Bottom water oxygenation prior to the Modern Warm Period
The intensive Planolites mottling that completely overprints the
primary sedimentary fabric (bioturbation index 4) in the pre-MCA sediments
clearly demonstrates efficient ventilation of the bottom waters, which
enabled macrobenthic fauna to inhabit the seafloor (Figs. 3a and 4). The
scarcity of discrete traces in this sediment interval could be attributed to
less intense pyritization along the burrow walls than in the overlying
sediments (Thomsen and Vorren, 1984) and to the overlapping of densely spaced
burrows (Virtasalo et al., 2011a, b). Macrofauna living in the area and
capable of completely mixing the seafloor sediment include the isopod
Saduria entomon, the polychaete Harmothoe sarsi, the
amphipods Monoporeia affinis and Pontoporeia femorata, and
the mysid shrimp Mysis relicta. The discrete Planolites are
produced by vermiform burrowers such as the priapulid Halicryptus spinulosus.
The enhanced preservation of the thinly bedded sedimentary fabric during the
MCA at ∼ 900–1200 AD, as demonstrated by the decrease in the
bioturbation index (Figs. 3b and 4), is indicative of declining near-bottom
oxygen levels during this period. The poorly developed grading of these beds
is ascribed to the predominantly lateral sediment transport in the basin
(Jokinen et al., 2015) and possibly results from the slow migration of thin
accretionary bedforms. The dominance of Planolites with a small
diameter and vertical extent in the trace fossil assemblage together with the
subtle decrease in the bioturbation index (Fig. 4) demonstrate low burrowing
activity under a considerable oxygen stress (Savrda and Bottjer, 1986, 1991),
especially during the first half of the MCA.
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 H2S concentration, which hampered
efficient Mo sequestration (Helz and Adelson, 2013) and Ph production by
methanogenic microbes (Sect. 6.3.2). This could explain why the Mo and
Pr / Ph profiles fail to record the modest deoxygenation during the MCA
indicated by the trace fossil data. Since ∼ 1200 AD, the increases in
the bioturbation index and in the abundance of Arenicolites trace
fossils, characterized by generally larger size and greater vertical extent
than Planolites, denote enhanced bottom water oxygenation and
a downward shift of the SMTZ, continuing throughout the LIA (Figs. 3c and 4).
Indeed, the greater burrow depth and size suggests a downward shift in the
annual mean depth of the redoxcline during this period (Savrda and Bottjer,
1986, 1991).
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 / K and grain size profiles and the
organic matter proxies (Sect. 6.1 and 6.2) that indicate amplified lateral
sediment transport (focusing) and primary productivity during the MCA in
comparison to the LIA (Figs. 4 and 5). In addition, we note a slight decrease
in LSR at ∼ 1380 AD (Fig. 2), coinciding with the MCA–LIA transition.
The temporal pattern is similar to the development of hypoxia in the Baltic
Proper (Jilbert and Slomp, 2013; Funkey et al., 2014; Jilbert et al., 2015;
Dijsktra et al., 2016; Hardisty et al., 2016; Papadomanolaki et al., 2018)
and in the Danish straits (van Helmond et al., 2017), where warm climatic
phases during the Holocene have been characterized by declining oxygen
levels. Yet, we note that the amplitude of these changes appears markedly
less pronounced at our study site, which is most likely attributed to the
lack of permanent halocline in this shallow coastal region. Collectively, the
similar trends in the intensity of hypoxia in different parts of the Baltic
Sea basin attest to regional forcing that was most likely related to changes
in the atmospheric circulation patterns such as the NAO, as suggested by
Jilbert and Slomp (2013).
Progressive intensification of hypoxia during the Modern Warm
Period
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 µM have been reported
to prevail in the uppermost centimeters of the sediment column within the
SMTZ at the study site (Karoliina Koho, University of Helsinki, personal communication, 2018). Assuming a salinity
and temperature of 5.6 and 9 ∘C, respectively (Virtasalo et al.,
2005), and a pH of 6.6 (our unpublished data) at the sediment–water
interface, this total sulfide concentration corresponds to
a H2Saq concentration of > 400 µM
(calculated with R package AquaEnv, Hoffmann et al., 2010), clearly exceeding
the requirement of 11 µM H2Saq for the
activation of the sulfide switch (Helz et al., 1996; Erickson and Helz,
2000). The annual accumulation rate of Mo is expected to be controlled by the
duration of the late summer period when the SMTZ is located closest to the
sediment surface (Adelson et al., 2001; Helz and Adelson, 2013), which, in
turn, is modulated by the balance between the delivery of labile OM and
electron acceptors at the seafloor (Middelburg and Levin, 2009). By
extension, when this period is longer in duration we expect seasonal hypoxia
in the bottom water to be more intense and thus that the Mo accumulation
rate provides a first-order proxy for bottom water hypoxia during the MoWP.
