The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated the depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygenation since the beginning of the 1900s was fueled by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.
The expansion of hypoxic dead zones is an ongoing global problem both in the
marine realm (Diaz and Rosenberg, 2008; Vaquer-Sunyer and Duarte, 2008;
Gooday et al., 2009; Rabalais et al., 2010, 2014) and in lacustrine
settings (Jenny et al., 2016a, b). Bottom water oxygen depletion
(< 2 mg L
Over the past century, the Baltic Sea has seen a marked expansion of benthic hypoxia (Jonsson et al., 1990; Conley et al., 2011; Carstensen et al., 2014a), and the Baltic Sea dead zone is often referred to as the largest anthropogenically induced hypoxic marine area in the world (Diaz and Rosenberg, 2008). Yet, although long-term trends in the expansion of hypoxia in offshore areas of the Baltic Sea have been widely studied, little is known about the past evolution of hypoxia in the shallow coastal areas, where episodic or seasonal oxygen deficiency is forced by thermal rather than salinity stratification (Virtasalo et al., 2005; Conley et al., 2011). Importantly, these coastal areas act as a filter for nutrients received from the catchment (Asmala et al., 2017). Thus, changes in biogeochemical cycles in coastal sediments may have an impact on nutrient transport to offshore areas of the Baltic Sea (Almroth-Rosell et al., 2016). Elucidating fluctuations in coastal hypoxia will aid in the understanding of the efficiency of the coastal filter and is therefore vital for understanding the expansion of hypoxia in the entire Baltic Sea. Indeed, it is still debated whether the decisive factor triggering the hypoxic event during the Medieval Climate Anomaly (MCA, 900–1350 AD) in the Baltic Proper was intensified land use in the catchment (Zillén and Conley, 2010) or anomalously warm climate (Kabel et al., 2012; Papadomanolaki et al., 2018).
In this study, we present a multiproxy reconstruction of the development of hypoxia in an enclosed coastal setting in the Finnish Archipelago Sea (northern Baltic Sea) over the past 1500 years, covering the known climatic oscillations of the MCA, the Little Ice Age (LIA, 1350–1850 AD), and the Modern Warm Period (MoWP, after 1850 AD) in order to assess how the coastal zone responds to centennial–millennial climate variability and potential past inputs of nutrients from the catchment areas. We use diverse bulk sediment geochemical proxies in combination with integrated sedimentological and ichnological analyses to elucidate temporal changes in the intensity of near-bottom water oxygen deficiency. In order to constrain the drivers behind the observed oxygenation changes, we assess past fluctuations in hydrodynamic conditions at the study site, and in the delivery of OM, and compare these with the past climate variability and changes in the anthropogenic nutrient loading from the catchment.
The Baltic Sea is a shallow (mean depth 54 m) semi-enclosed basin located on
a continental shelf (Fig. 1a) between maritime temperate and continental
sub-Arctic climate zones. Climatic conditions in the area are largely
modulated by the North Atlantic Oscillation (NAO) as well as the summer low
and winter high over Eurasia (e.g., Rutgersson et al., 2014). Winter mean air
temperature ranges from
Surface salinity exhibits an increasing trend from north to south, from 3–5 in the Gulf of Finland and Gulf of Bothnia to 8–10 in the southern Baltic (Leppäranta and Myrberg, 2009). This horizontal salinity gradient results from combined effects of high riverine freshwater input in the north and occasional inflows of saline water from the North Sea through the Danish straits in the south. The contrasting density of these two water masses leads to the formation of a strong 10–20 m thick pycnocline, which lies at a depth of 40–80 m depending on the sub-basin (Leppäranta and Myrberg, 2009). Irregular saline inflow events from the North Sea occasionally ventilate the deep stagnant bottom waters of the Baltic Proper, but this oxygen is readily exhausted with a net effect of stronger stratification and possibly even more severe oxygen depletion (Conley et al., 2002; Carstensen et al., 2014a).
Maps of the study area.
