Ishiwa et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0262-5(2019) 6:17

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Australian Summer Monsoon variability inthe past 14,000 years revealed by IODP
Expedition 356 sediments
Takeshige Ishiwa1,2,3* , Yusuke Yokoyama2,3, Lars Reuning4,8, Cecilia M. McHugh5, David De Vleeschouwer6 andStephen J. Gallagher7

Abstract
The International Ocean Discovery Program (IODP) Expedition 356 Site U1461 cored a Miocene to Holocene
sedimentary sequence in the upper bathyal carbonate offshore northwestern Australia (NWA). The siliciclastic
component of these strata is primarily derived from the Australian continent. Radiocarbon dating on macrofossils
and planktonic foraminifera shows that the upper 14 m section at Site U1461 preserves Holocene sediments,
recording regional climate variability. K/Ca ratios determined by X-ray fluorescence elemental analyses and %K
determined by shipboard natural gamma ray analysis are interpreted as indicators of riverine run-off from theAustralian continent. We document the consequences of the variability of the Australian Summer Monsoon (ASM)on the continental shelf of NWA. We report an increase in terrigenous input due to a riverine run-off after 11.5 ka,
which reaches a maximum at ~ 8.5 ka. This maximum is the result of the enhanced ASM-derived precipitation in
response to the southern migration of the Intertropical Convergence Zone (ITCZ). A decrease in riverine run-off due
to a weakening of precipitation in the NWA region after 8.5 ka was caused by the northern migration of the ITCZ.
We conclude that the ITCZ reached its southernmost position at 8.5 ka and enhanced precipitation in the NWA
region. This Holocene record shows that even during interglacial periods, monsoonal variability was primarily
controlled by the position of the ITCZ.
Keywords: International Ocean Discovery Program Expedition 356, Australian Summer Monsoon, Radiocarbon
dating, Holocene climate variability, Northwestern Australia

Introduction
Northwestern Australia (NWA) is located at the edge of
the Indo-Pacific Warm Pool (IPWP), which is the largest
source of moisture and heat on Earth (Gagan et al. 2004;
De Deckker et al. 2014; De Deckker 2016). NWA shelfunderlies the path of the Leeuwin Current, a regional
south-flowing current and branch of the IndonesianThroughflow, which brings warm and low salinity waterfrom the IPWP to the Indian Ocean (Gallagher et al.
2009, 2014). A strong rainfall seasonality characterizes
NWA climate variability during the Australian Summer
Monsoon (ASM), which is one of the largest monsoons* Correspondence: ishiwa.takeshige@nipr.ac.jp
1
National Institute of Polar Research, Tokyo, Japan
2
Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba,
Japan
Full list of author information is available at the end of the article

similar to the Indian Monsoon (Betzler et al. 2017) andthe Asian Monsoon (Wang et al. 2001; Tada et al. 2016).
ASM rainfall plays an important role for the hydrologyof the equatorial Pacific and Indian Ocean through theIndonesian archipelago deriving freshwater discharge
from rivers and changing water salinity in this region(De Deckker 2016). During glacial-interglacial cycles, theintensity of the ASM changed on ~ 100–1000-year timescales (Kuhnt et al. 2015; Eroglu et al. 2016). The ASM isstrongly influenced by the position of the Intertropical
Convergence Zone (ITCZ), which migrated with warming
and cooling conditions during the deglaciation period(Kuhnt et al. 2015; Mohtadi et al. 2011). Indonesian stalagmite δ18O data records the precipitation pattern of theASM during the last deglaciation, suggesting that the
ASM was synchronized with northern high-latitude climate events (Ayliffe et al. 2013; Denniston et al. 2013).

