Earth-Science Reviews 91 (2008) 77–92
Contents lists available at ScienceDirect
Earth-Science Reviews
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e a r s c i r ev
Past occurrences of hypoxia in the Baltic Sea and the role of climate variability,
environmental change and human impact
Lovisa Zillén a,⁎, Daniel J. Conley a, Thomas Andrén b, Elinor Andrén b, Svante Björck a
a
b
Geobiosphere Science Centre, Department of Quaternary Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden
School of Life Sciences, Södertörn University College, SE-141 89 Huddinge, Sweden
a r t i c l e
a b s t r a c t
i n f o
The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen
deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf
of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The
cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of
nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is
poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c.
50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and
explore the role of climate variability, environmental change and human impact. We present a compilation of
proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated
results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of
the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the
formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str.
Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP
and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the
Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide
with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content.
This study implies that there may be a correlation between climate variability in the past and the state of the
marine environment, where milder and dryer periods with less freshwater run-off correspond to increased
salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of
laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as
the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes
have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the
Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea
watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was
not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the
Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the
Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial
revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage
ditches intensified, and agricultural and forest industry expanded extensively.
© 2008 Elsevier B.V. All rights reserved.
Article history:
Received 7 December 2007
Accepted 13 October 2008
Available online 11 November 2008
Keywords:
Baltic Sea
Littorina Sea
hypoxia
Holocene
Medieval Warm Period
modern historical period
climate change and human impact
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . .
Baltic Sea characteristics . . . .
History of the Baltic Sea . . . .
Evidence for hypoxia in the past
4.1.
Laminated sediments . .
4.2.
Geochemistry . . . . . .
4.3.
Chronology . . . . . . .
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⁎ Corresponding author. Tel.: +46 46 222 7805; fax: +46 46 222 4830.
E-mail address: lovisa.zillen@geol.lu.se (L. Zillén).
0012-8252/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.earscirev.2008.10.001
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L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
5.
Occurrence of hypoxia . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
During modern time (last 50 years) . . . . . . . . . . . . . . . . . . . .
5.2.
During modern historical time (AD 1950–AD 1800) . . . . . . . . . . . .
5.3.
On geological time-scales (AD 1800–10 000 cal. yr BP) . . . . . . . . . . .
5.3.1.
Bothnian Sea, Bothnian Bay, Gulf of Finland and Archipelago Sea . .
5.3.2.
Gotland Basin . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3.
NW and NC Baltic Proper . . . . . . . . . . . . . . . . . . . .
5.3.4.
Landsort Deep and W Gotland Basin . . . . . . . . . . . . . . .
5.3.5.
Southern Baltic Sea . . . . . . . . . . . . . . . . . . . . . . .
6.
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.
Hypoxia in time and space . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1.
Bothnian Bay, Bothnian Sea, Gulf of Finland and the Archipelago Sea
6.1.2.
Baltic Proper . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.
Hypoxia and driving mechanisms . . . . . . . . . . . . . . . . . . . . .
7.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Hypoxia, defined as b2 mg/l dissolved oxygen, occurs in aquatic
environments when dissolved oxygen becomes reduced in concentration to a point harmful to aquatic organisms living in the environment.
Hypoxia is a globally significant problem with over 400 reported sites
suffering from its effects (Diaz and Rosenberg, 2008). Hypoxia not only
causes severe ecosystem disturbances (Diaz and Rosenberg, 1995) but
alters nutrient biogeochemical cycles (Vahtera et al., 2007) and forms
hydrogen sulfide which is hazardous to numerous fauna and flora
communities (Diaz and Rosenberg, 1995). Widespread oxygen
deficiency has for that reason severely reduced macrobenthic communities below the halocline in the Baltic Sea over the past decades
(Laine, 2003) and produced benthic “ecological deserts” that annually
cover over 30% of the seafloor (Karlson et al., 2002). Hypoxia also
affects food chain dynamics, fish habitats and fisheries (Karlson et al.,
2002; Bonsdorff, 2006).
The hypoxic zone in the Baltic Sea has increased about four times
since the 1960s (Jonsson et al., 1990), and currently covers an area
averaging 41000 km2 annually (e.g. Conley et al., 2002). Hypoxia has
for that reason developed into a serve environmental problem for the
Baltic Sea and its dependents. The increasing trend in hypoxia is
thought to be caused by enhanced eutrophication due to excess load of
waterborne and airborne nutrients (nitrogen and phosphorus) to the
sea from anthropogenic sources (Wulff et al., 2007). Eutrophication
has been of great concern for the countries in the Baltic region at
least since the 1980s, with ministerial level commitments to reduce
nutrients and improve water quality (Johansson et al., 2007).
The debate about improving the present state of the Baltic Sea
through implementation of the European Water Framework Directive
often refers to conditions prior to the turn of the last century (1900) as
an environmental reference status, when the Baltic is suggested to
have been an oligothrophic clear-water body with oxygenated deep
waters (Österblom et al., 2007). However, geological records show that
the Baltic Sea is a dynamic ecosystem that has undergone many
environmental changes over the last c. 16 000 years (Björck, 1995;
Andrén et al., 2000a,b; Berglund et al., 2005). Studies of the more
recent past, for instance, reveal that hypoxia has been present in some
basins for at least the last 100 years (Jonsson et al., 1990). Furthermore,
analyses of long sediment cores suggest that hypoxia in the Baltic Sea
has occurred intermittently in deep basins in the Baltic Proper over
thousands of years (Andrén et al., 2000a,b; Sohlenius et al., 2001;
Emeis et al., 2003) and that cyanobacterial blooms have occurred
during the last c. 7000 years (Bianchi et al., 2000; Kunzendorf et al.,
2001; Poutanen and Nikkilä, 2001). Various investigations also imply
that there may be a correlation between climate variability in the past
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and the state of the marine environment, where warmer periods
correspond to increased primary production and higher salinities
resulting in amplified hypoxia and enlarged distribution of benthic
mortality and laminated sediments (e.g. Thorsen et al., 1995; Fjellså
and Nordberg, 1996; Andrén et al., 2000a; Bianchi et al., 2000;
Nordberg et al., 2000; Emeis et al., 2003). In addition, the occurrence
of hypoxia in the deeper basins today need not necessarily be attributed to human activity but could be naturally driven by oceanographic, environmental and climate forcing (Filipsson and Nordberg,
2004a,b).
In addition, both climate variability and human impact have the
potential to greatly affect the environment in the semi-enclosed Baltic
Sea and its catchment. However, the long-term spatial and temporal
extent of hypoxia and its possible connections to these parameters are
poorly known. Anthropogenic forcing in the drainage area, such as,
changes in land use and population density, could indirectly have
affected the marine/brackish environment already in the Late
Holocene. It is known from numerous long-term studies of lake
sediments in Northwest Europe that population growth and agricultural development have impacted lakes for thousands of years and
that cultural eutrophication of lakes has a history longer than just
decades or centuries (e.g. Fritz, 1989; Renberg et al., 2001; Bradshaw
et al., 2005). Furthermore, the International Panel on Climate Change
(IPCC) has recognized that hypoxia is a problem of growing concern
with projected climate change (www.ipcc.ch) and recent studies
predict that cyanobacteria blooms will magnify with global warming
(Pearl and Huisman, 2008). It is thus essential to improve our
understanding about the timing, extent and mechanism(s) causing
hypoxia on millennial time-scales in order to understand the full
range of the natural variability and to put forward realistic measures
to improve the future environment of the Baltic Sea. It is also
important to put the recent human impact in a time-perspective in
order to understand the modern environmental issues.
This paper aims to review and synthesize the current knowledge in
the Baltic Sea about the appearance of hypoxia in modern time (last
50 years), modern historical time (50–200 years ago) and in the
geological past (last c. 10 000 years), based on previous publications.
We examined a large number of papers about Baltic Sea sediments,
but only present here papers which report or address hypoxia. We
present a compilation of several long sediment records covering most
of the Holocene (i.e. the last c. 10 000 years) and explore possible
connections with the presence of past hypoxia and the role of climate
and environmental variability and human impact. With the purpose to
spatially and temporally reconstruct the occurrence of hypoxia our
main objective is to answer three basic questions: where, when and
why was the Baltic Sea hypoxic in the past?
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
2. Baltic Sea characteristics
The Baltic Sea (area of 412 560 km2; volume of 21631 km3; average
depth of 52 m; maximum depth of 459 m; Seifert and Kayser, 1995) is
a semi-enclosed brackish water body consisting of a series of basins
where the Bothnian Bay, the Bothnian Sea, the Gulfs of Finland and
Riga and the Baltic Proper are the main water masses (Fig. 1). We will
not address hypoxia in the transition zone between the Baltic Sea and
the Skagerrak, nor in coastal areas. The modern record of hypoxia in
the Danish straits has been previously reported by Conley et al. (2007).
