Bioavailable Strontium in Northern Europe
Denmark and Southern Sweden Baseline
Denmark is characterized by a relatively young and rather homogenous
“basement” geology. About 50% of the country is constructed of Late
Cretaceous-Early Tertiary carbonate platforms, the other 50% by marine clastic
sediments, all covered by more or less thick sequences of diverse glaciogenic
sediments deposited during the two last Ice Ages. The Quaternary glaciogenic
sediments are composed, among other things, of various weathered
Precambrian granitoids (gneiss and granite) from Norway and Sweden. Almost
everywhere in Denmark, these glacial deposits are the source of strontium
isotopes for plants, animals, and people. There is very little bedrock exposure
anywhere in the country with the exception of the island of Bornholm in the
middle of the southern Baltic Sea.
We have measured a variety of materials to determine bioavailable levels of
87Sr/86Sr
from throughout Denmark, including small rodents in modern owl
pellets, modern snails, modern sheep’s’ wool, archaeological fauna, and
prehistoric human remains (Frei and Price 2012). The results of our study
indicated that there are two major terrestrial sources in Denmark. Two different
lobes of ice covered most of Denmark in the last glaciation. The island of
Zealand and the eastern quarter of the island of Funen were under a lobe from
the east, bringing rock and sediment from Sweden and the Baltic Basin into
Denmark. Western Funen and Jutland were covered by another lobe coming
from the north, bringing sediments from Norway and the North Sea Basin to the
country.
Our baseline measurements (Fig. S1) show slightly higher 87Sr/86Sr values on
Zealand and eastern Funen and we have suggested that a range from 0.7072 to
0.7119 likely defines the local bioavailable values for this area (Frei and Price
2012). For the rest of Funen and Jutland, values were slightly lower and a range
of 0.7081 to 0.7103 should define the bioavailable
87Sr/86Sr
in this part of
Denmark. Although these sources differ somewhat, they also overlap significantly
and cannot be used to ascertain mobility between the two regions of the country,
except perhaps in the most extreme cases. For Denmark as a whole, any human
87Sr/86Sr
values above 0.7119 are almost certainly non-local.
Fig. S1. Bioavailable strontium isotope ratios in Denmark and southern Sweden.
Both Denmark and southwestern Scania are located in a zone that is
characterized by soft sedimentary substratum and thick glacial deposits. The
geology of the southwestern Swedish province of Scania has three major
components. The region was subjected to multiple glaciations by ice lobes
coming from the north, northeast, east, southeast, and south. The boundary
between the most recent tills from the Northeastern and the Baltic lobes can be
identified across Scania (Lagerlund 1987). These glacial tills covering the surface
of much of southwestern Scania should have 87Sr/86Sr values different from the
underlying bedrock depending on the origins of the sedimentary load in the
glacial lobes.
Scania forms part of the boundary between the two major classes of the bedrock
of Europe. To the north and east lies an ancient craton, the Fennoscandian
Shield in Sweden, Norway, and Finland. To the south and west in the rest of
Europe there is a mobile belt of crustal blocks. The border zone between these
two regions is marked by the Tornquist Line, which extends 2000 km from the
North Sea to the Black Sea.
In Scania, this ca. 50 km wide zone runs diagonally through the middle of the
province on a NW-SE line from Helsingborg to Ystad and forms a typical horst
and graben landscape formed primarily during the Mesozoic (Graversen 2009).
To the north and east of the Tornquist Line lie the Proterozoic (2,500 to 542
million years ago) granites and gneisses of the Fennoscandian Shield. These
very old rocks generally have very high
87Sr/86Sr
values typical of a cratonic
landscape. To the south and west of the Tornquist Line are the phanerozoic (the
last 542 million years of earth’s history) sediments, largely of Mesozoic age. This
area contains the only Jurassic sedimentary deposits in Sweden. The bedrock in
this area also includes substantial marine deposits of Late Mesozoic Senonian
and Early Cenozoic Danien age.
Strontium isotope ratios in bedrock in this zone should largely follow the known
curve for seawater over time with values ranging between ca. 0.707 and 0.7085
(Vezier 1989). In addition to geological sources, the sea influences strontium
isotope ratios in Denmark and southern Sweden. Rainwater and sea spray with a
87Sr/86Sr
value of 0.7092, a constant for seawater, affect coastal areas.
Measurement of marine aerosols in southern Sweden showed transport across
almost the entire region, a distance of some 300 km, with a decrease in
concentration from west to east (Gustafsson and Franzén 2000).
There are several sources of information on 87Sr/86Sr levels in Sweden, including
several studies of strontium isotope ratios from archaeological sites in Sweden.
Sjögren et al. (2009) analyzed 87Sr/86Sr in human remains from megalithic burials
in the Falbygden region of western Sweden along with numerous bioavailable
samples from the surrounding region. Frei et al. (2009) recorded strontium
isotope ratios in sheep wool and soil leachates from several areas in Sweden
and Denmark. Arcini and Price (in prep.) have measured 87Sr/86Sr in human teeth
from the Viking period and various biological materials from southern Sweden.
