Nutrient cycles in the Arctic Ocean are being altered by
changing hydrography, increasing riverine inputs, glacial melt and sea-ice
loss due to climate change. In this study, combined isotopic measurements of
dissolved nitrate (δ15N-NO3 and δ18O-NO3) and silicic acid (δ30Si(OH)4) are used
to understand the pathways that major nutrients follow through the Arctic
Ocean. Atlantic waters were found to be isotopically lighter (δ30Si(OH)4=+ 1.74 ‰) than their polar
counterpart (δ30Si(OH)4=+ 1.85 ‰)
owing to partial biological utilisation of dissolved Si (DSi) within the
Arctic Ocean. Coupled partial benthic denitrification and nitrification on
Eurasian Arctic shelves lead to the enrichment of δ15N-NO3 and lighter δ18O-NO3 in the polar
surface waters (δ15N-NO3= 5.44 ‰,
δ18O-NO3= 1.22 ‰) relative to
Atlantic waters (δ15N-NO3= 5.18 ‰,
δ18O-NO3= 2.33 ‰). Using a
pan-Arctic DSi isotope dataset, we find that the input of isotopically light
δ30Si(OH)4 by Arctic rivers and the subsequent partial
biological uptake and biogenic Si burial on Eurasian shelves are the key
processes that generate the enriched isotopic signatures of DSi exported
through Fram Strait. A similar analysis of δ15N-NO3
highlights the role of N-limitation due to denitrification losses on Arctic
shelves in generating the excess dissolved silicon exported through Fram
Strait. We estimate that around 40 % of DSi exported in polar surface
waters through Fram Strait is of riverine origin. As the Arctic Ocean is
broadly N-limited and riverine sources of DSi are increasing faster than
nitrogen inputs, a larger silicic acid export through the Fram Strait is
expected in the future. Arctic riverine inputs therefore have the potential
to modify the North Atlantic DSi budget and are expected to become more
important than variable Pacific and glacial DSi sources over the coming
decades.
Introduction
The dissolved macronutrients nitrate (NO3-) and silicic acid
(Si(OH)4) are key nutrients in sustaining marine primary production in
the Arctic Ocean and have distinct sources from the Atlantic and Pacific
oceans (Tremblay et al., 2015).
Additionally, river and coastal erosion contribute dissolved silicon (DSi)
and nitrate which fuel approximately 30 % of Arctic-wide net primary
productivity (Terhaar et al., 2021). The Greenland
ice sheet has also been suggested as an important source of DSi to the
Arctic Ocean (Hatton et al.,
2019; Hawkings et al., 2017). It has been estimated that >85 %
of DSi from riverine sources is not consumed by phytoplankton
(Le
Fouest et al., 2013) and is exported out instead, but the controlling
processes of this remain unclear. Thus, an integrated understanding of the
relative importance of sources to the internal cycling of DSi and the
controls on the export of DSi to the North Atlantic is lacking. Stable
isotope measurements of nitrate (δ15N-NO3 and δ18O-NO3) and dissolved silicon (δ30Si(OH)4) can provide useful insights into nutrient
sources and cycling within the ocean
(Brzezinski
et al., 2021; Sigman et al., 2000; Varela et al., 2016), particularly when
both isotopes are combined
(Grasse
et al., 2016; De Souza et al., 2012). In this study, we present the first
full profiles of δ30Si(OH)4 measurements in Fram Strait
and over the East Greenland shelf in conjunction with nitrate isotopes to
examine the controls on DSi export through the Fram Strait and suggest
potential future scenarios.
In the Arctic Ocean, primary production is controlled by complex
interactions between light availability and nutrient limitation
(Giesbrecht
and Varela, 2021; Popova et al., 2012; Yool et al., 2015) which are highly
variable both spatially and temporally. Nitrogen is the primary limiting
nutrient for primary production in the Eurasian Arctic
(Krisch et al., 2020;
Tuerena et al., 2021a), and sedimentary denitrification on shallow Arctic
shelves play an important role in limiting nitrogen availability
(Fripiat
et al. 2018; Granger et al., 2018), making the Arctic Ocean a net sink of
nitrate overall (Yamamoto-Kawai et al., 2006). In
contrast, there is an excess of DSi in the Arctic Ocean and a
disproportionally large amount of DSi is exported to the North Atlantic via
Fram Strait and the Canadian Arctic Archipelago. Budget estimates have shown
that the Arctic Ocean contributes to more than 10 % of the DSi entering
the North Atlantic
(Torres-Valdés et al.,
2013).
The excess of DSi in the Arctic Ocean's DSi budget is attributed to Pacific
water, which enters the Arctic through the Bering Strait, but also
freshwater sources, as highlighted in Fig. 1. The Arctic Ocean receives a
disproportionally large volume of freshwater relative to its area
(>10 % of the world's riverine discharge) from several of the
world's largest rivers, such as the Ob, Yenisei, Lena and Kolyma rivers
which discharge onto the Eurasian shelves. These four rivers alone provide
1755 km3 of freshwater to Arctic shelves annually, along with 135×109 g of nitrate and 4816×109 g of DSi (Holmes et
al., 2012), which fuels coastal and Arctic-wide productivity, subsequently
transported through the Transpolar Drift (TPD)
(Charette
et al., 2020; Terhaar et al., 2021). Arctic glacial meltwaters provide a
potentially significant contribution to the Arctic's nutrient budget
(Hatton et al.,
2019; Meire et al., 2016), with DSi and amorphous Si inputs from the
Greenland ice sheet estimated to constitute around 37 % of riverine fluxes
in the coastal regions of Arctic seas (Hawkings et
al., 2017). However, large quantities of DSi are removed within fjords
(Hopwood et al., 2020) and the
fraction that is exported from Greenland and Svalbard fjords into the open
ocean remains poorly documented.
Map of the Arctic Ocean showing the study area of this research
and general surface circulation patterns within the Arctic Ocean. Red arrows
represent warm, saline currents of the Atlantic and Pacific, and blue arrows
represent fresh, cold water modified within the Arctic Ocean (adapted from
Tremblay et al., 2005). Orange triangles show the river deltas of four
rivers which constitute the largest freshwater source to the Atlantic–Arctic
sector: the Ob, Yenisei, Lena and Kolyma rivers. Shaded areas of the
central Arctic show September sea-ice extent for 2006 (dark blue), 2017
(light blue) and 2020 (grey-blue). Figure adapted from NSIDC, 2020. Inset:
nitrate isotope sample stations for this study are shown with blue dots
(JR17005) and green dots (FS2018). The station numbers for silicon isotope
profiles measured within this study are shown in red for FS2018 and black
for JR17005.
Atlantification is leading to changes in sea-ice cover and stratification of
the Eurasian Basin (Arthun et al.,
2012; Lind et al., 2018) and increasing nutrient availability in the surface
ocean (Randelhoff et
al., 2018; Tuerena et al., 2021a). Meanwhile, DSi concentrations from
Atlantic waters (AWs) are decreasing in the sub-Arctic regions
(Hátún et al., 2017) and the
inflow of Pacific water is increasing
(Woodgate,
2018). Riverine freshwater inputs have been increasing in the Eurasian
sector (Mcclelland et al., 2006) and nutrient fluxes are
increasing in rivers with degrading permafrost
(Frey
et al., 2007; Frey and McClelland, 2009; Zhang et al., 2021). All of these
changes have widespread impact on phytoplankton dynamics
(Ardyna and Arrigo, 2020). In response, the nutrient budgets
of the Arctic Ocean are expected to change, with potential repercussions on
downstream ecosystems and Atlantic nutrient budgets. In order to predict
such impacts, a better understanding of the relative importance of Arctic
nutrient sources and internal cycling is needed.
Fram Strait is both an inflow and outflow gateway and a key area of exchange
between the Arctic and the North Atlantic. On the eastern side, warm, saline
AW originating from the subpolar and subtropical gyre of the North Atlantic
flows northward in the surface intensified West Spitsbergen Current. On the
western side, polar surface water (PSW) carries cold, fresh
Arctic-originating water and sea ice into the subpolar North Atlantic Ocean
in the upper (ca. 250 m) part of the water column
(Dodd
et al., 2012; Rudels et al., 2002; de Steur et al., 2009). PSW is relatively
low in nitrate, carrying the signal of Pacific nutrient stoichiometry and
benthic denitrification to the Atlantic Ocean through low N : P ratio
(Dodd et al., 2012). In contrast to PSW, AW has
relatively high nitrate concentrations but is poor in DSi (≅5µM) as this key nutrient is depleted in the Atlantic through uptake by
silicifying phytoplankton species during its northward movement. The
stoichiometry of DSi availability compared to nitrate (DSi : N < 1) in
AW in Fram Strait suggests phytoplankton blooms experience DSi limitation
prior to nitrate limitation
(Krause
et al., 2018, 2019; Tuerena et al., 2021a).
