Nitzsche et al.
Progress in Earth and Planetary Science(2022) 9:58
https://doi.org/10.1186/s40645-022-00517-z

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Metal contamination in a sediment corefrom Osaka Bay during the last 400 years
Kai Nils Nitzsche1,8* , Toshihiro Yoshimura1, Naoto F. Ishikawa1, Hiroto Kajita1,2,3, Hodaka Kawahata3,4,
Nanako O. Ogawa1 , Katsuhiko Suzuki5 , Yusuke Yokoyama1,4,6,7 and Naohiko Ohkouchi1   

Abstract
Osaka Bay adjacent to the Kyoto–Osaka–Kobe metropolitan area was affected by severe metal pollution during thetwentieth century; yet little is known about the trace metal sources and pre-industrial human activities. We havedetermined the elemental concentrations and zinc stable isotope ratios (δ66Zn) in bulk sediments and the trace metal
concentrations in chemical fractions of a 9-m-long sediment core from Osaka Bay. Our goals were (1) to reconstruct
the historical trace metal contamination, and (2) to identify anthropogenic Zn sources and the solid phases of anthropogenic trace metals. The core provided a continuous environmental record of the last 2300 years based on radiocarbon dating of molluscan shells. Copper, Zn, and Pb showed an initial enrichment from the 1670s AD, which could
be caused by human activities due to an increasing population. In agreement with previous findings, the trace metal
concentrations slightly increased from the 1870s, strongly increased from the beginning of the twentieth century,and peaked around 1960 before environmental pollution control laws were enacted. Increasing trace metal concentrations in the acid-labile and reducible fractions obtained by the Community Bureau of Reference (BCR) sequential
extraction procedure toward the surface indicate carbonates and Mn oxyhydroxides were the primary fractions foranthropogenic trace metals. The δ66Zn values (1) were constant until the 1940s, suggesting that the average δ66Zn
of industrial sources was indistinguishable from that value of the natural background, (2) showed a slight decrease
from the 1950s and remained constant until the present, and (3) fell in a binary mixing process between a lithogenic(~ + 0.27‰) and an anthropogenic endmember (~ + 0.17‰), the latter likely representing a mixture of various Zn
sources such as road dust, tire wear, industrial effluents, and effluents from wastewater treatment plants. We conclude
the combination of Zn stable isotopes together with chemical fractions obtained by the BCR method represents apromising approach to assess the trace metal sources and their potential mobility in sediment cores from anthropogenically affected coastal areas.
Keywords: Trace metal, Enrichment factor, Zinc isotope, Osaka Bay, Sediment core, Sequential extraction
1 Introduction
Coastal bays adjacent to large cities are prone to trace
metal contamination from various points and diffusive
sources originating in their catchments, posing a threat
to aquatic life (Barletta et al. 2019; de Souza Machado*Correspondence: nitzsche@geo.tu-darmstadt.de1
Biogeochemistry Research Center, Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC), 2‑15 Natsushima‑Cho, Yokosuka, Kanagawa
237‑0061, Japan
Full list of author information is available at the end of the article

et al. 2016). Osaka Bay in the eastern part of the Seto
Inland Sea is located directly next to Osaka City, whichis part of the Kyoto–Osaka–Kobe metropolitan area,
the second largest metropolitan area in Japan. Rapid
industrialization and urbanization paired with lacking environmental standards for industrial wastewater,and emissions led to a dramatic increase in the trace
metal concentrations from the beginning of the twentieth century (Hosono et al. 2010; Yasuhara and Yamazaki2005). Yet, little is known about the metal sources, the© 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/.

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

speciation of trace metals in the sediment, and preindustrial metal pollution.
Early human activities such as mining, smelting of
weapons, and artifacts for worshipping can be trace metalsources. Pre-industrial mining and metallurgy could betraced in lake sediment cores from Europe and the SouthAmerican Andes (Cooke and Bindler 2015). Zong et al.
(2010) suggested smelting of metal tools increased the Cu
and Pb concentrations in the sediment of the river mouthof the Pearl River 2000 years ago. Urban activities and theconstruction of the Great Buddha statue in ancient Nara,
Japan, during the eighth century increased the Cu, Hg,
and Pb concentrations in adjacent soils (Kawahata et al.
2014). Osaka, formerly known as Naniwa, has a long history dating back to the Kofun period (250–592 AD) andwas the scenery of repeated warfare during the Sengoku
period (1467–1615 AD) (see Sect. 2.3); thus, humanactivities such as smelting presenting could be expected
as potential metal sources.
Trace metals deposited into the aquatic environment
are present in different solid phases in the sediment.
The trace metals in these phases can be extracted using
the Community Bureau of Reference (BCR) sequential
extraction procedure (Rauret et al. 1999), which provides
insights into their bioavailability, potential mobility, andtransferability. Yasuhara and Yamazaki (2005) suggested
metal pollution strongly contributed to the decline inostracode absolute abundance in a core close to the Yodo
River mouth at the inner part of the bay from the early
1910s to the 1970s AD. Similarly, metal pollution was
believed to have contributed to the decline in marineorganisms at other sites in the Seto Inland Sea (Irizuki
et al. 2018). Thus, trace metal chemical fractions could
provide insights into potential toxicological effects on
marine biota. Recently, Tonhá et al. (2020) showed the
combination of the trace metals extracted by the BCR
method with zinc stable isotopes (δ66Zn) was a promising tool to trace Zn contamination and redistribution in
sediments from coastal Sepetiba Bay, Brazil.
Industrial processes such as redox reactions, evaporation (e.g., during smelting, and coal combustion), anddissolution (e.g., in wastewater treatment plants) causea significant Zn isotopic fractionation (c.f. Desaulty andPetelet-Giraud 2020). Often, the δ66Zn values of theresulting Zn (by)products differ from that of the Upper
Continental Crust (UCC; + 0.28 ± 0.05 ‰, 2σ) (Chenet al. 2013). Typical contemporary anthropogenic materials with low δ66Zn values are road dust (+ 0.08 ‰to + 0.17 ‰) and tire wear (+ 0.08 ‰ to + 0.21 ‰) (Donget al. 2017; Souto-Oliveira et al. 2018; Thapalia et al.
2010), and industrial waters and effluents from wastewater treatment plants (− 0.03 ‰ to + 0.15 ‰) (Chen et al.
2008; Desaulty and Petelet-Giraud 2020; Sakata et al.

Page 2 of 192019). Anthropogenic materials with high δ66Zn values
are slags and effluents from smelters (up to + 1.5 ‰) (Juillot et al. 2011; Sivry et al. 2008) and Zn in electroplating
wastes after electrochemical reduction (up to + 3.5 ‰)(Kavner et al. 2008). As such, combining δ66Zn values
with changes in Zn concentrations has become a popular
tool for tracing anthropogenic Zn in sediment cores fromcoastal areas throughout the Anthropocene time (Araújo
et al. 2017, 2019a; b; Sakata et al. 2019; Tonhá et al. 2020).
In this study, we have investigated the temporal variations in the weathering intensities, and the trace metal
concentrations in bulk sediments and chemical fractions and Zn stable isotope ratios in a 9-m-long sediment core from Osaka Bay. Owing to the long history of
Osaka (Naniwa), we aimed for reconstructing the historical trace metal contamination. In addition, we compare
the historical trace metal contamination with sedimentcores from other sites in the Seto Inland Sea, and we
assess potential toxic effects on marine biota. Sakata et al.
(2019) suggested the contribution of Zn discharged fromelectroplating plants led to increasing δ66Zn values in acore from Tokyo Bay from the early 1950s. Such a pattern may also exist for Osaka Bay. Thus, our second goal
was to identify the anthropogenic Zn sources and thesolid phases of anthropogenic trace metals. We show thatthe combination of Zn stable isotopes with trace metalchemical fractions obtained by the BCR method represents a powerful tool to study the anthropogenic Znsources and their potential bioavailability.