The Baltic Sea watershed has a population over 85 million. The
drainage area is c. 4 times larger than the area of the sea itself, which
yields substantial freshwater input to the basin. Inflow of denser saline
bottom waters takes place through shallow and narrow connections
(i.e. the Danish belts and Swedish sound) from the Skagerrak/Kattegat
and brackish surface water is transported out over the same sills.
Inflow of saline water in the south and the occurrence of large rivers in
the north yield a south-north salinity gradient where the surface
salinity is c. 8–10 in the southern Baltic, c. 7–8 in the Baltic Proper
79
and c. 3–5 in the Gulf of Finland, Bothnian Sea and Bothnian Bay
(Matthäus, 2006).
The water column of the Baltic Proper is permanently stratified,
consisting of two water masses, an upper layer of brackish water with
salinities of c. 7–8 and more saline deep waters of c. 11–13. At the
transition zone between these water masses a strong permanent
halocline is formed which prevents vertical mixing of the water
column and ventilation of more oxygen rich waters to the bottom
(Matthäus and Schinke, 1999). The halocline is formed at depths
varying between c. 30 m in the Arkona Basin, 60 m in the Bornholm
Basin, c. 80 m in the Gotland Basin and Landsort Deep (Matthäus,
2006). The Bothnian Sea and, in particular, the Bothnian Bay have
weaker haloclines and a better ventilation due to less variable inflow
conditions (Stigebrandt, 2001). In combination with a lower productivity this makes these basins mostly oxic.
Large inflows (100–250 km3) of higher salinity and oxygen-rich
water from the North Sea represent an important mechanism by
which the Baltic deep water is displaced and renewed to a significant
degree. All inflows are substantially mixed with surface water, on
Fig. 1. Map of the Baltic Sea showing the major basins in black (BB = Bothnian Bay, BS = Bothnian Sea, GF = Gulf of Finland, GR = Gulf of Riga, BP = Baltic Proper, WB = West Baltic, K =
Kattegat and S = Skagerrak) and sub-basins in white (AS = Archipelago Sea, NWBP = Northwest Baltic Proper, NCBP = North Central Baltic Proper, LD = Landsort Deep, GB = Gotland
Basin, BB = Bornholm Basin, AB = Arkona Basin, MB = Mecklenburgian Bay, and GD = Gdansk Deep) mentioned in the text and coring sites for the sediment records studied in this
paper. Black and white circles represent localities containing laminated sediments (geological records of hypoxia) while white circles show the position of sediment cores with
homogeneous (oxygenated and bioturbated) sediments. Also, the locations of the lake sediment sequences referred to in the text (black circles).
80
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
average 3 times (Stigebrandt and Gustafsson, 2003). However,
frequent but small inflows (10–20 km3) are generally insufficient to
displace the deep bottom water because they have a much less density
and therefore do not penetrate into the deepest parts of the Baltic Sea
(Stigebrandt, 2001; Matthäus, 2006).
In summary, the physical setting of the Baltic Sea, the narrow and
shallow connections to the Skagerrak/Kattegat and relative large river
runoff influence the salinity and oxygen distribution in the basin
resulting in low salinity, long residence time, strong density
stratification of the water mass and limited water exchange (Wulff
et al., 1990; Gustafsson and Westman, 2002). This estuarine circulation retains nutrients and organic matter within the basin and leads to
high nutrient availability.
3. History of the Baltic Sea
The present Baltic Sea formed in the course of several events that
accompanied the onset of the last deglaciation of Scandinavia at c.
17 000–15 000 cal. yr. BP (e.g. Björck, 1995). The postglacial history of
the Baltic Sea and its drainage area is characterized by regionally
confined transgressions and regressions caused by the interaction of
the differential subsidence and uplift of land, deglaciation, ponding
and drainage of fresh-water stages and phases of eustatic sea-level
rise. By tradition, the late Quaternary history of the Baltic Sea is
divided into four main stages, i.e. the Baltic Ice Lake, the Yoldia Sea, the
Ancylus Lake and the Littorina Sea, representing time periods with
either brackish condition with connections to the Skagerrak/Kattegat
or isolated and up-dammed freshwater lake conditions (Björck, 1995).
The Littorina Sea can further be divided into three sub stages i.e. the
Early (or Initial) Littorina Sea (c. 10 000–8500 cal. yr BP) the Littorina
Sea sensu stricto (s. str.; c. 8500–3000 cal. yr BP) and Late Littorina Sea
(c. 3000 cal. yr BP to present; Berglund et al., 2005). We will apply the
latter terminology for the Littorina Sea, which is based on studies of
near shore sediment cores, because it is supported by high-resolution
chronologies established by 14C-dating of terrestrial plant macrofossils and environmental reconstructions constructed from multistratigraphical analyses (Berglund and Sandgren, 1996; Berglund et al.,
2005; Yu et al., 2007).
The Baltic Ice Lake began to form c. 16 000 cal. yr BP and was filled
by the accumulation of glacial waters from the waning ice sheet. It
existed until c. 11600 cal. yr BP when the dam broke open at Mount
Billingen in south central Sweden (Björck and Digerfeldt, 1984;
Strömberg, 1992; Andrén et al., 2002). This resulted in a considerable
fall of the Baltic lake-level (c. 25 m; Svensson, 1991; Björck, 1995). The
deep basins of the freshwater Baltic Ice Lake were characterized by
oxic conditions (due to seasonal overturn of the water column) and
low organic productivity (Andrén et al., 2002). During this period
glacial varved and non-varved clays were deposited.
After the final drainage of the Baltic Ice Lake the Yoldia Sea formed
(11600–10 700 cal. yr BP) and its level was determined by that of the
global ocean. The water of the Yoldia Sea was partly brackish for 200–
300 yr around 11200 cal. yr BP (Wastegård et al., 1995; Andrén et al.,
2002) and linked to Lake Vänern through a broad connection over the
central part of southern Sweden and to Skagerrak/Kattegat through
narrow fjords west of Lake Vänern. The brackish phase of the Yoldia
Sea stage had its highest salinities in depression areas between Lake
Vänern and Stockholm in central Sweden (Schoning, 2001). However,
it has been suggested that brackish bottom-waters also penetrated as
far south as the Bornholm Basin and Hanö Bay in southern Baltic
(Björck et al., 1990; Andrén et al., 2000b; Andrén et al., 2007). The
dominant sedimentary deposits from the Yoldia Sea stage consist of
organic-poor (silty) clays, often varved in and north of the Gotland
Basin (Andrén et al., 2002).
Through the continuous land-uplift, the fjord connections to the
Skagerrak/Kattegat shallowed and forced Baltic level to rise above sea
level (Björck, 1995). The Baltic changed once more to lake conditions,
the Ancylus Lake. During the Ancylus Lake stage (c. 10 700–10 000 cal.
yr BP) the relatively faster land uplift in the northern parts of the basin
caused it to be tilted towards the south and at c. 10 000 cal. yr. BP the
Ancylus Lake attained the level of the global ocean (which was also
rising) through a fluvial-lacustrine system in the lowlands of the
Fehmarn-Great Belt region (Björck, 2008). This connection to
Kattegatt was subsequently broadened and later the Öresund Strait
began to function as an important inlet of saltwater around c. 8500–
7800 cal. yr BP (e.g. Andrén et al., 2000a,b; Sohlenius et al., 2001;
Emeis et al., 2003). However, due to the lack of a robust geochronology, the timing of the first clear brackish/marine Littorina intrusion is
much debated. In the southern Baltic it has been dated to c. 6500 cal.
BP with the Optically Stimulated Luminescence (OSL) dating method
(Moros et al., 2002; Rößler, 2006; Kortekaas, 2007) and it is presumed
that a transitional phase (i.e. the Early Littorina Sea Stage or the
Mastogloia Sea) existed with episodic marine influxes starting at c.
9800 cal. yr BP based on traditional 14C-dating (Andrén et al., 2000b;
Yu et al., 2003; Berglund et al., 2005; Witkowski et al., 2005). The
saltwater intrusion during the Early Littorina Sea/Littorina Sea s. str.
transition (often referred to as the transition phase between the
Ancylus Lake and the Littorina Sea or the Mastogloia Sea) initiated the
establishment of a permanent halocline in the Baltic Sea and a more
marine flora and fauna (Andrén et al., 2000a; Sohlenius et al., 2001).
During this time, the inlet transect area in the Öresund Strait was
about twice its size compared to today (Gustafsson and Westman,
2002) and the surface water of the Baltic was more saline, i.e. 10–15 in
the Baltic Proper, than at present, i.e. 7–8, (Gustafsson and Westman,
2002; Emeis et al., 2003; Berglund et al., 2005). However a more
conservative estimate, c. 4 units higher then present, is given by
Donner et al. (1999) for the Gulf of Finland and Gulf of Bothnia based
on the stable isotope composition in bivalve shells. Furthermore,
surface salinity values of c. 7.3–10.3 and 4.8–10.3 (at present c. 3–5)
were reported by Widerlund and Andersson, (2006) between 6770
and 3070 cal. yr BP in the Bothnian Sea and Bothnian Bay, respectively.
