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\DOI{10.5802/crgeos.339}
\datereceived{2025-07-04}
\daterevised{2026-04-06}
\dateaccepted{2026-04-27}
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\section*{Declaration of interests}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliations other than their research organizations.}

\begin{document}

%\dateposted{2026-02-16}

\begin{noXML}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\CDRsetmeta{articletype}{research-article}

\TopicFR{G\'eochimie, cosmochimie}
\TopicEN{Geochemistry, cosmochemistry}
\TopicFR{G\'eologie et civilisation}
\TopicEN{Geology and civilisation}

\title{Accumulation and provenance of pollutant metals into a remote
shallow peat bog from the Massif Central mountains (France)}

\alttitle{Accumulation et origine des polluants m\'{e}talliques dans
une tourbi\`{e}re peu profonde du Massif Central (France)}

\author{\firstname{Alain} \lastname{V\'eron}\CDRorcid{0000-0002-0933-4053}\IsCorresp}
\address{Aix Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France}
\email[A. V\'eron]{veron@cerege.fr}

\author{\firstname{Nicolas} \lastname{Minvielle Larousse}\CDRorcid{0000-0001-8925-0708}}
\address{CNRS, Aix-Marseille Univ, LA3M, Aix-en-Provence, France}
\email[N. Minvielle Larousse]{nicolas.minvielle@cnrs.fr}

\author{\firstname{Fr\'ed\'eric} \lastname{Guiter}\CDRorcid{0000-0003-0044-2426}}
\address{Institut M\'{e}diterran\'{e}en de Biodiversit\'{e} et
d'\'{E}cologie, IMBE, Aix Marseille Univ., Avignon Univ, CNRS, IRD,
Europ\^{o}le M\'{e}diterran\'{e}en Arbois, 13545 Aix-en-Provence,
France}
\email[F. Guiter]{frederic.guiter@imbe.fr}

\author{\firstname{Abel} \lastname{Guihou}\CDRorcid{0000-0001-7347-378X}}
\addressSameAs{1}{CEREGE, Aix Marseille Univ, CNRS, 
IRD, INRAE, Coll. France, Aix-en-Provence, France}
\email[A. Guihou]{guihou@cerege.fr}

\author{\firstname{Julien\nobreakauthor Le}\nobreakauthor\lastname{Guirriec-Cornu}}
\address{IAUB, Universitat de Barcelona, Institut d'Arqueologica,
Montalegre 6-8, Soterrani 08001 Barcelona, Spain}
\email[J. L. Guirriec-Cornu]{julien.leguirriec@ub.edu}

\author{\firstname{Jacques-Louis} \lastname{de Beaulieu}\CDRorcid{0009-0001-4660-3216}}
\addressSameAs{3}{Institut M\'{e}diterran\'{e}en de Biodiversit\'{e} et
d'\'{E}cologie, IMBE, Aix Marseille Univ., Avignon Univ, CNRS, IRD,
Europ\^{o}le M\'{e}diterran\'{e}en Arbois, 13545 Aix-en-Provence, France}
\email[J.-L. de Beaulieu]{jacques-louis.debeaulieu@orange.fr}

\shortrunauthors

\keywords{\kwd{Massif Central}\kwd{Lead isotopes}\kwd{Trace
metals}\kwd{Ores}\kwd{Western Europe}}

\altkeywords{\kwd{Massif Central}\kwd{Isotopes du plomb}\kwd{M\'{e}taux
en trace}\kwd{Minerais}\kwd{Europe de l'Ouest}}

\dedicatory{\raggedleft This study is dedicated to the memory of
our friend, Fabrice Gr\'{e}goire, a most prevailing and inspiring
figure for the study of mires.\break\vspace*{1pc}}

\begin{abstract}
\looseness=-1
We examine the accumulation and provenance of non-ferrous metals in a
shallow peat sequence (60 cm) collected in the Valley of Chassezac
(Narcettes, Cevennes) in the French Massif Central (FMC), one of the
oldest mining districts in Europe. The core is dated between the Modern
period (<250 years old) and the 5th millennium BCE
(\tsup{14}C and pollens). Trace metal (Pb, As, Sb, Cu, Ni, Zn)
enrichments generally increase from mid-core at 31 cm [70 CE--180 BCE]
to the top, with maxima during the Modern period (0--18 cm, <250 yr).
Statistical analyses (Principal Component Analysis and a source
apportionment MixSIAR Bayesian model) applied to trace metals and
stable Pb isotopes enable to discriminate the excess Pb imprint and its
geographic provenance from Pb--Ag mining in France and surrounding
countries within the first 31 cm. The most striking findings are the
almost non-existent imprint from the numerous Spanish mines and the
overwhelming French and Italian source imprints. Few Greek mines also
contribute Pb enrichment, most particularly at depth, during the Roman
period and late Iron Age. As expected from its proximity to the
Narcettes core, the FMC (northern area and the mont Loz\`{e}re) and
mines from the Alps dominate the French contribution. The significant
input from the mont Loz\`{e}re extends its well-known medieval Ag--Pb
exploitation into the Roman, and possibly, Gallic periods. Finally, a
specific enrichment centered at 47--51 cm [3.2--4.1 ky BCE] is clearly
discerned for Zn, Cu and Ni that may be associated with indigenous Cu
smelting from nearby Cu mining districts in the Languedoc region
(Cabri\`{e}res and Roquemengarde). This finding would signify one of
the oldest atmospheric metal contamination at a remote
French site.
\vspace*{-2pt}
\end{abstract}

\begin{altabstract}
Nous pr\'{e}sentons l'accumulation et l'origine des m\'{e}taux
non-ferreux dans une tourbi\`{e}re peu profonde (60 cm)
pr\'{e}lev\'{e}e dans la Vall\'{e}e du Chassezac (Narcettes, Cevennes).
Cette vall\'{e}e est situ\'{e}e dans le Massif Central (France), une
des plus anciennes r\'{e}gions mini\`{e}res d'Europe. La s\'{e}quence
s\'{e}dimentaire est dat\'{e}e entre la p\'{e}riode Moderne (<250
ans) et le 5\textsuperscript{\`eme} mill\'{e}naire BCE
(\tsup{14}C et pollens). Les enrichissements en m\'{e}taux
(Pb, As, Sb, Cu, Ni, Zn) augmentent entre 31 cm [70 CE--180 BCE] et la
surface de la s\'{e}quence, avec un maximum pendant la p\'{e}riode
Moderne (0--18 cm, <250 ans). Des analyses statistiques (Analyse par
Composantes Principales et mod\`{e}le Bay\'{e}sien MixSIAR) sont
appliqu\'{e}es aux donn\'{e}es (concentrations en m\'{e}taux et
isotopes stables du plomb) afin de caract\'{e}riser les
enrichissements, en particulier ceux du plomb dont les isotopes
permettent de d\'{e}finir la source g\'{e}ographique des minerais dont
la signature est retrouv\'{e}e dans les premiers 31 cm de la
tourbi\`{e}re. Les r\'{e}sultats les plus remarquables sont la quasi
absence de l'empreinte de minerais de plomb espagnol et la domination
des sources de plomb fran\c{c}ais et italien. Quelques mines grecques
contribuent \'{e}galement aux apports de plomb, en particulier pendant
la p\'{e}riode romaine et l'\^{a}ge du bronze tardif. Comme attendu de
par sa localisation, la tourbi\`{e}re accumule une fraction importante
de m\'{e}taux provenant de mines du Massif Central (r\'{e}gion nord et
mont Loz\`{e}re). En particulier, la contribution du mont Loz\`{e}re
s'\'{e}tend au-del\`{a} de la p\'{e}riode m\'{e}di\'{e}vale bien
document\'{e}e avec une exploitation des mines pendant la p\'{e}riode
Romaine, voire durant l'\^{A}ge du Bronze. Enfin, un enrichissement en
Cu, Zn et Ni est d\'{e}cel\'{e} \`{a} 47--51 cm dans la tourbi\`{e}re
[3.2--4.1 ky BCE] qui serait possiblement associ\'{e} \`{a} des
\'{e}missions depuis des districts miniers languedociens riches en
cuivre (Cabri\`{e}res et Roquemengarde) situ\'{e}s \`{a} moins de 200
km de la Vall\'{e}e de Chassezac. Cette contamination atmosph\'{e}rique
au n\'{e}olithique serait une des plus anciennes observ\'{e}es en
France sur un site naturel.
\end{altabstract}

