Hallenberger et al.
Progress in Earth and Planetary Science(2022) 9:49
https://doi.org/10.1186/s40645-022-00507-1

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

The pteropod species Heliconoides inflatus
as an archive of late Pleistocene to Holocene
environmental conditions on the Northwest
Shelf of Australia
Maximilian Hallenberger1* , Lars Reuning2, Hideko Takayanagi3, Yasufumi Iryu3, Nina Keul2,
Takeshige Ishiwa4   and Yusuke Yokoyama5

Abstract
There is growing interest in the use of pteropods as potential archives of past changes in ocean chemistry. However,pteropods have rarely been used in studies of millennial-scale sedimentary records, especially in shallow-marine
environments. This study obtained core data for the last 16 kyr from the Northwest Shelf of Australia (NWS). Changes
in the carbonate saturation state were assessed based on the carbon isotope ratios (δ13C) of shells and the Limacinadissolution index (LDX) measured on the planktonic pteropod species Heliconoides inflatus. In addition, the calcification depth of the pteropods was estimated based on oxygen isotope values (δ18O) of pteropod shells and seawater. Our findings indicate that H. inflatus calcifies at a depth of 95–140 m. This confirms that H. inflatus records ashallow-marine signal on the NWS. The δ13C values of the pteropods record a notable decrease in carbonate ion
concentrations after 8.5 ka. This decrease is associated with the post-glacial onset of humid conditions on the NWS.
The studied pteropod shells are pristine throughout the 16 kyr section and have low LDX values. Therefore, the LDX
proxy appears to lack the sensitivity to be applicable in this highly supersaturated, shallow-marine environment. Until
this study, proxies derived from H. inflatus have been exclusively utilized in open-marine settings. Our results indicatethat the δ13C values of H. inflatus also represent a useful proxy for carbonate ion concentrations in shallow-marine
environments.
Keywords: Carbonate ion concentration, Pelagic gastropod, Indo-Pacific Ocean, Limacina dissolution index, Carbon
and oxygen isotopes, Sedimentary core record
1 Introduction
Ocean water chemistry parameters, such as carbonate
ion concentrations and the saturation states of carbonate minerals, have an important role in the production ofcarbonates in low-latitude shallow-marine environments.
This includes the (inorganic) precipitation of aragonite mud (Shinn et al. 1989; Robbins and Blackwelder*Correspondence: maximilian.hallenberger@emr.rwth-aachen.de
1
Energy and Mineral Resources Group (EMR), Geological Institute, RWTH
Aachen University, Wuellnerstrasse 2, 52062 Aachen, Germany
Full list of author information is available at the end of the article

1992; Purkis et al. 2017) and the formation of non-skeletal grains (Li et al. 2015; Trower et al. 2017; Harris et al.
2019), both of which are dependent on elevated carbonate saturation states. In addition, carbonate saturation isa controlling factor on the calcification rates and fitnessof calcifying organisms (Manno et al. 2017).
On the Northwest Shelf of Australia (NWS), cyclicalchanges in carbonate sedimentation have been linked toclimate-related changes in carbonate saturation states(Hallenberger et al. 2019, 2021). Low fluvial runoff(Ishiwa et al. 2019; Courtillat et al. 2020) and high ratesof evaporation have been proposed to have resulted in© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

(2022) 9:49b

125°0'0"E

SST [°C] - February

115°0'0"E

120°0'0"E

SST [°C] - July

SST (°C)
31

Site U1461

28
0SO257-18571
MD2361115°0'0"E

15°0'0"S

15°0'0"SSO257-18548

120°0'0"E

125 250500 km

125°0'0"E

SO257-18548

SST (°C)

Site U1463
20°0'0"S

20°0'0"S

MD01-2378

20°0'0"S

15°0'0"S

MD01-2378

Site U1463

125°0'0"E

15°0'0"S

120°0'0"E

27

Site U1461

23
0SO257-18571
MD2361115°0'0"E

120°0'0"E

125 250500 km

125°0'0"E

Fig. 1 Modern distribution of water temperature on the NWS. (a, b) Maps of the present-day sea surface temperatures (SSTs), as well as thelocation of important sites offshore of northwestern Australia and in the Timor Sea. Temperature data were acquired from the CSIRO Oceans andAtmosphere database, representing snapshots of the austral summer (February 2015, a) and austral winter (July 2015, b) SST distribution (Hobart2017). The approximate position of the 150 m isobath (black line) delineates the onshore region and shelf break

20°0'0"S

115°0'0"E

10°0'0"S

10°0'0"Sa(Juranek et al. 2003; Keul et al. 2017; Oakes et al. 2021).
More commonly, carbonate ion concentrations havebeen determined based on different proxies derived fromforaminiferal shells, including boron-to-calcium ratios(Yu and Elderfield 2007; Yu et al. 2010), uranium-to-calcium ratios (Raitzsch et al. 2011; Keul et al. 2013), andboron isotopic compositions (δ11B values; Yu and Elderfield 2007). While these proxies have been proven to yieldrobust constraints on carbonate ion concentrations (Yuet al. 2010), these proxies are of limited use in a shallowmarine setting, where temporal variations in oceanic andclimatic conditions combined with the ≤ 1 yr lifespan can
result in a strong seasonal bias.
To constrain the changes in ocean water chemistryover the last deglacial transition, we studied sedimentcore records covering the last 16 kyr from the International Ocean Discovery Program (IODP) Sites U1461 andU1463 (Fig. 1). Changes in carbonate ion concentrations
were determined based on the stable carbon isotope
ratios (δ13C values) of H. inflatus shells (Keul et al. 2017).
The H. inflatus shells were further screened for theirpreservation state (Limacina dissolution index [LDX]
values), which is a parameter that is directly linked to thearagonite saturation state of the ambient seawater (Gerhardt and Henrich 2001; Wall-Palmer et al. 2013, 2014).
The interpretation of H. inflatus-based proxy data iscomplicated by the limited knowledge of its calcification depth. Heliconoides inflatus inhabits a large range of
depths from the sea surface down to 1000 m (Wormuth

10°0'0"S

elevated ocean water carbonate saturation during comparatively dry glacial stages (Hallenberger et al. 2019).
This led to the formation of extensive platform systems
consisting of ooids (Gallagher et al. 2014, 2018; Hallenberger et al. 2021), as well as the deposition of large
volumes of aragonite needle mud (Dix et al. 2005; Hallenberger et al. 2021). In comparison, the interglacial stages
were defined by a more humid climate, and the deposition was dominated by skeletal calcite grains (Hallenberger et al. 2019, 2021). However, glacial to interglacial
proxy records for changes in ocean water chemistry on
the NWS, thereby impeding the significance of previous
interpretations linking climate and cyclical sedimentation. In this paper, we address this issue by utilizing proxy
data derived from the aragonitic shells of the cosmopolitan euthecosomatous pteropod (i.e., pelagic gastropod)
Heliconoides inflatus (d’Orbigny 1843), also known as
Limacina inflata (Janssen 2012). Aragonitic pteropods
are particularly useful in predicting the carbonate saturation state of shallow ocean waters due to their sensitivity to changes in ocean acidification and abundance inshallow water depths (Bé and Gilmer 1977; Bednaršek
et al. 2012; Keul et al. 2017). In addition, H. inflatus
is characterized by a 1-yr life cycle and diel migration(Wormuth, 1981; Almogi-Labin et al., 1988). Therefore,carbonate ion concentrations derived from the shells of
H. inflatus represent an integrated proxy record, making this species a promising target for the reconstruction of past ocean water conditions on shallow shelf areas