The transition is also associated with an abrupt increase in organic
matter content and a clear change from clay to clay-gyttja (often
laminated) in the sediment record (Fig. 2). Due to the continued
melting of the North American and Antarctic ice sheets the sea-level
rise continued until c. 6500–6000 cal. yr BP. Based on studies of
marginal lake basins along the coast of Blekinge, southeastern
Sweden, Yu et al. (2007) demonstrate that the Baltic Sea level rose
steadily from 8600 to 6500 cal. yr BP, with a rapid sea-level rise at c.
7600 cal. yr BP. After this stage the land-uplift was greater than the
almost stable global sea-level and a more continuous regression
Fig. 2. Photo of sediments from core 211660-1 recovered from a depth of 241 m in the
Gotland Deep showing laminated Littorina Sea s. str. clay-gyttja.
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
started. The connections via Öresund and the Danish Straits became
shallower, salinity decreased and present conditions in the Baltic Sea
gradually developed.
4. Evidence for hypoxia in the past
4.1. Laminated sediments
One of the environmental prerequisites for the formation and
preservation of laminated sediments in aquatic environments is, apart
from cyclic sedimentation, the absence of a relatively large benthic
fauna that bioturbate (vertically mix) the uppermost sediments (e.g.
Zillén et al., 2003). Laminated sediments, therefore, tend to be found
in relatively deep basins where vertical mixing of the water column is
limited and hypoxic conditions prevail, which restrict the presence of
bottom fauna (Zillén et al., 2003).
Laminated sediments form in sedimentation basins of the open
Baltic Sea below the permanent halocline, where the vertical mixing
of the water mass is weak (Stigebrandt, 2001; Matthäus, 2006). The
composition of individual laminae in Littorina Sea sediments1 (in the
deep basins of the Baltic Sea is either depositional or authigenic in
origin (Neumann et al., 1997; Burk et al., 2002). Depositional laminae
couplets (or sometimes triplets and quadruplets) can occur and
consist of alternating lithogenic (clay-rich) and diatomaceous (diatom-rich) mud. The diatom component forms mainly during the
spring bloom, while the lithogenic sediment input dominates in
autumn and winter when biogenic production is minimal and storms
redistribute sediment to the deeper basins.
Distinctly laminated authigenic Ca-rich rhodochrosite (Mn,Ca)CO3
laminae occur regularly throughout the Littorina Sea sediments of the
Gotland Deep (Neumann et al., 1997; Burk et al., 2002; Burke and
Kemp, 2004). The precipitation of Mn-carbonates takes place close to
the sediment–water interface and is a result of changing redoxconditions due to episodic oxic water inflows to the generally hypoxic
basin. For a more detailed description about the formation and
composition of laminations in Baltic Sea sediments see e.g. Sohlenius
et al. (1996), Neumann et al. (1997), Sternbeck and Sohlenius (1997),
Sohlenius and Westman (1998) and Burk et al. (2002).
Given that the majority of relatively large benthic faunas cannot
live under hypoxic conditions, the occurrence of laminae in sediment
records is a good indicator of past bottom water hypoxia in aquatic
environments. However, the preservation of laminations can be
affected by other post-depositional disturbances, such as erosion by
water movements created by currents (Larsen et al., 1998). Sediment records displaying alternating laminated and homogeneous
sequences could also be subjected to bioturbation at the transition
zone. When bottom environments change from hypoxic to more oxic
conditions benthic fauna communities are reestablished, which can
homogenize the upper layers of the earlier deposited laminated
interval. Therefore, the age approximation of the boundary between
laminated and homogeneous sediments (i.e. the change from hypoxic
to more oxic conditions) is a maximum estimate. In this review,
hypoxic conditions will be ascribed to the formation and preservation
of laminations in sediment records.
4.2. Geochemistry
The effect of redox conditions on the distribution of trace elements
(e.g. Mn, Mo, U, V, Cu and Zn) has been discussed in a number of
studies (e.g. Sohlenius and Westman, 1998; Sohlenius et al., 2001). It
has been demonstrated that trace elements accumulate in sediments
deposited under reducing conditions and several of these have been
1
From now on and throughout this paper the term Littorina Sea sediments refers to
sediment deposited after the Early Littorina Sea/Littorina Sea s. str. transition (i.e. after
c. 8500-8000 cal. yr BP based on traditional 14C-dating) if not stated otherwise.
81
reported to be enriched in laminated sediments of Littorina Sea age in
the Baltic Sea (Sternbeck et al., 2000). The presence of iron sulfides
(e.g. pyrite and greigite) in sediments is also thought to reflect reduced
or hypoxic bottom conditions (Snowball, 1993). However, the primary
distribution of both trace elements and Fe-sulfides may be altered by
diagenetic changes occurring after sediment deposition (Snowball
and Thompson, 1990). Studies show that changes in redox conditions
can lead to sulfide diffusion and pyritization of earlier deposited
sediments (Thompson et al., 1995) and that the primary signals can be
altered by diagenetic changes occurring several thousands of years
after sediment deposition (Sternbeck et al., 2000). There are also a
number of different mechanisms by which trace elements may be
enriched in sediments. Thus, the absence or presence of trace
elements in sediments should be used with caution when interpreting
ancient sedimentary environments. Therefore, we have only used the
presence of laminae in sediments as a proxy for hypoxic/anoxic
conditions since it has been shown to be the most prominent/
undisputable evidence of oxygen depletion (Sohlenius et al., 1996).
Studies of recent marine sediments have not shown any clear-cut
and systematic relationships between bottom water oxygen concentrations and the sediment organic carbon content (Burdige, 2007).
While enhanced preservation of organic material may occur due to
changes in macrofaunal abundance or changes in reducing equivalents, oxygen per se has no demonstrated effect on carbon preservation or remineralization (Cowie et al., 1995).
4.3. Chronology
Good chronological control is necessary to understand past
environmental and climate changes (Zillén et al., 2003). On millennial
time-scales, sediments from the Baltic sub-basins mostly rely on timescales based on measurements of the 14C content in organic material.
However, this dating method can be biased by unidentified marine/
brackish 14C reservoir effects (Köningsson and Possnert, 1988),
redeposition of older carbon from shallower areas to deep accumulation bottoms, and uncertainties arising from inferred ages derived
from interpolation between individually dated levels. The sediments
in the Baltic Sea are characterized by low organic carbon contents
(usually b10%) and deficiency in macrofossil remains (especially in the
deeper basins), which further makes it a delicate task to construct an
accurate chronology based on 14C-dating. In the majority of studies
dating is performed on bulk sediment samples, but should if possible
be performed on macrofossils, preferably terrestrial, since bulk
sediment samples are known to show too old ages, and poor temporal
resolution (at best c. 5–10 14C-dates in sediment sequences covering
the last c. 10 000 years) often resulting in great uncertainties in the
estimation of sedimentation rates.
The 14C-ages presented here are calibrated (1σ) by using the Oxcal
calibration program v4.0 (Bronk Ramsey, 2001) and expressed as cal.
yr BP (calendar years before 1950). In order to simplify the comparison
of the different marine records the ages are not corrected for marine
reservoir effects since they are inconsistent and poorly known in the
Baltic Sea. For example, Andrén et al. (2000a) corrected the calibrated
ages for a marine reservoir effect ranging from 100 to 300 years and
used a correction age of 400 years to compensate the contribution of
old resuspended carbon in the Gotland Basin, while Hedenström and
Possnert (2001) estimated a total reservoir effect of 700–1000 14C yr
from analyses of coastal Littorina Sea s. str sediments.
5. Occurrence of hypoxia
5.1. During modern time (last 50 years)
From water quality monitoring records, the bottom surface area
covered by hypoxic water averaged c. 41000 km2 over the time period
1970–2000 (Conley et al., 2002). The smallest hypoxic area occurred in
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L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
Fig. 3. Modern variability of hypoxia in the Baltic Proper (after Conley et al., 2002).
1993 during the peak of the “stagnation period” when major salt
water inflows were at their minimum (Stigebrandt and Gustafsson,
2007) with only 12 000 km2 of bottom covered by hypoxic waters. The
peak in hypoxic area was in 1971 following large salt water inflows in
previous years (Conley et al., 2002). While major salt water inflows are
well known to displace and renew bottom water oxygen supplies
(Matthäus, 2006), it is less well appreciated that they also create large
areas of stratification where oxygen can be depleted (Gerlach, 1994;
Conley et al., 2002). In modern times it has been typical that most of
the deep basins are continuously hypoxic including the Gotland Deep,
Landsort Deep, NW Baltic Proper, and Gdansk Deep (Fig. 3) with
permanent anoxia in the deepest parts of the Gotland Deep (Conley
et al., 2002). During years with large areas of hypoxia, low oxygen
zones migrate higher up in the water column and the different basins
become connected to form one large hypoxic area (Fig. 3).
