Introduction
The oceans play an essential role in the regulation of atmospheric CO2 and the buffering of the global climate system (e.g., Sabine,
2004) by removing carbon from the atmosphere via dissolution and
photosynthesis in the surface ocean, and storing it in the dissolved or
particulate forms. An important component of this oceanic sequestration is
the biological carbon pump, driven by sinking particles from the surface to
the deep ocean (e.g., Falkowski et al., 1998; Ducklow et al., 2001).
The magnitude of particulate organic carbon (POC) export flux from the upper
ocean was traditionally obtained from time-series sediment traps
(e.g., Honjo et al., 2008) and the natural radiotracer pair,
234Th/238U (e.g., Bhat et al., 1968; Buesseler et al., 1992).
Here we focus on the application of another natural radionuclide pair:
polonium-210 (210Po, T1/2=138.4 days) and its progenitor
lead-210 (210Pb, T1/2=22.3 years). The 210Po/210Pb pair
has a different particle-binding dynamic compared to the
234Th/238U pair since both isotopes are particle reactive, whereas
238U is conservative and remains dissolved in seawater
(Djogic et al., 1986). However, the nature of the
particle association differs between the isotopes. Lead-210 and 234Th
are only adsorbed to particle surfaces, whereas 210Po is both adsorbed
to surfaces and biologically reactive so it can be assimilated by organisms and
even bioaccumulated (Fisher et al., 1983; Cherrier et al., 1995; Stewart
and Fisher, 2003a, b). This behavior leads to a higher partitioning
coefficient (relative association between the isotope and the particulate
vs. the dissolved phase) of 210Po compared to that of 210Pb
(e.g., Masqué et al., 2002; Wei et al., 2014; Tang et al., 2017).
Lead-210 in the water column comes both from atmospheric deposition and in
situ production via the decay of 226Ra. The residence time of
210Pb in the atmosphere is only of days to weeks (Moore et al., 1974;
Turekian et al., 1977). Polonium-210 (produced by decay of 210Pb via
210Bi) activity in aerosols, and the subsequent fluxes to the surface
ocean, are only about 10 %–20 % of those of 210Pb
(Masqué et al., 2002). The large difference in their particle
reactivity and half-lives often leads to a disequilibrium between
210Po and 210Pb activities in the upper water column as
particles sink.
This deviation from secular equilibrium, often in the form of a deficit of
210Po activity with respect to 210Pb activity, can be used to
estimate POC export in a similar manner to the application of the
234Th/238U disequilibrium (Friedrich and Rutgers van der Loeff,
2002; Verdeny et al., 2009; Wei et al., 2011). Particle export fluxes
estimated from the 234Th/238U and the 210Po/210Pb
disequilibria integrate export that has occurred on timescales of weeks to
months prior to the sampling time, respectively. The use of both isotope
pairs could provide complementary information on the causes, timing, and
efficiency of export fluxes of POC (e.g., Murray et al., 2005; Stewart et
al., 2007; Roca-Martí et al., 2016).
In this study along the GEOTRACES GA01 transect in the North Atlantic, we
first used a traditional scavenging model with the assumptions of steady-state and negligible physical transport to derive 210Po fluxes over
different depths of the water column at 11 stations. Then, vertical
advection (primarily upwelling) was considered, and its impact on 210Po
flux was assessed. Using the POC concentration, and particulate 210Po
activity in the particles collected by in situ pumps, sinking fluxes of POC
were then calculated. The magnitude and efficiency of carbon export derived
from the 210Po/210Pb disequilibrium was considered in relation to
the composition of the phytoplankton community. Finally, the POC export
fluxes estimated from 210Po/210Pb disequilibria were compared
with those derived from 234Th/238U disequilibria.
Methods
Cruise track and hydrographic setting
The GEOVIDE cruise (GEOTRACES GA01 transect) was carried out in May–June 2014 from Lisbon to Newfoundland (Fig. 1). Seawater and particulate samples
for 210Po and 210Pb activity analysis were collected from the
water column at 11 stations (Fig. 1). The GA01 transect can be separated
into five sections according to its biogeochemical characteristics,
described in detail by Lemaitre et al. (2018). From east to west,
these are the Iberian Basin (stations 1, 13), the Western European Basin
(stations 21, 26), the Iceland Basin (stations 32, 38), the Irminger Basin
(stations 44, 60), and the Labrador Basin (stations 64, 69, 77).
Map of stations occupied during the GA01 transect in the North
Atlantic. The red squares indicate the stations where 210Po and
210Pb activities were measured, as discussed in this study. The transect is
divided into the Iberian Basin (stations 1, 13), the Western European Basin
(stations 21, 26), the Iceland Basin (stations 32, 38), the Irminger Basin
(stations 44, 60), and the Labrador Basin (stations 64, 69, 77).
Radionuclides sampling and analysis
Radionuclide data were produced by two collaborating laboratories to ensure
higher counting statistics for 210Po activity in the samples: the
Laboratori de Radioactivitat Ambiental at Universitat Autònoma de
Barcelona (UAB) (samples from stations 1, 13, and 21) and the Stewart
Laboratory at Queens College (QC) (samples from stations 26, 32, 38, 44, 60,
69, and 77). The sampling method for total and particulate 210Po and
210Pb samples and the determination of the radionuclide activity
was described in Tang et al. (2018). In brief, water
samples (5–10 L each) for total 210Po and 210Pb activity were
collected using Niskin bottles at 10 full water column stations (16–22 depths/station) and at 1 station to 1000 m (9 depths),
for a total of 200 samples. Particulate 210Po and 210Pb were collected at 3–10 depths per station between 15 and 800 m by using
McLane in situ pumps equipped with
a 53 µm PETEX screen to capture the large-size particles and a 1 µm quartz fiber QMA filter to capture small particles. The average
equivalent volume filtered for particulate 210Po and 210Pb samples
through the PETEX screen was 200 L and through the QMA filter was 70 L.