In addition, we have measured subfossil reindeer teeth from southern Sweden as
part of this project (Table S1). Two teeth from St. Slägarp and Hässleberga in the
province of Scania produced mean values of 0.7158 and 0.7116. Seasonal data
from the antler at Hässleberga suggest that at least some reindeer were resident
throughout the year (Larsson 1991, 1996, Larsson et al. 2002). Finds from
Denmark (Aaris-Sørensen et al. 2007) also indicate presence of reindeer in
South Scandinavia year round except during Younger Dryas when reindeer were
absent in the winter.
Table S1. Strontium isotope ratios for reindeer samples from Sweden. The values from St.
Slagarp and Hässleberga are an average of several samples from individual teeth (see Table 4).
A series of antler, bone, and enamel samples from a larger region to the north
around Gothenberg resulted in a mix of
87Sr/86Sr
values ranging from 0.7018 to
0.7098. The two higher values above 0.712 almost certainly reflect local
strontium isotope ratios in the older rock landscape of the region and perhaps are
due to diagenesis. The three values below 0.711, however, are clearly non-local
and very likely represent animals whose teeth formed in southwestern-most
Sweden, Denmark, or the North European Plain.
Baseline data for southern Sweden are also shown in Fig. S1. A general pattern
of lower values in southwestern Scania, similar to the landscape of neighboring
Denmark, has been observed. Since the southwestern corner of Scania is a
geological context of glacial moraine and outwash deposits very similar to
neighboring Denmark, 87Sr/86Sr values around 0.709 to 0.710 also characterize
this area. As one moves the north and east 87Sr/86Sr values become higher as
expected in a landscape of older rock and related glacial till. Values above 0.711
are common and even higher values are observed further to the north.
Northern and Central Germany
In order to understand strontium isotope variation in the potential geographic
range of movement of the reindeer from Stellmoor, the geology of northern and
Central Germany is of immediate interest. In general, the two areas contrast
sharply in terms of topography and geology. Northern Germany is a plain; much
of Central Germany is uplands. The surficial geology of northern Germany is
largely Pleistocene glacial and periglacial deposits of moraine, sands, and loess.
The geology of Central Germany is a complex mix of older and younger
sedimentary, igneous, and metamorphic rocks dating from the Tertiary to the
Mesozoic.
Northern Germany is a relatively well-defined flatland, often referred to as the
North German Plain or North European Plain. There are three major surface
deposits in northern Germany. The ground moraine and glacial deposits of the
last glaciation cover the northernmost parts of Germany and much of southern
Scandinavia. The moraine landscape is a mixture of rocks and sediments carried
south by glacial advance and left on the surface of the region when the ice
melted. The coversand region, stretching from the Netherlands across northern
Germany to Poland, is dominated by aeolian sands consisting largely of
reworked fluvial and glaciofluvial sediments. These Late Glacial coversands rest
on top of some of the glaciogenic materials deposited in this region during the
Saallian glaciation and lie between the Weichselian moraine deposits to the north
and finer wind-blown sediments known as loess to the south.
Loess and loess-related sediments cover wide parts of northern and Central
Germany in an almost continuous belt, trending roughly ESE-WNW (Wagner
2011). The southern boundary of the loess belt runs from south of the
coversands and drift sands to the northern edges of the Rhenish Massif, the Harz
Mountains, and the Ore Mountains (Eissmann 2002, Haase et al. 2007). This
band of loess, deposited against the Central German uplands, is 6 to 30 km wide
across this area, with depths of 2 or more meters, overlying older glacial
deposits.
Central Germany is primarily a region of uplands, defined arbitrarily in this study
as the area between the North German Plain and an east-west line through the
city of Frankfurt. Much of this region is designated as die Mittelgebirge, the
Central Uplands, with an eastern and western branch. The western Mittelgebirge
includes the Rhenish Massif, the Westphalian Lowlands, the Thuringian Basin,
the Vogelsberg Mountains, and the Harz Mountains. The region is covered by
extensive Pleistocene and Holocene sediments. The southern boundaries of the
Elsterian and Saalian glaciation cross through this region and separate the
periglacial Pleistocene sediments in the south from glacial, glaciofluvial,
periglacial, and fluvial deposits to the north. Quaternary sediments are mainly
accumulated in the valleys, on slopes, and in upland or marginal basins. The
underlying Tertiary to Mesozoic sedimentary rocks are exposed only in a few
ridges or hills (Wagner 2011). In the south of the Mittelgebirge region, Triassic
rocks outcrop, while in the northern section the bedrock geology is mainly
Jurassic to Cretaceous. The Rhenish Massif consists of metamorphic rocks,
mostly slates originating in sediments deposited during the Devonian and
Carboniferous periods. Tertiary and Quaternary igneous rocks are also found in
this area and occur most prominently in the Eifel, the Westerwald, and the
Vogelsberg, as part of the European Cenozoic Volcanic Province. Associated
basins include the Lower Rhine Graben, the Hessian Graben, and the Eger (Ore)
Graben, often completely covered with deep deposits of loess.
