Increasing occurrences of extreme weather events, such as
the 2018 drought over northern Europe, are a concerning issue under global
climate change. High-resolution archives of natural hydroclimate proxies,
such as rapidly accumulating sediments containing biogenic carbonates, offer
the potential to investigate the frequency and mechanisms of such events in
the past. Droughts alter the barium (Ba) concentration of near-continent
seawater through the reduction in Ba input from terrestrial runoff, which in
turn may be recorded as changes in the chemical composition (Ba/Ca) of
foraminiferal calcium carbonates accumulating in sediments. However, so far
the use of Ba/Ca as a discharge indicator has been restricted to planktonic
foraminifera, despite the high relative abundance of benthic species in
coastal, shallow-water sites. Moreover, benthic foraminiferal Ba/Ca has
mainly been used in open-ocean records as a proxy for paleo-productivity.
Here we report on a new geochemical data set measured from living
(CTG-labeled) benthic foraminiferal species to investigate the capability
of benthic Ba/Ca to record changes in river runoff over a gradient of
contrasting hydroclimatic conditions. Individual foraminifera (Bulimina marginata,
Nonionellina labradorica) were analyzed by laser-ablation ICP-MS over a seasonal and spatial
gradient within Gullmar Fjord, Swedish west coast, during 2018–2019. The
results are compared to an extensive meteorological and hydrological data
set, as well as sediment and pore-water geochemistry. Benthic foraminiferal
Ba/Ca correlates significantly to riverine runoff; however, the signals
contain both spatial trends with distance to Ba source and species-specific
influences such as micro-habitat preferences. We deduce that
shallow-infaunal foraminifera are especially suitable as proxy for
terrestrial Ba input and discuss the potential influence of water-column and
pore-water Ba cycling. While distance to Ba source, water depth, pore-water
geochemistry, and species-specific effects need to be considered in
interpreting the data, our results demonstrate confidence in the use of
Ba/Ca of benthic foraminifera from near-continent records as a proxy for past
riverine discharge and to identify periods of drought.
Introduction
Extreme weather events, such as drought and heat waves, have a devastating
impact globally (e.g., Sternberg, 2011). Droughts occur through an interplay
of below-average precipitation, high evapotranspiration, and hydrological
preconditions, such as soil moisture and water storage capacity (e.g.,
Tallaksen and Van Lanen, 2004). Alone in the 21st century, Europe was
affected by a series of exceptionally severe drought events (e.g., Bastos et
al., 2020a). With anthropogenic global warming accelerating the internal
processes driving hydroclimate variability, droughts are expected to keep
increasing in frequency and severeness in the future (e.g., Cook et al.,
2020). Thus, it is imperative to document droughts pre-dating
anthropogenically forced climate change to define a baseline of natural
hydroclimatic variability.
Hydrological drought, defined as deficiency in water supply including
below-normal river discharge, is one of the main comprehensive indicators
for accumulated hydrological responses to a reduction in precipitation on a
wide spatial and temporal scale (e.g., Tallaksen and Van Lanen, 2004). A
range of natural archives for historical droughts exist in the terrestrial
environment, including dendrochronological, ice-core- and speleothem-based
proxy records (e.g., Steiger et al., 2018). In aquatic systems, sediments
offer the potential to record hydrological conditions on land through
changing fluxes of materials across the land–water interface. One potential
sediment proxy capable of quantifying hydrological drought is the barium to
calcium ratio (Ba/Ca) in biogenic carbonates formed in the coastal marine
environment. Barium in seawater is dominantly sourced from fluvial input
(e.g., Wolgemuth and Broecker, 1970) and proportionally incorporated in
biogenic calcium carbonates as a substitute for the Ca2+ ion (e.g., de
Nooijer et al., 2017). Hence, changes in fluvial Ba inputs may be expected
to influence carbonate Ba/Ca in the nearshore environment.
While Ba/Ca is applied for paleoclimatic reconstructions, the focus so far
has been on corals in tropical environments (e.g., Saha et al., 2018) and
planktonic foraminifera (e.g., Bahr et al., 2013; Mojtahid et al., 2019).
Moreover, Ba/Ca of benthic foraminifera is dominantly interpreted as
a paleo-productivity proxy (e.g., Mojtahid et al., 2019; Ní Fhlaithearta
et al., 2010), due to the nutrient-like cycling of Ba in seawater (e.g.,
Dehairs et al., 1980; Paytan and Griffith, 2007). Benthic foraminifera are
ubiquitous in coastal sediments, and concurrently coastal sediment records
are important paleoclimate archives due to their high temporal resolution
and transitional position between the terrestrial and marine realm (review
by Howe et al., 2010; Murray, 2006). While rare applications of benthic
foraminiferal Ba/Ca as a runoff indicator exist (Groeneveld et al., 2018; Ni
et al., 2020), studies focusing on living benthic foraminifera are
distinctly lacking. In situ, core-top approaches with living foraminifera
are, however, essential to calibrate a proxy and create a reliable baseline
of empirical observations by correcting for species-specific biases and
seasonal-scale mechanisms influencing Ba/Ca in a coastal environment.
The drought and heat wave of 2018 that affected northern and central Europe,
including Scandinavia, was exceptional in its severeness (e.g., Peters et
al., 2020), and was followed by a warm and wet year in 2019 (Swedish
Meteorological and Hydrological Institute, SMHI). This contrast in
precipitation between 2018 and 2019 presents a unique opportunity to study
the response of benthic foraminiferal trace-elemental concentrations to
changes in riverine discharge. Here we test the hypothesis of benthic
foraminiferal Ba/Ca as a tracer for riverine discharge, assess the potential
of identifying drought events in sediment foraminiferal records, and
evaluate the broader implications for the interpretation of benthic Ba/Ca in
paleo-studies in coastal environments.
Location of the study area within Europe and on the west
coast of Sweden (marked by red rectangle) giving an overview of Gullmar
Fjord and Örekilsälven, including cumulative catchment area
(outlined in dark grey). The study sites GF 71 and GF 117, and environmental
monitoring sites Alsbäck, Björkholmen, and Släggö are
indicated. A transect of Gullmar Fjord with water masses of different
sources is given (S= salinity, t= typical residence
time in days (d) or years (yr); after Arneborg, 2004), with indication of
Örekilsälven input (blue arrow). Maps modified after SMHI and
Groeneveld et al. (2018).
Site description: Gullmar Fjord, Swedish west coast
Gullmar Fjord (GF; ca. 28 km by 1–2 km; Fig. 1) has a maximum water depth
of 120 m, located centrally at Alsbäck, and a sill depth of 43 m towards
the Skagerrak. The sediments in the fjord's deep basin are comprised of
silty clays with a low sand content and an organic carbon content of
approximately 3 % in surface sediments (e.g., Gustafsson and Nordberg,
2001). Phytoplankton blooms occur typically biannually in spring and autumn
(dominated by diatoms and dinoflagellates, respectively; e.g., Gustafsson
and Nordberg, 2001).
The river Örekilsälven is the main contributor of freshwater to the
fjord (annual average discharge of 24 m3 s-1 vs. 0.6–1.0 m3 s-1
inflow from catchment north and south of GF) and represents >80 % of the fjord's catchment area (1338 km2 out of ∼1600 km2; Fig. 1). The annual and seasonal discharge is proportional to
the respective hinterland precipitation. Örekilsälven's inflow
contributes to the fjord's brackish surface water layer (∼0.5 m; Fig. 1).
Further, the fjord's hydrography is governed by the water exchange with
Skagerrak (North Sea) and Kattegat (Baltic) (e.g., Arneborg, 2004). While
water masses exchange more readily above sill level with Kattegat waters,
the deep basin (>50 m) flushes approximately annually, typically
between January and April, and these ventilation events are driven by wind
direction and the winter NAO pattern (e.g., Filipsson and Nordberg, 2004).
