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Biogeochemical inferences of mobility of early Holocene fisher-foragers from the Southern Sahara Desert.

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Biogeochemical Inferences of Mobility of Early Holocene
Fisher-Foragers From the Southern Sahara Desert
Christopher M. Stojanowski* and Kelly J. Knudson
Center for Bioarchaeological Research, School of Human Evolution and Social Change,
Arizona State University, Tempe, AZ 85287
bioarchaeology; hunter-gatherers; radiogenic strontium isotope analysis; residential
mobility; North Africa
North Africa is increasingly seen as an
important context for understanding modern human evolution and reconstructing biocultural adaptations. The
Sahara, in particular, witnessed a fluorescence of
hunter-gatherer settlement at the onset of the Holocene
after an extended occupational hiatus. Subsequent subsistence changes through the Holocene are contrary to
those documented in other areas where mobile foraging
gave way to settled agricultural village life. In North
Africa, extractive fishing and hunting was supplanted by
cattle and caprine pastoralism under deteriorating climatic conditions. Therefore, the initial stage of food production in North Africa witnessed a likely increase in
mobility. However, there are few studies of paleomobility
in Early Holocene hunter-gatherer Saharan populations
and the degree of mobility is generally assumed. Here,
we present radiogenic strontium isotope ratios from
Early Holocene fisher-forager peoples from the site of
Gobero, central Niger, southern Sahara Desert. Data
indicate a relatively homogeneous radiogenic strontium
isotope signature for this hunter-gather population with
limited variability exhibited throughout the life course
or among different individuals. Although the overall signature was local, some variation in the radiogenic strontium isotope data likely reflects transhumance into the
nearby Aı̈r Massif. Data from Gobero were significantly
less variable than in other worldwide hunter-gatherer
populations, including those thought to be fairly sedentary. Strontium data from Gobero were also significantly
different from contemporaneous sites in southwestern
Libya. These patterns are discussed with respect to
archaeological models of community organization and
technological evolution. Am J Phys Anthropol 146:49–61,
2011. V 2011 Wiley-Liss, Inc.
Inference of residential mobility is central to reconstructions of prehistoric lifeways, subsistence practices,
and organization of human communities across the landscape. Radiogenic strontium isotope ratios are an
increasingly visible approach for inferring mobility in
the past. These ratios are determined in different tissues
at different times in an individual’s life, and can therefore be used to reconstruct permanent changes in residence throughout the life course (see Bentley, 2006). Initially used to identify migrants in sedentary agricultural
communities, more recent work uses radiogenic strontium isotopic data to infer mobility patterns in prehistoric hunter-gatherers (Tafuri et al., 2006; Haverkort
et al., 2008; Kusaka et al., 2009) among whom seasonal
transhumance and sedentary forager communities have
been identified. Therefore, it is no longer appropriate to
assume hunting-gathering-fishing populations in the
past were necessarily highly mobile, while at the same
time recognizing that different kinds of mobility need to
be considered that may not be reflected in the methods
we use to infer prehistoric migration (see Whallon,
In this article we present new data on hunter-gatherer
mobility from the Early Holocene Gobero site in the
southern Sahara of Niger. Radiogenic strontium isotopic
analyses of hunter-gatherer populations are relatively
recent (Tafuri et al., 2006; Haverkort et al., 2008;
Kusaka et al., 2009), and even more nascent in North
Africa (Tafuri et al., 2006; Buzon et al., 2007—see also
Lafrenz, 2004 for an overview). Therefore, this article
has two primary goals. First, we present new baseline
data from the southern Sahara and document underly-
ing geological variability in central Niger to complement
existing archaeological radiogenic strontium isotope data
sets. Second, we generate inferences about the specific
groups that lived in Niger during the Early Holocene,
thus contributing to broader hunter-gatherer research in
the Sahara and beyond. Data on 62 radiogenic strontium
isotope samples taken from archaeological human
remains, modern and archaeological faunal remains, and
soil samples are presented. We briefly review the
archaeological context of the Early Holocene Sahara,
describe the Gobero archaeological site, and review previous radiogenic strontium isotope studies in North
C 2011
Additional Supporting Information may be found in the online
version of this article.
Grant sponsor: National Science Foundation; Grant numbers:
BCS-0636066 and BCS-0820805; Grant sponsor: Wenner-Gren Foundation; Grant number: GR-7747; Grant sponsors: Arizona State University Institute for Social Science Research Catalyst Grant and
Arizona State University School of Human Evolution and Social
*Correspondence to: Christopher M. Stojanowski, Center for Bioarchaeological Research, School of Human Evolution and Social
Change, P.O. Box 872402, Arizona State University, Tempe, AZ
85287, USA. E-mail:
Received 29 December 2010; accepted 24 March 2011
DOI 10.1002/ajpa.21542
Published online 15 July 2011 in Wiley Online Library
Africa. New data are presented and interpreted with
respect to previous studies of paleomobility in Saharan
populations that are largely based on material culture
analyses detailing broad stylistic and functional similarities throughout North and East Africa during the Early
Holocene (Mohammed-Ali and Khabir, 2003; Garcea,
2006). Reconstructing paleomobility among these forager
peoples living under humid Sahelian conditions is critical for understanding broader cultural processes that led
to cattle and caprine pastoralism, an economic specialization practiced by many Saharan peoples today.
Early Holocene of North Africa
During the Late Pleistocene and Early Holocene,
North Africa experienced increasing humidity and was
rapidly re-colonized by human populations after a
50,000-year hiatus. These populations appeared across a
wide geographic area with a remarkably developed and
similar technology, seemingly de novo. Rock shelters and
campsites associated with lacustrine and riverine environments have been documented in the Nile Valley of
Egypt and Sudan, southwestern Libya, the Hoggar of
southern Algeria, and the Aı̈r of Niger (see reviews in
Roset, 1987; Close, 1995; Garcea, 2006; Kuper and Kröpelin, 2006). Material remains include pottery, microlithic stone tools, grinding equipment, and bone harpoons, a package which Sutton (1974, 1977) has termed
the ‘‘Aqualithic.’’ These ‘‘Aqualithic’’ peoples were hunters and foragers and produced pottery with remarkable
stylistic similarities across the Sahara (Roset, 1987;
Roset et al., 1990; Haaland, 1992, 1997; Close, 1995;
Haaland and Magid, 1995; Mohammed-Ali and Khabir,
2003; Van Neer, 2004), suggesting some degree of broadscale contact was evident. Although continued refinement of the chronology complicates broad interpretations, such similarities in subsistence and material culture adaptations over broad areas have been linked to
population expansions and the spread of major African
language families (see Sutton, 1974, 1977; MacDonald,
1998; Holl, 2005).