%\input{CR-pagedemetas}

\maketitle

\twocolumngrid

\end{noXML}

\dedication{This study is dedicated to the memory of our
friend, Fabrice Gr\'{e}goire, a most prevailing and inspiring figure
for the study of mires.}

\section{Introduction}

Metals are recognized as markers of past human activities, whether from
mining, metallurgy and/or the use of metal artefacts that are
responsible for the release of contaminated particles within ancient
urban and remote ecosystems \citep{Arnaudetal2005,
Baronetal2009,Delileetal2014, JouffroyBapicotetal2007,
KempterFrenzel2000,Mighalletal2002,Monnaetal2004,NriaguPacyna1988,
PacynaPacyna2001,Younesetal2024}.
To depict the transport and accumulation of trace metals is key, not
only to our understanding of past atmospheric circulation
\citep{Vleeschouwerdeetal2009,MartinezCortizasetal2007,
Shotyketal2002,Veronetal2014,Weissetal2002}, but also to unravel the
onset of human industries and its historic recurrences in Europe
\citep{Guyardetal2007, LeRouxetal2004, Monnaetal2000, Novaketal2003}.
Among these metals, lead (Pb) is of particular interest due to its
production associated with Silver (Ag) as a secondary cupellation
product \citep{Agricola1556, Doe1970}. It is also commonly found in
non-ferrous deposits like an oxide or carbonate with Copper (Cu), Gold
(Au) and Zinc (Zn). Its hardness but yet malleability, its resistance
to corrosion and its antibacterial and welding properties made Pb one
of the most common metal used by human societies since the 5{th}
millennium BCE \citep{Nriagu1983, Wertime1973, YahalomMacketal2007}. 
Greek and Roman societies significantly enhanced its mining and usage
between the 7{th}~c.\ BCE and the 5{th}~c.\ CE \citep{Lessler1988,
PlinyTheElder, SettlePatterson1980}. This exploitation resulted in a
factor 10 to 100 increase of Pb deposition in natural wetlands
\citep{Brannvalletal2001, Renbergetal2001, Shotyketal1998}. Lead also
has 4 stable isotopes, \tsup{204}Pb, \tsup{206}Pb, \tsup{207}Pb and
\tsup{208}Pb, the last three being the end product of the natural
Uranium (U)--Thorium (Th) decay chains. The relative abundance of these
isotopes in geological reservoirs vary according to the initial Th and
U concentrations and the age of the geological bodies \citep{Doe1970}.
As such, Lead Isotope Analysis (LIA) shall differ between various
geological reservoirs, and most particularly among ores
\citep{BrillWampler1967, GaleStosGale1982, GaleStosGale2000,
StosGaleGale2009}. The imprint of the latter in sedimentary deposits
allows for distinguishing the geographic provenance of the ores from
which contaminant Pb originates in natural reservoirs such as lakes and
peat bogs \citep{Eadesetal2002, LeRouxetal2004, Klaminderetal2003,
Monnaetal2000, Novaketal2003, Renbergetal2002, Shotyketal1998,
Vleeschouwerdeetal2007} that are suitable proxies for the record of
metal imprints \citep{Brannvalletal1997,
Monnaetal2004,Shotyk1996,Shotyketal2002,Shotyketal2003,
Vleeschouwerdeetal2010,Weissetal2002,Thevenonetal2011}.
As noted by \citet{Brannvalletal1997} and
\citet{Thevenonetal2011}, both proxies offer distinctive benefits
regarding dating, continuous atmospheric deposition record, geographic
situation and their capabilities to accumulate trace elements and to
preserve its distribution within the cores. Peats mirror atmospheric
metal transient input owing to their exclusive autochthonous organic
plant content and their seclusion from surface and ground water
drainage. Proper \tsup{14}C dating of its organic remains allow to
reconstruct deposition dynamics and to decipher the source and
accumulation rates of natural dust, pollutants and pollen
\citep{Chenetal2022,Vleeschouwerdeetal2010,
Jaraetal2017,MartinezCortizasetal1997,MartinezCortizasetal2002,
Romeyetal2015,Shotyketal1998,Shotyketal2003}. 
The latter are effective in order to
reconstruct vegetation cover in relation to known climate eras
\citep{Iversen1949, Penalba1994, VanGeel1978}, and therefore help
establishing proper age models in peat cores.\looseness=-1

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Location of the Narcettes peat core (filled black
circle) in the Chassezac Valley and main regions presented in this
research.}
{\vspace*{-.2pc}}
\end{figure*}

The French Massif Central (FMC, Figure~\ref{fig1}) is one of the oldest
mining areas in western Europe \citep{Baronetal2019,
Domergue1987,Milotetal2022}. Its
main mining activities were clearly identified during the Middle Ages
and the modern industrial period with Antimony (Sb), Au and, more
recently, U~{being} the prime mining products 
\citep{Briletal1994, Ploquinetal2010}.
Fluor (F), coal and kaolin were among the other
resources that complemented metal supplies \citep{Briletal1994}
and were intensively excavated since at least the 19{th} century.
Significant economic deposits of metals such as Pb, Zn, Cu and Au have
likely been extracted at earlier periods during the Iron Age
\citep{Baronetal2019,Cauuet2000,Trementetal2018},
prior to the Roman occupation of which little is known
regarding metal works from the FMC. Its influence on the production and
export of metals has been overshadowed by other sources from Spain,
Germany or Great Britain, most particularly during the Roman period.
Meanwhile, recent studies have shown that metals were mined and
exported from the FMC during the Roman Empire \citep{Baronetal2010,
Flauxetal2023, Pagesetal2022}. The Rh\^{o}ne River may have favoured
metal trades between southern Gaul and northern territories
\citep{Bonifayetal2007, DomergueRico2018}. Our main objectives are to
decipher regional and long-distance metal contamination in a remote
peat core in Massif Central and to assess their geographic source
imprints using the marker capability of Pb that was almost continuously
mined, at least since the Middle Ages, in the FMC \citep{Alleeetal2010,
BaillyMaitreetal2013, Baronetal2005, ElbazPoulichetetal2017}. We intend
to (i)~decipher the contribution of various regional, and foreign ore
imprints, and mix thereof, in a remote peatland within the FMC and
(ii)~contribute to our understanding of historical recurrences of metal
extraction from the FMC, most particularly prior to the Middle Ages. 
\looseness=-1

\section{Coring site and methodology} \label{sec2}
\subsection{The Narcettes core} \label{sec2.1}

Our coring site is located in the Narcettes peatland, on the Plateau de
Montselgues, abruptly overhanging the Chassezac Valley
(Figure~\ref{fig1}) where discontinued minor mining activities have
been described from the Middle Ages until the 20{th} century
\citep{BaillyMaitreetal2013}. The altitude of this site at 1036~m (NGF)
allows for the intercept of aerosols from regional and/or long-distance
provenance. The Narcettes is constituted of several discontinuous peat
areas over a total surface of $10^{4}$~km$^{2}$ with a \mbox{maximum}
accumulation of 0.9~m. Owing to the shallowness of the peat, we expect
a low stratigraphic resolution. Therefore, we shall not focus on a
continuous transient accumulation of metal imprints, but rather on
distinct historical periods to assess the significance of the FMC in
metal production, most particularly during and prior to the Middle
Ages. The occurrence of a \mbox{substantial} clay layer between a Triassic
aquifer (sandstone slab) and the underlying granitic bedrock has
greatly favored the emergence of the Narcettes humid area. The closest
mining district is located 5--10~km south of the Narcettes peatland, in
the villages of Sainte-Marguerite-Lafig\`{e}re and Montselgues where
was evidenced Pb and Ag mining between the 11{th} and
13{th} centuries \citep{BaillyMaitreetal2013}. {In situ} ore
crushing and calcination were clearly attested, while little or none
metallurgical processes were observed
\citep{MinvielleLarousseetal2019}. Although this mining district never
reached the importance of other medieval sites such as those located in
northeastern Europe (Alsace, Tyrol, Bohemia), we shall investigate its
potential imprint in our peat core record. Most interestingly, the
Narcettes peatland is situated just 50~km from the mont Loz\`{e}re
mountains where a significant Pb and Ag exploitation has been evidenced
during the Middle Ages \citep{Baronetal2005, Baronetal2010,Laurent2005,
Ploquinetal2010} and 20~km from the Largenti\`{e}re valley, where there
was another significant Pb and Ag mining activity in the Middle Ages,
and during the modern period \citep{Foglierinietal1980,
BaillyMaitre1996, Girard2003}. Our peat core (MS\tsub{1}) was collected
with a Russian GIK-type coring system (8~cm diameter, 62~cm length,
Lambert93 X780149.190 Y6379313.896) in a PVC plastic tube and kept in a
refrigerated chamber (at 6~{\textdegree}C) before sub-sampling for
palynological and geochemical analyses. The stratigraphic description
of the peat clearly attests to its ombrotrophic nature (Supplementary
Figure~1) with only two embedded organic clay layers that account for
less than 10\% of the core. More relevant information regarding the
Narcettes coring site can be found in
\citet{MinvielleLarousseetal2016}.

