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Journal of Archaeological Science 41 (2014) 322e332
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Journal of Archaeological Science
journal homepage: http://www.elsevier.com/locate/jas
Inferring animal husbandry strategies in coastal zones through stable
isotope analysis: new evidence from the Flemish coastal plain
(Belgium, 1ste15th century AD)
Gundula Müldner a, *, Kate Britton b, c, Anton Ervynck d
a
Department of Archaeology, University of Reading, Whiteknights, PO Box 227, Reading RG6 6AB, UK
Department of Archaeology, University of Aberdeen, St. Mary’s Building, Elphinstone Road, Aberdeen AB24 3UF, Scotland, UK
Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany
d
Flanders Heritage Agency, Koning Albert II laan 19, Box 5, B-1210 Brussels, Belgium
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 May 2013
Received in revised form
8 August 2013
Accepted 12 August 2013
In a proof-of-concept study, Britton et al. (2008) demonstrated that the isotopic composition of halophytic plants can be traced in the skeletal tissues of their animal consumers. Here we apply the method
to domestic herbivore remains (n ¼ 303) from nine archaeological sites in or near the Flemish coastal
plain (Belgium), where, prior to embankments, salt-marshes offered extensive pasture grounds for domestic herbivores. The sites span a period of w1500 years (Roman to late medieval period), during which
the coastal landscape was progressively transformed from little managed wetlands to a fully embanked
polder area. The bulk collagen data show variations between sites and over time, which are consistent
with this historical framework and are interpreted as reflecting environmental change and differences in
animal management in the coastal plain throughout the late Holocene. The study demonstrates the
immense value of faunal stable isotope analysis for characterising coastal husbandry strategies beyond
the means of traditional zooarchaeological techniques.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Isotope analysis
Carbon
Nitrogen
Animal bone
Collagen
Husbandry
Coastal environments
Roman
Medieval
1. Introduction
Before large-scale land reclamation and embankment schemes
changed the nature of its coastlines, north-west Europe possessed
vast coastal landscapes of mudflats (intertidal grounds) and saltmarshes (diverse areas of mainly herbaceous, halophytic vegetation on higher ground but still occasionally submerged by
seawater). These environments were extensively utilized by
humans for hunting, fishing, pasture or the manufacture of salt
since prehistoric times (Adam, 1990; Allen, 2000). Charting the
history of these rich natural habitats, i.e. their exploitation and
active modification by humans, is an active and important area of
research into subsistence, sustainability and the mutual dependence of humans and their natural environment (e.g. Rippon, 1997;
Ervynck et al., 1999; Bell, 2000).
* Corresponding author. Tel.: þ44 (0)118 378 7389.
E-mail addresses: [email protected] (G. Müldner), k.britton@
abdn.ac.uk (K. Britton), [email protected] (A. Ervynck).
0305-4403/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jas.2013.08.010
In a proof-of-concept study, Britton et al. (2008) demonstrated
that the isotopic composition of halophytic (salt-tolerating) plants,
which tend to be 15N-enriched compared to other terrestrial flora in
temperate environments, can be traced in the skeletal tissues of
their consumers. Stable isotope analysis of herbivore bone and
dentinal collagen therefore allows for the identification of saltmarsh grazing and has the potential to illuminate past animal
husbandry in coastal zones beyond the means of traditional
zooarchaeology (see also Prummel and van Gent, 2010; McManus
et al., 2013). In this paper, we aim to test this approach further by
exploring whether carbon and nitrogen stable isotopes are sufficiently sensitive to characterise different animal management
strategies and to reflect changes in the coastal environment as a
result of natural processes or human intervention. To this end, we
analysed sheep (Ovis aries) and cattle (Bos taurus) remains from a
number of archaeological sites in the Flemish coastal plain
(Belgium). These cover the period of approximately 1500 years
(1ste15th century AD, Roman to late medieval periods) during
which the coastal landscape underwent natural changes and
various episodes of human intervention until it was completely
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
transformed into a fully embanked polder (reclaimed land, protected from inundation by dikes). By the 13th century AD, these
events had almost entirely obliterated the extensive tracts of saltmarshes which had previously characterised the coastal landscape (see Ervynck et al., 1999; Verhulst, 1995). By examining faunal
isotope data from across the period of transition from coastal
wetland to polder, this paper explores animal husbandry practices
and especially the occurrence of salt-marsh grazing, along with any
potential differences in the management of different domestic
species.
2. Settlement and economy in the Flemish coastal plain
The Flemish coastal plain is part of the low-lying south-eastern
shore of the North Sea. It consists of an almost 70 km long and 10e
15 km wide expanse of mostly Holocene tidal sediments (modern
surface 0e3 m asl), which are limited on one side by the coastal
dune-belt (now continuous but formerly interrupted by tidal inlets)
and on the other by the Flemish central plain, a conglomeration of
Pleistocene sandy soils (Maréchal, 1992; Fig. 1). In the 1st millennium AD, prior to the transformation of the area into a polder, the
coastal region was a dynamic wetland landscape of mudflats, peat
deposits and tidal channels, all of it increasingly colonised by saltmarsh vegetation as the intertidal zone gradually silted up and the
landscape was raised to supra-tidal levels (Ervynck et al., 1999;
Baeteman et al., 1999, 2002; Tys, 2004b: 262). These processes
323
finally led to the development of vegetation types internationally
classified as ‘high salt-marshes’ but locally described as ‘saltmeadows’ (Dutch: zoutweide, French: pré salé, grassland in the high
marshes which is exposed to sea-spray but only flooded at exceptionally high tides and therefore particularly suitable for animal
pasture). Of numerous small streams and canals, only the river IJzer
and the Zwin tidal inlet remain today as permanent water courses
(Fig. 1).