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
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Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

Modelling studies show that the exposure of the
Sunda Shelf during the Last Glacial Maximum (LGM)
and its flooding during the Holocene global sea level
rise have influenced the hydrological cycle in thetropical Pacific and the eastern Indian Ocean andhave caused rainfall variation in NWA (Di Nezio andTierney 2013).
Terrigenous input from riverine run-off is a key
proxy for monsoonal rainfall variation in the NWA
region (De Deckker et al. 2014; Gallagher et al. 2014;
Kuhnt et al. 2015; Stuut et al. 2014). Previously, analyses of cores in the Timor Sea used riverine run-off
as a key proxy for monsoonal rainfall to determine
the relative ASM activity (Kuhnt et al. 2015). However, the source of sediment in the Timor Sea is bothfrom distal locations of the Australian continent andthe Indonesian archipelago. The International Ocean

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Discovery Program (IODP) Expedition 356 IndonesianThroughflow recovered a proximal ASM terrigenous
proxy record by coring a series of sites in the shallow
continental shelf to the upper bathyal region part ofoffshore NWA (Fig. 1), recovering a sequence of Miocene to Holocene sediments at Site U1461 over 1 km
thick (Gallagher et al. 2015, 2017a). Due to theirproximity to the Australian coast, the siliciclastic
component of the sediments is derived from theNWA continent via the Fortescue and De Grey rivers(Fig. 1; James et al. 2004). We date the upper 14 m ofSite U1461 using radiocarbon dating and describe thepast 14,000 years of climate variability in NWA. The
strata reveal evidence of the linkage between the
ASM and global climate changes, enhancing our understanding of the Holocene history of the ITCZ migration across the continental shelf of NWA.

Fig. 1 The location map of site U1461 with 200 m water depth contour from ETOPO dataset (Amante and Eakins 2009) and a monthly rate ofprecipitation in the tropical Indo-Pacific region. Rivers are shown with thin black lines (data obtained from a Global Self-consistent, Hierarchical,
High-resolution Geography Database after Wessel and Smith 1996). Austral summer daily precipitation data were obtained from the Global
Precipitation Climatology Project (Huffman et al. 1997), indicating that Site U1461 locates the southern limit of rain belt. Locations of the
discussed records from Ball Gown Cave (Denniston et al. 2013), MD00-2361, and FR10/19-GC17 (De Deckker et al. 2014; Stuut et al. 2014) are
shown on the map. Figures were drawn using The Generic Mapping Tools (Wessel et al. 2013)

Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

Methods/Experimental
IODP Expedition 356 Site U1461

IODP Expedition 356 Site U1461 (20° 12.8634′ S,
115° 03.9495′ E) cored a ~ 1 km thick section in 127
m water depth (Fig. 1) with 86% core recovery (Gallagher et al. 2015). This study focuses on the top 14 mof the core, which yielded well-preserved macrofossils(bivalves) and planktonic foraminifera for radiocarbon
dating.
The upper part of Site U1461 is divided into two units:
unit I and subunit IIa (Gallagher et al. 2017a). Unit I extends from the core top to 12.4 m CCSF-A and unit IIa
from 12.4 to 42.09 m CCSF-A, and subunit IIa was investigated to a depth of 14.0 m CCSF-A for this study.
Unit I is a homogeneous olive-grey to greenish grey
packstone (Fig. 2) with planktonic and benthic foraminifera, sponge spicules, and aragonitic pteropod shells(Gallagher et al. 2017a). The main carbonate mineral isaragonite (average ~ 35% of the sediment obtained fromfive samples in unit I) and the main non-carbonate mineral is quartz (average ~ 27% of the sediment) based on
the shipboard semi-quantitative XRD analysis (Gallagher
et al. 2017a). Subunit IIa is a creamy-grey mudstone towackestone (Fig. 2) with glauconite and abundantpeloids (Gallagher et al. 2017a). Macrofossils such as bivalves, gastropods, scaphopods, pteropods, echinoderms,
bryozoans, small benthic foraminifers, and solitary corals
are common (Gallagher et al. 2017a). The average content of aragonite of this interval is 79%. Quartz is rare,
and clay minerals are absent in subunit IIa (Gallagher et
al. 2017a).
Physical properties and major elemental analysis

