# A large-scale Sr and Nd isotope baseline for archaeological provenance in Silk Road regions and its application to plant-ash glass

Qin-Qin Lü, Yi-Xiang Chen, Julian Henderson, Germain Bayon

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Qin-Qin Lü, Yi-Xiang Chen, Julian Henderson, Germain Bayon. A large-scale Sr and Nd isotope baseline for archaeological provenance in Silk Road regions and its application to plant-ash glass. Journal of Archaeological Science, 2023, 149, 105695 (18p.). 10.1016/j.jas.2022.105695. hal-04203927

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# A large-scale Sr and Nd isotope baseline for archaeological provenance in Silk Road regions and its application to plant-ash glass

Qin-Qin Lü<sup>a,b,*</sup>, Yi-Xiang Chen<sup>c,d</sup>, Julian Henderson<sup>e</sup>, Germain Bayon<sup>f</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Bioavailable Sr isotopes

Detrital Nd isotopes

Archaeological provenance

Plant-ash glass

Silk Roads

Central Asia

West Asia

## ABSTRACT

Bioavailable Sr and detrital Nd isotopes are important tools for archaeological provenance. To apply Sr and Nd isotopes for provenance, regional isotope databases and baselines are generally needed. For the vast Silk Road regions of Mesopotamia, Iran, and Central Asia, detailed isotopic distribution patterns essential for determining provenance may not become available in the short term due to the severe deficiency of available data. In the present work, we investigate the geo-environmental factors controlling the Sr and Nd isotopic signatures and use published data from archaeology and Earth sciences, selected by rigorous criteria, to construct the first large-scale, semi-quantitative Sr-Nd isotope baseline for these Silk Road regions. Three isotopic zones are proposed for Central Asia: CA-1 (mountains), $ε_{Nd} < -7.5$, $^{87}$Sr/$^{86}$Sr > 0.7095; CA-2 (deserts), $ε_{Nd} = -5$ to $-2$, $^{87}$Sr/$^{86}$Sr ~ 0.709; CA-3 (loess), $ε_{Nd} = -5$ to $-2$, $^{87}$Sr/$^{86}$Sr > 0.710. General isotopic signatures are suggested for Iran: $ε_{Nd} = -8$ to $-4$, $^{87}$Sr/$^{86}$Sr = 0.7075-0.7090. Three isotopic zones are proposed for Mesopotamia: MP-1 (floodplain and foothill), $ε_{Nd} = -6.5$ to $-4$ (putative extended range MP-1N, $ε_{Nd} = -8$ to $-6$), $^{87}$Sr/$^{86}$Sr = 0.7080-0.7085; MP-2 (deserts), $ε_{Nd} < -8$, $^{87}$Sr/$^{86}$Sr > 0.7085; MP-3 (Syrian Euphrates), $ε_{Nd} = -5.5$ to $-2$, $^{87}$Sr/$^{86}$Sr = 0.7080-0.7085. Within the limitation of available data, these ranges indicate the overall trend of bioavailable Sr and detrital Nd isotopic signatures for each isotopic zone, which are controlled by their geological context, climate (e.g., precipitation), and various Earth surface processes (e.g., riverine versus aeolian transport). This baseline can be used as an essential guide for geochemical contexts to suggest or verify the provenance of plant-ash glass, serving as part of an integrative Sr-Nd isotopic approach. We illustrate the potential of this approach using two case studies. By investigating the isotopic compositions of Mesopotamian plant-ash glass, we suggest possible northern Mesopotamian origins for plant ash and silica raw materials used for Mesopotamian glass-making. By reassessing medieval plant-ash glass from San Lorenzo, Italy, we propose diverse origins including Central Asia and Mesopotamia. Additionally, as part of this isotopic approach, by examining the Nd isotope mixing lines in the context provided by the Nd isotope baseline, we reveal the occurrence of glass recycling for Islamic plant-ash glass from different production zones in association with a westward spread from Mesopotamia to the Eastern Mediterranean metropolises.

## 1. Introduction

Radiogenic Sr and Nd isotopes are among the most commonly used isotopic tools in Earth sciences. Initially used for geochronological purposes, they have become indispensable in the study of geological

processes from the deep Earth to surface environments. Sr and Nd isotope analyses are based on the respective decay of radioactive isotopes $^{87}$Rb and $^{147}$Sm into radiogenic isotopes $^{87}$Sr and $^{143}$Nd. The relative abundances of radiogenic isotopes in geological samples, expressed using the $^{87}$Sr/$^{86}$Sr and $^{143}$Nd/$^{144}$Nd ratios, are set by the initial parent-

<footnote><sup>a</sup> Department for the History of Science and Scientific Archaeology, University of Science and Technology of China, Hefei, 230026, China</footnote>

<footnote><sup>b</sup> McDonald Institute for Archaeological Research, University of Cambridge, Cambridge, CB2 3ER, UK</footnote>

<footnote><sup>c</sup> CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China</footnote>

<footnote><sup>d</sup> CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei, 230026, China</footnote>

<footnote><sup>e</sup> Department of Classics and Archaeology, University of Nottingham, University Park, Nottingham, NG7 2RD, UK</footnote>

<footnote><sup>f</sup> Univ Brest, CNRS, Ifremer, Geo-Ocean, Plouzané, F-29280, France</footnote>

<footnote>* Corresponding author. Department for the History of Science and Scientific Archaeology, University of Science and Technology of China, Hefei 230026, China. E-mail address: qinqinlu@icloud.com (Q.-Q. Lü).</footnote>
daughter compositions and the elapsed geologic time. In general, mantle-derived, younger, mafic rocks tend to exhibit relatively low $^{87}$Sr/$^{86}$Sr and high $^{143}$Nd/$^{144}$Nd ratios, while crust-derived, older, felsic rocks usually possess high $^{87}$Sr/$^{86}$Sr and low $^{143}$Nd/$^{144}$Nd ratios (White and Hofmann, 1982). Because of their role in geochemical fingerprinting, Sr and Nd isotopes are particularly useful for source tracing and have been employed as tracers in a wide range of surface processes, such as tracking the source of transported sediments, provenancing desert sands and dust, and tracing the circulation of oceanic currents (e. g., Banner, 2004; Frank, 2002; Goldstein and Jacobsen, 1988; Goldstein et al., 1984; Grousset and Biscaye, 2005).

Sr and Nd isotopes are applicable to archaeological provenance studies because archaeological samples are made of materials that are ultimately derived from geological and ecological processes. Sr isotopes have been extensively used for ancient diet reconstruction and human/ faunal mobility studies (e.g., Beard and Johnson, 2000; Bentley, 2006; Ericson, 1985; Price et al., 1994, 2002). Sr and Nd isotopes (sometimes in combination with Pb isotopes) have been applied to trace the origin of ceramics (e.g., Carter et al., 2011; De Bonis et al., 2018; Li et al., 2006, 2005; Zhang et al., 2004). Importantly, Sr and Nd isotopes have been proven beneficial for plant-ash glass provenance (e.g., Brill and Fullagar, 2009; Degryse et al., 2010a, 2010b; Freestone et al., 2003; Freestone, 2006; Henderson et al., 2005, 2010) and natron glass provenance (e.g., Degryse and Schneider, 2008; Freestone et al., 2003; Ganio et al., 2012; Henderson et al., 2005; Wedepohl and Baumann, 2000). The use of Sr and Nd isotopes for plant-ash glass provenance is noteworthy. Current research has reported the isotopic signatures of plant-ash glasses produced in Late Bronze Age (LBA) Egypt and Mesopotamia, southeastern Arabia (1st century), Iraq (Sasanian period), and the Islamic Levant and Syria. For the LBA and the Islamic period, by comparing isotopic compositions of glasses from different sites and of possible raw materials, researchers have proposed independent glass-making in the Eastern Mediterranean and Mesopotamia (Degryse et al., 2010a, 2010b, 2015; Freestone et al., 2003; Ganio et al., 2013; Henderson et al., 2005, 2009, 2010).

For archaeological provenance, the interpretation of Sr and Nd isotopes varies and depends on the types of samples under investigation. For instance, $^{87}$Sr/$^{86}$Sr ratios in human and faunal tooth enamel reflect the water and dietary intake (Bentley, 2006), and measured $^{87}$Sr/$^{86}$Sr ratios in plant-ash glass are inherited from the ashed halophytic plants used as flux in glass-making (Freestone et al., 2003). The Sr contents in these samples have a biological origin, thus, the Sr isotopic composition should be consistent with the local bioavailable Sr isotopic range. Meanwhile, for natron glass, if the Sr contents were mainly introduced by the aragonite-rich coastal sand used for glass-making, its $^{87}$Sr/$^{86}$Sr composition should reflect that of seawater (Freestone et al., 2003), and other lime sources in the sand and the addition of limestone can also affect the Sr isotopic composition of natron glass. For Nd isotopes, the Nd budget and corresponding $^{143}$Nd/$^{144}$Nd composition of archaeological glass are controlled by vitrifying materials such as quartz sand (Degryse and Schneider, 2008). Meanwhile, for pottery, the Nd contents derive mostly from clay and temper sources. Therefore, for provenancing glass and ceramics, the Nd isotopic signatures associated with regional clastic sediments (sand, gravel, clay, silt, etc.) are more relevant (referred to as 'detrital Nd').

Provenance studies typically require a reference database that can be used for comparison purposes. Unfortunately, available Sr-Nd isotopic data are still lacking in many regions. This is the case for the large area extending from Mesopotamia to Central Asia, which has a large spectrum of geo-environmental conditions and an immensely rich cultural heritage. This region was crucial for the communications and exchanges on the Silk Roads. However, to date, this area has been geochemically and archaeologically understudied. Considering the cost of sampling and analysis, the low density of data in this large region is not expected to be improved in the short term. This situation is especially true for plant-ash glass research. Existing chemical and isotopic analyses have

mainly focused on the Eastern Mediterranean. Meanwhile, isotope data of plant-ash glass and glass-making materials from Mesopotamia are currently limited, and to our knowledge, no glass-related Sr and Nd isotopic analyses have been formally reported from Iran and Central Asia (except for the mention of Sr isotope results in Brill and Fullagar, 2009). Without previously accumulated data for comparison, the potential of newly reported data cannot be fully exploited. For example, although Sasanian glass from Veh Ardashir, Iraq was studied with Nd and Sr isotopes (Ganio et al., 2013), the specific provenance of the glass was not suggested since no isotopic compositions of raw materials or comparable glass samples were available.

In the absence of an extensive reference database, identifying the general compositional variability of the regions under investigation represents a first step toward successful provenance determination. As already shown by metal provenance studies using lead isotopes, if large-scale Pb isotopic signatures are known for the regions, a geologically-informed inference about metal provenance can be made in the absence of isotope data from individual reference ores (Albarède et al., 2012; Artioli et al., 2020). When provenancing biological materials using Sr isotopes, even if a specific area of origin cannot be identified, eliminating the possibility of certain areas is still beneficial for provenance (Holt et al., 2021). Also, based on large-scale geologic histories of Pb ore formation, regions with ore deposits bearing indistinguishable Pb isotopic compositions can be indicated (Killick et al., 2020). These provenance studies using various isotope systems evidently show that a consideration of large-scale isotopic properties can significantly inform future research.

In the present work, we attempt to construct a general Sr-Nd isotope framework for archaeological provenance in the regions of Mesopotamia, Iran, and Central Asia to partly alleviate the current lack of a reference database. We investigate the geological and environmental factors affecting the isotopic signatures, and compile published bioavailable Sr and detrital Nd isotope data from archaeological and environmental samples that fit our selection criteria. Rather than finding exact isotopic values, we propose major isotopic zones and the likely isotopic compositional ranges of each zone. The validity of this semi-quantitative Sr-Nd isotope baseline is confirmed in the two case studies that follow.