The occupation history of the Flemish coastal plain in the first
millennium AD was long viewed as governed almost entirely by
sea-level changes, which allowed human activity only intermittently (Verhulst, 1980; De Moor and Ozer, 1985). This so-called
‘Dunkirk Transgression Model’ was only revised in the 1990s,
based on new geomorphological research and critical re-analysis of
the archaeological evidence (Ervynck et al., 1999; Baeteman, 1999;
Baeteman et al., 1999, 2002). Recent approaches to settlement
history now put greater emphasis on human agency and changing
economic and demographic conditions rather than focussing solely
on environmental determinants in order to explain variations in the
intensity of human exploitation (Ervynck et al., 1999; Tys, 2004b;
Loveluck and Tys, 2006).
The archaeological record for the Flemish coastal plain in the
Roman period (1ste4th century AD) demonstrates that a range of
activities took place, including fishing, salt production (with associated peat digging), crafts, arable farming, animal husbandry and
trade (Thoen, 1973, 1978, 1987), albeit probably at a smaller scale
Fig. 1. Map of the study area and sites (adapted from Baeteman et al. in Thoen, 1987; extent of dune-belt after www.kustatlas.be). The geography depicted is an ‘averaged view’ of a
coastal landscape evolving dynamically throughout the period from 1 to 1500 AD, presenting only a ‘snap shot’ of changes in course of the IJzer and the gradual silting up of the
Zwin. The numerous tidal gullies traversing the coastal plain before the medieval embankments are not shown.
324
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
than traditionally postulated (see Ervynck et al., 1999). However,
although it was long assumed that the coastal wetlands were
effectively unmanaged prior to the medieval period, recent excavations in Flanders and neighbouring Zeeland (The Netherlands)
have now shown the existence of infrastructural works of Roman
date, in the form of dikes and platform sites, demonstrating active
attempts to control the hydrology of at least parts of the area
(Vanhoutte et al., 2011; Dijkstra and Zuidhoff, 2011; Demey et al.,
2013 and references therein). Despite these finds, there is still little evidence for year-round settlements in the coastal plain and
most activities may have been carried out on a seasonal basis only.
It should be noted, however, that due to changes in sea-level in the
post-Roman period, large parts of the Roman coastline are now
submerged and substantial evidence may have been lost (Denys
and Baeteman, 1995; Baeteman et al., 2011; see Fig. 1).
Towards the end of the Roman period, a lack of maintenance of
the dikes and local lowering of the landscape through peat
extraction probably rendered areas in the coastal plain again
more vulnerable to the sea. Combined with economic instability
and population decline (Jansen, 1981), this perhaps explains why
evidence for occupation relating to the early medieval period
(5the9th century AD) within the coastal plain has long been
scarce (see Ervynck et al., 1999). Recent archaeological and historicalegeographical work, however, has identified a number of
sites that likely go back to at least the 6th and 7th centuries. In
addition to early medieval single farmsteads, which were typically situated on natural ridges of raised ground, the existence of
terps, multiple-household settlements on artificial mounds, has
now been demonstrated (Tys, 2004a; Loveluck and Tys, 2006;
Ervynck et al., 2012). These farms, which were probably permanently occupied and appear as ‘marisci’ in the documentary
sources from the end of the 8th century onwards, were operated
by free peasants, who evidently specialised in sheep husbandry
for the production of wool and textiles (Tys, 2004a; Ervynck et al.,
2012).
The history of the Flemish coastal plain in the high medieval
period (10the12th century AD) is characterised by the establishment of specialised ‘sheep estates’ (terrae ad oves), for which
extensive pasture was actively reclaimed by drainage. Initially,
relatively modest oval enclosures protected the settlements, but
these were followed by more ambitious schemes, consisting of
the construction of long dikes running alongside the tidal channels and, in the 12th century and later, the blockage of the
remaining channels by stone walls, which gradually transformed
the entire coastal plain into polder land (Tys, 2004b; Nicholas,
1992: 97e99).
These measures progressively reduced the influence of the tides
on the landscape. Consequently, the salt-marshes and -meadows,
which provided valuable pasture for sheep husbandry, were in
decline. In the coastal areas north of Brugge (Bruges), on both sides
of the Zwin tidal inlet, this process may have started from as early
as the 11th century, while in the south-western part of the plain,
around the mouth of the IJzer river, substantial embankments were
not built until later. In the latter area, large sheep farms or bercaria
existed into the 12th century. Eventually, however, and from at
least the 12th century onwards, reclaimed land throughout the
coastal plain was increasingly used for cattle husbandry and arable
farming instead of sheep grazing (Verhulst, 1998; Nicholas, 1992:
99e101, 124e126). The economic consequences of the decline of
sheep husbandry in the coastal plain were profound. It has been
suggested that the resulting shortage of raw wool from Flemish
domestic production initiated the expansion of the wool trade with
England, a most profitable venture that dominated the economy of
both countries for the remainder of the Middle Ages (Verhulst,
1998: 38; see Lloyd, 1977).
Drawing on the historical context outlined above, this paper will
principally examine two questions:
1. Are the stable isotope data of herbivore remains from the
Flemish coastal plain consistent with other evidence for the
development and change of the landscape in the first 1500
years AD?