For water samples, Po and Pb isotopes (including the added chemical yield
tracers of 209Po and stable lead) were co-precipitated with
cobalt–ammonium pyrrolidine dithiocarbamate (Co-APDC) (Fleer and Bacon,
1984) at sea, but digested using concentrated HCl and HNO3 back at the
home laboratories. Particulate samples were spiked with 209Po and
stable lead before acid digestion (UAB: HNO3/HCl/HF, QC:
HNO3/HCl). Polonium isotopes (209Po and 210Po) were plated by
deposition onto a sliver disc (Flynn, 1968) and their activity was
determined by alpha spectrometry. After removing any remaining Po isotopes
by running the plating solution through an anion exchange column, the
solution was respiked with 209Po and stored for at least 6 months.
Lead-210 activity was determined by plating the ingrowth of 210Po from
210Pb.
The activities of 210Po and 210Pb at the sampling date were
determined by correcting for nuclide decay, ingrowth, chemical recoveries,
detector backgrounds, and blank contamination (Rigaud et
al., 2013).
The 210Po flux method
The export flux of 210Po was estimated from total 210Po and
210Pb activities using a one-box model (Broecker et al., 1973;
Matsumoto, 1975; Savoye et al., 2006). The 210Po activity in the
surface ocean is the result of a balance between atmospheric input,
continuous production from the decay of 210Pb in seawater, radioactive
decay of 210Po, removal onto sinking particles, and transport into or
out of the box by advection and diffusion. Therefore, the general form of
the mass balance equation for 210Po between sources and sinks is as follows:
∂Po/∂t=FPo+λPoIPo-λPoIPo-P+V,
where ∂Po/∂t is the change in
210Po activity with time, FPo (dpm m-2 d-1) is the
atmospheric flux of 210Po to the sea surface, λPo is the
decay constant of 210Po (0.005 d-1), IPb and IPo
(dpm m-2) are the inventories of 210Pb and 210Po activities,
P (dpm m-2 d-1) is the removal flux of 210Po
via sinking particles, and V (dpm m-2 d-1) is the sum of the
advective and diffusive fluxes.
The atmospheric flux of 210Po is usually ignored as it represents only
∼2 % of the in situ production of 210Po from
210Pb in the upper water column of the open ocean (e.g., Cochran,
1992; Masqué et al., 2002; Murray et al., 2005; Verdeny et al., 2008).
We first used a steady-state (SS) model that assumes the negligible
atmospheric input of 210Po activity and ignores advection and
diffusion. In this case, the 210Po flux (P) can be simplified as
follows:
P=λPo(IPb-IPo).
The influences of advection and non-steady-state (NSS) processes on the
overall 210Po activity balance are discussed below in Sect. 4.1 and 4.2.
The mixed-layer depth (MLD, defined as a change in potential
density of 0.03 kg m-3 relative to the potential density at 10 m), the
depth of the euphotic zone (Z1%, defined as the depth at which
photosynthetic available radiation was 1 % of its surface value), the
primary production zone (PPZ, at which the fluorescence reaches 10 % of
its maximum), and the 234Th-238U equilibrium depth (ThEq) at each
station along the GA01 transect. Together with the 30-day (30 days prior to
the sampling date) average vertical velocity within the 20 m under the
corresponding depths (w20, 10-6 m s-1, downwards as positive
direction). Primary production (PP) and net primary production (NPP) rates
derived from 24 h bottle incubations (Fonseca-Batista et al., 2018;
Lemaitre et al., 2018) and from the VGPM products, respectively, are also
presented. Note that the NPP rates were averaged for the previous 138 days
(210Po half-life) prior to the sampling date. Dates are in mm/dd/yy format.
Integration depth (m)
w20 (10-6 m s-1)
Production (mmol C m-2 d-1)
St.