We also propose an integrative isotope approach for plant-ash glass provenance that is based on four pillars: chemical composition, Sr and Nd isotopic compositions, isotope mixing lines, and Sr-Nd isotope baseline. Chemical and Sr-Nd isotopic compositions are used to categorize glass artifacts into clusters that may indicate similar origins, as have been widely applied in current glass provenance works. Isotope mixing lines may suggest the occurrence of recycled glass and help explain compositional clustering results. The regional isotope baseline places compositional clusters into a geo-environmental context essential for verifying the sources of raw materials and the origin of glass. By seeking corroboration from the four lines of evidence, the best understanding of glass production and dispersion can be obtained. Using case studies of plant-ash glasses excavated from Mesopotamia and Italy, we demonstrate the benefit of using the Sr-Nd isotope baseline and this integrative approach for plant-ash glass provenance.

## 2. Method

### 2.1. Description

It is worthwhile to first look at how isotope baselines have been established in other regions. There have been considerable efforts to build quantitative regional isotope baselines ('isoscapes'), usually by processing a large amount of data from direct sampling. For example, many of the bioavailable Sr isotope maps are based on spatially dense empirical data (e.g., Blank et al., 2018; Evans et al., 2010; Frei and Frei, 2011; Hartman and Richards, 2014; James et al., 2022; Ladegaard-Pedersen et al., 2020; Laffoon et al., 2012; Nafplioti, 2011; Scaffidi and
Knudson, 2020; Snoeck et al., 2020; Wang et al., 2018; Wang and Tang, 2020; Willmes et al., 2018, 2014). An alternative approach is to predict $^{87}$Sr/$^{86}$Sr ratios by leveraging geochemical and environmental knowledge about bedrock age and lithology as well as chemical weathering (Bataille et al., 2014; Bataille and Bowen, 2012; Beard and Johnson, 2000; Hegg et al., 2013). However, accurate mechanistic modeling of complex Earth surface processes on a large spatial scale often poses serious challenges. Consequently, bioavailable Sr isoscapes that statistically incorporate empirical data and geo-environmental covariates have been under development (e.g., Bataille et al., 2020, 2018; Brennan et al., 2016; Janzen et al., 2020; Serna et al., 2020).

In contrast, there has been relatively less focus on Nd isotope baselines, although Nd isotope data of terrestrial surface rocks have been aggregated (e.g., the compilations by Blanchet, 2019; Jeandel et al., 2007; Robinson et al., 2021, and EarthChem http://portal.earthchem.org/) and can be used for sediment provenance. Compared to Sr isotope databases, Nd isotope databases have been developed more recently and have smaller coverages (Banner, 2004). Furthermore, unlike radiogenic Sr, which displays a uniform isotopic composition in seawater at a given point in geologic time (e.g., modern seawater $^{87}$Sr/$^{86}$Sr = 0.709179, Mokadem et al., 2015), Nd isotopic ratios exhibit a large variability in seawater due to a much shorter oceanic residence time (Lacan et al., 2012; Tachikawa et al., 2017), and a single global Nd isotope signal did not exist (Banner, 2004).

Establishing a quantitative baseline requires both systematic sampling and prior geological knowledge. In empirical studies of bioavailable Sr isoscapes, a geologic map is usually needed to guide the choice of sampling locations, and lithological grouping is commonly used as a useful constraint. On the other hand, the results of ab initio modeling need to be examined against empirical data to verify the model's predictability. In fact, an isoscape's potential to predict generally relies on geologic understanding and the density of data, both of which need to positively correlate with the complexity of regional geology (e.g., see case studies in Bataille et al., 2020). However, if detailed geologic surveys and environmental monitoring have not been conducted in a region and available isotope data are scarce or nonexistent, then neither geologically-informed models nor the interpolation of data points would have a prediction power larger than qualitative research.

To trace the origins of archaeological samples in areas with limited data, we need an approach that fully harnesses the information at our disposal. This approach should take heed of the underlying geological and environmental background and investigate the major controls of isotopic compositions. This would help demarcate the region into major geo-environmental groups (or isotopic zones), thereby accounting for the most general spatial variations. On top of this qualitative zoning, available empirical data can be used to anchor the representative isotopic value of each identified isotopic zone, although the exact isotopic ranges of these zones will be determined by future reported data. In this way, the established semi-quantitative baseline is still built on both actual data and geological/environmental knowledge. In essence, this is somewhat analogous to the domain mapping method of isoscape construction that was used in previous studies on bioavailable Sr isoscapes (e.g., Evans et al., 2010; Hodell et al., 2004; Ladegaard-Pedersen et al., 2020; Snoeck et al., 2020). The domain mapping approach links data (or data clusters) to geographical groups with similar lithology, and is best used for areas with homogeneous, well-understood lithological conditions and where superficial deposits do not significantly deviate from bedrock geology (Holt et al., 2021). In the present study, we attempt to relate both Sr and Nd isotope data to specific areas while considering not only the underlying lithologies but also exogenous (such as riverine and atmospheric) inputs. Although our data are not acquired from proactive sampling, which is impractical at this stage, we set rigorous criteria for selecting published data with the aim of archaeological provenance.

Finally, another subtlety about sampling arises from the nature of archaeology. In the provenance of synthesized ancient material culture, the artifact to be provenanced is compared to raw ingredients possibly

used to make it. Preferably, we should compare excavated ancient material culture with raw ingredients occurring near production centers and known to be relevant to ancient production activities. However, this is usually rather impractical. For instance, historical records of glass production or direct archaeological evidence for glass-making are lacking in many regions. As a result, scientists have conducted large-scale surveys of possible raw materials in the hope of narrowing the range of sources for provenance. Such examples include the investigation of the isotopic compositions of Syrian and Lebanese plants and sands for plant-ash glass provenance (Henderson et al., 2009, 2020b), and the search using chemical and Sr-Nd isotopic compositions for beach sands suitable for Roman glass-making (Brems et al., 2012, 2013a, 2013b). These studies show that, with well-defined sample selection and/or data screening strategies, the consideration of environmental samples from a large geographical range can inform the search for possible raw materials and help provenance material culture.

## 2.2. Data compilation

We assembled bioavailable Sr and detrital Nd isotope data from existing archaeological and geoscience publications. The sampling locations are displayed against the background of general bedrock geology in Fig. 1. The Sr isotopes are reported using the $^{87}$Sr/$^{86}$Sr ratio, and the Nd isotopes are expressed using the conventional $\epsilon_{Nd}$ notation:

$$ \epsilon_{Nd} = \left[ \left( \frac{^{143}\text{Nd}/^{144}\text{Nd}}{^{143}\text{Nd}/^{144}\text{Nd}} \right)_{\text{sample}} - 1 \right] \times 10^4 $$

where $(^{143}\text{Nd}/^{144}\text{Nd})_{\text{CHUR}} = 0.512638$. Note that all data used in this study have been normalized to the same $(^{143}\text{Nd}/^{144}\text{Nd})_{\text{CHUR}}$ value. The $\epsilon_{Nd}$ parameter denotes the extent to which the sample's Nd isotope ratio deviates from the present-day value of the Chondritic Uniform Reservoir (CHUR) (Jacobsen and Wasserburg, 1980).

For provenance studies using isotopes, it is paramount to use geologically screened data whose geologic significance has been carefully assessed (Artioli et al., 2020). Our data selection criteria are explained as follows. We focused on data from surface features and neglected geology deeper than near-surface. Because we are mainly concerned with large-scale trends among the regions, we prefer data representing average values for large areas when such data are available.

Sr is a mobile element that is preferentially leached from rocks during chemical weathering and erosional processes. The exchangeable (i.e., bioavailable) Sr fraction entering the hydrosphere is dominated by preferential dissolution of easily altered rocks (such as carbonates and evaporates) or rock-forming minerals (such as apatite and plagioclase), which are commonly enriched in Sr (e.g., Aubert et al., 2001; Capo et al., 1998). In addition, atmospheric inputs in the form of wet (rainfall, sea spray) and dry (aeolian dust) deposition, glacial deposition, and agricultural fertilizers also contribute to the exchangeable Sr pool in soils (e.g., Capo et al., 1998; Hartman and Richards, 2014; Holt et al., 2021; Pett-Ridge et al., 2009; Hedman et al., 2009). Therefore, despite major influence from the substrate, the bioavailable $^{87}$Sr/$^{86}$Sr in soils (and by extension, the terrestrial biosphere) does not exactly mirror the bedrock's isotopic value (Bentley, 2006), and substantial disparities in Sr isotopes among whole rocks, bulk soil, and the exchangeable fraction in soils may occur (Bataille et al., 2020; Sillen et al., 1998). The labile Sr in soils is taken up by plants and enters the food chain. However, we did not include soil leachates in the data used for baseline construction since they are usually very variable and may yield values inconsistent with plants (Evans and Tatham, 2004; Maurer et al., 2012; Ryan et al., 2018). We have, however, included results from surface water (but not groundwater), such as rivers and lakes, as suggested by existing research (e.g., Evans et al., 2010; Frei and Frei, 2011; Ladegaard-Pedersen et al., 2020; Maurer et al., 2012; Wang et al., 2018). Surface water reflects the average Sr isotopic composition of large catchment areas, serving as an
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (Color online) Map of Mesopotamia, Iran, and Central Asia showing simplified bedrock geology, sampling or excavated locations (sample types of vegetation, water, faunal or human samples for Sr isotopes, and surficial or riverine detrital sediments for Nd isotopes, and plant-ash glass), and major ancient cities. The map was created with QGIS (https://www.qgis.org/) by adapting two overlaid layers: CGMW Bedrock and Structural Geology and Natural Earth II with Shaded Relief, Water, and Drainages.</figcaption></figure>
easy-to-obtain signal to characterize the watershed when more extensive
sampling is absent. When spatially dense sampling becomes available,
data from large water bodies may be replaced but can still function as
useful references.

Because measured radiogenic isotopes are systematically normalized
to a given stable isotope ratio during analysis (for instance using
$^{86}$Sr/$^{88}$Sr = 0.1194), reported $^{87}$Sr/$^{86}$Sr values in the literature are not
determined by natural mass-dependent fractionation related to biolog-
ical or Earth surface processes. This implies that measured Sr isotopic
compositions for plant or animal samples faithfully correspond to local
bioavailable $^{87}$Sr/$^{86}$Sr signatures, e.g., as shown by relatively consistent
$^{87}$Sr/$^{86}$Sr ratios transferred up the food chain (Blum et al., 2000), except
for those samples that cannot be linked to any specific regional habitat,
such as migratory animals. Vegetation is one of the recommended ar-
chives for constructing Sr isoscapes since it is widely available and
directly represents the strontium that enters the food chain at a specific
locality (e.g., Evans et al., 2010, 2009; Evans and Tatham, 2004; Holt
et al., 2021; Maurer et al., 2012; Snoeck et al., 2020; Willmes et al.,
2018). A single plant may exhibit unrepresentative values if it absorbs Sr
from a confined or discrete cation pool, thus, the collection and com-
bination of multiple plants from near the same sampling point ('ho-
mogenized sampling') are suggested to reduce the effect of extremely
localized biases (Holt et al., 2021). Here, to construct Sr baselines, we
ensured that for plant data, each data point contains multiple plant
samples. Small, low-mobility animals have also been advocated as a
good choice of samples to represent the local bioavailable $^{87}$Sr/$^{86}$Sr
since they have a local feeding range (Price et al., 2002). Gathering
animal samples was not feasible for us, so we have included in the
dataset archaeological human and faunal samples. The data used are
from archaeologically excavated tooth enamels and eggshells (from os-
trich, a flightless bird). Ostrich shells from Mesopotamia have been
tentatively included considering the mention of ostrich exploitation in
Assyrian texts (Hodos et al., 2020) and that they generally match other
data from the region. Data from bones were not chosen because bones
typically undergo remodeling throughout the individual's lifespan and
are also prone to diagenetic contamination (Bentley, 2006; Hoppe et al.,
2003; Koch et al., 1997; Price et al., 2002). We did not use data from
snails because they are often biased toward the values of rainwater and
carbonates (Evans et al., 2010). To ensure the local origins of archaeo-
logical samples, we excluded any data that were indicated in the source
publications as immigrants or having nonlocal origins. Normally, Sr
isotope data from modern humans should not be used for archaeological
provenance due to their increased mobility and dietary variety. We
have, however, included data from modern Iranians and one modern
Afghan individual as secondary evidence because there are no or very
few data reported from these regions. Due to changes in environmental
conditions over the geologic timescale, fossils are not used as data
sources.