2. Do the isotope data reflect changes in husbandry strategies
over time, which may be related to these known environmental
changes?
3. Isotope analysis and coastal animal husbandry
Carbon and nitrogen stable isotope analysis of skeletal remains
for the reconstruction of diet is based on the well-established
premise that isotopic variation at the base of the food-chain can
be traced in the body tissues, including the skeleton, of consumers
(see review articles Ambrose, 1991; Sealy, 2001). Although the
focus of most applications in archaeology is currently on humans,
analyses of faunal remains have become increasingly important,
providing novel insights into animal ecology, herd management
and the reconstruction of past economies (e.g. Balasse and
Ambrose, 2005; Atahan et al., 2011; Britton et al., 2012;
Makarewicz and Tuross, 2012).
The relative abundances of the stable isotopes of carbon
(13C/12C, or d13C) and nitrogen (15N/14N, or d15N) vary characteristically across different environments and ecological zones, e.g. between plants of different photosynthetic pathways (see Smith and
Epstein, 1971; DeNiro and Epstein, 1978), or between terrestrial and
marine ecosystems (Schoeninger and DeNiro, 1984). Stable isotope
ratios of nitrogen (d15N) also vary systematically within foodwebs,
increasing by w3e5& with each step up the foodchain (Bocherens
and Drucker, 2003). The absolute d15N value of animal tissues can
nevertheless vary at the same trophic level, due to ‘baseline’ variation that can occur across and even within different biomes (e.g.
Ambrose, 1991; Kelly, 2000).
A recent study demonstrated that modern halophytes (both C3
and C4) tend to display significantly higher nitrogen isotope ratios
compared to freshwater and terrestrial plant species (Cloern et al.,
2002). Elevated d15N values have also been observed in other
coastal vegetation and plants from salt-pans, as well as those
growing on saline and coastal soils (Virginia and Delwiche, 1982;
Heaton, 1987; van Groeningen and van Kessel, 2002). There is
therefore an expected relationship between coastal location and/or
soil salinity and raised d15N in local flora and consequently also
herbivorous fauna. The reasons for the observed 15N-enrichment
are not fully understood, but the phenomenon must likely be
attributed to the input of marine nitrogen by flooding or sea-spray
(Virginia and Delwiche, 1982), as well as the effects of salinity and
pH on nitrogen transformations in the soil (Heaton, 1987; Ambrose,
1991; van Groeningen and van Kessel, 2002). Rainfall should dilute
salinity in the soils, and one would expect its impact on d15N to be
significantly dampened in high precipitation areas (e.g. Mulville
et al., 2009). Salinity also reduces discrimination against 13C during C3 plant photosynthesis, which can lead to substantial 13Cenrichment in salt-stressed plants and halophytes (Guy et al.,
1986a, 1986b; van Groeningen and van Kessel, 2002). However,
while elevated d15N values have been clearly observed in saltmarsh grazed fauna, variations in d13C (compared to fully-inland
C3 grazers) are perhaps more subtle (see Britton et al., 2008;
Britton and Müldner, 2013).
In addition to the ‘baseline’ and ecosystem-specific variations
explored above, other factors can influence the stable isotope
values of domestic herbivore tissues, such as dietary changes
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
through foddering or movement between different seasonal pasturelands (e.g. Balasse and Ambrose, 2005; Balasse et al., 2006).
Unlike dentinal collagen, which forms incrementally and gives information on the periodicity of certain aspects of herd management
(e.g. seasonal foddering, see Balasse et al., 2006), bone collagen
remodels slowly during life, and provides an indication of averaged
bulk diet over a number of years prior to death.
4. Materials and methods
In order to explore changes in livestock management in the
Flemish coastal plain across its transformation from coastal
wetland to polder, we selected herbivore (sheep and cattle) bones
from seven sites e and used published data from an eighth e situated in or on the edge of the coastal plain and dating from the
Roman period (1ste2nd century AD) to the end of the Middle Ages
(15th century AD) (see Fig. 1). For comparison, material was also
collected from the inland town of Ieper (Ypres). The nine sites used
in the analysis can shortly be described as follows:
(1) Stene: a Roman (late 1st to early 2nd century AD) ‘platform site’
connected to a dike in the coastal plain. The zooarchaeological
assemblage is completely dominated by sheep. It remains uncertain whether the site was occupied on a permanent or
seasonal basis (Demey et al., 2013).
(2) Oudenburg: a Roman military fort founded wAD 200 as part of
the ‘Saxon Shore’ defence and occupied until the start of the
5th century AD. The fort was situated on a sandy ridge protruding into the coastal plain. Samples were selected from a
context dated to the last quarter of the 4th century (Vanhoutte
et al., 2009).
(3) Leffinge: a terp (settlement built on an artificial mound) in the
coastal plain, yielding animal remains from the middle of the
7th to the beginning of the 11th century AD. A small (n ¼ 8) set
of isotope data was previously published (Ervynck et al., 2012)
and is used here.
(4) Uitkerke: a rural settlement in the coastal plain. Samples were
taken from the earliest occupation phase, dating to the late
8the10th century AD (Pype, 2002).
(5) Brugge (Bruges): town situated inland, on Pleistocene sand, but
only a few kilometres from the border to the coastal plain.
Samples were selected from consumption refuse dating to the
9the10th century AD, excavated at the ‘Brugse Burg’, the
stronghold of the Counts of Flanders at Brugge (De Witte, 1991;
Ervynck, 1991).