Remains of the activities of Early Holocene forager
communities were identified in Niger at three localities
near Adrar Bous (Adrar n’Kiffi, the well site, and Diatomite 1—Smith, 1973, 2008; Clark et al., 2008). This
work was supplemented by the excavations of Roset at
Tagalagal in the Aı̈r, Temet near Mt. Gréboun, and Tin
Ouaffedene located southeast of Adrar Bous (see Fig. 1)
(Échallier and Roset, 1986; Roset, 1987; Roset et al.,
1990; Haour, 2003). The existence of a distinct cultural
tradition (Smith’s Kiffian) is heavily debated due, in
large part, to the context in which these material
remains were recovered. Sites of this age are often
eroded due to significant landscape transformation over
the last 10 millennia resulting in the mixing of distinct
cultural strata at the surface; stratigraphic details have
yet to be firmly sorted out (see Roset, 1983; Smith, 2008;
Manning, 2009). Material remains at these sites include
a variety of lithics, grinding equipment, pottery, and
bone harpoon technology, all rarely in situ but often
affiliated with extinct lakebed deposits preserving a rich
terrestrial and lacustrine fauna (Roset et al., 1990; Gifford-Gonzalez and Parham, 2008). The pottery is among
the earliest in the Sahara and part of the ubiquitous
‘‘wavy-line,’’ ‘‘dotted wavy-line,’’ and rocker-stamped tradition found throughout North Africa (Roset, 1983, 1987;
American Journal of Physical Anthropology
Roset et al., 1990; Mohammed-Ali and Khabir, 2003;
Garcea, 2008). The lithic assemblage is particularly controversial, with Smith (2008) differentiating an earlier
macroblade industry (Ounanian) from an Early Holocene
microlith industry (Kiffian) from a younger, point-based
Tenerian pastoral technology. Roset (1983, 1987) and
Roset et al. (1990) is less certain about these distinctions.
These peoples were completely unknown skeletally
prior to fieldwork at Gobero; therefore, even basic
aspects of biocultural and bioarchaeological relevance
are unknown. Subsistence practices are largely inferred
from artifact function and reconstructions of paleolake
ecology. For example, Smith (2008) envisions the use of
crescent-shaped stone tools for hunting terrestrial and
aquatic fauna, bone harpoons for fishing, and grinding
stones to process wild grains. Although not interpreted
as necessarily sedentary, this reconstructed subsistence
profile does center activities around paleolake environments. Garcea’s (2008) analysis of the ceramics at Adrar
Bous suggests opportunistic manufacture using local
resources, consistent with the petrographic work of
Échallier and Roset (1986) who documented limited
movement of pottery from clay sources. Nonetheless,
Garcea (2008; p 284-285, 282) interprets the Early Holocene adaptation as ‘‘a mobile, lacustrine/riverine focused
settlement system’’ and views the deposits at Adrar
Bous as representing ‘‘open-air sites, which were probably ephemeral seasonal camps, located on the high
shorelines of ancient lakes’’ (Garcea, 2008). Garcea
(2008) views the Early Holocene in Niger as the local
manifestation of a broader process of trans-Saharan cultural development, linking communities together over
vast distances and bringing a similar pottery tradition
with them (see Roset, 1987; Roset et al., 1990; Haaland,
1992, 1997; Halid and Magid, 1995; Close, 1995; Garcea,
1998; Mohammed-Ali and Khabir, 2003).
The Gobero site complex in Niger
Gobero is located in central Niger in an area that preserves a series of paleodune cemeteries and lakebed
deposits (Sereno et al., 2008). Gobero was one of many
lakes that formed throughout the Sahara during the
Early Holocene (Faure et al., 1963; Petit-maire and
Riser, 1983) but was smaller than the mega lakes that
then existed, of which Lake Chad is the only remnant
(Bouchette et al., 2010). Zooarchaeological analyses of
surface and excavated fauna indicate an abundance of
lacustrine and terrestrial resources were present and coterminous with human reoccupation of the area. The
zooarchaeological inventory at Gobero (Sereno et al.,
2008; Table 5) is remarkably similar to that at Adrar
Bous (Roset et al., 1990; Gifford-Gonzalez and Parham,
2008; Table 11.4), suggesting environmental and subsistence parallels.
Radiocarbon dating of human remains indicates two
distinct periods of burial activity. The earliest occupation, the focus of this article, dates to 9,500 to 8,200 cal
BP with a majority of burials clustering within a narrow
horizon circa 9,500 cal BP (mean 5 9,507 6 98 years cal
BP, n 5 5—Sereno et al., 2008), generally coterminous
with archaeological sites further north (see above). These
burials were dark stained (pyrolusite) due to aqueous
submergence and primarily placed within a discrete area
on one specific dune (site G3, Fig. 2), suggesting a formal
cemetery (Miller and Stojanowski, 2008). These dark-
Fig. 1. Map of the central Sahara Desert showing underlying bedrock geology. Site numbers are as follows: 1 5 Gobero, 2 5
Alallaka Quarry, 3 5 Tagalagal, 4 5 Adrar Bous, 5 5 Temet, 6 5 Tin Ouaffadene. Libyan Sahara sites are indicated by the boxed
area (upper right corner). Lake Chad is in the lower right corner. A north-south line roughly dividing the Aı̈r separates the Chad
Basin and Iullemeden Basin. Geological data based on Schlüter (2006) and Cratchley et al. (1984).
stained burials were distinct from more recent burials at
Gobero in several ways (postcranial robusticity, stature,
craniofacial structure, burial positions, friability, susceptibility to sun bleaching, and calcrete encrustation)
thus allowing chronological allocation of all excavated
remains. The tight radiocarbon chronology of most buri-
als suggests ceremonial use of the site by an individual
band of hunter-gatherers for a few generations. A similar site usage pattern is suggested by the formal nature
of the cemetery with evidence of spatial structuring of
graves along the lines of kinship (Miller and Stojanowski, 2008).