\vspace*{-4pt}

\subsection{Subampling and analyses} \label{sec2.2}

\vspace*{-2pt}

\looseness=-1
Peat samples for pollen analyses were collected each 1.5~cm along the
core (41 samples). Samples of 10~g each were prepared according to
\citet{Nakagawaetal1998} using 10\% HCl, 40\% HF, 10\% KOH and
acetolysis. Palynological identification was performed using IMBE
reference collections and photographic atlases \citep{Reille1992,
Reille1995, Reille1998}. Taxa dynamics were statistically sequenced
with cluster analysis using the FORTRAN 77 CONISS program from the
rioja package \citep{Grimm1987, Juggins2012}.

Fifteen peat samples were collected along with pollen samples and
preserved in cleaned plastic bags before drying at 60~{\textdegree}C
before geochemical analyses (trace metals and stable Pb isotopes).
Approximately 50~mg of dried peat sample were digested in a microwave
nitrogen pressurized sealed chamber (UltraWave Thermo-Scientific) at 
high temperature (250~{\textdegree}C) and pressure (100~bar) with a
mixture of ultrapur HNO\tsub{3}, HCl and HF concentrated acids. Blanks
and standards were processed along with the samples. A galena sample
from a nearby mining district (Le Colombier) was also processed with
diluted HNO\tsub{3} (1:5) before LIA. A fraction of the digested solution
was analyzed for trace metals by Inductively Coupled Plasma Mass
spectrometry (ICP-MS quadrupole Perkin Elmer Nexlon 300X). Processed
blanks accounted for less than 0.1\% of the sample content while the
Analytical Relative Deviation (RSD) was less than 5\% for measured
elements. Accuracy is verified using the NRCC HISS-1 sediment standard
(Supplementary Table~1). All trace elements are within the range of the
certified value with the exception of Titanium (Ti) which is 10 to 15\%
lower than the certified value. Stable Pb isotope ratios were measured
using a Neptune${+}$ MC-ICPMS (Multi-Collector Inductively Coupled Plasma
Mass Spectrometer) at CEREGE (CNRS, France), after Pb extraction and
purification on AG1X8 resins \citep{Manhesetal1978}. The isotope
ratios were normalized by multiple analyses of the NBS981 standard.
Analytical uncertainties (one standard deviation) for the measured Pb
isotope ratios were 0.01\% and 0.03\% for \tsup{206}Pb/\tsup{207}Pb,
\tsup{208}Pb/\tsup{206}Pb and \tsup{206}Pb/\tsup{204}Pb,
\tsup{207}Pb/\tsup{204}Pb, \tsup{208}Pb/\tsup{204}Pb ratios
respectively for 1 to 10~v signals. Trace metals and LIA are presented
in Supplementary Tables~2 and~3 respectively. Geochemical univariate and
multivariate statistical analysis (Principal Component Analysis,
correlation, comparison tests, and descriptive statistics) were
performed with Past4 statistical package software
\citep{Hammeretal2001}.

\begin{table*}[t!]%tab1
\caption{\label{tab1}\tsup{14}C calibrated ages in the Narcettes peat
core. The calibrated age at 12--13~cm is likely out of range with dates
younger than 250 years old.\vspace*{-4pt}}
\begin{tabular}{ccccc}
\thead
Depth (cm) & Material dated & Lab. reference & Age uncal.\ BP & Age cal.\ (2~sig.)\\
\endthead
12--13 & Peat & Poz-105082 & \0\0$85 \pm 30$ & Out of range*\\
22--24 & Peat & Poz-105083 & \0$420 \pm 30$ & 331--523\\
32--35 & Peat & Poz-105125 & $2410 \pm 30$ & 2354--2491\\
42--44 & Peat & Poz-105126 & $4305 \pm 35$ & 4831--4960\\
57--58 & Peat & Poz-84219 & $5960 \pm 40$ & 6675--6888
\botline
\end{tabular}
{\vspace*{-4pt}}
\end{table*}

Radiocarbon dating was performed on 5 organic samples at the Poznam
Radiocarbon Laboratory (Poland) and is presented in Table~\ref{tab1}.
Age models were calculated using the R Clam package
\citep{Blaauw2010} that are discussed in the next section according to
pollen records. Modelled ages are presented in year BP or age [BCE-CE]
with their best estimates. Our chronology is based upon five \tsup{14}C
dates from which we calculated a smooth spline continuous age model
(Supplementary Figure~2). Because of lacking accurate recent dating,
all dates from the smooth spline model for depths above 23~cm were
speculative assessments with dates younger than 250 years old
(0--18~cm) and 500 years old (19--22~cm) based on \tsup{14}C dates and
the smooth pline model (Supplementary Table~4).

\section{Palynology and age model} \label{sec3}

The pollen diagram of the MS\tsub{1} core is shown in Figure~\ref{fig2}.\ 
It describes the transient distribution of pollen grains from various
taxa expressed in percentages.\ The fern \textit{Osmunda regalis} was
excluded from the pollen sums because its over-representation would
have concealed other taxa.\ We identified 7 biostratigraphic domains
that included from bottom up:

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Pollen diagram (\%) from the Narcettes peat core
with corrected \tsup{14}C dates, corresponding biostratigraphic domains
and lithology.}
{\vspace*{-3pt}}
\end{figure*}

{\smallskip\noindent}
LPAZ1 (62.5--53.5~cm), a mixed temperate coniferous forest type dominated
by \textit{Pinus} with \textit{Quercus} and \textit{Corylus}.
\textit{Ulmus} and \textit{Tilia} also contribute this woodland with
\textit{Betula} that slowly declines toward the top of the sequence;

{\smallskip\noindent}
LPAZ2 (53.5--43.5~cm), characterized by a significant decline in
\textit{Pinus} and increase in \textit{Betula} that may highlight
peatland expansion that seems to be characterized by oscillations
between humid (\textit{Betula}) and drier (\textit{Calluna}) periods;

\looseness=-1
{\smallskip\noindent}
LPAZ3 (43.5--36.5~cm), a humid period during which \textit{Pinus}
almost vanishes and \textit{Fagus} is clearly established with
prevailing humid taxa (\textit{Betula}, \mbox{\textit{Alnus}}) and the
disappearance of \textit{Calluna}. It should be noted that the fern is
abundant from 40--42~cm on. The onset of \textit{Olea} (olive tree)
evidenced in LPAZ3 is likely imported from the southern Mediterranean
hillside of the C\'{e}vennes mountains, and may not characterize the
birth of arboriculture in the vicinity of the peatland;

{\smallskip\noindent}
LPAZ4 (36.5--25.5~cm), an optimum for \textit{Pinus} and
\textit{Fagus}, and a sharp decline in \textit{Ulmus}, \textit{Corylus}
and \textit{Tilia}. The alternate sequences of \textit{Betula},
\textit{Alnus} and \textit{Calluna} depict hydrological oscillations
within the peatland. Markers of pastoralism (\textit{Rumex},
\textit{Plantago}) and cultivation (\textit{Cerealia},
\textit{Juglans}) clearly arise;

{\smallskip\noindent}
LPAZ5 (25.5--14.5~cm), woodland declines (except for \textit{Pinus}),
while taxa associated with agro-pastoralism rise, as well as
\textit{Calluna} that pervades into deforested non cultivated areas.
\textit{Juglans} is at its peak and pollen from olive trees cultivated
at lower altitudes also significantly rises;

{\smallskip\noindent}
LPAZ6 (14.5--3.5~cm), \textit{Calluna} is at its peak while
agro-pastoralism declines (except for \textit{Olea} from lower
altitudes).\ \textit{Pinus} increases with small \textit{P.~sylvestris}
pollens along with large \textit{P.~nigra} or \textit{maritima} that
may originate from artificial reforestation during the 19th century.
The latter is corroborated by the occurrence of planted \textit{Picea}
(spruce) that was not encountered in the FMC since the last glacial
maximum \citep{deBeaulieuetal1988};

{\smallskip\noindent}
LPAZ7 (3.5--0~cm), is characterized by a decline in \textit{Calluna}
for the benefit of grassland taxa and the onset of chestnuts.