2 Study area and materials
2.1 Osaka Bay and its catchment

Osaka Bay is located at the eastern end of the Seto Inland
Sea in southwestern Japan (Additional file 1: Fig. S1). The
bay covers approximately 1500 ­km2 and has a mean depthof approximately 30 m and is partly enclosed by the Awaji
Island in the West. The whole catchment of the bay covers a total area of approximately 10,700 ­km2 and it consists of several subbasins. The Yodo River in the Osakaplain is the primary freshwater source with an annual
estimated discharge of 5.14 ­km3 ­year−1 and a catchment
area of 8240 ­km2 including the catchment area of Lake
Biwa (3170 ­km2). Minor rivers are the Yamato River(0.43 ­km3 ­year−1, 1070 ­km2), the Muko River (496 ­km2),
and the Ina River (383 ­km2). In addition, numeral smaller
rivers with steep gradients have their origins in Mount
Rokkō northeast of the lowlands near Kobe City and in
the Kongō Range in the South.
The geology of the whole catchment is complex consisting mainly of Quaternary unconsolidated sedimentsof alluvial plain and terrace deposits primarily of fluviolacustrine origin (Itihara et al. 1988), and clastic(meta)sedimentary rocks of the Jurassic and CretaceousNitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

accretionary complexes (Fig. 1). Cretaceous granitoid
rocks are mostly exposed in the eastern part of thecatchment. Felsic volcanic and volcanoclastic rocks
mostly occur in the northwest and in the southeast.
Gneisses and schists of the Ryoke Belt, Carboniferous
to Permian limestones and basalts, and Cretaceous gabbros are mostly present in the eastern part.
Large cities in the direct vicinity are Osaka, Kobe,
Sakai, Nishinomiya, and Amagasaki, while Kyoto islocated further northeast. Consequently, large urban
and build-up land areas are present in the catchment(Fig. 1).

Page 3 of 19

2.2 Sediment sampling

A piston core and a multiple core were collected at station
OS5B from the central part of Osaka Bay (34° 31.632 N,
135° 12.606 E) at a water depth of 24 m during cruise
KT-11-13 on July 1, 2011 (Additional file 1: Fig. S1). The
piston core consists of gray homogenous muddy sediments with no distinguishable substantial changes in thesedimentary facies as shown by observations of the continuous section of the core using X-ray computed tomography (CT). The core was carefully cut in 2 cm slices fromthe surface to the final core depth of 884 cm. The multiple
core was 33 cm long and was cut into 1 cm slices.

Fig. 1 Location of the Osaka Bay catchment in Japan (left). Geological map of the Osaka Bay catchment (top) and land-use map (bottom) withsubbasins of the Yodo, Muko, and Yamato Rivers, other major rivers, the locations of the sediment cores collected at station KT-11-13 OS5B, and of
Osaka Castle. The geological information was obtained from the Seamless Geological Map (Geological Survey of Japan 2015). Land-use data wereobtained from Land Use Fragmented Mesh Version 2.5.1 from the National Land Numerical Information, created by Ministry of Land, Infrastructure,
Transport and Tourism, the Government of Japan

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

Page 4 of 19

2.3 Historical events in the catchment

2.3.2 Transition (Kofun) period (ca. 250 to 592 AD)

In the following, we summarize major transformations of
the Osaka Bay catchment and human activities that havepotentially affected the sedimentation rates, the sedimentsources, the weathering intensities, and the trace metal
input into the bay.

During the fifth century AD, a canal, Naniwa no Horie,
was dug through the sandbar which connected the sea
with Kawachi Lake (Fig. 2). A port facility, called Naniwatsu, with large warehouses was constructed at the tip of
the Uemachi Plateau, which allowed for trade with the
Chinese mainland and with the Korean peninsula, promoting the growth of the city. Furthermore, huge Mozu
tombs (Kofun) were constructed.

2.3.1 Yayoi period (ca. 930 BC to 250 AD)

The sea level changes and the evolution of Osaka Bay
were firstly summarized by Kajiyama and Itihara (1972).
In the present study, we refer to more recent data on theevolution of Osaka Bay (Matsuda 2008). During the Yayoi
period, large parts of the present Osaka city were underwater (Fig. 2). Osaka Bay reached further east to the
Uemachi Plateau, a peninsula connected to the landmassin the South. The brackish Kawachi Lagoon was locatedbetween the Uemachi Plateau in the West and the Ikoma
mountain range in the East. The Yodo River in the Northeast and the Yamato River (formerly called Nagase River)in the Southeast emptied into the bay creating alluvial
fans. Eventually, due to a growing sand bar north of the
Uemachi Plateau, Kawachi Lagoon turned into a freshwater lake from 400 BC to around 1 BC. From as earlyas 700 BC, rice cultivation started leading to permanent habitation on the Uemachi Plateau and around the
Kawachi plain (c.f. Pearson 2016).

2.3.3 Imperial and aristocracy period (592 to 1185 AD)

Owing to the increasing sedimentation and propagationof the Yodo and Yamato Rivers by the beginning of the
Heian period (from 794 AD), the original Kawachi Lake
had largely disappeared and turned into a fertile marshland (Kawachi plain) with smaller lakes and numerous
rivers suitable for rice cultivation. Naniwa-tsu continued
to grow as an urban center including the digging of canals
and rice irrigation. However, from the ninth century AD
Naniwa lost its importance for trade.
2.3.4 Feudal period (1183 to 1868 AD)

During the Kamakura (1183–1333) and Muromachi
periods (1336–1573), the development of rice fields
continued. From 1570 to 1580, Ishiyama Hongan-ji, an
important temple constructed in 1496, was besieged

Fig. 2 Kawachi Lake during the third and fourth centuries AD with location of the inset in the Osaka Bay catchment. The locations of the Naniwa
no Horie canal, the port Naniwa-tsu (constructed in the fifth century AD), of Naniwa Palace, and of the present Yodo River and Yamato River are also
shown. Modified after Kajiyama and Itihara (1986), Kajiyama and Itihara (1972), and Kusaka (2012)

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

by Nobunaga Oda. In 1583, the construction of Osaka
Castle began, which was completed in 1597 by Hideyoshi Toyotomi on the grounds of the former Ishiyama
Hongan-ji. Several flood control measures around Osaka
Castle, Yodo River, and its tributaries, were initiated by
Hideyoshi Toyotomi. Following the destruction of Osaka
Castle in 1615 at the beginning of the Edo period (1603–
1868), Osaka became an important economic hub. In
1704, the rerouting of the Yamato River into Osaka Bay
was completed (Fig. 2). As a consequence, the Kawachi
plain (the current Osaka plain) was spared from flooding,leading to the formation of new arable land for rice andcotton cultivation.
2.3.5 Modern period (1868 AD to present)

During the early Meiji era, large parts of the Osaka plain
were still used for rice cultivation (Additional file 1: Fig.
S2), but most agricultural lands disappeared during thefollowing decades. Following the opening of the Osaka
harbor in 1868, the Modern period began, and the industrialization was initiated. Rapid industrialization, urbanization, and population growth took place from about
1890 (the start of Japan’s industrial revolution) and during Japan’s post-war period of economic growth (1955–
1973). Furthermore, the Osaka metropolitan area washeavily bombed in 1945.