The Harz Mountains in the northeast corner of the Western Mittelgebirge are the
highest mountain range extending across Lower Saxony, Saxony-Anhalt and
Thuringia. The region is geologically diverse and the most common rocks lying
on the surface are argillaceous shales, slate greywackes and granite intrusions.
There are also important deposits of limestone and gabbro. The eastern
Mittelgebirge in Central Germany is dominated by the Thüringisch-Fränkisches
Mittelgebirge of porphyritic lava and the Erzgebirge range, a fault block mountain
range of crystalline slate and gneiss.
In terms of 87Sr/86Sr, the geology of Northern and Central Germany contrasts
sharply. The north is predictably more homogeneous due to large areas of glacial
and periglacial sediments. Measurements of surface water (Voerkelius et al.
2010) in the North German Plain normally exhibit ratios between 0.709 - 0.710.
Strontium isotope ratios in the loess have been measured in various parts of
Germany and are consistently between 0.7085–0.710 (Price et al. 2004). In
Central Germany, strontium isotope ratios of volcanic features range from 0.7030.705 (Lustrino and Wilson 2007, Stosch and Lugmair 1986), while older granites
and bedrock generally have values greater than 0.711. In general, as is the case
everywhere with strontium isotope ratios, older rocks have higher ratios while
younger rocks and sediments generally exhibit lower values.
Fortunately there have been a number of strontium isotope studies providing
baseline information in Germany to date, albeit the majority in the south of the
country (e.g., Grupe et al. 2003, Knipper 2011, Price et al. 1994a, 2001,
Schweissing and Grupe 2003). There are some published data from the north
(Grupe et al. 2009, 2010, Gillmaier et al. 2009, von Carnap-Bornheim et al. 2007,
De Jong et al. 2010). This information is summarized in Table 2 and has been
combined with our own analyses of a number of bioavailable samples from
northern and Central Germany on a map (Fig. S2).
Fig. S2. Bioavailable 87Sr/ 86Sr values in Central and Northern Germany.
There is also some information on bioavailable
87Sr/86Sr
from Central Germany.
Maurer et al. (2012) recently published a detailed study of bioavailable strontium
isotopes in the Thuringian Basin and the Thüringisch-Fränkisches Mittelgebirge
in East-Central Germany. The study area is a complex geological mix of
Buntsandstein, Keuper, Muschelkalk, small deposits of Oligocene sand and clay,
with much of the area overlain by Pleistocene loess deposits. The highest values,
0.7090 to 0.7110 are found on the Buntsandstein and the lowest values come
from the Muschelkalk and Holocene alluvium that fall in the range from 0.70800.7090. The loess sediments around the two archaeological cemeteries included
in this study had values ca. 0.7090-0.7100.
The Linearbandkeramik settlement of Nieder-Mörlen is located in the southern
part of Central Germany just north of Frankfurt and lies adjacent to the Taunus
Mountains and the Vogelsberg to the northeast (Nehlich et al. 2009). High
strontium ratios would be expected from The Taunus area while the basaltic
rocks of the Vogelsberg Mountains should provide lower ratios. The immediate
location of the site is in a basin filled with loess that has a strontium isotope ratio
of 0.708–0.710. 87Sr/ 86Sr values from a series of human teeth at Nieder-Mörlen,
however, average 0.712.
Kronseder (2008) has analyzed a series of baseline samples from archaeological
fauna from sites largely in Westphalia and provided bioavailable 87Sr/ 86Sr values.
These data are summarized as averages and plotted on the baseline map for
Central and Northern Germany (Fig. 6). 87Sr/ 86Sr values in Westphalia reported
in this study generally lie between 0.708 and 0.709. Brettell et al. (2012) report
strontium isotope ratios (mean = 0.7083 ± 0.0007) for human remains from a
medieval cemetery at Hannover-Anderten.
In addition to these examples we have measured a series of samples from the
major geological areas of Northern and Central Germany. Samples are normally
from faunal remains from archaeological sites in the region. These data are
depicted in Fig. S2 and provided in Table S2. In Northern Germany, many of the
values we observed fall in the range of 0.709 – 0.710 as expected. At the same
time, there are more than a few anomalous values >0.711 found in the region.
These higher values were a surprise and their source remains uncertain in our
analysis, perhaps coming from the older Saalian glacial deposits. 87Sr/ 86Sr
values in Central Germany also show considerable variation with values <0.709
to the east in Westphalia, ca. 0.709 in the area west of the Harz Mountains, and
clearly higher in the Erzgebirge region, including the highest values recorded in
this study.
Figures
Fig. S1. Bioavailable strontium isotope ratios in Denmark and southern Sweden.
Fig. S2. Bioavailable 87Sr/ 86Sr values in Central and Northern Germany.
Tables
Table S1. Strontium isotope ratios for reindeer samples from Sweden. The
values from St. Slagarp and Hässleberga are an average of several
samples from individual teeth (see Table 4).
Table S2. Bioavailable strontium isotope values from central and northern
Germany.