Natural gamma radiation (NGR) is emitted when 40K,
232
Th-series, and 238U-series nuclides decay to theirdaughter isotopes. Shipboard measurements of NGR detect these emissions and provide the means to quantify
the K, U, and Th contents of the recovered sediments(Gallagher et al. 2017b; De Vleeschouwer et al. 2017). In
the upper part of the cored sequence considered in thisstudy, shipboard NGR measurements were carried out at
a 10 cm resolution in Holes U1461B and D and at a 20 cm
resolution in Holes U1461A and C. We use the freely
available MATLAB algorithms of De Vleeschouwer et al.
(2017) to decompose the NGR energy spectra and quantify the K (wt.%), U (mg/kg), and Th (mg/kg) contents(https://doi.org/10.1594/IEDA/100668).
The top 14 m CSF-A (equivalent to 15 m CCSF-A) of
Hole U1461C was also subject to an Avaatech X-ray
fluorescence (XRF) scanning at 5 cm intervals, which
was conducted at the International Ocean Discovery
Program’s Gulf Coast Repository. The core sections werecovered with 4 μm-thick ultralene, and the samples wereanalysed twice at 9 kV and 30 kV. Here, we interpret only

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calcium (Ca), potassium (K), and titanium (Ti), which
are more robust compared to lighter elements, as thelatter is affected by the water content and the split-coresurface (Kido et al. 2006; Ishiwa et al. 2016; Nakamura
et al. 2016).
Age-depth model based on radiocarbon dating

Radiocarbon dating of macrofossils and planktonic
foraminifera (Table 1) was carried out using
single-stage accelerator mass spectrometry at Atmosphere and Ocean Research Institute, The Universityof Tokyo (Yokoyama et al. 2016). Macrofossils were
etched by 10 M HCl to remove contamination andsecondary carbonate (Ishiwa et al. 2016). Foraminifera
were washed in an ultrasonic bath prior tographitization. The method of graphitization follows
Yokoyama et al. (2007).
Calendar ages were obtained using a Bayesian age
modelling approach (Bchron: Haslett and Parnell 2008;
Parnell et al. 2008) with Marine 13 (Reimer et al. 2013)as a calibration curve. We set the local reservoir correction to 0 at Site U1461, since a local reservoir correctionin the NWA continental shelf was previously reported tobe minor (O’Connor et al. 2010; Bowman 1985). The
Bchron algorithm uses Monte Carlo methods to produce
a continuous age-depth model and interpolates ages ofoutlier intervals using obtained ages.

Results%K variation by natural gamma radiation analysis

The %K is ~ 0.2% from 14.0 to 12.4 m CCSF-A and increases to 9.6 m CCSF-A. The maximum %K is ~ 0.6%
at ~ 9.6 m CCSF-A. Above 9.6 m CCSF-A, the %K decreases to 0.2% to the core top (Fig. 2). The %K value at
8.31 m CCSF-A in Hole U1461C is interpreted to be an
outlier because there was an insufficient volume of sediments in the core liner at this level.
Major elemental analysis by XRF scanning

The Ca counts are ~ 800,000 from 14.0 to 11.5 m
CSSF-A and gradually decrease from 11.5 to 9.4 m
CCSF-A. There is a marked reduction at 1.6 m CCSF-A(Fig. 2). The K counts are ~ 2000 from 14.0 to 11.5 m
CCSF-A, increasing to 10,600 counts at 10.2 m CCSF-A.
The K counts gradually decrease from 8.2 m CCSF-A to4000 counts in the core top (Fig. 2). The Ti counts are~ 500 from 14.0 to 12.4 m CCSF-A, with a marked increase to 6000 counts that occurs from 12.4 to 10.2 m
CSF-A. The Ti counts gradually decrease to 1000 counts
from 8.2 m CCSF-A to the core top. K and Ti counts
show a similar pattern of variability through the studiedinterval.

Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

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Fig. 2 Lithology, %K, and major elemental (Ca, K, and Ti) variation at Site U1461 for the top 1400 cm. The shipboard analysis generated the lithology,
units, and bulk mineralogy. The %K contents were obtained from a shipboard NGR analysis and the MATLAB code from De Vleeschouwer et al. (2017).
The major elemental analysis (Ca, K, and Ti) on Hole C sediments was conducted by XRF scanning at the IODP Texas Gulf Repository

Age-depth model

Radiocarbon dating results are shown in Table 1. The
age-depth model established by Bchron suggests that theinterval from 15.0 m CCSF-A to the core top corresponds
to the last 25,000 years (Fig. 3). An extremely low

sedimentation rate (~ 0.07 m/kyr) occurred before 13.5 ka,
suggesting that the past climate variability may not be recording at the accurate timing. Subsequently, relatively
high sedimentation (~ 1.0 m/kyr) is observed to the core
top. Ages obtained from foraminifera at 7.81 m CCSF-A

Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

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Table 1 Age results of Site U1461
Lab. ID

Name

Depth
CSF-A (m)Depth
CCSF-A (m)Radiocarbon age(year BP)Calendar age (cal year BP) 95%
highest density regions

YAUT-034215

356-U1461C-1H-1 W-9/11

0.1

0.1

1460 ± 40

925

1084

94.2

F

YAUT-019123

356-U1461A-1F-1W-80/84

0.82

0.82

44,530 ± 310

46,62648,02994.7

F

YAUT-034216

356-U1461C-1H-1 W-109/111

1.1

1.1

2580 ± 50

2142

2336

94.3

F

YAUT-034209

356-U1461A-2F-2W-90/94

4.02

4.02

4850 ± 30

5049

5257

94.5

F

YAUT-034217

356-U1461C-1H-4 W-49/51

555290 ± 30

5584

5703

93.8

F

YAUT-034211

356-U1461A-3F-1W-80/84

7.12

7.62

6830 ± 40

7284

7409

94

F

YAUT-019119

356-U1461C-1H-6 W-29/32

7.81

7.81

6020 ± 40

6334

6517

94.1

M

YAUT-034213

356-U1461B-2H-3 W-69/71

8.1

8.8

6950 ± 40

7406

7516

93.7

F

YAUT-034212

356-U1461B-2H-3 W-109/111

8.5

9.2

7570 ± 40

7955

8119

94.1

F

YAUT-034218

356-U1461C-2H-1 W-109/111

9.5

10.5

8020 ± 40

8390

8548

94.2

F

YAUT-030602

356-U1461C-2H-4A-10/11

13.01

14.01

11,990 ± 50

13,335

13,526

94.2

M

YAUT-030603

356-U1461C-2H-4A-36/37

13.27

14.27

14,620 ± 50

17,155

17,458

94.6

M

YAUT-030604

356-U1461C-2H-4 W-38/40

13.29

14.29

20,470 ± 60

23,99924,257

94.4

M

YAUT-030605

356-U1461C-2H-4A-40/41

13.31

14.31

16,920 ± 60

19,786

20,056

94.4

M

YAUT-019120

356-U1461C-2H-4 W-46/52

13.39

14.39

20,160 ± 60

23,64523,923

94.5

M

YAUT-030607

356-U1461C-2H-4A-99/100

13.90

14.90

12,130 ± 60

13,461

13,708

94.5

M

YAUT-030611

356-U1461A-4H-3A-78/79

14.64

15.42

48,760 ± 770

–

–

–

M

YAUT-030609

356-U1461B-3H-1A-62/63

14.53

15.50

49,300 ± 820

–

–

–

M

YAUT-030612

356-U1461B-3H-1 W-77/78

14.68

15.65

54,750 ± 1540

–

–

–

M

YAUT-030637

356-U1461A-4H-3A-117/118

15.03

15.81

49,220 ± 830

–

–

–

M

YAUT-030639

356-U1461B-3H-2A-3/6

15.45

16.42

40,980 ± 330

–

–

–

M

YAUT-030613

356-U1461C-2H-6 W-2/3

15.93

16.93

51,470 ± 1030

–

–

–

M

YAUT-030615

356-U1461C-2H-6 W-6/7

15.97

16.97

49,550 ± 830

–

–

–

M

YAUT-030617

356-U1461B-3H-2 W-63/64

16.04

17.01

41,580 ± 370

–

–

–

M

YAUT-030618

356-U1461B-3H-2 W-68/68

16.08

17.05

45,090 ± 510

–

–

–

M

YAUT-019122

356-U1461C-2H-7 W-54/56

17.55

18.55

2040 ± 40

–

–

–

M

YAUT-030621

356-U1461C-3H-1 W-93/94

18.84

20.63

57,090 ± 1990

–

–

–

M

YAUT-030624

356-U1461C-3H-2 W-88/90

20.29

22.08

42,350 ± 450

–

–

–

M

YAUT-030625

356-U1461A-5H-1 W-4/7

20.56

22.67

12,310 ± 40

–

–

–

M

YAUT-030626

356-U1461C-3H-3 W-120/121