American Journal of Physical Anthropology
Fig. 2. Map of Africa and Niger (A) showing major landmarks and modern cities. Gobero plan-view (B) showing human burial
sites (G1–7) and intact lakebed deposits (shaded gray areas—modified after Sereno et al. (2008)). (C) Burial plan of site G3 showing
excavated grid squares and burial positioning and number. Dark circles are Early Holocene burials, white circles are Middle Holocene burials, with burials numbers in gray. Radiocarbon dates (labeled in black) are presented for those burials directly dated based
on enamel apatite (Sereno et al., 2008), calibrated midpoints as date BP.
Radiogenic strontium isotope analysis and
archaeological human mobility
Biogeochemical approaches to archaeological residential mobility utilize the fact that enamel and bone incorAmerican Journal of Physical Anthropology
porate isotopic signatures during mineralization that
reflect the isotopic signatures of the foods consumed and
the water imbibed during enamel and bone formation.
Radiogenic strontium isotope analysis utilizes geologic variability. The radiogenic isotope of strontium, 87Sr,
decays from Rb, so that Sr/ Sr varies according to
the age and initial composition of the bedrock (Faure
and Powell, 1972). As strontium moves through the ecosystem, 87Sr/86Sr does not fractionate appreciably
(Åberg, 1995; Blum et al., 2000), so that the strontium
isotope value measured in tooth enamel and bone
reflects the strontium isotope composition of the strontium sources in the diet (Ericson, 1985; Price et al.,
1994). If the strontium sources in the diet are local,
Sr/86Sr values measured in tooth enamel or bone will
reflect the geological region in which a person lived during the formation of the respective tissue (see overview
in Bentley, 2006).
Identifying and minimizing diagenetic, or postdepositional, contamination in archaeological samples is a key
aspect of isotopic mobility studies. It is clear that bone is
more susceptible than enamel to diagenesis and there
are various approaches to minimizing and identifying
diagenetic contamination (Nelson et al., 1986; Price et
al., 1992; Iacumin et al., 1996; Wright and Schwarcz,
1996; Koch et al., 1997; Sharp et al., 2000; Lee-Thorp,
2002, 2008; Hedges, 2002; Berna et al., 2003; Lee-Thorp
and Sponheimer, 2003; Shahack-Gross et al., 2003).
While a weak acid wash can remove diagenetic strontium (Sillen, 1989; Sillen and Sealy, 1995), major, minor,
and trace element concentrations can also be used to
identify hydroxyapatite in which elements like strontium
have likely been altered (e.g., Williams and Marlowe,
1987; Price et al., 2002; Grün et al., 2008; Tütken et al.,
Radiogenic strontium isotope
signatures in the Sahara
Little archaeological isotopic research on prehistoric
mobility has been undertaken in West and North Africa,
although this is slowly changing (Tafuri et al., 2006;
Buzon et al., 2007). There are many complications to
such inferences. First, Saharan radiogenic strontium isotope signatures today are relatively homogenous due to
aeolian dust transport (Grousset et al., 1992; Rognon et
al., 1996; Krom et al., 1999; Frumkin and Stein, 2004).
This is not relevant for investigations of prehistoric mobility at Gobero because strontium sources would reflect
the underlying bedrock deposits before the desertification of the region. However, establishing local baselines
using modern faunal samples may be biased by recent
Saharan dust, which generally has a high radiogenic
strontium isotope value (mean 87Sr/86Sr 5 0.722—Weldeab et al., 2002), depending on particle size (Grousset
et al., 1992, 1998; Rognon et al., 1996). Measured radiogenic strontium isotope ratios from dune samples
throughout northwestern Africa exhibited values ranging from 87Sr/86Sr 5 0.713 to 87Sr/86Sr 5 0.737 (Grousset
et al., 1998). Second, the Sahara has undergone significant transformation since the Early Holocene, including
the disappearance of major inland lakes and the deflation of the landscape over the last 4,500 years. As such,
inference of ecological and landscape variability is restricted to only permanent features that may have
offered resources different from those available at
Gobero. These include major massifs and inselbergs, plateaus, and most visibly Mega Lake Chad that reached
its optimum during the early occupation phase of Gobero
(Drake and Bristow, 2006; Bouchette et al., 2010). More
geologically ephemeral attractions (such as Gobero itself)
are no longer visible to us as possible migration structur-
ing features of the landscape. Finally, although not as
complex as the southern half of Africa (Schlüter, 2006;
Begg et al., 2009), the geology of the southern Sahara
does preserve sediments of multiple ages including PreCambrian basement, Silurian through mid-Jurassic volcanic granites, Jurassic through Cretaceous sandstones,
and recent volcanic rocks that are only centuries old. For
this reason it is critical that we focus on strontium bioavailability when possible. With these issues in mind, we
present new data from the Gobero region below and
focus on two previous studies from the Sahara here.
Tafuri et al. (2006) presented radiogenic strontium isotopic data on a series of individuals from the southwestern Libyan Fezzan that preserves a west-to-east monocline of Lower Silurian through Jurassic (Paleozoic-Mesozoic) sandstones. The range of human radiogenic
strontium values presented by Tafuri et al. (2006) vary
from 87Sr/86Sr 5 0.70975 to 87Sr/86Sr 5 0.71206. These
data are consistent with estimates of radiogenic strontium isotope sandstone values from eastern Libyan and
western Egyptian desert contexts reported by Schaaf
and Müller-Sohnius (2002), which range from 87Sr/86Sr
5 0.70910 to 87Sr/86Sr 5 0.71053. Buzon et al. (2007)
also presented archaeological human radiogenic strontium isotope data from North Africa. These data derive
from much younger New Kingdom populations from the
Third Cataract region of the Nile Valley and reflect
lower radiogenic strontium isotope values that range
from 87Sr/86Sr 5 0.70712 to 87Sr/86Sr 5 0.70912 (Buzon
et al., 2007). Buzon et al. (2007) also reconstruct radiogenic strontium isotope variability along the Nile River’s
course which ranges from 87Sr/86Sr 5 0.7075 (Cairo) to
Sr/86Sr 5 0.7090 (Thebes) to 87Sr/86Sr 5 0.7060 (White
Nile). These values are all lower than those observed in
the Libyan Fezzan and are consistent with other
reported radiogenic strontium isotope values from the
Upper Nile Valley (Gerstenberger et al., 1999; Talbot et
al., 2000; Pe-Piper and Piper, 2001; Weldeab et al., 2002)
and East Africa, where reported values are even lower
(87Sr/86Sr 0.703–0.705; e.g. Betton and Civetta, 1984;
Ayalew et al., 1999). The combined East African, Nile
Valley and Libyan Desert data suggest a dichotomy
between lacustrine and riverine contexts in the east
where recent volcanic input lowers expected radiogenic
strontium isotope values and desert contexts further
west which consist of Mesozoic and Cenozoic sandstones
and some Pre-Cambrian exposures.