3 Material and methods
3.1 Core dating

Twenty-seven molluscan shells were selected from thepiston core for age dating. They were dated via radiocarbon measurement by accelerator mass spectrometry (AMS) at the Micro Analysis Laboratory of the
University of Tokyo (Matsuzaki et al. 2004; Yokoyama
et al. 2019). The employed technique for sample preparation was reported previously (Yokoyama et al. 2007),
yet described briefly as follows. After ultrasonic cleaning, approximately 15 mg of shell samples was weighed,and weak acid was added to remove secondary C­ aCO3(> 15% of outer shell weight). After vacuum drying overnight, the cleaned sample was converted to C
­ O2 withphosphoric acid under vacuum conditions. The product­CO2 was passed through a vacuum line and target graphite was formed on iron powder in a hydrogen atmosphereheated at 650 °C. The graphite was pressed into target
holders and analyzed via AMS.
All radiocarbon age results were calibrated to calendar years using the OxCal ver. 4.4 software (Bronk
Ramsey 2009) with the Marine 20 dataset (Heaton et al.
2020). A regional-specific reservoir (DR) correction of
135 ± 20 years was used, which was estimated for Beppu
Bay, located in the Seto Inland Sea, by Kuwae et al.
(2013).

Page 5 of 19

3.2 Total organic carbon and total nitrogen analysis

The total organic carbon (TOC) and total nitrogen (TN)concentrations were determined from 0 to 40 cm core
depth of piston core (n = 20), and for each cm until 19 cm
and then for every second cm until 33 cm core depth ofthe multiple core (n = 26), respectively. With respect tothe multiple core, the TOC concentrations were determined after weighing approximately 20 mg of powdered sample into a silver capsule, decalcification with
1 M HCl, and drying at 80 °C, using a Flash 2000 CHNS
elemental analyzer at the Geological Survey of Japan,
Tsukuba (National Institute of Advanced Science andTechnology). The TN concentrations were determinedon the unacidified samples. For the piston core, the TOC
and TN concentrations were determined after weighing
samples into pre-cleaned smoothed wall tin capsules,
decalcification with 0.5 M HCl, and drying on a hotplate
at 80 °C. The tin capsules with the dried samples weresealed and analyzed by a sensitivity-improved elemental
analyzer (Flash EA1112, Thermo Finnigan, Bremen, Germany) connected to an isotope ratio spectrometer (Delta
plus XP, Thermo Finnigan, Bremen, Germany) at the Biogeochemistry Research Center, JAMSTEC, according to
Ogawa et al. (2010).
3.3 Grain size analysis

The mean grain size of 100 selected horizons of the piston core was analyzed on a Mastersizer 2000 laser diffraction particle size analyzer (Malvern Panalytical, UK)at Kochi Core Center, JAMSTEC. The wet sediment samples were pretreated with 1.2 M HCl and 2 M N­ a2CO3 toremove carbonates and biogenic opal, respectively. Afterchecking under the microscope for any remaining biological source particles, the samples were dispersed with
0.01 M ­(NaPO3)6.
3.4 Digestion of the bulk sediment

For chemical analyses of the piston core, subsamples ofeach horizon (without the surface sediment from 0 to2 cm) were taken until 132 cm core depth, every 10 cm
until 482 cm, and every 50 cm until 878 cm. The subsamples were freeze-dried, and the < 125 µm fraction was
manually dry-sieved and powdered. The powdered bulk
sediment samples were digested with H­ NO3–HF–HClO4
similar to the protocol described in Yokoyama et al.
(1999). The samples were evaporated to dryness and redissolved in 1 M ­HNO3 for further analysis.
3.5 Sequential extraction

We determined the distribution of trace metals in the
acid-soluble (F1), reducible (F2), oxidizable (F3), andresidual (F4) fractions of 56 samples using a modification of the revised Community Bureau of ReferenceNitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

(BCR) method (Table 1) (Rauret et al. 1999; Sahuquillo
et al. 1999). In the current study, single extractions
were performed using a 100 mg sample while maintaining the same sample/solution ratios, the supernatant
was removed by pipetting, and the residue was washedthree times with methanol and evaporated to dryness at
60 °C. To ensure the complete reaction of the acid-soluble fraction with acetic acid, step F1 was repeated as
we observed a decline in the pH value after the first F1
extraction indicating that carbonates were still present.
To account for exchangeable metals, ten samples covering a wide range in bulk Cu, Zn, and Pb concentrations
were extracted with 1 M N­ H4Cl and shaken for 3 h. Replicate extraction (n = 4) of the Tokyo Bay sediment JMS-1
standard yielded an uncertainty of typically < 5% (1σ) for
Cu, Zn, Pb, Co, and Ni.

Page 6 of 19

conditions with low-resolution mode using a Neptune Plus Multicollector ICP-MS (Thermo Scientific,
Bremen, Germany) at the Submarine Resources Center,
JAMSTEC. The samples were analyzed using the standard–sample–standard bracketing method with the newly
developed AA-ETH standard (Archer et al. 2017). Specifically, three separate analyses of the same sample solution
were conducted, for which uncertainties were reported
as two standard deviations (2σ). The instrumental massfractionation was corrected using Cu-doping (Maréchal
et al. 1999) after adjusting the Cu/Zn ratios to 1:1 inthe sample solutions. To allow for comparison with thepreviously published literature, the 66Zn/64Zn ratios are
expressed in delta notation relative to the JMC-LYON
standard according to the following Eq. (1):66