Sampling
Basin
MLD
Z1%
PPZ
ThEq
MLD
Z1%
PPZ
ThEq
PP
±
NPP
±
date
1
5/19/14
Iberian Basin
15
40
136
90
-1±5
-1±4
-3±2
-2±6
33
2
69
43
13
5/24/14
Iberian Basin
35
40
90
110
0.1±3.1
0.1±3.1
-2±5
0.4±54.6
79
3
61
32
21
5/31/14
Western European
15
32
64
110
-1±2
-1±4
-1±5
0.1±5
135
2
109
112
Basin
26
6/4/14
Western European
30
30
98
100
-2±3
-2±2
-5±5
-5±4
174
19
58
57
Basin
32
6/7/14
Iceland Basin
30
31
70
120
-1±9
-1±9
-4±20
-3±20
105
11
48
36
38
6/10/14
Iceland Basin
30
30
69
80
1±3
1±3
3±4
3±5
68
7
44
37
44
6/13/14
Irminger Basin
26
22
44
40
1±2
1±2
2±3
2±3
137
2
46
44
60
6/18/18
Irminger Basin
17
20
36
100
-14±20
-14±20
-36±40
-11±70
166
32
50
51
64
6/19/14
Labrador Basin
20
47
80
80
2±6
7±7
3±7
3±7
54
18
47
49
69
6/22/14
Labrador Basin
20
28
44
40
-2±1
-2±3
-2±3
-2±3
27
5
46
56
77
6/26/14
Labrador Basin
15
20
59
80
4±5
4±7
7±10
9±20
80
21
50
56
Many previous studies have used a single fixed integration depth for export
calculations at all sampling locations (e.g., 100 m in the Antarctic
Circumpolar Current, Rutgers van der Loeff et al., 1997; 120 m in the
central equatorial Pacific, Murray et al., 2005). The GA01 transect,
however, crossed diverse physical and biogeochemical conditions. Thus,
investigating export at a single fixed depth for every station may bias the
spatial comparisons of particle export. In this study, four site-specific
integration depths were used for each station: the mixed-layer depth (MLD),
the depth of the euphotic zone (Z1%), the primary production zone
(PPZ), and the 234Th-238U equilibrium depth (ThEq). The MLD was
defined as a change in potential density of 0.03 kg m-3 relative to the
potential density at 10 m (Weller and Plueddemann, 1996). Z1%
was defined as the depth at which photosynthetic available radiation was 1 %
of its surface value (Jerlov, 1968). PPZ was the depth at which the
fluorescence reaches 10 % of its maximum (Owens et al.,
2015). ThEq was the depth at the bottom of the total 234Th water column
deficit, where the activity of 234Th equals that of 238U
(data from Lemaitre et al., 2018). These depths were used both to
calculate 210Po and POC export and in order to compare the POC export
fluxes estimated from the 210Po/210Pb disequilibria to those
derived from the 234Th/238U disequilibria. Among the 11 stations,
the depths of the MLD (23±7 m) were similar to those at Z1%
(31±9 m), whereas the depths of the PPZ (72±29 m) and ThEq
(95±43 m) were deeper and comparable to each other. For the depths
of MLD, Z1%, PPZ, and ThEq at which total radionuclides data are not
available, the measured values of total 210Po and 210Pb activities
were linearly interpolated (Table 1).
The 210Po flux was then used to derive the flux of POC by multiplying
the deficit of 210Po by the ratio of POC concentration to 210Po
activity (POC/210Po) of the total and large particulate material. Particulate
210Po and POC data were not always available at the depths of the MLD,
Z1%, PPZ, and ThEq at our study sites. To estimate POC/210Po
ratios at these depths, a regression was performed between the measured
POC/210Po ratios and depth for each basin using a single power law
function.
Quantification of the influence of the vertical advection on 210Po
export
Cyclonic or anticyclonic eddies constantly impact the horizontal velocity
fields at our study sites (Zunino et al., 2017), changing the current
directions and making it difficult to estimate the magnitude of horizontal
velocities. This constant variability, together with the patchiness of
sampling resolution, meant we could not assess the influence of horizontal
advective processes on 210Po export estimates.
However, because we had relatively high depth resolution at each station, we
did attempt to assess the influences of vertical advection on 210Po
inventories at all the investigated depths by measuring the vertical
gradient of 210Po activity and multiplying it by a time-averaged
vertical velocity. Because the water column inventory of 210Po
represents an integration of the changes over approximately the mean life of
the isotope, we did not use the vertical velocity measured by the acoustic
doppler current profiler (ADCP) at the sampling time, but a time-averaged
vertical velocity from the Estimating the Circulation and Climate of the
Ocean, Phase II (ECCO2). The activity gradient of 210Po below the depth
z (i.e., the MLD, Z1%, PPZ, and ThEq) at each station was calculated from
the depth z (using the average activity in the layer of 0-z m) as
starting point (APo1) and linearly interpolated through the
measurements 20 m below z (APo2) at each station. A positive
gradient (APo2-APo1 > 0) was defined as higher
activity at the depth of (z+20 m) than the starting point. We labeled
the vertical velocity w20, which was the 30-day (30 days prior to the
sampling date) average vertical velocity between the depths of z and (z+20 m). The flux of 210Po due to vertical advection (Fw) was
calculated as the following:
Fw=w20×APo2-APo1.
Total 210Po fluxes at each depth, therefore, are the sum of the steady-state values based only on the 210Po deficit (Eq. 2), λPo(IPb-IPo), and vertical advective flux (Eq. 3), w20×APo2-APo1.
The ECCO2 vertical velocities were obtained from the Asia-Pacific
Data-Research Center (APDRC,
http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=1,
last access: 18 January 2019). The ECCO2 model
configuration uses a cube–sphere grid projection with 18 km horizontal grid
spacing and 50 vertical levels among which there are 12 equal vertical
layers from the surface to 120 m (Menemenlis et al., 2008). We selected the
ECCO2 grid points closest to the station and extracted vertical velocities
from the depths between z and (z+20 m) during 30 days prior to the
sampling date at each station. Because the deficit of 210Po activity in
the water column weighs the changes that occurred shortly prior to the
sampling time more heavily than those that occurred further back in time
(Verdeny et al., 2009), we chose to average the vertical
velocity over 1 month rather than over the mean life of 210Po (200 days). The 30-day averaged vertical velocity was then used to calculate
vertical advective 210Po export flux via Eq. (3) at each station.
Satellite-based net primary production and phytoplankton
composition
The 8-day net primary production (NPP) data with a spatial resolution of
0.083 by 0.083∘ were obtained from the
Oregon State University Ocean Productivity standard products (http://www.science.oregonstate.edu/ocean.productivity/,
last access: 18 January 2019), wherein NPP was
estimated by the Vertically Generalized Production Model (VGPM)
(Behrenfeld and Falkowski, 1997). Due to some missing data
between November 2013 and February 2014, NPP for each station was averaged
for the previous 138 days (210Po half-life) instead of 200 days
(210Po mean life).
