Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 21-24 |
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Modern geological research into the late and postglacial his-
tory of the inner Danish waters (i.e. Kattegat, Bælthavet and
Øresund, plus the adjoining fjords and estuaries) began at the
turn of the last century. Since then most investigations have
focused on the timing of the initial marine inundation of the
area, the early to mid-Holocene changes in land-sea confi -
guration and sea level changes during the mid-Holocene
Littorina period. Research on the late Holocene marine envi-
ronment has received less emphasis, undoubtedly due to
problems in finding continuous marine sediment records, as
sedimentation in large areas of the Danish waters seems to
have been characterised by complex spa-
tial and temporal patterns of deposition
and non-deposition (e.g. Lykke-An dersen
et al . 1993).
tory of the inner Danish waters (i.e. Kattegat, Bælthavet and
Øresund, plus the adjoining fjords and estuaries) began at the
turn of the last century. Since then most investigations have
focused on the timing of the initial marine inundation of the
area, the early to mid-Holocene changes in land-sea confi -
guration and sea level changes during the mid-Holocene
Littorina period. Research on the late Holocene marine envi-
ronment has received less emphasis, undoubtedly due to
problems in finding continuous marine sediment records, as
sedimentation in large areas of the Danish waters seems to
have been characterised by complex spa-
tial and temporal patterns of deposition
and non-deposition (e.g. Lykke-An dersen
et al . 1993).
In an ongoing project we aim to ex -
plore the continuous development of
Da nish coastal environments over the last
9000 years using a variety of proxy data,
including molluscs, dia
Da nish coastal environments over the last
9000 years using a variety of proxy data,
including molluscs, dia
toms, foramini
-
fera, algal pigments, plant macrofossils
and physical properties of sedi ments. The
project spans both environmental and
cultural history, and addresses the impor-
tant links between them, as the nature of
the coastal environment has exerted major
influences on cultural and societal expres-
sion and activity from Meso lithic to mod -
and physical properties of sedi ments. The
project spans both environmental and
cultural history, and addresses the impor-
tant links between them, as the nature of
the coastal environment has exerted major
influences on cultural and societal expres-
sion and activity from Meso lithic to mod -
ern times. This paper presents some of the first results from the
project concerning environmental changes in the Roman
Warm Period ( c . 2000-1600 years B.P.) as shown by changes
in molluscan faunas at two coring sites in Horsens Fjord and
Tem pelkrog in southern Isefjord (Fig. 1).
project concerning environmental changes in the Roman
Warm Period ( c . 2000-1600 years B.P.) as shown by changes
in molluscan faunas at two coring sites in Horsens Fjord and
Tem pelkrog in southern Isefjord (Fig. 1).
Hydrography
The present-day circulation pattern in the inner Danish
waters is dominated by a two-layer estuarine flow, driven by
outflow of low-salinity surface water from the Baltic Sea and
waters is dominated by a two-layer estuarine flow, driven by
outflow of low-salinity surface water from the Baltic Sea and
Environmental change in Danish marine waters during
the Roman Warm Period inferred from mollusc data
the Roman Warm Period inferred from mollusc data
Peter Rasmussen, Kaj Strand Petersen and David B. Ryves
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 21-24. Available at:
www.geus.dk/publications/bull
21
Fig. 1. Map of Denmark showing the location
of the two study sites Horsens Fjord and Tem -
pelkrog in Isefjord and the present day sea
surface salinities (psu; annual mean) in the
Danish waters. The red dots indicate fjords and
estuaries with Iron Age shell middens.
(modified from Dahl
et al
. 2003). Bælthavet
includes Storebælt and Lillebælt and the sea
between the islands south of Fyn and Sjælland.
inflow of high-salinity bottom water from the North Sea and
Skagerrak. Due to high inflows of freshwater to the Baltic Sea
from rivers there is a strong surface-salinity gradient from
west to east: from> 30 psu (practical salinity units, equivalent
to ppt) in Skagerrak decreasing eastwards to <5 psu in the
Baltic Sea (Fig. 1; Al-Hamdani et al . 2007 - this volume).
There is also a strong salinity stratification (a halocline)
within the water column, with a wedge of higher salinity,
North Sea-derived water underlying less dense, less saline sur-
face water.
Skagerrak. Due to high inflows of freshwater to the Baltic Sea
from rivers there is a strong surface-salinity gradient from
west to east: from> 30 psu (practical salinity units, equivalent
to ppt) in Skagerrak decreasing eastwards to <5 psu in the
Baltic Sea (Fig. 1; Al-Hamdani et al . 2007 - this volume).
There is also a strong salinity stratification (a halocline)
within the water column, with a wedge of higher salinity,
North Sea-derived water underlying less dense, less saline sur-
face water.