66 Zn/64 Zn

sample3.6 Chemical and isotopic analyses

δ ZnJMC - LYON =Aliquots of digested bulk sediments and of chemical fractions were analyzed for elemental concentrations (Co,Ni, Cu, Zn, Pb, Na, K, Ca, Al, Fe, Ti, Mn, Ba, Sr) using aquadrupole inductively coupled plasma mass spectrometry (iCAP Q ICP-MS; Thermo Scientific, Bremen, Germany) at JAMSTEC. The analytical precision (1σ) basedon replicate analysis of the JMS-1 standard during themeasurements was typically < 3%.
Zinc was purified from solutions that contained 2.4 µg
Zn with the BioRad anion-exchange resin AG1-X8(200–400 mesh) in Cl form using a modified version of
the protocol described in Sossi et al. (2015). Briefly, Zn
was eluted with 2 mL of Milli-Q water after washing theresin with 5 mL of 8 M HCl, 5 mL of 3 M HCl, and 4 mL
of 0.4 M HCl. To ensure Zn solutions were free of any
matrix elements (e.g., Al, Ti), the separation was repeated
one more time. The final Zn extract was evaporated todryness, re-dissolved in 0.5 mL ­HNO3 and 0.05 mL ­H2O2
and heated at 140 °C to digest organics from the resin,
again evaporated to dryness, and finally re-dissolved in
2% ­HNO3. The Zn recovery was 99 ± 6% based on the Zn
concentrations analysis of the Zn solutions.
The Zn concentration in the purified samples was
adjusted to 300 ppb. The Zn stable isotope ratios
were measured on these solutions under wet plasma(1)
An error propagation associated with the conversion of
δ66ZnAA-ETH to δ66ZnJMC-LYON was not performed as theanalytical uncertainty was usually greater than the error
related to the conversion. The quality control of the δ66Zn
values was assured by analyzing the NIST 682 highpurity Zn standard. The repeated measurement of NIST
682 yielded a δ66Zn value of − 2.42 ± 0.04 ‰ (n = 12, 2σ),which corresponds to values reported elsewhere (Conway et al. 2013; John et al. 2007). Furthermore, the accuracy of the measurements was assessed by analyzing thereference sediments JLk-1 (Lake Biwa), JMS-1 (Tokyo
Bay), and MESS-4 (Beaufort Sea, Arctic Canada), which
underwent the same purification protocol as the samples. The repeated measurement of JLk-1 yielded a δ66Zn
value of +0.27 ± 0.03 ‰ (n = 11) and that of JMS-1 was+0.29 ± 0.04 ‰ (n = 10). The δ66Zn value of MESS-4 was+0.26 ± 0.05 ‰ (n = 8), which agreed well with a previously reported value of +0.26 ± 0.02 ‰ (n = 4, 2σ) (Jeonget al. 2021).

66 Zn/64 Zn· 1000 + 0.28

AA - ETH

3.7 Enrichment factor and chemical alteration index

The enrichment factor (EF) of the trace metals was calculated as an index for the anthropogenic contamination

Table 1 Overview of the sequential extraction procedure with nominal target phases
Step

Fraction

Nominal target phases

Extractant

1

Acid soluble

Soluble and exchangeable species, carbonates

0.11 M acetic acid (repeated)

2

Reducible

Fe and Mn oxyhydroxides

3

Oxidizable

Organic matter and sulfides

0.5 M hydroxylamine hydrochloride, pH = 1.5

4

Residual

Silicates and well-crystalline oxides

8.8 M hydrogen peroxide followed by 1 M
ammonium acetate, pH = 2

Nitric acid, hydrofluoric acid, perchloric acid

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

and to account for textural differences across the samples(Andrews and Sutherland 2004; Chen et al. 2007). For
this purpose, the concentration of a trace metal of interest (M) is normalized to the concentration of a conservative element according to Eq. (2): MAl sample

EFM =  

MAl UCC(2)

Aluminum was chosen as the conservative element
because Al is widespread in aluminosilicate minerals andrelatively immobile during most weathering regimes. The
concentration ratio of the sample (M/Alsample) was thennormalized to that ratio in the Upper Continental Crust(M/AlUCC​) using the following elemental concentrationsof the UCC reported by Rudnick and Gao (2014): Al
81,497 µg ­g−1, Zn 67 µg ­g−1, Cu 28 µg ­g−1, Pb 17 µg ­g−1,
Co 17.3 µg ­g−1, and Ni 47 µg ­g−1. The EF was interpreted according to Chen et al. (2007): EF < 1: no enrichment, EF < 3 minor enrichment, EF = 3–5 moderateenrichment, EF = 5–10 moderately severe enrichment,
EF = 10–25 severe enrichment, EF = 25–50 very severeenrichment, and EF > 50 extremely severe enrichment.
The chemical index of alteration (CIA) of the residual
fraction (F4) was calculated as a measure of the degreeof weathering that rocks have experienced (Nesbitt andYoung 1982) according to Eq. (3):
Al2 O3· 100 %CIA =Al2 O3 + CaO + Na2 O + K2 O
(3)The CIA usually ranges from about 50 for fresh rocks
comprising “primary minerals” to around 100 for completely weathered rocks comprising secondary clay minerals (Young and Nesbitt 1998).
3.8 Statistical analyses

The Pearson product–moment correlation analysis wasused to explore relationships between the Zn concentrations and the Zn stable isotope ratios. We explored
relationships between the metal concentrations of thecompositional (closed) bulk elemental data by performing a Spearman product–moment correlation analysis of symmetric coordinates that were obtained after
an isometric log-ratio (ilr) transformation of the data.
Using symmetric coordinates for correlation analysis provides a better representation of the relationshipbetween two variables than classical correlation analysis between two pairs of variables as the latter neglects
the influence of all other variables (Garrett et al. 2017;
Kynčlová et al. 2017; Reimann et al. 2017). The statistical analyses were performed using R (version 4.1.2, R

Page 7 of 19

Foundation for Statistical Computing, Vienna, Austria,
http://​w ww.R-​proje​ct.​org/). Spearman correlation coefficients were computed for the symmetric coordinatesof the data using the function “gx.symm.coords.r” fromthe R package “rgr” (Garrett 2018). The Spearman correlation coefficients were visualized in a heatmap withdifferent colors for positive and negative correlations
after arranging the columns and rows using hierarchical clustering (average linkage using Euclidean distances) with distances based on the correlations assuggested by Reimann et al. (2017). This allows for visualizing clusters of positive and negative correlations,
respectively. The heatmap was drawn with the function“heatmap.2” from the package “gplots.”

4 Results
4.1 Age model

The age model for the piston core was constructedusing a P-sequence deposition model (Bronk Ramsey
2008; Bronk Ramsey and Lee 2013), which was implemented in the computer program OxCal. In constructing the age model, we excluded five 14C age values
based on relatively poorly preserved shell samples andthe 14C value that exceeded 1950 AD when calibrated(Table 2). The boundary of the change in the sedimentaccretion rate (SAR) was set to a core depth of 310 cm
because the slope between the uncalibrated 14C age andthe core depth was changing, and the mean grain size
showed a slight decrease from 270 cm (65 ± 7 µm, average ± standard deviation, n = 31) compared to greater
depths (73 ± 8 µm, n = 70) (Fig. 3). The drop in the
SAR could be caused by changes in the coastal currents(Yasuhara et al. 2002).
The piston core provided a continuous environmental
record of the last 2300 years as shown by the calendar
age-depth profile (Fig. 3). The calibrated age of a shellat 16 cm depth exceeded 1950 AD indicating that thissample was affected by the radiocarbon bomb pulse
peaked in 1963 AD in the Northern Hemisphere when
the Partial Test Ban Treaty took effect. To account fora possible loss of the upper part of the piston core, we
compared the TOC and TN concentrations between
the piston core and the multiple core (Additional file 1:
Fig. S3). TOC and TN showed a slight offset between
the piston core and the multiple core of around 8 cm
indicating that approximately the top 8 cm of the piston
core were lost during the sampling. Assuming that theage of the surface layer of the multiple core is 2011 AD,
we interpolated the age of the horizons from 0 to 33 cmof the piston core revealing that 0 cm core depth corresponded to approximately 1997 AD.