The Archipelago Sea, located in the southwestern coastal area of Finland in
the northern Baltic Sea (Fig. 1a), is a mosaic of thousands of islands and
small bays within an area of
The complex topography of the Archipelago Sea results in restricted water exchange between the inner archipelago and the open-sea areas (Mälkki et al., 1979), and numerous small basins with contrasting bottom water conditions exist in close proximity (Virtasalo et al., 2005). Water exchange in the area mostly occurs through deep straits following the fault-lines of the crystalline bedrock, which are mostly aligned in a north–south direction. In enclosed basins, a strong thermocline impedes mixing of dissolved oxygen to the bottom waters during summer, which together with high delivery of reactive OM to the seafloor commonly results in seasonal hypoxia (Virtasalo et al., 2005; Jokinen et al., 2015). Mixing of the water column through thermal convection takes place in spring and autumn due to the lack of a permanent halocline (Leppäranta and Myrberg, 2009).
The sediment fill of the Archipelago Sea since the deglacial to present
comprises a succession of ice-proximal tills and outwash, glaciolacustrine
rhythmites, patchily distributed debrites, postglacial lacustrine clays, and
brackish-water mud drifts (Virtasalo et al., 2007, 2014). The study area was
deglaciated at
Haverö is a small, extremely enclosed basin in the middle of the Archipelago Sea (Fig. 1). The Proterozoic bedrock surface in the area is dominated by microcline granites, with small patches of gneisses, amphibolite, and granodiorite (Bedrock of Finland, DigiKP). Due to the relatively high elevation of the surrounding islands, Quaternary deposits have been largely removed by erosion after the islands were uplifted above the wave base (Maankamara, DigiKP). The distance to the mouths of the largest rivers in the area, Aurajoki River and Paimionjoki River, is 25 and 38 km, respectively. A small brook drains into the basin from a lake located on the Haverö island. Sedimentation in the Haverö basin is dominated by reworking of previously deposited late- and postglacial clays and organic-rich brackish-water muds during autumn and winter, and by rapid settling of organic-rich aggregates during spring and summer (Jokinen et al., 2015). This seasonal contrast in the sedimentation, accompanied by severe seasonal hypoxia and consequent deterioration of macrobenthic fauna, has enabled the formation and preservation of annual laminations (varves) over the past decades (Jokinen et al., 2015). Population around Haverö is sparse, and the dominant direct anthropogenic nutrient loading is sourced from two local fish farming cages that were operational from 1987 to 2008 AD (Fig. 1b).
Sediment grain size depends on sediment inputs and hydrodynamic conditions. Given that sediment provenance remains fixed, temporal variation in grain size distribution in the coastal zone of the Baltic Sea, excluding river mouths, is mainly governed by changes in wind stress and local seafloor morphometry that modulate the bottom water energy flux (Lehmann et al., 2002b; Jönsson et al., 2005a; Ning et al., 2016). In general, periods with enhanced near-bottom currents become recorded in sediments through increased proportion of coarse grains (e.g., Hjulström, 1939).
In fine-grained sediments, potassium (K) is mainly associated with illite
((
Due to the contrasting composition of vascular plants in comparison to
phytoplankton, carbon (C) to nitrogen (N) ratios of sediment OM can be used
to estimate relative contributions of OM originating from terrestrial and
marine compartments (Meyers, 1994, 1997, and references therein). This
difference arises from the high abundance of proteins in algae, while
vascular plants are rich in cellulose instead. Accordingly, marine and
terrestrial OM are characterized by molar C
Stable isotope composition of sediment organic carbon
(
The stable isotope composition of nitrogen (
Founded on the observation that branched glycerol dialkyl glycerol
tetraethers (GDGTs I–III) are mainly sourced from terrestrial environment
(soil OM), whereas crenarchaeol originates predominantly from the marine
environment (a characteristic lipid for aquatic Thaumarchaeota),
Hopmans et al. (2004) defined the branched isoprenoid tetraether (BIT) index
as a proxy for the relative abundance of terrestrial OM:
Sedimentary-fabric analysis is focused on the preservation of primary sedimentary structure, its mixing by macrofaunal bioturbation, and the characteristics of identifiable bioturbation structures (trace fossils). Trace fossil assemblages of the organic-rich brackish-water muds of the Baltic Sea are well applicable for reconstructing past bottom water redox shifts (Virtasalo et al., 2011a, b). Benthic faunal responses to bottom water hypoxia include avoidance or even mortality of large species, loss of diversity, and shoaling of penetration depth or emergence from sediment (Levin et al., 2009). Consequently, the vertical extent, diameter, and diversity of burrows constructed by macrobenthic fauna decrease with declining bottom water oxygenation, which is thought to be the decisive factor shaping biogenic sedimentary fabrics in the Baltic Sea (Savrda and Bottjer, 1986, 1991; Virtasalo et al., 2011a, b), although other factors such as salinity, substrate consistency, and food supply also affect trace fossil assemblages in the area (Virtasalo et al., 2006, 2011a). Importantly, the behavior of macrobenthic fauna responds rapidly to changes in the bottom water environmental conditions, and these responses can be readily recorded in the trace fossil assemblages (Savrda and Bottjer, 1986; Wetzel, 1991). The magnitude of this response is governed by the intensity and duration the deoxygenation as well as by the recovery time between consecutive hypoxic events (Levin et al., 2009).