Several well-dated pollen sequences were determined in the vicinity of
the Narcettes peatland which could be compared to our record. They
showed the onset of beech wood between LPAZ3 and LPAZ4. According to
our smooth spline model it would have occurred circa 3100 BP.
Meanwhile, this beech transition is dated to $4770 \pm 100$ and $3862
\pm 100$ uncal BP in the nearby Velay and Ard\`{e}che areas
\citep{deBeaulieuetal1984, Dendieveletal2019}. These results suggest that
our smooth spline model may provide younger calculated ages for the
level 34--36~cm.\ The calibrated age at 32--35~cm
({2354--2491}~cal.\ BP) ({Table}~\ref{tab1}) is well
comprised within the modelled smooth spline age at this depth
{interval} ({2200--2870}~BP) \mbox{suggesting} that the smooth
spline model remains valid at this depth. Including a hiatus would
reduce the uncertainties in the age model. On the contrary, in LPAZ4,
the relative abundance of \textit{Abies} that rises during the late
Iron Age (suboreal-subatlantic transition) and the onset of
\textit{Carpinus} at 33~cm, generally found around 2000~BP in the FMC,
is found at 2270--2570~BP with our calculated age model. In this former
occurrence the smooth spline model provides slightly older calculated
ages. Owing to these uncertainties, we assume that both palynology and
the recorded lithography (Supplementary Figure~2) cannot resolve
possible lessening peat accumulation that may have resulted from less
favorable geomorphological and climate settings. We therefore consider
that the less accurate smooth spline age model reasonably fits the
well-dated anthropogenic influence during LPAZ3 and LPAZ4
biostratigraphic zones. Supplementary Table~4 shows the corresponding
modelled ages at each sample depth being analyzed for metals and stable
Pb isotopes in the core. The levels from 3 to 18~cm (D\tsub{1} to
D$_{4}$) are younger than 250 years old, while the 19~cm level (D5) is
younger than 500 years old based on the uncertainties of the smooth
spline model and the \mbox{biostratigraphy}.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Ratios of Metals (Me) to Aluminum (Al) in the
Narcettes peat core. Me/Al ratios are multiplied by $10^{3}$ (Pb),
$10^{4}$ (As, Cu, Ni, Zn) and $10^{5}$ (Sb).}
\vspace*{-3pt}
\end{figure*}
 
\section{Metal imprint and accumulation} \label{sec4}
\subsection{Metal enrichments} \label{sec4.1}

Trace metal concentrations are shown in Supplementary Table~2.\ 
Concentration of trace elements is \mbox{significantly} higher in the
top 25~cm, with the \mbox{exception} of crustal Ti and Aluminum (Al)
(Supplementary Table~2) that are known to vary according to the mineral
content within a core. This can be seen in the Narcettes core where Al
and Ti concentration increase by about a factor~2 (Supplementary
Table~2) in two organic clay layers at 21--23~cm and 58--62~cm
(Supplementary Figure~2) as expected from the clay contribution. Its
effect on trace element concentration is significant at 58--60~cm for
Pb and Sb concentration that increase by a factor 2 to 4 (Supplementary
Table~2).\ We need to take into account this mineralogical contribution
and its effect on trace metal concentration in the core. Here we use
the ratios of metals to a crustal-derived element to best evidence
these enrichments above the natural background. This normalization
using crustal elements allows to take into account various lithologies
in the core that are characterized by different trace metal
concentration. Both Ti and Al can be used as crustal markers. They are
significantly correlated ($r=0.9$; $p < 0.001$) suggesting that we do
not encounter major mineral shift in the core that would not be taken
into account by either of these crustal-derived elements. This
normalization allows for characterizing trace metal background and to
calculating excess fractions that may originate from human activities
(for concentration and Pb isotopes). We choose Al as a crustal marker
rather than Ti that displays a lesser accuracy than Al (Supplementary
Table~1). The ratios of metals to Al are shown in Figure~\ref{fig3}
where specific shifts can be noticed that include from top down: (A)~an
enrichment of all trace metals above 18~cm, i.e., younger than 250~yr
(estimated age model), that would correspond to atmospheric deposits
since at least the 18{th} century. While Pb, Nickel (Ni), Cu and Zn
keep increasing up to the top of the core, Sb and Arsenic (As) display
a peak between 6 and 11~cm followed by a decline in the top core.
Because of uncertainties with young 14C dates and our smooth spline age
model, we cannot describe with accuracy these transient shifts. It
would have been necessary to use adequate radiomarkers for recent
deposition stratigraphy such as \tsup{210}Pb and \tsup{137}Cs.
Meanwhile these variations in metal content are consistent with the
known pre-1950s industrial activities that may indicate both the rise
and phasing out of regional mining (marked by Sb and As exploitation),
and the imprint of regional contaminants; (B)~slight but noticeable
contamination imprints for Pb and Sb can be seen down to 31~cm at
1880--2130~BP [70~CE--180~BCE, best at 50~BCE], covering the Late Iron
Age and the Early Roman period; (C)~there is no further visible
enrichment for As, Pb and Sb down to the bottom of the core; (D)~a peak
centered at 47--51~cm, i.e.\ 5400--6030~BP [3.2--4.1~ky BCE] is clearly
discerned for Zn, Cu and Ni that shall be discussed considering
regional metal mining during the late Neolithic period. Trace metal
trends~A and~B clearly signify the input of metal excesses caused very
likely by human activities. It should be noticed that the Coefficient
of Variation of these ratios (CV: ratio of standard deviation to
corresponding mean) vary from $85 \pm 10\%$ to $48 \pm 17\%$ for Pb,
As, Sb and Cu, Ni, Zn respectively indicating the more sensitive
response of the priors to anthropogenic input. 

\subsection{Lead imprint} \label{sec4.2}

Lead isotopes of which ratios are efficient markers of anthropogenic
enrichments into peat cores \citep{LeRouxetal2004, Klaminderetal2003,
Monnaetal2000, Monnaetal2004, Shotyketal1998, Vleeschouwerdeetal2007}
allow to signify the main natural and excess fractions in the Narcettes
core (Supplementary Figure~3). The \tsup{207}Pb/\tsup{204}Pb versus
\tsup{206}Pb/\tsup{204}Pb graphic significantly seclude the top core
sample (D1, 3~cm) from the rest of the samples that are aligned upon
the same isochron (Supplementary Figure~3). This should be taken into
account when investigating the mines from which may originate Pb
accumulated at 3~cm. Furthermore, we could predict a more
\mbox{significant} crustal fraction that is consistent with the mean
crustal imprint line at a depth below\break 34~cm.%%%%%

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}Principal Component Analysis (PCA) for Pb/Al,
\tsup{206}Pb/\tsup{207}Pb and \tsup{208}Pb/\tsup{206}Pb ratios.
Components PC1 and PC2 account for 99.7\% of total variability.}
{\vspace*{-2pt}}
\end{figure*}