In particular, to provenance plant-ash glass, we are most concerned
with the Sr isotopic compositions of plants. In mobility studies, plant
samples are typically used to derive a range that interprets the origin of
animals or humans. Here, we explain the motivation behind using
excavated faunal and human samples to provenance plant-ash glass.
Since animals acquire their Sr isotopic signatures from averaging their
food intake, they have been observed to possess $^{87}$Sr/$^{86}$Sr ratios that are
less variable than the plants they consume, thus, faunal samples should
provide a conservative constraint for the local biome (Price et al., 2002).
In other words, faunal/human samples can inform, in an average sense,
the overall trend of the local plants' isotopic compositions (Holt et al.,
2021) and, by extension, the isotopic composition of plant-ash glass. In
this context, whether the $^{87}$Sr/$^{86}$Sr ratio of a specific glass artifact is
slightly outside the range given by faunal/human samples is less
important since we only need to provide an anchor for bioavailable
$^{87}$Sr/$^{86}$Sr ranges instead of obtaining the full range.

Applying Nd isotopes for provenancing archaeological materials
such as glass and ceramics is largely analogous to using them for

sediment provenance. The Sm and Nd contents in these archaeological
materials are typically associated with REE-bearing accessory minerals
(e.g., monazite, allanite, apatite, titanite, zircon) and some of the rock-
forming minerals (e.g., feldspar, pyroxene), and these minerals are
contained in detrital particles as raw ingredients derived from the
erosion of crustal rocks (Bayon et al., 2015; Brems et al., 2013b). For
siliciclastic sediments, chemical weathering and mineral sorting during
sediment transport generally result in negligible Nd isotope decoupling
(Bayon et al., 2015; Meyer et al., 2011). The Nd isotope ratio, therefore,
acts as a robust provenance proxy that is generally unaffected by Earth
surface processes and preserves the primary isotopic signature of the
source area (Banner, 2004; Bayon et al., 2015; Degryse and Schneider,
2008; Grousset and Biscaye, 2005). Therefore, reported Nd isotopic
analyses for surface detrital sediment can generally be used as reliable
local Nd isotopic signatures in archaeological provenance studies. Here,
we have included data from riverine and wetland sediments, desert
sand, soil, and the top surface of drilled sediment cores. Data from ore
deposits are not used since they are irrelevant to the provenance of
siliciclastic-sediment-based artifacts (e.g., glass and ceramics).

It is worth emphasizing that Nd isotopes in non-silicate components of sediments are not considered in this study. Marine precipitates such as carbonates are typically more radiogenic in Nd isotopes than the associated silicates and are also preferentially weathered (Faure and Mensing, 2005; Goldstein and Jacobsen, 1988; Hindshaw et al., 2018). Sedimentary Fe-oxides may display different $\epsilon_{Nd}$ signatures than detrital fractions (Bayon et al., 2020). Other non-terrogenous sediment phases (e. g., Fe-Mn oxyhydroxides and organic matter) may contain Nd as well (Bayon et al., 2002). It is therefore nontrivial to clarify that the Nd isotope data used here are specifically of the terrigenous fraction, i.e., the silicate residues after the organic and carbonate fractions have been removed by the leaching process. This implies that if a significant portion of the Nd contents in the archaeological material to be provenanced have a non-continental origin, this baseline may not fully apply.

We additionally discuss aeolian and riverine samples as archives for Sr and Nd isotopes. Aeolian deposition can be significant and may sometimes dominate the Sr and Nd budgets in the surface layer (Bataille et al., 2020; Hartman and Richards, 2014; Pett-Ridge et al., 2009; Weldeab et al., 2002). In dust, $^{87}$Sr/$^{86}$Sr typically strongly increases as grain size decreases, indicating that it is sensitive to the effect of wind sorting, but the Nd isotope ratio does not vary much with grain size and is considered more pertinent to the dust origin (Feng et al., 2009). The sources of aeolian dust can be highly complex and are not necessarily local (see discussion in the online Supplementary Material – SM). Aeolian dust from exposed marine carbonates has a very different impact than that from siliciclastic sediments on the $^{87}$Sr/$^{86}$Sr signature of the surface (Capo et al., 1998). Therefore, it is necessary to consider aeolian deposition as one of the controlling processes, but we do not recommend directly using isotopic compositions of dust to establish the local range.

River systems and other water bodies integrate the isotopic compositional variability of fluxes in their catchments, thus fitting our purpose of examining large-scale trends. Therefore, dissolved Sr and detrital sediment Nd in rivers and water bodies can be used to construct Sr and Nd isotope baselines on the premise that the geo-environmental conditions of the watershed are known. The weathering of carbonaceous and other marine-derived sedimentary rocks generally shifts $^{87}$Sr/$^{86}$Sr in water bodies toward less radiogenic values (Bayon et al., 2021) and causes the dissolved load's $\epsilon_{Nd}$ value to differ from the suspended particulates (Bayon et al., 2015). The preferential weathering of volcanic rocks in the drainage basin can also lead to a positive $\epsilon_{Nd}$ offset for fine-grained sediments (Bayon et al., 2015) and a negative $^{87}$Sr/$^{86}$Sr shift for dissolved cations (Voerkelius et al., 2010). In riverine sediments, the Sr isotopic composition of the fine detrital fraction may deviate toward more radiogenic levels compared to the coarse fraction, while Nd isotopes are not significantly dependent on grain size (Bayon et al., 2015, 2021).
We have compiled Sr isotope data (Clauer et al., 2000; Elliott et al., 2015; Gregoricka, 2013; Henderson et al., 2009; Hodos et al., 2020; Kenoyer et al., 2013; Kutterer and Uerpmann, 2017; Pokrovsky et al., 2017; Posey, 2011; Wang et al., 2018) in Table S1 and Nd isotope data (Bayon et al., 2015; Chen et al., 2007; Cullen et al., 2000; Henderson et al., 2020b; Kumar et al., 2020; Sharifi et al., 2018; Sirocko, 1994) in Table S2 in SM Section 1. Since the data are not from preplanned sampling, they are inevitably distributed unevenly. As an intuitive reference for our estimation of isotopic ranges, we have also summarized, in Tables S3 and S4 (SM Section 5), the distinct isotopic groups identified in three studies (bioavailable Sr isotopes for France, Willmes et al., 2018, and the Maya region, Hodell et al., 2004, and Nd isotopes of sediments in rivers with medium or small watershed sizes, Bayon et al., 2015).

### 3. Isotope baseline

We investigate the isotopic signatures for each region following the Silk Road route from east to west by surveying the tectonic, erosion, and transport controls that are at work. In the following, for Central Asia, Iran, and Mesopotamia, we describe corresponding isotopic zones and their respective approximate ranges for Nd and Sr isotopes. A detailed discussion of the characteristic geological/environmental conditions of each isotopic zone is given in SM Sections 2–4. The isotopic ranges are decided by considering published data (Table S1, Table S2, Fig. 2). In addition to these data, additional data as indirect or secondary evidence are occasionally mentioned in the following discussion. The results are given in Table 1 and Fig. 2. Since our intention is to look for broad characteristics rather than exact values, the upper and lower bounds of these estimated ranges are set arbitrarily.

#### 3.1. Central Asia

The distribution patterns for bioavailable Sr and detrital Nd isotopes in Central Asia are mainly controlled by the combined effects of rock erosion, aeolian deposition, and river transport (see SM Section 2). Based on the observed variability in distinct geo-environmental conditions, Central Asia can be divided into three main isotopic zones. Zone 1

<figure><img src="image_5.png" /></figure>

**Fig. 2.** (Color online) Estimated detrital $ε_{Nd}$ and bioavailable $^{87}$Sr/$^{86}$Sr ranges are displayed together with reported data (median value if consisting of multiple data) from environmental and bioarchaeological samples. The gray-colored circled numbers correspond to the data numbers in Tables S1–S2 (Section 1 in SM). The following isotopic zones are proposed: Central Asia (CA) 1–3, Iran, and Mesopotamia (MP) 1–3. Isotopic ranges of these zones are described in Table 1. Major geological features controlling the isotopic distribution patterns are indicated: Cenozoic & Mesozoic marine carbonate sediments, the Zagros, Mesopotamian deserts/semi-deserts, the CAOB, and the Tian Shan & Pamir Mountains. This illustration is only intended for indicating relations between the zones, and the ranges should not be taken as exact. See the text and the SM for data sources and details.

consists of mountainous areas including the Tian Shan and Pamirs, dominated by old continental rocks of Paleozoic and Precambrian ages. The glacier-fed rivers Amu Darya and Syr Dara may also exhibit isotopic signatures similar to those of Zone 1. Zone 2 includes deserts and sedimentary basins, which are mostly covered by Mesozoic and Cenozoic carbonaceous sedimentary rocks deposited in the Tethys and the Paratethys seas. Zone 3 is the piedmont loess in eastern Central Asia that resulted from aeolian deposition. Wind sorting is likely important in areas with significant aeolian deposition (Zones 2 and 3). Many locations of historical and archaeological significance, such as the Fergana Valley and Samarkand, are close to all three isotopic zones (Fig. 1).

Limited Nd and Sr isotope data have been reported for Central Asia. On a larger scale, siliciclastic materials of the massive Central Asian Orogenic Belt (CAOB) should overall have negative, relatively moderate $ε_{Nd}$ values (such as several northern Chinese deserts in the CAOB with an average $ε_{Nd}$ of -4.4, Li et al., 2011), but variations among blocks are expected to be significant. The $ε_{Nd}$ values for deserts and loess in northern Central Asia may be comparable to sand in the nearby Gurbantunggut Desert in the Junggar Basin, northern Xinjiang, whose $ε_{Nd}$ values were reported at -4.0 to -1.2 (Chen et al., 2007).

For the piedmont loess at the foot of mountains, because bioavailable Sr data are unavailable, soil in Kyrgyzstan from foothill locations such as Bishkek could be used as a strained reference, which has an average $^{87}$Sr/$^{86}$Sr of 0.71579 (digested) (Dewan et al., 2015). The silicate component in the loess from the upper reaches of the Naryn River (a major tributary of the Syr Darya) was reported to have $^{87}$Sr/$^{86}$Sr of approximately 0.720 (not bioavailable data) (Bujakaite et al., 2016). As a comparison, the mean $^{87}$Sr/$^{86}$Sr of loess leachates in the Chinese Loess Plateau is 0.71122 (Wang and Tang, 2020).