Page 2 of 15

10°0'0"S

Hallenberger et al. Progress in Earth and Planetary ScienceHallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

1981; Juranek et al. 2003; Bednaršek et al. 2012; Keulet al. 2017). Heliconoides inflatus undergoes diel verticalmigration, with the highest population densities at < 75 m
depth during the night and at > 200 m depth during theday (Wormuth 1981). The main driver for this day–nightcycle, which can reach a vertical migration depth range
of > 250 m (Chen and Bé 1964; Wormuth 1981), is foodavailability and the presence of predatory species (Wormuth 1981; Almogi-Labin et al. 1988; Juranek et al. 2003;
Oakes and Sessa 2020). Heliconoides inflatus also displays
ontogenetical migration patterns, with juveniles staying
near the sea surface, and adults found in deeper water(Almogi-Labin et al. 1988).
To determine the depth range recorded by the pteropods, oxygen isotope data for H. inflatus were comparedwith the present-day conditions on the NWS, as well as
more established proxy data, including for benthic (Uvigerina spp.) and planktonic (Globorotalia menardii)
foraminifera.

2 Modern oceanographic setting
The climate and oceanography of the NWS are influenced
by strong seasonal changes between the austral summer(December–March) and austral winter (June–August)(Sudmeyer 2016). In general, the NWS is characterized
by subtropical arid conditions, with an annual rainfallof ~ 320 mm (Sudmeyer 2016). Most (~ 60%) of the precipitation in northwestern Australia occurs seasonallyduring the Australian Summer Monsoon (Lough 1998;
Hesse et al. 2004). In addition, strong cyclones occur during austral summer months, resulting in torrential rainfall, flash floods, and rapid discharge of fluvial sediments(Fandry and Steedman 1994; Condie et al. 2003). Surface
air temperatures are highest during the austral summer
months, with average values > 30 °C. In comparison, airtemperatures drop to an average of 20 °C during the austral winter (Sudmeyer 2016).
Water temperatures on the NWS vary significantly
depending on the season, water depth, and distance fromshore (Fig. 1 and Additional file 1: Fig. S1A). During the
austral winter, the coastal region is characterized bycold water, which follows a sharp frontal gradient, starting with the lowest temperatures inshore (~ 23 °C) andshifting toward higher temperatures offshore (25–27 °C;
Fig. 1B). The elevated density of this narrow band ofcold and saline (salinity = 35) water leads to a cross-shelfdensity gradient that results in the offshore transport ofcoastal water along the ocean floor (James et al. 2004;
Mahjabin et al. 2020). This so-called denser shelf watercascade presently extends down to a depth of 150 m,
transporting cold coastal waters over distances of > 50 km
offshore (Mahjabin et al. 2020). During the austral summer months, strong solar heating results in high sea

Page 3 of 15

surface temperatures (SSTs; > 30 °C) in the nearshorearea, which gradually become lower further offshore (28–30 °C; Fig. 1A). The lateral and depth gradients of watertemperature are less pronounced as compared with the
austral winter months (Fig. 1B and Additional file 1: Fig.
S1B). In addition, depth profiles display seawater salinities that are ~ 0.1 higher at the sea surface (< 40 m depth)
as opposed to deeper waters during the austral summer(Additional file 1: Fig. S1C). This is opposite to the case in
the austral winter months, when salinity increases steadily from 34.85 to 35.00 between the sea surface and 150 m
depth (Additional file 1: Fig. S1C).
The water masses are mainly sourced by the Indonesian Throughflow (ITF), which originates in the IndoPacific Warm Pool and transports warm and oligotrophic
seawater to the NWS. These waters form the Holloway
Current, which feed into the Leeuwin Current around
Cape Range (northwestern Australia; Pattiaratchi 2006;
D’Adamo et al. 2009). Interannually, the oceanography of
the NWS is modified by the El Niño–Southern Oscillation (ENSO). During El Niño events, the ITF and Holloway Current strengths are weakened due to modulationsin the equatorial western Pacific wind stress (Feng et al.
2013). The weakened current system results in negative
SST anomalies along the northwestern to western coastof Australia (Feng et al. 2004, 2013). In turn, La Niña
events have been shown to trigger strong heatwaves innorthwestern Australia, with water temperatures risingby several degrees (Feng et al. 2013; Tozuka et al. 2014;
Zinke et al. 2015).

3 Materials and methods
This study is based on core material retrieved from IODP
Sites U1461 (20°12.863′ S, 115°3.950′ E) and U1463(18°57.918′ S, 117°37.422′ E). Site U1461 is located in thewesternmost part of the NWS, 100 km northwest of Barrow Island at a water depth of 127 m (Fig. 1). Site U1463
is situated ~ 300 km northeast of Site U1461 in a slightly
deeper water depth of 147 m (Fig. 1).
At Site U1461, samples were taken from the upper 10 mof holes U1461B and U1461C. The sediments are unlithified, olive to dark gray, skeletal wackestones to packstones, which were described in detail by Hallenberger
et al. (2019). Samples from Site U1463 were collectedfrom the upper 11.5 m of holes U1463B and U1463C.
This section is broadly equivalent to Unit 1 as describedin the IODP shipboard report (Gallagher et al., 2017a, b).
Sediments in this section are creamy gray to light greenish gray, unlithified, wackestones to packstones, whichwere deposited in a neritic to hemipelagic environment.
The calcium carbonate content is high (~ 87 wt%), withthe microfossil assemblage consisting predominantlyof pteropods and a variety of planktonic and miliolidHallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

benthic foraminifers (Gallagher et al. 2017a, b). Bioturbation is moderate and only slight drilling disturbanceoccurs throughout the studied interval (Gallagher et al.
2017a, b). Measured total organic carbon (TOC) contents vary between 0.5 and 1.0 wt% at Site U1461 (Gallagher et al. 2017a, b) and 0.9–1.0 wt% at Site U1463(Gallagher et al. 2017a, b). The depths of all samples are
given as core composite depth below seafloor (CCSF)
to account for differences in the individual depth scalesof single holes. A total of 52 sample wedges filled with
soft sediment (20 ­cm3) were collected from Site U1461(n = 14) and Site U1463 (n = 38). Each sample was subsequently wet sieved to separate the fine (< 63 µm) andcoarse (> 63 µm) fraction. From the coarse fraction, pteropod shells of H. inflatus, benthic foraminiferal tests ofthe genus Uvigerina (Uvigerina spp.), and different planktic foraminiferal tests were handpicked.
3.1 Accelerator mass spectrometry 14C dating