In order to discern the enriched and natural fractions in the core, we
perform a Principal Component Analysis (PCA) that includes both Pb
normalized concentration and stable isotopes (\tsup{206}Pb,
\tsup{207}Pb and \tsup{208}Pb isotopes). This statistical approach has
shown its usefulness in recent provenance studies using Pb and its
stable isotopes \citep{Albaredeetal2024, TomczykZabinski2023}. Two main
components are extracted that can explain most of the data variability
(PC\tsub{1}: 94\%; PC\tsub{2}: 5.7\%). In Figure~\ref{fig4}, samples are
identified by their mean depth and are grouped according to their
scores, i.e., the contribution of each variable to PC\tsub{1} and
PC\tsub{2} on component axes. All depths that are clearly enriched in
Figure~\ref{fig3} display positive PC\tsub{1} scores (3--31~cm, trends~A
and~B) with the top depths (3--11~cm) being noticeably defined. Upper
sections of the core and its large enrichments could not be properly
dated and likely correspond to the past 250 years in coincidence with
the modern industrial rise in western Europe. The A--B cluster between
15 and 31~cm covers most of the period from the so-called ``industrial
revolution'' to the Roman period.\ The C~trend (Figure~\ref{fig3}) can
be subdivided into 2 clusters, C\tsub{1} and C\tsub{2}, in the PCA,
(Figure~\ref{fig4}). Cluster C\tsub{1} could be altered by the D~trend
that shows Cu, Ni and Zn enrichments at the same depths during the
4{th} millennium BCE (Figure~\ref{fig3}) while C\tsub{2} may represent
the depths at which the background is best\break \mbox{expressed}.

To best investigate accumulation spikes observed in the Narcettes core
(Figure~\ref{fig3}), one can calculate the contribution of excess Pb
(Pb$_{\mathrm{xs}}$) above the natural imprint in the core using the background
as defined by the Pb/Al ratios and the PCA:
{$$
\mathrm{Pb}_{\mathrm{xs}} = ([\mathrm{Pb}]_{\mathrm{t}} - ((\mathrm{Pb}/\mathrm{Al})_{\mathrm{c}}
\times [\mathrm{Al}]_{\mathrm{t}}))/[\mathrm{Pb}]_{\mathrm{t}}
$$}\unskip
Pb$_{\mathrm{xs}}$: relative contribution of excess Pb for each sample;
[Pb]$_{\mathrm{t}}$: total measured Pb (ppm) for each sample;
[Al]$_{\mathrm{t}}$: total measured Al (ppm) for each sample;
(Pb/Al)$_{\mathrm{c}}$: mean background Pb/Al ratio (trend~C in Figure~\ref{fig3}).

The Pb$_{\mathrm{xs}}$ (\%) fraction at each level is displayed in
Figure~\ref{fig5} along with corresponding PC\tsub{1}\ $z$-scores,
\tsup{206}Pb/\tsup{204}Pb and \tsup{207}Pb/\tsup{204}Pb ratios.\ The
crustal isotopic imprint in Figure~\ref{fig5} is calculated from the
largest possible natural imprint using the combined
\tsup{206}Pb/\tsup{204}Pb and \tsup{207}Pb/\tsup{204}Pb ratios as
calculated from cluster C\tsub{2} (see PCA analysis,
Figure~\ref{fig4}).\ All of the markers show significant metal Pb
excess during the past 2000 years. The human Pb imprint during the Late
Iron Age and Early Roman period at 31~cm [70~CE--180~BCE, best at
50~BCE] is clearly expressed with a calculated Pb$_{\mathrm{xs}}$ contribution
of 50\% and isotopic ratios noticeably different ($p < 0.001$,
\mbox{student} $t$~test) from the mean crustal
\tsup{206}Pb/\tsup{204}Pb and \tsup{207}Pb/\tsup{204}Pb ratios ($18.886
\pm 0.031$ and $15.698 \pm 0.003$ respectively).\ There is a 20\%
Pb$_{\mathrm{xs}}$ enrichment at 39~cm [1570--2010~BCE, best at~1840~BCE] with
\mbox{isotopic} ratios that are not statistically different from expected
crustal imprint.\ This discrepancy raises the limits of these
calculations with weak metal enrichments that are to be used
cautiously. This is why we only consider possible human imprints when
corroborated by several markers. At last, the D~trend (46--52~cm) is
clearly defined by \tsup{206}Pb/\tsup{204}Pb and, to a lesser extent,
\tsup{207}Pb/\tsup{204}Pb ratios that are statistically different from
the crustal isotopic imprint ($p < 0.05$, student $t$~test) and a
significant increase in the PC1 $z$-scores (Figure~\ref{fig5}).
Meanwhile there is no Pb$_{\mathrm{xs}}$ enrichment at these depths. While
plausible, this human imprint during the 4{th} millennium BCE remains
speculative.

\begin{figure}
\includegraphics{fig05}
{\vspace*{-3pt}}
\caption{\label{fig5}Calculated Pb$_{\mathrm{xs}}$ fraction, PC1 $z$-scores,
\tsup{206}Pb/\tsup{204}Pb and \tsup{207}Pb/\tsup{204}Pb ratio (along
with crustal isotopic range) in the Narcettes core. A, B, D are
enriched levels from Figure~\ref{fig3}. See Section~\ref{sec5} for
explanations regarding calculated indices.}
{\vspace*{-6pt}}
\end{figure}

The significant Cu, Zn and Ni excesses at 46--52~cm (D~trend in
Figure~\ref{fig2}) includes the D12 and D13 samples that are best dated
to 4080--3450 BCE with the smooth spline model (Supplementary Table~4).
While it is characterized by significantly non crustal
\tsup{206}Pb/\tsup{207}Pb ratios (Figure~\ref{fig5}), it could not be
explained with Pb systematics as there are no \mbox{significant}
Pb$_{\mathrm{xs}}$ at these depths. This lack of Pb enrichment and a non-crustal
Pb isotope imprint suggest a contamination source possibly from
volcanic activity and/or Cu ingots smelting rather than Pb mining. The
Pb isotope radiogenic signatures from the main eruptive areas in
western Europe do not comply with LIA in the Narcettes peat at these
depths \citep{Holmetal2001, Thirlwalletal2004, Wittigetal2007}. The
inception of a significant Cu metallurgy in western Europe was commonly
found during the III{rd} millennium BCE onwards \citep{Hamonetal2020,
RaackRisch2008}. Earlier Cu smelting occurrences are
scarce that could explain such contamination during the 4{th}
millennium BCE in central and eastern Europe
\citep{Radivojevicetal2010, Ryndinaetal1999}. Meanwhile there is an
occurrence, just 100~km away from the Chassezac Valley, in the
Cabri\`{e}res mining district (Figure~\ref{fig1}), where Cu extraction
has been evidenced since at least the late 4{th} millennium BCE
\citep{Ambert1996,Ambert1999,Bouquetetal2006,
Prangeetal2003}. The oldest calibrated \tsup{14}C date for this
chalcolithic mining district is 3310--3230 BCE at the Capitelle du Boum
\citep{Ambertetal2002} where analyzed Cu objects were isotopically
associated with Cabri\`{e}res Cu mines, and displayed the highest Pb
content \citep{Ambertetal2009}. This result along with the finding of
Cu tools dated to 3530--2880 BCE at the Neolithic settlement of
Roquemengarde \citep{Guilaine1992}, 15~km from the Cabri\`{e}res
district, unveils early protohistoric Cu smelting that may have been
recorded regionally during the 4{th} millennium BCE, most particularly
in the absence of nearby Pb mining. The pollen record shows a likely
exogenous \textit{Olea} intrusion from the southern Cevennes as early
as 3200 BCE that would support a northward regional transport of
aerosols during the 4{th} millennium BCE (Figure~\ref{fig2}) toward
the Chassezac Valley.\looseness=-1

\section{Lead geographic provenance} \label{sec5}
\subsection{Mine selection (isotope imprints and{\hfill\break}
Euclidian Distances)} \label{sec5.1}