Monthly average concentrations of diatoms, coccolithophores, cyanobacteria,
chlorophytes, and total chlorophyll with the spatial resolution of 0.67×1.25∘ were obtained from the Goddard Earth Science
Data and Information Services Center Interactive Online Visualization and
Analysis Infrastructure (Giovanni)
(https://giovanni.gsfc.nasa.gov/giovanni/ last access: 18 January 2019, Acker and Leptoukh,
2007). Time-series (October 2013–July 2014, covering > 200 days before sampling) data are averages over longitude for each month. We
extracted data for the five basins individually and calculated the fraction of
each phytoplankton group at each station as the ratio of their concentration
to total chlorophyll concentration.
Satellite-derived monthly average fraction of major phytoplankton
groups from October 2013 to July 2014 along the GA01 transect: fdia,
fcoc, fcya, and fchl are the fractions of
diatoms (purple), coccolithophores (blue), cyanobacteria (gray), and
chlorophytes (orange), respectively. Data are from the Giovanni online data
system https://giovanni.gsfc.nasa.gov/giovanni/ (last access: 18 January 2019).
Results
Satellite-derived seasonal NPP and phytoplankton composition
The VGPM modeled NPP data along the GA01 transect was averaged over
∼138 days prior to the sampling date (see Sect. 2.5, Table 1). Seasonal NPP at each station varied from low values of
44–79 to a maximum value of 109 mmol C m-2 d-1 at
station 21. The Western European Basin had the highest seasonal NPP,
followed by the Iberian Basin, while the Iceland Basin, the Irminger Basin,
and the Labrador Basin all had similar NPP values in the range of 45–49 mmol C m-2 d-1. There was a shift in the biological community
towards larger phytoplankton (e.g., diatoms) from east to west along the
transect (Fig. 2). The basins for which diatoms were the dominant phytoplankton
group did not necessarily have higher seasonal production relative to the
basins where smaller phytoplankton (e.g., coccolithophores) were more
abundant. Indeed, the Iberian Basin had the second highest seasonal NPP,
despite the fact that the majority of chlorophyll was produced by
coccolithophores. Despite the evidence that earlier blooms may have been
driven by diatoms (see Sect. 4.2), these observations highlight the
possible contribution of small particles to production, and possibly to
export (proportional to their role in production according to
Richardson and Jackson, 2007), along the transect. Moreover, this could also be
due to shorter blooms in the Irminger and Labrador basins where the
phytoplankton growth was light-limited during winter compared to the
conditions in the Iberian and Western European basins.
The satellite-derived phytoplankton species composition demonstrated unique
features within the basins (Fig. 2). The Iberian Basin was dominated
(> 60 %) by coccolithophores between October 2013 and July 2014,
but had a gradual increase in the contribution of diatoms until April 2014
and a decreasing contribution after that. In the Western European Basin,
station 26 was dominated by diatoms all year round, while station 21 was
dominated by diatoms, except in October 2013 and July 2014 when the
combination of chlorophytes and coccolithophores contributed 35 %–77 %
to the total chlorophyll concentration. The stations in the
Iceland, Irminger, and Labrador basins were all dominated (> 98 %) by diatoms between October 2013 and July 2014.
One-month averaged vertical velocity w20
The 1-month averaged vertical velocities w20 ranged from -36×10-6 to 9×10-6 m s-1 along the transect
(negative is upwelling, positive is downwelling, Table 1). The standard
deviations of w20 were generally of the same order as the values of
w20. Particularly large standard deviations, which exceed the typical
values of the vertical velocity by a full order of magnitude, were found at
stations 13 (35–55, 110–130 m) and 21 (110–130 m). These high
standard deviations suggest that the data on w20 should be used with
great care. Downwelling was seen at stations 38, 44, 64, and 77 with the
velocities in the range of 1×10-6 to 9×10-6 m s-1. Upwelling
was seen at the remaining stations, with highest intensity at station 60
near Greenland (absolute value: 11–36×10-6 m s-1).
The upwelling velocities were roughly equivalent at stations 1, 13, 21, 26,
32, and 69 (absolute value: 1–5×10-6 m s-1).
Section plots of water column 210Po deficits (dpm 100 L-1,
total 210Pb activity minus total 210Po activity) across the GA01
transect. Panel (a) is the upper 500 m. Panel (b) is 500–5500 m.
Station numbers and basins are shown at the top of the upper panel.
Total 210Po deficits
The vertical profiles of total 210Po and 210Pb activity at each
station have been described in a companion article (Tang et
al., 2018). Here we show the section view of the water column 210Po
deficit (dpm 100 L-1), which was calculated as total 210Pb
activity minus total 210Po activity (Fig. 3). There were small
210Po deficits in the upper 100 m (including the majority of the depths
of MLD, Z1%, PPZ, and ThEq at all stations) at stations 1, 13, and
21, whereas a relatively large excess of 210Po was observed at 100–400 m depth. Station 60 had the highest deficits of 210Po
(∼8 dpm 100 L-1, n=5) at 40–120 m depth. A large
surface deficit of 210Po was found at station 64 (8 dpm 100 L-1)
and a surface excess was found at station 38 (-3.5 dpm 100 L-1). There
were positive 210Po deficits throughout most of the water column at
stations in the Irminger and Labrador basins, whereas large 210Po
excesses (negative deficits) below 100 m were generally seen in the Iberian
Basin and Western European basins. Such 210Po excess was likely related
to the Iberian upwelling, which may have provided a source of 210Po
activity.
The total 210Po flux as the sum of the flux calculated from
the deficit and vertical advection, together with POC/210Po ratios in
particles > 1 µm (derived from the power law function in
Fig. 5) and POC fluxes derived from 210Po at the corresponding depths.
The uncertainties of 210Po export flux are associated with the activity
uncertainty of the radionuclides. The error for the calculated particulate
POC/210Po ratio in each basin is the standard error of regression. The
uncertainties of the 210Po-derived POC flux were estimated based on
the propagation of error.