In the Gulf of Finland salt water inflows create stratification, and
together with the small volume of water under the halocline and
coupled with high productivity, oxygen is rapidly depleted below the
halocline (Laine et al., 2007). Trends in hypoxia in modern times in the
Gulf of Finland are related to variations in salt water inflows, with less
stratification and less hypoxia during “stagnation periods” when salt
water inflows are reduced.
Fig. 4. Figure showing the expansion of laminated bottoms in the Baltic Proper during
the last 100 years (after Jonsson et al., 1990). Laminations deposited before 1940 are
considered as natural while post-1960 laminae formation is regarded as a result of
human impact. The study was based on results from 29 coring sites and ages of the
sediments were obtained by counting the individual laminae.
5.2. During modern historical time (AD 1950–AD 1800)
Historical proxy records of hypoxia, defined here as the sediment
archives deposited during the last 50–200 years in the Baltic Sea, show
that laminations are prevalent in the sediments (Gripenberg, 1934;
Ignatius et al., 1968; Jonsson et al., 1990; Hille, 2005). Jonsson et al.
(1990) reported laminations in 8 out of 29 coring sites in the Gotland
Basin, Landsort Deep and Bornholm Basin with the upper 40 cm of
sediment consisting of 100–200 continuously and annually deposited
laminae. Based on the distribution of laminations in surficial sediments,
Jonsson et al. (1990) summarized the expansion of laminated bottoms in
the Baltic Proper with rapid increases in the presence of laminated
sediments following 1960 (Fig. 4), the time period when eutrophication
rapidly increased in the Baltic Sea. Hille (2005) sampled the upper 1 m of
sediments at 53 stations at depths deeper than 150 m in the Gotland
Basin and mapped the spatial distribution of laminated sediments. At
almost all stations the uppermost sediments were distinctly laminated,
and had accumulated over homogeneous non-laminated sediments.
Fig. 5. Bathymetric map over the Gotland Deep showing the extension of laminated
sediments deposited 100 years ago to the present (after Hille, 2005). The result is based
on samples of the upper sediments at 53 stations. The age estimates are based on the
210
Pb dating technique.
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
Based on 210Pb dating, Hille (2005) observed that the laminated
sediment structures started to form, basin wide, at depths greater
than 150 m, 100 years ago basin-wide in the Gotland Basin (Fig. 5).
Laminated sediments were reported from a few stations at shallower
depths less than 150 m, which suggest that the area with lamina
formation could be even greater than shown in Fig. 5. Gripenberg (1934)
recovered about 80 short (b30 cm) sediment cores during the years
1924–1928. She describes sediments with stratifications (some rich in
hydrogen sulfide) in clayey muds in both coastal and deeper basins
within the Bothnian Bay, Bothnian Sea, Gulf of Finland and N Baltic
Proper. Although the ages of these laminated sediment cores have not
been estimated, they should represent the period from the turn of the
century to the time of collection in 1924–1928 to be consistent with Hille
(2005) and Jonsson et al. (1990).
5.3. On geological time-scales (AD 1800–10 000 cal. yr BP)
A compilation of the geological records reporting the occurrence
of Holocene laminated sediments (i.e. the proxy used for occurrence
83
of past hypoxia in this paper) in the Baltic Sea during the last
c. 10 000 cal. yr BP is presented in Fig. 6. Owing to the fact that
accumulation basins have the potential to provide long undisturbed
sediment sequences, the majority of the sediment cores were
therefore recovered from the deeper basins in the Baltic Sea (see
Fig. 1.) The core lengths covering the last c. 10 000 years vary between
c. 4 and 10 m. In most studies, data from the uppermost sediments (i.e.
the last c. 100–200 years) are missing due to sediment lost during the
coring operation.
All studies suggest that laminations started to form in the deeper
depressions of the Baltic Sea (i.e. Gotland Deep, Landsort Deep, NW
Baltic Proper, North Central Baltic Proper, W Gotland Basin and
Gdansk Deep) c. 8500–7800 cal. yr BP ago, in association with the
formation of a permanent halocline at the transition between the
Early Littorina Sea and the Littorina Sea s. str. (e.g. Witkowski, 1994;
Sohlenius et al., 1996; Lepland and Stevens, 1998; Andrén et al., 2000a;
Harff et al., 2001; Emeis et al., 2003). The deeper basins have
experienced intermittent hypoxia during most of the Holocene except
at depths N250 m in the Landsort Deep where sediment cores show
Fig. 6. Figure showing the occurrence of laminated sediment plotted on time scales based on calibrated uncorrected 14C-ages (1σ; Oxcal calibration program v4.0, Bronk Ramsey,
2001), predominantly preformed on bulk-sediment samples. In a few studies, the ages of the sediment are based on a combination of calibrated 14C-dates and palaeomagnetic secular
variations or biological/physical correlations. Note that the calibrated uncorrected 14C-ages presented here, may vary 500–1000 years in relation to terrestrial dates due to reservoir
effects (e.g. Andrén et al., 2000a; Hedenström and Possnert, 2001). In cases of questions regarding lithological descriptions a question mark is shown in order to illustrate the
uncertainties in the stratigraphy. The records are presented in order of water depth at the coring site with greater water depths to the right. The data are obtained from various
research papers, which are presented in Table 1. Most of the long sediment records are missing the uppermost sediments during the last 200–100 years.
84
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
more continuous laminated structure (Fig. 6). A more detailed
description divided by basin is presented below.
5.3.1. Bothnian Sea, Bothnian Bay, Gulf of Finland and Archipelago Sea
Studies of the Bothnian Sea and Bothnian Bay reveal that laminated
sediments have been deposited at various depths within these basins at
least during the beginning of Littorina Sea s. str. (i.e. from c. 6900 cal. yr
BP). The upper limit of the laminated sequences is not dated, but the
thickness of the deposit is about 1 m, which, considering the estimated
sedimentation rates of 1.76–2 mm yr− 1 (Jerbo, 1961; Ignatius et al., 1968)
corresponds to about 500–600 years. In the Bothninan Sea continuous
laminated sequences have been reported in coastal areas in sediments
of the age of c. 8000–1000 cal. yr BP (Ignatius et al., 1968; Fig. 6.).
However, the substandard sediment description (white question mark)
and the dating (based on correlations to pollen zones) make the
interpretation of that record uncertain. In both the Bothnian Sea and
Bothnian Bay, sulfide banding is frequently recorded in both Early
Littorina clays and in younger sediments (Jerbo, 1961; Fig. 6).
In parts of the Gulf of Finland, laminated sediments were deposited
from the Early Littorina Sea-Littorina Sea s. str. transition at c. 8000 cal.
yr BP until at least c. 5000 cal. yr BP (Åker et al., 1988; Fig. 6). In this
area, sulfide bands are reported in sediments older than Littorina Sea
s. str. age. Periods of laminated sediments have been recorded in the
Archipelago Sea during the majority of the Littorina Sea s. str. stage
(i.e. c. 7600–2800 cal. yr BP; Virtasalo et al. 2005, 2006; Fig. 6). It
should be noted that some of the above mentioned sediment cores
were recovered from areas situated in relative shallow and sheltered
coastal areas (see Fig. 1).
5.3.2. Gotland Basin
We show six records from the Gotland Basin, which in most of the
cases are a compilation of several sediment cores (see Table 1.). The
majority of the sediment sequences display periods of alternating
laminae and homogeneous sediment deposition. Laminae deposition
started to form between 8500 and 7800 cal. yr BP. Records 2, 3, 5 and 6
show three periods of laminae formation and preservation during the
last c. 8500 cal. yr BP (Fig. 6; Table 1). These periods are centered
around c. 8000–7000, 6000–4000 and 2000–800 cal. yr BP. Records 1
and 4, display continuous laminae formation between 8300–5800 and
8500–4600 cal. yr BP, respectively (younger sediments deposited
above the laminated sequences were not sampled). However, a
substandard sediment description makes it difficult to determine
whether the latter sediment sequences contain homogeneous sediments between 6000 and 7000 cal. yr BP or not (Fig. 6).
Four of the records (i.e. 2–5) display iron sulfide banding in
sediments pre-dating Littorina Sea s. str. age.