Nitrate removal processes within the Arctic Ocean are reflected in the
nitrate isotopic signatures of 5.5 ± 0.4 ‰ for
δ15N-NO3 and 1.3 ± 0.4 ‰ for
δ18O-NO3 of PSW (Tuerena et
al., 2021a), which are significantly different from incoming AW signatures
of 5.1 ± 0.2 ‰ for δ15N-NO3 and
2.4 ± 0.3 ‰ for δ18O-NO3. The
difference between these two water masses reflects benthic denitrification
on shallow Eurasian shelves, also termed coupled partial
nitrification–denitrification (CPND), which increases δ15N-NO3 while decreasing δ18O-NO3, producing an
associated increase in the parameter Δ(15–18), defined as δ15N-NO3-δ18O-NO3, through the release of
isotopically heavy ammonia from sediments
(Fripiat et
al., 2018; Granger et al., 2018). PSW transports high DSi concentrations
from Pacific and riverine influence: δ30Si(OH)4 of Pacific
water is ≅+1.4 ± 0.2 ‰
(Reynolds et al., 2006) and the
isotopically light source of DSi is traced through the Bering Strait and
into the upper halocline waters of the Arctic Ocean
(Giesbrecht et al., 2022). Pacific δ30Si(OH)4 is lighter than North Atlantic signatures (δ30Si(OH)4≥+ 1.7 ‰) which are enriched
to a greater extent from the Southern Ocean source signal as DSi is depleted
through partial uptake and subsequent burial of DSi in the North Atlantic
(Brzezinski
and Jones, 2015; De Souza et al., 2012). Siberian rivers have high seasonal
and regional variability in their isotopic signatures, which are
isotopically light from weathering processes in Arctic rivers, leading to
fractionation of the isotope from the local bedrock
(Pokrovsky
et al., 2013; Sun et al., 2018). The mean δ30Si(OH)4 of
DSi from major rivers to the Arctic Ocean is estimated at +1.3 ± 0.3 ‰ (Sun et
al., 2018). In addition to these sources, benthic supply of DSi to Arctic
shelves was recently documented in the Barents Sea which adds light isotopes
to shelf waters (Ward et al., 2022a, b),
although the magnitude of this flux on a basin-wide scale is currently
unknown.
While Arctic sources of DSi are isotopically light, Arctic polar surface
waters are isotopically heavy (δ30Si(OH)4≅+1.8±0.1 ‰) with isotopically heavy deep basins
(Brzezinski
et al., 2021; Giesbrecht et al., 2022; Varela et al., 2016). This heavy
isotopic enrichment is attributed to physical processes
(Liguori et al.,
2020) and biological modification within surface waters
(Giesbrecht
et al., 2022; Varela et al., 2016). A recent study however highlights the
importance of biological productivity and biogenic Si burial of riverine DSi
in generating these enriched Arctic signatures
(Brzezinski et al.,
2021). Although isotopic signatures have been measured up to 60∘ N
in the Atlantic Ocean
(De Souza
et al., 2012; Sutton et al., 2018), no direct measurements of δ30Si(OH)4 are available from Atlantic–Arctic Gateways such as the
Fram Strait. Therefore, δ30Si(OH)4 signatures of modified
inflowing AW in the Arctic Ocean and outflowing PSW and the contributions
from East Greenland shelves are unknown.
This study fills this crucial gap in the Arctic silicon isoscape,
documenting isotope signatures and nutrient cycling processes in Fram
Strait, focussing on the upper water masses. We use a combination of
geochemical parameters (δ30Si(OH)4, δ15N-NO3, δ18O-NO3, Δ(15–18), N* and
Si*) alongside hydrographic data (salinity, temperature, mixed layer depth)
to explore the sources and internal cycling of DSi in the water masses
exported through the Fram Strait. We then proceed to put these in the
context of pan-Arctic isotope datasets and evaluate the implications of
Arctic nutrient cycling on how nutrient export is likely to change in the
future with ongoing climate change.
MethodSample collection
Samples were collected from two CTD sections across the Fram Strait (JR17005
and FS2018) and from CTD profiles near the Ile-de-France between 2017–2019
(Table 1). The CTD package was equipped with a SBE911plus CTD system
recording multiple parameters (conductivity, temperature, pressure and
salinity). Salinity was calibrated on-board using an Autosal 8400B
salinometer (JR17005) and a Guildline Portasal salinometer (FS2017–2019).
Samples for dissolved inorganic nutrient analysis were collected from Niskin
bottles and stored in pre-cleaned HDPE bottles which were frozen at -20 ∘C immediately after collection. Samples for isotopic analysis
were filtered inline using Nuclepor polycarbonate membranes (0.4 µM
porosity) into acid-cleaned polypropylene bottles and stored at
-20 ∘C (nitrate isotopes) or acidified at 0.1 % v/v with 12M HCl
and stored at 4 ∘C (silicon isotopes).
Summary of sections along which samples were collected.
Dissolved inorganic nutrient concentrations for JR17005 were determined from
frozen samples on autoanalysers following standard colorimetric methods on a
Bran and Luebbe QuAAtro 5-channel autoanalyser at the National Oceanographic
Centre UK (Brand et al., 2020). Detection limit
for nutrient analysis was 0.1 and 0.03 µM for DSi and nitrate
respectively with accuracy with respect to CRMS (certified reference materials) of 2.75 % and 0.91 %
(Brand et al., 2020). For FS2018, nutrients were
analysed following methods from
Hansen and
Koroleff (1999) and Schnetger and Lehners (2014) on a SmartChem 200
discrete analyser at the Technical University of Denmark (FS2017–2019) and
calibrated using OSIL nutrient standards. Analytical precision is of 2 %
and the detection limit was of 0.4 µM for nitrate and 0.1 µM for
DSi. While measurement from frozen is suboptimal for silicic acid
concentrations, separate non-frozen samples could not be collected for
nutrients due to sampling and shipping restrictions. DSi concentrations were
independently checked at the University of Edinburgh from the silicon
isotope samples (acid preserved) during analysis with the HACH reagent
method. Both datasets from frozen and acidified were in very good agreement
and frozen samples were not found to have lower DSi concentrations. DSi
concentrations from FS2018 also closely align with concentrations measured
in the same water masses in JR17005 below the seasonal layer in the upper
500 m of the water column and align with published concentrations in the
literature.
Nitrate isotope analysis
δ15N-NO3 and δ18O-NO3 were measured using
the bacterial strain of P. aureofaciens following the denitrifier method
(Casciotti et al., 2002; Sigman et
al., 2001). Measurements were corrected using international reference
standards IAEA-N3 and USGS-34 in each run, as well as an internal standard
of North Atlantic deep water (δ15N-NO3= 4.92 ± 0.12 ‰, δ18O-NO3= 1.88 ± 0.45 ‰) for inter-run comparability, with standard
reproducibility across runs of ± 0.1 ‰ and ± 0.4 ‰ for δ15N-NO3 and δ18O-NO3 respectively. Final values were corrected using the
correction scheme described in
Weigand et al. (2016) and following
Tuerena et al. (2021a, b) for inter-comparability of datasets in the Atlantic–Arctic
region.
Silicon isotope analysis
DSi concentrations are very low in the Arctic Ocean (<10µM);
as such, previous protocols from
Brzezinski et al. (2003) and
Reynolds et al. (2006), originally based on the Magnesium Induced
Coprecipitation (MAGIC) method described in
Karl and Tien (1992), were adapted to
allow measurements at concentrations below 10 µM. DSi of seawater (40 mL
of sample) was co-precipitated in two steps along with brucite using 1 M NaOH. Precipitate was recovered after 24 h by centrifugation, re-dissolved
using 6 M HCl and diluted to 2 ppm Si. The solution was further purified by
loading 0.5 mL of the solution onto pre-cleaned 1.8 mL Biorad AG50W-X8
cation-exchange resin columns.
The isotopic composition of the prepared solution was determined by
MC-ICP-MS on a Nu Plasma II instrument at the University of Edinburgh using
standard-sample bracketing and calculated from the permil deviation from
isotopic reference material NBS28
(Georg et al., 2006),
calculated as the following:
δxSi=xSi28SisamplexSi28SiNBS28-1×1000[‰],
where δxSi is either δ29Si or δ30Si.
As per
Fripiat
et al. (2011a, b) and Liguori et al. (2021), δ29Si were converted to δ30Si to improve
reliability and global comparability of datasets
(Cardinal et al.,
2003, 2005), using the theoretical conversion factor of 1.96, calculated
from the kinetic fractionation law (Young
et al., 2002). The method of analysis and interferences are discussed in
further detail in Supplement S1.