Material and methods
Two sediment core lengths of 6 m and 13.5 m were extracted
from respectively Horsens Fjord and Tempelkrog, both at a
water depth of around 5 m and both consisting of homoge-
neous clay-gyttja. The two sediment records were Accelerator
Mass Spectrometry (AMS)
from respectively Horsens Fjord and Tempelkrog, both at a
water depth of around 5 m and both consisting of homoge-
neous clay-gyttja. The two sediment records were Accelerator
Mass Spectrometry (AMS)
14
C-dated using terrestrial plant
material, thus avoiding marine-derived material which suf-
fers from uncertain reservoir and hard-water effects that can
cause serious dating problems in Danish fjord and marine
waters (Heier-Nielsen et al . 1995). The Horsens Fjord and
Tempelkrog sediment cores were subsampled at intervals of
fers from uncertain reservoir and hard-water effects that can
cause serious dating problems in Danish fjord and marine
waters (Heier-Nielsen et al . 1995). The Horsens Fjord and
Tempelkrog sediment cores were subsampled at intervals of
1 cm and 2 cm, respectively, which is equivalent to a resolu-
tion of 10-15 years in both records for the time period dis-
cussed in this paper. Molluscs were extracted from the cores
by wet-sieving of known sediment volumes through a sieve
with a mesh size of 0.1 mm. Macrofaunal specimens were
identified and counted and species numbers were calculated
for 100 ml of sediment (Fig. 2).
Results and discussion
Molluscan faunas
Based on analogies with their present-day ecological require-
ments in relation to salinity, temperature, depth and sub-
strate, Quaternary molluscs are useful tools as indicators of
environmental and climatic changes through time. Changes
in surface salinity (and temperature) have often been linked
with fluctuations in different marine populations, for exam-
ple stocks of fish and molluscs, which tolerate only certain
ranges of salinity and temperature. In the transitional area
between the Skagerrak and the Baltic Sea, the salinity gradient
seems to be the main limiting factor in the geographic distri -
bution of mollusc species (Sorgenfrei 1958; Petersen 2004).
ments in relation to salinity, temperature, depth and sub-
strate, Quaternary molluscs are useful tools as indicators of
environmental and climatic changes through time. Changes
in surface salinity (and temperature) have often been linked
with fluctuations in different marine populations, for exam-
ple stocks of fish and molluscs, which tolerate only certain
ranges of salinity and temperature. In the transitional area
between the Skagerrak and the Baltic Sea, the salinity gradient
seems to be the main limiting factor in the geographic distri -
bution of mollusc species (Sorgenfrei 1958; Petersen 2004).
22
Fig. 2. Summary stratigraphic, macrofaunal
data from two sediment cores from Horsens
Fjord and Tempelkrog. Abundances of mol -
luscs are expressed as specimens per 100 ml
of fresh sediment. Remains of echinoids are
shown as present/not-present. The dotted
lines delimit the Roman Warm Period.
Note the change of x-axis scale.
The summary stratigraphic, macrofauna data from Hor -
sens Fjord and Tempelkrog during the period 2500 to 1500
years B.P. are shown in Fig. 2. At both sites there is a distinct
increase in mollusc abundance at the beginning of the
Roman Warm Period between c . 2000 and 1850 years B.P.
and at Tempelkrog again at the end of the Roman Warm
Period. At Horsens, this increase is mainly due to Mytilus
edulis (blue mussel), Rissoa albella, R. albella var. sarci,
Bittium reticulatum, Cerastoderma edule (common cockle)
and Mysella bidentata , and at Tempelkrog, Hydrobia ulvae , H.
ventrosa, H. neglecta and Rissoa albella . At both sites, this
increase in mollusc abundance is accompanied by the appear-
ance of Ostrea edulis (European flat oyster) and a more steady
presence of remains of echinoids (sea urchins), both indicat-
ing more saline conditions than in the period before or after
(Fig. 2). Ostrea edulis needs at least 25 psu in order to repro-
duce (Jensen & Spärck 1934) and is not present in the inner
Danish waters today, except for the western part of Lim -
fjorden which is connected to the North Sea. Furthermore,
the reproductive success of Ostrea edulis is very sensitive to
temperature with optimal conditions around 20-22°C
(Spärck 1924). Echinoids are generally stenohaline and
therefore disappear when the water becomes brackish ( c . 10
psu). In addition, other mollusc species, which also indicate
an increase in salinity to> 25 psu, appear at Horsens Fjord,
although with low abundance. These include Parvicardium
scabrum, Abra nitida, Rissoa violacea, Gari fervensis, Velutina
velutina, Odostomia umbilicaris and Abra prismatica (Sorgen -
frei 1958).