Sedimentary molybdenum (Mo) content is a well-established proxy for past
redox fluctuations in bottom waters overlying marine sediments (e.g., Algeo
and Lyons, 2006; Scott and Lyons, 2012; Helz and Adelson, 2013). It has been
successfully applied to Baltic Sea sediments for bottom water redox
reconstructions, especially in deep areas (Mort et al., 2010; Jilbert and
Slomp, 2013; Jilbert et al., 2015; Dijkstra et al., 2016; Hardisty et al.,
2016; van Helmond et al., 2017). The sensitivity of sedimentary Mo content to
redox fluctuations is due to the conversion of the relatively inert molybdate
ion (
The application of the pristane to phytane ratio (Pr
The study site was selected based on previous studies by Jokinen et al. (2015), where it was found that the sediment in the basin comprises thick varves since the beginning of the 20th century, providing a high-resolution archive of environmental change for the corresponding period. Due to this apparent sensitivity of the basin to bottom water hypoxia, as manifested in the continuous laminations, together with the central location in the middle of the Archipelago Sea, we expected the site to be representative of the past environmental changes in the area. Importantly, despite the contrasting bottom water oxygenation between adjacent sub-basins in the area (Virtasalo et al., 2005), the long-term trends in environmental conditions are largely congruent over the entire Archipelago Sea, excluding areas close to prominent nutrient point sources (Suomela, 2011).
Two replicate sediment cores (HAV-KU-5 and HAV-KU-6) were retrieved using a
5 m long piston corer onboard R/V
Retrieved sediment cores.
Age constraints for the age model were obtained based on visual varve
counting (1900 AD onwards),
For X-radiography, plastic boxes (
Classification scheme for assessing bioturbation index, modified from Behl and Kennett (1996).
Lids of the polystyrene sample boxes were removed, the samples were
freeze-dried, and the dry bulk density was calculated. Subsequently, the
sediment samples were ground in an agate mortar and analyzed for total carbon
(C
In the Laboratory of Chronology at the University of Helsinki, selected
freeze-dried and ground samples were measured for stable isotope compositions
of carbon (
Selected freeze-dried and homogenized samples were analyzed for various
biomarkers at the Department of Marine Geology in the Leibniz Institute for
Baltic Sea Research (IOW) following Kaiser and Arz (2016). Briefly,
0.5–1.0 g of sediment was used for accelerated solvent extraction (Dionex
ASE 350, Thermo Fisher Scientific) with a 9 : 1 volumetric mixture of
dichloromethane and methanol using high pressure (100 bar) and temperature
(100
Elemental contents of the freeze-dried and homogenized samples were estimated
by a combination of ICP-OES (inductively coupled plasma optical emission
spectrometry) and ICP-MS (inductively coupled plasma mass
spectrometry) analysis. Initial ICP-OES analysis for K and Ti was performed
at the Department of Food and Environmental Sciences at the University of
Helsinki. A portion of 0.1–0.2 g of dry
sediment was dissolved in 2.5 mL of HF (38 %) and 2.5 mL of a mixture
of
Accuracy of the initial ICP-OES results was checked by digestion and analysis
of a subset of 28 samples at IOW together with the international reference
material SGR-1b (USGS). A total of 50 mg of dried and ground sediment was
first treated in open Teflon vessels (PDS-6; Heinrichs et al., 1986) with
1 mL
ICP-MS was used to determine the contents of Mo, Pb, and the ratio of stable
Pb isotopes (
All measured elemental contents were corrected for the weight of the salt in
the pore water using the ambient salinity and porosity (Lenz et al., 2015).