22.11

23.90

55,830 ± 1750

–

–

–

M

YAUT-030628

356-U1461A-5H-3A-72/73

24.23

26.34

50,710 ± 1060

–

–

–

M

YAUT-030629

356-U1461C-3H-6 W-3/4

25.44

27.23

> 37,964

–

–

–

M

YAUT-030631

356-U1461C-3H-6 W-29/30

25.70

27.49

50,330 ± 0

–

–

–

M

YAUT-019125

356-U1461C-4H-1 W-13/18

27.56

31.06

44,730 ± 320

–

–

–

M

YAUT-030632

356-U1461C-4H-2 W-96/97

29.81

33.31

> 39,963

–

–

–

M

YAUT-030635

356-U1461C-4H-2 W-122/124

30.07

33.57

> 40,470

–

–

–

M

YAUT-030636

356-U1461C-4H-3 W-60/61

30.95

34.45

> 39,907

–

–

–

M

Calendar age is calculated using Bchron model (Haslett and Parnell 2008; Parnell et al. 2008) with a Marine 13 curve (Reimer et al. 2013). YAUT is a laboratory
number of the single-stage accelerator mass spectrometry at the University of Tokyo
F planktonic foraminifera, M macrofossils

and macrofossil ages at 7.62 m CCSF-A show an inverse
age-depth relationship. We interpret radiocarbon date at
0.8 m CCSF-A as an outlier due to the secondary deposition or coring disturbance, since the date at this intervalshows an extremely older age than proximal ages.

Discussion
Riverine run-off records at Site U1461

The %K contribution to the total NGR on the shelfof NWA is interpreted to indicate the presence offluvial-derived clays and feldspar, reflecting theIshiwa et al. Progress in Earth and Planetary Science(2019) 6:17

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Fig. 3 Site U1461 age-depth model by radiocarbon dating. Bchron (Haslett and Parnell 2008; Parnell et al. 2008) age-depth models withcalibrated ages using Marine 13 (Reimer et al. 2013). The black line is the mean calibrated chronology; the lighter grey shade indicates the 97.5%
range, and the darker grey shade indicates the 90% range

relative variability of continental moisture (Christensen etal. 2017; Groeneveld et al. 2017). The %K from NGR datashows a similar pattern to K counts derived fromXRF scanning (Figs. 2 and 3). %K variation from NGR
complements the gap of K counts from 8.17 to 9.45
m CCSF-A (Fig. 2). The K and Ti counts also exhibit
a similar pattern of variability (Fig. 2). Biogenic or

precipitated carbonate dominates at Site U1461, withhigh Ca intensity and minor variation. We focus on
the pattern of K normalized against Ca. It has beenpreviously reported that K/Ca ratios are interpreted
to reflect the variation in riverine run-off from theAustralian continent in NWA (Kuhnt et al. 2015;
Stuut et al. 2014).

(2019) 6:17

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Variability in the Australian Summer Monsoon from the
Pleistocene and the Holocene

have enhanced erosion, caused greater drainage by rivers, and increased the amount of riverine run-off bringing terrigenous components to the NWA continental
shelf. Site U1461 directly receives sediments from theNWA continent through the Fortescue and De Grey rivers (James et al. 2004). The enhanced precipitation in
the drainage catchment of the Fortescue and De Grey
rivers is interpreted to have been caused by the southward migration of the ITCZ that began during the Early
Holocene.
Stalagmite records from Ball Gown Cave (Denniston
et al. 2013) (Fig. 1) reveal the trend of enhanced ASM
from 11 to 8.4 ka, whereas the variation in the terrigenous input from marine sediment off the Cape Range region (Fig. 1, MD00-2361, Stuut et al. 2014) suggests anincrease to 8 ka (Fig. 4). The ITCZ southern migration
shifted the rain belts to the south, explaining the increased pattern of precipitation in the NWA region. This
ITCZ migration is consistent with the records from thelatitudinal cores across the Timor Sea (Kuhnt et al.
2015).