For the Central Asian basins, the Aral Sea has been previously analyzed for $^{87}$Sr/$^{86}$Sr in sediments (0.70992, Dewan et al., 2015) and in carbonates and water (both at 0.70914, Pokrovsky et al., 2017), and the latter value can be used as a regional bioavailable Sr signature for the sedimentary basin. The main source of exchangeable Sr in the Aral Basin is the Cretaceous–Paleogene calcareous strata derived from the Tethys and Paratethys sedimentary rocks (Pokrovsky et al., 2017), which should yield low $^{87}$Sr/$^{86}$Sr ratios characteristic of seawater at that time (approximately 0.708, see Burke et al., 1982; McArthur et al., 2012). However, the weathering of silicate components brought by wind and rivers likely elevates the bioavailable $^{87}$Sr/$^{86}$Sr.

For the Central Asian mountains, reported Sr data from similar geologic zones in the vicinity are mentioned here as likely parallels: bioavailable $^{87}$Sr/$^{86}$Sr is 0.7105 ± 0.0007 (1σ) for the Tian Shan and northern Tarim and 0.7110 ± 0.0009 (1σ) for the eastern Pamirs, both of which were reported from river waters on the Chinese side of the mountains (Wang et al., 2018). The riverine sediment from the final stretch of the Amu Darya in Uzbekistan has been analyzed: the $^{87}$Sr/$^{86}$Sr ratio is 0.718455 for clay particles (not bioavailable data) (Bayon et al., 2021), and the $ε_{Nd}$ value for the silt fractions is measured at -9.2 (Bayon et al., 2015), showing influence from mixed continental sources. Due to the efficient mixing of fine particles during riverine transport, the fine-grained sediment load in the lower course of rivers (or at the river mouth) provides integrated provenance information at the catchment scale. However, mountains often contribute significantly to the total Sr flux due to higher weathering rates at high altitudes and constant weathering of mountain-derived suspended particulates along the river's course (Bataille et al., 2020). The $^{87}$Sr/$^{86}$Sr ratio of the Syr Darya water is 0.7094, indicating a strong influence from weathered silicates, and the $^{87}$Sr/$^{86}$Sr ratio of the Amu Darya water is expected to be similar, if not higher, since both rivers are significantly affected by sediments from mountains and drain geologically comparable areas (Pokrovsky et al., 2017) and the Amu Darya may be especially prone to the influence of upstream detrital sediments (see Section 2 in SM). In addition, the Nd isotope ratios of the silicate fractions of cryoconites sampled from glaciers in the Tian Shan of Kyrgyzstan and the Pamir of Tajikistan range from -11.2 to -7.5 and -9.4 to -9.1, respectively, and the $^{87}$Sr/$^{86}$Sr
<table><caption>**Table 1**
Estimated detrital εNd and bioavailable<sup>87</sup>Sr/<sup>86</sup>Sr ranges.</caption><thead><tr><th>Region</th><th>Isotopic zone</th><th>Zone description</th><th>εNd</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>Main geo-environmental characteristics</th></tr></thead><tbody><tr><td rowspan="3">Central Asia (CA)</td><td>Zone 1</td><td>mountains, Amu Darya, Syr Darya</td><td>&lt; -7.5</td><td>&gt;0.7095</td><td>Precambrian & Paleozoic crustal rocks, river transport</td></tr><tr><td>Zone 2</td><td>deserts and sedimentary basins</td><td>-5 to -2</td><td>~0.709</td><td>Mesozoic & Cenozoic calcareous sediments, aeolian deposition</td></tr><tr><td>Zone 3</td><td>loess plateau, mountains</td><td>-5 to -2<br/>-8 to -4</td><td>&gt;0.710<br/>0.7075-0.7090</td><td>aeolian deposition<br/>Mesozoic & Cenozoic calcareous sediments, mix of mafic and felsic rocks</td></tr><tr><td rowspan="3">Iran</td><td>details unknown</td><td></td><td></td><td></td><td></td></tr><tr><td>Zone 1</td><td>foothills, Tigris River, floodplain</td><td>-6.5 to -4 (putative Zone 1 N: -8 to -6)</td><td>0.7080-0.7085</td><td>river transport, fluvial deposition</td></tr><tr><td>Zone 2</td><td>deserts, semi-deserts, and uplands</td><td>&lt; -8</td><td>unknown, assumed &gt;0.7085</td><td>aeolian deposition, erosion of Mesozoic & Paleozoic outcrops, Mesozoic & Cenozoic calcareous sediments</td></tr><tr><td>Mesopotamia and environs (MP)</td><td>Zone 3</td><td>Euphrates Valley (Syria)</td><td>-5.5 to -2</td><td>0.7080-0.7085</td><td>Cenozoic mafic formations, river transport</td></tr></tbody></table>

ratios of soluble, organic, and carbonate fractions in the cryconites from the Tian Shan are all slightly above 0.710 (Nagatsuka et al., 2014). These values may result from relatively local sources, since long-distance aeolian transport is considered less significant in Central Asia mountains (Li et al., 2018).

Central Asia's southwestern corner may be associated with slightly lower isotopic values, as the $^{87}$Sr/$^{86}$Sr ratio for the Kara-Bogaz-Gol Bay is 0.708153 (Clauer et al., 2000), although this value is affected by water inputs from the nearby Caspian Sea. Isotopic signatures similar to southern mountainous Central Asia may also apply to Afghanistan. It has been reported that $^{87}$Sr/$^{86}$Sr is 0.70904 for modern human tooth enamel from Afghanistan (Posey, 2011). The εNd for the dust from the Sistan Basin — which receives alluvial clasts from rivers originating in the Hindu Kush and traversing central Afghanistan — is estimated at -11.5 (Kumar et al., 2020).

Combining these reported analyses, we propose that Zone 1 may have relatively high bioavailable $^{87}$Sr/$^{86}$Sr (more than 0.7095) and relatively low εNd values (for instance, lower than -7.5). Zone 2 should show slightly lower $^{87}$Sr/$^{86}$Sr (~0.709) and relatively high εNd values (probably -2 to -5). Zone 3 may exhibit high $^{87}$Sr/$^{86}$Sr (likely larger than 0.71) and relatively high εNd values (probably -2 to -5). It has been previously observed that in areas dominated by silicate lithologies (e.g., mudstone, sandstone, gneiss, and granite), bioavailable $^{87}$Sr/$^{86}$Sr ratios are typically more variable, while carbonate-dominant, lithologically homogeneous areas usually display a tight bioavailable $^{87}$Sr/$^{86}$Sr range (Bataille et al., 2020; Evans et al., 2010). Additionally, Sr isotopic signatures in topographically varying areas are harder to predict than generally level areas (Evans et al., 2009). Consequently, we expect a more variable isotopic range for geochemically complex, high-relief areas such as Zone 1 compared to carbonate-rich, flat territories such as Zone 2.

## 3.2. Iran

Iran's highland landscape was formed as a consequence of the Cimmerian Orogeny and the Alpine-Himalayan Orogeny. Much of Iran is covered by Mesozoic and Cenozoic sediments, interspersed by old and young igneous outcrops including extrusive and intrusive rocks and ophiolites. We can theoretically divide Iran into four major zones on the grounds of geologic history: northern Iran, central Iran, western Iran (the Zagros Mountain), and southeastern Iran (see SM Section 3). Iran is dominated by arid and semi-arid climates. Considering the absence of major glacier systems and large rivers, bioavailable Sr and detrital Nd isotopic distribution patterns for each zone should be mostly controlled by relatively local surficial sediments, although all four zones are marked by complex and diverse lithologies. The Zagros and Alborz Mountains are among the regions receiving the highest precipitation in Iran and are expected to dominate the Sr and Nd isotopic patterns in western and northern Iran through rock erosion/weathering and river

transport. Soils in Iran are generally highly calcareous, with calcium carbonate usually above 25% (Roozitalab et al., 2018).

Isotope data reported from Iran are extremely inadequate. Fine-grained river sediments of the Sefid Rud in northwestern Iran, which originates in the Zagros and flows out of the Alborz into the Caspian Sea, have been analyzed for sediment Nd isotopes, showing relatively radiogenic εNd values ranging between -6.5 and -4.6 (Bayon et al., 2015; Tudryn et al., 2016), a result from integrating varied lithologies (see Table S4). Soil from Fars Province in southern Iran is characterized by a lower εNd value of -7.4 (Kumar et al., 2020). Because the small rivers draining the Zagros carry clastic sediments to the Persian Gulf and the Gulf of Oman, the top surface of the sediment core extracted from the Gulf should roughly reflect the gross composition of rocks in the Zagros, which shows an εNd range of -6.9 to -6.0 (Sirocko, 1994). The sand dunes in the northern UAE, which are close to the Zagros extension in northern Oman, have εNd = -5.3 (Kumar et al., 2020). Notably, there is a significant lack of available data for central Iran.

There is only one site with faunal tooth enamel data publicly reported, which is Tepe Yahya ($^{87}$Sr/$^{86}$Sr = 0.708132(Q1)-0.708301(Q3)) in Kerman Province, southern Iran (Gregoricka, 2013). In northern Iran, the Sr isotope ratios from the Caspian Sea and the rivers discharging into the Caspian from its southern coast are 0.708183 and 0.708293, respectively (Clauer et al., 2000), although the value of the Caspian reflects all of its catchment (including outside Iran). Unpublished Sr isotope data of faunal samples from the Atrek River valley in Kopet Dagh, northern Iran also align with the low Sr isotopic signatures mentioned above (personal communication with Zihua Tang). Modern human tooth enamels from cities across Iran show a large variation, although most of the data center around the approximate range of 0.70809-0.70840 (Posey, 2011), which concurs with Tepe Yahya and the Caspian waters. In this dataset of modern humans, no significant variations exist among different cities. This could support an overall uniformity of low $^{87}$Sr/$^{86}$Sr in Iran but could also result from a diverse diet for modern humans that smooths out contrasting isotopic values.

Currently available data from Iran do not allow for a delineation of distinct ranges for each zone. Nonetheless, we can use educated guesses to suggest Iran's most general isotopic signature. The bioavailable Sr isotopes in Iran most likely derive from the erosion of marine and continental deposits in surficial sediments. This most likely involve Mesozoic marine-derived carbonates, which should hence exhibit $^{87}$Sr/$^{86}$Sr ratios between 0.707 and 0.708 (McArthur et al., 2012). Additional endmembers likely include Tertiary volcanic rocks (usually low in $^{87}$Sr/$^{86}$Sr) and the widely occurring Cenozoic/Mesozoic siliciclastic sediments (usually high in $^{87}$Sr/$^{86}$Sr). We therefore tentatively propose a bioavailable $^{87}$Sr/$^{86}$Sr range of 0.7075-0.709. Estimating the Nd isotopic signature is even more challenging. The regional terrigenous source of Nd likely corresponds to a mixture of Cenozoic and Mesozoic silicate sediments and Tertiary volcanic rocks, meaning that the variability is potentially large. The reported εNd values of surficial sediments
range between -8 and -4, which may be used to represent the approximate range of regional values, although this relatively stable range could also be a consequence of efficient sediment mixing by rivers or in soils from which our data come. It is possible that southern Iran is on the lower end of this $ε_{Nd}$ range with the Zagros dominating the geology. Certainly, actual isotopic results from Iran may show local variations due to geological and/or environmental complexity. Nevertheless, if our estimates about Iran's overall isotopic ranges hold, Iran, in general, may actually show a broad uniformity, which could be attributed to the buffering effect of widely occurring calcareous contents in weathered rocks and soils for Sr isotopes, as well as a natural mix of felsic and mafic rocks that cancels out extreme Nd isotopic compositions. The absence of long-range transport (e.g., compared to Central Asia) may have also prevented distinct isotopic pools from being funneled and amplified.