Mass accumulation rates (MARs) of the individual elements were calculated as
follows:
The three independent varve counts for the continuously laminated recent
sediments (3–76 cm) suggest that this section covers
The apparent temporal fluctuations in the magnitude of the bulk sediment
reservoir age as indicated by the downcore reversals in
Simplified lithology, constructed age model, and
To estimate the content of atmospheric pollution Pb in the sediment, we
applied a simple two-component mixing model following Brännvall et
al. (1999):
The background isotope composition of Pb was calculated as the mean prior to
the apparent onset of Pb pollution at 900 AD (Fig. 2), which yielded an
estimate of 1.397. We used the age model based on
We found a remarkable consistency between our Pb record and the pollution Pb profiles of varved lakes in eastern Sweden, where the preindustrial Pb fallout was mainly sourced from mainland Europe and from the British Isles (Brännvall et al., 1999; Fig. 2). Hence, we used the main Pb pollution features found consistently in all of these lakes to constrain an age model for our sediment core (Fig. 2; Table 3). The onset of medieval Pb pollution (at 900 AD, Lougheed et al., 2012) and the medieval pollution maximum (at 1200 AD, Lougheed et al., 2012, 2017; Zillén et al., 2012) have previously been used as age constraints in sediments from the Baltic Proper. However, our study is, to our best knowledge, the first to identify the 1530 AD pollution peak and the pollution minima of 1350 and 1600 AD (Brännvall et al., 1999; Renberg et al., 2002) as age–depth points in Baltic Sea sediments.
Age constraints given as an input for the age model constructed
using OxCal 4.2 software. Pb pollution features were obtained from
Brännvall et al. (1999). The IDs are as in Fig. 2. Measured bulk
sediment AMS-
A conservative 1
The obtained age constraints (Table 3) were used to construct a Bayesian age
model in OxCal 4.2 software (Bronk Ramsey, 2009), using the
The sediment in the core HAV-KU-6 is characterized by organic-rich mud, which conforms to the brackish-water mud drift sensu Virtasalo et al. (2007), indicative of recent deposition in the basin. Based on lithology, the core can be divided into four units as described below (Figs. 2 and 3). These units approximately correspond to the pre-MCA, MCA, LIA, and MoWP intervals (Fig. 3).
X-radiographs of different lithological units together with their
interpretations.
The basal part (393–324 cm,
The thinly bedded mud (324–201 cm,
Profiles of median grain size and Ti
The thinly bedded mud is gradationally overlain by greenish-grey mud roughly
corresponding to the LIA interval (201–76 cm,
The sharply laminated mud (76–0 cm) roughly corresponds to the MoWP and comprises rhythmically alternating light brown, black, and grey laminae. The thickness of the individual lamina successions varies from 2 to 13 mm, with a generally upward increasing trend, which partly results from decreasing compaction. These laminites correspond to the varves described by Jokinen et al. (2015). Trace fossils are virtually absent in this unit (bioturbation index of 1–2), although occasional blurring of the laminations is observed (Figs. 3d and 4). Black sulfide staining becomes predominant from 53 cm upwards.
Median grain size (range 1.8–2.4
The proxies for organic matter contents and composition correlate strongly
with each other and show profiles very similar to those in Ti
Geochemical profiles reflecting the delivery and preservation of
organic material in the basin. Fractions of C
Sedimentary Mo content and MAR (ranges 2–8 mg kg
Pr
Spearman rank correlation (
The recent changes observed in our proxies during the MoWP are complex and
are described here in more detail. The LIA–MoWP transition (1800–1900 AD)
was characterized by low Mo and C
Comparison of possible climatic and anthropogenic drivers of
eutrophication in the study area versus geochemical profiles reflecting the
sources and delivery of organic
matter to the sediment (C
Despite declining rapidly after the onset of continuous laminations,
Pr
Although the Haverö basin has been an enclosed basin throughout the study
period (Fig. 1b), the glacio-isostatic rebound has resulted in progressively
calmer depositional conditions in the area, as evidenced by the generally
decreasing trajectories in Ti
Superimposed on this general trend towards calmer sedimentary conditions
towards the present, climatic oscillations have exerted a prominent control
on the bottom water hydrographic conditions at the study site. We ascribe the
elevated Ti
The bulk sediment
We note that the excessively old bulk sediment radiocarbon dates for the
NaOH-soluble fraction suggest a marked input of old reworked OM (Fig. 2).