Integration depth (m)
210Po flux (dpm m-2 d-1): 210Po/210Pb term
210Po flux (dpm m-2 d-1): vertical advection term
St.
MLD
Z1%
PPZ
ThEq
MLD
±
Z1%
±
PPZ
±
ThEq
±
MLD
±
Z1%
±
PPZ
±
ThEq
±
1
15
40
136*
90*
1.1
0.3
1.5
0.8
-4.5
2.2
-0.9
1.6
2.4
19.7
3.6
14.7
6.8
4.8
4.6
16.2
13
35*
40
90*
110*
3.4
0.9
4.1
0.9
4.3
1.8
3.7
2.0
-0.2
5.2
-0.2
5.6
3.7
10.0
1.0
10.6
21
15
32*
64*
110*
-0.6
0.5
-0.7
0.8
2.2
1.2
3.5
1.8
-1.1
4.0
-0.4
1.7
2.7
9.9
0.01
0.40
26
30
30
98*
100
4.8
1.5
4.8
1.5
15.2
3.1
26.4
4.8
-0.9
3.2
-0.9
3.2
4.0
4.0
2.8
4.0
32
30
31*
70*
120*
4.7
0.9
4.8
0.9
9.1
1.4
8.5
2.2
-1.6
12.2
-1.6
12.0
7.9
33.4
3.0
23.3
38
30
30
69*
80
-0.5
1.3
-0.5
1.3
3.7
2.5
5.2
2.6
0.4
1.8
0.4
1.8
-1.0
3.5
-0.9
4.9
44
26*
22*
44*
40
1.5
1.0
1.0
1.0
4.2
1.4
3.6
1.4
0.9
2.1
1.1
2.5
0.9
2.2
1.5
2.8
60
17*
20
36*
100
3.1
1.1
3.8
1.1
9.8
1.6
37
5.4
-24.9
49.6
-40.4
74.9
-36.2
69.0
14.1
87.1
64
20*
47*
80
80
5.8
0.8
9.8
2.1
17.8
3.2
18
3.2
-0.7
2.9
-4.3
8.8
-0.5
3.7
-0.5
3.7
69
20*
28*
44*
40
4.0
0.7
6.1
0.8
8.5
1.6
8.3
1.5
1.9
3.3
3.4
5.8
5.8
7.9
6.7
8.9
77
15*
20
59*
80
2.2
0.6
2.9
0.7
7.0
2.4
9.8
2.9
-0.6
5.2
0.3
6.4
3.0
9.9
-15
29
210Po flux (dpm m-2 d-1): total flux
POC/210Po (mol dpm-1)
St.
MLD
±
Z1%
±
PPZ
±
ThEq
±
MLD
±
Z1%
±
PPZ
±
ThEq
±
1
3.5
19.7
5.1
14.7
2.3
5.3
3.6
16.2
540
67
305
67
150
67
190
67
13
3.2
5.3
3.9
5.7
7.9
10.1
4.7
10.8
330
67
305
67
190
67
169
67
21
-1.7
4.1
-1.1
1.8
4.9
10.0
3.5
1.9
542
89
389
89
287
89
227
89
26
3.9
3.5
3.9
3.5
17.7
5.1
29.2
6.2
400
89
400
89
238
89
236
89
32
3.0
12.2
3.2
12.1
17.0
33.4
11.6
23.4
367
111
363
111
265
111
216
111
38
-0.2
2.3
-0.2
2.3
2.7
4.3
4.2
5.6
367
111
367
111
267
111
252
111
44
2.5
2.3
2.1
2.7
5.1
2.6
5.1
3.1
310
107
330
107
254
107
263
107
60
-21.8
49.6
-36.6
74.5
-26.4
69.0
51.2
87.2
364
107
342
107
274
107
187
107
64
5.1
3.0
5.5
9.0
17.4
4.9
17.4
4.9
675
152
375
152
261
152
261
152
69
5.9
3.4
9.4
5.8
14.4
8.0
15.0
9.0
675
152
536
152
393
152
419
152
77
1.5
5.2
3.1
6.4
10.1
10
-4.8
29.0
822
152
675
152
321
152
261
152
210Po–POC flux (mmol C m-2 d-1): 210Po/210Pb term
210Po–POC flux (mmol C m-2 d-1): total flux
St.
MLD
±
Z1%
±
PPZ
±
ThEq
±
MLD
±
Z1%
±
PPZ
±
ThEq
±
1
0.6
0.2
0.4
0.3
-0.7
0.4
-0.2
0.3
1.9
10.7
1.5
4.5
0.3
0.8
0.7
3.1
13
1.1
0.4
1.3
0.4
0.8
0.4
0.6
0.4
1.0
1.8
1.2
1.7
1.5
2.0
0.8
1.9
21
-0.3
0.3
-0.3
0.3
0.6
0.4
0.8
0.5
-0.9
2.2
-0.4
0.7
1.4
2.9
0.8
0.5
26
1.9
0.7
1.9
0.7
3.6
1.5
6.2
2.6
1.5
1.4
1.5
1.4
4.6
2.0
6.9
3.0
32
1.7
0.6
1.7
0.6
2.4
1.1
1.8
1.1
1.1
4.5
1.1
4.4
4.5
9.1
2.5
5.2
38
-0.2
0.5
-0.2
0.5
1.0
0.8
1.3
0.9
-0.1
0.8
-0.1
0.8
0.7
1.2
1.1
1.5
44
0.5
0.4
0.3
0.4
1.1
0.6
1.0
0.5
0.8
0.8
0.7
0.9
1.3
0.9
1.4
1.0
60
1.1
0.5
1.3
0.5
2.7
1.1
6.9
4.1
-7.9
20
-12.5
25.9
-7.2
19.1
9.6
17.2
64
3.9
1.0
3.7
1.7
4.7
2.8
4.7
2.8
3.5
2.1
2.1
3.5
4.5
2.9
4.5
2.9
69
2.7
0.8
3.3
1.0
3.4
1.4
3.5
1.4
4.0
2.5
5.1
3.4
5.7
3.8
6.3
4.4
77
1.8
0.6
1.9
0.6
2.3
1.3
2.5
1.7
1.3
4.3
2.1
4.3
3.2
3.6
-1.3
7.6
* For the depths at which total radionuclides data are not available,
the measured values of total 210Po and 210Pb activities were
linearly interpolated at the missing depths.