### 3.3. Mesopotamia

Mesopotamia, meaning "between rivers", is the region in western Asia dominated by the Tigris-Euphrates river system. Here, we consider the isotopic background for Iraq and the adjacent areas of eastern Syria, Kuwait, and Khuzestan in Iran. In Mesopotamia, large rivers — mainly the Tigris and the Euphrates — as well as aeolian dust play important roles in controlling the regional distribution patterns of detrital Nd and bioavailable Sr (see SM Section 4). The Mesopotamian Plain, combined with the Persian Gulf, forms a large foreland basin to the southwest of the Zagros (Fouad, 2010). We propose that Mesopotamia and its environments can be divided into three distinct isotopic zones. Zone 1 consists of the areas of northeastern Iraq (an extension of the Zagros), most of the Tigris drainage, the Mesopotamian Floodplain in southern Iraq, as well as Kuwait and the Khuzestan Plain. This zone is subject to the influence of the Zagros geology either directly or indirectly (as conveyed by the Tigris and other rivers). Zone 2 is the deserts and semi-deserts in western Iraq and eastern Syria, where dust storms have a significant effect. Mesozoic and, to a lesser extent, Paleozoic outcrops can be found in this zone. Zone 3 is the section of the Euphrates Valley in northern Mesopotamia.

Mesopotamian sediments from the southern floodplain and wetland have been analyzed, which yield an $ε_{Nd}$ range of -4.3 to -6.0 (Kumar et al., 2020; Sharifi et al., 2018) (for comparison, $ε_{Nd}$ = -5.5 for the dust collected in Baghdad, Cullen et al., 2000). The $ε_{Nd}$ value for Kuwait soil, reported as -5.1 (Kumar et al., 2020), also falls into this range. Sites in southern Iraq have reported faunal and human samples with $^{87}$Sr/$^{86}$Sr close to or very slightly higher than 0.708 (Hodos et al., 2020; Kenoyer et al., 2013). These numbers are indeed comparable to data from Iran, but the $ε_{Nd}$ values from the floodplain seem to be on the more radiogenic end of the Iranian Zagros. Most of northeastern Iraq is geologically analogous to the folded region of the Iranian Zagros and should bear similar isotopic signatures, and pre-Jurassic outcrops are present. We therefore propose that there could be a sub-zone (Zone MP-1N) in northeastern Iraq with slightly lower $ε_{Nd}$ values than the Mesopotamian Plain. This is, however, mostly speculative at this stage without reported Nd isotope data. For Sr isotopes, only a small dataset of plants from a foothill location in Iraqi Kurdistan is available for northeastern Iraq (0.708031(Q1)-0.708152(Q3)) (Elliott et al., 2015), which is comparable to southern Iraq. The Sr isotope ratios of ostrich egg shells from Nineveh in northern Iraq are slightly higher (0.708237(Q1)-0.708520 (Q3)) (Hodos et al., 2020). This slight discrepancy could arise from its location situated upstream of the confluence of the Tigris with eastern tributaries, or alternatively because some of the ostriches may have relied on mixed food/water sources partially related to the semi-deserts nearby (the ostriches were inferred to be wild rather than captive). Overall, the $^{87}$Sr/$^{86}$Sr ratios for Zone 1 should be slightly over 0.708.

Sands in the Syrian semi-desert east of Iraq display a very different $ε_{Nd}$ of -12.3 (sidenote: $^{87}$Sr/$^{86}$Sr = 0.709-0.710, nonbioavailable) (Henderson et al., 2020b). This $ε_{Nd}$ value points toward a geologically

older provenance, possibly related to siliciclastic materials derived from nearby uplifted formations of the Mesozoic and, to a lesser extent, Paleozoic ages, and/or aeolian deposition involving regional or remote dust sources. We expect similar scenarios for $ε_{Nd}$ values in western Iraq, although no data have been reported from there. The arid areas in western Iraq and eastern Syria (Zone 2) have no bioavailable Sr isotope data reported. It is possible that the exchangeable Sr mainly originates from aeolian deposition as previously suggested for arid regions (Capo and Chadwick, 1999). On the other hand, Mesopotamia was once part of the Tethyan Seaway during the Miocene (Bialik et al., 2019). The strata of marine deposits should contribute low $^{87}$Sr/$^{86}$Sr ratios (approximately 0.7085-0.7090 for seawater of the time, McArthur et al., 2012) to the labile Sr pool in eastern Syria or western Iraq (similar strata are beneath thick layers of Quaternary fluvial sediments in the Mesopotamian Plain, Yacoub, 2011). We thus tentatively suggest a combined effect of marine carbonate deposits and weathered silicate sediments for the regional bioavailable $^{87}$Sr/$^{86}$Sr signature (analogous to Central Asian Zone 2), which should show a higher $^{87}$Sr/$^{86}$Sr range than Mesopotamian Zone 1.

In eastern Syria, the sands of the Euphrates near Raqa have $ε_{Nd}$ values of approximately -2.1 (sidenote: $^{87}$Sr/$^{86}$Sr around 0.708, nonbioavailable) (Henderson et al., 2020b), which is likely caused by the occurrence of mafic formations in the river's upper catchment in the mountains of southeastern Turkey. Soil from the archaeological site of Tell Leilan in the Khabur Plain in northeastern Syria yields an $ε_{Nd}$ of -4.9 (Cullen et al., 2000). Plants from locations along the Euphrates and its branches in eastern Syria (including Raqa) show $^{87}$Sr/$^{86}$Sr of 0.70809(Q1)-0.70834(Q3) (Henderson et al., 2009).

Accordingly, due to sediment transport and fluvial deposition dominated by the Tigris, Zone 1 should feature relatively uniform isotopic signatures that are similar to those of the Zagros. The proposed $ε_{Nd}$ range is from -4 to -6.5, while the regional bioavailable $^{87}$Sr/$^{86}$Sr signature should fall around or slightly higher than 0.7080 but likely less than 0.7085. Hypothetically, northeastern Iraq may exhibit slightly lower $ε_{Nd}$ values for detrital sediments (for instance, -8 to -6, using data from southern Iran as a reference). For Zone 2, aeolian deposition or erosion of old outcrops could have provided low-$ε_{Nd}$ sand (for instance, <-8). Bioavailable Sr isotopic values for this zone can be assumed to be higher than 0.7085, but a better estimate is difficult due to a lack of data. In Zone 3, the Euphrates Valley before the river meets the deserts should largely retain isotopic signatures inherited from the river's upper course (for instance, $ε_{Nd}$ = -2 to -5.5, $^{87}$Sr/$^{86}$Sr approximately 0.7080-0.7085). The $^{87}$Sr/$^{86}$Sr ranges proposed for Zone 1 and Zone 3 are relatively narrow, but we opt to keep these ranges for now to correspond with current data.

### 3.4. Short summary

We summarize the proposed isotopic zones and the estimated bioavailable Sr and detrital Nd isotopic ranges in Table 1 and illustrate them in Fig. 2.

Based on these ranges, we note that there are considerable overlaps between the different regions considered in this study, often reflecting similarities in geological and environmental settings among these regions (pure coincidences are possible; some are mentioned below). For example, a mix of mafic and felsic sources likely set the keynote for sediments derived from the Taurus-Zagros orogen, resulting in largely comparable Nd isotopic values in Mesopotamian Zones 1 and 3 and parts of Iran. Also, a repeatedly occurring mechanism is aeolian deposition, which needs to be considered for West and Central Asia but is still not well understood.

In many of the studied regions, geologic history and high evaporation (the latter is due to the arid/semi-arid climate) provide conditions for the buildup of calcareous contents in the surface horizons, profoundly affecting bioavailable Sr isotope ratios. In these areas, the common undertone is the shared 'geologic heritage' of once being part
of the Tethys or the Paratethys Sea. When these areas were cut off from the open ocean, shallow lagoonal conditions also widely occurred. In recent epochs, most of these regions have witnessed climate aridification and intensified aeolian dust events. Consequently, large parts are covered by sedimentary deposits composed of carbonate, evaporite, and siliciclastic layers. For instance, Cretaceous limestone and Jurassic shale are the most important parent rocks for soils in central and southern Iran, and are responsible for their high carbonate contents (Toomanian et al., 2001). In addition, the ubiquitous occurrence of gypsum in the arid soils of the Iranian central plateau can be traced to Lower Cretaceous marine evaporites (Khademi et al., 1997). These calcareous contents were typically originally introduced by colluvial fans and mountain runoffs, and their accumulation and distribution in soils are the result of dissolution and precipitation processes under arid/semi-arid conditions (Khademi et al., 1997; Khormali and Toomanian, 2018; Toomanian et al., 2001). Furthermore, a calcrete crust, which is developed by dissolution and precipitation of calcite under an arid/semi-arid climate, has been commonly observed in the surface facies of Euphrates deposits in Syria (Stow et al., 2020), although the origin of the calcium is not entirely clear (possibly aeolian dust). Based on the association of calcareous contents in the surface horizons with marine-derived calcareous sediments and aeolian deposits in these areas, we propose that their bioavailable Sr isotopic signatures likely reflect the Sr isotope ratio of past ambient seawater at the time of sediment deposition, supplemented by the effect of aeolian sorting and deposition which usually elevates the Sr isotope ratio.

The overlap between Mesopotamia (MP-1 and MP-3) and Iran in both Nd and Sr isotopic ranges potentially makes it difficult to discriminate between raw material sources in these areas. Also, the εNd values for Central Asia outside the mountains (CA-2 and CA-3) may coincide with Iran and Mesopotamia, although they may mostly be differentiated using corresponding bioavailable Sr isotopic compositions. Similarly, the εNd values for the Central Asian highlands (CA-1) and Iraqi/Syrian deserts (MP-2) also largely overlap, making bioavailable Sr isotopes particularly useful for separating them. On the other hand, thanks to contrasting εNd signatures, it is possible to distinguish raw material sources in western Iraqi and eastern Syrian deserts (MP-2) from other Mesopotamian and Iranian sources. Siliciclastic materials from the Central Asian mountains (CA-1) also stand out from other Central Asian locations by their εNd values.

We additionally note that isotopic signatures for peripheral regions may potentially overlap with the regions considered in this study. For instance, the Nd isotopic range for Levantine sands has been reported (Levantine coastal sands εNd = -7 to -1, Henderson et al., 2020b; the Belus River mouth εNd = -4.8 to -1, Brems et al., 2014), reflecting the Nile's sediment inputs into the eastern Mediterranean Sea. In Egypt, the Nile has a radiogenic sediment load characterized by an average εNd = -3.3 (Goldstein et al., 1984; Weldeab et al., 2002) or εNd values fluctuating between approximately -6 and -2 (Bastian et al., 2021), strongly influenced by fine materials from volcanic formations of the Ethiopian Highlands. Sands from the inland western deserts along the Nile in Egypt show εNd = -8.7 to -6.0 (Brems et al., 2014), and sediments from Alexandria on the Mediterranean coast exhibit εNd = -8 to -6 (Brems et al., 2013b). Meanwhile, Saharan dust is marked by its low εNd value of approximately -13 (Grousset et al., 1998). The Indus Valley in Pakistan may be represented by the Indus River's sediments (εNd = -12.2, Jeandel et al., 2007). The Nd isotopic signatures of these detrital sediment sources overlap with several zones in Mesopotamia, Iran, and Central Asia. In addition, human and faunal samples from inland Egyptian sites along the Nile exhibit a local average ⁸⁷Sr/⁸⁶Sr of 0.7077 (Buzon and Simonetti, 2013; Stantis et al., 2019). Samples from coastal locations may report ⁸⁷Sr/⁸⁶Sr ratios that are similar to that of seawater, thereby overriding any regional variation. For example, the southeastern tip of the Arabian Peninsula, including the northeastern UAE and northern Oman, exhibits ⁸⁷Sr/⁸⁶Sr ratios slightly lower than seawater (Gregoricka, 2013, 2014; Kutterer and Uerpmann, 2017).