(6) Koksijde: samples were selected from contexts associated with
two high medieval (10the12th century) farmsteads. The remains of an old dike, possibly erected in the 11the12th century,
are still visible close to the site (Lehouck et al., 2011).
325
(7) Veurne: a fortification of the Counts of Flanders in the western
coastal plain, which evolved from a large circular fort, probably
erected in the 9th century, to a motte-and-bailey castle and
associated settlement. Bones were sampled from three occupation phases, dating from the 10the11th (Phase 1), 11the12th
(Phase 2), and the 13th century AD (Phase 4) (De
Meulemeester, 1982; Van Doorslaer, 1985; Maenhaut van
Lemberge, 1985).
(8) Ieper (Ypres): principal centre of the medieval Flemish cloth
industry. While the town is located in inland Flanders,
approximately 20 km from the inland border of the coastal
plain, it was supplied with wool, possibly also sheep, from the
coastal plain (Verhulst, 1998). Samples were selected from
excavations in the former St. Michael’s quarter and date to the
late 13th and first half of the 14th century AD (Dewilde and Van
Bellingen, 1998; Ervynck, 1998).
(9) Raversijde: a late medieval fishermen’s village in the central
part of the Flemish coastal area, which was established in the
early 15th century, directly behind the dune belt and parallel
dike. Samples were selected from 15th century contexts
(Pieters et al., 2013).
While the focus of this study is on sheep, as these are thought to
have been preferentially grazed in the marsh areas, a smaller
number of cattle samples were also processed, in order to explore
possible differences between the species. Sampling procedures and
analytical techniques are described in the Online Supplementary
Material (S.1).
5. Results
Based on established quality indicators (DeNiro, 1985; Ambrose,
1990; van Klinken, 1999), four samples from Stene and seven from
Ieper yielded too little, or not sufficiently well preserved collagen
for valid isotopic determinations (Table S.1). This leaves a data-set
of 223 sheep and 80 cattle samples, including eight samples from
Leffinge previously published by Ervynck et al. (2012) and nine
additional cattle samples from Raversijde and Veurne published by
Ervynck et al. (2007). These latter samples fit in well with our data
and demonstrate that comparisons with previously published data
from Flanders are valid. Summary statistics for the nine sites and
the combined data-set are provided in Tables 1e2, individual data
in Table S.1. Individual data for the nine sites are plotted in Figs. 2e
10 and average values in Figs. 11e12.
Statistical tests (see Table S.2 for details and results) demonstrate that the d15N values of the sheep were significantly higher
than those of the cattle at seven of the sites. Differences at the
remaining two sites, Stene and Leffinge, could not be tested as no/
only one cattle were available. However, based on visual inspection,
Table 1
Descriptive statistics for sheep from the nine sites.
Site
Species
n
Mean d13C (&)
1 s.d.
Mean d15N (&)
1 s.d.
Min d13C
Max d13C
Min d15N
Max d15N
All
Stene
Oudenburg
Leffinge
Uitkerke
Brugge
Koksijde
Veurne (all)
Phase 1
Phase 2
Phase 4
Ieper
Raversijde
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
Sheep
223
15
17
7
38
18
22
58
20
19
19
23
25
20.9
21.3
21.5
19.9
20.3
21.0
20.4
20.7
20.8
20.9
20.5
22.0
21.8
1.0
0.9
1.0
1.0
1.1
0.7
0.6
0.8
0.8
0.9
0.7
0.4
0.5
9.6
8.8
8.2
10.6
9.9
8.7
10.0
10.5
10.6
10.1
10.7
8.8
8.8
1.4
1.2
2.2
1.0
0.9
1.3
0.9
1.0
1.1
0.9
0.8
1.3
1.1
23.1
22.4
23.0
21.2
22.1
22.3
21.6
22.5
22.5
22.2
21.7
22.8
23.1
18.2
19.0
18.6
18.2
18.3
20.0
19.1
19.1
19.1
19.4
19.1
21.4
20.4
4.1
5.8
4.1
8.6
8.1
6.0
8.0
8.2
8.5
8.2
8.7
7.0
7.3
12.6
10.8
11.6
11.7
11.9
11.1
12.1
12.6
12.6
11.0
11.7
11.5
11.3
326
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
Table 2
Descriptive statistics for cattle from the nine sites.
Site
Species
n
Mean d13C (&)
1 s.d.
Mean d15N (&)
1 s.d.
Min d13C
Max d13C
Min d15N
Max d15N
All
Oudenburg
Leffinge
Uitkerke
Brugge
Koksijde
Veurne (all)
Phase 1
Phase 2
Phase 4
Ieper
Raversijde
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
Cattle
80
8
1
11
8
9
20
7
8
5
9
14
21.5
21.8
21.4
20.8
21.5
21.3
21.6
21.6
21.8
21.2
22
21.8
0.6
0.6
e
0.6
0.7
0.3
0.4
0.3
0.2
0.4
0.5
0.3
7.4
5.4
6.4
8.1
6.9
7.6
8.1
7.9
8.0
8.6
6.9
7.3
1.4
1.6
e
1.2
1.4
0.6
1.6
0.9
1.5
2.7
0.6
0.7
23.0
23.0
e
21.9
22.7
21.8
22.3
21.0
22.3
21.7
22.3
22.6
20.0
21.0
e
20.9
20.7
20.9
20.7
22.0
21.0
20.7
20.5
21.3
2.6
2.6
e
5.0
4.3
6.7
5.9
6.7
5.9
6.3
5.5
6.0
13.0
6.9
e
9.5
9.3
8.4
13.0
9.2
9.7
13.0
7.8
8.4
similar differences between the species existed at Leffinge, too (see
Fig. 4). At several sites d13C also varied significantly between species
although not consistently in one direction: at Veurne, Koksijde and
(by visual assessment only) Leffinge, sheep d13C were higher than
those of cattle, while at Ieper they were more negative and at
Uitkerke they were bimodally distributed either side of the cattle
data (Table S.2; Figs. 4e5 and 7e9). Site-by-site comparisons of
sheep and cattle separately demonstrated a number of statistically
significant differences between sites (Tables S.3e4, results discussed below). Furthermore, comparisons of animals from different
chronological phases at Veurne revealed a small but significant
shift towards higher d13C in cattle from Phase 2 (11the12th century) to Phase 4 (13th century) (Table S.5).