Gobero is located on a broad plain of partially exposed
(Tegama Formation; Wright et al., 1985) that form the
western part of the Chad Basin (Begg et al., 2009) and
extend into the Iullemeden Basin east of the Aı̈r (see
Fig. 1). Mapping of geological features at Gobero indicates a predominant exposure of these sandstones overlain sporadically with Pleistocene (Quaternary) dunes
that sat directly on top of the bedrock (Sereno et al.,
2008). Because Gobero is located within the Lake Chad
drainage, the Pleistocene dunes are likely weathered
sediments from the nearby Aı̈r Massif; however, these
are not a major component of the Gobero region. Aeolian
sand covered most of these features but this is not relevant to this timeframe. Based on the age and type of
bedrock, we hypothesize that individuals who obtained
strontium from food sources near Gobero will have radiogenic strontium isotope signatures that are likely
Sr/86Sr 0.710 based on radiogenic strontium isotope
signatures from similar exposed bedrock and archaeologAmerican Journal of Physical Anthropology
ical human remains in western Egypt and Libya (Schaaf
and Müller-Sohnius, 2002; Tafuri et al., 2006).
We consider two areas of possible movement away
from the Gobero paleolake. First, given the proximity of
Gobero to the Aı̈r Massif, the lack of visible geological
features to the immediate south or east of Gobero, and
the presence of contemporaneous sites near the Aı̈r
(Roset, 1987; Roset et al., 1990; Sereno et al., 2008;
Smith, 2008), movement away from Gobero may have
followed an lowland-upland pattern similar to that documented in Libya (Tafuri et al., 2006). Second, Lake Chad
reached its recent maximum during the Kiffian occupation phase at Gobero. Mega Lake Chad was located far
to the southeast, however, but was connected to the
Tibesti Massif by the Angamma delta (Drake and Bristow, 2006; Bouchette et al., 2010), thus offering a number of resources that may have attracted people to the
region. Whether such movement from the Aı̈r to Lake
Chad would be visible isotopically is difficult to discern,
however, given the composition of the Chad Basin bedrock.
The Chad drainage is a large, intracontinental basin
formed by uplifting events at its margins (Burke, 1976).
Its interior sediments are all post Pan-African in age
(\550 mya), which contrast with the circumferential,
uplifted massifs. Today, large sections of the basin’s
northern segment are covered by the Ténéré Desert that
consists of Holocene dune cover unrelated to prehistoric
mobility at Gobero. These dunes probably cover Pleistocene-age (Quaternary) sediments that are visible
throughout much of the southern portion of the basin
(Burke, 1976) and are partially eroded by wave action
near the borders of Mega Lake Chad (Drake and Bristow, 2006; Bouchette et al., 2010). These Pleistocene-age
dunes were present at Gobero where they formed
between 12 and 17 kya; the human remains were placed
within these sediments (Sereno et al., 2008). The bedrock of the Chad basin, however, consists of Cretaceous
sandstones that are exposed only in the western portion
of the drainage, near the Aı̈r and Gobero (The Tegama
Formation). Genik (1992) and Hartley and Allen (1994)
date sandstones in the Niger segment of the Chad Basin
(Termit and Ténéré rift structures) as Miocene-recent in
age. In general, exposed sediments trend from younger
near Lake Chad (due to greater accumulation of Pleistocene sand and clays—the Chad Formation) to exposures
of older bedrock near the Aı̈r; however, most of this variation manifests only near the Aı̈r itself. The older
exposed bedrocks are expected to yield higher radiogenic
strontium isotope values (see Price et al., 2006); however
this may be offset by greater contribution from weathering of recent volcanic materials that are deposited within
the Chad watershed. As such, much of the Chad drainage is likely relatively homogenous and expected to produce strontium bioavailability signatures similar to that
seen at Gobero (87Sr/86Sr 0.710). We therefore
hypothesize that movement throughout the Chad drainage would be difficult to detect using radiogenic strontium isotopic analysis.
Unlike the relatively homogenous Chad Basin, the Aı̈r
Massif presents a complex geological history. It is part of
the Tuareg Shield located on the West African Mobile
belt that was deformed during the Pan-African orogeny
circa 550 mya (Black et al., 1994). Mapped exposures
include Pre-Cambrian basement basalts, Paleozoic (Silurian and Devonian) subvolcanic ring structures (not
mapped in Fig. 1), and Cenozoic volcanic rocks (Black
American Journal of Physical Anthropology
et al., 1994; Liégeois et al., 1994; Schlüter, 2006; Williams, 2008). The Aı̈r is divided along its north-south
axis by a major sheer zone related to more recent volcanism in the region. Some of the recent volcanic flows
may be only a few centuries old. In addition, sediments
of Devonian and Carboniferous age (Paleozoic) outcrop
along the Aı̈r’s western side, while the eastern terrane
(Aouzeguer) surrounding the Tafidet basin (an extension
of the Tegama Formation) consists of Proche-Ténéré
molassics (see Fig. 1) (Liégeois et al., 1994). Given the
different geological exposures of the eastern and western
sides of the Aı̈r Massif and the clustering of younger basaltic flows in the southern half of the massif, we expect
microregionalization of radiogenic strontium isotope signatures. Nonetheless, this complex geological history
confounds attempts to convert reported 87Sr/86Sr values
into predicted values for humans. This difficulty reflects
the relative surface exposure of different granites and
basalts, differential weathering of these materials, and
differences in source strontium content (Bentley, 2006).