years B.P. are shown in Fig. 2. At both sites there is a distinct
increase in mollusc abundance at the beginning of the
Roman Warm Period between c . 2000 and 1850 years B.P.
and at Tempelkrog again at the end of the Roman Warm
Period. At Horsens, this increase is mainly due to Mytilus
edulis (blue mussel), Rissoa albella, R. albella var. sarci,
Bittium reticulatum, Cerastoderma edule (common cockle)
and Mysella bidentata , and at Tempelkrog, Hydrobia ulvae , H.
ventrosa, H. neglecta and Rissoa albella . At both sites, this
increase in mollusc abundance is accompanied by the appear-
ance of Ostrea edulis (European flat oyster) and a more steady
presence of remains of echinoids (sea urchins), both indicat-
ing more saline conditions than in the period before or after
(Fig. 2). Ostrea edulis needs at least 25 psu in order to repro-
duce (Jensen & Spärck 1934) and is not present in the inner
Danish waters today, except for the western part of Lim -
fjorden which is connected to the North Sea. Furthermore,
the reproductive success of Ostrea edulis is very sensitive to
temperature with optimal conditions around 20-22°C
(Spärck 1924). Echinoids are generally stenohaline and
therefore disappear when the water becomes brackish ( c . 10
psu). In addition, other mollusc species, which also indicate
an increase in salinity to> 25 psu, appear at Horsens Fjord,
although with low abundance. These include Parvicardium
scabrum, Abra nitida, Rissoa violacea, Gari fervensis, Velutina
velutina, Odostomia umbilicaris and Abra prismatica (Sorgen -
frei 1958).
The two study sites are located 120 km apart and the sim-
ilar pattern in macrofaunal assemblages, with an increase in
mollusc abundance and the appearance of a suite of almost
fully marine taxa at various times during the Roman Warm
Period, thus testifies to a widespread change to more saline
and productive conditions across the inner Danish waters.
mollusc abundance and the appearance of a suite of almost
fully marine taxa at various times during the Roman Warm
Period, thus testifies to a widespread change to more saline
and productive conditions across the inner Danish waters.
The fairly regular presence of
Ostrea edulis
(see below) may
also suggest an increase in sea surface temperature during this
period. The reason for two (or three) `episodes' with changes in
faunal compositon at Tempelkrog and only one at Horsens
Fjord is unclear but could be a result of spatial or temporal
differ ences in substrate, local current conditions or post-
mortem (taphonomic) processes.
also suggest an increase in sea surface temperature during this
period. The reason for two (or three) `episodes' with changes in
faunal compositon at Tempelkrog and only one at Horsens
Fjord is unclear but could be a result of spatial or temporal
differ ences in substrate, local current conditions or post-
mortem (taphonomic) processes.
Sea and society
Inferences of higher salinity and productivity during the
Roman Warm Period from the palaeoenvironmental data are
also supported by an independent line of evidence from the
archaeological record. Large shell middens dated to the cen-
turies around 2000 years B.P., and predominantely com-
posed of Mytilus edulis, Cerastoderma edule, Littorina littorea
(common periwinkle) and to a lesser extent Ostrea edulis , are
recorded along several Danish and North German fjords
(Figs 1, 3; Anger 1973; Harck 1973; Poulsen 1978; Petersen
1985). The synchroneity between the inferred changes in the
marine environment and the appearance of these Iron Age
shell middens strongly suggest a causal connection between
the two, implying that people responded to the increased
productivity in the marine environment by a comprehensive
and targeted gathering and processing of shellfish.
Roman Warm Period from the palaeoenvironmental data are
also supported by an independent line of evidence from the
archaeological record. Large shell middens dated to the cen-
turies around 2000 years B.P., and predominantely com-
posed of Mytilus edulis, Cerastoderma edule, Littorina littorea
(common periwinkle) and to a lesser extent Ostrea edulis , are
recorded along several Danish and North German fjords
(Figs 1, 3; Anger 1973; Harck 1973; Poulsen 1978; Petersen
1985). The synchroneity between the inferred changes in the
marine environment and the appearance of these Iron Age
shell middens strongly suggest a causal connection between
the two, implying that people responded to the increased
productivity in the marine environment by a comprehensive
and targeted gathering and processing of shellfish.
