The application of strontium isotope analysis to historic cemetery contexts: a case study for the creation of robust individual identifications

Shannon K. Freire and Alexis M. Jordan

Introduction

The primary goal of the Milwaukee County Institutional Grounds Cemetery project writ

large, and of our particular research, is to restore the identities of individuals interred at this

cemetery, reversing the trend of collective neglect, marginalization, and social amnesia in life

and in death. Following the 1991-1992 excavation, approximately 190 individuals were

preliminarily identified using historical documentation, material culture, and geospatial analysis.

Subsequent bioarchaeological analyses have provided an additional line of evidence for the

identification of these individuals. The cemetery population of Western European immigrants

and local/nonlocal native born Americans is composed of paupers, the institutionalized, and the

unidentified of the city of Milwaukee during this period. Archaeological evidence, including the

limited recovery of discrete burial markers and the complex depositional sequence of interments,

as well as analysis of historical archival material, revealed that burial practices were more

complex than the original records suggested, thus necessitating a multifaceted approach for the

secure identification of the interred.

In an effort to create more robust identifications of these individuals, we have recently

completed a natal strontium isotope pilot study. Our study was designed to 1) test measured

individual 87Sr/86Sr signatures against the natal regions provided by putative non-local historical

identifications and 2) test the applicability of 87Sr/86Sr isotope research as a line of evidence to

support individual identification within historic cemetery contexts.

Radiogenic strontium isotopes have been used in archaeological contexts to create

models of likely residence locations (Price et al. 2002). While bioavailable 87Sr/86Sr ranges can

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be extremely specific for a given geographical locale, strontium signatures should not

intellectually be construed as having the ability to provide a ‘post-code’ of residence (Bentley

2012:9328; Pollard 2011). Rather, the use of radiogenic strontium isotope analysis is most useful

when differentiating between local and non-local populations (Hedman et al. 2009; Slater et al.

2014). Within our project, strontium isotope analysis was applied by comparing dental enamel

87Sr/86Sr signatures of preliminarily identified individuals with geologies of their historically

defined natal regions. As strontium isotopes vary with types of bedrock and sediment, it has been

possible to isolate geological regions within these historic countries as locations of likely origin

(Price et al. 2004).

Background

The area of the cemetery that is the focus of our research, Area II, was in use between

1918-1925, and was excavated between 1991-1992 following the onset of construction at the

Milwaukee Medical Grounds Complex in Wauwatosa, Wisconsin (Richards 1997). Preliminary

historical identifications have been established with caveats and various levels of strength based

on the types and efficacy of various lines of evidence drawn from osteological data, historical

documentation, material culture, and excavation data. The nuance of our case study is different

from most previous strontium isotope studies in that a component of our hypothesis is the

expectation that all of our individuals sampled, and the majority of individuals from the MCIG

cemetery, will have natal regions outside Southeast Wisconsin (Richards 1997:126; Schwarcz et

al. 2010:345).

For all individuals in this study the historical information and osteological analysis were

in agreement. We cannot preferentially favor determined 87Sr/86Sr signatures that suggest an area

of geological origin different from the historically provided natal region if we do not have a

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reason to dismiss the historical identification out of hand. Each of these lines of evidence must

be characterized as working in tandem, and providing give-and-take as we conduct our analysis.

Strontium Isotope Analysis

There are four naturally occurring isotopes of Strontium and our attention focuses

specifically on the single radiogenic isotope, 87Sr. The abundance of radiogenic 87Sr is expressed

as a ratio relative to the abundance of non-radiogenic 86Sr, and this ratio varies with geology,

water, and bioavailability (Pye 2004:220). In the course of an animal’s regular biological activity

(eating and drinking), the local isotope composition of the water, plants, and animals consumed

is captured in its skeletal tissues when strontium substitutes for calcium during element

mineralization (Slater et al. 2014:118). Pye describes 87Sr/86Sr as an “ideal tracer for where an

organism lived and derived the bulk of its diet” (2004:218) as there is no metabolic fractionation,

or change in relative abundance, as a result of an animal’s body size, metabolic processes, or diet

(Slater et al. 2014:118).