Except for periods of water renewals, the fjord's waters are stratified in
layers of distinct salinity and renewal times (e.g., Arneborg, 2004; compare
Fig. 1).
Sampling sites and number of analyzed specimens per species
and sediment interval for each site and sampling occasion, before and after quality control of
measurements. Median Ba/Ca with median average deviation (MAD) of n-chambers
in µmol mol-1 per species, site, and season (number of data points,
i.e., analyzed specimens, indicated in brackets) for top 1 cm sediment.
SiteSpeciesSampling dateNumber of After quality control Median Ba/Ca(water depth)analyzed specimens (n-chamber spots) with MADposition(µmol mol-1)0–0.5 cm0.5–1.0 cm0–0.5 cm0.5–1.0 cm0–1 cmGF 117N. labradorica17 September 20189∗11892.77 ± 1.13 (17)inner fjord24 February 2019108873.43 ± 1.53 (15)(115–117 m)11 June 201979762.89 ± 0.71 (13)58∘19.695′ N,B. marginata17 September 201817∗101591.82 ± 1.04 (24)11∘33.147′ E24 February 201998456.84 ± 1.98 (9)11 June 2019111010106.63 ± 2.25 (20)GF 71N. labradorica17 September 20181010991.97 ± 0.26 (18)outer fjord24 February 201910108104.33 ± 1.07 (18)(69–71 m)11 June 2019111110114.10 ± 0.72 (21)58∘17.116′ N,B. marginata17 September 20181310480.55 ± 0.13 (12)11∘30.546′ E24 February 20191111872.82 ± 1.09 (15)11 June 20191011892.19 ± 1.15 (17)
∗ marks samples
compiled from two duplicate cores.
Material and methodsSampling
Our study is based on three seasonal sampling campaigns (September 2018,
February and June 2019) in Gullmar Fjord, Swedish west coast (Table 1).
During each campaign two sites were sampled, GF 71 (water depth
∼70 m b.s.l.) and GF 117 (∼116 m b.s.l.). A CTD with
O2-probe and Niskin water-collection system was deployed to record
temperature, dissolved oxygen concentration ([O2]), and
salinity (reported using the practical salinity scale) throughout the water
column and to collect bottom water. Bottom water was sampled in bottles of
25–40 mL and refrigerated until further analyses. Sediment cores were
collected with a GEMAX® twin-barrel corer (modified Gemini
corer, 9 cm diameter, from Oy Kart AB, Finland) in two casts. For
foraminiferal analyses, the top 1 cm of two duplicate cores from the first
casts was sliced in 0.5 cm sections (ca. 32 mL sediment; 0–0.5,
0.5–1.0 cm). Each sample was placed into an HDPE (high-density
polyethylene) bottle with roughly equal amounts of ambient bottom water and
36 µL of CellTracker™ Green (CTG) CMFDA dye
(5-chloromethylfluorescein diacetate; with dimethyl sulfoxide (DMSO); both
ThermoFisher Scientific) to reach a final CTG concentration of 1 µM.
The samples were incubated at 6 ∘C (in situ bottom-water
temperature 6.5–7.1 ∘C) for 10 h for living cells to
take up the CTG epifluorescent tracer for later observation (e.g., Bernhard
et al., 2006), and thereafter preserved in 40 mL of ethanol.
At site GF 117, one set of duplicate cores was sampled for bottom-,
pore-water, and solid-phase geochemical analyses. During each campaign, the
first core was used for bottom-water and pore-water sampling using Rhizons™
at 2 cm vertical resolution. The samples were collected into 10 mL
polyethylene syringes through pre-drilled holes (diameter 4 mm; e.g.,
Jokinen et al., 2020) immediately after core retrieval. Within a few hours
after retrieval, all water samples were transferred into 15 mL polypropylene
centrifuge tubes and acidified to 1 M HNO3 for elemental analyses.
The second core from each pair was sampled for solid-phase parameters. In
September 2018, the second core was immediately sampled on deck after core
retrieval. In February and June 2019, the second core was sampled a few
hours after core retrieval inside a cool room. Each core was sliced at 1 cm intervals (0–10 cm) and 2 cm intervals (10–30 cm). Each sediment slice was
transferred into plastic bags, which were carefully sealed under water, and
transferred into gas-tight glass jars. The jars were flushed with N2
and stored in the dark at 4 ∘C prior to further processing to avoid
oxidation of the samples. In preparation for solid-phases analyses, each wet
sediment slice was subsampled inside a N2-flushed glove bag and frozen
for 24 h at -20∘C. Subsequently, each sample was freeze-dried
for 48 h, homogenized, and weighed in between each step to determine the
water [g] and salt contents [g], and porosity [cm3 cm-3] using the
bottom-water salinity and the assumed solid-phase density of 2.65 g cm-3 (Burdige, 2006). The water and salt contents were used to
determine the salt-free weight of the dry sediment, which is needed to
correct the solid-phase elemental concentrations for salt dilution.
Geochemical analyses of benthic foraminifera
Living (CTG-labeled) specimens of Bulimina marginata d'Orbigny, 1826 and Nonionellina labradorica (Dawson, 1960) were
selected for laser-ablation (LA) ICP-MS trace element analysis, according to
their availability in each sample (i.e., abundance). To extract specimens
the samples were washed over a set of sieves (63, 100 µm) and the
>100µm fraction was wet-picked under epifluorescent
light (Nikon SMZ 1500 with Nikon Intensilight C-HGFI). When a single core
produced an insufficient number of specimens for analyses, specimens from
the duplicate cores were also analyzed. An average of 10 living specimens
– identified by their CTG-label – per sediment interval and site were
selected for geochemical analyses (Table 1). The tests were treated with
NaOCl (5 %) for 2–5 h to remove organic materials including cytoplasm
(e.g., Mashiotta et al., 1999), following a triple rinse with Milli-Q
(Milli-Q Integral, EMD Millipore Corporation, Billerica, MA, USA) water and
dried at room temperature.
Laser ablation ICP-MS analyses were carried out at the Department of Geology,
Lund University, Sweden, using a Bruker Aurora Elite (quadrupole) ICP-MS and
a 193 nm Cetac Analyte G2 excimer laser installed with a two-volume HelEx2
sample cell. Helium was used as carrier gas (approximately 0.8 L min-1) in
combination with a down-stream addition of argon (approximately 0.95 L min-1)
after the sample chamber. The counts of the isotope masses 138Ba, as
well as 27Al, 43Ca, 55Mn and 66Zn, were acquired at 5 Hz
(i.e., laser shots per second). To obtain a stable isotope signal a
“squid” was inserted downstream of the sample chamber. The scan time of
the ICP-MS was synchronized with the laser repetition rate to minimize
artificial fluctuations of the isotope signals. Spot sizes were adapted to
chamber sizes of the investigated species (75×75, 55×75,
50×70µm) but kept constant within individual sessions and applied
to both samples and standard material. Previous studies indicate that using
variable spot sizes does not influence elemental fractionation during LA
measurements on foraminiferal tests (e.g., Eggins et al., 1998). The samples
were analyzed at an energy density of 1 J cm-2 to avoid breaking of
foraminiferal tests and to obtain a longer ablation signal. The conditions were
mimicked for the secondary carbonate standards. Glass standards, US
National Institute of Standards and Technology SRM NIST610 and 612 (see
below), were analyzed at 3 J cm-2 for optimized ablation efficiency. The
LA ICP-MS was operated in manual mode for sample analyses. Elemental
baseline levels before each spot analysis were measured for at least 30 s.