We use the statistical MixSIAR model, a hierarchical Bayesian mixing
model to decipher the geographical provenances of Pb ore imprints and
mix thereof measured in the Narcettes core \citep{Longmanetal2018}.
This model uses the well-known capabilities of Pb and its stable
isotopes to label the imprint of \mbox{non-ferrous} ores and decipher their
geographic origin (see references in the introduction paragraph).
Meanwhile, this isotopic approach may be hindered by (i)~the
heterogeneity of ore deposits (2)~isotopic fractionation during
smelting that can affect isotopic ratios and, (3)~recycling and pooling
of various ores from different locations to produce ingots. Geological
heterogeneity is generally below 0.3\% for ores of archaeological
significance within a given ore field \citep{Barnesetal1974,
StosGaleGale2009}. Isotopic fractionation during smelting impedes the
use of Pb isotopes for provenance studies \citep{Buddetal1995a,
PollardHeron2008}. This fractionation has been predicted from
thermodynamic models \citep{MullikenHarkins1922}. Experimental models
show that no measurable fractionation takes place during Pb smelting
\citep{Barnesetal1978,GaleStosGale1996,Macfarlane1999,
Baronetal2009,StosGaleGale2009,CuiWu2011}.
Metal recycling, remains an issue that cannot be easily resolved, most
particularly for the characterization of artefacts \citep{Pernicka1995,
Sayreetal1995, Buddetal1995b, Gale2001}. To achieve this statistical
analysis, we first need to calculate the excess Pb isotopic imprint
(IC$_{\mathrm{xs}}$) for each sample depths in order to compare them to isotopic
ore signatures measured from various Pb mines as compiled in a recent
database \citep{Tomczyk2022}:
{$$
\mathrm{IC}_{\mathrm{xs}} = [\mathrm{IC}_{\mathrm{m}} 
- (\mathrm{IC}_{\mathrm{b}} \times
\mathrm{Pb}_{\mathrm{b}})]/[\mathrm{Pb}_{\mathrm{xs}}]
$$}\unskip
IC$_{\mathrm{xs}}$: Isotopic Composition of the excess fraction;
IC$_{\mathrm{b}}$: Isotopic Composition of the background;
IC$_{\mathrm{m}}$: measured Isotopic Composition;
Pb$_{\mathrm{b}}$: background Pb fraction (relative contribution ${=}$ 1 ${-}$
Pb$_{\mathrm{xs}}$);
Pb$_{\mathrm{xs}}$: excess Pb fraction (relative contribution ${=}$ 
1 ${-}$ Pb$_{\mathrm{b}}$).

The Pb$_{\mathrm{b}}$ relative contribution for each sample is
calculated from Pb$_{\mathrm{xs}}$ while IC$_{\mathrm{b}}$ is
determined from the background defined by PCA analysis (cluster
C\tsub{2}, Figure~\ref{fig4}) and shown in Figure~\ref{fig5}. 

In order to minimize the number of mine entries in the MixSIAR model,
we ought to reduce the ore database to a restricted number of sources
that are compatible with the calculated IC$_{\mathrm{xs}}$ at each level. We
consider as a potential source any mine imprint that is within a
two-sigma multivariate distribution of each calculated IC$_{\mathrm{xs}}$.\ To
do so, we \mbox{calculate} \mbox{Euclidian} Distances (EDs), i.e., the
distance between two points using cartesian coordinates and the
Pythagoream theorem. Here, EDs are determined from two-dimensional
isotopic systems that include
\tsup{206}Pb/\tsup{204}Pb--\tsup{207}Pb/\tsup{204}Pb,
\tsup{206}Pb/\tsup{204}Pb--\tsup{208}Pb/\tsup{204}Pb, and
\tsup{206}Pb/\tsup{207}Pb--\tsup{208}Pb/\tsup{206}Pb ratios. EDs are
calculated at each depth between IC$_{\mathrm{xs}}$ and ore imprints from
various European regions. Both IC$_{\mathrm{xs}}$ and isotopic ore imprints need
to be normalized to 1 before ED calculation using $X_i$ and $Y_i$ min
and max for each ratio, e.g., for
\tsup{206}Pb/\tsup{204}Pb--\tsup{207}Pb/\tsup{204}Pb isotopic system:
{\begin{eqnarray*}
{X}_{n} &=& [({X} - {X}_{\mathrm{min}})/({X}_{\mathrm{max}} -
{X}_{\mathrm{min}})]\\
{Y}_{n} &=& [({Y} - {Y}_{\mathrm{min}})/({Y}_{\mathrm{max}} -
{Y}_{\mathrm{min}})]
\end{eqnarray*}}\unskip
where $(X, Y)$ are cartesian coordinates of \tsup{206}Pb/\tsup{204}Pb
and \tsup{207}Pb/\tsup{204}Pb respectively with $(X_{n}, Y_{n})$
being the corresponding normalized coordinates.

Then EDs are calculated for each enriched sample relative to the
isotopic imprints of all the major Pb ores (from the isotopic
databases):
{$$
\mathrm{ED}(i,j) = \sqrt{({X}_{ni} - {X}_{nj})^{2} + ({Y}_{ni} -
{Y}_{nj})^{2}}
$$}\unskip
where ($X_{ni}$, $Y_{ni}$) are the normalized coordinates of
IC$_{\mathrm{xs}}$ in one of the isotopic systems, and ($X_{nj}$,
$Y_{nj}$) the normalized coordinate of a given ore imprint in the same
isotopic system, e.g.\ 
\tsup{206}Pb/\tsup{204}Pb--\tsup{207}Pb/\tsup{204}Pb.

Lead mines are selected from France (FR, 491 data) and the neighboring
significant Pb producers in western Europe, i.e., Spain (SP, 568 data),
Italy (IT including Sardinia, 366 data), Germany (GE, 466 data) and
Great Britain (GB, 251 data) to which we have added the Greek (GR)
mines to explore the significance of long-distance transport of (and/or
import from) Pb ore imprints from an historical well-known Pb--Ag
producer, most particularly during the Mediterranean Antiquity. Only
the mines with major Pb resources were selected from the data bases.
The French database is subdivided into several regions including the
Massif Central (northern MCN, eastern MCE and southern MCS regions with
a focus on the mont Loz\`{e}re mountain LO), the Alps, the Armorican
basin, the Vosges and the Pyrenean mountain areas, the last three being
abbreviated as OTH for ``other''. The FMC domain is considered apart
from the rest of France and, most particularly, the mont Loz\`{e}re
owing to its proximity to the Narcettes peatland and its
well-investigated mining/metallurgical activities \citep{Baronetal2006,
Baronetal2010, Ploquinetal2010}. The distance between ore imprints and
IC$_{\mathrm{xs}}$ is calculated for each isotopic system and considered
significantly proximate with ED ${<}$ 0.02. This difference corresponds to a
$\Delta_{\mathrm{iso}} \leq 0.06\%$, i.e., the highest analytical
uncertainty (two standard deviation) on \tsup{206}Pb/\tsup{204}Pb,
\tsup{207}Pb/\tsup{204}Pb and \tsup{208}Pb/\tsup{204}Pb ratios.
Although the same geographic source may appear several times with ED
${<}$ 0.02 within the same or several isotopic systems at a given depth,
it is only considered once at each sample depth (Di). The number of
mines at Di is reported in Supplementary Tables~5 and~6 for all
considered countries and French regions. The reported number of mines
issued by EDs is not intended to define a single ore provenance but
rather identify trends that could signify the relative influence of a
region. We report EDs down to 39~cm (D10), i.e.\ [1570--2010, best at
1840~BCE], below which IC$_{\mathrm{xs}}$ calculation is highly
speculative due to small Pb enrichment (Pb$_{\mathrm{xs}}$ below 20\%)
and/or Pb concentration below 30~ppm.