For Site U1461, a previously published age model was utilized to convert core depth into time before present (Hallenberger et al. 2019; Ishiwa et al. 2019). The chronologyof Site U1463 is based on 10 accelerator mass spectrometry (AMS) radiocarbon (14C) dates. Radiocarbon datingof a mixture of different planktic foraminifer species was
performed by the Single Stage Accelerator Mass Spectrometer at the Atmosphere and Ocean Research Institute, The University of Tokyo, Japan (Yokoyama et al.,2019). Calibration to calendar years was undertaken witha Bayesian age modeling approach (Ramsey 2009) using
the Marine20 database (Heaton et al. 2020). Utilizing
the marine reservoir database (http://​calib.​org/​marine/;
Reimer and Reimer 2001), 10 known reservoir ages wereaveraged in a 1000 km radius (Bowman 1985; O’Connor
et al. 2010). The resulting reservoir age is small (ΔR
57 ± 27), and thus, the local correction was set to zero. A
continuous age model was calculated with Bchron, which
is an age–depth modeling package implemented in “R”
software (version 4.1.2) (Team 2018), using the algorithmof Haslett and Parnell (2008).
3.2 Limacina dissolution index

Pteropod shells were examined according to the Limacina dissolution index (LDX) (Gerhardt et al. 2000; Gerhardt and Henrich 2001). The LDX methodology involves
the analysis of the surface of single H. inflatus shells.
Depending on the preservation, each specimen is evaluated on a scale of 0–5, with 0 being a shell that is transparent and lustrous, with a perfect outer aragonite layer,and 5 being a shell that is opaque white in color, totally
lusterless, and perforated, with extensive corrosion of theouter aragonite layer (Gerhardt and Henrich, 2001). For
each sample, at least 10 shells of adult H. inflatus of a size

Page 4 of 15

of ≥ 300 μm were assigned a preservation value basedon optical microscopy observations. The LDX value foreach sample was then calculated according to the following equation (Gerhardt et al. 2000; Gerhardt and Henrich2001):LDX =(np × p)/np(1)

where np is equal to the number of tests per preservation stage p (0–5) with the sum of np being at least 10.
Dissolution along the shell surface was further evaluatedusing a scanning electron microscope (SEM; Keyence, VE8800).
3.3 Stable carbon and oxygen isotopes

Stable carbon (δ13C) and oxygen (δ18O) isotopic compositions were determined for all the collected H. inflatus andUvigerina spp. samples. Prior to measurement, each sample was cleaned in an ultrasonic bath to remove contamination. This process was repeated once with methanol
and twice with ultra-pure deionized water. The quality of
the cleaning process was subsequently evaluated by optical microscopy. In addition, a random subset of tests waschecked for cementation by optical microscopy and witha SEM. A total of 10 specimens were pooled and crushed
for subsequent measurement of δ13C and δ18O values.
Measurements of the carbonate powders were conductedwith a Thermo Fisher Scientific DeltaV Advantage mass
spectrometer coupled to a ThermoQuest Kiel-III automated carbonate device at the Institute of Geology andPaleontology, Tohoku University, Japan. The samples(~ 0.1 mg per sample) were reacted with 100% phosphoric acid at ~ 72 °C. A value of 1.01025 was used as thefactor to account for the δ18O fractionation occurring
during phosphoric acid digestion (Sharma and Clayton
1965). All results are reported in the δ notation in permil (‰) calibrated to the NBS-19 international standard relative to the Vienna Pee Dee Belemnite (VPDB).
The mean external error and reproducibility (1σ) wereassessed by replicate measurements of an in-housecarbonate standard (JCp-1; δ13C = − 1.63‰ ± 0.03‰;
δ18O = − 4.71‰ ± 0.03‰) (Okai et al. 2004; Nakayama
et al. 2008) and was determined to be low for both δ13C(± 0.03‰) and δ18O (± 0.05‰) values. Isotopic values
determined on a control sample of 10 pteropods will
deviate from the parent population of all pteropods in asingle sediment sample. To estimate the uncertainty created by the selection of a subset of pteropods, we undertook repeat sampling and measurement of 10 pteropods
from each parent population (i.e., coarse fraction of asediment sample). The averaged measured deviations
between both pteropod subsets were 0.12‰ for δ18O and0.07‰ for δ13C.

Hallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

3.4 Calcification depth of H. inflatus

Euthecosomatous pteropods have been found to precipitate aragonite close to oxygen isotope equilibrium(δ18Oeq) with ambient seawater (Grossman et al. 1986,
Almogi‐Labin et al. 1991, Fabry and Deuser 1992, Juranek
et al. 2003, Keul et al. 2017). For H. inflatus, differences
between the calculated δ18Oeq and δ18O values measured
in shells have been shown to be minor, ranging from 0
to 0.1‰ (Fabry and Deuser 1992). Based on this observation, the depth of calcification of H. inflatus can be
determined by comparing the δ18O measured in H. inflatus with δ18Oeq depth profiles of the ambient seawater(Grossman et al. 1986; Wall-Palmer et al. 2018). δ18Oeq
profiles were approximated as a function of water temperature and the oxygen isotopic composition of seawater (δ18Osw; Grossman and Ku 1986, Juranek et al. 2003,
Keul et al. 2017), where δ18Osw was determined based on◦  0.245 −
T C =



Page 5 of 15

3.5 Conversion of ice volume‑corrected oxygen isotope
data to temperature

The regression of Böhm et al. (2000) was further used toestimate past changes in water temperature by replacing the δ18Oeq values with the δ18O values measured in
the H. inflatus shells using Eq. (3). Since there are nodata on the past δ18Osw or salinity values available for the
NWS, δ18Osw was approximated based on modern values (Additional file 1: Fig. S1D). Calculated δ18Osw values
range from 0.15 to 0.25‰, with limited fluctuations withdepth or season (Additional file 1: Fig. S1D). Based on
this distribution, we set the δ18Osw value used in the temperature calculations to be equivalent to the overall meanobserved at both sites (mean = 0.21‰).
Past seafloor water temperatures recorded at Site
U1463 were reconstructed based on the δ18O values ofthe benthic foraminifera Uvigerina spp. using the regression of Marchitto et al. (2014):


0.045461 + 0.0044 δ18 O − δ18 Osw − 0.47
0.0022a regional correlation between δ18Osw and seawater salinity (S; Pei et al. 2021a, b):

δ18 Osw = −11.66 + 0.34 × S.

(2)

Seasonal profiles of water temperature and salinity at Sites U1461 (Station 273179) and U1463 (Station 278667) were derived from the World Ocean Atlas
2018 (Locarnini et al. 2018). δ18Oeq was then calculated
based on a linear regression devised for generic aragonitic marine organisms, which calcify in equilibrium with
δ18Osw (Böhm et al. 2000):◦ T C = 20 − 4.42 × δ18 Oeq − δ18 Osw .
(3)
The temperature–δ18O relationship devised by Böhm
et al. (2000) combines several sources of biogenic andsynthetic aragonite data (Tarutani et al. 1969; Grossman and Ku 1986; Rahimpour-Bonab et al. 1997; Böhm
et al. 2000), which thus extends the temperature window
and reduces the overall error present in previous studies (Böhm et al. 2000). It has been successfully applied
to calculate water temperatures from H. inflatus shells
and was therefore favored over other calibrations in theliterature (Oakes et al. 2021). This correlation was alsopreferred over the only existing species-specific correlation for H. inflatus (Keul et al. 2017), as it includes
the high water temperatures typical of the NWS (Böhmet al. 2000).