Coincidentally, materials from some locations in the continental hinterland (such as Central Asia Zone 2) also exhibit ⁸⁷Sr/⁸⁶Sr ratios in this range. Last, faunal samples from Bahrain (Gregoricka, 2013) display similar Sr ranges with Iran and, to a lesser extent, Iraq, which may be attributed to aeolian deposition by dust storms that sweep across the Middle East (see Section 4 in SM). Considering these potential overlaps, we need to keep geologically and archaeologically informed when interpreting similar Nd or Sr isotope ratios from different locations.

## 4. Provenancing plant-ash glass

Since ancient glass was often involved in long-distance trade, composition-based provenance is especially meaningful to reveal sociocultural links between the locations of production and usage (Lü et al., 2021). With the estimated isotopic ranges for the regions from Mesopotamia to Central Asia, we will now apply this Sr-Nd isotope baseline to plant-ash glass provenance. Our goal is twofold: first, since our baseline has been constructed entirely from environmental and bioarchaeological information without using any glass data, we will verify whether this baseline contributes to an understanding that is consistent with established provenances of archaeological glass. Second, we will see if our integrative Sr-Nd isotope approach can bring a new understanding of archaeological glass provenance (i.e., ascertaining or narrowing down possible origins). We will show that the Sr-Nd isotope baseline indeed provides essential geochemical guidance, with which the proposed origin of glass artifacts or raw materials can be verified.

In plant-ash glass, Sr is mostly brought in with the lime content in plants (minor contributions from minerals such as calcite, feldspar, and pyroxene in the silica raw material are possible), reflecting the bioavailable Sr of the environment where the plants grew (Degryse et al., 2010c; Freestone et al., 2003). Nd in plant-ash glass mostly originates from the non-quartz minerals in silica material (Brems et al., 2013b), such as detrital clay minerals. The Nd levels in halophytic plants are negligible (Barkoudah and Henderson, 2006). The potential contribution of Nd contents from clay adhering to the plants or contamination from the crucible or grinding tools is minor (Degryse et al., 2015), and such contamination was also unlikely to happen for any large-scale manufacture of glass. Therefore, the Sr and Nd isotope ratios serve as unique indicators for the origins of flux and silica raw materials respectively, allowing for independent provenance for the two main ingredients.

To properly provenance glass artifacts, both chemical and isotopic compositions need to be considered. Isotopic information alone is not a one-size-fits-all tool. For example, as mentioned above, the εNd range of Levantine coastal sand largely coincides with that of Mesopotamia detrital sediments. However, for plant-ash glass provenance, the chemical composition of Levantine glass is typically distinct from Mesopotamian glass (e.g., distinguished by the MgO/CaO ratio, Henderson et al., 2016). In echoing previous researchers who advocated a multi-proxy approach in mobility studies (e.g., Holt et al., 2021; Knudson and Price, 2007; Wu and Zhang, 2022), we stress that a combination of chemical and multiple isotopic compositions is more likely to reduce the ambiguity in glass origins that are often observed with one type of tracer alone.

The availability of Sr and Nd isotope data also opens fresh opportunities to investigate glass 'recycling'. Isotope mixing models have been used in archaeology, for example, with Sr isotopes for mobility studies (e.g., Bentley, 2006; Lengfelder et al., 2019; Montgomery et al., 2007), Sr, Pb, and Nd isotopes for glass (Degryse et al., 2006; Henderson et al., 2015), Sr isotopes for ceramics (Henderson et al., 2020a), and Pb isotopes for metallurgy (Pernicka, 2013; Pollard and Bray, 2015). Here, using two-endmember mixing lines of εNd vs 1/Nd, we examine potential glass recycling practices associated with medieval plant-ash glass from the Silk Road regions. This is a more appropriate way than the εNd vs Nd plot to present Nd isotope data because mixing relationships between two separate endmembers generate linear trends in the εNd vs 1/Nd plot.
graph. The geochemical context provided by the isotope baseline can also be very indicative of the mixing endmembers. We suggest that an integrative approach, based on the complementary use of the Sr–Nd isotope baseline and isotope endmember mixing plots, in addition to the widely used chemical and isotopic composition clustering, offers the most consistent provenance results.

We also underscore the unique bioavailable $^{87}$Sr/$^{86}$Sr signatures (>0.7095) for Central Asia Zone 1 and Zone 3. Almost no isotopic investigation of Central Asian excavated glass has been reported, except for the brief mention of ‘Central Asian chemical type’ glass from Xinjiang, China, which bears $^{87}$Sr/$^{86}$Sr ratios of 0.710–0.716 (Brill and Fullagar, 2009). This range falls into our proposed Central Asian Zones 1 and 3, suggesting that the plant ash used to make these glasses or the glass products themselves may have originated from the mountainous or piedmont loess areas in eastern Central Asia. This is consistent with existence of historical population centers (such as the Fergana Valley) in eastern Central Asia and the close contact of this area with Xinjiang. We emphasize that our proposed $^{87}$Sr/$^{86}$Sr range was obtained by using qualitative reasoning and data from environmental proxies. Therefore, this match with glass data also confirms the validity of our baseline construction method.

## 4.1. Case study 1: plant-ash glass from Mesopotamia

In this study, we apply our integrative Sr–Nd isotope approach to investigate the provenance of Mesopotamian plant-ash glass. Islamic plant-ash glass from the Levant has been included for comparison. The compared datasets include plant-ash glass from Raqqa, Syria (Islamic) (Henderson et al., 2009), Veh Ardashir, Iraq (Sasanian) (Ganio et al., 2013), Dibba, UAE (1st C. CE) (Van Ham-Meert et al., 2019), Tell Brak, northeastern Syria, Nuzi, northeastern Iraq, and Nippur, southern Iraq (LBA) (Degryse et al., 2010a, 2015; Henderson et al., 2010), Damascus, Beirut, and Serçe Limani (Islamic) (Brill, 1999; Henderson et al., 2020b), and Tyre and Banias (Islamic) (Degryse et al., 2010b). Here, we have distinguished between raw glass (raw furnace glass or glass ingots/chunks), which indicates the occurrence of glass production, and manufactured glass objects. Fig. 3 displays the comparison of $^{87}$Sr/$^{86}$Sr ratios for biological and glass samples. The $\epsilon_{Nd}$ comparison, as well as the relationships given by Sr–Nd isotopes, is illustrated in Fig. 4.

In Fig. 3, almost all samples show $^{87}$Sr/$^{86}$Sr ratios below modern seawater. A relative homogeneity of $^{87}$Sr/$^{86}$Sr is observed: $^{87}$Sr/$^{86}$Sr ratios in Fig. 3 are actually within a relatively small interval for a variety of samples from a vast area, especially when compared to bioavailable $^{87}$Sr/$^{86}$Sr ranges in many other regions that are known to display a much

greater $^{87}$Sr/$^{86}$Sr variability (for example, China, Wang and Tang, 2020). As discussed above, this relative uniformity of $^{86}$Sr/$^{87}$Sr compositions most likely reflects the fact that bioavailable Sr signatures in these regions are controlled by weathering of marine-derived sedimentary rocks. For Mesopotamia and Iran, we calculated the bioavailable Sr isotopic range (average ± 2 s.d. = 0.707728–0.708631, blue lines in Fig. 3) from excavated faunal and human samples (western deserts are likely outside this range). Note that 2 s.d. are inherently relatively conservative (i.e., broad) for the actual local ranges. In Fig. 3, almost all Mesopotamian glass samples, including most raw glasses from Raqqa, Veh Ardashir, and Tell Brak, and most vessels from Raqqa, Nuzi, and Veh Ardashir, fall into this Mesopotamian-Iranian isotopic range indicated by the blue lines. This range corresponds to the isotopic zones of MP-1, MP-3, and Iran, suggesting that the majority of Mesopotamian glass was probably made using plant ash from Mesopotamia (except for MP Zone 2) or Iran, and that Mesopotamian deserts/semi-deserts (Zone 2) were likely not among the main origins for the ashed plants. Compared to Iran, Mesopotamia is more likely the origin of the plant ash with more excavated glass reported from there, but glass-making using Iranian plants may not be distinguished on the basis of Sr isotopes. Not all halophytic plant species are suitable for fluxing in glass-making. The fact that large quantities of glass objects demonstrate similar $^{87}$Sr/$^{86}$Sr ratios may indicate either common sources of plant ash or similar geology/ecology from which the plants derived. The first possibility implies one or more supply centers of plant ash, while the second possibility suggests a likely consistent practice of ashing similar species of plants harvested from similar environments.

Although our proposed bioavailable Sr isotopic ranges for MP-1 and MP-3 overlap, there are still minor but discernible differences in Sr isotopic distribution patterns: based on reported biological samples from MP-1 (Ur and Mashkan Shapir in southern Iraq, and Bestansur in northeastern Iraq, but excluding Nineveh ostrich egg shells) and from MP-3 (plants from locations in the Euphrates Valley in Syria), most of MP-1 likely has a lower average bioavailable $^{87}$Sr/$^{86}$Sr than MP-3. In Fig. 3, almost no glass corresponds isotopically to the southern Iraqi (or Bestansur) biological samples (below the gray dashed-line box in Fig. 3), implying that the plants used for glass-making likely did not come from the floodplain (MP-1 regions). The Sr isotopic composition of most LBA glasses (Tell Brak, Nippur, Nuzi) seems to better align with that of the Euphrates Valley or Nineveh biological samples (inside the gray box in Fig. 3), which suggests that river valleys in northern Mesopotamia could be the main source of plant ash for LBA glass-making. Interestingly, even glass discovered in Nippur, located in southern Iraq, does not quite align with southern Iraqi faunal/human samples in $^{87}$Sr/$^{86}$Sr ratios, meaning

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (Color online) Sr isotopic composition of glass, plant, faunal and human samples in West Asia. The range of Mesopotamian and Iran faunal samples (average ± 2 s.d., blue dashed line) and the most concentrated isotopic range of the glasses (gray box and gray line) are indicated. Refer to Table S1 (Section 1 in SM) and the text for data sources.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. (Color online) Nd–Sr isotopes for raw glass and glass vessels: (a) relationship between εNd and ⁸⁷Sr/⁸⁶Sr and (b) εNd vs 1/Nd. Dotted box X: most of LBA glass; dot-dashed box Y: most Sasanian glass from Veh Ardashir; dashed box Z: a small group of LBA glass; brown dotted line AB, green dashed line CD, green dot-dashed line EF: mixing lines indicating remelting practices for Raqqa, Damascus, and Beirut vessels, respectively. Vessels are indicated by open symbols while raw glass is indicated by solid symbols. Refer to Table S2 (Section 1 in SM) and the text for data sources. Not all samples have Nd concentrations reported.</figcaption></figure>

that glass from Nippur was possibly also made using plant ash from northern Mesopotamia. The use of this northern source of plant ash may have continued or revived later, since Islamic glass vessels from Raqqa show similar Sr isotopic signatures.