\looseness=-1
According to EDs, Spain contributes the most numerous mines to the
Narcettes core imprint with a mean contribution of 49\%, followed by
France (22\%) and Italy (18.5\%) while Great Britain and Germany mean
apportionment is 5\%. Greece contribution remains not significant
(Supplementary Table~5). These relative contributions reflect the
density of existing mining districts where Pb isotopes have been
measured in these countries \citep{BlichertToftetal2016,
StosGaleetal1995, Tomczyk2022} and therefore should be considered with
caution for provenance assessment. The top core (D1, 3~cm) displays a
smaller number of European mines (less than 30, Supplementary Table~5)
than the immediate underlying layers, owing likely to the predominance
of other worldwide mine sources in most recent years that are not
included in this study. This result is consistent with the isotopic
imprints reported in Supplementary Figure~3 where D1 is clearly apart
from the 300~My isochron, suggesting a different ore mixture than that
of the other samples to explain its isotopic imprint. The D2 to D4
layers (i.e., younger than 250 years old) display the same relative
contribution with Spain and France accounting for about 70--80\% of the
total mine inventory. This apportionment is also found at D7 and D8
that covers the Late and Early Roman period with an almost similar
number of mines. It should be noticed that the German mines
contribution is at its peak (18~mines) during this period, possibly as
a response to the rise of Roman~control of German mines during the
Empire \citep{Herediaetal2025, KempterFrenzel2000, Monnaetal2000}.
The medieval era (D5 and D6) is characterized by the most contributing
mines (150 to 169 reported mines) with a peak of the Spanish and
Italian imprints. This would be consistent with the Italian and Spanish
economic wealth during the 15{th} and 16{th} centuries owing to their
demographic and economic growth, as well as colonization in Asia and
America that paved the road to capitalism \citep{Braudel1946,
Luzzatto2013, Malanima2020, Marechaux2023, Vives2015}. It should be
noted that half of the Italian contribution arise from Sardinia. The
last investigated two levels (D9 and D10) only report 20 different
mines, mostly from Spain and Italy, that explain IC$_{\mathrm{xs}}$
(Supplementary Table~5). This is the lowest mine occurrence in the
Narcettes core. While Pb--Ag artefacts and metallurgy were evidenced
since the Nuragic period (1800--500~BCE) in Sardinia
\citep{Atzenietal1990, MattaVandkilde2023}, and during the 2{nd}
millennium BCE in the Iberic peninsula \citep{Cortizasetal2016,
Leblancetal2000, Noceteetal2005}, its likely low intensity and not
extensive aspect make it hazardous to fully establish these
fingerprints in our core at these depths that would infer a
long-distance conveyance by means of atmospheric and/or trade into the
Massif Central region. Furthermore, there is no other metal enrichment
that corroborates these findings (Figure~\ref{fig5}). Therefore, source
provenance remains highly speculative at D9 and D10 and shall not be
considered with the MixSIAR statistical model.

\looseness=-1
In Supplementary Table~6, we display the number of French Pb mines that
may relate to Narcettes ICxs at each depth, with a specific interest in
the Massif Central. The Northern Massif Central (MCN) contributes the
most numerous mines in the Narcettes core (mean contribution 50\%),
followed by the southern Massif Central (MCS, 27\%) and the
mont Loz\`{e}re (LO, 23\%) while the Alps and other regions (OTH) vary
from 13 to 8\% respectively. As for EDs analyses from western European
countries (Supplementary Table~5), the uppermost D1 level likely
includes worldwide influences and is only \mbox{explained} by 6 mines from
France with no significant trend. The total number of mines reaches it
maximum during the Middle Ages (D6), and, to a lesser extent, the Late
Medieval-Early Modern period (D5) with an overwhelming contribution
from the northern Massif Central (MCN, Supplementary Table~6). The well
investigated mont Loz\`{e}re mine field does not appear to
significantly contribute to this maximum whereas intense smelting
activities have been evidenced in this district during the Middle Ages
(985--1280 CE) \citep{Baronetal2006}. Here, the mont Loz\`{e}re maximum
contribution to French mines occurs during the Modern period (16 mines
at D3--D4, ${<}$250~yr) and at Late Roman--Early Medieval and Late Iron
Age--Early Roman periods (14 mines at D6--D7) (Supplementary Table~6).

\looseness=-1
The concern raised by local contribution from the Chassezac Valley to
the Narcettes peat core is addressed with the isotopic analysis of a
galena ore (USS 23-238 collected in 2014) collected from the nearby
mine of the Colombier that is comprised within the
Sainte-Marguerite-Lafig\`{e}re and Montselgues mining district
(Supplementary Table~3). According to EDs, none of the Narcettes core
IC$_{\mathrm{xs}}$ can be explained by this mining district (the Colombier EDs
are significantly above 0.02 with a median of 0.15). Based on this
result, it is therefore reasonable to assume that Pb mining from the
Chassezac Valley didn't significantly influence the isotopic imprint
recorded in the Narcettes core. While specific mining districts are
identified from the ED analyses, it is not possible to infer which ones
have been exploited in the past. The provenance estimates from EDs
should be considered with caution as it may rely, to some extent, on
the number of existing mines from which isotopic imprints are
available. Indeed, some regions are more investigated than others due
in part to the most favored cost-effective exploration of large ore
bodies reopened during the 19{th} and 20{th}~c.\ and the lack of data
from hard to access forested and mountainous mining areas. As such,
significant smaller mining districts like those located in the Massif
Central remain to be explored. This flaw may yield a possible
erroneously substantial perception of some highly explored regions and
should be considered, most particularly when metal enrichments and/or
models are indecisive. This is why we only emphasize results that are
either corroborated by several proxies and/or statistically
significant.

\subsection{The MixSIAR Bayesian model} \label{sec5.2}

The MixSIAR Bayesian model is a flexible model structure that uses MCMC
(Monte Carlo Markov Chains) algorithms to calculate posterior
\mbox{probabilities} with Bayes rule \citep{Lambert2018,
Semmensetal2009, Wardetal2010}. It was initially coded for stable
isotopes in trophic chains and then applied to Pb isotopes
\citep{Longmanetal2018,Stocketal2018}. We use the R programming language
\citep{Rcoreteam2025}. As such, the MixSIAR model calculates the most
probable candidates for the geographic source of Pb ores. To perform a
MixSIAR Bayesian model, all the mines from each French regional and
European sources are pooled to minimize the influence of the number of
mine isotopic imprints available from each region or country and, the
number of variables that may affect the outcome of the model. MixSIAR
outcomes are density probabilities reported as mean and statistical
uncertainty for each source. These pooled sources allow to refine
provenance estimates and constitute the candidate distributions used
for Bayesian modelling using MixSIAR. The model is run with
uninformative priors (each source given the same statistical weight), a
3 million iterations Markov Chain Monte Carlo and null random mixing
factors. The model output is the literal mix of composition of mixture
from several mine imprints. As such, the posterior probability density
of potential source signatures shall diverge from the density
distribution of the experimental data, e.g., a distribution similar to
the uninformative prior. The latter can be estimated for densities
below $1/N$, $N$ being the number of potential regional sources as
determined from EDs (${<}$0.02). As explained in the previous paragraph,
D9 and D10 are not considered for this modelling. We use the
\tsup{206}Pb/\tsup{204}Pb, \tsup{207}Pb/\tsup{204}Pb and
\tsup{208}Pb/\tsup{204}Pb isotopic ratios as variables in the model.

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Scaled posterior densities (0 to 1) of studied
groups using MixSIAR. We only show countries with a significant
proportion of mix in top layers (D1 to D4, ${<}$250~yr).}
\end{figure*}

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Scaled posterior densities (0 to 1) of groups
investigated using MixSIAR. We only show countries with a significant
proportion of mix for the sample depths D5--D6~(a) and D7--D8~(b) in
the Narcettes core.}
{\vspace*{-1.5pt}}
\end{figure*}

The density distribution for each country at various sample depth is
presented in Supplementary Table~7. Figures~\ref{fig6}, \ref{fig7}a and
\ref{fig7}b show the distribution of density probabilities with means
above the uninformative prior for sample depths D1--D4 (${<}$250~yr),
D5--D6 (Medieval period) and D7--D8 (the Late Iron Age--Early Roman and
Late Roman--Early Medieval periods). During the Modern period
(${<}$250~yr), the French, Italian and Greek sources are the most
significant. The distribution of the German and Spanish probability
densities follows an almost centered uninformative null distribution
that weakens the likeliness of their single occurrences
(Figure~\ref{fig6}).

During the Medieval period (Figure~\ref{fig7}a), the French source
appears the most significant, \mbox{followed}, to a lesser extent, by
the Italian source. As for Germany and Spain, the Great Britain source
follows an almost centered uninformative null distribution that weakens
the likeliness of its single occurrence during this period. The D7 and
D8 sample depths that correspond to the Late Roman--Early Medieval and
Late Iron Age--Early Roman epochs respectively are characterized by
only two significant sources from Italy and Greece
(Figure~\ref{fig7}b).