Despite this contribution of pre-aged carbon, likely related to intensive
reworking and lateral sediment advection in the basin (Jokinen et al., 2015),
we assume that the C
Although inferences about past productivity based solely on the
phytoplankton-derived C
The fluctuations in the input of phytoplankton-derived OM to the basin
generally coincide with the past climatic oscillations (Fig. 5) and with
paleoproductivity records from the Baltic Proper (Leipe et al., 2008; Kabel
et al., 2012; Jilbert and Slomp, 2013). Indeed, the MCA and MoWP are typified
by relatively high input in comparison to the LIA, implying enhanced
productivity under warm climatic phases. However, we note that
The temporal trend in OM input observed in our data is similar to the trend
in temperature recorded in millennial-scale regional climate model
simulations (Schimanke et al., 2012), and with dendroclimatic summer
temperature reconstructions for southeastern Finland (Helama et al., 2014)
as well as with a TEX
Importantly, we note that the long-term trends in land use and precipitation
in the catchment show no similarity to our record of phytoplankton-derived OM
input prior to the MoWP, suggesting that external nutrient inputs did not
force past increases in productivity. Indeed, in agreement with the
population growth records for Finland (Kuosmanen et al., 2016, and references
therein), marked human-induced land-use changes in the catchments of varved
lakes in southwestern and south-central Finland became discernible at
The progressive increase in the
By contrast to the increase in OM accumulation in the early 20th century, the
shift to unprecedentedly heavy
The intensive
The enhanced preservation of the thinly bedded sedimentary fabric during the
MCA at
Despite the decline in the bottom water oxygen concentration during the MCA,
we infer that the pore water chemistry was typified by a relatively deep and
poorly developed SMTZ with low
We attribute the multicentennial-scale fluctuations in bottom water
oxygenation associated with the MCA and LIA to climatic variability that
modulated both hydrographic conditions and accumulation of OM at the
seafloor. This is supported by the Ti
The MAR of Mo in the sediments at our study site increased during the MoWP
(Fig. 6). Due to the fact that the study site is seasonally hypoxic, rather
than permanently anoxic or euxinic, the most likely mechanism for Mo
enrichment is via diffusion of seawater Mo into the sediment towards the SMTZ
(see Helz and Adelson, 2013), which may be amplified by the shuttling of Mo
associated with Mn oxides (Algeo and Lyons, 2006; Scheiderich et al., 2010;
Scott and Lyons, 2012; Sulu-Gambari et al., 2017). It is thus plausible that
the scavenging of Mo mostly takes place close to the sediment surface at the
end of summer stratification period when the SMTZ reaches its shallowest
position in the sediment column (Mogollón et al., 2011) and shuttling and
refluxing of Mn is expected to be at its annual maximum. Indeed, total
sulfide concentrations of > 600
During the MoWP, the Haverö basin has undergone a progressive aggravation
of bottom water hypoxia, typified by two distinct regime shifts. First, a
marked shoaling of the sediment redoxcline at 1900 AD is manifested in the
contemporaneous occurrence of continuous lamination (near-complete cessation
of macrobenthic activity), a decrease in Pr
Considering the negligible variation in the proxies for the source of OM
prior to 1930 AD (Fig. 6), the onset of seasonal hypoxia and the resulting
preservation of continuous lamination since the beginning of 20th century was
apparently not solely forced by potential changes in primary productivity
linked to human-induced eutrophication. Instead, we postulate that this
deoxygenation was forced by the following complex interplay of factors:
(1) increased source-to-sink ratio, combined with intensified lateral
sediment transport especially during early winter due to the warming climate,
leading to enhanced sediment focusing and higher MAR of C
Another marked redox shift is observed at 1950 AD, where a rapid increase in
Mo MAR accompanied by a prominent decrease in Pr
Contour plots for water column dissolved oxygen concentration in August over the past decades at four intensive monitoring stations (data from HERTTA database) located around the study site (HAV-KU-6). The interpolations were produced with the Ocean Data View software (Schlitzer, 2017). White dots represent the original measurement data.