6. Discussion
6.1. Baselines
The proof-of-concept study (Britton et al., 2008) demonstrated
that sheep and cattle from Bronze Age sites in the Severn Estuary
(U.K.), which (by evidence of fossil footprints and coprolites) appear
to have been grazed in the local salt-marshes on at least a seasonal
basis, had significantly elevated nitrogen isotope ratios compared
to domestic herbivores from other (non-salt-marsh) British sites.
These results were consistent with the established 15N-enrichment
of plants growing in coastal and saline soils (see above, Section 3). A
trend towards higher d13C in the salt-marsh animals compared to
the other herbivores was also observed, although the differences
were thought too small at the time to be interpreted with confidence (Britton et al., 2008; see Britton and Müldner, 2013).
Fig. 2. Individual stable isotope data for sheep and cattle from the Roman platform site
at Stene (1st to early 2nd century AD) compared to mean (1 s.d.) values for Flemish
inland animals.
In order to assess whether herbivores from the Flemish coast
show similar enrichments, it is necessary to establish ‘baseline
values’ for animals that were raised inland. Unfortunately, isotope
data from Flanders are (compared to Britain) relatively scarce and
only 23 faunal samples, mostly cattle, from three inland sites, the
castles of Ename (w1000 AD) and Londerzeel (13the14th century
AD, Ervynck et al., 2007) and the town of Aalst (16the18thcentury
AD, Quintelier et al., 2013), are currently available (see Fig. S.1).
Using these assemblages to characterise inland environments is not
unproblematic, because of their small size and since it is possible
that the sites were supplied with non-local animals. The considerable data variability at some of the sites would indeed be
consistent with this. The site averages in Figs. 2e12 should therefore be taken as a rough indication of inland values rather than as a
robust baseline. Mean isotope values (2 s.d.) of the 18 cattle
(sheep from Aalst were excluded because of their high variability)
are 22.2 1.2& (d13C) and 6.9 2.0& (d15N). Based on these data
and given the location and context of the sites, we propose that an
increasing coastal component in the diet is the most likely explanation for herbivores with d15N of w9.0& and greater (i.e. outside
the 2 s.d. range of the inland cattle). It should be noted that this
threshold of 9& may be a generous estimate, at least compared to
data from Britain (see Fig. S.1). Even among the salt-marsh grazed
animals from the Severn Estuary, only two out of 21 samples have
d15N > 9.0& (Britton et al., 2008).
6.2. Isotope variation between sheep and cattle
At eight of the nine Flemish sites (at Stene no cattle were
available for testing), sheep are 15N-enriched over cattle, and only
Fig. 3. Individual stable isotope data for sheep and cattle from the late Roman military
fort at Oudenburg (late 4th century AD) compared to mean (1 s.d.) values for Flemish
inland animals.
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
327
Fig. 4. Individual stable isotope data for sheep and cattle from the early to high medieval terp settlement at Leffinge (7the11th century AD) compared to mean (1 s.d.)
values for Flemish inland animals (Data: Ervynck et al., 2012).
Fig. 6. Individual stable isotope data for sheep and cattle from the early medieval
castle at Brugge (9the10th century AD) compared to mean (1 s.d.) values for Flemish
inland animals.
10 cattle (13%) in the study have d15N 9.0&, compared to 160
(72%) of the sheep (see Results, Figs. 3e10). Both species are
ruminant grazers (‘bulk and roughage eaters’) and there is no
reason to assume systematic isotopic variation between them on
grounds of their digestive anatomy (Hofmann, 1989; van Soest,
1994). While the two still differ in their grazing behaviour, as
sheep more readily include non-herbaceous plants into their diet
(Hodgson et al., 1991; Dumont, 1997), this rarely results in
discernible isotopic differences between the species in temperate
C3 environments, and we know of no case where these are as
clearly expressed as in the Flemish data-set. The data therefore
almost certainly reflect deliberate differences in livestock management rather than ecological niche separation. This pattern is
indeed consistent with later historical sources which indicate that
the coastal marshes and salt-meadows were primarily used for
grazing sheep, while cattle were preferentially kept on fresh (or at
least less saline) pastures inland (Lindemans, 1952).