NIST610 was used for instrument tuning to ascertain high and stable signal
counts on the studied elements, low oxide production (<0.5 %
monitoring 238U /238U16O and 232Th /232Th16O),
and low elemental fractionation (232Th /238U ratios close to 1).
NIST610 was used as external calibration material with GeoReM (Geological
and Environmental Reference Materials, 2021; Jochum et al., 2005) composition
values (via http://georem.mpch-mainz.gwdg.de, last access: 30 April 2021), giving an average internal
relative precision for Ba of 4.4 % (based on the relative standard
deviation of raw data (cps) per analysis session; Table S1), and to correct
for instrumental drift. Primary standard NIST610 and secondary standards,
NIST612, MACS-3 (MicroAnalytical Carbonate Standard, United States
Geological Survey, 2012; Jochum et al., 2012), and JCt-1 and JCp-1 (AIST Japan
calcium carbonate pellets), were run at the beginning and end of each session
and the data used to monitor the quality of the analyses (Table S1). A
selection of these – NIST610, MACS-3, JCp-1 – were additionally run after
each 10–15 sample spots.
For each specimen, the final (n), penultimate (n-1), and antepenultimate
(n-2) chambers (as far as possible) were targeted and ablated from the
outside to inside of the wall. For B. marginata also the signal of the initial chambers
was measured with a laser-spot covering proloculus and following chambers
and ablating through these chambers (referred to as “p”). We consider this
to represent a bulk signal of an extended growth period, including the
primary signal of the juvenile chambers as well subsequently added layers of
calcite with each new chamber addition (e.g., Erez, 2003).
Benthic foraminiferal trace-element data processing
Raw counts were converted to elemental concentrations (ppm) with the
software package Igor Pro 6.37 with Iolite v3.5 (Paton et al., 2011). The
element ratios were calculated based on the average concentration measured
during the ablation period of the primary test calcium carbonate. This
signal interval was determined through manual examination of each trace
element profile. Constant counts of 43Ca were used to find integration
intervals of calcite ablation. Intervals of enriched signals of 27Al,
55Mn, and/or 66Zn, indicative of secondary surface coatings and/or
contaminations, were excluded. Such were typically measured at the beginning
and/or end of each profile (corresponding to the outer and inner test wall
surfaces, respectively), with relatively constant raw elemental counts
intermediate (see also e.g., Ni et al., 2020). Trace elemental ratios
(TE/Ca) were calculated with the assumption of the TEs being bound as
carbonate (assuming 40 % wt Ca in the CaCO3 matrix) and normalized to
calcium.
In total, 486 ablation profiles (>15 data points of the ICP-MS
per signal) were performed on chambers (n, n-1, n-2 and p) of B. marginata and 346 on
chambers (n, n-1, n-2) of N. labradorica specimens and evaluated according to the
following criteria (i.e., quality control): (1) [Ba] higher than limit of
quantification (LOQ = baseline signal +10×σ; σ= standard deviation of baseline signal) (9 signals), (2) exclusion of
ablation signals with potential clay contamination (i.e., Al/Ca>0.4 mmol mol-1; Wit et al., 2010) (30 signals), (3) application of a 95 %
confidence limit on Ba/Ca, for each species, site and season, to remove
outliers (i.e., >2× standard deviation) (39 signals). In total,
78 of 832 (9.4 %) ablation profiles were rejected from further analyses.
From the benthic foraminiferal Ba/Ca values the median and MAD were
calculated per species, season, and site, to limit the influence of extreme
outliers. As no systematic differences between sediment intervals were
observed (Welch ANOVA, Table S2), Ba/Ca data were pooled. If not indicated
otherwise, only data on n-chambers were considered, assuming these were
precipitated in close temporal proximity to the time of sampling.
Pore-water and sediment trace-element analyses
The acidified bottom-water and pore-water samples were analyzed for elemental
concentrations of Ba by inductively coupled plasma-mass spectrometry
(ICP-MS; Thermo Scientific XSeries 2; Utrecht University, Department of
Earth Sciences) and of manganese (Mn), iron (Fe), and sulfur (S) by
inductively coupled plasma-optical emission spectrometry (ICP-OES; Thermo
Scientific iCAP 6000; Helsinki University, Ecosystems and Environment
Research Program). For the determination of total solid-phase Ba, Mn, and Fe
concentrations, an aliquot of 100–125 mg of freeze-dried and powdered
sediment was digested in 2.5 mL solution of 3:2 HClO4 (70 %) and
HNO3 (65 %) and HF (40 %). The Teflon vessels were left with closed
lids overnight under a fume hood with water trap on a hotplate at
90 ∘C. After 12 h, the lids were removed, and the extracts
were heated to 140 ∘C to evaporate the acids. The evaporate was
re-dissolved in 25 mL HNO3 (4.5 %) and left overnight on a hotplate at
90 ∘C. The next day, the Teflon vessels were weighed again to
determine the dilution of the final solutions. Subsequently, the sample
residues were analyzed by ICP-MS (Ba) at Utrecht University and by ICP-OES
(Mn, Fe) at the University of Helsinki. Based on sediment sample duplicates
the average analytical uncertainties were 6 ppm (Ba), 3.5 ppm (Mn), and 18.6 ppm (Fe).
Meteorological and hydrological data
All meteorological, hydrographical, and hydrological data of the Swedish
west coast, the sill fjord, and its catchment area are publicly available
from SMHI (SMHI, 2021). We used the SHARK hydrographic database
(https://sharkweb.smhi.se/hamta-data/, last access: 10 February 2021; SMHI, 2021) to obtain monthly data on salinity, [O2],
and/or chlorophyll a across 2018 and 2019 from three fjord stations
Släggö (58∘25.98′ N, 11∘43.57′ E, 70 m b.s.l.), Alsbäck (58∘19.40′ N, 11∘32.80′ E, 120 m b.s.l.), and Björkholmen (58∘23.26′ N,
11∘37.60′ E, 50 m b.s.l.) (Fig. 1), of which Alsbäck is
closely located to GF 117. Daily runoff and sediment load data were
collected from the hydrological database “vattenweb”
(https://www.smhi.se/data/hydrologi/vattenwebb, last access: 10 February 2021), partly based on the
S-HYPE (Hydrological Predictions for the Environment) model maintained by
SMHI. For both hydrological and meteorological data, the averages were
calculated for 30, 60, 90, and 120 d periods preceding the sampling
occasions directly or by 1 month, respectively (i.e., four periods ending
on and four periods ending 1 month prior to the sampling dates).
Statistical analyses
All statistical analyses were performed with the software package PAST 4.05
(Hammer et al., 2001), considering a p value of <0.05 as
significant. Since parts of the data were non-normally distributed
(Shapiro–Wilk's test) and heteroscedastic (Bartlett's test), we performed
ANOVA Welch or Kruskal–Wallis tests associated with Mann–Whitney post hoc
tests to compare Ba/Ca between seasons, sediment intervals, species, and
chambers (non-parametric tests). To determine strength and direction of
monotonic relationships between categorial and continuous variables (e.g.,
chamber number and Ba/Ca) Spearman rank correlations were tested. We used
Pearson's r to determine the relationship between two continuous variables
(e.g., Ba/Ca and river discharge; dissolved TE concentrations), using
normalized data if testing data across several sites and species (0–1
scaling).