Outlook
The evidence of salinity increase in the inner Danish waters
during the Roman Warm Period only seems to be explicable
by the more frequent inflow of high-salinity North Sea water,
which travelled through the Danish straits and further east
into the Baltic Sea. This scenario is supported by the presence
of Ostrea edulis in Iron Age shell middens as far into the Baltic
as the head of Flensburg Fjord and Eckernförder Bucht
during the Roman Warm Period only seems to be explicable
by the more frequent inflow of high-salinity North Sea water,
which travelled through the Danish straits and further east
into the Baltic Sea. This scenario is supported by the presence
of Ostrea edulis in Iron Age shell middens as far into the Baltic
as the head of Flensburg Fjord and Eckernförder Bucht
23
Fig. 3. Section of an Iron Age shell midden at
Horsens Fjord. The Iron Age middens are
mainly composed of shells, charcoal and pot
boilers (stones used in cooking) and are
interpreted as specialised coastal sites used
for gathering and processing of shellfish
(Poulsen 1978). Mollusc analysis from North
German middens suggests that they are
seasonal sites used in the late summer or
autumn (Anger 1973). Photograph courtesy of
Karen Løkkegaard Poulsen.
(North Germany; Anger 1973; Harck 1973); this is only pos-
sible through a combination of a more frequent input of
high-salinity water and a higher rate of water exchange than
today. The increased inflow of North Sea water seems to have
penetrated as far as the central Baltic Sea where diatom and
isotopic data also suggest a salinity rise during the Roman
Warm Period (Emeis et al . 2003). Diatom-based sea-surface
salinity reconstructions from southern Skagerrak also indi-
cate higher salinity during the period (Hebbeln et al . 2006),
in good agreement with our results from the Danish waters
and evidence from the Gotland Basin (Emeis et al . 2003).
sible through a combination of a more frequent input of
high-salinity water and a higher rate of water exchange than
today. The increased inflow of North Sea water seems to have
penetrated as far as the central Baltic Sea where diatom and
isotopic data also suggest a salinity rise during the Roman
Warm Period (Emeis et al . 2003). Diatom-based sea-surface
salinity reconstructions from southern Skagerrak also indi-
cate higher salinity during the period (Hebbeln et al . 2006),
in good agreement with our results from the Danish waters
and evidence from the Gotland Basin (Emeis et al . 2003).
Future work
Work is ongoing to analyse the other proxy records from the
two sites over the last 9000 years to provide additional inde-
pendent environmental information and to test the infer-
ences made from the subfossil mollusc data. For instance, a
model to relate fossil diatom assemblage composition to past
surface-water salinity is being developed using a large range
of contemporary samples from the western Baltic Sea, Lim -
fjorden and coastal brackish lakes and fjords (the MOLTEN
project: http://craticula.ncl.ac.uk/Molten/jsp/; Ryves et al .
2004). Coastal sediments contain important natural archives
of past environmental changes and palaeoecological tech-
niques can provide a powerful means of revealing the natural
variability of the marine environment and the links between
environmental and socio-cultural changes over time. Further -
more, this approach can establish the nature of environmen-
tal conditions in nearshore marine areas prior to the impact
of modern society on coastal regions. It is especially impor-
tant to establish ecological baseline conditions before realistic
goals for environmental management of coasts can be set.
two sites over the last 9000 years to provide additional inde-
pendent environmental information and to test the infer-
ences made from the subfossil mollusc data. For instance, a
model to relate fossil diatom assemblage composition to past
surface-water salinity is being developed using a large range
of contemporary samples from the western Baltic Sea, Lim -
fjorden and coastal brackish lakes and fjords (the MOLTEN
project: http://craticula.ncl.ac.uk/Molten/jsp/; Ryves et al .
2004). Coastal sediments contain important natural archives
of past environmental changes and palaeoecological tech-
niques can provide a powerful means of revealing the natural
variability of the marine environment and the links between
environmental and socio-cultural changes over time. Further -
more, this approach can establish the nature of environmen-
tal conditions in nearshore marine areas prior to the impact
of modern society on coastal regions. It is especially impor-
tant to establish ecological baseline conditions before realistic
goals for environmental management of coasts can be set.
Acknowledgements
The ongoing research project
Denmark's coastal environment over the
last 9000 years: linking cultural and hydrographic change
is co-financed
by the Danish Research Council for the Humanities (FKK) and the
Danish Natural Science Research Council (FNU), whose support is grate-
fully acknowledged. Additional funding for the project has been pro-
vided by Loughborough University, UK. Karen Løkkegaard Poulsen is
thanked for information about Iron Age shell middens.
Danish Natural Science Research Council (FNU), whose support is grate-
fully acknowledged. Additional funding for the project has been pro-
vided by Loughborough University, UK. Karen Løkkegaard Poulsen is
thanked for information about Iron Age shell middens.
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Authors' addresses
P.R. & K.S.P.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
per@geus.dk
D.B.R.,
Department of Geography, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK.
24