Ishiwa et al. Progress in Earth and Planetary Science

It has been suggested that the ASM was weak or inactive
during glacial periods (Wyrwoll and Miller 2001; Gallagher et al. 2014; De Deckker et al. 2014). This relative
inactivity explains the extremely low sedimentation rateprior to 13.5 cal kyr BP at Site U1461 (Figs. 3 and 4),
suggesting a weak or absence of terrigenous input fromthe continent during this period.
The rapid increase in the sedimentation rate is at ~13.5 ka (Figs. 3 and 4), when the %K and K/Ca ratios at
Site U1461 remain relatively low from ~ 13.5 to 11.5 ka(Fig. 4), which reveal that the ASM intensification occurred at 11.5 ka during the transition from the Late
Pleistocene to the Holocene. This intensification of theASM corresponds to a change in lithological units fromsubunit IIa to unit I, marked by an increased quartz content. These increases in quartz content and %K also suggest that the terrigenous input from the continent
increased due to the enhanced ASM-driven precipitation. A marine sediment core off the Cape Range region(core Fr10/95-GC17, Fig. 1) records evidence of the
ASM intensification/onset at ~ 13 ka, which was interpreted to be due to a southward shift in the ITCZ (De
Deckker et al. 2014). In addition, an increase in thechlorite content in core Fr10/95-GC17 indicates that
Leeuwin Current strengthened at this time since chlorite
originated from the Fortescue and De Grey rivers (Gingele et al. 2001a, 2001b), and Leeuwin Current transported these clays westward to the Cape Range region(De Deckker et al. 2014). Wyrwoll and Miller (2001)
analysed the lacustrine and fluvial sediments from theKimberley region and suggested that ASM was active
after 14 ka. Similarly, peat sediments in north Kimberleyrecord the onset of monsoonal activity after ~ 14 ka, as
evidenced by the increasing organic content in a spring(Field et al. 2017). The Kimberley region is the northern
region compared with Site U1461, suggesting that thetiming of onset/intensification of ASM due to the southern shift of ITCZ was earlier than Site U1461.
The transgression associated with the deglacial sea
level rise shifted Site U1461 from an inner- (0–50 m) toa mid-ramp (50–120 m) position at around 14 ka (Fig. 4).
On the modern shelf, the siliciclastic content decreases
from the inner- to the mid-ramp due to the impact ofstorm waves and tidal currents (James et al. 2004). The
observed increase in K/Ca ratios therefore cannot be explained by a change in the depositional environment due
to sea level variations.
Climate variability in the NWA during the Early Holocene

Increasing K/Ca ratios and %K from 11.5 to 8.5 ka reveal
the terrigenous input from the Australian continent(Fig. 4). Intensified precipitation during the ASM would

Climate variability in the NWA after the Early Holocene

The decrease in the K/Ca ratios and %K content revealed a reduction in terrigenous input after 8.5 ka at
Site U1461 (Fig. 4), indicating a weakening of erosion
and riverine run-off in the drainage area due to reduced
precipitation. The northern migration of the ITCZ
moved the rain belt away from NWA after 8.5 ka, while
the northern cores across the Timor Sea show the shift
to dry condition between 8.1 and 7.3 ka (Kuhnt et al.
2015). However, this apparent dry shift is not strongly
observed in the record from the Cape Range region(Fig. 4, Stuut et al. 2014). The stronger covariation between the record at Site 1461 with continental archives
may be related to its proximal position at the continental
shelf, which is directly influenced by the continental climate variability in NWA.