Although the decline in Mo content and concomitant increase in Pr
The recovery in Pr
Previous studies have suggested that the eutrophication of the Archipelago
Sea began in the late 1960s (Bonsdorff et al., 1997a, b; Hänninen et al.,
2000; Suomela, 2011). Our data show that environmental conditions around the
study site in the Archipelago Sea likely deteriorated several decades prior
to this and therefore also prior to the establishment of water quality monitoring
campaigns in the 1960s. This highlights the use of sediment-core studies for
the long-term reconstruction of environmental conditions in such settings.
Our
Despite the decreased loading of sewage waters since the 1980s, following the establishment of a wastewater treatment plant for the city of Turku (Suomela, 2011; Fig. 6), the continued leakage of nutrients from agriculture to the Archipelago Sea (Ekholm et al., 2015) together with intensive P regeneration from surface sediments mainly upon the dissolution of Fe-bound P (Puttonen et al., 2014) has sustained the trend toward increasing eutrophication and shoaling of hypoxia until the present (Figs. 6 and 7). In addition, the recent trajectory towards further aggravation of hypoxia has likely been amplified by the progressively increasing summer temperatures (Fig. 6), which is also supported by the increased importance of climatic effects in the forcing of oxygen depletion in the Swedish coast of the Baltic Sea since the late 1970s (Savage et al., 2010). Hence, while reductions in nutrient loading appear to have improved bottom water oxygenation in the Stockholm Archipelago since the 1990s (Karlsson et al., 2010), we observe no signs of recovery in the Archipelago Sea so far.
This study shows that multicentennial-scale climatic oscillations affect near-bottom water oxygenation of a shallow coastal basin in the northern Baltic Sea currently suffering from severe seasonal hypoxia. During warm phases, increased export production of labile, phytoplankton-derived OM combined with effective sediment focusing to the deepest part of the basin drives deoxygenation of the near-bottom waters in summer. Accordingly, decreased oxygen levels are observed during the MCA and MoWP, but the intensity of the MoWP hypoxia, typified by complete deterioration of the macrobenthic community, is unprecedentedly severe. The progressive deoxygenation during the 1900s was originally triggered by gradual shoaling of the basin due to glacio-isostatic uplift and basin infilling that, together with warming climate and anthropogenic nutrient input, promoted the vulnerability of the basin to hypoxia and intensified OM accumulation. By contrast, the marked aggravation of hypoxia in the 1950s was unequivocally attributed to the excessive anthropogenic nutrient loading from the catchment, which substantially stimulated autochthonous primary production. Our results demonstrate that the markedly more severe hypoxia during the MoWP in comparison to the MCA is not only attributed to the excess anthropogenic nutrient loading, but also to the gradual changes in the basin configuration that have increased the sensitivity to deoxygenation towards the present. Such natural changes should be considered when elucidating anthropogenic contribution to hypoxia. Furthermore, signs of eutrophication in the area are readily discernible in our sediment record already in the beginning of 1900s, implying that the water quality diverged from natural conditions decades prior to the establishment of monitoring campaigns. This has important implications for the assessment of reference conditions for water quality in the area. Despite the recent measures taken to reduce anthropogenic nutrient loading to the area, we find no evidence of recovery from hypoxia, suggesting that further measures are needed to alleviate oxygen depletion.
The data we have produced ourselves can now be found in
Pangaea:
The supplement related to this article is available online at:
SJ devised the study, conducted field and laboratory work, interpreted the data, produced the figures, and drafted the paper. JV devised the study, interpreted the data, and assisted with trace fossil analysis and writing the paper. TJ assisted with laboratory work, interpreted the data, and assisted with writing the paper. JK assisted with the biomarker analyses, interpreted the data, and assisted with writing the paper. OD carried out the ICP-OES and ICP-MS analyses at IOW, interpreted the data, and assisted with writing the paper. HA interpreted the data and assisted with writing paper. JH conducted fieldwork, interpreted the data, and assisted with writing the paper. LA carried out the IRMS analysis and assisted with writing the paper. MC assisted with the ICP-OES analysis in Helsinki. TS conducted field and laboratory work and assisted with writing the paper.
The authors declare that they have no conflict of interest.
We acknowledge the crew on R/V