Williams (2008) notes that three-fourths of the Aı̈r Massif consists of Paleozoic anorogenic ring structures where
it is expected that 87Sr/86Sr 5 0.705–0.706 (Brown et al.,
1989; Demaiffe et al., 1991). Younger Cenozoic volcanic
rocks in the Aı̈r Massif are less common and expected to
yield values of 87Sr/86Sr 5 0.703–0.706 which are similar
to expected values from the geologically related Hoggar
of southern Algeria (Leger, 1985; Petters, 1991; Dupuy
et al., 1993; Black et al., 1994). However, detailed maps
of the Aı̈r Massif indicate extensive exposures of PreCambrian granites where the radiogenic strontium signature should be much higher (see Fig. 1). We report
one value from Capra below that suggests greater input
from these older sediments, and we hypothesize that
extended stays in the Aı̈r Massif would result in significantly higher radiogenic strontium values in those tissues that formed during that time period.
Sampling strategy
Baseline radiogenic strontium isotope data were
obtained from soil samples and archaeological and modern faunal material (Supp. Info., Table S1). All soil samples consist of gray, fine-grained, unconsolidated sand
that comprised the Pleistocene-age dunes in which the
burials were placed. Soils samples were collected from
fill immediately below the burial. Faunal samples were
collected opportunistically at or near Gobero during the
course of fieldwork. Comparison of archaeological and
modern species allows assessment of potential diagenetic
contamination, and the different home ranges of the species sampled can elucidate underlying local and regional
strontium variability. Adult males, females, and juveniles were sampled, reflecting a broad demographic segment of the population. Sex was determined using cranial and pelvic morphology. Adult age estimation used
pubic symphysis, auricular surface, and cranial suture
closure (Boldsen et al., 2000) supplemented with dental
attrition seriation. Juvenile age estimates were based on
dental eruption and formation, and to a lesser extent
long bone lengths.
To understand changes in paleomobility throughout
the life course, adult first, second and third molars were
selected from each burial and matched with a non-diagnostic segment of long bone or rib to provide isotopic evidence from dental and skeletal elements that formed at
TABLE 1. Raw and summary data for human radiogenic strontium isotope data from Gobero
Burial #
Summary Data
95% CI
0.0000531–0.000312 0. 000101–0.000365 0. 000144–0.000629 0.0000256–0.000138
different times in that individual’s life (Tables 1 and 2).
Dental and skeletal elements were selected that had not
been exposed prior to their recovery, and burials that
were significantly exposed due to erosion were excluded
from the sampling design. In addition, careful sampling
ensured that elements were not selected if they had
been exposed to bulking (Rhoplex) or conservative agents
(Acryloid B-72).
Laboratory methodology for radiogenic
strontium isotope analyses
Archaeological enamel and bone samples were prepared at the Archaeological Chemistry Laboratory at Arizona State University (Knudson, 2007; Knudson and
Price, 2007; Torres-Rouff and Knudson, 2007). Radiogenic strontium isotope ratios were obtained at the W.M.
Keck Foundation Laboratory for Environmental Biogeochemistry at Arizona State University. The strontium
was separated from the sample matrix with EiChrom
SrSpec resin based on published methodologies
(Knudson, 2007; Knudson and Price, 2007; Torres-Rouff
and Knudson, 2007). At Arizona State University, strontium isotopes were measured on a Neptune multi-collector inductively coupled plasma mass spectrometer (MCICP-MS), where SRM-987 has exhibited 87Sr/86Sr 5
0.710265 6 0.000010 (2r, n 5 25). In addition, samples
were analyzed for major, minor, and trace element concentrations, including calcium, phosphorus, uranium and
neodymium, in order to identify the presence of diagenetic contamination; these samples were analyzed on a
quadrupole inductively-coupled plasma mass spectrometer (Q-ICP-MS) at Arizona State University.
Evaluating diagenetic contamination
Major, minor, and trace element concentrations in
archaeological human enamel were analyzed as a proxy
for identifying diagenetic contamination (e.g., Williams
and Marlowe, 1987; Price et al., 2002; Grün et al., 2008;
Tütken et al., 2008). In biogenic samples, Ca/P 5 2.1; at
Gobero, mean Ca/P 5 2.14 6 0.01(n 5 70, 1r), which we
interpret as evidence of little diagenetic contamination.
In addition, mean concentrations of uranium (U) and ne-
odymium (Nd) are low; mean U/Ca 5 1.7 3 1024 6 4.0
3 1024 (n 5 21, 1r) and mean Nd/Ca 5 1.2 3 1024 6
1.5 3 1024 (n 5 21, 1r). These values are within the
range of published U/Ca enamel data that were interpreted as uncontaminated (Price et al., 2002).
In addition, the bone radiogenic strontium isotope
data do not reflect a trend toward the local soil signatures as expected if the former were affected by diagenetic contamination. In fact, the mean human bone
Sr/86Sr value is significantly different from the burial
matrix soil sample data (P 5 0.00070). Although it is important to differentiate geological versus bioavailable
strontium when interpreting these kinds of data, that
the bone data do not trend toward the soil values is suggestive of their biogenic nature (see Fig. 3).
Further evidence that the data we present are biogenic can be inferred from the patterns of variation
within and among individuals at Gobero. For example, if
enamel and bone signatures are very similar and show a
local value this may suggest replacement of the relevant
molecules in the post-depositional environment. At first
glance, the data from Gobero seem to present this pattern because there is little difference between tooth and
bone signatures (Table 1). However, individual variation
in 87Sr/86Sr values suggests a more complex pattern. For
example, the standard deviation in radiogenic strontium
isotopes values for dental and bone signatures by individual is on average 0.00005 for juveniles, 0.00020 for
female adults, and 0.00030 for male adults. The samples
from adults are an order of magnitude more diverse
throughout the life course suggests the bone data are
not reverting to a local average because there is no reason to expect age-specific bone value patterning if diagenetic contamination were evident. Furthermore, examining the average enamel radiogenic strontium isotope
value minus the bone radiogenic strontium isotope value
in each individual indicates change in the adult versus
pre-adult 87Sr/86Sr value. A correlation between the average tooth-bone difference and age at death returns a
strong, positive result (r 5 0.70100, P 5 0.03500). This
suggests that as an individual’s age increased, the difference between adult and juvenile radiogenic strontium
isotope signatures increased; this is exactly what one
would expect if the data were biogenic. To the contrary,
if the bone data represented local values due to diageAmerican Journal of Physical Anthropology
less variable; Gobero higher
less variable
less variable; Gobero higher
less variable
Fig. 3. Radiogenic strontium isotope values in archaeological
human and zooarchaeological samples from Niger. Humans
listed by burial number seriated from youngest to oldest from
left to right based on the midpoint of their respective age intervals (see Table 1). Age of individual is listed before the before
number. Burials preceded by ‘‘99’’ were adults whose age could
not be determined. Black diamond 5 bone, open circle 5 M1,
hashed circle 5 M2, gray circle 5 M3. Zooarchaeological data
(stars) represent the following subdivisions: 1) ZooarchAir 5
data from Alallaka quarry and camel specimens near Agadez on
the road to Gobero, 2) ZooarchFault 5 owl pellet rodent data
and several unidentified species of modern herbivore collected
from jackal dens, 3) ZooarchLocal 5 archaeological and modern
faunal samples collected from the Gobero paleodunes.