(4).

To account for the δ18O offset caused by changing ice
volume, measured values for H. inflatus and Uvigerina
spp. were adjusted by − 0.1‰ for each 10 m of sea-level
fall (Shackleton and Opdyke, 1973), following the global
Quaternary sea-level curve of Lambeck et al. (2014).
3.6 Conversion of carbon isotope data to carbonate ion
concentrations

The δ13C values of pteropods are inversely correlatedwith the carbonate ion concentration (­ [CO32–]) of seawater (Juranek et al. 2003; Keul et al. 2017). In this study,­[CO32–] was calculated based on the δ13C values of H.
inflatus using the species-specific correlation presented
by Keul et al. (2017):

carbonate ion concentration =δ13 C − 3.067
.
−0.01

(5)

4 Results
4.1 Age model for site U1463

Radiocarbon ages acquired for Site U1463 are provided
in Table 1 and Fig. 2. The chronological profile suggests that the upper 12 m of core corresponds to thelast 16 kyr (Fig. 2). Between 16 and 7 ka, sedimentation
rates were comparatively low (30–90 cm/kyr). After 7 ka,
the sedimentation rate exhibited a substantial increase(60–230 cm/kyr), with a marked peak observed between
7 and 5 ka (230 cm/kyr). Subsequently, relatively lowHallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

Page 6 of 15

Table 1 Age results of Site U1463
Laboratory CodeSample Name

Depth CSF-A [m]Depth CCSF-A[m]

YAUT-034202

356-U1463B-1H-1A 0/4

0.02

0.02

YAUT-034203

356-U1463B-1H-1A 68/72

0.7

0.7

YAUT-034204

356-U1463B-1H-2A 18/22

1.7

1.7

YAUT-034205

356-U1463B-1H-2A 118/122

2.7

2.7

YAUT-034206

356-U1463B-1H-3A 68/72

3.7

3.7

YAUT-051514

356-U1463C-2H-3A 17/21

7.31

8.31

YAUT-055628

356-U1463C-2H-3A 55/59

7.69

8.69

YAUT-056104

356-U1463C-2H-3A 130/134

8.44

9.44

YAUT-056105

356-U1463C-2H-5A 17/21

10.31

11.31

YAUT-051515

356-U1463C-2H-5A 130/134

11.44

12.44

Radiocarbon age[year BP]Calendar age[cal year BP] 95%
highest density
regions

1216 ± 37

1775 ± 34

2887 ± 31

3632 ± 29

4789 ± 296552 ± 53

7136 ± 289143 ± 36

10,626 ± 47

13,914 ± 85

501

730

1029

1297

2326

2662

3215

3514

4689

5034

6635

7026

7306

7563

9522

9883

11,506

11,997

15,667

16,269Calibration of calendar years was based on a Bayesian age modeling approach (Ramsey, 2009) using the Marine20 database (Heaton et al., 2020). Depth of individual
samples is given as core depth below sea floor (CSF-A) and core composite depth below sea floor (CCSF-A), as defined by the IODP Depth Scale Terminology (IODP-MI
2011). Calendar ages are represented by the lower and upper boundary of the highest density region10

15Site U1463

200

5

150
100

10

0

250

50
0

Depth [m CCSF-A]

Depth [m CCSF-A]0

b 0

Age [cal ka BP]
5

Sedimentation rate [cm/kyr]a0

Age [cal ka BP]
5

10

15

20

25

Site U1461

5

10

15

Fig. 2 Age–depth relationships at (a) Site U1463 and (b) Site U1461, including the sedimentation rate and confidence interval (2σ). The age modelfor Site U1461 was previously published by Hallenberger et al. (2019)sedimentation rates characterized the uppermost part ofthe studied core, where only 2 cm of sediment represents
the last 600 yr (3 cm/kyr). This may be the result of atechnical issue that is typical of IODP drilling operations,whereby the uppermost sediment in a core is often lost.
4.2 Limacina dissolution index

Pteropod data for Sites U1461 and U1463 are shown in
Fig. 3 and listed in Additional file 1: Tables S1–S2. Pteropod preservation at both sites ranged between pristine(LDX = 0) and excellent (LDX = 1), with LDX values
never exceeding 1 (Fig. 3). At Site U1461, the pteropods
were pristine (LDX = 0) throughout the studied coresection, without any notable trends or changes (Fig. 3).
At Site U1463, the pteropods were mostly pristine,with little to no evidence of dissolution (Fig. 4A). An

exception was found in an interval deposited between
8 and 6 ka, where the average preservation state of thepteropods was slightly deteriorated (LDX = 1; Fig. 3).
Shells occurring within this interval were often partially opaque white or slightly cloudy, indicating initialdissolution of the shell surface (Fig. 4B). In part, thisdissolution removed the outer layer of the pteropod
shells, exposing the underlying rod-like internal structure (Fig. 4C–D).
4.3 Oxygen isotopes and water temperatures

Measured δ18O values are shown in Fig. 3 and listed in
Additional file 1: Tables S1–S2. Pteropod δ18O values
displayed a clear trend over time at Site U1463 (Fig. 3).
Prior to 10 ka, the δ18O values were elevated as compared with the rest of the section, with δ18O = 0.2–0.8‰

Hallenberger et al. Progress in Earth and Planetary Science

5

10

15

-0.8
Site U1463

a

Site U1461
1

-1

b
1.5

0.2

Site U1461

c

Site U1463
0.7

2

d

LDX

δ CPtero. [‰]Site U1463

δ OUvi. [‰]δ OPtero. [‰]0(2022) 9:49

Site U1463
0

Site U1461
0

5

10

15

Age [ka BP]
Fig. 3 Isotope values and Limacina dissolution index (LDX) data
for IODP Sites U1461 and U1463. (a) δ18O values of pteropods (H.
inflatus) from Sites U1461 and U1463. (b) δ18O values of the benthic
foraminifera Uvigerina spp. from Site U1463. (c) δ13C values ofpteropods (H. inflatus) from Sites U1461 and U1463. (d) LDX values ofpteropods from Sites U1461 and U1463(mean = 0.4‰). At ca. 10 ka, the δ18O values decreased
rapidly from 0.2‰ to − 0.4‰ (Fig. 3). Pteropods ofyounger age have consistently low δ18O values of − 0.6‰
to − 0.2‰ (mean = − 0.3‰). The δ18O values for H. inflatus from Site U1461 are largely the same as those from
Site U1463 within the selected age segment, with values
varying between − 0.7‰ and − 0.2‰ (mean = − 0.4‰).
Water temperatures calculated from pteropods at bothsites fluctuate between 21.5 and 24.2 °C, with an averagetemperature of 22.8 °C (Fig. 5B; Additional file 1: Tables
S3–S4). As opposed to the δ18O profiles, there were no
discernable trends or systematic changes in water temperature over time or between the two sites (Fig. 5B).
δ18O values of Uvigerina spp. from Site U1463 vary
between − 0.9‰ and 0.0‰ (mean = − 0.6‰) without
any discernible trends over time (Fig. 3; Additional file 1:
Table S2). Calculated temperatures were highest (26.7 °C)
for the oldest measured sample at 16 ka, and decreased
steadily until the present-day, when water temperaturesat Site U1463 were ~ 18 °C (Fig. 5C).
4.4 Carbon isotopes and carbonate ion concentrations