At a number of the sites (Koksijde, Leffinge, Veurne and Uitkerke), the differences in d15N are associated with significant 13Cenrichment of the sheep over the cattle (Figs. 4e5 and 7e8;
Table S.2). The magnitude of enrichment observed in some of the
animals is remarkable, at least for herbivores from Northwest
Europe: 46 (20%) of the sheep have d13C 20.0& (maximum
value 18.2&), when in the substantial herbivore data-set
available from Britain, d13C of sheep or cattle rarely even
exceeds 20.5& (see captions Fig. S.1 for references). It is well
established that salinity reduces discrimination against 13C during
C3 plant photosynthesis, leading to an elevation in d13C of sometimes several per mil in the tissues of salt-stressed plants and
halophytes (Guy et al., 1986a, 1986b; van Groeningen and van
Kessel, 2002). Depending on local conditions, a ‘salinity effect’
alone could therefore account for the raised d13C in the Flemish
sheep. Nevertheless, the contribution of other, 13C-enriched fodder
to the diet cannot be excluded. Another dietary end-member with
significantly higher d13C than the bulk of the diet, which was only
consumed by some of the animals, would indeed be the simplest
explanation for the bimodal distribution of the Uitkerke sheep,
which are grouped either side of the cattle (Fig. 5).
Of possible high d13C food sources, a contribution from millet
(Panicum miliaceum), the only C4 cultigen present in the Low
Countries in antiquity (Bakels, 1991), is least likely, as it does not
grow in saline soils (van Zeist et al., 1976) and seems to disappear
from Flanders in medieval times (see its absence from the overview
given by Lindemans, 1952). Millet has never been found in
meaningful quantities in archaeological contexts in Flanders and
would therefore not have been easily available in the sites studied.
While the sandy Flemish coast also does not support the growth
of seaweed (another 13C-enriched feed), floating seaweed (see
Fig. 5. Individual stable isotope data for sheep and cattle from the early medieval
settlement at Uitkerke (8the10th century AD) compared to mean (1 s.d.) values for
Flemish inland animals.
Fig. 7. Individual stable isotope data for sheep and cattle from the high medieval
farmsteads at Koksijde (10the11th century AD) compared to mean (1 s.d.) values for
Flemish inland animals.
328
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
Vandendriessche et al., 2006: 104) is cast up as drift on beaches,
especially after storms. It could therefore have been available as
supplementary fodder at least on occasion.
Salt-marshes are also the only ecosystems in Northwest Europe
with wild C4 plants, in past contexts namely Spartina maritima
(small cordgrass), a perennial grass colonising the mudflats and
lower marshes. S. maritima is now all but extinct on southern North
Sea coasts, having been outcompeted by modern invasive species
(Marchant and Goodman, 1969; Beeftink, 1975; Adam, 1990: 78ff.;
Castillo et al., 2008); however, its presence on the Flemish coast is
attested in historical times (Van Landuyt et al., 2006). Although it
has been suggested that S. maritima was only introduced to
Northern European salt-marshes relatively recently (see Wolff,
2005), well stratified finds from the Humber Estuary (England)
attest the presence of Spartina, sp. maritima or another as yet unidentified species, as early as w2600 BP (McGrail, 1981). Archaeobotanical finds from the Low Countries are missing, however, so it
is unclear when it was introduced here and whether it was present
in the early first millennium AD. Although the high marshes and
salt-meadows, which are no longer commonly flooded, are generally preferred for livestock grazing, traditional sheep breeds appear
to readily consume lower marsh cordgrasses if they are within
reach (Ranwell, 1967: 249). Apart from seaweed, it is therefore also
possible that a C4-plant, S. maritima, consumed in low quantities,
perhaps on a seasonal basis, contributed to the more markedly
elevated carbon isotope values observed in some of the Flemish
sheep.
6.3. Inter-site variation and site-specific patterns: sheep
Mean isotope values for sheep from the nine study sites are
plotted in Fig. 11. Visually, the sites divide into two groups: the
sheep from Veurne, Koksijde, Uitkerke and Leffinge plot together
with higher average d13C and/or d15N values than the remaining
five sites. Statistical tests mostly confirm the visual assessment
(Table S.3aeb) but it should be noted, that the employed tests
(Kruskal Wallis with Dunn-Bonferroni post-hoc tests) are conservative in order to control for family-wise error during the large
number of pair-wise comparisons required. They may therefore
lack the power to detect all differences in the data-set (Sheshkin,
2007).
Overall, the sheep from the early and high medieval sites (7the
12th century AD), which were located directly in the coastal plain,
therefore have the most elevated carbon and nitrogen isotope
values in the data-set. This is consistent with the assumption that
they were grazing on the extensive salt-marshes and -meadows
that existed before the construction of large-scale embankments
(see above, Section 2). The sometimes substantial 13C-enrichment
in sheep from these sites is probably the most intriguing aspect of
the data and, if attributed to the effect of salt-stress on the plants,
indicate that the sheep were kept in areas where the marine influence was felt more strongly than at the other sites, possibly
because of more frequent marine incursions or since they were
grazing on newly reclaimed land. Both suggestions would be
consistent with the known historical context of these farmsteads
(see Section 2). The early and high medieval sites are also thought
to be the first permanently occupied settlements in the coastal
plain. The need to bring livestock through the winter would have
required a more intensive exploitation of the natural surroundings,
and the fodder may, at times, have included seaweed or possibly C4
cordgrass (see above).
Such practices could at least explain the bimodal distribution of
the d13C of the sheep from Uitkerke (Fig. 5). The cultural artefacts
from the different contexts from which the samples were taken
demonstrate that the differences between the two groups are not
chronological (Pype, pers. comm.). The best explanations for the
clear grouping of the data are therefore either that there was a very
strict separation of pasture grounds at this multi-household settlement, most likely to do with ownership of land and flock.
Alternatively, the sheep with the most 13C-enriched isotope values
could have had an additional component in their diet, fodder,
perhaps given only on a seasonal basis, with a high enough d13C to
effect a complete separation of the two groups. Seasonal data obtained from the dental serial sections as part of a future study will
hopefully resolve this question.
