

2010). The number of typhoons almost doubles and occurs

throughout the seasons. Reduced vertical wind shear combined

with warmer SSTs lead to a wide spread increase in tropical

cyclones (hurricanes and typhoons), producing ocean vertical

mixing to 120-200 m depth at the present.

The records for the interannual climate characteristics

based on the Pliocene fossil Porites coralδ18O by Watanabe et al.

(2011) support the hypothesis that stronger winds in the tropical

Pacific act as a possible driving force behind the El Niño/Southern

Oscillation (ENSO) when SSTs are higher than present levels

(Ravelo et al., 2004; Fedorov et al., 2010).

5.4. Deep trench as sediment trap Most fluvial input of terrigenous materials including

extinct freshwater diatoms at DSDP Site 436 is trapped by deep

trenches, which surround the northwestern Pacific rims, and is

limit contamination of the materials with hemipelagic sediments

which move offshore in suspension.

On the other hand, along the eastern margin of the mid-

latitude North Pacific, deep trench as sediment trap are not

recognized. Instead, the freshwater diatom abundances in marine

sediments off the coasts of Oregon and California in the northeast

Pacific have been used to infer massive discharges of freshwater

from the Columbia River and sea-surface salinity anomalies

(Lopes and Mix, 2009). In core MD02-2499 off northern

California, the freshwater diatoms abundantly occurred at 30.5,

27.0, 23.0, 20.0 and 17.5 ka during most of marine oxygen isotope

stage (OIS) 2 and constituted more than 40% of assemblages.

6. Conclusion

1. The Pliocene extinct fossil diatom of freshwater species

Aulacoseira praeislandica is abundant exclusively during the

interval from 4.9 to 4.1-3.7 Ma in the early Pliocene of DSDP Site

436 on the abyssal plain off northeast Japan.

2. The sedimentation rate at Site 436 during this early

Pliocene interval was 30 m/m.y., during between 7.0 and 3.7 Ma,

substantially less than during the interval between 3.7 and 1.3 Ma,

where it is ~55 m/m.y., suggesting that this early Pliocene interval

was marked by reduced biologic productivity.

3. The early Pliocene increases of fossil diatoms of

freshwater and near-shore marine species coincides with increased

abundance of volcanic debris, quartz and clay minerals, and fecal

pellets, suggesting deposition by settling through the water

column after being transported to the area through the atmosphere

by the prevailing typhoons and through the ocean water by the

Kuroshio-Kuroshio Extension system, which shifted further to the



north than its present location, driven by wind.

4. Modeling studies suggest that tropical cyclones

(hurricanes and typhoons) and ocean wind-driven circulation

increased during the early Pliocene in the tropical to mid-latitudes

because of a vast pole-ward expansion of the tropical warm pool

associated with expanded El Niño-like conditions.

Acknowledgment

We gratefully acknowledge Drs. Yoshiro Tanimura and

Akihiro Tuji of the National Science Museum for identification of

extinct fossil freshwater diatom species. We acknowledge with

special thanks Dr. John A. Barron of the USGS at Menlo Park for

critical reviews and suggestions for improving the manuscript. We

acknowledge Dr. Andrey Yu. Gladenkov of the Russian Academy

Sciences and Mr. Hirofumi Yamamoto of the Japan Agency for

Marine–Earth Science and Technology (JAMSTEC) for providing

information on the topics. We thank also reviewers Drs. Yusuke

Okazaki and Jonaotaro Onodera, and a member of the editorial

staff Dr. Minoru Kitamura of JAMSTEC for their suggestions for

improving that helped to make this better paper.

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Series, 176, 591-617.



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Allochthonous Diatoms in DSDP Site 436 on the Abyssal ... · 2. Materials and methods . 2.1. DSDP Site 436 Site 436 is located near the crest of the outer swell seaward of the Japan