Inter-run comparability and method reproducibility of measurements was
checked with the international solid standard Big Batch and both high and
low concentration seawater standards Aloha1000 and Aloha300.
Average standard measurements for the period of this study is
Aloha1000=+0.67± 0.03 ‰, +1.32 ± 0.06 ‰ (n=16), BigBatch =-5.33 ± 0.02 ‰, -10.50 ± 0.04 ‰ (n=7)
for δ29Si(OH)4 and δ30Si(OH)4
respectively (uncertainties of 1 SD). Long-term reproducibility of converted
δ30Si(OH)4 is BigBatch =-10.49 ± 0.09 ‰ (n=58), Aloha1000=+1.29± 0.08 ‰ (n=58) and Aloha300=+1.70± 0.05 ‰ (n=30) compared to inter-laboratory measurements
of BigBatch =-10.48 ± 0.2 ‰, Aloha1000=+1.25 ± 0.2 ‰, Aloha300=+1.66± 0.35 ‰ (Grasse et al., 2017; Reynolds
et al., 2007). The reproducibility of the full chemical and analytical
procedure, including chemical preparation and analytical measurements in
separate MC-ICP-MS sessions, was additionally estimated on a subset of
duplicate seawater samples (n= 8). The mean absolute difference between
duplicate samples analysed was ± 0.04 ‰ (1 SD).
Derived parameters
Mixed layer depth (MLD) is identified as the maximum depth at which the
potential density was within 0.1 kg m-3 of the shallowest measurement
(Peralta-Ferriz
and Woodgate, 2015). MLD governs the depth for which nutrients resupply
surface waters and to which planktons are mixed
(Yool et al., 2015). In this study,
PSW is defined as potential temperature (θ) < 0 ∘C and potential density (σθ) <27.7 kg m-3,
and AW is defined as θ>2∘C and 27.7<σθ<27.97 kg m-3 or σθ<27.7 kg m-3 and salinity >34.92 psu, as per
Richter et al. (2018).
The semi-conservative tracers N* and Si* were calculated from inorganic
nutrient concentrations where N*= NOx- PO4-×16,
adapted from Gruber and Sarmiento (1997), and Si*= DSi - NOx (Sarmiento et al., 2004). Both tracers are
indicative of nutrient deviation from typical Redfield ratio, and highlight
additional sources or processes through which nutrients become deficit (i.e.
negative N* shows nitrate deficit in comparison to phosphate). The isotopic
parameter Δ(15–18) is calculated as Δ(15–18) =δ15N-NO3-δ18O-NO3. Δ(15–18) captures
variation in both isotopes, tracing sources and modification of nitrate
(Rafter et al., 2013).
ResultsHydrography and mixed layer depth
Figure 2 shows temperature and salinity across Fram Strait in July–August 2018. The hydrographic situation is typical of the late summer season. Warm
inflowing and recirculating sub-tropical originating AW is found primarily
between 2.5∘ W and the eastern end of the section at 10∘ E, in the
upper 500 m. Its core flows northward within the West Spitsbergen Current at
6–8∘ E. Over the East Greenland shelf, PSW dominates the upper
150 m, extending from the western end of the section to the AW/PSW interface
at about 3∘ W. Re-circulating Atlantic waters (RAWs), underlay PSW
on the East Greenland shelf, while Arctic Atlantic water (AAW) is also found
below AW at the foot of the East Greenland shelf. We refer the reader to
Rudels et al. (2002) for an overview of
the properties of water masses found in Fram Strait.
The MLD did not exceed 100 m for FS2018. Late-season MLD was deeper in AW
than in PSW, occurring between 30–60 m. MLD is significantly shallower over
the East Greenland shelf, occurring at 5–10 m. Hydrography and nutrient
distribution of JR17005 follow roughly similar patterns as FS2018 apart from
seasonal variations, and are previously described in
Tuerena et al. (2021).
Hydrography of Fram Strait cruise FS2018 from August–September
2018 presented for full depth (c, d) and upper water column (a, b) for temperature (a, c) and salinity (b, d). The mixed-layer depth is
shown by a cyan line (calculation of MLD is described in Method section
below). Isotherms (a, c) and isohalines (b, d) are also displayed.
Atlantic water (AW), polar surface water (PSW) and Arctic Atlantic water
(AAW) are marked.
Nutrient concentrations
Panels (a) and (b) of Fig. 3 show the nitrate and DSi concentrations in the
upper 400 m of the water column along the late-summer 2018 Fram Strait
section. P-values reported are for t-tests between AW and PSW water masses.
Nitrate concentrations were low across the section in the upper 50 m of the
water column from phytoplankton utilisation and dilution by low-nitrate
freshwater sources. Below the mixed layer depth, NOx is higher in AW
(12.10 ± 0.98 µM) than in PSW (8.08 ± 2.19 µM, p<0.01), consistent with export of low nitrate waters from the Central Arctic.
Nutrients and isotopes across Fram Strait section of late summer
2018 (a) NOx (where NOx= nitrate + nitrite), (b) DSi, (c) N* (where N*= NOx-16*PO4-), (d) Si* (where Si*= Si(OH)4-- NOx), (e)δ15N-NO3, (f)δ18O-NO3, (g)Δ(15–18)
(where Δ(15–18) =δ15N-NO3-δ18O-NO3), (h)δ30Si(OH)4. Cyan line displays MLD
for the section (calculation described in Method section).
Negative N* reflecting a deficit of nitrate are evident on the East
Greenland shelf in panel (c) of Fig. 3. In contrast, N* reaches near
positive values in AW with an average of -0.55 ± 0.38 below the MLD
(Table 2). This highlights that nitrate is more depleted in PSW relative to
dissolved phosphate, becoming potentially limiting to primary production
towards the end of the biological growth season as nitrate concentrations
approach 0 µM.
Averaged water mass signatures of the Fram Strait (a) excluding the
mixed layer depth and (b) within the mixed layer in spring (JR17005) and
summer (FS2018). Water mass definitions based on
Richter et al. (2018). AW = Atlantic
water, PSW = polar surface water, wPSW = warm PSW, AAW = Arctic
Atlantic water, DW = deep water, DSOW = Denmark Strait overflow water.
N* is defined as N*= NOx–16*PO4-) and Si* is defined as Si*= Si(OH)4-–NOx).
DSi concentrations were low across the section above the MLD with stronger
depletion at shallower depths and further west (Fig. 3b). Comparison of
DSi concentrations measured during May–June 2018 and August–September 2018
(Table 2) reveal that DSi concentrations were similarly higher in PSW (4.28 ± 2.93 µM) and in AW (3.19 ± 1.20 µM) in the mixed
layer at the start of the season, and fell to similarly depleted
concentrations by the end of the summer (1.03 ± 0.98 and 1.26 ± 1.11 µM for AW and PSW respectively).
Below the mixed layer, DSi is low in AW (5.42 ± 0.71 µM) from DSi-poor Atlantic waters of subpolar origins. DSi in PSW is higher than in AW
(6.64 ± 1.71 µM, p<0.01), potentially reflecting Arctic
sources of DSi to PSW. In the deep Fram Strait, DSi concentrations vary
locally, but generally increase with depth up to a concentration of 9.45 ± 2.48 µM in deep waters (Fig. 4). On Fig. 3d, strongly
negative Si* in AW reflect the strong DSi deficit relative to nitrate in
Atlantic-originating waters, while Si* closer to phytoplankton requirements
in PSW illustrate excess DSi in PSW.
DSi concentrations (a, c) and dissolved silicon isotope profiles
(b, d) for spring (JR17005, circles) and late summer (FS2018, triangles) of
the 2018 growth season in Fram Strait. Colour scale represents salinity
(psu).
Isotopic signatures
Measured profiles of δ30Si(OH)4 across Fram Strait are
shown in panel (h) of Fig. 3 (late summer data, for spring data see
Supplement S2) and Fig. 4. Positive signatures of δ30Si(OH)4 were measured throughout the water column, ranging from
+1.34 ‰ to +3.16 ‰ for the entire
section (Fig. 3h). The heaviest δ30Si(OH)4 signatures were measured in the upper 100 m of the section (Fig. 4),
consistent with fractionation from diatom uptake during growth. Below the
MLD, mean δ30Si(OH)4 for AW was +1.74 ± 0.06 ‰ (Table 2), which aligns closely with measurements
of waters from North Atlantic origin
(Brzezinski
and Jones, 2015; De Souza et al., 2012). Conversely, DSi in PSW was
isotopically heavier than DSi in AW (p<0.02), the mean δ30Si(OH)4 value for PSW was +1.85 ± 0.09 ‰. This is comparable to measurements of the upper
halocline layer in the Canadian Basin (δ30Si(OH)4=+1.84 ‰) from
Varela
et al. (2016), and outflowing surface measurements from
Brzezinski et al. (2021) in the TPD where δ30Si(OH)4=+1.92 ‰, and aligns with the heavy signatures of
Arctic-originating waters in the North Atlantic
(De Souza
et al., 2012; Sutton et al., 2018).