Compared to the three medieval farmsteads (Leffinge, Uitkerke,
Koksijde), the isotope data from the castle at Veurne are much more
wide-spread, and while this result may partly be due to the fact that
more samples were obtained from here than from the other sites, it
is also consistent with the central functions of the castle which
would have received animals from the various sheep farms in the
IJzer estuary (Fig. 8). Interestingly, 13th century samples from
Veurne (Phase 4) show no apparent lessening of the coastal influence in comparison with earlier data, even though the salt-marshes
should have much declined by then, based on historical evidence e
although not yet to the same extent as in the north-eastern part of
the coastal plain (Verhulst, 1998). Still, the greater clustering of the
data in the Phase 4 samples, could be interpreted as evidence that
areas for grazing livestock were now more restricted and that
Veurne castle was supplied from a smaller number of farms.
It has been suggested that early medieval farms were the first
permanently occupied settlements in the coastal plain, while Roman platform-sites, such as Stene, were only in use seasonally (Tys,
2004a; Demey et al., 2013). If this was indeed the case and the
animals from Stene therefore spent relatively shorter times grazing
in the marshes, it could explain the isotopic differences between
sheep from Stene, whose d15N are only moderately high (mean d15N
8.8&), and the medieval sites (mean d15N 10.0& or higher). Alternatively, the lack of a stronger coastal signal at Stene could be a
reflection of the effectiveness of Roman dikes and tidal management. Pollen and plant macrofossils from Stene attest to the cultivation of turnip-rape, turnip and field bean (Brassica rapa subsp.
campestris and/or rapa; Vicia faba) at or near the site, indicating the
presence of sufficiently desalinated soils. Analyses of dung fragments suggest that the crops were also used as fodder for the sheep
(Demey et al., 2013), which, especially in the case of V. faba, may
have had a measurable impact on their d15N: Legumes acquire nitrogen directly from the atmosphere and their d15N are therefore
Fig. 8. Individual stable isotope data for sheep and cattle from the early to later medieval castle at Veurne (10the13th century AD) compared to mean (1 s.d.) values for
Flemish inland animals. Phase 1: 9the10th century AD; Phase 2: 11the12th century
AD; Phase 4: 13th century AD.
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
329
consistently low, around 0& (Virginia and Delwiche, 1982; Shearer
and Kohl, 1986). Field beans are moderately salt tolerant and were
widely cultivated along the Dutch and German coasts from the Iron
Age to the early medieval period, probably for animal and human
consumption (Bakels, 1991; Behre, 2004). Evidence from the
Flemish coastal plain is still limited, however (see Ervynck et al.,
1999: 109).
The mean isotope values for sheep from late Roman Oudenburg
are very similar to those from Stene some 300 years earlier (Fig. 11).
Even so, the individual data (Fig. 3) imply that this military fort at
the edge of the coastal plain was provisioned with a mixture of
animals from the coast and inland: only about one third of the
sheep have high enough d15N values (i.e. 9.0&) to suggest a clear
coastal or salt-marsh influence, although the possible role of legumes (V. faba) in the diet of the animals needs to be explored,
before these data can be interpreted with confidence. Individual
data for sheep from the castle in Brugge (Fig. 6), also situated just
outside the coastal plain, show a similar pattern to Oudenburg and
also suggest that at least in the 9the10th century, the Counts of
Flanders did not rely solely on their coastal estates for meat
production.
Average isotope values for sheep from the later medieval sites,
late 13theearly 14th century Ieper and 15th century Raversijde, are
very similar to those from Oudenburg, Stene and Brugge (Fig. 11).
However, the contrast between Ieper and Raversijde and the early
and high medieval settlements in the coastal plain (Leffinge, Uitkerke, Koksijde and Veurne) is stark, and indicates that significant
changes had occurred between these two periods, in the environment or the management of the animals, or both. Given the known
historical context, it is reasonable to assume that these changes are
related to the transformation of the coastal plain into a polder area.
The majority of the sheep from the inland town of Ieper cluster
below 9.0&, but fit in well with the bulk of the samples from the
fishing village of Raversijde, which is directly on the coast (Figs. 9e
10). While it cannot be excluded that Ieper was supplied with animals from the coast, these similarities suggest that livestock bred
in the coastal plain were now mostly indistinguishable from those
raised inland. A smaller group of sheep from Ieper have higher d15N
(10& and above) and a coastal origin for these is, again, possible. It
must not be forgotten, however, that there are several environmental, anthropogenic and physiological mechanisms which can
result in elevated d15N in herbivores including and most notably in
the present context, as they relate to intensity of land use, high
stocking density and manuring (Schwertl et al., 2005; Fraser et al.,
2011; see also Kelly, 2000; Fuller et al., 2005; Robbins et al., 2010).
Especially given the inland location of Ieper and the fact that sheep
with high d15N from this site are also among the most 13C-depleted
of the entire data-set, one of these factors, rather than the effects of
coastal grazing, may be responsible for the 15N-enrichment. More
isotope data from inland sites would be useful in order to explore
this issue further.
At Raversijde there is greater overlap between the isotope values
of sheep and cattle than at any other site inside the coastal plain
(Fig. 10), suggesting that the animals were now grazed in similar
environments, and it is indeed known that cattle husbandry was
increasingly moved into the coastal plain in the later Middle Ages
(see Nicholas, 1992). Nonetheless, just under half (48%) of the sheep
from Raversijde still have high d15N (9&), and it is interesting that
these samples also show a greater spread in d13C. This could be
consistent with some of the sheep grazing on or before the dike and
dune belt, where the influence of the sea was still more strongly
felt, while others were kept, with the cattle, on the reclaimed
polder land.