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Table 2 Depth and radiocarbon-derived dates (± 1σ) of the molluscan shells retrieved from the piston core. The range of calendar
ages defines a 68.2% confidence interval
Sample typeLab. code

Depth

Conventional 14C age

Calibrated age (unmodeled)

Calibrated age (modeled)(cm)(year BP)(Cal. year BP)(Cal. year BP)− 627 ± 41

Snail

MTC-15894

16

Mollusca

MTC-16218

34

Bivalve

MTC-16219

68

Mollusca

MTC-16220

70

Mollusca

MTC-16221

100

Mollusca

MTC-16222

148

Bivalve

MTC-15951

152

Mollusca

MTC-15896

178

Snail

MTC-16223

212

Bivalve

MTC-15897

222

Echinus

MTC-15952

310

Echinus

MTC-16224

336

Echinus

MTC-15899

338

Echinus

MTC-16225

378

Mollusca

MTC-16226

459

Echinus

MTC-15953

481

Bivalve

MTC-15901

541

Mollusca

MTC-15954

571

Mollusca

MTC-16228

599

Bivalve

MTC-15902

623

Echinus

MTC-16230

677

Mollusca

MTC-16231

689

Bivalve

MTC-15955

691

Mollusca

MTC-15903

777

Echinus

MTC-16232

807

Echinus

MTC-15904

839

496 ± 63898 ± 64399 ± 52887 ± 66

1282 ± 68
857 ± 58

1287 ± 52

1576 ± 65

1442 ± 52

1816 ± 62

2145 ± 65

1946 ± 57

1915 ± 70

2370 ± 652164 ± 592285 ± 54

2379 ± 75

2615 ± 652448 ± 52

2518 ± 71

2455 ± 73

2785 ± 72

2687 ± 56

2810 ± 72

2818 ± 55

Out of calibration

Out of calibration

49 ± 40

21 ± 23217 ± 99

40 ± 30

207 ± 99567 ± 76

178 ± 91570 ± 66826 ± 89703 ± 781075 ± 931413 ± 93

1201 ± 851169 ± 95

1660 ± 105

1431 ± 89

1561 ± 95

1672 ± 114

1959 ± 112

1753 ± 96

1840 ± 114

1763 ± 114

2169 ± 116

2051 ± 106

2198 ± 116

2207 ± 99

Commenta

136 ± 29

143 ± 30265 ± 30466 ± 31–b

582 ± 31714 ± 32751 ± 33

1090 ± 44–b1148 ± 411230 ± 38–b1444 ± 32

1569 ± 31

1632 ± 32–b1740 ± 34

1852 ± 38

1877 ± 39–b2064 ± 49

2128 ± 53

2195 ± 58

a: Excluded from the calculation of the sedimentation age model using P-sequence because the calibrated age exceeded 1950 AD
b: Excluded from the calculation of the sedimentation age model using P-sequence because the analyzed shells are of poor preservation

4.2 Trace metal concentrations and enrichment factorsof the bulk sediment, and chemical index of alteration

The Cu, Zn, Pb, Co, and Ni concentrations remained relatively constant from the bottom of the core (~ 300 BC)
until the 1670s AD (Fig. 4), from when the Cu, Zn, andPb concentrations showed a slight increase (Fig. 5). The
Cu concentrations further increased until approximately
1800 AD. From the mid-1870s, Zn, Cu, Pb, and Co
showed a slight increase until the mid-1930s, from whena rapid increase was observed until approximately1960
AD. The trace metal concentrations then declined until
the mid-1970s from when they remained constant until
approximately 1990 AD. In contrast, Ni showed a substantial increase from the early 1910s until the early
1940s AD, from when a rapid decrease was observed.
Nickel and Co usually showed no enrichment (EF < 1),
Cu and Pb showed a minor enrichment (EF < 3), andZn showed a moderate enrichment (EF < 5) (Fig. 6 andAdditional file 1: Fig. S4). The temporal trends of theenrichment factors generally agree with those of the trace
metal concentrations. The heatmap of correlation coefficients based on the symmetric coordinates showed acluster of Mn, Pb, Cu, and Zn (Fig. 7).
The CIA showed only mineral variation throughout thewhole core (75–78%) (Additional file 1: Fig. S5).
4.3 Trace metals concentrations in the chemical fractions

The temporal trends of the Cu, Zn, and Co concentrations in the acid-labile, reducible, and oxidizable fractions
generally agreed with those in the bulk sediments, i.e., Cuand Zn showed an initial increase from the 1670s AD, aslight increase from the mid-1870s and a strong increase
from the mid-1930s peaking around 1950 (Fig. 8). In
case of Pb, the reducible fraction reflected the temporal
trend of the bulk sediments. For Ni, a rapid increase wasobserved for the residual fraction from the early 1910s,

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 3 a Core depth (cm) versus calibrated age of the piston core. For each 14C age controlling point, P-sequence modeled and unmodeled age
probability distributions are shown. The error bars represent the 1σ error of the P-sequence modeled age. b Average grain size

decreasing from the early 1940s, while the other fractions
showed only marginal changes.
Owing to the temporal trends in the chemical fractions, the proportion of trace metals in the different
fractions has changed across the core (Fig. 9). Zinc wasusually hosted in the residual fraction (31–79%), followed by the acid-labile (3–35%), the reducible (8–26%)
and the oxidizable fraction (9–12%). Copper was mainly
hosted in the residual fraction (40–73%), followed by theoxidizable (17-26%), the reducible (6–26%), and the acidlabile (1–11%) fractions. Cobalt primarily occurred in theresidual fraction (52–64%), followed by the acid-labile(11–24%), reducible (10–16%), and oxidizable (8–12%)
fractions. In contrast, most Pb occurred in the reducible(usually 52–75%, followed by the residual (17–47%) fractions, while only little Pb was present in the acid-soluble(1–8%) and oxidizable (0.3–3%) fractions. Nickel was
typically found in the residual fraction (74–86%). Furthermore, Cu in the exchangeable fraction accounted for

up to 55% in the acid-soluble fraction, Zn represented up
to 18%, Ni and Pb usually up to 10%, and Co up to 2% for
the ten samples tested (Additional file 1: Table S1). The
proportion of exchangeable Cu was on average 1.5 andthat of Zn 3.0 times higher, respectively, in the five pre1890 samples without anthropogenic contamination.
4.4 Zinc stable isotopes

The δ66Zn values of the bulk sediment were relatively constant until the early 1940s with on average + 0.27 ± 0.01‰ (n = 15, 2σ) (Fig. 6, Additional file 1:
Table S2). The δ66Zn values then showed an abrupt
decrease to on average + 0.22 ± 0.01‰ (n = 13), which
was constant until around 1990. The δ66Zn values were
negatively correlated with ­EFZn (r = − 0.79, P < 0.01;
Fig. 10a) and with the logarithm of the sum in Zn concentrations of the acid-soluble and the reducible fractions (r = − 0.83, P < 0.01).