In the deep waters of Fram Strait, δ30Si(OH)4 are lighter
than PSW (Fig. 4), aligning with the gradient decrease of
-0.15 ‰ over the full depth profile reported in
Brzezinski et al. (2021). The measured signatures also align with measurements in the North
Atlantic of Nordic-originating endmembers (DW-δ30Si(OH)4=+1.65 ± 0.13 ‰ and
DSOW-δ30Si(OH)4=+1.75 ± 0.08 ‰, De
Souza et al., 2012). Light δ30Si(OH)4 values were measured
at the sediment interface of deep basins (Fig. 4), showing potential
interaction of benthic efflux of DSi from isotopically light porewaters
(Ehlert et al.,
2016; Ward et al., 2022a, b), and remineralisation of isotopically
lighter biogenic Si in the deep. This is also observed in
Brzezinski
et al. (2021) and Liguori et al. (2020) who found isotopically light
measurements in the deep Nansen and Amundsen basins. Low sampling
resolution within our dataset and the strong influence of local circulation
precludes quantification of such local recycling processes from advective
signals in Fram Strait with certainty.
Figure 5 displays the full water column profiles of nitrate isotopes
measured along the spring (JR17005) and late-summer (FS2018) sections.
δ15N-NO3 is enriched in PSW (5.44 ‰)
compared to AW (5.18 ‰, p<0.01) while the
δ18O-NO3 is lighter in PSW (1.22 ‰)
than in AW (2.33 ‰, p<0.01, Table 2), following
trends identified in Tuerena et al. (2021a).
Panel (g) of Fig. 3 illustrates the decoupling of both isotopes reflected in
diverging Δ(15–18), indicative of CPND. A high confidence in
accuracy and reproducibility of nitrate isotopes measurements in this study
is obtained as the dataset aligns and follow the same trends as profiles
reported in Tuerena et al. (2021a).
Nitrate concentrations (left), δ15N-NO3
(middle-left), δ18O-NO3 (middle-right) and Δ(15–18) profiles (right) for FS2018 (triangles) in Fram Strait. Typical
profiles for PSW (cyan) and AW (red) from JR17005 are shown in circles for
comparison between studies (Tuerena et al., 2021). Colour scale represents
salinity (psu).
In surface waters, δ15N-NO3 increase with reducing nitrate
concentrations, which is consistent with biological uptake (Fig. 5). This
is also observed in most δ18O-NO3 profiles apart from PSW
profiles measured far onto the East Greenland shelf in FS2018 (Fig. 3),
where remote signals of denitrification dominates over biological uptake
signals, even in the upper water column. As shown in Fig. 3 and
summarised in Table 2, isotopic signatures of both dissolved silicon and
nitrate isotopes closely follow the hydrography of water masses in spring
and summer. In Fram Strait, a key area of exchange with the North Atlantic
where inflowing and outflowing water masses show strong differences in
their physical properties, and dissolved nitrate and silicon isotopes
measurements provide insights into nutrient sources and cycling within the
Arctic Ocean and the pathways through which nutrient modification and
exchange occurs.
DiscussionUsing nutrient isotopes to examine Arctic nutrient cyclingTrends between <inline-formula><mml:math id="M518" display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">15</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>N-NO<inline-formula><mml:math id="M519" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula>, <inline-formula><mml:math id="M520" display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">30</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>Si(OH)<inline-formula><mml:math id="M521" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula> and nutrient utilisation
In this section, we compare δ15N-NO3 and δ30Si(OH)4 measurements with the fraction of nitrate and DSi
remaining (f) in PSW and AW in Fram Strait (Fig. 6a and b). We
define f as the fraction of nutrient remaining in the surface layer relative
to concentrations below the MLD (Table 2). f=1 indicates no nitrate or DSi
has been used and f=0 indicates complete depletion of the nutrient inventory.
Nitrate utilisation vs. δ15N-NO3 for
AW (a, depth <600 m) and PSW (b, depth <150 m).
DSi utilisation vs. δ30Si(OH)4 for AW (c,
depth <600 m) and PSW (d, depth <150 m). Circles
denote measurements from JR17005 (spring) and triangles from FS2018
(summer). Red symbols show measurements within the mixed layer. Black line
follows the closed fractionation model and grey line an open fractionation
model. f is the fraction of nutrient remaining, calculated from the nutrient
concentrations of water masses AW and PSW below the MLD (Table 2).
The fractionation of nitrate and DSi during phytoplankton uptake can be
modelled by Rayleigh systematics
(Altabet and
Francois, 2001; Mariotti et al., 1981), and is often linked to local
hydrography. Rayleigh systematics assume a closed system, i.e. there is no
import/export of the nutrient from the euphotic zone while it is being
utilised by phytoplankton. In late spring and summer, the PSW layer in Fram
Strait is largely a closed system as it is highly stratified. Nitrate and
DSi are mainly replenished during winter destratification
(Altabet and Francois, 2001).
In this environment, δ15N-NO3 and δ30Si(OH)4 can be expected to fall on a trend based on their
isotopic effect (∼ 2 ‰–6 ‰ for nitrate and
∼ 1 ‰ for DSi globally), and are described
by exponential trend lines in the Rayleigh field on Fig. 6
(Varela et al.,
2004). In areas of upwelling, or in a case where the resupply of nutrients
to the euphotic zone occurs due to multiple stratification and
destratification events throughout growth season, conditions are better
modelled as open system, described by a linear trend line in the Rayleigh
field. However, in low-nutrient zones such as the PSW layer in Fram Strait,
nutrient uptake stoichiometry can be dictated by nutrient-limitation itself
rather than by the physical re-supply of nutrients
(Hutchins
and Bruland, 1998; Moore et al., 2013), which in turn can lead to a shift
from open to closed system dynamics as the source of nutrients switches from
new to remineralised.
During the growth season of 2018, nitrate in AW and PSW enriched in δ15N-NO3 at lower nitrate concentrations, consistent with
fractionation associated with nutrient uptake by phytoplankton (Fig. 6).
AW follows the traditional isotopic effect of 5 ‰ and PSW
follows the particularly low isotopic effect of 2 ‰.
Nitrate fractionation in AW behaves between closed and open system kinetics,
with a shift from more closed conditions in spring towards more open
conditions in summer. This corroborates with the relatively weak
stratification of AW (Rudels
et al., 2005), facilitating re-supply of nitrate and other
nutrients over the spring and summer growth season through destratification
events such as those described by
Tuerena et al. (2021a). In PSW,
δ15N-NO3 fractionation follows an exponential trend and
behaves as a closed system in spring, indicative of the strong salinity
stratification of PSW. A shift towards a mostly linear trend in summer is
observed (Tuerena et al., 2021a), suggesting open
system kinetics below the mixed layer. While a shift from open to closed
system could be expected due to strengthening of stratification over the
summer season, we observe a shift from closed to open systems instead. This
is unlikely to reflect a change in hydrographic conditions, but indicates a
shift towards consumption of regenerated nitrogen in nitrate-depleted waters
instead, thereby lowering ambient δ15N-NO3 from expected
trends. This is further supported by the equivalent trends observed in
δ15N-PN (Fig. S3, Supplement S3).
The relationship between apparent remaining DSi and δ30Si(OH)4 in AW follows closed system kinetics during the growth
season (Fig. 6) after DSi was drawn down to 1 µM in AW in summer 2018
(Fig. 4). DSi is strongly depleted in surface AW in summer, preventing
direct δ30Si(OH)4 measurements within the MLD and
observation of shifts within the isotopic system. Our observations remain
consistent with other studies which find that DSi is one of the limiting
nutrients to diatom growth in AW in the eastern Fram Strait along with Fe
(Krisch et al., 2020). In contrast to
nitrate, biogenic Si is recycled less within the upper water column,
preventing a switch to recycled nutrient sources later in the seasons. As
DSi becomes fully utilised and ambient conditions become unfavourable to
diatom growth, a shift towards non-siliceous species is expected in late
summer along with a shift towards open system kinetics.
δ30Si(OH)4 in PSW does not show a good fit with either of the
fractionation models and measurements from within the MLD are not consistent
with fractionation associated with nutrient uptake by phytoplankton at lower
nutrient concentrations. Trends of δ30Si-PSi are also
inconsistent with any model (Fig. S3, Supplement S3). This
suggests that unlike nitrate, DSi in PSW is not primarily controlled by
biological processes, and its variations are more likely to be driven by
physical mixing and dilutive effects instead. The decoupling of N and DSi
isotopic systems is indicative that N is strongly limiting in the highly
stratified PSW and prevents extensive DSi utilisation locally.