We note that the amount of anthropogenically sourced Mo in our sediment
record is likely negligible in comparison to the enrichment caused by
authigenic processes. Indeed, it has been shown that modern sediment
sequestration of Mo in the area shows no spatial trends but is largely
controlled by bottom water oxygenation (Peltola et al., 2011). Furthermore,
the similarity between raw Mo content and Mo / Al profiles (Fig. S3)
implies that the sediment Mo content at the study site is determined by
authigenic Mo sequestration.
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 / Ph, and an increase in the
MAR of Mo (Fig. 6). Although the onset of the increased Mo MAR is hard to
constrain due to the scarcity of age constraints prior to the preservation of
continuous laminations, the appearance of subtle enrichments in Mo content
evidences intermittent shoaling of the SMTZ and consequently intensified
sequestration of Mo in the sediments since 1900 AD (Adelson et al., 2001;
Helz and Adelson, 2013). Likewise, the preservation of laminated
sedimentary fabric suggests upward migration of the redoxcline towards the
sediment–water interface, inhibiting burrowing by macroinfauna, whereas the
occasional blurring of laminations (Fig. 3d) is ascribed to subtle mixing by
meiofauna or transient colonization by opportunistic nectobenthos (Virtasalo
et al., 2011b; Jokinen et al., 2015). Although annual recovery of macrofauna
is common to systems prone to recurring seasonal hypoxia (Diaz and Rosenberg,
1995; Levin et al., 2009), such rapid recolonization was effectively
inhibited in the Haverö basin since 1900 AD, possibly owing to increased
porewater H2S concentration (Diaz and Rosenberg, 1995). A critical
threshold for defaunation in areas experiencing seasonal hypoxia is often
around 0.7 mg L-1 (Llansó, 1992; Diaz and Rosenberg, 1995),
pointing to severe near-bottom water oxygen depletion at the study site
already at 1900 AD.
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 Corg
(Fig. 7; Sect. 6.2.2) – together with the prior accumulation of organic-rich
brackish-water muds in the basin at least since ∼ 500 AD, this likely
led to progressive depletion of electron acceptors (“oxygen debt”) at the
seafloor (Pamatmat, 1971); (2) decreased bottom water volume due to the
gradual shoaling of the basin, resulting in increased vulnerability to
hypoxia (Caballero-Alfonso et al., 2015); (3) strengthened summer thermocline
caused by global warming and gradual isolation of the
basin hampered the replenishment of the
bottom water oxygen reservoir (Hordoir et al., 2017); (4) increased
anthropogenic nutrient loading from the catchment potentially stimulated
primary productivity and the delivery of OM to the seafloor (Sect. 6.2.2).
Accordingly, the onset of recurring seasonal hypoxia at around 1900 AD can
be at least partly attributed to the naturally increased vulnerability to
deoxygenation, which alongside global warming and direct anthropogenic
forcing irreversibly tipped the ecosystem over a threshold, inducing a regime
shift commonly associated with coastal oxygen deficiency (e.g., Conley et
al., 2009b).
Another marked redox shift is observed at 1950 AD, where a rapid increase in
Mo MAR accompanied by a prominent decrease in Pr / Ph suggest
unprecedentedly reducing conditions at the sediment–water interface
(Fig. 6). This shift likely denotes shoaling of the redox zonation as a
response to eutrophication in the area, which has been reported in previous
studies of the Baltic Sea (Slomp et al., 2013; Egger et al., 2015; Rooze et
al., 2016; Jilbert et al., 2018) and reflects intensified delivery of labile
OM to the seafloor with respect to the supply of electron acceptors
(Middelburg and Levin, 2009). Considering the decreasing trend in the local
summer temperatures over this interval, promoting less intense sediment
focusing, weaker thermal stratification, and suppressed productivity, we
infer that the upward migration of the SMTZ was more likely driven by the
increased anthropogenic nutrient loading from the catchment than by climatic
factors (Fig. 6). Similar exacerbation of bottom water hypoxia around the 1950s
has been reported in the coastal areas of Sweden (Persson and Jonsson, 2000;
Savage et al., 2010) and in the Baltic Proper (Fig. 6; Jonsson et al., 1990;
Carstensen et al., 2014a), reflecting synchronous increases in the
anthropogenic nutrient loading around the Baltic Sea (Conley et al., 2009a;
Savage et al., 2010; Gustafsson et al., 2012; Carstensen et al., 2014a). This
deoxygenation around the 1950s conforms to the global spread of hypoxia in
coastal areas (Vaquer-Sunyer and Duarte, 2008) following the “Great
Acceleration” (Steffen et al., 2015). Accordingly, the steepest gradient in
the δ15N profile at our study site is reached between 1950 and
1995 AD, paralleled by intensified agricultural practices and fast
population growth rate (Fig. 6).