Results and discussionContrasting hydroclimatic conditions in 2018/2019 and Ba/Ca shift in
benthic foraminifera
In the year 2018 north-western Europe experienced severe heat and drought,
ascribed to an interplaying Rossby Wave-7 circulation and positive North
Atlantic Oscillation index (NAO) across the Northern Hemisphere (e.g.,
Kornhuber et al., 2019). The occurrence of this large-scale synoptic weather
pattern has increased within the last three decades, being responsible also
for the 2003, 2006, 2012, and 2015 drought events, in response to
anthropogenic global warming (e.g., Kornhuber et al., 2019).
(a) Maps of Sweden showing annual average runoff in 2018
and 2019 compared to reference period 1961–1990 (SMHI), with study area
marked by red rectangle. (b) Average monthly river discharge of
Örekilsälven across 2018 and 2019 with median Ba/Ca with MAD per
investigated species (n-chambers) and site indicated.
In Sweden, the drought was expressed as above-normal mean air temperatures
and below-normal precipitation and distinctly imprinted in the seasonal
runoff conditions across the country (Fig. 2a; SMHI).
While the hydrological cycle naturally experiences seasonal variability –
high runoff in winter, spring, and autumn (150 mm each), low in summer (50 mm) – in 2018 already the spring was exceptionally dry. The very dry
spring conditions likely contributed to amplifying the following summer
drought (Bastos et al., 2020b), with summer and autumn precipitation being
50 %–75 % of the 1961–1990 average (southwestern Sweden).
On the contrary, the year 2019 registered above-average annual precipitation
(120 %–130 % in southwestern Sweden), although strongly varying monthly
(e.g., record-low in April, highest precipitation in May), and was warmer
than average both in Sweden and globally (Dunn et al., 2020).
In the study area the contrasting hydroclimate conditions of 2018 and 2019
is expressed in annual average discharges of Örekilsälven to GF of 18.5
and 29.0 m3 s-1, respectively (Fig. 2). Coinciding, the benthic
foraminiferal Ba/Ca shifted significantly in most samples, corresponding to
an increase of up to 5 times (Table 1; Figs. 2b, 3, S1).
Box and whisker plots of benthic foraminiferal Ba/Ca from
Gullmar Fjord (this study), Anholt (Kattegat) and Hanö Bay (Baltic Sea)
(Groeneveld et al., 2018). Grey boxes represent samples from conditions
without direct influence of terrestrial Ba addition, white boxes
terrestrial-influenced samples. Number of analyzed specimens (for LA ICP-MS
analyses) or bulk samples (of >8 specimens each, ICP-OES
analyses, marked by ∗) given in brackets. Note that Ba/Ca is derived from
different species (marked by dashed rectangles).
Correlations of foraminiferal Ba/Ca with the shift in hydroclimatic
conditions of the consecutive years imply Ba/Ca representing the discharge
60–90 d prior to sampling (r=0.36–0.40) or discharge conditions
preceding the Ba/Ca record by 1 month (r=0.46–0.48; testing Ba/Ca
across species and sites; Table S3). On the one hand, a potential lag time
in the transport of riverine Ba to coastal sediments is conceivable,
determined by marine processes influencing the sinking and deposition of Ba.
This may be influenced by environmental conditions including the degree of
primary productivity and/or bottom-water oxygenation state and thus
temporally variable (further discussed in Sect. 4.2). On the other hand, the
lagging aims to account for the uncertainties in the exact timing of
foraminiferal growth. In this respect, the lag is probably most applicable
to the foraminiferal samples collected in February 2019, assuming the majority of
foraminiferal calcite was precipitated in connection to the preceding autumn
phytoplankton bloom (e.g., Gustafsson and Nordberg, 2001; Fig. S2). Temporal
differences in reproduction and growth of the species could explain the
differing strength of correlation to the observed hydroclimatic conditions,
as well as species-specific intrinsic factors that can influence benthic
foraminiferal Ba/Ca (further discussed in Sect. 4.3). While riverine Ba
concentrations may also be temporally variable and thus could conceivably
affect foraminiferal Ba/Ca signals, we consider such variations negligible
in comparison to the contrasting Ba supply in response to the hydrological
conditions of 2018 and 2019.
The range of Ba/Ca across 2018/19 varies between the inner-fjord and the
outer-fjord site in all species (1–4 times vs. 2–5 times, respectively).
The Ba/Ca signal of the outer-fjord site is generally lower than in the
inner fjord in B. marginata, although the same is not resolved by N. labradorica (Fig. 3). To explain
the trend in B. marginata, we hypothesize that the outer-fjord site experiences larger
differences between periods of contrasting influence of Ba-rich river water
versus low-Ba seawater of Kattegat and Skagerrak. This is plausible as the
influence of Ba addition by Örekilsälven discharge diminishes, and
the frequency of water exchange events increases, from the inner to outer
fjord (Fig. S3). The fjord topography could be an additional factor, with
the deep basin acting as a Ba sink. While the effects are minor compared to
those of prolonged drought periods, these spatial factors should be of
consideration when inferring hydroclimatic gradients from foraminiferal
Ba/Ca.
The Ba/Ca signals of 2018 compared well to values expected for environments
with negligible terrestrial influence (e.g., Anholt site in Kattegat; see
Fig. 1, compilation of data from this study and Groeneveld et al., 2018 in
Fig. 3); thus they are anomalously low considering the close spatial proximity to
a river mouth (<25 km; Fig. 1) and seasonally restricted water
exchanges with the open sea (Fig. S3). On the other hand, Ba/Ca of the 2019
fjord samples ranges distinctly higher, consistent with the expectation for
coastal environments such as the Baltic Sea (Fig. 1 for location; Fig. 3).
We deduce that the drought prevailing in spring and summer 2018 caused a
distinct lack of riverine input, interrupting terrestrial Ba addition to the
fjord. The results imply that benthic foraminiferal Ba/Ca in the coastal
environment responds on short timescales to the prevailing hydrological
conditions on the adjacent continent.
Sediment evidence for Ba cycling in Gullmar Fjord
Barium is incorporated into foraminiferal tests proportionally to ambient
concentrations (e.g., de Nooijer et al., 2017). Hence, contrasting Ba2+
availability in the foraminifera's microenvironment is a prerequisite for
the observed benthic Ba/Ca signals. For planktonic foraminifera it has been
shown that Ba incorporation is temperature- and salinity-independent (e.g.,
Hönisch et al., 2011). As both parameters were stable in the fjord (T=6.7±0.2∘C, S=34.4±0.1), an
influence on benthic Ba/Ca can be excluded. Bottom waters stayed oxygenated,
albeit over a large gradient (ambient [O2] during sampling = 70–217 µmol L-1; Fig. S3), and [O2] shows no correlation to benthic
Ba/Ca. This implies that Ba2+ concentrations close to the
sediment–water interface must have changed between the sampling moments.
(a) Solid phase and (b) dissolved trace element
profiles of site GF 117 at each sampling occasion.
The expected difference in Ba2+ availability between 2018 and 2019,
however, is not clearly supported by the Ba sediment profiles obtained
during sampling (Fig. 4; based on GF 117). Sedimentary Ba contents range
from 3.11–3.53 µmol g-1, indicating a strong enrichment relative to
sediments of the Skagerrak (ca. 0.41 µmol g-1 particulate Ba; Lepland
et al., 2000), confirming a near-shore accumulation due to terrigenous
inputs. However, no significant changes were recorded in the surface
sediment Ba concentration. Meanwhile, pore-water Ba profiles indicate
diagenetic remobilization of Ba at depth in the sediment column. Values in
excess of 1 µmol L-1 Ba2+ at 20 cm depth – distinctly higher than
marine water column values (Kattegat [Ba] = 0.02–0.06 µmol L-1,
Groeneveld et al., 2018; global open ocean surface [Ba] = 0.04–0.05 µmol L-1, Hsieh and Henderson, 2017) – support the concept of Ba
release into pore waters fueling potential uptake into benthic foraminifera.