Nutrient uptake in surface AW is constrained by low DSi concentrations in
limiting conditions for diatom growth, while uptake in PSW is constrained by
strong nitrate limitation and DSi is only partially utilised. This indicates
that the extent to which DSi is taken up is regulated by nitrate
availability in PSW at Fram Strait. δ15N-NO3 and δ30Si(OH)4 show a strong link between the silicon and nitrogen
cycles in Fram Strait as they regulate each other through availability,
contributing to the asymmetry observed in nutrient exports across the strait
(Torres-Valdés et al.,
2013).
Upstream transformation of nitrate and DSi in PSW and AW in Fram
Strait
The nutrient composition of polar waters exported through Fram Strait
reflect their nutrient cycling history within the Arctic Ocean through
altered DSi : N ratio and isotopic signatures. Figure 7a shows trends for
δ15N-NO3 vs. δ18O-NO3, displaying that
fractionation due to uptake by phytoplankton assimilation follows the
established fractionation ratio of 1:1
(Granger et
al., 2004; Sigman et al., 2005) but on separate fractionation lines. δ15N-NO3 and δ18O-NO3 of PSW plots on a
fractionation line consistent with isotopically lighter sources of δ18O-NO3, while δ15N-NO3 and δ18O-NO3 measured in surface AW follow a line consistent with
isotopically heavier sources, suggesting different nutrient sources in AW
and PSW.
The modification of nitrate in the Arctic Ocean is readily apparent when
plotting N* against δ18O-NO3 (Fig. 7b); as salinity
decreases and the influence of polar-originating waters increases, N*
decreases, indicating a nitrogen deficit in relation to phosphate in PSW.
Water samples with lower N* are accompanied by lighter δ18O-NO3 signatures. This relationship is attributed to CPND in
the Arctic Ocean (Granger et al.,
2011): settling particulate organic nitrogen from coastal productivity
degrades at the sediment interface of the extensive shallow shelves and
produces large sources of sedimentary ammonium. In shelves where sedimentary
denitrification preferentially consumes isotopically light NO3, the
NH4+ thus released into the water column is isotopically heavy in
δ15N. Subsequently, during nitrification, this benthic efflux
of isotopically heavy NH4+ is combined with light oxygen isotopes
nearing local δ18O-H2O into the nitrate pool. This
decouples the two isotopes by decreasing the δ18O-NO3 of
nitrate overall whilst increasing δ15N-NO3.
Fram Strait measurements of (a)δ15N-NO3 vs. δ18O-NO3 (solid and dotted lines show 1:1 fractionation lines). (b)δ18O-NO3 vs. N*. (c)Δ(15–18) vs. δ30Si(OH)4 excluding samples
from within the mid-layer depth to remove seasonal variation. (d) Si* vs.
δ30Si(OH)4. In all figures, circles represent spring (JR17005) and
triangles show late summer (FS2018). Color scale for all plots show salinity
(psu).
CPND is a widespread process in Arctic shelves and has been observed in the
Chukchi Sea
(Brown
et al., 2015; Granger et al., 2018) and the East Siberian Sea
(Fripiat et al., 2018) and
contributes to the observed Arctic-wide nitrogen deficit in relation to
phosphate
(Torres-Valdés et
al., 2013; Yamamoto-Kawai et al., 2006). This shelf-derived signal is
exported into the Arctic halocline (Granger
et al., 2018), namely through the TPD, and can be traced in the outflowing
water masses in Fram Strait (Tuerena et al.,
2021a), reflecting the impact of shelf processes on PSW nutrient ratios.
Thus, the low N*, light δ18O-NO3 and heavy Δ(15–18) signal exported in the PSW is the signature of N loss on Eurasian
shelves and the Chukchi Sea.
DSi concentrations in outflowing PSW are 1.2 µM higher and δ30Si(OH)4 is isotopically heavier by +0.11 ‰
relative to inflowing AW (Table 2). Documented Pacific and meteoric sources
of DSi are isotopically light
(Hawkings
et al., 2017; Pokrovsky et al., 2013; Reynolds et al., 2006; Sun et al.,
2018) but DSi behaves non-conservatively across the Arctic Ocean. DSi uptake
by phytoplankton in the Arctic Ocean and loss due to biogenic Si burial
fractionate the upper water-column δ30Si(OH)4 towards
heavier signatures
(Brzezinski
et al., 2021; Liguori et al., 2020; Varela et al., 2016).
Varela
et al. (2016) suggest the heavy signal observed in the deep Arctic is
sourced from intermediate Atlantic-originating waters but we observe no
significant enrichment of δ30Si(OH)4 in the intermediate
water masses of Fram Strait (Figs. 3, 4). Given that the inflowing
AWs are already too poor in DSi to contribute to isotopic enrichment, the
observed increase in DSi concentrations may point to riverine DSi sources
subject to enrichment due to biogenic si production and burial instead
(Brzezinski et al., 2021).
As seawater is undersaturated with respect to biogenic Si at all depths in
the ocean (Archer et al., 1993), biogenic Si dissolution occurs
in the water column and at the sediment–water interface. Upon burial,
biogenic Si will continue to dissolve until pore-waters are saturated
(Kamatani, 1982;
Nelson et al., 1995). Arctic shelves are characterized by a shallow water
column with relatively high sedimentation rates influenced by river and
biogenic fluxes, conditions favourable to reduced biogenic Si exposure to
dissolution, and rapid burial. Therefore, it is expected that Arctic shelf
seas are particularly efficient at removing biogenic Si through opal burial.
Although some studies have reported low opal burial rates and rapid
recycling within the seafloor of the Barents Sea shelf
(Ward et al., 2022a, b), this may not be
the case with shallower Eurasian shelves with higher sedimentation rates and
stronger riverine influence (Kara Sea, Laptev Sea and East Siberian Sea).
This suggests geochemical cycling of Si can strongly vary from one Eurasian
shelf to another
(Macdonald et al.,
2010). In areas of low nitrate concentrations such as the Eurasian sector of
the Arctic Ocean, DSi is only partially utilised in the surface as
productivity is limited by N deficit, leading to fractionation. The
isotopically lighter biogenic Si is preferentially buried leaving water
column δ30Si(OH)4 heavy overall. This contrasts with deep
Arctic basins with low productivity and long water residence times which
provide opportunities for more remineralisation in the water column and at
the water–sediment interface, leading to relatively small modification in
the water-column δ30Si(OH)4
(Brzezinski
et al., 2021; Liguori et al., 2020). The heavy δ30Si(OH)4 signatures of PSW thus records the partial utilisation and the loss of
lighter Si through burial in the Arctic shelves.
Figure 7c shows the relationship between Δ(15–18) and δ30Si(OH)4 in samples below the MLD which should not be affected
by seasonal biological fractionation. A gradient is observed between AW and
PSW, with a gradual increase in Δ(15–18) from 2 ‰
to 4 ‰ as salinity decreases, and an increase in δ30Si(OH)4 from +1.7 ‰ to
+2 ‰, linking the processes of denitrification
(Fripiat
et al., 2018; Granger et al., 2011, 2018) and removal of isotopically light
DSi sources through biogenic Si burial in the shelves
(Brzezinski
et al., 2021; Liguori et al., 2020) contributing to the evolution of the
dual isotope signal of PSW. The combination of both CPND and biogenic Si
burial indicated by the isotopic signatures of N and DSi can only occur in
areas which receive a direct high influx of terrestrial DSi and hosts CPND,
namely the Bering Sea and Eurasian shelves.
AW entering the Arctic is poor in DSi which limits biological uptake in AW
(Agustí
et al., 2018; Krause et al., 2018, 2019). Any excess DSi (e.g. from Pacific
and shelf waters supplied to AW) will be consumed during the growth season
until nitrate is exhausted. The enrichment of δ30Si(OH)4
in Arctic waters exported out of Fram Strait points towards partial
utilisation of DSi, constrained by the availability of nitrate within the
TPD
(Brzezinski
et al., 2021; Liguori et al., 2021). The combination of supply and use of
these nutrients is reflected in panel (d) of Fig. 7, where PSW is distinct
from AW with positive Si* (DSi sources from terrestrial runoff and Pacific
influence where nitrate is in deficit relative to phytoplankton
requirements) and heavy δ30Si(OH)4 signatures whereas the
relationship with salinity reflects the mixing of these distinct water mass
signatures. While AW signals remain clustered, large variability in PSW Si*
and isotopic signature highlight the regional variation and complexity of
the Si budget around the Arctic Ocean (Table 2).