5.3.3. NW and NC Baltic Proper
The five records from the NW and NC Baltic Proper show similar
alternating periods of laminated and homogeneous sediment
Table 1
Number, core label, water depth (m), lithological description, dating method, analyses preformed and references presented in Fig. 6
No Core label
Water depth Lithological
(m)
description
Dating
Bothnian Bay (BB), Bothninan Sea (BS), Gulf of Finland (GF) and Archipelago Sea (AS)
1
J5, J1, 19, 20, 22, 27
50–100
2
n/a
n/a
Graphical
Inferred from other source
3
AS6-VH1; AS6-VH2
66;64
Text, graphical
PM, 14C
14
C
4
Core C
40
Table, B/W Photo
Gotland Basin
1
GD0101
2
G94-5; G95-3
3
20048-1; 20048-4;
20007-1
4
n/a
5
6
211660-1; 211660-2
211660-5; 211660-6
Gdansk Deep
1
96
200
206
240
238
Table, graphical
Table, graphical
Digital core scan,
graphical
Table, graphical
241
241–249
Table, graphical
Graphical
101
Table
NW Baltic Proper (NWBP) and NC Baltic Proper (NCBP)
1
9106
73
Graphical
2
9208
83
Graphical
14
Analyses
References
X-ray diffraction
MS, LOI, X-ray, fossils
Diatoms, LOI, pollen
Jerbo, 1961; Jerbo and Hall, 1961 (BB, BS)
Ignatius et al., 1968 (BS)
Virtasalo et al., 2005;2006 (AS)
Åker et al., 1988 (GF)
14
C
C,
14
C
210
Diatoms, pigments, vce
Diatoms, TOC, d13C
Diatoms, TOC, vce
Borgendahl and Westman, 2007
Andrén et al., 2000a
Sohlenius et al., 2001; Emeis et al., 2003; Miltner et al.,
2005
14
C
Diatoms, TOC, vce
Sternbeck et al., 2000; Sternbeck and Sohlenius 1997;
Sohlenius et al., 1996
14
210
C,
Pb
Diatoms, TOC, vce
Andrén et al., 2000a
PM, and correlation to 211660-1 Fossils, chlorophyll, TOC, Harff et al., 2001; Dippner and Voss 2004; Kunzendorf
vce, geo statistic
and Larsen, 2002; Alvi and Winterhalter, 2001; Brenner,
2001a; Voss et al., 2001; Kotilainen et al., 2000; Nytoft
and Larsen, 2001
14
210
Diatoms,vce
C,
Pb
Pb
14
Diatoms, TOC, vce
Diatoms, pigments,
TOC, vce
14
C
Diatoms, TOC, vce
No direct datings, PM, 14C, both Fossils, vce
inferred from other cores
MS
PM, 14C
Westman and Sohlenius 1999 (NWBP)
Bianchi et al., 2000; Westman and Sohlenius 1999
(NWBP)
Westman and Sohlenius 1999 (NWBP)
Brenner 2001b; Nytoft and Larsen 2001 (NCBP)
Landsort Deep (LD) and W Gotland Basin (WGB)
9205
106
Graphical
14
Bianchi et al., 2000; Westman and Sohlenius 1999 (LD)
2
Psh-2537
155
Table, graphical
14
3
n/a
221
Text, graphical
14
4
9302
250
Graphical
14
Text, graphical
Text, graphical
14
3
4
9102
211670-4; 211670-7
111
175
Graphical
Graphical
5
GC-4
175.8 m
Graphical
5
6
14754-4
n/a
252
459
C
C
Witkowski, 1994
14
C
C
C
C
C
C
14
Diatoms, pigments,
TOC, vce
Diatoms, pollen, Grain
size, C. org, Vce
TOC, vce
Diatoms, pigments,
TOC, vce
Diatoms, pollen
TOC, vce
Kotilainen et al., 2001 (NWBP)
Elmelyanov et al., 2001 (WGB)
Lepland and Stevens 1998; Böttcher and Lepland, 2000
(LD)
Bianchi et al., 2000; Westman and Sohlenius 1999 (LD)
Thulin et al., 1992 (LD)
Lepland and Stevens 1998 (LD)
PM = Palaeomagnetic dating; MS = Magnetic susceptibility; LOI = Loss on ignition; TOC = Total organic carbon; vce = various chemical elements.
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
deposition as those in the Gotland Basin (Fig. 6). However, two of the
records (i.e. 1 and 3) are missing data for the last c. 6000 and 4300 cal.
yr BP, respectively, due to non-deposition or erosion of sediments.
Laminations begin to form between 8400 and 8000 cal. yr BP. A short
period of homogeneous sedimentation is evident between 8000 and
7000 cal. yr BP in record 1 and 3, while records 4 and 5 from the
NC Baltic Proper display longer periods of homogeneous sedimentation during the early Littorina Sea s. str. i.e. between c. 8000 and
6000 cal. yr BP. In the three continuous sediment sequences homogeneous sediment deposition dominates after c. 5000 cal. yr BP until
c. 2600 cal. yr BP (record 2) and c. 1400 cal. yr BP (records 4 and 5),
where after a period of c. 500 year of laminated sediments are
deposited. Record 4 also displays laminae formation during the last
c. 100 years.
5.3.4. Landsort Deep and W Gotland Basin
Three sediment records (i.e. 1, 3 and 4) from the Landsort Deep are
characterized by alternating laminated and homogeneous sedimentation as in the Gotland Basin and NW and NC Baltic Proper, while the
remaining records (i.e. 2, 5 and 6) display only one long interval of
laminae formation extending the majority of the sediment sequences
analysed (Fig. 6). Laminations start to form in all complete records
between 8400 and 7800 cal. yr BP. In records 1 and 3, laminations dominate the sediment structure from that period until
c. 5000 cal. yr BP, while record 4 is very similar to 4 in the NC Baltic
Proper and displays a dominant homogeneous sedimentation during
the early Littorina Sea s. str. Homogeneous sedimentation dominates
after c. 5000 cal. yr BP, where after a period of Late Littorina Sea
laminae formation occurs between c. 2000 and 1600, 3300 and 1600,
1800 and 1000 cal. yr BP in records 1, 3 and 4, respectively. Record 2,
from the West Gotland Basin (south of Landsort Deep) may imply that
laminated sediments were deposited there persistently, at a depth of
155 m, between c. 7800 and 2000 cal. yr BP, but sediment descriptions
are not sufficient enough to rule out the presence of non-laminated
phases. Records from the deepest part of the Landsort Deep (no. 5 and
6) show continuous laminae deposition from 6800 and 5200 cal. yr BP
to present, respectively, although these shorter cores do not have the
complete Holocene record. In core 6 two Mn(II)-precipitate rich
sapropel intervals 1390–1990 cal. yr BP are found indicating an
expansion of bottom water anoxic conditions. One record from
these areas report the occurrence of sulfide banded Early Littorina
clay (i.e. 3).
5.3.5. Southern Baltic Sea
Investigations from the shallower southern Baltic suggest that the
Bornholm and Arkona basins were frequently oxygenated during the
Holocene and that sediments from that region were bioturbated,
although geochemical records imply more reduced conditions after
the Early Littorina–Littorina s. str transition (Sohlenius et al., 2001;
Moros et al., 2002). Andrén et al. (2000b) reports frequent occurrence
of Macoma shell throughout a sediment core from the deepest part of
the Bornholm Basin since c. 6500 cal. yr BP. Frequent shell debris of
Macoma baltica is also reported in a long sediment sequence from the
Gdansk Deep at a water depth of 88 m (Witkowski, 1994). However, in
the deeper Gdansk Deep, at a depth of 101.5 m Witkowski (1994)
describes continuous laminated sediments between c. 9000 and
3300 cal. yr BP (Fig. 6). No long laminated sediment sequences have
been reported from Mecklenburgian Bay or Gulf of Riga (e.g.
Kowalewska, 2001; Rößler, 2006).
6. Discussion
In the very deepest areas of the Baltic Sea, below 250 m in the
Landsort Deep, and in some coastal areas, hypoxia has been present
more or less continuously during the last c. 8500–7800 cal. yr BP. At
shallower water depths (above 250 m), but below the permanent
85
halocline, there are 3 major periods of hypoxia: during the early
Littorina Sea s. str. (c. 8000–4000 cal yr. BP), during the middle Late
Littorina Sea (c. 2000–800 cal. yr BP) and during the last c. 100 years,
although an intensification of hypoxia after 1950 has occurred. Three
periods of homogeneous sedimentation are prominent in most of the
records: between c. 7000–6000, c. 4000–2000 and c. AD 1200–1900.
In the following sections we will discuss the spatial and temporal
variability in hypoxia within the Baltic Sea and explore possible
driving factor(s) for hypoxia especially focusing on the role of climate
and anthropogenic pressures.
6.1. Hypoxia in time and space
6.1.1. Bothnian Bay, Bothnian Sea, Gulf of Finland and the Archipelago
Sea
Short laminated sequences have been registered (between c. 500
and 600 yr in duration) in early Littorina s. str. sediments in the
Bothnian Sea and Bothninan Bay by Jerbo (1961). High relative sealevel (c. 100–150 m above present) together with limited freshwater
input (Gustafsson and Westman, 2002) lead to higher salinities,
increased stratification (reduced mixing) enhancing the conditions
needed for the occurrence of hypoxia and the deposition of laminated
sediments in the northern basins during the early development of the
Baltic Sea. The salinity gradually decreased throughout the Littorina
Sea and the halocline in the northern Baltic diminished. To our
knowledge, no laminated sediments from the open sea of younger age
have been reported in this region except for those of modern historical
age by Gripenberg (1934).