Additionally, we observe that the ⁸⁷Sr/⁸⁶Sr ratios for Veh Ardashir samples (and some Nuzi, Tell Brak, and Raqqa samples) are above the data plotted in the gray box in Fig. 3. It is possible that a different source of plants was used for making these glasses during the LBA, the Sasanian period, and the Islamic period. The ⁸⁷Sr/⁸⁶Sr ratios typically exhibited by these samples are near the upper end of the MP-1/MP-3 ranges, but are not radiogenic enough to be attributed to MP-2. Meanwhile, Veh Ardashir samples are comparable in ⁸⁷Sr/⁸⁶Sr to the upper half of the range given by Nineveh ostrich egg shells, implying a possible link. Also, several Veh Ardashir objects exhibit even higher ⁸⁷Sr/⁸⁶Sr ratios deviating toward 0.709. Geographically, Veh Ardashir is adjacent to Ctesiphon south of today's Bagdad, and its local Sr isotopic signature should align mostly with the ⁸⁷Sr/⁸⁶Sr range of Mesopotamian Floodplain, but this is apparently not the case reflected in Veh Ardashir glass. This hints that the plants used for glass-making could have grown in areas with slightly more radiogenic ⁸⁷Sr/⁸⁶Sr signatures in northern Mesopotamia, for example, semi-arid environments near the river valleys where aeolian-sourced strontium probably has a minor contribution, and the area near Nineveh is a possible candidate. Also, the similar ⁸⁷Sr/⁸⁶Sr ratios in some of the glass artifacts from Tell Brak and Raqqa indicate that this plant ash source was possibly also accessible for the Euphrates Valley settlements in Syria. Considering that both Nineveh and Veh Ardashir are located on the Tigris, one possibility is that the movement of materials or products was facilitated by the river. If, however, plant ash was from areas near the Euphrates Valley in northern Mesopotamia, transportation of goods via the Euphrates would also be viable. The 9th-century Islamic capital, Samarra, where compositional evidence is considered supportive of glass production (Henderson et al., 2016; Schibille et al., 2018), is to the north of Baghdad on the Tigris. Future Sr isotopic analyses on Islamic glass from Samarra and Ctesiphon could help delineate the temporal and geographical extent of the use of this type of plant ash.

Levantine glasses vary widely in ⁸⁷Sr/⁸⁶Sr ratios. As already noted in

previous research, both coastal and inland plants had been exploited for Levantine glass-making (Degryse et al., 2010b; Henderson et al., 2020b). The broadly variable Nd–Sr isotopic signatures of Beirut and Damascus vessels (Figs. 3 and 4(a)) show that the metropolitan centers likely attracted glass merchandise from different regions. Significantly, in Fig. 3, a considerable portion of the Beirut, Damascus, and Tyre glasses do not align with coastal Levantine or western Syrian plants but fall into the Mesopotamian-Iranian range. Mesopotamia might have provided plant ash or glass products for the Levant (Iran is less likely due to its distance from the Levant). We will return to this later with a discussion based on further evidence from Nd isotopes. Nonetheless, it has been reported that the ⁸⁷Sr/⁸⁶Sr ratios of plants and invertebrates in northern Israel vary from 0.7074 to 0.7090, mainly resulting from the Tethys sedimentary deposit and varied lithologies (Hartman and Richards, 2014), which overlaps with the range for Mesopotamia. Hence, the possibility of using ashed plants mostly from near the Levant cannot be excluded.

Concerning Nd isotopes, LBA glass vessels from Nuzi and Nippur and raw glass from Tell Brak (dotted-line box X in Fig. 4(a)) mostly fall into the εNd ranges of MP-1 and MP-3. With the overlapping isotopic ranges, we are unable to ascertain the exact sources of silica based on current evidence. However, considering the generally low impurity levels of LBA Mesopotamian glass (e.g., Tell Brak glass Nd ~ 1 ppm, Henderson et al., 2010; Nuzi glass Al₂O₃ < 1%, Fe₂O₃ < 0.5%, Degryse et al., 2010a; Nippur glass Al₂O₃ < 1.5%, Fe ~ 5000 ppm, Cr ≤ 30 ppm, Walton et al., 2012), silica materials (e.g., quartz pebbles) from northern Mesopotamia were more likely to have been involved than silt-rich sands from the fluvial deposits in southern Mesopotamia.

Notably, most Sasanian raw glass and glass vessels from Veh Ardashir fall into the intermediate εNd range of -8.5 to -6.0 (dot-dashed-line box Y in Fig. 4(a)). The εNd values of these artifacts agree with the range for Iran or MP-1N — an extended range of MP Zone 1 potentially existing in northeastern Iraq and showing slightly lower εNd signatures. Data for Veh Ardashir raw glass are within the εNd ranges of MP-1 and the putative MP-1N, likely indicating that Sasanian glass production indeed exploited silica sources related to the Zagros geology. Previously, it has been pointed out that different silica materials were used for
different groups of Veh Ardarshir glasses (Mirti et al., 2009). It is
possible that the silica materials used to make Veh Ardashir glasses (and
other glasses related to MP-1N) were from the same general geological
background (thus having similar εNd values) but with different weath-
ering or erosion conditions.

A small amount of LBA Mesopotamian glass, mostly from Nuzi, shows εNd values between -10 and -8.5 (dashed-line box Z in Fig. 4(a)), consistent with the range for MP Zone 2. These artifacts include one glass ingot from Tell Brak and one ingot from Nuzi, implying that this type of silica or raw glass was likely within their reach. This suggests that desert/semi-desert sands in northern/western Iraq and eastern Syria were probably exploited for glass-making as early as the Bronze Age. During the Islamic period, Raqqa raw glass, possibly made using Syrian semi-desert sand, has εNd values lower than -10 (Henderson et al., 2020b). The continued or revived use of this type of silica material also concurs with the use of northern plant ash suggested above, showing that Raqqa's Islamic glass-making industry probably employed similar local raw materials to those used during the LBA.

Curiously, in Fig. 4(a), vessels from Beirut, Damascus, and Raqqa are not close to Levantine and Raqqa raw glasses in Sr and Nd isotopic compositions, and were suggested to be products from either a different type of primary glass or mixed glass (Henderson et al. 2020b). Here, we suggest that this disparity in isotope ratios is due to glass mixing/-recycling, in which the isotopic signature of imported glass was gradually taken over by locally made glass that was added with each remelting (the original chemical compositions were also diluted in this process). We demonstrate this in Fig. 4(b) with εNd vs 1/Nd relations, in which the distribution of data points on such ‘binary mixing lines’ are suggestive of glass recycling practices and displays the endmembers that were mixed in.

For Raqqa vessels (brown dotted line AB), the mixing of glasses likely occurred, starting with glass at Point A (9th century vessel), which is characterized by εNd ~ -8.5 and a very low Nd concentration (~2 ppm), and going on a trend toward εNd values and Nd levels that are typical of Raqqa raw glass (Point B, 9th–12th century vessels). This shows a recycling process in which imported high-quality glass made using low-impurity silica materials, possibly crushed quartz pebbles from MP-1N, was gradually diluted by repeated addition of glass vessels (or less likely, raw glass) that were made using local sands from MP-2 semi-deserts. The occurrence of glass mixing was previously suggested for some of Raqqa glass (Henderson et al., 2004). The glass with an extremely low Nd level and a possible Iraqi origin, represented by Point A, is reminiscent of the high-quality Islamic glass from Samarra (Henderson et al., 2016; Schi-bille et al., 2018), a potential link that is worth further investigation.

For Damascus vessels (green dashed line CD), a similar process likely took place, starting with Raqqa-type glass with very low ε<sub>Nd</sub> values (Point C, 12th century vessels). Glass characterized by ε<sub>Nd</sub> ~ -9 and Nd concentrations ~ 5 ppm, possibly locally made, was then added through repeated remelting (Point D, 12th–14th century vessels). Since western Syria to the east of the Anti-Lebanon Range was marked by deserts and the Palmyride fold belt, which are largely comparable to MP-2's conditions for detrital sediments, it is possible that silica sources from large parts of western Syria exhibit ε<sub>Nd</sub> signatures similar to MP-2 (although their bioavailable <sup>87</sup>Sr/<sup>86</sup>Sr ratios could be considerably different, as shown by plant samples from western Syria in Fig. 3). For Point D, although silica materials (or glass products) from northern Iraq (MP-1N) also possess comparable ε<sub>Nd</sub> levels, the constant use of imported glass or raw materials from distant locations for remelting when glass from nearer locations was available seems unlikely.

In addition, Beirut vessels may have also undergone recycling pro-
cesses, starting from imported Mesopotamian glass and replenished by
local Levantine glass (green dot-dashed line EF). The compositional
signatures of Beirut vessels are more variable on the Mesopotamian end
(Point E), shown as a string of scattered points on the Raqqa mixing line
AB. A possible explanation is that Raqqa's reworked glass products
containing varying portions of northern Iraqi glass were possibly

brought to Beirut, which were subsequently reused by adding Levantine
glass. Most of the Beirut data are from the 12th–14th century vessels.

If the above deductions about glass mixing hold, we may further
suggest a westward diffusion pattern of Islamic plant-ash glass from
Mesopotamia to Syria-Palestine during the period of the 9th–14th cen-
tury. In this diffusion, glasses from Mesopotamia were recycled in a melt
of locally made glass at major Middle Eastern metropolises in the trade
network.

Glass recycling was likely widespread and common in history but is still difficult to ascertain in provenance research (Rehren and Freestone, 2015), and its occurrence during the Islamic period was also affirmed by the cullet carried by the 11th-century Serçe Limani shipwreck (Bass et al., 2009). Benefiting from isotopic ranges for different zones and the use of Nd isotope mixing lines, we have provided compositional evidence for the recycling practice of Islamic plant-ash glass. It was likely that, simultaneously with primary glass production in the Islamic world, recycling of broken glass widely occurred, involving glass products that reached the end of their use life. It is almost certain that recycling was not limited to the above scenario in which imported glass was mixed into locally made glass. Glass products made entirely from recycled local glass should have been common. However, remelted glasses from different production zones allow us to identify the occurrence of glass recycling precisely because only in such cases can a mixing line be drawn between drastically different chemical/isotopic signatures.

The above analysis of plant-ash glass recycling also implies that part of the glasses traded were probably remade products from an intermediate location on the trade route while the chemical and/or isotopic signatures of the original primary glasses were ‘masked’. This confirms that the dispersion and recycling of glass may have been intertwined processes. This has ramifications on composition-based provenance studies and prompts us to suggest that an examination of ε<sub>Nd</sub> vs 1/Nd relations should be included in isotopic provenance studies when possible.

To summarize, using our integrative Sr–Nd isotope approach, we have suggested that northern Mesopotamia may have been a supply center for glass-making raw materials over a long period, likely having provided at least two types of plant ash from different environments near river valleys (MP-3 or MP-1) and two types of silica materials related to mountain-derived clastic rocks and desert sands (MP-1N and MP-2). Using Nd isotope mixing line, we have also revealed the practice of glass recycling for Islamic plant-ash glass.

4.2. Case study 2: provenance of San Lorenzo glass

In this study, we assess the provenance of the medieval plant-ash
glass from San Lorenzo, Apulia, Italy, dated to the 12th-14th century.
Sr and Nd isotopic compositions were reported for 12 samples (Gliozzo
et al., 2021). The 12 samples from San Lorenzo include SLC3 (goblet),
SLC7 (from prunted beaker group 1), SLC9-10 (from prunted beaker
group 3), SLC12 (beaker), SLC13-14 (bottles), SLC21-24 (lamps), and
SLC25 (vessel base). Based on a comparison with European and Middle
Eastern plant-ash glasses, Gliozzo et al. inferred possible Mediterranean
or Mesopotamian origins for the San Lorenzo artifacts, where a Medi-
terranean provenance was used to broadly indicate Levantine, Syrian, or
Adriatic origins, while a Mesopotamian provenance mostly referred to
central Iraq. Our use of these geographic terms is somewhat different.
Most notably, we use ‘the Eastern Mediterranean’ to refer to the coastal
strips only (mainly Egypt and the Levant) and consider Raqqa as an
inland site in northern Mesopotamia. For SLC3, SLC9, SLC10, and
SLC21, Gliozzo et al. also suggested partial similarities with certain
Italian samples. Three samples, SLC14, SCL22, and SLC23, were without
a match and no provenances were suggested.