In summary, low availability of nitrogen in the Eurasian sector of the
Arctic Ocean appears to regulate the extent to which DSi is utilised and
subsequently exported through PSW. At Fram Strait, PSW carries the isotopic
signals of DSi and N modification within Eurasian shelves through processes
such as benthic denitrification and partial utilisation of DSi.
Si cycling in the Arctic Ocean and sources of dissolved silicon exported
through Fram Strait
The Arctic exports significant amounts of DSi but not N through Fram Strait
(Torres-Valdés et al.,
2013). Modification of PSW as a result of shelf processes can be traced
across the Arctic simultaneously using N and DSi isotopes (Fig. 7c). Here
we use δ30Si(OH)4 against ln[DSi] plots to examine the
pathway of this transformation from DSi sources to Fram strait (Fig. 8).
Broad negative trend lines are observed in Fig. 8, but on separate
trend lines for AW and the pan-Arctic, with PSW in between. Decreasing DSi
and heavier δ30Si(OH)4 suggest that mixed riverine and
Pacific sources of DSi are transported across the Arctic towards Fram Strait
through PSW.
Pan-Arctic trends of δ30Si(OH)4 against
ln(DSi). Coloured dots show measurements from within AW (red, max. depth = 600 m) and PSW water masses (blue, max.depth = 150 m) from this study based
on water mass definitions in Table 2. Grey symbol sets are published δ30Si(OH)4 from major DSi sources to the surface Arctic domain
and surface water masses. Triangles: N. Pacific (<100 m, stations
1–6); stars: transpolar drift (<60 m, stations 30–38 from Brzezinski
et al., 2021). Crosses: Bering Strait (max. depth = 60 m, stations 4–6 from
Brzezinski et al., 2021). Squares: Canadian Arctic (surface and intermediate
water mass signatures of the Canadian Arctic sector, from Table 2 in
Giesbrecht et al., 2022). Octagons: glacial runoff from Greenland and
Svalbard glaciers (Hatton et al., 2019). Diamonds: Lena River (Sun et al.,
2018). Stars show average endmember composition of AW (red) and Pacific and
riverine sources (Grey). Red dotted trend line is the least squared
regression for δ30Si(OH)4 vs. the natural logarithm of DSi
within AW, and blue and grey dotted trend lines are the equivalent for PSW and
pan-Arctic (excluding Fram Strait) respectively. These trend lines show
fractionation from partial utilisation of DSi consistent with fractionation
models.
The Bering Strait inflow
δ30Si(OH)4 in the upper 100 m of the water column in the North
Pacific is relatively light (∼+1.5 ‰),
with high DSi concentrations (∼ 40 µM)
(Reynolds et al., 2006).
Pacific-originating nutrients are strongly modified in the Bering Strait by
riverine input with high DSi : N ratio from the Yukon River, by benthic
denitrification, and significant biological consumption over the broad
shallow shelves in the Bering and Chuchki seas. The combined processes lead
to increasing δ30Si(OH)4 following biological uptake and
fractionation
(Brzezinski et
al., 2021; Giesbrecht et al., 2022). Thus the Pacific endmember measured in
the Bering Strait is heavily modified by biogeochemical cycling on shelves,
and Arctic inflow here has lower DSi concentrations relative to the Pacific
Ocean. At lower ln(DSi) on the pan-Arctic trend line, riverine and Pacific
sources become indistinguishable in surface water masses of the Arctic
Ocean and the TPD, reflecting that mixing and biogeochemical cycling in the
high Arctic homogenises both nutrient sources prior to export in PSW.
The Eurasian shelf signal
Siberian rivers have isotopically light δ30Si(OH)4 from
clay mineral weathering
(Mavromatis
et al., 2016; Pokrovsky et al., 2013; Sun et al., 2018). However,
terrestrial DSi input and biological consumption occurs simultaneously on
shallow Eurasian shelves. Riverine inputs support one-third of the net
primary productivity of the Arctic Ocean (Terhaar et
al., 2021), most of which occurs on the Eurasian shelves (MacDonald et al,.
2010). Phytoplankton uptake further reduces DSi concentrations and leads to
isotopically heavier δ30Si(OH)4. This inference follows
Brzezinski et al. (2021), as it is also reflected in the TPD. Thus, in Fig. 8, the broad
negative trend line from riverine and Pacific sources across the TPD to Fram
Strait reflects the progressive depletion of DSi through biological uptake
and biogenic Si burial resulting in isotopic enrichment as it travels
through the Arctic. Partial DSi utilisation modifies both the Si budget and
its isotopic composition. DSi transported from Eurasian shelves through the
TPD towards Fram Strait is reflected in isotopically heavy δ30Si(OH)4 in PSW which aligns with the broad Rayleigh field in
Fig. 8.
In Fram Strait, δ30Si(OH)4 fractionation involves separate
trend lines for AW and PSW. The trend for AW is statistically significant at
Fram strait (R2>0.7) but shifted downwards indicating a
distinct Atlantic source. In contrast, the PSW trend line is shifted upwards
from AW towards heavier isotopic values and higher DSi concentrations,
following more closely the broader Arctic trends. In addition, larger
variability in Si isotope signatures of PSW (R2>0.3)
reflects the combined effects of local biological uptake and mixing between
Arctic and Atlantic source signatures around Fram Strait.
Glacial influence on <inline-formula><mml:math id="M674" display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">30</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>Si(OH)<inline-formula><mml:math id="M675" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula> exported from the
Arctic Ocean via Fram Strait
Glacial and sea-ice inputs have been suggested to significantly impact
Arctic Si budgets (Fripiat
et al., 2014; Hawkings et al., 2017), this is evaluated further in Fig. 8.
Isotopic studies in Greenland and Svalbard glaciers have shown isotopically
light signatures with low DSi concentrations
(Hatton et al., 2019). Benthic studies in SW
Greenland fjords found a significant diffusive flux of isotopically light Si
into overlying shelf waters (Ng et al.,
2020), although export from fjords remains to be characterised. Si inputs
from Greenland and Svalbard have been suggested as significant contributors
to the Arctic Si budget which is exported through PSW to the North Atlantic
(Hatton et al., 2019;
Hawkings et al., 2017, 2018), though the glacial freshwater content of PSW
at 79∘ N is relatively small
(<13 %, Stedmon et
al., 2015).
In Fig. 8, we show that light δ30Si(OH)4 signatures from
Greenland and Svalbard glacial sources also have low DSi concentrations and
do not align in the Rayleigh field with the Arctic trend observed. This
suggests Greenland and Svalbard glaciers are not significantly impacting the
Si budget of outflowing PSW at Fram Strait. This implies in situ studies of
glacial streams in Greenland may overestimate glacial contribution of Si to
Eurasian Arctic nutrient budgets. A possible explanation for this is
amorphous phases of Si represent >95 % of the total Si flux
(Hawkings et al., 2017) and a large fraction of this
may be buried in the sediments of Arctic fjords prior to dissolution,
reducing the impact of glacially-sourced DSi.
The particularly low apparent isotopic effect of the Arctic Ocean has been
attributed to the influence of sea ice and sea-ice diatoms drawing down DSi
(Giesbrecht
et al., 2022; Varela et al., 2016). Sea-ice brine is heavier or equal to
surrounding waters δ30Si(OH)4
(Fripiat et
al., 2007, 2014) and may contribute to the isotopically heavy signature of
polar waters
(Liguori
et al., 2020; Varela et al., 2016). A recent study from
Brzezinski et al. (2021) did not find direct evidence of such an impact on a basin-wide scale.
Here we evaluate the role of sea ice in influencing Arctic δ30Si(OH)4 signatures in a region influenced by brine rejection.
In Fig. 9, we present hydrography and late-summer profiles of DSi and
δ30Si(OH)4 collected from Ile-de-France section between
2017–2019 (section location is shown in Fig. 1). This area is
characterised by perennial sea-ice cover (Schneider and
Budeus, 1995). A PSW layer extends down to 125 m of the water column and is
influenced by brine released during winter sea-ice formation
(Budeus and Schneider, 1995). In the freshwater layer, a
small peak in DSi concentration (2–3 µM) is observed at ∼ 40 m. The small increase in DSi concentration at this depth suggests a DSi
source from sea-ice processes. However, there is no distinct isotopic
enrichment (Fig. 9) associated with this source. Thus DSi inputs from sea
ice cannot be the reason for enriched δ30Si(OH)4
signatures of PSW. This inference is consistent with studies suggesting that
sea ice and sea-ice brine tend to be relatively low in DSi
(Fripiat et al., 2017),
with no significant impact on pan-Arctic isotope signatures
(Brzezinski et al.,
2021).