δ13C values of pteropods from Sites U1461 and U1463
are listed in Additional file 1: Tables S1 and S2 andPage 7 of 15

shown in Fig. 3. In general, the δ13C values of H. inflatus vary between 0.8 and 1.5‰ (mean = 1.2‰). A directcomparison between values obtained from equivalent age ranges at both sites revealed a statistically significant (student t-test; p < 0.05) higher mean for Site
U1461 (mean = 1.3‰) as compared with Site U1463(mean = 1.1‰). δ13C values of pteropods from Site U1461
exhibit no clear trends over time, with δ13C ratios ranging
between 1.2 and 1.5‰ (mean = 1.3‰). At Site U1463, theδ13C values vary from 0.8 to 1.4‰ (mean = 1.1‰), withthe values increasing significantly (p < 0.05) after 8.5 ka(Fig. 3). Prior to 8.5 ka, the δ13C values were comparatively low (0.8–1.0‰; Fig. 3). At ca. 8.5 ka, δ13C values
display an abrupt increase from 0.9 to 1.2‰ between two
data points (Fig. 3). Thereafter, the δ13C values remainhigh (1.0–1.4‰).
Calculated ­[CO32–] for Sites U1461 and U1463 arelisted in Additional file 1: Tables S3 and S4, respectively.
The ­[CO32–] profile displays an inverse trend over time
as compared with the measured δ13C profiles (Fig. 6A).
Prior to 8.5 ka, calculated values for Site U1463 are consistently high, with ­[CO32–] of 200–230 µmol/kg of seawater (sw) (Fig. 6A). At ca. 8.5 ka, [­CO32–] decreased
notably by almost 40 µmol/kg sw (Fig. 6A). Following this decrease, ­[CO32–] fluctuated but remained atlower levels as compared with pre-8.5 ka, with valuesof 160–200 µmol/kg sw (Fig. 6A). At Site U1461, calculated ­[CO32–] were consistently slightly lower (p < 0.05) ascompared with those at Site U1463, with values of 160–
190 µmol/kg sw (Fig. 6A).

5 Discussion
5.1 Calcification depth of H. inflatus

A comparison between the δ18Oeq profiles and δ18O values measured in H. inflatus reveals that the annual average calcification depths were 95–140 m (Fig. 7). Seasonalchanges in δ18Oeq are minor, with shallower calcification
depths for the summer months (85–130 m) as comparedwith the winter months (115–155 m; Fig. 7). These results
are indicative of slightly deeper calcification as comparedwith most open marine H. inflatus records, which haveestimated calcification depths to be 50–100 m (Fabry andDeuser 1992; Jasper and Deuser 1993; Keul et al. 2017;
Oakes et al. 2021). Notably, the calcification depths are at
the shallow end of the range proposed by Juranek et al.
(2003) for H. inflatus from the Sargasso Sea (50–650 m).
Past seawater temperatures calculated from the δ18O
values of H. inflatus range between 21.5 and 24.2 °C, withlittle apparent change from 16 ka onward (Fig. 5). This
is inconsistent with regional records from northwestern
Australia (Spooner et al. 2011; De Deckker et al. 2014; Peiet al. 2021a, b) and the Timor Sea (Xu et al. 2008; Sarnthein et al. 2011; Pei et al. 2021a, b), which indicate anHallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

aPage 8 of 15b

250 μmc

250 μmd

125 μm

45 μm

Fig. 4 Optical microscopy and SEM images of selected H. inflatus samples. (a) Most pteropods in the studied core material are very well preserved(LDX = 0), including lustrous and transparent shells (Site U1463; 12 ka). (b) The preservation of some shells from Site U1463 (6–8 ka) exhibits slightdeterioration (LDX = 0–1). Single shells partly have an opaque milky white color or a cloudy appearance (white arrow) (Site U1463; 7 ka). (c) SEM
image of a slightly corroded pteropod. The black square indicates the location of (d) (Site U1463; 7 ka). (d) Dissolution of a pteropod as evidenced
by the rough surface of part of the shell (white arrow) (Site U1463; 7 ka)

increase in SSTs ranging between 2 and 4 °C from 16 ka
to the present. However, records from water depthsof > 100 m or from the thermocline level indicate there
was no notable increase in temperature during the same
period (Fig. 5F–G). This is consistent with the estimatedcalcification depths being below 100 m (Fig. 7).
Temperatures at a depth equivalent to the seafloor,
as recorded by the benthic foraminifera Uvigerina spp.,
display a clear trend over time, with values decreasing considerably from 16 ka (27 °C) to 8.6 ka (20 °C;
Fig. 5C). After 8.6 ka, the seafloor temperatures arelargely stable, with values equivalent to those measured at the present-day at Site U1463 (Additional file 1:
Fig. S1). The post-glacial decrease in seawater temperature was likely the result of a progressive rise in

sea level (Fig. 5A). Site U1463 is presently situated ata water depth of 147 m and was characterized by shallow-marine inner ramp conditions (< 40 m) prior to thepost-glacial transgression (Fig. 5A). The shelf surrounding Site U1463 was likely affected by the strong seasonality in seawater temperatures on the coastal NWS(Fig. 1A, B). This is particularly relevant for Uvigerinaspp., because it can display a seasonal bias based on theavailability of nutrients (Ohga and Kitazato 1997; Fontanier et al. 2003; Scourse et al. 2004). On the NWS,
nutrient availability is largest during the austral summer months (Hallegraeff and Jeffrey 1984; Rochford
1988; James et al. 2004), implying a spike in Uvigerina
spp. abundance during the austral summer months. As
such, the δ18O values of Uvigerina spp. likely record

Hallenberger et al. Progress in Earth and Planetary Science0bSite U1463

27

prec. max. 10acSite U1461

300

2

22

MD01-2378e

20

MD2361 (100 m)

f

30

MD2361 (150 m)
SO257-18548

25

15SO257-18571

25

0

5

Age [ka BP]