In Fig. 5, we compare the Sr and Nd isotopic compositions of San
Lorenzo glass with Levantine and Mesopotamian Islamic/Sasanian
plant-ash glasses (Degryse et al., 2010b; Ganio et al., 2013; Henderson
et al., 2009, 2020b), and compare their chemical compositions with
<figure><img src="image_8.png" /><figcaption>Fig. 5. (Color online) (a) Nd–Sr isotope ratios for San Lorenzo glass in the context of the isotopic zones. (b) εNd vs 1/Nd. (c) Al₂O₃ vs MgO/CaO. (d) Cr vs Fe₂O₃. San Lorenzo samples are marked by red-colored sample numbers. Refer to the text for data sources for these plant-ash glasses. Some of the datasets are represented by average values and standard deviations (underlined and italicized in the legend) so that the graph is not overcrowded. (r) and (v) in the legend indicate raw glass and vessels, respectively.</figcaption></figure>

Islamic plant-ash glasses from Egypt (Groups E1 and E2) (Schibille et al., 2019), the Levant (Beirut and Khirbat al-Minya, Henderson et al., 2016; Damascus, Henderson et al., 2016; Ramla, Phelps, 2018; Tyre, Freestone, 2002; Banias, Freestone et al., 2000), Mesopotamia (Raqqa, Henderson et al., 2004, 2016), and Central Asia (Ghazni, Fiorentino et al., 2019; Akhsiket and Kuva in Fergana, Rehren et al., 2010), as well as Sasanian glass from Veh Ardashir in Mesopotamia (Mirti et al., 2008, 2009).

With uniquely high ⁸⁷Sr/⁸⁶Sr ratios, SLC22 sits close to CA-3 in Fig. 5 (a), suggesting it was possibly made in the piedmont loess area (the use of sands from CA-2 and plant ash from CA-3 is also a possibility), while SLC23 appears closer to CA-1, i.e., the mountainous area. The chemical compositions of these samples also fall into Central Asian glasses, which are rich in Al and Fe, as shown in Fig. 5(c)(d). With high Nd concentrations, SLC22 and SLC23 lie in the upper left corner in Fig. 5(b). SLC14's εNd value is in the range of the Levant or Mesopotamia, and its ⁸⁷Sr/⁸⁶Sr ratio is similar to Levantine raw glass but still close to the Mesopotamian ranges (Fig. 5(a)). Its Al, Cr, and Fe levels, typically considered related to the silica source, are consistent with Mesopotamian glass, but its MgO/CaO is distinctly low, a signature for Levantine glass (Fig. 5(c)(d)). Thus, SLC14 can be identified as Mesopotamian glass but with possible Levantine influence. SLC21's Sr and Nd

isotopic compositions imply a Mesopotamian (especially Raqqa) origin (made using MP-2 sands and MP-1/MP-3 plant ash), and its chemical composition in Fig. 5(c)(d) is also close to Mesopotamian glass (although almost on the edge with Central Asian glass, possibly due to variation in impurities in Mesopotamian desert sands).

SLC3, SLC9, and SLC10 are in the same isotopic range as Raqqa raw glass in Fig. 5(a), whereas in Fig. 5(c)(d), they align with Levantine glasses and are closest to Damascus vessels. We have discussed above that the analyzed Damascus vessels were probably made from mixing Raqqa glass with locally made glass. SLC3, SLC9, and SLC10 could have been produced from a similar process, as they are also on the mixing line (colored green) in Fig. 5(b). In Gliozzo et al. SLC12 was inferred to have a Mesopotamian origin since its εNd value matches that of Veh Ardashir glass. In fact, SLC12's isotopic signatures in Fig. 5(a) can lead to different explanations. We note that its Al, Cr, and Fe concentrations are often in line with Central Asian glasses. In Fig. 5(b), SLC12 can be considered to be on a hypothetical mixing line connecting Central Asian glasses (SLC22–23) with Mesopotamian glass (e.g., Veh Ardashir glass). Therefore, SLC12 may possibly have been produced by mixing Central Asian and Mesopotamian glasses. Another explanation, considering the possibility of plant ash trade, is that SLC12 was made using silica materials from CA-1 and plant ash from Iran or CA-2.
For SLC7, SLC11, SLC13, and SLC24, a probable connection with
Raqqa glass was suggested based on Nd isotope ratios. From Fig. 5(c)(d),
we note that their Al, Cr, and Fe concentrations are not consistent with
Raqqa Type 1 or Type 4, but most closely align with Egyptian glass. In
Fig. 5(a), their εNd values agree with the ranges for Egyptian inland
western desert and the Sahara Desert (Brems et al., 2014; Grousset et al.,
1998), and their ⁸⁷Sr/⁸⁶Sr ratios can be explained by a potential plant
ash source under the influence of sea spray or similar to the plant ash
used to make Serçe Limani glass (also within the nonbioavailable
⁸⁷Sr/⁸⁶Sr range for Egyptian desert sands, Brems et al., 2014). If these
artifacts were indeed produced in Egypt, their isotopic signatures are
separated from LBA Egyptian plant-ash glass (⁸⁷Sr/⁸⁶Sr =
0.70780–0.70817, Brill and Fullagar, 2009; Degryse et al., 2015; Hen-
derson et al., 2010; and εNd = –7.8 to –9.5, Degryse et al., 2015;
Henderson et al., 2010), indicating that different glass-making materials
were used.

The identification of Central Asian glass in San Lorenzo is note-
worthy. It was perhaps not uncommon for Central Asian glass to have
arrived in medieval Europe despite the geographical separation. A
wheel-cut bowl in Venice with an assigned date of approximately 1000
AD has an inscription of ‘Khurasan’ on the bottom (Kröger, 1995),
indicating its likely origin from the Khorasan region between Iran and
Central Asia. Here we have also indicated that some of the San Lorenzo
glass was probably made from a mix of remelted glasses from different
origins, again suggesting that glass recycling at the time was likely
common. It was likely that many Central Asian and Mesopotamian glass
pieces underwent recycling processes and were transformed into Eastern
Mediterranean products before they reached Europe. Perhaps, part of
the glasses received by Italy from the Eastern Mediterranean were
reworked glass products, which were then remelted again to manufac-
ture objects that exhibit Italian, French, and other European designs
such as those found in San Lorenzo.

Using the integrative isotope approach, our analysis has reached
similar conclusions for some of the San Lorenzo glasses to those pro-
posed in Gliozzo et al. However, we have suggested alternative possi-
bilities for others in the assemblage. More reported data will help
ascertain their exact origins. It should still be noted that Italy has a
history of primary glass production. However, since very different
compositions have been identified in San Lorenzo glass vessels of similar
styles, this compositional diversity implies that primary raw glass was
probably not constantly available for the workshops in the region. In the
above analysis, our isotope baseline provides an essential geochemical
context for comparing isotopic compositions and has been a useful guide
when chemical information is incomplete or ambiguous. In particular,
with a lack of isotope data for glass from Central Asia, the baseline fills a
crucial gap and proves to be helpful.

5. Conclusion

We have established the first semi-quantitative bioavailable Sr and
detrital Nd isotope baseline for the Silk Road regions of Central Asia,
Iran, and Mesopotamia. Three isotopic zones for Central Asia and three
isotopic zones for Mesopotamia have been proposed, although only one
general range for Iran has been proposed due to limited data. The iso-
topic signatures of these zones are controlled by geologic history,
environmental conditions, and Earth surface processes. To varying ex-
tents, Mesozoic-Cenozoic marine and continental sedimentary deposits
predominantly set the isotope geochemical background for most of these
zones. Our exploration showcases the potential of utilizing reported data
from Earth and environmental sciences for the benefit of archaeological
research. Except for plant-ash glass, the regional isotope baseline may
also be used for other archaeological materials such as ceramics.

Aided by this isotope baseline as an indispensable geochemical guide
and applying isotope mixing lines, we have suggested that glass recy-
cling occurred in Islamic plant-ash glass. We have inferred northern
Mesopotamian origins for the raw materials used to make

Mesopotamian glass, and suggested diverse provenances in Meso-
potamia and Central Asia for glasses found at San Lorenzo. Our analysis
also indicates that Nd isotopes may have great potential as a useful tool
in plant-ash glass provenance, especially if more reference data become
available in the future.

Admittedly, the available dataset is still sparse for such a vast and
geochemically varied region, and we have only treated spatial vari-
ability on a very coarse level. For Central Asian basins and the Meso-
potamian Floodplain, where lithology and environmental conditions are
relatively homogeneous, sparse data coverage may be acceptable.
However, for geologically complex areas such as the Central Asian
mountains and Iran, currently available data may not sufficiently reflect
the spatial variability of isotopic compositions. For certain areas such as
western Iraq, northeastern Iraq, and Central Asian loess, due to the lack
of data, the results have been mostly obtained by drawing parallels with
nearby areas and by qualitative investigations. More data and a better
understanding of surface conditions are needed to improve the quality of
the baseline. In this regard, we note that this work should be viewed in
the context of generating useful information with limited available data,
a scenario often encountered in archaeology. We consider our estimate
of isotopic signatures as a crude sketch to demonstrate only the
macroscopic isotopic patterns, or as a zeroth-order approximation
awaiting higher-order corrections. We anticipate our baseline as a
starting point that will be refined by future regional studies on specific
materials that are archaeologically more relevant.

The impact of climate and environmental change may significantly
alter the regional isotopic signature. One such example is the Nile.
Sediment cores show that the change in rainfall caused the contributions
from major sediment sources to fluctuate significantly, leading to
marked changes in Sr and Nd isotope ratios in the Nile floodplain and
delta over the Holocene (Bastian et al., 2021; Stanley et al., 2003;
Woodward et al., 2015). This underscores the future need to incorporate
paleoenvironmental studies and take temporal shifts in isotopic patterns
into consideration. Furthermore, we have not considered sampling
depths for the data. It has been known that roots taking up Sr at different
depths affect the ⁸⁷Sr/⁸⁶Sr ratio of plants (Poszwa et al., 2004), and Nd
isotopic compositions may also differ for soils at different depths (Aubert
et al., 2001). Although not practical for ‘retrospective’ data aggregation,
a consideration of vertical profiles should be included for future ‘pro-
spective’ sampling.

CRediT author statement

Qin-Qin Lü: Conceptualization; Data curation; Formal analysis; Funding acquisition; Investigation; Methodology; Project administration; Resources; Validation; Visualization; Writing - original draft; Writing - review & editing. Yi-Xiang Chen: Formal analysis; Methodology; Validation; Writing - review & editing. Julian Henderson: Methodology; Validation; Writing - review & editing. Germain Bayon: Methodology; Validation; Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgements

The authors would like to thank Prof. Zihua Tang (Institute of Ge-
ology and Geophysics, CAS) for constructive comments and Prof. Ruizhi
Yang (USTC) for important facility support. The authors appreciate
helpful suggestions from Prof. Changsui Wang (UCAS), Prof. Liqun Dai
(USTC), Prof. Xingxiang Zhang (USTC), Dr. Xiaotong Wu (USTC), and
David Redhouse (University of Cambridge). The authors gratefully
acknowledge support from the University of Cambridge. Q.-Q. Lü would
like dedicate this work to Dr. Hai-Wei Long (1984–2015) of Shunde, Guangdong, a promising high energy physicist, an art and music lover, a guardian for the underprivileged, a true friend, and a noble soul. Dr. Long will be sorely missed.

This research was supported by the National Natural Science Foundation of China (Grant No. 12005222) and USTC Research Funds of the Double First-Class Initiative (Key Innovation Project for Early Career Investigators No. YD2110002005), both awarded to Dr. Lü.

## Appendix A. Supplementary material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.jas.2022.105695.

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