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.
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Although the decline in Mo content and concomitant increase in Pr / Ph
suggest a slight improvement in the bottom water oxygenation since 1990 AD
(Fig. 6), this is an unlikely scenario. Instead, the MAR of Mo has remained
high since 1950 AD and reached the highest values of the record as late as
2005 AD, suggesting that aggravation of bottom water hypoxia has continued
up to the present, as evidenced by the local monitoring data directly at the
study site (Fig. S4). In line with this, monitoring data in other parts of
the Archipelago Sea consistently demonstrate progressive deoxygenation in the
area during last two decades (Fig. 8; Suomela, 2011; Caballero-Alfonso et
al., 2015). Therefore, we ascribe the decrease in Mo content at 1990 AD to
dilution by the concurrently enhanced sediment MAR (Fig. 6). We postulate
that, in addition to effective sediment focusing, the increased sediment MAR
was likely fueled by ballasting effects, whereby eutrophication-induced
increase in the OM production in the euphotic zone drives the sedimentation
of fine-grained lithogenic material through aggregation (Passow, 2004; Passow
and De La Rocha, 2006; De La Rocha et al., 2008), as suggested by the close
covariation between sediment and Corg MARs (Fig. 6). In line with
this, it has been shown that rapid sedimentation events during vernal
phytoplankton blooms in the study area are caused by the formation of
organomineralic aggregates adhered together by sticky transparent exopolymers
(TEP) excreted by phytoplankton (Jokinen et al., 2015), coupling the
sedimentation of lithogenic material to autochthonous primary production.
Therefore, while Corg content remained fairly constant over the
shift to enhanced sediment MAR at 1985 AD due to intensified sediment
focusing and ballasting effects, the content of Mo was diluted as the depth
and intensity of the H2S front remained relatively unaffected.
The recovery in Pr / Ph at the core top (Fig. 6), which is observed
despite the obvious aggravation of hypoxia until the present, suggests that
the distinct minimum in Pr / Ph between 1950 (53.5 cm) and 1990 AD
(29 cm) is caused by a post-depositional overprint. Although the mechanism
causing a diagenetic decline in Pr / Ph in this depth interval at the
present day remains speculative, a likely candidate is the excess Ph
production by methanogenic microbes (Brassel et al., 1981; Venkatesan and
Kaplan, 1987; Duan, 2000). Indeed, the current position of the SMTZ at our
study site is likely located in the uppermost 10–20 cm below the sediment
surface (see Sawicka and Brüchert, 2017 and Jilbert et al., 2018, for
comparable systems), below which methane concentrations are expected to
increase to the millimolar range. As such, Pr / Ph may not be a direct
proxy for bottom water oxygenation at our study site. Instead, it likely
records the rate of methanogenesis below the SMTZ, reflecting the amount of
labile OM that escapes aerobic degradation. Importantly, the invariably high
Pr / Ph prior to the 20th century (Fig. 4) could denote that intensive
methane formation, due to increased productivity and subsequent shoaling of
the redox zonation over the past few decades (Slomp et al., 2013; Egger et
al., 2015; Rooze et al., 2016; Jilbert et al., 2018), is unprecedented in our
sediment record. However, the low Mo content of < 10 mg kg-1
(Fig. 6) suggests that the basin has remained non-euxinic until the present
(Scott and Lyons, 2012). This is also evidenced by the slope of ∼ 2.7
for the linear regression between Mo and Corg (Fig. S1), being
slightly shallower than reported for continental margin upwelling systems
characterized by persistently hypoxic, but non-euxinic bottom water
conditions (Algeo and Rowe, 2012, and references therein). Furthermore, the
MAR of Mo has remained in less than half of the MAR reported for the
seasonally mildly euxinic Chesapeake Bay (Helz and Adelson, 2013).