10

15

Fig. 5 Water temperature proxies at Sites U1461 and U1463 ascompared with regional data. The gray band indicates a precipitation
maximum on the NWS (Hallenberger et al. 2019; Ishiwa et al.
2019). (a) Paleo-water depth at both studied sites based on the
sea-level curve of Lambeck et al. (2014). The relative sea level at
Sites U1461 and U1463 was not corrected for isostatic deformation.
(b) Water temperature based on the δ18O values of H. inflatus. (c)Water temperature calculated from δ18O data for Uvigerina spp. at
Site U1463. (d) δ18O data for the planktic foraminifera G. menardii(Courtillat et al. 2020) used to calculate past changes in upper
thermocline water temperatures at Site U1461. (e) Past variationsin thermocline water temperatures based on the Mg/Ca ratioof P. obliquiloculata at Site MD01-2378 (Xu et al. 2008). (f) Water
temperature record from the North West Cape of Australia (MD-2361)
at 100 and 150 m depth based on the modern analogue technique(Spooner et al. 2011). (g) Sea surface temperature changes basedon the Mg/Ca ratios of G. ruber in the Timor Sea (Site SO257-18548)
and offshore of northwestern Australia (Site SO257-18571; Pei et al.,2021b)

high, nearshore, austral summer water temperaturesduring lowstand conditions (Fig. 1A). During the postglacial rise in sea level, Site U1463 became progressively more distal and was thus less affected by thestrong seasonal fluctuations in nearshore water temperatures (Fig. 1A, B). This is clearly reflected by thepresent-day conditions, which indicate comparatively
stable water temperatures at Site U1463 on the seafloor
throughout the year (Additional file 1: Fig. S1A).

humid

d

0

Site U1461

-4

0

g

Site U1461-2

log (Ti/Ca)

25

160

b

180

dry

Site U1461

230

Site U1463

Site U1463
Temp.Menardii [°C]

26

17

15

c

Temp. MAT [°C]

TWTMg/Ca [°C]215Site U1463

d

SSTMg/Ca [°C]a25Site U1461

0

Carbonate [μmol/kg-sw]

Site U1463

Site U1461

Age [ka BP]

15

CO2 [ppmv]

prec. max. 10

Page 9 of 15

LDX

Age [ka BP]

Temp.Ptero. [°C]5160Temp.Uvi. [°C]

Water depth [m]0(2022) 9:49

5

10

15

Age [ka BP]

Fig. 6 Selected downcore records of regional and global proxies
compared with data from IODP Sites U1461 and U1463. The
gray band indicates a regional precipitation maximum on the
NWS (Hallenberger et al. 2019; Ishiwa et al. 2019). (a) Carbonate
ion concentration calculated from δ13C values of pteropods. (b)Atmospheric ­CO2 concentration determined from the Dome
Concordia (Antarctica) ice core (Monnin et al. 2001). (c) Limacinadissolution index (LDX) for pteropods from Sites U1461 and U1463.
(d) The log ratio of Ti/Ca, which is a proxy for arid (low values) andhumid (high values) conditions in northwestern Australia. Site U1461(Hallenberger et al. 2019) records the onset of humid conditions afterca. 10 ka

In comparison, H. inflatus calcified at considerably
lower temperatures than Uvigerina spp. from 16 to 10 ka(Fig. 5B, C). This temperature difference decreased steadily until 10 ka, after which the pteropods display a constant 2 °C offset toward higher temperatures as comparedwith Uvigerina-based seafloor temperatures (Fig. 5B, C).
This can be interpreted to be the result of a post-glacial
rise in sea level. While the calcification depth of benthic
Uvigerina spp. increased, H. inflatus appeared to calcify atconstant temperatures regardless of the sea level (Fig. 5B).
Given that depth is the main control on seawater temperature across the shelf, it may be concluded that until ca.
10 ka H. inflatus calcified at greater depths as comparedto the Site U1463 seafloor and were thus sourced fromfurther offshore. This interpretation is supported by thecomparatively small amount of pteropods present in thissection of the core (Gallagher et al. 2017a, b). After 10 ka,
the pteropods calcified in warmer and shallower water ascompared with the seafloor of Site U1463 (Fig. 5).
At Site U1461, an existing δ18O record for the planktic foraminifera Globorotalia menardii (Courtillat et al.
2020) was converted into water temperatures using theHallenberger et al. Progress in Earth and Planetary Science0

Dec. - Mar.

Apr. - May(2022) 9:49

Page 10 of 15

Jun. - Aug.

Sep. - Nov.

Annual Average

20

Depth [m]

40
60
80

80 - 140 m

δ18Oara. [‰]δ18Oara. [‰]δ18Oara. [‰]95 - 140 m

115 - 155 m

140

100- 145 m

120

85 - 130 m

100

160
-1.80 -1.40 -1.00 -0.80 -0.40 0.20 -1.80 -1.40 -1.00 -0.80 -0.40 0.20 -1.80 -1.40 -1.00 -0.80 -0.40 0.20 -1.80 -1.40 -1.00 -0.80 -0.40 0.20 -1.80 -1.40 -1.00 -0.80 -0.40 0.20

δ18Oara. [‰]δ18Oara. [‰]Fig. 7 Seasonal and annual profiles of the theoretical aragonite δ18O equilibrium values for Site U1463. A direct comparison with the values
measured for H. inflatus (gray bands) indicates calcification depths between 80 and 155 m, with annual averages of 95–140 m

regression presented by Spero et al. (2003). The resulting temperatures are 22.6–26 °C, with no discernible
trend over time (Fig. 5D). Globorotalia menardii calcifies primarily at the upper thermocline level (Sautter
and Thunell 1991; Ravelo and Fairbanks 1992; Niebler
et al. 1999; Spero et al. 2003; Wejnert et al. 2010), witha limited seasonal flux in tropical and subtropical waters(Jonkers and Kučera 2015; Venancio et al. 2017). On thepresent-day NWS, the depth of the thermocline can beapproximated from the vertically and seasonally resolved
World Ocean Atlas seawater temperature database(Locarnini et al. 2018). The thermocline is defined here
as the zone of highest depth-dependent gradient in seawater temperature, which is at its shallowest between
December–May (40–100 m; Additional file 1: Fig. S1B).
In comparison, the thermocline is only weakly developed
between June–November, where substantial increases
in the temperature gradient only become evident below
100 m depth (Additional file 1: Fig. S1B). This observation is consistent with previous studies, which suggested
that the water column is well-stratified during the australsummer and strongly mixed during the austral winter(Hallegraeff and Jeffrey 1984; James et al. 2004). On average, G. menardii records significantly (p < 0.05) higher(+ 1.6 °C) seawater temperatures as compared with H.
inflatus (Fig. 5). The constant offset in temperatures
between both archives indicates that H. inflatus calcifies in deeper waters as compared with the upper thermocline dwelling G. menardii. The lack of variability of
G. menardii δ18O values further supports the assumption that the water temperatures at the thermocline levelremained largely unaffected by post-glacial increases intemperature.