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 4 Trace metal concentrations of the bulk sediment in the piston core during the last 2300 years. *Azuchi-Momoyama period

Fig. 5 Trace metal concentrations of the bulk sediment in the piston core during the last 450 years. The horizontal dashed lines indicate theJapanese periods. The vertical dotted lines mark the ERL (effect range—low) values (Long et al. 1995)

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 6 Time profiles of δ66Zn and of the enrichment factor (EF) of Zn, Cu, Pb, Ni, and Co in the piston core during the last 450 years

5 Discussion
5.1 Trace metals before the Edo period

The relatively constant trace metal concentrations andenrichment factors before the Edo period (Fig. 4 andAdditional file 1: Fig. S4) indicate that early human activities were negligible trace metal sources. Although there
was a constant inhabitation of the Osaka area with ricecultivation during the Yayoi and Kofun periods, smelting was not much known yet, and many metal tools werelikely imported from the Korean peninsula (Rhee et al.
2007). Iron smelting (tatara) for metal tools, weapons,
and armor was performed from the end of the fifth century (Pearson 2016), but did not affect the trace metal
concentrations, and neither did the warfare during the
Sengoku period (1467–1615 AD).
5.2 Metal pollution during the Edo period

The increase in the Zn, Cu, and Pb concentrations fromthe 1670s AD (Figs. 5, 8) is later than the reconstructionof Osaka Castle (1620–1629 AD) after its destructionin 1615 AD by the Tokugawa forces. Thus, it is possiblethat human activities owing to an increasing population in the Osaka area at the beginning of the peaceful Edo period led to a trace metal input into the bay.
The relatively constant Zn and Pb concentrations until
the 1870s agree with the stable population in the Osaka
area during the Edo period (Saito 2002). The increasing
Cu concentrations until the beginning of the early nineteenth century AD could be caused by emissions fromore smelting by the Sumitomo Copper Refinery established about 1 km southwest of Osaka Castle in 1655
AD and expanded in 1690 AD, which processed copperfrom the Besshi Copper Mines, Ehime Prefecture, from1690 AD (Suzuki et al. 1998). Although significant correlations between the trace metal concentrations andthe CIA exist (Additional file 1: Fig. S6), it is unlikelythat slightly enhanced weathering intensities caused
the trace metal enrichment from the 1670s AD and the1870s AD. Instead, human activities have likely affected
the weathering intensities due to deforestation and ricecultivation, while the trace metal enrichment was due
to anthropogenic metal inputs.

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

Fig. 7 Heatmap of correlation coefficients based on the symmetriccoordinates for the bulk sediment. Blue colors indicate negative andbrown and red colors indicate positive correlations. The elements
were sorted based on the cluster analysis as suggested by Reimann
et al. (2017). The cluster of Cu, Zn, and Pb in the lower left indicatesthat these elements could share the same origin

5.3 Metal pollution during the modern age

The gradual increase in the Zn, Cu, Pb, and Co concentrations in the bulk sediments (Fig. 5) and leachate fractions(Fig. 8) from the 1870s also observed for the inner bay(Yasuhara and Yamazaki 2005) marks the beginning of
early industrialization in the Osaka area. The second halfof the nineteenth century AD in Japan was characterized
by the industrial revolution and urbanization promotedby the Meiji government (from 1868 AD). The industrialization in the Osaka area included coal combustion, ore
smelting, shipbuilding, cotton spinning, and others. For
instance, several Cu smelters started operating at the endof the nineteenth century AD. Furthermore, the population of Osaka City increased rapidly from the end of
the nineteenth century AD (Fig. 11). The rapid increasein the Zn and Pb concentrations from the 1930s agrees
with those previously found in a sediment core from theinner bay (Yasuhara and Yamazaki 2005) and cores fromthe central bay (Hosono et al. 2010), and falls within the
period of economic growth following the Japanese-Russo
War (1904–1905 AD) and World War I (1914–1918 AD).
Furthermore, the recovery stage (1946–1954 AD) and theperiod of rapid economic growth (1955–1973 AD) led toincreasing trace metal emissions. Due to the enactmentof environmental pollution laws during the late 1960s
and 1970s AD, the trace metal concentrations have been
decreasing. In the present study, the peaks in the trace

Page 12 of 19

metal concentrations around 1960 agree well with thoseobserved for other sediment cores in Osaka Bay (Hosono
et al. 2010; Yasuhara and Yamazaki 2005) and for the
Osaka Castle moat (Inano et al. 2004).
In comparison with other sediment cores from the Seto
Inland Sea, metal pollution in Osaka Bay started earlier(1870s AD) than Harima-Nada (from around 1900 AD)(Hoshika et al. 1983), Hiroshima Bay (from the 1930s
AD) (Yasuhara et al. 2003), Beppu Bay (from the early
1920s AD) (Irizuki et al. 2022), and than Hiuchi-nada(from the 1950s AD) (Irizuki et al. 2018). This is not surprising because the Osaka area was rapidly industrialized following the opening of its port. However, the trace
metal pollutions typically peaked during the 1960s for allsites in the Seto Inland Sea due to the legal restrictions
on wastewater and remain still high at present indicating that attempts to reduce trace metal pollution are notenough and that the redistribution of legacy metals is still
problematic.
The cluster of correlations between Mn, Pb, Cu, andZn (Fig. 7) indicates a similar origin of Cu, Zn, and Pb,
as well as a strong association with Mn phases. Trace
metal containing smelter and other industrial effluentswere directly transferred into rivers, which discharge into
the bay. Furthermore, Zn, Cu, and Pb released into theatmosphere by coal combustion and ore processing haveentered the aquatic domain by atmospheric deposition.
Fe–Mn oxyhydroxides formed in the water column could
scavenge these trace metals, while up to 55% of Cu and
18% of Zn within the acid-soluble fraction, respectively(Additional file 1: Table S2), were weakly adsorbed onto
clay surfaces such as of kaolinite and smectite. Calcifying organisms such as foraminifers incorporate Zn and asmall amount of Cu into their shells and into their biomass indicated by the increasing trace metal concentrations in the acid-labile and oxidizable fractions from the1870s (Figs. 8, 9). For instance, some benthic foraminifera
incorporate more Zn with increasing Zn concentrationsin the ambient seawater (Smith et al. 2020). Furthermore,organic matter degradation and the reductive dissolution
of Fe–Mn oxyhydroxides in subsurface sediments release
initially bound trace metals into the porewater, which
can subsequently re-adsorb to carbonates, clay minerals, and sulfides (Canavan et al. 2007; Dang et al. 2015).
In our previous study on a sediment core from nearby
Lake Biwa, we also found that the acid-labile and reducible fractions were the main hosts of anthropogenic trace
metals (Nitzsche et al. 2021) highlighting the role of Mn
oxyhydroxides for trace metal association in sedimentcores from freshwater and coastal environments.
Not only are the trace metal concentrations higher during the 1980s than in pre-industrial times albeit the discharge restrictions from the 1970s (Fig. 5), but they areNitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 8 Time profiles of trace metal concentrations in the leaching fractions in the piston core during the last 450 years. The horizontal lines refer tochanges in trace metal concentrations

also in more bioavailable and mobile fractions (Fig. 8)
with possible toxic effects to marine biota. Potential toxicological effects on marine organisms can be assessed
based on the ERL (effect range—low) and ERM (effectrange—medium) values of the sediment guidelines of the
US Environmental Protection Agency (USEPA) (Longet al. 1995). The ERL values for Cu, Zn, and Pb are 34,
150, and 46.7 µg ­g−1, and the ERM values are 270, 410,
and 218 µg ­g−1, respectively. Thus, the Cu concentrations slightly exceeded the ERL value from the 1930s
until the 2000s, while Pb only exceeded the ERL around1960 (Fig. 5). Zinc started to exceed the ERL value fromthe early 1950s but remained below the ERM value.
Thus, metal pollution exceeding the ERL values in these
periods could have affected Osaka Bay’s marine organisms and ecosystem negatively. More toxic effects wereobserved in the inner bay (Yasuhara et al. 2007; Yasuhara
and Yamazaki 2005).
5.4 Zinc stable isotopes as tracer for Zn contamination