The Gulf of Finland has experienced periods of severe hypoxia in
modern times (Laine et al., 2007). The geological records from Gulf of
Finland and the Archipelago Sea demonstrate that these regions were
affected by hypoxia during most part of the more saline Littorina Sea s.
str (Åker et al., 1988; Virtasalo et al., 2005, 2006). However, well
described long sediment records from these regions are sparse
(Heinsalu et al., 2000) and those presented here are recovered from
shallower coastal areas, which may reflect hypoxic conditions
influenced by local environmental factors, such as topography and
water currents, rather than regional signals of past hypoxia.
6.1.2. Baltic Proper
The Baltic Proper is the main basin affected by hypoxia during the
Holocene. However, due to the large dating uncertainties, it is difficult
to determine precisely if the separate periods with lamina formation
in this area occurred synchronously (Fig. 6). Better chronological
control is needed for sediment cores from the deeper basins, as most
studies are based on 14C dating on bulk sediment samples with the
associated problem of contamination from old reworked carbon and
unknown brackish/marine reservoir ages and their temporal variability. It can, however, be argued that the alternating periods of
laminae and homogeneous sediment deposition in the Gotland Basin,
NW and NC Baltic Proper and Landsort Deep coincide within the error
estimates associated of the individual chronologies and that such
deposition has occurred basin wide at depths varying between 73 and
250 m in the Baltic Proper during the last c. 8500–7800 cal. yr BP
(Fig. 6).
In order to generalize the discussion about the Baltic Proper and
possible trigging mechanism for open sea hypoxia, we show data from
only two sediment cores (211660-1 and 20048-1), covering the last
c. 10 000 years, recovered from the Gotland Deep (e.g. Andrén et al.,
2000a; Emeis et al., 2003). These two sediment cores (Fig. 7) are
representative of conditions in the Gotland Deep at a water depth of
c. 240 m (they show almost identical stratigraphy in comparison with
at least 6 other cores; see Table 1) and were selected because they
provide (i) well established chronologies, (ii) detailed sediment
descriptions, and (iii) data from various analyses, which can be used
in the analysis for the driving mechanisms for hypoxia. Terrestrial
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L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
palaeoclimate data obtained from lake sediments in the Baltic Sea
drainage area (see Fig. 1) were compiled for comparison to the hypoxia
records (Fig. 7).
6.1.2.1. Littorina Sea s. str. (8500–4000 cal. yr BP). From c. 8500–7800
to c. 4000 cal. yr BP laminated sediments were formed in all the
deeper basins (except for a shorter period between 7000 and 6000 cal.
yr BP) in the Baltic Proper (i.e. Gotland Deep, Landsort Deep, NW and
NC Baltic Proper, W Gotland Basin and Gdansk Deep; Fig. 6), which
suggest that these areas were affected by hypoxia during most of the
Littorina Sea s. str. At this time, the relative salinity in the Baltic Proper
was high (10–15; Gustafsson and Westman, 2002; Emeis et al., 2003),
most likely influenced by high sea-levels between c. 8000 and
6500 cal yr BP (Yu et al., 2007), as well as dry conditions (Snowball
et al., 2004) minimizing the freshwater discharge to the basin, and the
upper limit of the hypoxic zone was probably determined by a strong
stratification. Laminae formation also corresponds to high TOC-values
(6–8%; reflecting high organic accumulation) in the Baltic, and less
negative δO18 values (reflecting dry conditions and low net precipitation), low lake-levels, high annual mean temperatures and low χhf
values (indicating low precipitation in the form of snow and
associated reduced catchment erosion rates during spring snowmelt; Fig. 7) in the terrestrial surroundings.
The time period between 9000 and 5000 cal. yr BP is known as the
Holocene Thermal Maximum (HTM), which in Northwest Europe is
characterized by relatively high summer insolation, high atmospheric
temperatures and dry conditions (Snowball et al., 2004). The HTM
caused the possible disappearance of glaciers in the Scandinavian
mountains (Dahl and Nesje, 1994; Snowball and Sandgren, 1996; Nesje
et al., 2001), high sea surface temperatures in the North Atlantic (Koç
et al., 1993), a pine tree-limit c. 300–400 m higher than today
(Kullman, 1999; Barnekow, 2000), lower lake-levels and decreased
humidity in southern Sweden (Digerfeldt, 1988; Hammarlund et al.,
2003) and increased summer atmospheric temperatures as recorded
by pollen analyses (Seppä and Birks, 2001; Seppä et al., 2005) and
chironomid reconstructions (Korhola et al., 2002).
Fig. 7. Figure showing the occurrence of laminated sediments (Andrén et al., 2000a), total organic carbon content (TOC; Andrén et al., 2000a) and salinity estimates (Emeis et al.,
2003) derived from 13C/12C ratio of organic carbon, plotted against a calendar year time scale, which is corrected following Andrén et al. (2000a) i.e. we have applied a marine
reservoir effect of 100–300 yr in sediments older than 8000 cal. yr BP, a marine reservoir effect of 300 yr in sediments younger then 8000 cal. yr BP and due to the present of old
resuspended carbon in the Baltic Sea an additional correction age of 400 yr is applied throughout the sequence. Also shown are modelled salinity estimates (shaded curve) after
Gustafsson and Westman, (2002) based on a compilation of records. The chronology of the latter curve is based on 14C dates of various materials, including macrofossil remains, and is
adjusted for a marine reservoir effect of 300 years. The sediment records suggest that during the last c. 8500–7800 cal. yr BP hypoxia has occurred intermittently in sub-basins in the
Gotland Deep. Note that periods of laminated sediments coincide with intervals of high TOC content and high salinity estimates. Marked are also the timing of the Holocene Thermal
Maximum (HTM) and the Medieval Warm Period (MWP). Also, terrestrial palaeoclimate data obtained from Swedish lake sediments i.e. (1) oxygen isotopes reflecting effective
humidity/net precipitation (Hammarlund et al., 2003), (2) regional reconstruction of lake-level fluctuations reflecting past hydrology changes (Digerfeldt, 1988), (3) pollen-based
annual mean temperature reconstructions (Seppä et al., 2005), and (4) high-field magnetic susceptibility (χhf) which is a signal of catchment erosion (Snowball et al., 2002). The timescales for Lake Igelsjön, Bysjön and Flarken sediment sequences are based on 14C dating of terrestrial macrofossil remains while the chronology for Lake Sarsjön sediment sequence,
which is annually laminated, is established by varve counting.
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
The interval between 7000 and 6000 cal. yr BP is characterized by
homogeneous sedimentation and corresponds to a decline in the
salinity estimates (Emeis et al., 2003), lower TOC-values, increased
effective humidity (more negative δO18-values and rising lake-levels),
enhanced catchment erosion (a peak in the χhf record) and a c. 0.5 °C
drop in temperature (Fig. 7). This event has also been recorded in
varved lake sediments in west central Sweden (Zillén, 2003), where it
was interpreted as a response to colder climate conditions with
increased winter snow accumulation. It also coincides with one of the
major ice-rafting episodes in the North Atlantic (Bond et al., 1997) and
glacier advances in northern Sweden (Karlén and Kuylenstierna,
1996).
6.1.2.2. Late Littorina Sea (4000 cal. yr BP–AD 1800).
A period of
homogeneous sediment accumulation is evident in most records
between c. 4000 and 2000 cal. yr BP (Fig. 6), and corresponds to lower
salinity estimates (c. 4–10; Gustafsson and Westman, 2002; Emeis
et al., 2003) and lower TOC concentrations (c. 3–4%; Andrén et al.,
2000a; Fig. 7). The terrestrial data-sets show a marked change around
4000–3700 cal. yr BP and coincide with the Late Holocene climate
development in Northwestern Europe, which was characterized by
decreasing temperatures and more moist conditions (e.g. Hammarlund et al., 2003; Snowball et al., 2004; Seppä et al., 2005; Fig. 7). In
Scandinavia the start of this period (4000–3500 cal. yr BP) was
marked by tree-line decline in the Scandinavian mountains (Barnekow, 2000) and glacier advances (Nesje and Kvamme, 1991; Snowball,
1996). During this period, bottom water in the Baltic Proper became
oxic promoting the re-establishment of a benthic fauna and
bioturbation of sediments.
From c. 2000 to 800 cal. yr BP laminated sediments were again
deposited, basin wide, in the Baltic Proper (Fig. 7). The laminae
deposition corresponds to maximum TOC concentrations (8.4%)
around 900–800 cal. yr BP in the Gotland Basin, which was interpreted
by Andrén et al. (2000a) as a result of high primary production.