netic contamination, no correlation with age at death
would be evident. Finally, we note that the average
human bone 87Sr/86Sr value is significantly lower than
the average human tooth 87Sr/86Sr value (P 5 0.01040).
Source: Haverkort et al. (2008).
Source: Kusaka et al. (2009).
Source: Buzon et al. (2006).
Source: Tafuri et al. (2006).
*Significant at alpha 5 0.05 level.
Establishing a local baseline and regional
variability patterns
Global Hunter-Gatherer sites
North African sites
Libya—Late Acacusd
Libya—Late Pastorald
Libya—Final Pastorald
0.7078 (0.0005); F test 0.04946*; T test \0.00001*
0.7113 (0.0008); F test 0.00070*; T test 0.20870
0.7113 (0.0005); F test 0.03870*; T test 0.04430*
0.7112 (0.0003); F test 0.03230*; T test 0.52880
0.7112 (0.0007); T test 0.00130*
Gobero less variable
Gobero less variable
0.7104 (0.0006); F test \0.00001
0.7089 (0.0002); F test 0.01400*
0.7123 (0.0022); F test \0.00001*
0.7092 (0.0004); F test 0.52910
Sr/86Sr, mean (SD)
Sr/86Sr, mean (SD)
TABLE 2. P values for comparisons of Gobero dental and bone data with other hunter-gatherer samples
American Journal of Physical Anthropology
Based on our radiogenic strontium isotope analyses of
soil and modern faunal samples from the region, the bioavailable strontium isotope signatures at Gobero are
high, as expected, and distinct from other parts of the
study area. More specifically, soil samples from Gobero
burials exhibit 87Sr/86Sr 5 0.71290 6 0.00064 (n 5 7,
1r) while modern faunal samples exhibit 87Sr/86Sr 5
0.71261 6 0.00116 (n 5 21, 1r) (Sereno et al., 2008).
Further consideration of the faunal data reveals interesting patterns. First, there are five archaeological and
modern faunal samples selected directly from the Gobero
archaeological site complex. Both archaeological and
modern samples show similar 87Sr/86Sr values with little
variability and exhibit 87Sr/86Sr 5 0.71160 6 0.00062 (n
5 5, 1r) (Supp. Info., Table S1). The similarity of modern
and archaeological faunal 87Sr/86Sr data further supports
the biogenic nature of our radiogenic strontium isotope
In addition to these samples from the area of the
Gobero cemeteries, a number of modern samples were
collected from the Mazelet fault that forms the southern
boundary of the Gobero region (Sereno et al., 2008).
These data consist of modern rodent samples from owl
pellets and modern herbivores collected from jackal
dens. The Mazelet fault data (87Sr/86Sr 5 0.71275 6
0.00012 (n 5 11, 1r)) are significantly higher than the
human (P 5 0.01040) and faunal data (P 5 0 0.02050)
collected from Gobero paleodunes. The owl pellet data
are particularly variable (87Sr/86Sr 5 0.71268 6 0.00144
(n 5 7, 1r)), which is not surprising given that each pellet represents one to two meals of small bodied rodents.
Although we do not know the species of these owls, those
endemic to the Sahara (e.g., Bubo sp., Tyto sp.) generally
have maximum hunting ranges of five kilometers (Lovari
et al., 1976; Rifai et al., 2000). These data, therefore,
suggest significant microlocal variation is present in the
Gobero region that has been averaged in humans.
In addition to these local data, we obtained one sample
from a juvenile goat (Capra hircus) raised near the Alallaka quarry site (Sereno et al., 2008) in the Aı̈r Massif
(see Fig. 1). We also collected several camel specimens
along the Agadez-Bilma piste (desert track) north and
northwest of Gobero. Both the piste (87Sr/86Sr 5 0.71293
6 0.00047 (n 5 4, 1r)) and Aı̈r Massif data (87Sr/86Sr 5
0.71470 (n 5 1)) indicate higher 87Sr/86Sr values in comparison with the human data (P 5 0.0085 for the piste
sample-human T-test). These data from areas north of
Gobero are consistent with expectations that exposed
sediment age increases as one approaches the Aı̈r. Combined, then, the faunal data reflect regional variability
in 87Sr/86Sr values as expected based on differences in
underlying bedrock geology, as discussed above.
Paleomobility at Gobero during
the Early Holocene
Results of radiogenic strontium isotope analyses of
archaeological human tooth enamel and bone samples
are shown in Table 1 and Supporting Information, Table
S2. Mean archaeological human enamel and bone
Sr/86Sr 5 0.71159 6 0.00030 (1r, n 5 32) and ranged
from87Sr/86Sr 5 0.71129 to 87Sr/86Sr 5 0.71266. Mean
archaeological human enamel 87Sr/86Sr 5 0.71171 6
0.00031 (1r, n 5 20) and mean archaeological human
bone 87Sr/86Sr 5 0.71138 6 0.00089 (1r, n 5 12). These
values are similar to the baseline strontium isotope data
discussed above.
When these data are further considered by tooth type
several interesting observations are apparent (Table 1).
First, mean 87Sr/86Sr values for M1, M2, and M3 are
identical to the fourth decimal place (mean 87Sr/86Sr 5
0.7117) and interindividual standard deviations between
tooth classes are roughly the same (0.0002, 0.0004,
0.0003, respectively). The 95% confidence intervals based
on 1,000 bootstrapped standard deviations indicate no
significant differences in 87Sr/86Sr variability among
tooth classes and there does not appear to be a significant weaning effect reflected in the enamel data. These
data suggest a similar mobility experience for individuals throughout the juvenile age interval. Second, the
Sr/86Sr bone (mean 87Sr/86Sr 5 0.7114) values are significantly different from the tooth (mean 87Sr/86Sr 5
0.7117) values (P 5 0.01040); however, this difference is
likely not relevant from an anthropological perspective.