(a) Integrated hydrography of the Ile-de-France section for
2017–2019. Isotherms are shown in white and isopycnals in black. MW = meteoric water, rAW = recirculated Atlantic water. Bottom panels: DSi
concentrations (b) and δ30Si(OH)4(c) for late summer 2017
(circle), 2018 (upwards triangle) and 2019 (downwards triangle) of the
Ile-de-France section. Colour scale represents salinity (psu).
Processes affecting the export of Arctic DSi to the Atlantic Ocean
Figure 8 reveals that DSi exported to the Atlantic in PSW in 2018 was
sourced from incomplete utilisation of DSi on Eurasian shelves. This leads
to the question as to what limits the complete utilisation of DSi in the
Arctic. In Fig. 10, we plot δ15N-NO3 versus DSi : N
ratios, excluding measurements within the MLD at Fram Strait to remove
seasonal uptake trends (this was not applied to measurements in the TPD and
shelf seas due to the shallow nature of the water masses). The figure
reveals the three mixing components of the Arctic N budget; namely, the very
heavy δ15N-NO3 values generated on the shelves with
high DSi : N ratios due to removal of light nitrate by CPND, the input of
terrestrial riverine N with relatively light δ15N-NO3
signatures (δ15N-NO3= 2.3 ‰,
Francis, 2019) with variable but high DSi : N ratios, and Atlantic
sources (δ15N-NO3= 4.8 ± 0.2 ‰, Tuerena et al., 2015, and DSi : N = 0.6, World Ocean Database, 2013). AW sources become important in Fram
Strait and contribute to nitrate by mixing across the halocline in basins
where AAW underlies below PSW. Pan-Arctic N isotopic trends, shown in Fig. 10, are dominated by mixing of sources rather than fractionation by
biological uptake; a striking contrast to the δ30Si(OH)4
trend (Fig. 8). This is arguably caused by the near-complete utilisation of
nitrate on Eurasian shelves and above the halocline, leading to limited
overall fractionation from source signatures. This widespread nitrate
limitation in the Arctic is attributed to fixed N loss from benthic
denitrification on the shallow shelves which constitute approximately 50 %
of the Arctic Ocean area. A significant portion of N loss from
denitrification is derived from organic matter from the overlying water
column (Mctigue et al.,
2016; Tuerena et al., 2021c), leading to a net deficit of N exported out of
the Arctic
(Torres-Valdés et
al., 2013; Yamamoto-Kawai et al., 2006).
Pan-Arctic trends of δ15N-NO3 against Si : N ratio.
Triangles = Atlantic water at Fram Strait; circles = polar surface water
at Fram Strait (this study). Stars = transpolar drift (Doncila, 2020);
diamonds = East Siberian shelf (Fripiat et al., 2018). Dotted lines shows
the regression line between AW and shelf endmembers, dotted lines are for
1 SD. Data are plotted below the mid-layer depth in Fram Strait to remove
seasonal variation. This could not be applied to the transpolar drift and
East Siberian shelf due to the shallowness of the water masses. Colour scale
shows temperature. δ15N-NO3 endmember for summertime Siberian rivers
is obtained from Francis (2019) from ArcticGRO measurements.
Furthermore, Arctic rivers are a larger source of DSi than N
(Holmes et al., 2012) and the N supplied is quickly removed in
river deltas (Tuerena et al.,
2021c). For example, on the Laptev sea shelves, it is estimated that
62 %–76 % of riverine dissolved organic nitrogen is removed within a couple
of months by denitrification and biological utilisation
(Thibodeau et al., 2017). This is evident from the
very low nitrate concentrations in the TPD and high DSi : N ratios
(∼ 5 µM and 1.8, respectively, Doncila, 2020)
which is heavily influenced by riverine inputs and modification over the
Eurasian shelves. The near absence of nitrate in surface waters overall
contributes to the higher DSi : N output observed in PSW in Fram Strait. We
conclude that incomplete utilisation of DSi in the Arctic Ocean and its
subsequent export through the Fram Strait is governed largely by widespread
N limitation due to the rapid removal of nitrate in the Arctic Ocean, namely
on Eurasian shelves.
Evaluating contribution of DSi sources at Fram Strait
With decreasing ln(DSi) in Fig. 8, riverine and Pacific sources of δ30Si(OH)4 form a homogenous pan-Arctic trend line driven by
partial utilisation of DSi, separate from AW in the Rayleigh field. The
δ30Si(OH)4 of PSW plots between these two trend lines, as
a mixture of AW and Arctic-sourced nutrients instead (Figs. 6,
8).
As the pan-Arctic relationship is strong (R2=0.67), the extent of
utilisation of DSi sources and their relative contribution to PSW can be
estimated from the apparent pan-Arctic isotopic effect 30ε
(regression of trend line in the Rayleigh field). The two models are
displayed on Fig. 11. Considering the multiple nutrient pathways and
physical mixing as waters are transported from Arctic shelves to the Fram
Strait in the Arctic Ocean, it is expected that the pan-Arctic dataset fits
an open system. This is what is observed on Fig. 11. The open model is
more coherent with a 30ε close to global estimates of
-1 ‰ and a stronger R2 for the open system model
(R2= 0.83) than for a closed system (R2= 0.67). Using
the pan-Arctic dataset, we estimate 30ε=-0.58 ‰ for closed system fractionation and
30ε=-1.09 ‰ for open system
fractionation (Fig. 11, Supplement S4). This is in close
agreement with measured isotopic effects in the Canadian Arctic sector
(-0.59 ‰ and -1.19 ‰ for closed and
open systems respectively, Giesbrecht et al., 2022) which
may be lower than the global average due to sea-ice diatoms but also
dilutive effects.
Estimate of the apparent δ30Si isotopic effect
across the Arctic Ocean from source to export to Fram Strait through the
transpolar drift. The lines and equations are the result of linear
regression of δ30Si(OH)4 vs. the natural logarithm of f
(where f= measured DSi / DSi source prior to any biological consumption),
representative of closed system dynamics (left) and δ30Si(OH)4 vs. 1-f, representative of open system dynamics. AW and
PSW are not included in this regression as it is assumed they originate from
different nutrient sources. Coloured dots show measurements from within AW
(red, max. depth = 600 m) and PSW water masses (blue, max. depth = 150 m)
from this study based on water mass definitions in Table 2. Grey symbol sets
are published δ30Si(OH)4 from major DSi sources to the
surface Arctic domain and surface water masses. Triangles: N.Pacific
(<100 m, stations 1–6); stars: transpolar drift (<60 m,
stations 30–38 from Brzezinski et al., 2021). Crosses: Bering Strait (max.
depth = 60 m, stations 4–6 from Brzezinski et al., 2021). Squares: Canadian
Arctic (surface and intermediate water mass signatures of the Canadian
Arctic sector, from Table 2 in Giesbrecht et al., 2022). Octagons: glacial
runoff from Greenland and Svalbard glaciers (Hatton et al., 2019). Diamonds:
Lena River (Sun et al., 2018). The y intercept of both trend lines provides a
δ30Si(OH)4 estimate of DSi sources in the Arctic Ocean
ranging from 1.27 ‰–1.31 ‰.
Using the PSW signature calculated in Table 2 (1.84 ± 0.09 ‰), the apparent remaining nutrient fraction
(fPSW) within PSW can be estimated from the isotopic effects calculated
above. We estimate fPSW= 0.37 ± 0.06 and fPSW= 0.51 ± 0.08 for closed and open systems respectively. Considering the
large-scale patterns of transport, circulation and mixing within the Arctic
Ocean, we can assume the system is open as nutrients are likely to be
frequently resupplied, and closed system assumptions would lead to an
overestimation of nutrient consumption, but highlight that care needs to be
taken when applying fractionation models to the open ocean. Modelled DSi
within PSW in an open system would thus be 7.8 ± 0.6 µM and 1.89 ± 0.02 ‰ (based on parameters detailed in
Supplement S4).
Based on the calculation above, riverine sources contribute to 40 ± 4 % of the total DSi inventory at Fram Strait, with Pacific sources
contributing to 8 ± 1 %. Although sea ice has been proved to play an
important role for DSi cycling in other parts of the Arctic Ocean
(Giesbrecht et al., 2022; Liguori
et al., 2021), this calculation assumes sea ice only dilutes ambient DSi
concentrations and has no net isotopic effect based on our observation
locally (Sect. 4.2.4). Nevertheless, this basic calculation produces a
rough estimate based on DSi concentrations and isotopic signatures measured
within PSW. It illustrates two things: (1) a mixture of heavily utilised
riverine DSi and partially utilised Pacific-originating nutrients control
the nutrient inventory exported through the PSW; and (2) PSW DSi inventory is
highly sensitive to the extent of utilisation of riverine DSi on Arctic
shelves due to the high initial concentration. This calculation highlights
that modification of riverine nutrient sources and removal on Arctic shelves
is likely to have a large influence on the Atlantic DSi export and its
isotopic budgets. To improve the above estimate, accurate understanding of
DSi consumption on shelves prior to export in the central Arctic Ocean is
required, with improved isotopic signature determination of riverine sources
and shelf water masses.