5.2 Development of shallow‑marine aragonite saturationon the NWS

δ13C values of the pteropods from Site U1463 record
a clear trend over time, with values increasing markedly after 8.5 ka (Fig. 3). This increase is broadly coeval
with a fundamental change in shallow-marine carbonate
sedimentation on the NWS (Hallenberger et al. 2019).
After 10 ka, the dominant mode of carbonate production
across large parts of the NWS changed from inorganic
aragonite to skeletal calcite (James et al. 2004; Dix et al.
2005; Hallenberger et al. 2019). This change was proposed to be the result of a more humid climate causedby the onset of the Australian Summer Monsoon, which
in turn led to a decrease in the saturation state of aragonite (Hallenberger et al. 2019; Ishiwa et al. 2019). The
saturation state of aragonite (Ωarag) is closely linked tothe ­[CO32–], which in turn is inversely correlated with theδ13C values of H. inflatus (Juranek et al. 2003; Keul et al.
2017). As such, both the δ13C values of the pteropods andsedimentary composition indicate there was a change inseawater Ωarag at a similar time on the NWS.
In addition, the δ13C values of pteropod shells can be
influenced by several factors, which can obscure any
potential signal caused by variations in [­ CO32–]. To avoid
any misinterpretation, possible alternative factors influencing the δ13C values are now briefly discussed.
Fluctuations in riverine influx can substantially alter
the chemical and isotopic composition of tropical coastal
waters (Zeebe and Wolf-Gladrow 2001; Moyer and Grottoli 2011; Utami et al. 2021). A previous study suggested
an abrupt increase in riverine influx across the NWS after
10 ka (Hallenberger et al. 2019). This is not reflected bythe δ13C values measured at Sites U1461 and U1463. In

Hallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

particular, at Site U1463, the δ13C values were higher rather
than lower after 10 ka, and therefore opposite to what
would be expected from a potential riverine influx (Fig. 6).
Enhanced productivity can result in higher δ13C values of dissolved inorganic carbon (δ13CDIC), which could
affect the δ13C values of the pteropods shells (Lazar andErez 1992). However, published data for offshore northwestern Australia suggest that productivity remainedlargely unaffected during the last glacial to interglacialtransition (Martı́nez et al. 1999, Spooner et al. 2011), withuniformly oligotrophic conditions present during the lastglacial as well as in the Holocene (Murgese and De Deckker 2007). This is consistent with limited sampling of
TOC at Site U1463, which did not reveal an increase during the last glacial period as compared with the presentday interglacial (Gallagher et al., 2017a, b).
Finally, there were global shifts to lower δ13CDIC valuesof ocean waters during glacial stages as compared withinterglacial stages caused by differences in the terrestrial
storage capacity of 13C (Curry et al. 1988; Duplessy et al.
1988; Peterson et al. 2014). This mechanism is proposed
to modify the δ13CDIC values of the entire ocean by up to
0.5‰ (Curry et al. 1988; Duplessy et al. 1988; Peterson
et al. 2014). However, in shallow tropical oceans (< 500 m
depth), glacial to interglacial changes are more limited,with cited values averaging out at 0.0‰ (McGee et al.
2007; Broecker and McGee 2013).
After 8.5 ka, the ­[CO32–] derived from the δ13C values of
H. inflatus display no significant changes and thus likely
reflect the modern conditions on the NWS. Both deep
water and shelf records of climate display a slight increasein aridity following ~ 6 ka (Kuhnt et al., 2015; Ishiwa et al.,2019; Hallenberger et al., 2019). This trend is not reflectedin ­[CO32–] values (Fig. 6), indicating a potential lack in
sensitivity of H. inflatus proxy data. Values for presentday ­[CO32–] on the NWS have been estimated using thepython toolbox PyCO2SYS (Humphreys et al. 2020) andwere compared with those determined from H. inflatus.
PyCO2SYS solves the carbonate system for all major components, including [­CO32–], if two out of nine defining
parameters as well as temperature and salinity are available(Humphreys et al. 2020). In this study, the carbonate system was resolved by utilizing a combination of alkalinity(GLODAP Station 36379; Key et al. 2004), pH (GLODAP
Station 36379; Key et al. 2004), salinity (WOA Station
278667; Zweng et al. 2019), and H. inflatus based-seawatertemperatures (Fig. 5; Additional file 1: Tables S3–S4). The
upper and lower boundaries of the water depth for whichthe ­[CO32–] was calculated are defined by the estimatedcalcification depths of H. inflatus (95–140 m). Using these
constraints, we calculated a modern [­CO32–] of 158 and211 µmol/kg sw for 140 and 95 m water depth, respectively. In comparison, ­[CO32–] determined from H. inflatus

Page 11 of 15

at ≤ 8.5 ka are 160–200 µmol/kg sw, with an average of
185 µmol/kg sw (Fig. 6; Additional file 1: Tables S3–S4).
Therefore, the calculated present-day ­[CO32–] are the sameas those determined from the H. inflatus data.
5.3 
Limacina dissolution index as a shallow‑marine proxy

The LDX was originally utilized to track the locationand past variations of the aragonite lysocline (Gerhardt
et al. 2000; Gerhardt and Henrich 2001). Based on its
depth, pteropods undergo strong corrosion at the sediment–water interface (Gerhardt and Henrich 2001).
Recent research has expanded the application of the LDX
to glacial to interglacial changes in Ωarag of ocean waters
caused by fluctuations in atmospheric ­CO2 (Wall-Palmer
et al. 2013, 2014; Sreevidya et al. 2019) or changes inwater mass properties (Klöcker et al. 2006). Better preservation typically characterizes glacial stages, when lower
atmospheric ­CO2 results in higher Ωarag (Wall-Palmer
et al. 2013, 2014). Unlike its original application, dissolution is proposed to occur “in-life” due to the pteropods
interacting with surrounding ocean water (Wall-Palmer
et al. 2013, 2014). Comparable features have beenreported for modern pteropods, where in-life shell dissolution has been linked to anthropogenic changes inatmospheric ­CO2 (Orr et al. 2005; Roger et al. 2012).
For shelf sediments offshore of northwestern Australia,
LDX values remain low regardless of the post-glacial
increase in global atmospheric ­CO2 (Fig. 6). The pristinecondition of most pteropods implies a near absence of inlife dissolution or dissolution at the sediment–water interface. LDX values have also been used to track the intensityof aragonite dissolution in marine porewaters during shallow burial (Reuning et al. 2022). They have shown that
dissolution of relatively Sr-rich aragonite, as indicated byincreased LDX values, leads to a concomitant increase in
Sr ion concentrations in the porewater. The slight increaseof LDX values at ca. 8–6 ka (Fig. 3) was not associated withan excess Sr concentration in a porewater sample fromthis interval (Gallagher et al., 2017a, b). As such, there isno evidence for aragonite dissolution during burial in thestudied section. Instead, the overall excellent pteropod
preservation (Fig. 6) indicates that the shallow-marine
environment of the NWS was characterized by protracted
elevated Ωarag. This is supported by the strong aragonite
supersaturation on the modern NWS (Ωarag ≈ 4) (Rogeret al. 2012). In the past, reduced amounts of atmospheric­CO2 (Monnin et al. 2001), as well as elevated regional aridity (Hallenberger et al. 2019; Ishiwa et al. 2019), are likely
to have resulted in even higher levels of supersaturation.
An exception was recorded for the period between 8
and 6 ka, when the preservation of pteropods was slightly
deteriorated at Site U1463 (Fig. 6). This was broadly coeval
with the recorded decrease in carbonate ion concentrations

Hallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

(ca. 8.5 ka), implying that the lower [­CO32–] may haveaffected shell preservation (Fig. 6). The duration of thisperiod matches a phase of elevated humidity reported
for the NWS and Western Australia (Van der Kaars et al.
2006; Hallenberger et al. 2019; Ishiwa et al. 2019; Pei et al.
2021a, b). It is possible that the associated increase in riverine influx and precipitation led to a decrease of the Ωaragin the shelf waters, resulting in the slight corrosion of thepteropods formed in this period. After ca. 6 ka, the climate
proxies record a gradual return to more arid conditions innorthwestern Australia (Kuhnt et al. 2015; Hallenberger
et al. 2019; Ishiwa et al. 2019). Consequently, pteropod
preservation again became pristine. However, such interpretations should be made with caution as there are no
similar trends recorded for pteropods at Site U1461 (Fig. 6).