The 210Po flux calculated from the deficit of 210Po
alone
Using the data of total 210Po and 210Pb activities, the amount of
210Po escaping from the surface ocean via particles (210Po
fluxes, dpm m-2 d-1) was calculated using Eq. (2) assuming
steady state and ignoring advection and diffusion (Table 2,
210Po/210Pb term). The 210Po fluxes were negligible or very
low at stations 1, 21, and 38. At the other stations the 210Po fluxes
averaged 3.7±1.4, 4.6±2.6, 9.5±4.9, and 14.4±12 dpm m-2 d-1 at the MLD, Z1%, PPZ, and ThEq, respectively.
The 210Po fluxes tended to increase with depth at 7 out of 11
stations (26, 38, 44, 60, 64, 69, and 77). At the MLD, Z1% and PPZ,
the largest 210Po fluxes were all found in the Labrador Basin. The
other four basins had relatively similar 210Po export fluxes (2.1–2.8 dpm m-2 d-1) at the MLD and Z1%. The West European Basin had
much higher 210Po flux (8.7 dpm m-2 d-1) relative to that in
the Iberian Basin (-0.1 dpm m-2 d-1) at the PPZ. At the ThEq, on
the other hand, the Irminger Sea had the highest 210Po fluxes followed
by the West European Basin. The lowest 210Po fluxes at all investigated
depths were generally found in the Iberian Basin.
The ratio of POC concentration to 210Po activity
(POC/210Po) in the particles collected by in situ pumps. SSF is small-size fraction (1–53 µm); LSF is large-size fraction (> 53 µm); TPF is total particulate fraction (> 1 µm).
POC/210Po (µmol dpm-1)
Station
Depth (m)
SSF
±
LSF
±
TPF
±
1
30
276
32
414
58
296
30
1
80
166
28
1040
159
355
44
1
550
41
4
31
4
39
4
1
800
18
17
19
4
19
10
1
120
108
14
222
42
117
13
1
250
65
7
63
9
65
6
13
60
289
29
216
26
281
25
13
100
206
20
132
14
198
17
13
200
79
7
50
8
76
7
13
450
73
7
35
7
69
6
21
80
622
51
13 405
2599
1280
96
21
120
133
18
2398
407
380
44
21
250
85
9
482
133
109
10
21
450
54
6
117
14
60
6
26
30
377
70
310
34
350
42
26
83
271
41
289
37
280
28
26
153
275
94
118
14
209
43
26
403
67
21
43
19
62
17
32
30
492
60
733
382
500
59
32
60
379
43
337
87
376
40
32
100
311
39
376
56
326
33
32
200
145
17
133
30
144
15
32
450
41
5
55
9
42
4
32
800
25
4
55
7
29
4
38
20
254
38
345
108
258
37
38
60
339
51
284
66
333
46
38
109
157
15
196
23
163
13
44
20
1025
115
3085
798
1176
124
44
40
463
58
1379
1787
475
59
44
80
140
14
90
23
137
13
44
150
102
18
97
56
102
17
44
300
47
7
25
7
45
6
60
8
306
30
1003
150
422
36
60
60
232
33
851
193
272
36
60
100
197
33
303
72
209
31
60
250
61
7
294
84
72
8
64
30
525
77
656
83
580
58
64
60
455
75
286
77
434
64
64
100
439
49
319
44
420
41
64
150
107
36
158
28
129
24
64
400
40
5
48
8
41
4
69
20
347
44
879
164
397
46
69
60
78
6
657
216
84
7
69
100
257
26
359
44
268
24
69
150
125
14
127
25
125
13
69
410
30
3
71
8
34
3
77
10
1281
309
917
150
1181
213
77
50
1372
357
1020
412
1339
320
77
80
512
63
544
103
516
57
77
200
84
13
217
79
92
13
77
460
22
3
59
6
27
3
POC/210Po ratios in particles
Most of the ratios of POC concentration to 210Po activity
(µmol dpm-1) in the large-size fraction of particles
(POC/210Po_LSF, > 53 µm) were
comparable to or higher than those in the small-size fraction
(POC/210Po_SSF, 1–53 µm), although a few
samples at stations 13, 26, 44, 64, and 77 had lower values of
POC/210Po_LSF than those of
POC/210Po_SSF (Table 3, Fig. 4a). The POC/210Po
ratio in the total particles (> 1 µm, the combination of
small and large particles, POC/210Po_TPF) was similar to
that in the small particles (SSF), within about 97 % (Table 3, Fig. 4b).
This is because over 80 % of the particulate 210Po activity was
associated with the small-size fraction (Tang et al., 2018)
likely due to the large surface area of abundant small particles. Because of
the possible link between small particles and export along the transect
discussed in Sect. 3.1, and the results that scavenging of 210Po was
governed by the small particles (Tang et al., 2018), we
propose to use this total particulate fraction in addition to the more
commonly used large-size fraction to calculate POC export along this cruise
track.