However, the pore-water profiles also do not show significant changes
between sampling years and most of the enrichment of Ba2+ in
pore waters is observed deeper in the sediment column.
To explain the observed changes in foraminiferal Ba/Ca between 2018 and 2019
in the absence of whole-core sediment or pore-water geochemical evidence, we
infer that rapid sub-annual processes at the sediment–water interface must
be responsible. With sedimentation rates at the study sites being 0.7–0.9 cm yr-1 (Filipsson and Nordberg, 2004), only millimeters of new sediment
material are expected to accumulate between the sampling occasions.
Sub-annual changes in reactive Ba supply within this material, however, may
dictate pore-water Ba release close to the sediment–water interface and
uptake into benthic foraminifera. Such changes are unlikely to be captured
in the pore-water profiles obtained at 2 cm resolution on the three sampling
occasions. We infer therefore that during wet years such as 2019, reactive
Ba is transported into the fjord in short-timescale pulses directed by
continental rainfall and riverine discharge events, resulting in transient
high fluxes of reactive Ba, which can be recorded by benthic foraminiferal
Ba/Ca. Such temporally restricted fluxes may also explain the high relative
standard deviation of Ba/Ca (in n-chambers of B. marginata= 46 %–71 %; N. labradorica= 17 %–69 %). While chamber precipitation in foraminifera is generally
completed within a few hours (e.g., de Nooijer et al., 2009), population
signals represent cumulative seasonal trends in terrestrial Ba input and
benthic availability. In the following we explore potential sources of
reactive Ba.
Potential sources of short-timescale changes in Ba/Ca: biogenic barite
cycling
In marine environments Ba can be associated with various particulate phases,
including detrital or “biogenic” barite (BaSO4), and adsorbed to
organic matter, carbonates, or metal oxides (e.g., Coffey et al., 1997;
Dehairs et al., 1980; McManus et al., 1998). Of these, biogenic barite is
the most commonly studied and therefore we must consider the potential of
biogenic barite cycling to influence the observed signals in foraminiferal
Ba/Ca.
In open-ocean settings primary productivity acts as the main facilitator of
downward Ba transport: dissolved Ba is adsorbed or bound to primary
producers and forms biogenic barite upon organic matter degradation in the
water column (e.g., Dehairs et al., 1980; Martinez-Ruiz et al., 2019).
Thereby, a Ba–productivity relationship develops that is frequently used to
infer paleo-productivity changes from benthic foraminiferal Ba/Ca (e.g.,
Mojtahid et al., 2019; Ní Fhlaithearta et al., 2010). For example,
shifts from low to high paleo-productivity periods reflect in benthic Ba/Ca
increases by 2–3 times in the Mediterranean (before and during sapropel S1;
Mojtahid et al., 2019; Ní Fhlaithearta et al., 2010) and <1.5
in the Bahama Banks and Caribbean Sea (Last Glacial Maximum to deglaciation;
Hall and Chan, 2004). However, this relationship is a function of water
depth due to barite crystal growth rates and settling rates and cannot be
reliably extended to shallow-water deposits as studied here (<1000 m water depth; e.g., Von Breymann et al., 1992). Still, episodic removal of
Ba2+ by barite formation in association with phytoplankton blooms is
known from estuaries, although typically showing rapid Ba regeneration with
riverine discharge surges (e.g., Stecher and Kogut, 1999).
A theoretical mechanism for short-timescale release of Ba from biogenic
barite in sediments is variable pore-water sulfate undersaturation (e.g.,
McManus et al., 1998). However, no evidence for this mechanism is observed
in the fjord. Pore-water total S concentrations, primarily reflecting
sulfate, are stable throughout the sediment column (Fig. 4b). Organo-clastic
sulfate reduction (e.g., McManus et al., 1998) is primarily activated once
more energetically beneficial electron acceptors are consumed (oxygen,
nitrogen, manganese (Mn), iron (Fe); e.g., Froelich et al., 1979). In the
sediments of GF the anaerobic reduction of Mn and Fe (oxyhydr)oxides
(hereafter referred to as Mn or Fe oxides) dominates, as evidenced by
pore-water Mn and Fe enrichments (down to ca. 12.5–25 cm depth; Fig. 4b).
The fjord's high sedimentary Mn content makes sulfate reduction and
diagenetic Ba2+ release within the habitat of benthic foraminifera
generally unlikely (e.g., Vandieken et al., 2012). Hence, both biogenic and
detrital barite can be considered as unreactive in this setting.
Ba and metal oxide cycling
In transition zones between fluvial and marine environments, such as GF,
estuarine mixing behavior modulates Ba addition from the continents through
the desorption of Ba2+ from suspended matter of fluvial origin
(terrestrial aluminosilicates and other detrital material) by cation
exchange at low to medium salinities (e.g., Coffey et al., 1997; Hanor and
Chan, 1977). However, Ba may then become associated with Fe and Mn oxides
(e.g., Coffey et al., 1997), due to the oxides' affinity for Ba and its
chemical analogues (e.g., Balistrieri and Murray, 1986). This association
potentially allows Ba to be sedimented in the near-shore environment,
providing a mechanism for short-term changes in Ba supply to the seafloor.
An overall coupling of Ba to Mn (and Fe) cycling in the GF sediments is
supported by the pore-water profiles showing coinciding pore-water maxima of
these elements (Fig. 4) and positive inter-element correlations (Fig. S4).
We propose that short-timescale pulses of Fe and Mn oxide sedimentation in
association with discharge events also lead to rapid release of Ba into
solution at the seafloor as oxides are utilized in the remineralization of
fresh organic matter. Based on experimental estimates of Fe/Mn oxide
particle settling velocity in the Baltic, oxides shuttle through the
water column in the order of weeks, considering the water depths of the
study sites (Glockzin et al., 2014). This constitutes a transport mechanism
with a short temporal delay between surface Ba addition and foraminiferal
incorporation at depth that is in line with the correlation of Ba/Ca to
riverine fluxes strengthening with increasing time between sampling and
river discharge period (see Sect. 4.1; Table S3).
The influence of Ba2+ liberated from recently accumulated Fe and Mn
oxides appears to decrease below the water–sediment interface, where
Ba2+ availability is dictated more by processes deeper in the sediment
column. Barium is remobilized at depth in the sediment column during oxide
reduction, as indicated by the upward diffusion of Ba2+ from below ca.
10 cm depth (Fig. 4b). This liberation of Ba2+ from Mn oxides appears
to be constant, as well as negligible in magnitude compared to the
terrigenous sedimentary Ba background signal, showing the dominance of
non-reactive Ba in the system (Fig. 4a). Therefore, shallow-infaunal species
such as B. marginata are expected to be exposed to greater variability in Ba2+
availability than deep infaunal species.
Proxy potential of N. labradorica and B. marginata
Ba/Ca of N. labradorica and B. marginata are significantly different for most seasons at each site
(Table S4; Welch ANOVA). Partly, this may be an effect of species-specific
differences in trace element incorporation, which has been documented for
several taxa of foraminifera including Bulimina and Nonionellina (e.g., Barras et al., 2018; de
Nooijer et al., 2017; Koho et al., 2017).