The lake records show less negative δO18-values, a lowering of
lake-levels, increased temperatures and low χhf values between
c. 1500 and 800 cal. yr BP (Fig. 7). The laminae deposition and the
marine and terrestrial paleoclimate and palaeoenvironmental changes
presented here, overlap with the timing of the climate anomaly
known as the MWP (or Medieval Climate Optimum-MCO) when
temperatures were probably c. 0.5–0.8 °C higher than today (e.g. Briffa
et al., 1990). However, the salinity reconstructions by Gustafsson and
Westman (2002) and by Emeis et al. (2003) are inconsistent during
this time period and onwards. The estimates by Emeis et al. (2003)
show a steady increase in salinity from about 3000 cal. yr BP, although
higher salinities during this period are not supported by diatoms. The
reconstruction of salinity by Gustafsson and Westman (2002), which
are based on modeling of the physical process of saltwater input from
an expanded opening of the Danish Straits and physical mixing of
water masses, imply low and relatively stable values from c. 2000 cal.
yr BP to the present and agrees better with the diatom records. In the
absence of multiple high-resolution salinity reconstructions, it is
therefore difficult to determine if the salinity increased during this
time interval or not, even though the terrestrial palaeoclimate records
suggest decreased freshwater discharges.
The period of hypoxia overlapping the MWP also correlates with
population growth and large-scale changes in land use (see Fig. 8) that
occurred in much of the Baltic Sea watershed. Watershed studies show
large increases in nutrient loading with cutting of forests (Likens et al.,
1970). Reconstructed phosphorus concentrations and associated
eutrophication of lakes (Fig. 8) began during the Bronze and Iron
Ages (1700 BC–AD 1050) with rapid increases in nutrient loading
associated with major changes in agriculture during the Medieval
period (Bradshaw et al., 2005). From high diatom-inferred total
phosphorous values, Renberg et al. (2001) concluded that Lake
Mälaren, Sweden, was culturally eutrophicated already from Medieval
87
time and that even more nutrient-rich conditions developed after c.
AD 1850. The early-medieval expansion was followed by a period of
stagnation and population decline in the late 14th and early 15th
centuries (Fig. 8) mainly due to the Black Death. In Sweden the
population decreased from 1100 000 to 347 000 from AD 1300 to AD
1413 (Andersson Palm, 2001; Fig. 8). This decline is known as the latemedieval crisis and was characterized by decreased total production
and abundance of farms (Lagerås, 2006). It affected most parts of
Europe and had an impact on all levels of society (Lagerås, 2006).
During this period (after c. AD 1200), the bottom waters in the
Baltic Sea became more oxic and homogeneous sediments were
deposited until the beginning of the 19th century (Fig. 7). In
Northwestern Europe, this period is characterized by a climate
deterioration with the onset of the Little Ice Age (LIA) in the 14th
century when glaciers advanced in the Scandinavian mountains and it
was c. 1 °C cooler than today (Matthews and Briffa, 2005). This
alteration is also visible in the Baltic Proper as a change in the diatom
composition dated to c. 650–750 cal. yr BP (Andrén et al., 2000a).
6.1.2.3. Modern historical time period to present (AD 1800 to present).
Laminae of historical age have been reported from the Baltic Proper
with appearance of laminae around 200–100 years ago with a modern
expansion from 1940 to the present (Jonsson et al., 1990). However,
the age assumptions were based on counting individual laminae in
short sediment cores. Applying that technique requires identification
of an annual sedimentation cycle that explains the seasonally
deposited laminae and their composition (Zillén et al., 2003); no
such sedimentological model is presented in their paper and their age
estimates should therefore be considered with caution. However, Hille
(2005) showed that continuous laminae formation are present basin
wide at depths N150 m in the Gotland Basin beginning 100 years ago
based on 210Pb dating. Interestingly, Gripenberg (1934) reported
laminated sediments and hydrogen sulfide formation of approximately the same age (i.e. 100 years) in the Bothninan Bay, Bothnian
Sea, Gulf of Finland and N Baltic Proper. In addition, the historical
sediment records imply that large areas of the Baltic Sea were hypoxic
at least from AD 1900 (Jonsson et al., 1990) and that the deeper bottom
waters of the Baltic Proper have been depleted of oxygen since then
and up to the present time.
The start of the industrial revolution in Northwestern Europe around
AD 1850 dramatically increased human impact in the Baltic Sea
drainage area. The Swedish population grew about 0.8% per year from
AD 1801–1900 (i.e. from 2.35 to 5.14 million people; Andersson Palm,
2001; Fig. 8) and the agricultural production increased with 0.5% per
year and capita between AD 1750 and AD 1850 (Larsson and Olsson,
1992). Around AD 1850 digging of drainage ditches intensified in
Fennoscandia (often recorded as a massive clay layer in individual lake
sediments; Petterson, 1999; Ojala, 2001; Zillén et al., 2003) and the
forest industry rapidly expanded. Between AD 1850 and1880 Swedish
export of wood increased about 4 times (from 667000 to 2763000 m3
per year; Larsson and Olsson, 1992). In Finland the same number
increased 3 times between AD 1830 and 1870 (Schybergson, 1973). The
forests were heavily exploited and numerous saw mills were built along
the main rivers in Fennoscandia. For example, in Finland, the number of
saw mills doubled between AD 1840 and 1870 (Schybergson, 1974).
Maps show that during the 19th century and the beginning of the
20th century, large areas in southern Sweden were sparsely vegetated.
Grazing, cultivation and deforestation were very intensive. It is known
from historical sources that the most treeless period in the area
occurred during this period (Emanuelsson, 1993). As an example, at
the end of the 18th century oak stands had been destroyed in many
areas and oaks were therefore protected by a Royal Ordinance. Studies
of Swedish lake sediments show that after the reorganization of the
agrarian system during 19th century soil erosion and accumulation of
allochtonous matter in the lake increased substantially and subsequent bottom water anoxia developed around AD 1900 (Olsson et al.,
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L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
Fig. 8. (A) The Swedish population AD 1300–2000 as estimates by Andersson Palm, (2001). (B) The reconstructed P profiles on a calendar year time-scale BC/AD from Dallund Sø,
Denmark after Bradshaw et al. (2005). (C) Vegetation/land use changes in Southern Sweden on a BC/AD time-scale illustrated as approximate proportions of three main land use
categories based on pollen diagrams after Berglund et al. (1991). Coast refers to the coastal landscape and inland to the inner landscape.
1997). Significant changes in diatom assemblages attributed to
eutrophication dated to c. AD 1850–1900 have been observed in
estuaries of the Baltic (Clarke et al., 2006) and in the southern Baltic
Sea (Witkowski and Pempkowiak, 1995; Andrén et al., 1999).
6.2. Hypoxia and driving mechanisms
From the discussion in Section 6.1., it is clear that climate and
anthropogenic pressures both have played a role as drivers of hypoxia
through time in the Baltic Sea. Climate can be a short-term driver
influencing hypoxia on the scale of years through variations in deep
water inflows (Conley et al., 2002) and on longer time-scales with
influencing freshwater inflow and the overall salt balance of the Baltic
(Gustafasson and Westman, 2002).
The occurrence of hypoxia in the Baltic basin on geological timescales (prior to more intense human impact) as reconstructed in this
study, seems to be related to two primary forcings: increased salinity
and increased productivity in a warmer climate. In contrast, homogeneous sedimentation corresponds to more oxic conditions and
weakening of those two driving factors.
The salinity in the Baltic Sea varies due to changes in both saltwater
inflow and freshwater input (Stigebrandt, 2001; Gustafsson and
Westman, 2002), where the former is controlled by the depth of the
salinity surface in the inlet areas (Stigebrandt, 2001) and the latter by
net precipitation (precipitation-evaporation) in the drainage area.
Unfortunately, there are no data of saltwater inflow variability, beyond
the time of instrumental records, which makes the interpretation of
such changes and their relation to hypoxia on long time-scales
difficult. However, freshwater variability should be reflected in the
terrestrial sediment proxies of effective humidity/net precipitation,
lake-level and catchment erosion (Fig. 7). Due to the good coherence
between the stratigraphy in the sediment records from the open Baltic
Sea and the reconstructed hydrology changes in the catchment area
(Fig. 7) it is our hypothesis that freshwater variability has been one of
the factors, in addition to saltwater inflow from the Kattegatt/
Skagerrak and sea-level changes, controlling the deep oxic conditions
in the basin and that such changes have followed the general
Holocene climate development in Northwest Europe. It is noteworthy,
that the period of oxic bottom conditions between c. 7000 and
6000 cal. yr BP occurs during a phase of generally high salinity and
high sea-levels (Gustafsson and Westman, 2002). The evidence of such
a condition suggest that climate driven hydrological changes in the
drainage area (Fig. 7) managed to turn a hypoxic system into a more
oxic state for a period of c. 200–1000 years.