Interindividual variability in the bone data (0.00009) is
an order of magnitude lower than the comparable tooth
data. This difference is statistically significant for the
M2-bone comparison (P 5 0.00002) and for the average
tooth-bone comparison (P 5 0.00047). A pattern of
greater tooth rather than bone interindividual strontium
variability has been documented elsewhere and interpreted as reflecting the time averaging effects of skeletal
tissues (Haverkort et al., 2008; Kusaka et al., 2009).
Although diagenetic contamination would also be
expected to decrease the variability of the bone data, as
discussed above this is unlikely (Supp. Info., Tables S1,
S2, and Fig. 3). The overall pattern is one of remarkable
homogeneity, both throughout the life course of single
individuals as well as among different individuals that
were buried at Gobero.
Consideration of the patterning of values in Figure 3
refines this interpretation, however. The enamel-bone
differences clearly pattern by age-at-death, which is
unexpected. Individuals who died as juveniles have very
similar enamel and bone values. However, individuals
who died as adults have enamel strontium values that
are higher than their respective bone values, without
exception. This pattern is established somewhere in the
15 (G3B35) to 21 (G3B3) year age interval and is maintained in all adults who died at older ages. While juvenile and adult bone values are not significantly different
(P 5 0.442), subadult and adult enamel values are (P 5
0.00018). This variation seen in the enamel values is difficult to explain. If the converse was observed the likely
interpretation would be diagenesis or related to adult
age status within the community and changing behavioral patterns related to subsistence or post-marital residence. Indeed there is some sex-specific patterning to
enamel-bone differences; males returned an average difference of 0.0005 while females returned an average difference of 0.0003. The result was not significant owing to
the small sample size (P 5 0.363).
Results of inferential statistical tests comparing
Gobero to other hunter-gatherer sites are presented in
Table 2. Parametric F-tests comparing aggregate enamel
and bone data indicate Gobero is significantly less variable than Lake Baikal (Haverkort et al., 2008), Jomon
(Kusaka et al., 2009), and Late Acacus (Tafuri et al.,
2006) hunter-gatherers. Since the Jomon and Late Acacus are generally considered sedentary hunter-gatherers,
these comparisons suggest limited mobility for the Kiffians at Gobero. This interpretation must be tempered
by differences in underlying geological variability, however.
Radiogenic strontium isotopic ratio variability was low
among humans buried at Gobero and was significantly
lower than other hunter-gatherer groups, including
those thought to be fairly sedentary. This pattern was
documented in both dental and skeletal tissues and is
unlikely related to diagenetic contamination. Individuals
at Gobero also demonstrated limited radiogenic strontium isotope variability throughout the life course and
little difference in life course patterning among individuals. In other words, the overall radiogenic strontium isotope signature was local, enamel-bone differences within
individuals were minimal, and the same basic pattern
was found for most individuals. This consistent homogeneity suggests group composition was fairly stable with
no evidence for in-migration or mate exchange from
regions with vastly different geochemical signatures.
Likewise, Gobero individuals did not appear to spend
extended periods of the juvenile age interval in areas
with significantly higher or lower radiogenic strontium
American Journal of Physical Anthropology
isotope values. In other words, if time was spent in the
Aı̈r Massif it was likely related to seasonal transhumance rather than permanent changes in residence. The
group moved in concert as a single social unit on the
landscape. This, of course, does not preclude the kinds of
specialized mobility and exchange documented among
hunter-gatherers (Whallon, 2006), but such interaction
was not spatially or temporally coarse enough to be
reflected in the biogeochemical signatures.
There is, however, one striking feature of Figure 3 that
requires further comment and which deviates from the
overall patterning discussed above. Given the homogeneity of juvenile radiogenic strontium isotope values compared to adult data (see Fig. 3), it appears that individuals who died as adults lived in a larger number of different geochemical zones as juveniles, and then moved to the
Gobero paleolake for the last years of their lives. On the
contrary, those who died as juveniles appear to have lived
in the area near Gobero for their entire lives. There are
different interpretations of these data. First, given the
tight distribution of radiocarbon dates (Sereno et al.,
2008) the burial record may define a short-lived (several
generation) occupation of the Gobero region. In other
words, despite the overall homogeneity of signatures,
micro-level variation may reflect a frontier community
that repopulated Gobero for a short period of time; adults
grew up in a wider range of environments but all died
with a local Gobero signature. Juveniles were born, lived
and died near Gobero and show less variability among tissues as a result. This interpretation relies heavily on the
radiocarbon record, however, and it is not parsimonious to
assume we have identified the first migrant community to
this region. Finally, it is also possible that individuals
who died as juveniles consumed strontium from a smaller
range of sources; this hypothesis could be tested through
stable strontium isotope analysis to identify the trophic
levels of strontium consumed (Knudson et al., 2010).
The second interpretation relates to postmarital residence practices. Here, some adults who died at Gobero
were living in their non-natal environment reflecting
residence after marital migration had occurred. This process explains the variety of enamel signatures seen in
adults as well as the variation among adults in terms of
the degree of difference between enamel and bone signatures—some were born and died locally, others were
born extralocally but died locally. Juvenile enamel-bone
similarity reflects a sedentary existence in their natal
community. Despite the intriguing potential insights into
Early Holocene community structure, these inferences
are based on limited data and the overall degree of
radiogenic strontium isotope variability we are dealing
with is very small, comparatively speaking.
Interestingly, there is one individual that does not conform to the tight dating of most Early Holocene burials
and is also aberrant isotopically. Burial G3B28 is over
1,000 years younger than other burials from the Early
Holocene occupation phase, was not buried near the formal cemetery on site G3 that comprised the majority of
this dataset (see Fig. 3), and was buried in a different
burial posture (highly extended). This burial shows the
only nonlocal radiogenic strontium isotope datum point
(for the M2, Fig. 3) and the highest bone value of any
individual, suggesting this male may have spent more
time in environments such as the Aı̈r Massif. The higher
enamel and bone signatures, combined with the earlier
date, may reflect adaptation to a deteriorating climate in
which aridity favored a more nomadic lifestyle.