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Scaled posterior densities (0 to 1) of groups
investigated using MixSIAR. We only show French regions with a
significant proportion of mix for the upper layers (D1 to D4,
${<}$250~yr) in the Narcettes core.}
\end{figure*}

The distribution of posterior density probabilities for country sources
(Supplementary Table~7) is not overwhelming (maximum of 0.25)
suggesting that the model cannot strongly signify single sources, and
therefore we likely face a well distributed and mixed input. Meanwhile,
significant distributions (Figures~\ref{fig6} and~\ref{fig7}) emphasize
the prominence of French, Italian and Greek sources. The Italian source
is \mbox{relevant} \mbox{during} all of the periods while the French
one is identified within the upper core and during the Medieval period.
The Greek imprint is encountered during the Modern and the Antique
periods. It should be noticed that the Greek imprint is driven by a
single mine field (Supplementary Table~5) from the Peloponnese
peninsula (Mola\"{i}), but accounts for a significant density
probability still.\ Most amazingly, while the Spanish mines are the most
numerous from the EDs, almost none is revealed with the MixSIAR model.
Aggregating the sources into single isotopic imprints and corresponding
standard deviation for each source does not preclude the French and
Italian influence, while it emphasizes the Greek source and revokes the
Spanish one. The likely rather intense mixed imprint resulting from
various input to the Narcettes core does not allow MixSIAR model to
\mbox{discriminate} highly significant single sources, but it enables
to reveal the slight, but statistically noteworthy, prominence of the
Greek, Italian and French sources over the other countries.

The density distribution for various French regions (as defined in
Section~\ref{sec5.1}) at each sample depth in the Narcettes core is
presented in Supplementary Table~7.\ Figures~\ref{fig8}, \ref{fig9}a
and~\ref{fig9}b show the distribution of density probabilities with
means above the uninformative prior for sample depths D1--D4
(${<}$250~yr), D5--D6 (Medieval period) and D7--D8 (Antique period)
respectively. The uppermost level significantly displays OTH (for other
French sources than the Massif Central and the Alps) as the most
probable source with a mean mix distribution of 0.72
(Figure~\ref{fig8}). The Armorican Massif (mine field of
Porte-aux-Moines in French Britany) is the single OTH source.
Archeological field investigation would be necessary to substantiate
mining activities for the past 250 years. As mentioned earlier,
worldwide Pb emissions may overcome French sources in the uppermost,
most recent, section of the core, and therefore other mines that are
not taken into account in this study may have significantly contributed
to the isotopic imprint at sample depth~D1. The Alps and the Massif
Central (MCN and, to a lesser extent, the mont Loz\`{e}re) constitute
the most probable mix during the past 250 years (Figure~\ref{fig8}).
These occurrences are consistent with DE analyses (Supplementary
Table~6), although the ALPS posterior densities are significantly
higher than expected from the rather low number of identified mines
(mostly from Montchabert, Saint-Pierre-Peone and La Plagne). This
\mbox{latter} result \mbox{substantiates} again the need to strengthen
provenance studies beyond the use of graphic-based estimates (like EDs)
with a more advanced statistical model such as MixSIAR that is not as
much biased by the number of available isotopic imprints available from
the databases. During the Medieval period, the French apportionment is
dominated by the Massif Central, and, to a lesser extent, the Alps
(Figure~\ref{fig9}a; Supplementary Table~7). The mont Loz\`{e}re mine
field displays significant mean density during the Modern period and
the Middle Ages (Figures~\ref{fig8}, \ref{fig9}). These results
corroborate EDs estimates (Supplementary Table~6) and the known recent
mining activity during the 19{th} and 20{th} centuries, as well as
earlier mining activity during the Middle Ages \citep{Baronetal2009,
Baronetal2010} that are discussed in Section~\ref{sec5.1}. The northern
Massif Central (MCN) not only displays the most numerous mines from the
EDs analyses, but also the highest probable density (Figure~\ref{fig8})
comforting as such archaeological findings regarding the importance of
mining activities in this mountainous region during the Middle Ages
\citep[see][]{MinvielleLarousse2025}.

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}Scaled posterior densities (0 to 1) of groups
investigated using MixSIAR. We only show French regions with a
significant proportion of the mix for the sample depths D5--D6~(a) and
D7--D8~(b) in the Narcettes core.}
\vspace*{-2pt}
\end{figure*}

The D7--D8 levels, which comprise the Late Iron Age, the Early and Late
Roman and the Early Medieval periods, highlights the Alps and the
mont Loz\`{e}re as the main probable sources to explain the mixed
isotopic imprint recorded in the Narcettes core (Figure~\ref{fig9}b).
This result suggest that the mont Loz\`{e}re mining district may have
been exploited during the Roman period \citep[][book~IV,
2.2]{Strabon0000}, and the Late Iron Age.\ Further LIA in
mont Loz\`{e}re peat \mbox{sediments} near metallurgical sites showed
significant Pb enrichments during the Late Iron Age (20~CE to 300~BCE)
\citep{Baronetal2005}. The latter is corroborated by our pollen records
that show the onset of local agriculture and pastoralism during the
Iron Age circa 800~BCE (Figure~\ref{fig2}) \citep{Dendieveletal2019}
and a sharp deforestation initiated during the 8--9{th}~c.\ BCE in
the Massif Central. Mine distribution from EDs is almost equally
distributed between the various mining regions in Massif Central and
the Alps at D7--D8. The statistical MixSIAR approach completes this
latter estimate and provides valuable insights regarding regional
assessment of the geographical mining district that explains pollutant
Pb input into our peat core.

\section{Conclusion} \label{sec6}

The Narcettes peat core records the accumulation of non-ferrous metals
that extents from the Modern period to the Neolithic, at the onset of
the chalcolithic period in western Europe. Metal excesses are discussed
on the basis of normalized data and statistical approaches to infer
human related contamination. An algebrical approach based on Euclidian
Distances between Pb isotope ratios measured in the peat and source
imprints from European mines allows to discriminate possible Pb--Ag
mines that could contribute to the recorded Narcettes Pb imprint. This
analysis evidences the overwhelming influence of Spanish mines and, to
a lesser extent, French and Italian ones. When considering French
sources only, the FMC (MCN, and, to a lesser extent, mont Loz\`{e}re
and MCS) mines prevail. While this method permits to identify specific
mines, it should be considered with caution when it comes to
deciphering the relative influence of mining districts. This is why we
use a Bayesian model (MixSIAR) that is exclusively dedicated to assess
the geographic origin of pollutant Pb accumulated into the sediment
core. This statistical approach emphasizes possible mining ore
districts from which Pb could originate. As for EDs, it is based on
stable Pb isotope markers. The most remarkable results from this
provenance model are (i)~the significant signature from France and
Italy that overwhelms the Spanish provenance in the core, (ii)~the
prominent Greek imprint, most particularly in the Late and Early Roman
periods, (iii)~the preponderance of the French sources from Massif
Central and the Alps and, most especially, (iv)~the mont Loz\`{e}re
imprints during the Roman period and the Iron Age suggesting that
mining exploitation of this site has significantly predated its
well-known medieval metallurgical activities. At last, Cu, Ni and Zn
enrichments exhibit one of the earliest metal contaminations at a
French remote site during the 4{th} millennium BCE that may result
from the atmospheric transport of these metals from regional Neolithic
copper metallurgical districts located at Roquemengarde and
Cabri\`{e}res\break (Languedoc). 

\section*{Acknowledgments}

We thank the institutions and colleagues who made this study possible:
the Service R\'{e}gional de l'Arch\'{e}ologie Rh\^{o}ne-Alpes (French
Ministry of Culture), the Conservatoire d'Espaces Naturels
Rh\^{o}ne-Alpes and the Groupe d'\'{E}tude des Mines Anciennes. We
acknowledge the support of Pierre Deschamps and H\'{e}l\`{e}ne Mariot
(CEREGE ENVITOP). Comments from three anonymous reviewers help to
greatly improve this manuscript.

\printCOI

\section*{Supplementary materials}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-339-suppl.pdf}}

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