Fig. 9. Individual stable isotope data for sheep and cattle from the late medieval town
Ieper (13the14th century AD) compared to mean (1 s.d.) values for Flemish inland
animals.
Fig. 10. Individual stable isotope data for sheep and cattle from the late medieval
fishing village at Raversijde (15th century AD) compared to mean (1 s.d.) values for
Flemish inland animals.
6.4. Inter-site variation and site-specific patterns: cattle
In comparison to the sheep data, there is only modest isotopic
variation between the cattle from the individual sites (Fig. 12). It is
interesting that the mean cattle values from this present study are
consistently more 13C-enriched than those of the inland ‘controls’
from Londerzeel, Ename and Aalst, although it is only the differences between these inland sites and the early medieval farmsteads
in the coastal plain (Uitkerke and Koksijde) which are statistically
significant (Table S.4a). This may indicate that even though cattle at
these coastal sites were managed differently from the sheep, they
were still grazed on moderately saline pastures and/or given 13Cenriched fodder in small amounts.
Of the individual data, the cattle from Oudenburg appear most
unusual as there are two exceptionally low (<3.5&) d15N values in a
relatively small data-set (Fig. 3). While this raises the question of
how far-reaching the supply network of the late Roman Army was
(see Thomas and Stallibrass, 2008), these data should not be overinterpreted before the role of legumes in the local economy is
better understood (see above).
7. Conclusions
The stable isotope data from the Flemish coastal plain are
remarkable for the large variability that is displayed by herbivore
330
G. Müldner et al. / Journal of Archaeological Science 41 (2014) 322e332
Fig. 11. Mean (1 s.d.) stable isotope values for sheep from the nine sites compared to
average data for Flemish inland animals.
samples from a relatively small geographical area and in an
(almost) exclusively C3 environment. In this, they probably reflect
the dynamic and isotopically heterogeneous nature of coastal
wetlands (see Cloern et al., 2002), as well as diverse and specific
animal husbandry and herding strategies. Within this context, this
study successfully identified a number of diachronic trends and
site-specific patterns, which are overall consistent with the major
historical and environmental changes that occurred in the region
from the Roman to the later medieval period: The bulk collagen
data are consistent with suggestions based on the historical and
archaeological evidence, that coastal exploitation in the Roman
period was limited and perhaps primarily seasonal, but shifted to
year-round occupation in the early medieval period. Here, the
herbivore values appear to reveal an increase in marine influence,
possibly a reflection of more numerous marine incursions or the
grazing on newly reclaimed land. The data also clearly reflect
different husbandry strategies for sheep and cattle, consistent with
descriptions in historical sources which indicate that the saltmarshes and -meadows were primarily used for grazing sheep,
but with interesting variations between individual sites indicating
different adaptations. The effects of the progressive transformation
of the area into a polder and disappearance of the coastal marshes
are mostly seen in the late medieval samples: differences between
sheep and cattle are greatly reduced and isotope values from
coastal and inland sites are much more similar than before,
although again, site-specific investigations suggest greater variation than expected. Overall, the results of this project therefore
demonstrate that carbon and nitrogen stable isotope analysis is a
powerful approach to investigating animal husbandry and environmental change in coastal zones which can complement existing
evidence with novel, site-specific information.
Methodologically, it appears that d15N values are a useful but, for
now at least, somewhat blunt tool in the investigation of coastal
grazing. With reference to isotope data from inland animals, d15N
could be broadly categorised as ‘elevated’ (probably because of a
‘coastal component’ in the diet) or not, but beyond this, very little
patterning was observed. Conversely, the variation in d13C appeared
much more systematic, at least at some of the sites, even if the exact
mechanisms behind this were not always transparent. Plants acquire their carbon and nitrogen through different pathways (see
e.g. Marshall et al., 2007), and while isotope ratios of the two elements can be correlated, especially in coastal environments where
both may be influenced by salinity, they need not be. The results
presented here clearly bear this out (see also Cloern et al., 2002).
More isotope studies on modern coastal ecosystems, including flora
and local fauna, are needed in order to better understand their
relationships and thereby to interpret patterns in the data.
The forthcoming seasonal evidence (from dental serial sections)
will no doubt assist in further interpretations and the use of other
isotopic systems (e.g. sulphur) may be able to verify when elevated
d15N in herbivores are indeed due to coastal grazing and when they
should be attributed to different mechanisms (see Richards et al.,
2001), although the sulphur isotope systematics of salt-marshes
are complex and results may not be clear-cut (see Peterson and
Fry, 1987). Nevertheless, these additional analyses could be
particularly important when the method is applied to less-well
understood time periods, e.g. prehistory. This study has clearly
demonstrated, however, that even on their own, carbon and nitrogen isotope ratios from bulk collagen can give significant new
insights into ancient husbandry practices.
Acknowledgements
We thank the British Academy (SG101625) for funding, the
Flanders Heritage Agency and Ten Duinen Abbey Museum for
samples, An Lentacker (Flanders Heritage Agency) for help with the
identification of the animal remains, Yan Gao, Tina Moriarty and
Charlotte Scull for assistance with sample preparation and analysis
and Sarah Lucas (all University of Reading) for graphics support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jas.2013.08.010.
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