Materials and Methods

Of the three primary components of a human tooth, enamel is the hardest and most

inorganic. By substituting for Calcium in inorganic apatite, Strontium is incorporated into human

enamel (Radhakrishan 2011). The skeleton actively remodels over the course of an individual’s

life, thus 87Sr/86Sr signatures from archaeological bone will reflect the last years of life history,

rather than a natal region, in the case of an immigrant (Schwarcz 20010:336). The exclusive use

of bone as a material for strontium isotope analysis is thus dependent on the nature of the

research question and comes with additional complications and concerns with respect to diagenic

contamination (Budd et al. 2000; Lee-Thorp 2008; Price 2007). When compared with bone or

dentine, the greater bond strength of enamel makes it an ideal material for isotope analysis, as it

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is less prone to environmental exchange and post-depositional diagenesis (Pye 2004). By

incorporating the permanent M1 and M3 molars, an environmental picture encapsulating the first

few months before birth through approximately 15 years of age is created (Hillson 1996; Pye

2004; White and Folkens 2005). The additional use of the permanent third molar enables the

consideration of two important factors; 1) the possibility of early childhood migration, or

migration of the mother, in that 2) the first permanent molar, because of pre-natal and early life

mineralization, will receive strontium input from the mother through pregnancy diet and

lactation (Dupras and Tocheri 2007; Schwarcz et al 2010:336).

A total of 18 human teeth from 9 individuals from the 1991-1992 MCIG collection

representing 9 distinct mortuary contexts were analyzed for 87Sr/86Sr isotope ratios. The sample

was obtained from the tooth enamel of select individuals with robust preliminary historic

identifications, congruent osteological profiles, and first and third permanent molars in good

condition. These molars were analyzed for each individual, allowing us to highlight any

differences in 87Sr/86Sr isotope ratios. Biological profiles were completed according to standard

osteological methods (Buikstra and Ubelacker 1994; Spradley and Jantz 2011; White, Black and

Folkens 2012). Sample preparation was undertaken following the procedures outlined in Hedman

et al. 2009 and Slater et al. 2014. Mass spectrometry was performed using a Nu Plasma HR

multicollector ICP-mass spectrometer at the University of Illinois Champaign-Urbana.

Results

Like strontium’s bioavailability, finding a match in natal regions is an organic process,

complicated by differing availability of geological surveys, hydrological surveys, and

archaeological strontium isotope studies. We determined whether each individual’s strontium

signatures fell within published bioavailable ranges of their putative natal regions, based on

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geological bedrock and sediment, water, fauna, and human dental enamel. Together, these

factors comprise an isoscape of a geographic region. We isolated 87Sr/86Sr isoscapes in each

historically identified region that matched the individuals’ natal 87Sr/86Sr signatures. For the

purposes of our discussion, we will be referring to our putatively identified individuals by name

in keeping with our intent to restore identity, while acknowledging that these identifications,

while robust, are in still in development. Our profile identifications fell into three categories.

Frequently, it was the case that we were able to pinpoint geologic formations that would

generate a 87Sr/86Sr range similar to that which was determined via MC-ICP-MS, but we were not

able to definitely preference one region of a nation over another; for example, central over

southwest Germany in the case of contiguous geologic formations between these regions. This is

illustrated by two individuals, Emil Richardt and Ernst Gutzhke, both with natal German regions

and relatively more radiogenic 87Sr/86Sr signatures. These signatures correspond with Triassic age

geological formations, such as Keuper and Bundsandstein, which are found extending from

southwest Germany through east-central Germany. This is contrasted with our other putatively

German individual, Gustav Neumann, who has a less radiogenic 87Sr/86Sr signature that fits

distinctly with the younger loess soils that are specifically seen in Pre-Alpine lowlands of

Southeastern Germany.

The 87Sr/86Sr signatures of Ernst Gutzhke necessitate further elaboration. The signatures

of his permanent M1 and M3 differ by 0.001, a difference that has been cited in the literature as

indicating potential migration (Price et al. 2004). This difference can mean one of two things:

either Ernst himself migrated in childhood (5.5 months to 16 years) before the complete

mineralization of his third molar, or Ernst’s mother migrated while pregnant or in the pre-

weaning period. Given our knowledge that Ernst immigrated to the U.S. at age 22, this migration

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was likely within Germany from a more radiogenic location to the lowlands (Drew 2015,

personal communication).