While B. marginata's Ba/Ca reliably records fluctuations in terrestrial-derived Ba
input, N. labradorica shows seasonal shifts in Ba/Ca only in the outer-fjord site. Against
expectation, Ba/Ca of the inner-fjord site does not deviate substantially
from values recorded during the drought period in 2018 in N. labradorica. This might be
connected to a deeper effective living depth in this site, exposing the
specimens to a constant, low-level supply of Ba2+ (as discussed in
Sect. 4.2.2). Factors determining migration and distribution of this species
comprise food supply and the bottom-water oxygenation regime (e.g., Alve and
Bernhard, 1995), which conceivably could vary between the inner and outer
fjord. Nonionellina labradorica was less abundant in the inner fjord. Generally, its abundance has
been decreasing within GF since the early 1980s, likely driven by the
increase in periods of bottom-water hypoxia ([O2] <64µmol L-1) (e.g., Filipsson and Nordberg, 2004). This potential sensitivity of
N. labradorica's Ba/Ca signal to factors other than terrestrial input-driven Ba
variability should be considered in proxy interpretations. Oppositely, B. marginata is
particularly promising as a proxy for riverine discharge.
Incorporation of trace elements, including Ba2+, is further known to be
a function of ontogeny in some taxa, as calcification rates typically
decrease and control of element partitioning increases over the growth of
tests (e.g., de Nooijer et al., 2017). Analyses across successive chambers
of B. marginata and N. labradorica do show significant differences between chambers for some samples
(Mann–Whitney test, Table S5). Particularly the proloculus area of B. marginata exhibits
almost consistently higher Ba/Ca ratios than later chambers (i.e., n, n-1,
n-2; Fig. S1). Because the strength and direction of the inter-chamber
correlations vary (Spearman rank correlation, Table S5) we infer a combined
influence of ontogeny and seasonal differences in pore-water [Ba] imprinting
in successive chambers. As the sharp trend across the studied years is seen
both in most recently formed chambers (n) and prior-formed chambers
(n-1, n-2; p for B. marginata; Fig. S1), environmental changes appear to overprint
ontogenetic effects on Ba/Ca at least across strong hydroclimatic gradients
as experienced between 2018 and 2019.
Implications for applications of benthic Ba/Ca as paleo-river discharge
and drought proxy
Our study highlights the potential of benthic foraminiferal Ba/Ca as
indicator of drought and river discharge conditions in near-continent
records, based on signals of living foraminifera populations. We demonstrate
that the input of terrestrial Ba through riverine discharge and runoff is
the dominant control on surface sediment Ba availability and foraminiferal
Ba/Ca, in the investigated coastal region. As shallow-infaunal foraminifera
(e.g., B. marginata) show a stronger response in Ba/Ca to continental hydroclimatic
conditions, we deduce that their micro-habitat exposes them to higher
Ba2+ variability from terrestrial sources, and therefore that these
species may be more suitable as recorders of drought. The rapid cycling of
bioavailable Ba at or close to the water–sediment interface appears to be
modulated by the sedimentation and dissolution of Fe–Mn oxide carrier phases
on short timescales. Therefore, we caution that a specific set of conditions
may be required for the proxy to faithfully record hydroclimatic conditions.
Most importantly, GF is characterized by high rates of Fe and Mn oxide
accumulation under a generally oxic water column, which favors a strong role
of dissimilatory oxide reduction in diagenesis and therefore enhanced
oxide-mediated Ba cycling. Other systems with lower oxide fluxes or
persistent anoxic conditions may not be suitable to record Ba/Ca signals of
hydrodynamic changes.
In our data the variations in Ba/Ca are foremost explained by changes in
physico-chemical parameters of the ambient environment during test
precipitation but seem to be partly controlled by species-specific vital
effects. While the seasonal trend in Ba/Ca across opposing river discharge
conditions is significant even when pooling the two investigated taxa, we
highlight that monospecific analyses are preferable. The data set suggests
that ontogenetic trends in Ba incorporation of the studied species are
probable. Nevertheless, as all investigated chambers show comparable trends
in relation to continental hydroclimate conditions, at least on annual
timescales, bulk trace-elemental analyses (of specimens of restricted test
sizes) are likely sufficient for Ba/Ca proxy applications in paleo-studies.
High-temporal-resolution interpretation of hydrodynamic changes in
paleo-applications (e.g., sub-annual timescales) should be considered with
caution unless uncertainties regarding the duration of Ba downward
transport in the water column and lag between Ba input and foraminiferal
incorporation can be resolved. Further, the sensitivity of the technique to
more subtle changes than experienced between the extreme and contrasting
weather conditions during 2018 and 2019 remains to be determined.
Further studies considering the interplay of riverine discharge, open-ocean
water exchange, vertical and horizontal Ba transport, and hinterland
precipitation are needed to assess the ability of the proxy to reconstruct
river discharge quantitatively.
Data availability
The foraminiferal trace element data generated during this study are stored at the Swedish National Data Service (SND) under Brinkmann (2022).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-19-2523-2022-supplement.
Author contributions
The concept was developed by IB and HLF. IB, TJ, KMP, MS, and HLF
participated in the sampling campaigns. IB, TJ, TN, and KMP performed the
measurements. CB, TJ, and TN validated the data. IB analyzed the data and
prepared the manuscript draft. All co-authors reviewed and edited the
manuscript. HLF administered the project, provided resources, and
supervised.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors thank the captain and crew of the RV Oscar von Sydow and RV Skagerak for technical assistance. We thank Sami Jokinen and Hanna Nilsson for their help during the cruises. We acknowledge the staff of the Kristineberg Marine Research Station for their support during the field campaigns. Further, the hydrographic data used in this project are from SMHI's database, including SHARK. The SHARK data collection is organized by the Swedish environmental monitoring program and funded by the Swedish Agency for Marine and Water Management (SwAM). Finally, we thank the editor and the two anonymous referees for their feedback.
Financial support
This research has been supported by the Vetenskapsrådet (grant no. 2017-04190), the Kungliga Fysiografiska Sällskapet i Lund (grant no. –), the Academy of Finland (grant nos. 317684 and 319956), and the Crafoordska Stiftelsen (grant no. –).
Review statement
This paper was edited by Tyler Cyronak and reviewed by two anonymous referees.