The POC/210Po in total particles (POC/210Po_TPF)
varied from 19 to 1300 µmol dpm-1 with a mean of 290±320 µmol dpm-1 (n=51, upper 800 m). The variability of
POC/210Po_TPF ratios in this study is in line with
previous observations in the Antarctic Circumpolar Current (300–1200 µmol dpm-1 for particles > 1 µm)
(Friedrich and Rutgers van der Loeff, 2002) and the central
Arctic (90–1900 µmol dpm-1 for particles > 53 µm) (Roca-Martí et al., 2016). The average ratio of
290 µmol dpm-1 is comparable to those observed in the central
equatorial Pacific (202±90 µmol dpm-1 for particles
> 0.45 µm) (Murray et al., 2005), the North
Atlantic (290±70 µmol dpm-1 for particles > 1 µm) (Rigaud et al., 2015), and the South Atlantic
(113±80 µmol dpm-1 for particles > 0.7 µm) (Sarin et al., 1999).
The measured POC/210Po ratios in total particles at each station and
depth were grouped into the five basins and fitted against depth using a single
power law function in each basin (Fig. 5). The fit equations were used to
calculate total particulate POC/210Po ratios at the investigated depths
at each station (Table 3).
Plots of the ratios of POC concentration to 210Po activity in
(a) the large (> 53 µm) particles
(POC/210Po_LSF) against the small (1–53 µm)
particles (POC/210Po_SSF), and in (b) the total
(> 1 µm) particles (POC/210Po_TPF)
against the small particles. The black lines indicate the 1:1 line.
The ratios of POC concentration to 210Po activity in the total
particles vs. depth in each basin along the GA01 transect. Power law
regression (red line) was fitted for POC/210Po against depth in each
plot: the Iberian Basin (stations 1, 13), West European Basin (stations 21,
26), Iceland Basin (stations 32, 38), Irminger Basin (stations 44, 60), and
Labrador Basin (stations 64, 69, 77). The data points denoted as filled by
black circles were outliers (points at a distance greater than 1.5 standard
deviations from the power law model) and excluded from the power law
regression.
Discussion
Physical advection effects on 210Po export fluxes
In the study region, there were consistent patterns of circulation traveling
through and near our sampling sites during the GEOVIDE cruise. From east to
west the cruise track crossed the North Atlantic Current, the East
Reykjanes Ridge Current, the Irminger Current, the Irminger Gyre, the
eastern and western boundary currents and the Labrador Current (Fig. 1 in García-Ibáñez et al., 2018). Additionally, short-lived
eddies and fronts were also observed during the cruise, particularly in the
OVIDE section from Portugal to Greenland (García-Ibáñez
et al., 2018; Zunino et al., 2017). In this dynamic region advective
influences may be important to include in calculations of 210Po export.
Despite this knowledge, we could not include horizontal advection in our
model because the horizontal resolution of our sample sites was not
sufficient to constrain reliable horizontal gradients of 210Po activity
in the study region. This assumption of negligible horizontal physical
transport has been made in most 210Po studies because of a similar lack
of spatial resolution (e.g., Kim and Church, 2001; Stewart et al., 2010;
Rigaud et al., 2015), and may be justified in some open-ocean settings where
horizontal gradients in 210Po activity are small (e.g.,
Wei et al., 2011). For more dynamic regimes, such as along the GA01 transect,
however, this assumption needs to be carefully evaluated, and the relative
importance of advective 210Po flux should be assessed if possible.
We did, however, have enough sampling depths at each station to assess the
vertical variability in 210Po activity and to estimate the impact of
vertical advection on the 210Po flux. The range of 210Po activity
flux due to vertical advection (-40 to 14 dpm m-2 d-1, Table 2)
was of the same magnitude as the steady-state fluxes calculated from the
deficit alone (-5 to 37 dpm m-2 d-1, Table 2). The magnitude of
the uncertainty of the 210Po export flux due to vertical advection was
influenced by the large variance in vertical velocity field mentioned in
Sect. 3.2. When excluding the three depths at stations 13 and 21, where the
monthly vertical velocity average had substantial standard deviations (an
order of magnitude greater than w20), the uncertainty of the 210Po
export flux was on average 2-fold larger than the calculated 210Po
export flux. The largest positive vertical advective 210Po fluxes were
at station 1, where the Iberian upwelling increased the calculated flux by
150 %–500 %. The largest negative vertical advective 210Po fluxes
were seen at station 60 where upwelling decreased the 210Po flux by 370 %–1100 % at the depths of the MLD, Z1%, and PPZ. This is because
the upwelling velocity was high at those depths (14–36×10-6 m s-1, Table 1) and the upwelled water was depleted in
210Po activity. The vertical advective transport was smaller at the MLD
and Z1% at station 13, at the ThEq at station 21, and at the PPZ and
ThEq at station 64, with contributions lower than 6 % of the total
210Po fluxes. Including vertical advection in our flux estimates at all
other depths, however, increased or decreased the 210Po fluxes by
10 %–180 %. Like with vertical advection, neglecting horizontal advection can
result in either an underestimate or overestimate of 210Po export flux
depending on whether the advected water is enriched or depleted in
210Po. However, because our study region was characterized by distinct
water masses separated over tens to hundreds of meters in the vertical plane,
whereas those same water masses covered huge distances (hundreds to thousands of
kilometers) in the horizontal plane (Fig. 4 in
García-Ibáñez et al., 2018), vertical advection would most
likely result in more change in physical and chemical parameters over the
scale of sampling than horizontal advection would. Because the advective
210Po export flux was calculated as the product of the velocity of the
water mass and the gradient of 210Po activity in the corresponding
direction, horizontal advection would most likely contribute a much smaller
range of advective 210Po flux estimates.