Run-off variability in the catchment area could be explained by
oceanographic and atmospheric changes over the North Atlantic,
L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
where periods of strong westerlies across northern Europe in
association with more positive North Atlantic Oscillation (NAO)
index would increase the precipitation and wind stress over the
Baltic Sea (Hurrell, 1995). Such conditions would cause a greater
outflow and a smaller and fresher inflow due to lower surface salinity
in the inlet areas (Stigebrandt, 2001) and subsequently leading to a
freshening of the basin (Samuelsson, 1996). This freshening, in
combination with increased wind stress (wind stress is one of the
major physical parameters that controls the vertical mixing of the
upper layers in the Baltic Sea; Stigebrandt, 2001), over the Baltic Sea
would result in a weakened halocline and enhanced vertical mixing
(Stigebrandt, 2001) allowing more efficient exchange of oxygen across
the halocline (Conley et al., 2002). This scenario would promote more
oxic bottom water conditions and the deposition of homogeneous
sediments.
Changes in the Baltic Sea and the connection to climate and
environmental variability have previously been explored by Emeis
et al. (2003) who postulate that high salinity resulted in an increased
density contrast and a stronger stratification of the water column,
which promoted hypoxia in the bottom waters with the subsequent
deposition of laminated sediments. The changes in salinity were
hypothesized to be linked to climatic fluctuations over the North
Atlantic, where high salinity phases coincided with warmer and drier
periods of low river runoff and prominent atmospheric high-pressures
over Scandinavia and northeastern Europe. Similar relationship
between NAO indices and the salinity has been recognized in marine
waters in fjords on the Swedish west-coast (Nordberg et al., 2000,
2001). Based on comparison between instrumental measurements
and sediment records, the latter studies showed that there was a close
connection between higher bottom-water salinity during negative
NAO index and the deposition of laminated sediments.
Also, Zorita and Laine (2000) analyzed the relationship between
annually average salinity and oxygen concentrations during the last
30 years in the Baltic Sea and changes in NAO indices. They show that
on such time-scales the salinity and oxygen concentrations were
negatively correlated within each layer, i.e. low salinity corresponded
to higher than normal oxygen contents and vice versa. Stronger than
normal westerly winds were related to lower than normal salinity
in the upper and lower layers in almost all areas in the Baltic Sea
together with higher than normal oxygen concentrations. They
suggest that negative salinity anomalies may be caused by increased
precipitation in the Baltic Sea catchment during positive NAO indices
and that this process may be more important at longer time-scales
than the inflow of saltier North Sea waters as suggested by Matthäus
(2006). This is supported by Gustafsson and Westman (2002) who
demonstrate that changes in net freshwater input from the catchment area explains the major part of salinity variability in the Baltic
Sea during the last 8500 years and that periods with larger than
normal freshwater run-off in the drainage area result in decreased
salinities.
It is well established that recent anthropogenic enhanced
eutrophication during the last c. 50 years has caused expansion of
hypoxia in bottom waters of the Baltic Sea (Jonsson et al., 1990; Conley
et al., 2002). However, it is our hypothesis that hypoxia occurring as
early as the MWP are also associated with the first large-scale impact
of man in the Baltic Sea watershed. Large land use changes, as those
described in Sections 6.1.2.2. and 6.1.2.3., were probably common in
most of the countries around the Baltic Sea. Such changes (i.e.
agricultural development and population growth) would most likely
have caused increased erosion and nutrient input to the Baltic Sea and
promoted eutrophication and, in combination with the warmer
climate, triggered hypoxia (TOC concentrations display maximum
values during this time period).
The implication for this scenario would be that the historical
records of laminated sediments in the Baltic Sea may not only be of
natural origin, but also partly anthropogenic, and that the modern
89
expansion of laminated sediments is a result of further human impact.
Vahtera et al. (2007) have hypothesized that there are internal
feedbacks within the Baltic Sea that act to sustain hypoxia. These
include enhanced P release (Conley et al., 2002) and increases in
denitrification potential with hypoxia (Vahtera et al., 2007), leading to
an acceleration of eutrophication and fueling of ever larger cyanobacteria blooms. In addition, once the system becomes hypoxic it
modifies the benthic communities, reducing the abundance of large
animals. These benthic organisms both mix organic material downward and irrigate the sediments, and with hypoxia there is a
subsequent loss of reducing equivalents that can oxidize organic
matter (Karlson et al., 2005). Therefore, repeated hypoxic events can
lead to an increase in the susceptibility to eutrophication increasing
the vulnerability to further hypoxia perpetuating hypoxia. Once
hypoxia occurs, reoccurrence is likely and may be difficult to reverse
demonstrating the sensitivity of the Baltic Sea to anthropogenic
alterations. We hypothesize that minor increases of nutrients from
forest clearance and agricultural development, together with climate
warming during e.g. the MWP created hypoxic conditions. Once the
Baltic Sea has switched into a hypoxic mode it is difficult to reoxidize
because of internal feedbacks in the system.
An important question is what factors are responsible for
terminating hypoxic conditions in the Baltic Sea? Why did hypoxia
end after nearly 4000 years of near continuous hypoxia (c. 8000–
4000 cal. yr BP) during the Holocene Thermal Maximum? Was the
cooling that followed the HTM together with decreasing salinity
enough to end hypoxia? Why did the Baltic become oxic 200–
1000 years during that time period? The Little Ice Age with really cold
winters, less nutrients coming in from less agriculture (in addition to
the plague reducing population and impact; Fig. 8), is believed to be
the reason for hypoxia ending. The Baltic was oxic again until c.
100 years ago when man's influence was large enough again during
the industrial revolution and development of the forest industry, in
combination with a warmer climate, to cause widespread hypoxia.
Will proposed reductions in nutrient inputs (Johansson et al., 2007) be
sufficient to switch the Baltic back to an oxic situation in the face of a
substantial global warming?
In summary, environmental changes in the Baltic Sea catchment,
either triggered by climate change or human impact, may have been of
more significance for the development of hypoxia in the past than
previously thought. Land–sea interactions are thus not just modern
phenomenon but have been important on long time-scales. More
research is needed to determine the relative importance of the main
driving forces, i.e. climate change, human impact, and internal
feedback mechanisms. In the light of global warming and increased
anthropogenic pressures in the Baltic Sea region, it is essential to
include these issues into the discussion of the present state and the
future of the Baltic Sea.
7. Conclusions
During the early and more saline development of the Littorina Sea
s. str. (c. 8000–6000 cal. yr BP), laminated sediments were deposited
in the northern Baltic Sea (Bothnian Sea and Bothnian Bay) where
hypoxic bottom conditions prevailed.
Laminae deposition has occurred basin wide, at depths varying
between 73 and 250 m during three major periods in the Baltic Proper
the last c. 8500–7800 years: i.e. between c. 8000–4000, 2000–800 cal.
yr BP and subsequent to AD 1800. These periods overlap the HTM
(c. 9000–5000 cal. yr BP), the MWP (c. AD 750–1200) and the modern
historical period (AD 1800 to present). Laminae formation coincides
with increases in salinity (at least during the HTM) and organic carbon
accumulation. Hypoxia appears to have been more or less continuous
at greater water depths (N250 m).
Oxic bottom conditions were common in the Baltic Proper
between c. 7000–6000, c. 4000–2000 and c. 800–200 cal. yr BP and
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L. Zillén et al. / Earth-Science Reviews 91 (2008) 77–92
correspond to estimates of low salinity, low organic carbon accumulation and to Holocene climate development in Northwestern Europe
characterized by decreasing temperatures and more moist conditions.
The southern Baltic Sea (e.g. the Bornholm Basin and Arkona Basin)
was frequently oxygenated during the Holocene, except for the
deepest part in Gdansk Deep.
Large hydrological changes in the Baltic Sea catchment as a
response to climate changes, have most probably affected the
environmental conditions in the basin. Long-term freshwater discharge variability may have been an important factor controlling the
stratification of the water column and the deposition of laminated
sediments in the Baltic Sea during the last c. 8000 cal. yr BP.
Laminated sediments spanning the MWP correlate with population growth and large-scale changes in land use that occurred in the
Baltic Sea watershed during the early-medieval expansion. This
implies that the change from homogeneous to laminated sediment
deposition in the Baltic Sea was probably caused by a combination of
climate changes and human impact.
Large areas of the Baltic Sea (i.e. the Bothnian Sea, Bothnian Bay,
Gulf of Finland and Baltic Proper) were hypoxic around AD 1900,
which coincides with the beginning of the industrial revolution at
AD1850 in Northwestern Europe when population grew, agricultural
production increased, cutting of drainage ditches intensified and
forest industry expanded explosively, as well as with the beginning of
the on-going global warming. The deep waters of the Baltic Proper
have been depleted of oxygen since then and up to present time.
Once widespread areas of the Baltic Sea become hypoxic there are
processes, such as enhanced P flux and reduced denitrification, which
act to sustain and continue hypoxic conditions. This demonstrates the
sensitively of this large enclosed sea to anthropogenic perturbations.
Acknowledgments
This work was partially supported by a grant from Baltic Sea 2020
and by a Marie Curie Chair to DJC (MEXC-CT-2006-042718). We thank
Bo Gustafsson and Maren Voss for comments on earlier versions of the
manuscript. We also acknowledge the valuable reviews from Erik
Bonsdorff and Anders Stigebrandt.
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