References
Alve, E. and Bernhard, J. M.: Vertical migratory response of benthic
foraminifera to controlled oxygen concentrations in an experimental
mesocosm, Mar. Ecol. Prog. Ser., 116, 137–151, 1995.Arneborg, L.: Turnover times for the water above sill level in Gullmar
Fjord, Cont. Shelf Res., 24, 443–460, 10.1016/j.csr.2003.12.005, 2004.Bahr, A., Schönfeld, J., Hoffmann, J., Voigt, S., Aurahs, R., Kucera,
M., Flögel, S., Jentzen, A., and Gerdes, A.: Comparison of Ba/Ca and
δ18O as freshwater proxies: A multi-species core-top study on
planktonic foraminifera from the vicinity of the Orinoco River mouth, Earth
Planet Sc. Lett., 383, 45–57, 10.1016/j.epsl.2013.09.036, 2013.Balistrieri, L. S. and Murray, J. W.: The surface chemistry of sediments
from the Panama Basin: The influence of Mn oxides on metal adsorption,
Geochim. Cosmochim. Ac., 50, 2235–2243, 10.1016/0016-7037(86)90078-5,
1986.Barras, C., Aurélia, M., Pia, N. M., Edouard, M., Jassin, P., Carole,
L., Filipsson, H. L., and Frans, J.: Experimental calibration of manganese
incorporation in foraminiferal calcite, Geochim. Cosmochim. Ac., 237, 49–64,
10.1016/j.gca.2018.06.009, 2018.Bastos, A., Fu, Z., Ciais, P., Friedlingstein, P., Sitch, S., Pongratz, J.,
Weber, U., Reichstein, M., Anthoni, P., Arneth, A., Haverd, V., Jain, A.,
Joetzjer, E., Knauer, J., Lienert, S., Loughran, T., McGuire, P. C.,
Obermeier, W., Padrón, R. S., Shi, H., Tian, H., Viovy, N., and Zaehle,
S.: Impacts of extreme summers on European ecosystems: a comparative
analysis of 2003, 2010 and 2018, Philos. T. Roy. Soc. B, 375,
20190507, 10.1098/rstb.2019.0507, 2020a.Bastos, A., Ciais, P., Friedlingstein, P., Sitch, S., Pongratz, J., Fan, L.,
Wigneron, J. P., Weber, U., Reichstein, M., Fu, Z., Anthoni, P., Arneth, A.,
Haverd, V., Jain, A. K., Joetzjer, E., Knauer, J., Lienert, S., Loughran,
T., McGuire, P. C., Tian, H., Viovy, N., and Zaehle, S.: Direct and seasonal
legacy effects of the 2018 heat wave and drought on European ecosystem
productivity, Sci. Adv., 6, eaba2724, 10.1126/sciadv.aba2724, 2020b.Bernhard, J. M., Ostermann, D. R., Williams, D. S., and Blanks, J. K.:
Comparison of two methods to identify live benthic foraminifera: A test
between Rose Bengal and CellTracker Green with implications for stable
isotope paleoreconstructions, Paleoceanography, 21, PA4210, 10.1029/2006PA001290, 2006.Burdige, D. J.: Geochemistry of marine sediments, Princeton University
Press, Princeton, NJ, USA, 10.2307/j.ctv131bw7s, 2006.Brinkmann, I.: Trace-elemental data (Ba/Ca) of benthic foraminifers from core-top sediments of Gullmar Fjord, Swedish West coast, Swedish National Data Service [data set], 1, https://doi.org/10.5878/eb67-9v72, 2022.Coffey, M., Dehairs, F., Collette, O., Luther, G., Church, T., and Jickells,
T.: The behaviour of dissolved Barium in estuaries, Estuar. Coast Shelf Sci.,
45, 113–121, 10.1006/ecss.1996.0157, 1997Cook, B. I., Mankin, J. S., Marvel, K., Williams, A. P., Smerdon, J. E., and
Anchukaitis, K. J.: Twenty-first century drought projections in the CMIP6
forcing scenarios, Earth's Future, 8, e2019EF001461, 10.1029/2019ef001461, 2020.de Nooijer, L. J., Toyofuku, T., and Kitazato, H.: Foraminifera promote
calcification by elevating their intracellular pH, P. Natl. Acad. Sci. USA, 106,
15374–15378, 10.1073/pnas.0904306106, 2009.de Nooijer, L. J., Brombacher, A., Mewes, A., Langer, G., Nehrke, G., Bijma, J., and Reichart, G.-J.: Ba incorporation in benthic foraminifera, Biogeosciences, 14, 3387–3400, 10.5194/bg-14-3387-2017, 2017.Dehairs, F., Chesselet, R., and Jedwab, J.: Discrete suspended particles of
barite and the barium cycle in the open ocean, Earth Planet. Sc. Lett., 49,
528–550, 10.1016/0012-821X(80)90094-1, 1980.
Dunn, R. J. H., Stanitski, D. M., Gobron, N., and Willett, K. M.: Global Climate, in: State of the Climate in 2019, edited by: Blunden, J. and Arndt, D. S., B. Am. Meteorol. Soc., 101, 9–128, https://doi.org/10.1175/BAMS-D-20-0104.1, 2020.Eggins, S. M., Rudnick, R. L., and McDonough, W. F.: The composition of
peridotites and their minerals: a laser-ablation ICP–MS study, Earth Planet. Sc. Lett., 154, 53–71, 10.1016/S0012-821X(97)00195-7, 1998.Erez, J.: The source of ions for biomineralization in foraminifera and their
implications for paleoceanographic proxies, Rev. Mineral. Geochem., 54,
115–149, 10.2113/0540115, 2003.Filipsson, H. L. and Nordberg, K.: Climate variations, an overlooked factor
influencing the recent marine environment, An example from Gullmar Fjord,
Sweden, illustrated by benthic foraminifera and hydrographic data,
Estuaries, 27, 867–881, 10.1007/BF02912048, 2004.Froelich, P. N., Klinkhammer, G., Bender, M. L., Luedtke, N., Heath, G. R.,
Cullen, D., Dauphin, P., Hammond, D., Hartman, B., and Maynard, V.: Early
oxidation of organic matter in pelagic sediments of the eastern equatorial
Atlantic: suboxic diagenesis, Geochim. Cosmochim. Ac., 43, 1075–1090, 10.1016/0016-7037(79)90095-4, 1979.GeoReM: Geological and Environmental Reference Material: http://georem.mpch-mainz.gwdg.de, last access: 30 April 2021.Glockzin, M., Pollehne, F., and Dellwig, O.: Stationary sinking velocity of
authigenic manganese oxides at pelagic redoxclines, Mar. Chem., 160, 67–74,
10.1016/j.marchem.2014.01.008, 2014.Groeneveld, J., Filipsson, H. L., Austin, W. E., Darling, K., McCarthy, D.,
Krupinski, N. B. Q., Bird, C., and Schweizer, M.: Assessing proxy signatures
of temperature, salinity, and hypoxia in the Baltic Sea through
foraminifera-based geochemistry and faunal assemblages, J. Micropalaentol.,
37, 403–429, 10.5194/jm-37-403-2018, 2018.Gustafsson, M. and Nordberg, K.: Living (stained) benthic foraminiferal
response to primary production and hydrography in the deepest part of the
Gullmar Fjord, Swedish West Coast, with comparisons to Höglund's 1927
material, J. Foraminiferal. Res., 31, 2–11, 10.2113/0310002, 2001.Hall, J. M. and Chan, L. H.: Ba/Ca in benthic foraminifera: Thermocline and
middepth circulation in the North Atlantic during the last glaciation,
Paleoceanography, 19, PA4018, 10.1029/2004PA001028, 2004.Hammer, O., Harper, D. A. T., and Ryan, P. D.: PAST: Paleontological
statistics software package for education and data analysis, Palaeontol.
Electron., 4, 9 pp., http://palaeo-electronica.org/2001_1/past/issue1_01.htm (last access: 30 April 2021), 2001.Hanor, J. S. and Chan, L. H.: Non-conservative behavior of barium during
mixing of Mississippi River and Gulf of Mexico waters, Earth Planet. Sc. Lett.,
37, 242–250, 10.1016/0012-821X(77)90169-8, 1977.Hönisch, B., Allen, K. A., Russell, A. D., Eggins, S. M., Bijma, J.,
Spero, H. J., Lea, D. W., and Yu, J.: Planktic foraminifers as recorders of
seawater Ba/Ca, Mar. Micropaleontol., 79, 52–57, 10.1016/j.marmicro.2011.01.003, 2011.Howe, J. A., Austin, W. E., Forwick, M., Paetzel, M., Harland, R., and Cage,
A. G.: Fjord systems and archives: a review, Geol. Soc. Spec. Publ., 344, 5–15,
10.1144/SP344.2, 2010.Hsieh, Y.-T. and Henderson, G. M.: Barium stable isotopes in the global
ocean: Tracer of Ba inputs and utilization, Earth Planet. Sc. Lett., 473,
269–278, 10.1016/j.epsl.2017.06.024, 2017.Jochum, K. P., Nohl, U., Herwig, K., Lammel, E., Stoll, B., and Hofmann, A.