Conclusions
The IODP Expedition 356 cored the upper slope to theouter shelf carbonates at Site U1461 at a present-day
latitude of 20° S. The site is close to the Australian coast,
which makes Site U1461 ideally located to record the
ASM intensity. Indeed, this area has received fluviallysourced siliciclastic sediment over the last five million
years. The focus of this work is the upper 14 m, where anear-complete 14,000-years record of the variability of
the ASM is preserved. Shipboard NGR (%K) andXRF-derived K/Ca ratios chart a detailed LGM to Holocene record of terrigenous input through riverine
run-off.
Increased %K and K/Ca ratios from 11.5 to 8.5 ka reveal the enhanced ASM precipitation due to the ITCZ

Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

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Fig. 4 Comparison of paleoclimatic records related to ASM variability in the past 15,000 years. a Ball Gown Cave stalagmite records (Denniston etal. 2013). b Fe/Ca records of core MD00-2361 recovered from the continental slope of Western Australia (Stuut et al. 2014). c K/Ca records
obtained from Site U1461. d %K at Site U1461 measured by shipboard NGR measurement. e Sedimentation rate of Site U1461 established by the
Bchron model. f Global sea level curve from Lambeck et al. (2014)

Ishiwa et al. Progress in Earth and Planetary Science(2019) 6:17

southward shift. This increased precipitation trend isalso observed in a speleothem record from Ball Gown
Cave and sediment records from the Cape Range region.
This trend is caused by the southern shift of the ITCZ,
which brings the seasonal rain belt to the NWA. A reduction of %K and K/Ca ratios after 8.5 ka is interpreted
as a shift to more arid conditions in the NWA, causedby the northward shift of the ITCZ.
Abbreviations
ASM: Australian Summer Monsoon; IODP: International Ocean Discovery
Program; ITCZ: Intertropical Convergence Zone; LGM: Last Glacial Maximum;
NGR: Natural gamma radiation; NWA: Northwestern Australia
Acknowledgements
This study used samples and data from the International Ocean Discovery
Program Expedition 356 Indonesian Throughflow. We truly appreciate themembers of this expedition, including the JOIDES Resolution Science
Operator and the Siem Offshore crew and the Expedition 356 science party,
and Drs. T. Aze, C. Sawada, and Y. Miyairi for laboratory assistance. We also
thank the handling editor Ken Ikehara and the two anonymous reviewers
whose comments greatly improved the manuscript.
Funding
Funding was provided by the Australian IODP office and the ARC Basins
Genesis Hub (IH130200012) to S.J.G., the Japan Society for the Promotion of
Science (JSPS) KAKENHI (16J04542) to T.I. and KAKENHI (15KK0151, 17H01168)
to Y.Y., the DFG (German Science Foundation, Project 320220579) to L.R., NSF- United States Science Support Program (USSSP), OCE-1450528 for C.M., andthe Deutsche Forschungsgemeinschaft (VL96/1-1) to D.D.V.. This work is partof a DAAD project DANA between S.J.G. and D.D.V.. The production of thispaper was supported by a National Institute of Polar Research publication
subsidy.
Availability of data and materials
Radiocarbon date are available in Table 1. Please contact author for otherdata requests such as NGR and XRF.
Authors’ contributions
TI carried out this research with input from the co-authors. CMM and LR
conducted the XRF core scanning, and YY proceeded with the radiocarbon
measurements. YY, CMM, LR, DDV, and SJG contributed to the discussions
and improved the manuscripts. TI, CMM, LR, DDV, and SJG were part of the
IODP Expedition 356 Science Party. All authors approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
National Institute of Polar Research, Tokyo, Japan. 2Atmosphere and Ocean
Research Institute, The University of Tokyo, Chiba, Japan. 3Department ofEarth and Planetary Science, Graduate School of Science, The University ofTokyo, Tokyo, Japan. 4Energy and Mineral Resources Group, Geological
Institute, RWTH Aachen University, Aachen, Germany. 5School of Earth andEnvironmental Sciences, Queens College (C.U.N.Y.), Flushing, New York, USA.
6
MARUM-Center for Marine and Environmental Sciences, University ofBremen, Bremen, Germany. 7School of Earth Sciences, University of
Melbourne, Melbourne, Victoria, Australia. 8Present Address: Institute of
Geosciences, CAU Kiel University, Kiel, Germany.

Page 9 of 10

Received: 8 August 2018 Accepted: 21 January 2019

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