American Journal of Physical Anthropology
The nature of Early Holocene Saharan
community interaction
Comparing 87Sr/86Sr values at Gobero to other human
data throughout the Sahara Desert helps clarify the nature of Early Holocene human adaptation in North
Africa (Table 2). Unsurprisingly, the aggregate tooth
data from Gobero were significantly different from aggregate tooth data from Tombos in the Nile Valley (P \
0.00010) (Buzon et al., 2007). Pottery styles clearly
moved farther than people. However, comparison with
Libyan Fezzan data revealed a number of robust patterns (Tafuri et al., 2006). Samples of large enough size
for inferential comparison were available for three temporal subsets of the Libyan data: Late Acacus huntergatherers (9,800–9,000 BP), Late Pastoral period pastoralists (5,000–3,500 BP), and Final Pastoral period pastoralists (3,500–2,700 BP) (Table 2). Interindividual
radiogenic strontium isotope variability at Gobero was
significantly lower than Late Acacus hunter-gatherers (P
5 0.00070), although the values were not significantly
different (P 5 0.20870). In comparison with Late Pastoral burials from Libya, Gobero exhibited significantly
less interindividual variability (P 5 0.03870) and was
also significantly different in terms of the radiogenic
strontium isotope signature itself (P 5 0.04430). Finally,
interindividual variability at Gobero was not significantly less variable than the Libyan Final Pastoral sample (P 5 0.52880); however, the radiogenic strontium isotope values were significantly different (P 5 0.03730).
When all dental data from the Libyan Fezzan are combined, enamel data from Gobero were significantly different (P 5 0.00130) suggesting somewhat non-overlapping
patterns of movement.
That radiogenic strontium isotope values from Gobero
were higher and generally less variable than those from
Libyan Sahara sites suggests two things about the
hunter-gatherers buried at Gobero. First, people living
at Gobero appear to have been less mobile than their
contemporaries in southwestern Libya—the Late-Acacus
hunter-gatherers. Settlement patterning for the Late
Acacus focused on upland environments in which aridity
favored a broad-based but sedentary approach to subsistence with an emphasis on hunting and tending Barbary
sheep (Cremaschi and di Lernia, 1996; Garcea, 2004).
Fish was not a major component of the Late Acacus diet.
Given the relatively similar underlying bedrock geology
in both places (and if anything Libya is more variable,
see Fig. 1), decreased variability suggests some degree of
sedentism for the foragers at Gobero. Second, both the
Libyan sites and Gobero are located on sandstone formations; the former are Paleozoic-Mesozoic, the latter Mesozoic-Cenozoic in age. The older sediments should provide higher strontium isotope values; however, this was
not the case. Significantly higher strontium isotope values at Gobero are consistent with input from Pre-Cambrian sediments from the Aı̈r Massif (see Fig. 1), which
could reflect actual movement into the Aı̈r or a greater
contribution of weathered sediments from the Aı̈r to the
Chad Formation. Both factors likely contribute to the
elevated strontium isotope ratios. We do note, however,
at least one contemporaneous site in the Aı̈r Massif
(Tagalagal) with a similar material culture inventory to
Gobero (Roset, 1983, 1987). Interestingly, Tagalagal is a
high elevation (1,850 masl), open air rock shelter not
associated with a paleolake, with no harpoons, poor
quality lithic resources and no locally available clay for
pottery (Roset, 1983, 1987; Échallier and Roset, 1986). If
a lowland-upland pattern of mobility was practiced then
one wonders what attracted people away from the
Gobero paleolake into the highlands of the Aı̈r Massif.
Human occupation of central Niger is also attested by
several sites near Adrar Bous, Temet near Mt. Gréboun
in the Aı̈r, and a dune site similar to Gobero further
north at Tin Ouaffadene (Roset, 1983, 1987). Therefore,
it is reasonable to infer mobility into these upland environments, and as Roset (1987) noted the northwestern
portion of Niger near the Aı̈r Massif is littered with surface scatters of similarly aged sites, suggesting a dense
network of communities had once existed in what is now
a ‘‘desert within a desert.’’
Finally, the significant difference between the data from
central Niger and Libya speaks to archaeological models
of settlement structure and the spread of ceramic technologies throughout the Sahara. Radiogenic strontium isotope data suggest the presence of relatively discrete
groups of people locally adapted to specific environmental
resources that ranged within a relatively well-defined territory. Échallier and Roset’s (1986) ceramic petrographic
work is particularly relevant here given their findings of
clay sourcing within 75 km at Tagalagal and 15 to 20 km
at Adrar Bous; the latter they inferred was a long term if
not permanently occupied site. In Libya subsistence
resources involved Barbary sheep and collected grasses
while in Niger there was heavy emphasis on aquatic and
terrestrial fishing and hunting. Therefore, people returned
to the Sahara during the Early Holocene humid phase
and quickly adapted to local conditions in ways that
allowed for mobility within these niches, but not a wide
ranging settlement structure. Broad similarities in material culture over wide areas resulted from inter-group contact and the exchange or copying of ideas and technology
at the periphery rather than highly mobile subsistence
adaptations. These would come later under arid conditions with the advent of pastoralism.
The authors thank members of the Gobero excavations
and subsequent analyses for invaluable collaborations.
In the Archaeological Chemistry Laboratory, Katie Miller
and Meridith Masoner performed invaluable laboratory
assistance in sample preparation. In the W.M. Keck
Foundation Laboratory for Environmental Biogeochemistry, the authors are extremely grateful for the expertise
of Dr. Gwyneth Gordon and laboratory access granted by
Dr. Ariel Anbar. Charisse Carver provided valuable assistance with the construction of the figures. Preliminary
interpretations of some of these data were presented at
the 19th Biennial Meeting of the Society of Africanist
Archaeologists in Frankfurt, Germany on September 8–
11 2008 in a presentation entitled, ‘‘Recent Bioarchaeological and Biogeochemical Research at Gobero: Paleodiet and Residential Mobility in the Early and Middle
Holocene’’ by Kelly J. Knudson and Christopher M. Stojanowski; the authors thank the audience members, particularly Dr. Judith Sealy, for valuable input. The
authors are also grateful to two anonymous reviewers
for their helpful comments.
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