Conversely, we were able to positively rule out regions that would have produced

87Sr/86Sr ranges that were either too high or too low based on their geology, hydrology, and

overall bioavailable strontium levels. The 87Sr/86Sr signature of Harry Newton fits within a very

specific region of Ohio, the Scioto River watershed, located on Silurian-aged bedrock. The

87Sr/86Sr signature of Gertrude West fits within a specific area of Indiana in the Southwest

characterized by Pennsylvanian-aged bedrock. It should be noted that the human dental enamel

comparison used to identify this isoscape was derived from a match in neighboring Illinois with

comparable geologies, as no human enamel strontium studies have been published for this area

of Indiana. The identification of areas of matching bioavailable 87Sr/86Sr within Poland was fairly

straightforward; Poland can essentially be characterized as having a relatively homogenous and

less radiogenic area to the north and a geologically heterogeneous and more radiogenic area in

the south. Identified individuals Bruno Barkovich and John Zinich can both be associated with

the area of northern Poland characterized by young Cenozoic bedrock overlain with coversands,

loess, and gravel.

Because geological formations frequently appear across national boundaries and political

boundaries are dynamic, extra care must be taken in the case of individuals from regions

characterized by historically shifting political boundaries. Two individuals provide specific

examples of this concern. Vasila Abrodavich died in 1924. His natal region was listed as

Czechoslovakia, which was not an extant political unit until 1918 (Briggs and Clavin 2003). At

the time of his dental mineralization, his matching isotopic region would have been part of

Austro-Hungarian Empire. Wolfgang Aschenbrenner also presents a complex case. His natal

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region is listed as Austria/Hungary. Our first step was to eliminate Hungary, based on the

relatively less radiogenic bioavailable 87Sr/86Sr ranges seen in the homogenous geology.

Aschenbrenner’s relatively high 87Sr/86Sr values match well with the metamorphic bedrock

overlain with loess sediments of Weinviertel region of lower Austria. However, his 87Sr/86Sr

signatures also match ranges seen within the Czech Republic, in an area north of the Elbe with a

similar geological profile. Given that there was no discernable standardization in the assignment

of natal regions in our historical records with respect to then current political boundaries and

despite linguistic affiliation of individual names with each of the natal regions provided, these

individuals may have grown up in an area of matching geology but differing political designation

than the stated burial record entry.

Discussion

There are several research questions this case study contributes to and suggests areas for

further investigation. First, it was our intention to establish the utility of 87Sr/86Sr analysis in later

historical contexts. This is a relatively under-researched time period, and is accompanied by

additional unique complications that are not generally a consideration in prehistoric isotopic

studies. First, there is the issue of a more ‘cosmopolitan’ diet as a result of changing availability

of dietary sources through transportation and agrarian practices associated with industrialization

(Keita et al. 2010). The companion concern to the ‘cosmopolitan’ diet is the greater utilization of

water reservoirs and deep wells in conjunction with public health concerns in the face of

burgeoning populations and industrial pollution (Leavitt 1997). While refrigerated train cars are

not an explicit concern for our population at this time, general concerns regarding the

urbanization of diet can be mitigated by providing more generalized localities for natal origin as

we have done in this study. Exploration into the influence of aquifer geologies through the

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introduction of well water to the diet can be explored in Southeast Wisconsin, as well water has

an appreciably lower 87Sr/86Sr range than surface water, which brings us to areas for future

research.

One element of our planned future research is the addition of an increased level of

specificity in our identification of natal regions through 18O analysis. When strontium research is

complemented by oxygen isotope analysis, the clinal variations created by precipitation narrow

the ranges of the geologic natal regions identified with 87Sr/86Sr ratios. Generation of a more

specific geographic profile will in turn allow us to incorporate additional localized historical

documentation. This will aid in strengthening preliminary identifications of these individuals and

contribute to our understanding of population demographics and immigrant populations in turn-

of-the-century Southeast Wisconsin. It also highlights a second area for further research, the

creation of a viable isotopic study model for future immigrant cemetery analyses in Southeast

Wisconsin. In the future, this can be accomplished by testing the first and third permanent molars

of definitively local individuals in order to establish a bioavailable 87Sr/86Sr range for this historic

time period in Southeast Wisconsin. Lastly, in an effort to continue our stated (research) goal of

providing an additional line of evidence through isotopic analysis for generating robust

individual identifications, we cannot state strongly enough the importance of receiving curational

responsibility for the individuals recovered during the 2013 excavation, to continue this

important work of restoring names and identities to the individuals that were interred at the

Milwaukee County Institutional Grounds Poor Farm Cemetery. Thank you.

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