In this study, we have used Zn stable isotope ratios to assess
anthropogenic Zn sources. Because the anthropogenic Zn

was primarily associated with carbonates and with Fe–Mn
oxyhydroxides in the sediment core (Figs. 8, 9), Zn isotopic
fractionation during the adsorption onto mineral surfaces
must be considered (Komárek et al. 2022). Zinc adsorbed
on calcite is isotopically heavy (Dong and Wasylenki 2016)
and the heavy Zn isotopes are preferentially incorporated
into calcite (Mavromatis et al. 2019). Similarly, the adsorption onto Fe–Mn oxyhydroxides favors the heavy isotopes(Balistrieri et al. 2008; Bryan et al. 2015; Juillot et al. 2008).
Thus, it is likely that the decreasing δ66Zn values toward
the surface were due to changes in metal sources rather
than isotopic fractionation during complexation with surfaces. Future studies will have to determine the δ66Zn values in the trace metal chemical fractions to test why thedetected isotopic fractionation during adsorption found inlaboratory studies does not affect δ66Zn values in the bulk
sediments.
Although a doubling of ­EFZn from the 1870s until the
1940s was observed, the relatively constant δ66Zn values(+ 0.27 ± 0.01‰) (Fig. 6) indicated that the average δ66Zn
value of the anthropogenic Zn overlapped with that of
the UCC (+ 0.28 ± 0.05 ‰, 2σ) (Chen et al. 2013). For

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 9 Distribution of Co, Ni, Pb, Cu, and Zn in the chemical fractions (expressed as % of the sum of fractions)instance, the estimated δ66Zn values of fly ash particles
from coal combustion span a wide range from − 0.31
to + 1.26‰ depending on the δ66Zn of the coal (Desaulty
and Petelet-Giraud 2020; Ochoa Gonzalez and Weiss2015). Particles emitted by Zn smelters tend to have low
δ66Zn values (− 0.39 ± 0.05‰), while slags and waterleaching the slag tailings tend to be enriched in 66Zn(+ 0.51 to 1.18‰) (Desaulty and Petelet-Giraud 2020).

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

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Fig. 10 a Scatter plot of δ66Zn against the Zn enrichment factor ­(EFZn), and b against 1/Zn in the piston core from Osaka Bay, in a core from Lake
Biwa (Nitzsche et al. 2021), in a core from Tokyo Bay (Sakata et al. 2019), in a core from the Loire Estuary, France (Araújo et al. 2019b), in the coresfrom Sepetiba Bay, Brazil (Araújo et al. 2017; Tonhá et al. 2020). The black lines represent regression lines through all points. The δ66Zn values of theanthropogenic endmembers in b were estimated from the intercepts of the regression lines. Extending the regression line in a to + 0.17 ‰ yields
an average ­EFZn of the anthropogenic endmember of ~ 5.5. The δ66Zn value of the upper continental crust (UCC) was obtained from Chen et al.
(2013). The range in δ66Zn values of urban and industrial sources includes industrial effluents (+ 0.10 ‰ to + 0.15 ‰) (Desaulty and Petelet-Giraud2020), road dust (+ 0.08 ‰ to + 0.17 ‰), tire wear (+ 0.08 ‰ to + 0.21 ‰) (Dong et al. 2017; Souto-Oliveira et al. 2018; Thapalia et al. 2010), andwastewater treatment plants (WWTP; + 0.06 ‰ to + 0.08 ‰) (Chen et al. 2008; Desaulty and Petelet-Giraud 2020)

Fig. 11 Total population of Osaka City. Note that the sudden decline
in the population around 1920 was caused by a tripling of the cityarea due to an amalgamation with other areas. Source: Osaka City

The abrupt decrease to on average + 0.22 ± 0.01‰
from the early 1950s indicates the enhanced contribution of a 66Zn-depleted source. Such a decrease
was also observed for Lake Biwa from the late 1950s(Nitzsche et al. 2021). The negative correlation
between δ66Zn and E­ FZn (r = − 0.79, P < 0.01; Fig. 10a)indicates a binary mixing between a natural endmember (+ 0.27‰) and urban anthropogenic sources withlower δ66Zn values as previously suggested for urban
lakes across the US (Thapalia et al. 2015, 2010) andthe Loire estuary, France (Araújo et al. 2019b). We
have estimated the average δ66Zn value of the anthropogenic endmember as + 0.17 ± 0.01‰ based on theintercept of the regression line between δ66Zn and
1/Zn (Fig. 10b). This value was close to the value
observed for Lake Biwa + 0.14 ± 0.02‰ (Nitzsche
et al. 2021) and slightly higher than the ranges ofurban and industrial sources observed in other studies. Typical anthropogenic sources with low δ66Zn
values include industrial effluents (hospitals, chemical industries, agro-food industry, surface treatment
industry; + 0.10‰ to + 0.15‰) (Desaulty and Petelet-Giraud 2020), road dust (+ 0.08‰ to + 0.17‰),tire wear (+ 0.08‰ to + 0.21‰). Other sources suchas gasoline (− 0.50‰ to − 0.23‰) (Dong et al. 2017;
Gioia et al. 2008; Souto-Oliveira et al. 2019; Thapalia et al. 2010) and atmospheric industrial emissions(− 0.60 ‰ to + 0.15 ‰) (Ochoa Gonzalez et al. 2016;
Souto-Oliveira et al. 2018) could be of minor importance as they are more depleted in 66Zn. Low δ66Zn
values in rainwater in the Uji area, south of Lake Biwa,
were attributed to road dust and Zn emitted via hightemperature processes such as smelters and fire power
plants (Takano et al. 2021).

Nitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

Using the δ66Zn value of the anthropogenic endmember (+ 0.17‰), we can estimate the anthropogenic Zncontribution in a simple binary mixing model (Araújo
et al. 2021) according to Eq. (4):Znanthropogenic =δ66 Znsample − δ66 Znnatural
δ66 Znnatural − δ66 Znanthropogenic(4)
In this equation, δ Znsample corresponds to the δ Zn
value of a sample, δ66Znnatural is the δ66Zn value of thenatural endmember (+ 0.27‰), and δ66Znanthropogenic represents the δ66Zn value of the anthropogenic endmember(+ 0.17‰). We found the contribution of the anthropogenic endmember increased from approximately 10% in
the mid-1940s up to 80% around 1980 (Additional file 1:
Fig. S7).
Furthermore, the reduction in untreated domestic
wastewater (+ 0.28 ± 0.02‰, n = 1, Chen et al. 2008)
and of industrial effluents, and the contribution of waterfrom wastewater treatment plants (WWTPs; + 0.05‰
to + 0.11‰) (Chen et al. 2008; Desaulty and PeteletGiraud 2020; Sakata et al. 2019) from the early 1970s
did not have a significant effect on the δ66Zn values until
around 1990. The constant δ66Zn values could be due tosimilar δ66Zn values of anthropogenic sources and/or thecontribution of contaminated river sediments and runoff
from industrial and urban areas (Andronikov et al. 2021).
In contrast to the positive relationship between δ66Zn
and 1/Zn observed for Osaka Bay, a negative relationship was observed for a core from Tokyo Bay owing toincreasing δ66Zn values from the 1950s until + 0.47‰ inthe early 1970s (Sakata et al. 2019). Sakata et al. (2019)
estimated a δ66Zn value of + 0.51‰ for the anthropogenic endmember, and the authors suggested the contribution of Zn discharged from electroplating plants,
which started to operate in the early 1950s. Similarly,
Araújo et al. (2017) suggested the δ66Zn value of the
major anthropogenic Zn source associated with electroplating wastes was + 0.86 ± 0.15‰ in Sepetiba Bay,
Brazil. Tonhá et al. (2020) showed that the anthropogenic Zn was mainly hosted in the acid-soluble fraction
in Sepetiba Bay. Similarly, we found a negative correlation between the δ66Zn values with the logarithm of thesum of the Zn concentrations of the acid-soluble and the
reducible fractions (r = − 0.83, P < 0.01) implying that theanthropogenic Zn was hosted in these two bioavailable
and mobile fractions. Today, there are 302 electroplating companies in Tokyo prefecture and 195 companies in
Osaka prefecture with a smaller number of companies inneighboring prefectures that are part of Tokyo and Osaka
Bays, respectively (Japan Federation of Electro Plating
Industry Association 2003). Thus, it is likely that effluents
66