Future implications
Our results highlight some important connections between nutrient cycling
and the control on the exchange of nutrients between the Arctic and the
Atlantic Ocean. This study has identified a link between the Arctic N and Si
cycles: low Nitrogen availability regulates the extent of DSi drawdown in
exported PSW and is traced to Arctic shelf processes. The nitrogen deficit
is generated by biological Arctic processes such as CPND. This along with
the extent of utilisation of DSi sources controls the excess DSi exported
out of the Arctic Ocean through gateways such as the Fram Strait. In the
changing Arctic Ocean, this has far-reaching implications to ecosystems and
nutrient budgets as discussed below.
Using δ30Si(OH)4 signatures, we have estimated that over
40 % of DSi exported out through Fram Strait is of riverine origin.
Freshwater inputs to the Arctic Ocean from the Eurasian sector are expected
to increase in response to climate change
(Mcclelland et
al., 2006; Rawlins et al., 2010). Increasing riverine discharge and
permafrost degradation is accelerating the transport of terrestrial material
to Eurasian shelves and likely increasing the export of major nutrients
(Zhang et al., 2021). As
NO3 delivery from rivers is low, riverine sources of DSi are increasing
faster than N inputs.
Light, DSi and NO3 availability all play an important role in dictating
the complex patterns of diatom production around the Arctic Ocean
(Giesbrecht
et al., 2019; Krause et al., 2019). Our study illustrates that NO3
availability plays an important role for biogenic Si production in the
Eurasian Arctic. Nitrogen is quickly removed in Siberian rivers at low
salinities (Sanders et al., 2021;
Tuerena et al., 2021c) through benthic denitrification, with roughly 70 %
of terrestrial N removed before reaching the seawater endmember
(Letscher et al., 2013) depleting N in relation to DSi in the deeper water column. Such rapid N
removal implies additional terrestrial NO3- inputs are not
likely to significantly impact N availability. Nitrogen deficiency on Arctic
shelves is currently limiting DSi consumption to only 14.3 % of its net
riverine input
(Le
Fouest et al., 2013). This implies that as terrestrial DSi inputs increase,
a larger proportion of terrestrial DSi will remain unutilised and ultimately
get transported through the TPD out to Fram Strait. This will increase the
export of DSi to the North Atlantic, but also alter the δ30Si(OH)4 of PSW which is derived from the partial biological
utilisation of DSi. Terrestrial DSi inputs increasing in the future combined
with increased N limitation will reduce the percentage of DSi consumption in
the Arctic Ocean, leading to lighter isotopic signatures of DSi exported
towards the North Atlantic.
Locally, the larger export of DSi through the TPD has implications for
nutrient dynamics in Fram Strait. N limitation is strong in PSW and is
predicted to increase in AW (Tuerena et al.,
2021a). Increasing primary production in the Arctic shelves as sea ice melts
and light availability increases
(Arrigo et al., 2008; Arrigo and
van Dijken, 2015) will increase N demand and further N losses through
denitrification which could reduce DSi uptake further and limit net
productivity from silicifying species and impact carbon drawdown in Fram
Strait. A decline in diatoms and a shift towards smaller phytoplankton
assemblages is already observed with warming in Fram Strait
(Lalande et al., 2013). Such changes will
be accentuated further with N limitation.
We also recognise there are competing influences on the future nutrient
status of the Fram Strait. The higher export of DSi can compensate for
decreasing DSi supply through AW to the Arctic Ocean resulting from
Atlantification (Arthun et al.,
2012; Lind et al., 2018) which leads to decreasing DSi concentrations in AW
(Hátún et al., 2017). While
terrestrial increase in DSi input and reduced utilisation in the Arctic will
supersede this signal in PSW over time, this can potentially lead to a
decrease in the DSi inventory of intermediate and deep waters of the Arctic
ocean influenced by AW, while increasing DSi export out of the Arctic
through the PSW.
The far-reaching consequences of the predicted future increases in Arctic
DSi export to the North Atlantic imply changes to primary production
patterns and DSi concentrations in deep water masses formed here. Waters in
the North Atlantic are richer in nitrate than DSi, and available evidence
indicate DSi limitation of diatom spring blooms due to limiting
concentrations of silicic acid in the region
(Henson et
al., 2006; Leblanc et al., 2005). This envisioned additional supply of DSi
can impact the duration of diatom blooms in the sub-Arctic North Atlantic
(Allen et al., 2005), and possibly enhance diatom
production with subsequent implications for carbon export to the deep ocean.
In longer timescales, this can also increase the preformed DSi inventories
in the North Atlantic deep waters, with an impact on the nutrient status of
the deep water masses worldwide.
Conclusions
Previous understanding of the importance of physical (water-mass mixing) vs.
biological (production and dissolution) controls in setting δ30Si(OH)4 distribution across the Arctic was limited by the lack
of direct measurements at major gateways
(Brzezinski et al.,
2021). This study provides the first full depth profiles of δ30Si(OH)4 in Fram Strait, in combination with δ15N-NO3 and δ18O-NO3, closing gaps in the
Arctic isoscape and confirming mechanisms of transformation.
Isotopic measurements document the transformation of PSW outflowing through
Fram Strait, with isotopic signatures Δ(15–18) = 4.22 ± 0.89 ‰ and δ30Si(OH)4=+1.85±0.09 ‰. δ30Si(OH)4 is
significantly enriched by +0.11 ‰ in PSW compared to
inflowing AW, while Δ(15–18) is enriched by
1.37 ‰, showing significant source modification of the
nutrients between the inflow and outflow waters.
Further examination of DSi and N isotopes trace nutrient sources and
modification processes in PSW primarily to Eurasian shelves; the increase in
DSi concentration and enrichment of δ30Si(OH)4 is traced
to biological uptake of DSi and partial burial of biogenic Si on the
shelves, sustained by the high DSi load from Eurasian rivers. Export of DSi
out of the Arctic through Fram Strait is ultimately regulated by
N limitation resulting from N-poor input from terrestrial sources combined
with efficient removal of N through assimilation and denitrification on
shelves. This is documented in PSW through decoupling of the oxygen and
nitrogen isotopes of nitrate from traditional 1 : 1 relationship. Glacial
influence from Greenland and Svalbard glaciers and Pacific inflow appeared
of smaller influence at Fram Strait in PSW, with riverine sources
contributing to ∼ 40 % of the DSi exported out of Fram
Strait.
The measurement of DSi and N isotopes provides the first insights into the
coupling of the N and Si cycle in the Arctic Ocean. Nitrate limitation
during primary production generates excess DSi which is subsequently
exported to the North Atlantic. As riverine nutrient sources of DSi are
expected to increase faster than N with climate warming, this can enhance N
limitation within the Eurasian Arctic Ocean and increase the export of DSi
to the North Atlantic Ocean.
Data availability
Nutrient
(10.5285/b61d58df-b8e8-11c4-e053-6c86abc0246c, Brand et al., 2020) and
nitrate isotope data (10.5285/b93fb7c0-110e-2470-e053-6c86abc05d60,
Tuerena and Ganeshram, 2021) for JR17005 are publicly available from the
British Oceanographic Database website. Silicon isotope dataset is publicly available on the British Oceanographic Database (10.5285/e92f0984-be44-28bb-e053-6c86abc036a3, Debyser et al., 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-19-5499-2022-supplement.
Author contributions
MCFD wrote the paper. MCFD, LP, RET, PAD
and RSG designed the study. MCFD, RET and AD analysed nitrate isotope
samples. MCFD and LP analysed silicon isotope samples. All authors
contributed to field work implementation and to the final version of the
paper.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank the crew and participants of Changing
Arctic Ocean cruises onboard the RRS James Clark Ross and Fram Strait Arctic
Outflow Observatory cruises onboard RV Lance and RV Kronprins Haakon for
support in sampling. We also thank the ARISE team for the collaborative
sampling effort and sharing scientific ideas.
Financial support
This work was supported by a Natural Environment Research
Council (NERC) Doctoral Training Partnership (grant no. NE/L002558/1) and from
the ARISE project (grant no. NE/P006310/1) awarded to Raja S. Ganeshram, part of the
Changing Arctic Ocean programme, jointly funded by the UKRI NERC and the
German Federal Ministry of Education and Research (BMBF).
Review statement
This paper was edited by Emilio Marañón and reviewed by Damien Cardinal, Zhouling Zhang, and one anonymous referee.
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