6 Conclusions
A 16-kyr-long sedimentary record from the NWS wasstudied to determine past ocean water conditions, basedon different proxy records from H. inflatus. δ18O-based
calcification depths indicate that H. inflatus records ashallow-marine shelf signal, with comparable conditions
between 95 and 140 m depth on the modern NWS. A
direct comparison with different archives further revealed
that H. inflatus calcified at lower temperatures than theupper thermocline dwelling Globorotalia menardii, but at
higher temperatures (post-10 ka) than benthic foraminifera of the genus Uvigerina. This places lower (Site U1463
seafloor; 147 m depth) and upper (upper thermocline)
boundaries on the calcification depth of H. inflatus. Basedon these conditions, a preferential calcification in the
nutrient-rich thermocline is proposed for the NWS. Thisconclusion is supported by an overall lack of temperature changes since 16 ka, which is analogous to regional
foraminifera-based thermocline records.
Past variations in shallow-marine ­[CO32–] and the saturation state of aragonite were tracked using the LDX andδ13C values of H. inflatus. LDX values indicate limitedchanges during the last 16 kyr, irrespective of the coeval
global increase in atmospheric ­CO2 and/or regional fluctuations in the hydrological cycle. As such, it is likelythat the sensitivity of the LDX proxy is not sufficient torecord glacial to interglacial changes in the saturation
state of aragonite (Ωarag) in a highly supersaturated shallow-marine environment, such as the NWS. In comparison, the δ13C values of H. inflatus showed a post-glacialdecrease in [­CO32–] at ca. 8.5 ka. This can be linked tothe proposed onset of monsoonal rainfall and humid
conditions on the NWS. It further coincides with the previously recorded shelf-wide cessation of inorganic aragonite production. Our study supports the hypothesis thatclimate-related changes in aragonite saturation states

Page 12 of 15

were responsible for the cyclical change in carbonate
facies deposition across the extensive ramp system.
Abbreviations
NWS: Northwest Shelf of Australia; LDX: Limacina Dissolution index; IODP:International Ocean Discovery Program; ITF: Indonesian Throughflow; SST:
Sea surface temperature; CSF-A: Core depth below seafloor; CCSF-A: Core
composite depth below seafloor; AMS: Accelerator mass spectrometry; δ13C:
Carbon isotope ratio; δ18O: Oxygen isotope ratio; δ18Oeq: δ18O of carbonates
precipitating in equilibrium with seawater; δ18Osw: δ18O of seawater; [CO32–]:
Carbonate ion concentration; sw: Seawater; Ωarag: Saturation state of aragonite; δ13CDIC: δ13C of dissolved inorganic carbon (DIC).

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00507-1.
Additional file 1: Table S1. Limacina dissolution index and carbon andoxygen isotope data for pteropods (Heliconoides inflatus) from IODP Site
U1461. Table S2. Limacina dissolution index and carbon and oxygenisotope data for pteropods (Heliconoides inflatus) and benthic foraminifera (Uvigerina spp.) from IODP Site U1463. Table S3. Calculated watertemperatures and carbonate ion concentrations based on the δ13C andδ18O values of pteropods (Heliconoides inflatus) from IODP Site U1461.
Water temperatures were calculated based on a linear regression devisedfor generic aragonitic marine organisms, which calcify in equilibrium with
δ18Osw (Böhm et al, 2000). Carbonate ion concentrations are based on
the species-specific correlation presented by Keul et al. (2017). Table S4.
Calculated water temperatures and carbonate ion concentrations basedon the δ13C and δ18O values of pteropods (Heliconoides inflatus) fromIODP Site U1463. Water temperatures were calculated based on a linear
regression devised for generic aragonitic marine organisms, which calcify
in equilibrium with δ18Osw (Böhm et al, 2000). Carbonate ion concentrations are based on the species-specific correlation presented by Keul et al.
(2017). Fig. S1. Seasonal profiles of water temperature (a.1–a.4), watertemperature gradient (b.1–b.4), and salinity (c.1–c.4) at Site U1461 (gray;Station 273179) and Site U1463 (black; Station 278667). All temperatureand salinity data were derived from the World Ocean Atlas 2018 (Locarnini
et al. 2018). Also shown are profiles of the δ18O values of seawater (δ18Osw;
d.1–d.4) derived from a regional calibration of the seawater salinity (Peiet al. 2021a, b).
Acknowledgements
This research was conducted on samples collected during IODP Expedition
356 (Indonesian Throughflow) and was supported by a grant from the German Science Foundation (DFG; RE 2697/4-1) and the Japan Society for the
Promotion of Science (JSPS; PE19752). We thank the JOIDES Resolution Science Operator staff and Siem Offshore crew for their invaluable assistance andskill during IODP Expedition 356. Finally, we would like to thank the staff of the
Kochi Core Center, Japan, for sampling and shipping the sedimentary samples
on which this research is based.
Author contributions
MH acquired the LDX and isotope data, analyzed the results, and drafted theoriginal manuscript. NK supervised and reviewed the analysis of the pteropod
proxy data. HT supervised the acquisition and processing of the C–O isotope
data. LR and YI conceptualized and supervised the study and reviewed themanuscript. TI and YY acquired the radiocarbon data. All authors read andapproved the final manuscript.
Funding
Open Access funding enabled and organized by Projekt DEAL. This research
was supported by a Postdoctoral Fellowship for Research in Japan (Grant
PE19752) to MH from the Japan Society of the Promotion of Science (JSPS).
MH was further funded by a German Science Foundation grant (DFG Project320220579) awarded to LR.

Hallenberger et al. Progress in Earth and Planetary Science(2022) 9:49

Availability of data and materials
The datasets supporting the conclusions of this article are included within thearticle and its additional files.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Energy and Mineral Resources Group (EMR), Geological Institute, RWTH
Aachen University, Wuellnerstrasse 2, 52062 Aachen, Germany. 2 Instituteof Geosciences, CAU Kiel, Ludewig‑Meyn‑Strasse 10, 24118 Kiel, Germany.
3
Institute of Geology and Paleontology, Graduate School of Science, Tohoku
University, Aramaki‑aza‑aoba 6‑3, Aoba‑ku, Sendai 980‑8578, Japan. 4 National
Institute of Polar Research, Midori‑cho 10‑3, Tachikawa‑shi, Tokyo 190‑8518,
Japan. 5 Atmosphere and Ocean Research Institute, The University of Tokyo,
Kashiwanoha 5‑1‑5, Kashiwa‑shi, Chiba 277‑8564, Japan.
Received: 27 January 2022 Accepted: 19 September 2022

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