66

Page 16 of 19

from electroplating plants enriched in 66Zn were also discharged into Osaka Bay to some extent, but quantification remains difficult. Consequently, albeit the Tokyo andthe Osaka metropolitan areas are the two largest metropolitan areas in Japan, which suffered from severe metal
pollution during the twentieth century, the Zn stable isotope data reveal that the Zn sources differed based on theindustries in the catchment.

6 Conclusions
This study showed the temporal variation of trace metals
and Zn stable isotopes in a sediment core from Osaka Bay
during the last 2300 years. The BCR sequential extraction
procedure showed that carbonates and Mn oxyhydroxides were the primary hosts for anthropogenic trace metals in the core. An initial trace metal enrichment fromthe 1670s indicated pre-industrial activities. Althougha doubling of E
­ FZn from the 1870s until the mid-1940s
AD was observed, the constant δ66Zn values showedthat the early anthropogenic Zn sources (e.g., coal combustion) were indistinguishable from the natural background highlighting the limitation of Zn stable isotopes
as tracers for early industrialization. Instead, more contemporary Zn sources depleted in 66Zn (e.g., road dust,
tire wear, industrial effluents, effluents from WWTPs)(Desaulty and Petelet-Giraud 2020) caused a decreasein the δ66Zn values from the early 1950s and throughout
the later twentieth century. This decrease is opposite to
the increasing δ66Zn values observed for Tokyo Bay fromthe 1950s suspected due to effluents from electroplatingcompanies enriched in 66Zn (Sakata et al. 2019). Consequently, Zn stable isotopes allow tracing different anthropogenic Zn sources in bays adjacent to metropolitan
areas, while the BCR method can provide further insights
into the potential mobility of Zn and other trace metals.
Abbreviations
AD: Anno Domini; AMS: Accelerator mass spectrometry; BC: Before Christ; BCR:
Community Bureau of Reference; BP: Before present; CIA: Chemical index ofalteration; CT: X-ray computed tomography; δ66Zn: Zinc stable isotope ratio;
EF: Enrichment factor; ERL: Effect range—low; ERM: Effect range—medium;
JAMSTEC: Japan Agency for Marine-Earth Science and Technology; SAR: Sediment accretion rate; TOC: Total organic carbon; TN: Total nitrogen; UCC​: Upper
Continental Crust; WWTP: Wastewater treatment plant.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00517-z.
Additional file 1: Table S1. Cu, Zn. Pb, Co, and Ni concentrations in theexchangeable fraction extracted by 1M NH4Cl and their proportion (%) in
the acid-soluble fraction extracted two times with 0.11M acetic acid (Hac)of ten selected samples. Table S2. Bulk Cu, Zn, Pb, Co, and Ni concentrations and δ66Zn values of samples analyzed for Zn stable isotopes of theNitzsche et al. Progress in Earth and Planetary Science(2022) 9:58

core OS5B. *ND: not determined. Fig. S1. Catchment of Osaka Bay (boldblack line) with prefectural boundaries (thin black lines) and location of
the sediment cores collected at station OS5B. Fig. S2. Swampy lowlands
during the early Meiji era. Source: Technical Report of the Geospatial
Information Authority of Japan (Technical Report D1-No.633, Marsh datain the early Meiji Era (Kinki District)). Fig. S3. Comparison of total organiccarbon (TOC) and total nitrogen (TN) concentrations between the piston
core and the multi core. Fig. S4. Time profiles of the enrichment factor(EF) of Zn, Cu, Pb, Ni, and Co in the piston core during the last 2300 years.
*Azuchi-Momoyama period. Fig. S5. Time profiles of the chemical index
of alteration (CIA) calculated from the residual fraction (F4). *AzuchiMomoyama period. Fig. S6. Biplots of trace metal concentrations against
chemical index of alteration (CIA) calculated from the residual fraction (F4).
Fig. S7. Time profiles of the Zn contribution estimated based on a simple
binary mixing model following equation (3).
Acknowledgements
We thank Ami Togami (AORI), Yosuke Miyairi (AORI), Masafumi Murayama(Kochi Core Center), Hiroyuki Matsuzaki (MALT), and the onboard scientistsof the KT-11-13 cruise for their support in core drilling, slicing, grain size, andradiocarbon analysis, and Y. Yoshikawa and N. Kudo (both JAMSTEC) for theirsupport in chemical analysis.
Author contributions
KNN, HKawahata, and TY designed the study. KNN carried out the experimental work, analyzed the data, and wrote the first manuscript draft. HKawahataorganized the sampling of the sediment core and provided the sediment
samples. YY conducted the radiocarbon analysis. HKajita contributed to
stratigraphical and chronological interpretations. KNN, TY, and KS conducted
the metal concentration and zinc isotope analysis. NOO conducted the TOC
and TN analysis of the piston core. KNN and NFI did the literature research. All
authors were involved in the data interpretation and edited and approved thefinal manuscript.
Funding
K. N. Nitzsche was supported by the JAMSTEC Young Research Fellowship.
The analytical work was partly supported by JSPS KAKENHI Grant Number
20H00193 to YY.
Availability of data and materials
The research data are available from the corresponding author upon request.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Biogeochemistry Research Center, Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC), 2‑15 Natsushima‑Cho, Yokosuka, Kanagawa
237‑0061, Japan. 2 Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo‑Cho, Hirosaki, Aomori 036‑8561, Japan. 3 National Instituteof Advanced Industrial Science and Technology, 1‑1‑1 Higashi, Tsukuba, Ibaraki
305‑8567, Japan. 4 Atmosphere and Ocean Research Institute, The Universityof Tokyo, 5‑1‑5 Kashiwanoha, Kashiwa, Chiba 277‑8564, Japan. 5 Submarine
Resources Research Center, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2‑15 Natsushima‑Cho, Yokosuka, Kanagawa 237‑0061,
Japan. 6 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑Ku, Tokyo 113‑0033, Japan.
7
Research School of Physics, The Australian National University, Canberra, ACT​
2601, Australia. 8 Present Address: Department of Soil Mineralogy and Soil
Chemistry, Institute of Applied Geosciences, Technical University of Darmstadt,
Schnittspahnstraße 9, 64287 Darmstadt, Germany.
Received: 30 May 2022 Accepted: 15 October 2022

Page 17 of 19

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