W.: GeoReM: A new geochemical database for reference materials and isotopic
standards, Geostand. Geoanal. Res., 29, 333–338, 10.1111/j.1751-908X.2005.tb00904.x, 2005.Jochum, K. P., Scholz, D., Stoll, B., Weis, U., Wilson, S. A., Yang, Q.,
Schwalb, A., Börner, N., Jacob, D. E., and Andreae, M. O.: Accurate trace
element analysis of speleothems and biogenic calcium carbonates by
LA-ICP-MS, Chem. Geol., 318, 31–44, 10.1016/j.chemgeo.2012.05.009,
2012.Jokinen, S. A., Koho, K., Virtasalo, J. J., and Jilbert, T.: Depth and
intensity of the sulfate-methane transition zone control sedimentary
molybdenum and uranium sequestration in a eutrophic low-salinity setting,
Appl. Geochem., 122, 104767, 10.1016/j.apgeochem.2020.104767, 2020.Koho, K. A., de Nooijer, L. J., Fontanier, C., Toyofuku, T., Oguri, K., Kitazato, H., and Reichart, G.-J.: Benthic foraminiferal Mn/Ca ratios reflect microhabitat preferences, Biogeosciences, 14, 3067–3082, 10.5194/bg-14-3067-2017, 2017.Kornhuber, K., Osprey, S., Coumou, D., Petri, S., Petoukhov, V., Rahmstorf,
S., and Gray, L.: Extreme weather events in early summer 2018 connected by a
recurrent hemispheric wave-7 pattern, Environ. Res. Lett., 14, 054002, 10.1088/1748-9326/ab13bf, 2019.Lepland, A., Sæther, O., and Thorsnes, T.: Accumulation of barium in
recent Skagerrak sediments: sources and distribution controls, Mar. Geol.,
163, 13–26, 10.1016/S0025-3227(99)00104-8, 2000.Martinez-Ruiz, F., Paytan, A., Gonzalez-Muñoz, M. T., Jroundi, F., Abad,
M. M., Lam, P. J., Bishop, J. K. B., Horner, T. J., Morton, P. L., and
Kastner, M.: Barite formation in the ocean: Origin of amorphous and
crystalline precipitates, Chem. Geol., 511, 441–451, 10.1016/j.chemgeo.2018.09.011, 2019.Mashiotta, T. A., Lea, D. W., and Spero, H. J.: Glacial–interglacial changes
in subantarctic sea surface temperature and δ18O-water using
foraminiferal Mg, Earth Planet. Sc. Lett., 170, 417–432, 10.1016/S0012-821X(99)00116-8, 1999.McManus, J., Berelson, W. M., Klinkhammer, G. P., Johnson, K. S., Coale, K.
H., Anderson, R. F., Kumar, N., Burdige, D. J., Hammond, D. E., Brumsack, H.
J., McCorkle, D. C., and Rushdi, A.: Geochemistry of barium in marine
sediments: implications for its use as a paleoproxy, Geochim. Cosmochim. Ac.,
62, 3453–3473, 10.1016/S0016-7037(98)00248-8, 1998.Mojtahid, M., Hennekam, R., De Nooijer, L., Reichart, G.-J., Jorissen, F.,
Boer, W., Le Houedec, S., and De Lange, G. J.: Evaluation and application of
foraminiferal element/calcium ratios: Assessing riverine fluxes and
environmental conditions during sapropel S1 in the Southeastern
Mediterranean, Mar. Micropaleontol., 153, 101783, 10.1016/j.marmicro.2019.101783, 2019.
Murray, J.: Ecology and Applications of Benthic Foraminifera, Cambridge
University Press, Cambridge, UK, ISBN 9780521828390, 2006.Ní Fhlaithearta, S., Reichart, G.-J., Jorissen, F. J., Fontanier, C.,
Rohling, E. J., Thomson, J., and De Lange, G. J.: Reconstructing the seafloor
environment during sapropel formation using benthic foraminiferal trace
metals, stable isotopes, and sediment composition, Paleoceanography, 25,
PA4225, 10.1029/2009pa001869, 2010.Ni, S., Krupinski, N. B. Q., Groeneveld, J., Fanget, A. S., Böttcher, M.
E., Liu, B., Lipka, M., Knudsen, K. L., Naeraa, T., Seidenkrantz, M. S., and
Filipsson, H. L.: Holocene hydrographic variations from the Baltic-North Sea
transitional area (IODP site M0059), Paleoceanogr. Paleoclimatol., 35,
e2019PA003722, 10.1029/2019pa003722, 2020.Paton, C., Hellstrom, J., Paul, B., Woodhead, J., and Hergt, J.: Iolite:
Freeware for the visualisation and processing of mass spectrometric data, J.
Anal. At. Spectrom., 26, 2508–2518, 10.1039/C1JA10172B, 2011.Paytan, A. and Griffith, E. M.: Marine barite: Recorder of variations in
ocean export productivity, Deep-Sea Res. Pt. II, 54,
687–705, 10.1016/j.dsr2.2007.01.007, 2007.Peters, W., Bastos, A., Ciais, P., and Vermeulen, A.: A historical,
geographical and ecological perspective on the 2018 European summer drought,
Philos. T. Roy. Soc. B, 375, 20190505, 10.1098/rstb.2019.0505, 2020.Saha, N., Rodriguez-Ramirez, A., Nguyen, A. D., Clark, T. R., Zhao, J.-X.,
and Webb, G. E.: Seasonal to decadal scale influence of environmental
drivers on Ba/Ca and Y/Ca in coral aragonite from the southern Great Barrier
Reef, Sci. Total Environ., 639, 1099–1109, 10.1016/j.scitotenv.2018.05.156, 2018.Stecher, H. A. and Kogut, M. B.: Rapid barium removal in the Delaware
estuary, Geochim. Cosmochim. Ac., 63, 1003–1012, 10.1016/S0016-7037(98)00310-X, 1999.Steiger, N. J., Smerdon, J. E., Cook, E. R., and Cook, B. I.: A
reconstruction of global hydroclimate and dynamical variables over the
Common Era, Sci. Data, 5, 180086, 10.1038/sdata.2018.86, 2018.Sternberg, T.: Regional drought has a global impact, Nature, 472, 169–169,
10.1038/472169d, 2011.SHARK: Svenskt HavsARKivs hydrographic database: https://sharkweb.smhi.se/hamta-data/, last access: 10 February 2021.SMHI: Swedish Meteorological and Hydrological Institute's database [data set], https://www.smhi.se/, last access: 10 February.
Tallaksen, L. M. and Van Lanen, H. A. (Eds.): Hydrological drought:
processes and estimation methods for streamflow and groundwater, Dev. Water
Sci., Elsevier, Amsterdam, Netherlands, 48 pp., 2004.Vandieken, V., Pester, M., Finke, N., Hyun, J.-H., Friedrich, M. W., Loy, A.,
and Thamdrup, B.: Three manganese oxide-rich marine sediments harbor similar
communities of acetate-oxidizing manganese-reducing bacteria, ISME J., 6,
2078–2090, 10.1038/ismej.2012.41, 2012.
Vattenweb hydrological database: https://www.smhi.se/data/hydrologi/vattenwebb, last access: 10
February 2021.Von Breymann, M. T., Emeis, K.-C., and Suess, E.: Water depth and diagenetic
constraints on the use of barium as a palaeoproductivity indicator, Geol. Soc.
Spec. Publ., 64, 273–284, 10.1144/gsl.Sp.1992.064.01.18, 1992.Wit, J., Reichart, G.-J., Jung, S., and Kroon, D.: Approaches to unravel
seasonality in sea surface temperatures using paired single-specimen
foraminiferal δ18O and Mg/Ca analyses, Paleoceanography, 25,
PA4220, 10.1029/2009PA001857, 2010.Wolgemuth, K. and Broecker, W. S.: Barium in sea water, Earth Planet. Sc. Lett.,
8, 372–378, 10.1016/0012-821X(70)90110-X, 1970.