Overall, the influence of physical advection on 210Po activity may
range from relatively unimportant to dominant depending on the study area. In
this study, we observed physical processes influencing 210Po fluxes, in
particular at stations 1 and 60. For future studies of 210Po and
210Pb activity in regions of established upwelling or ocean margins, we
suggest designing the sampling plan so that the magnitude and variability of
these processes may be incorporated into 210Po export models. At ocean
margins, in particular, more water samples should be taken to improve the
resolution of horizontal features.
Non-steady-state effects on 210Po export fluxes
To our knowledge, three time-series studies of 210Po and 210Pb
activities have been conducted to date and have assessed the NSS effects on
210Po fluxes. First, in the upper 500 m of the Sargasso Sea,
Kim and Church (2001) found that the SS model may have
overestimated and underestimated the 210Po export fluxes in May and
July 1997, respectively. Second, at the DYFAMED site of the northwestern
Mediterranean Sea, the ∂Po/∂t term
accounted for ∼50 % of 210Po flux estimated by using
the SS model (Stewart et al., 2007). Last, in the South China
Sea, the 210Po export fluxes at 1000 m calculated from the SS
and NSS models had similar values within the uncertainties (Wei et
al., 2014). In fact, the SS model generally results in an underestimation of
the 210Po flux under conditions of decreasing 210Po activity
in the water column (i.e., when blooms switch from the productive phase to
the export phase), whereas the SS model overestimates the flux for conditions
of increasing 210Po activity (i.e., high atmospheric deposition).
Time-series (1 January–12 July 2014) satellite estimates of net
primary production (NPP) between 1 January and 12 July in 2014 at each
station along the GA01 transect (VPGM algorithm,
http://www.science.oregonstate.edu/ocean.productivity/, last access: 18 January 2019). The
shaded rectangle in each plot denotes NPP for about 2 months prior to the sampling
date. Two months of NPP data are needed because this timescale could ensure
the sensitivity for NSS estimates (Friedrich and Rutgers van der Loeff,
2002; Stewart et al., 2007). The vertical red line in each plot indicates
the sampling date at each station.
Atmospheric aerosol deposition along the GA01 transect was reportedly low
without the significant influence of the Saharan plume (Shelley
et al., 2017). The influence of atmospheric deposition on the SS estimates
obtained in this study, therefore, can be ignored. However, it is important
to assess the ∂Po/∂t term that
was associated with the site-specific bloom events during the cruise.
Satellite estimates of net primary production (VGPM model) for the eight
8-day periods prior to the sampling date (∼2 months) were
calculated at each station (Fig. 6). Two months of NPP data are needed because
such a timescale could ensure the sensitivity for NSS estimates
(Friedrich and Rutgers van der Loeff, 2002; Stewart et al., 2007). NPP values
for the 2-month period were in the ranges of 51–184, 39–403, 22–131, 18–204, 16–210 mmol C m-2 d-1 in the Iberian Basin, the
West European Basin, the Iceland Basin, the Irminger Basin, and the Labrador
Basin, respectively, indicating the occurrence of blooms during this time
period along the transect that might have influenced the 210Po fluxes
derived from Eq. (1).
SS may have underestimated the 210Po export along the GA01
transect depending on the stage of the bloom before sampling. For example,
at station 21 the largest NPP peak (403 mmol C m-2 d-1) occurred 2 weeks before our sampling date and diminished rapidly
(∼100 mmol C m-2 d-1 at sampling time). The combination of high
phytoplankton export and a sudden decrease in NPP may have significantly
lowered the 210Po activity in the upper waters, resulting in a negative
∂Po/∂t, and thus the SS model may
have underestimated the true 210Po flux. Temporal variations were also
seen in the time-series phytoplankton community composition, in particular
at stations 1 and 13 (Fig. 2). Both stations were dominated (> 60 %) by coccolithophores between October 2013 and July 2014, but appeared
to have a diatom bloom in April 2014 before sampling. Polonium-210 and
210Pb tend to bind to specific biopolymeric functional groups, leading
to fractionation during their sorption onto particles (Quigley et al.,
2002; Chuang et al., 2013; Yang et al., 2013). The temporal variation of
phytoplankton composition could therefore also lead to non-steady-state
effects on the overall 210Po activity balance, which are difficult to
assess but deserve more attention.
The NSS effect on the 234Th fluxes at the ThEq were evaluated during
the same cruise along the GA01 transect in Lemaitre et al. (2018) by
using the NSS model developed in Savoye et al. (2006). Because the
cruise plan did not allow an opportunity to reoccupy the study areas over
time, the authors made the assumption that 234Th activity was in
equilibrium with 238U activity at the starting date of the bloom. Their
results suggested that the NSS 234Th fluxes were about 1.1 to 1.3 times
higher than the SS estimates in the Iberian and West European basins, and
1.4 to 2.1 times higher in the Iceland, Irminger, and the Labrador basins.
We did not attempt to apply the same technique to estimate NSS 210Po
fluxes in this study because the assumption of equilibrium between
210Po activity and 210Pb activity at the starting date of the
bloom may be inappropriate and the 210Po deficit integrates over a
longer time period (months) than a typical bloom event (days to weeks).
POC flux calculated from 210Po flux
The POC export fluxes were calculated by multiplying both the 210Po
export fluxes calculated from the deficit alone (SS without advection) and
the total 210Po fluxes (sum of the fluxes calculated from the
210Po deficit and vertical advection) by the total particulate
(> 1 µm) POC/210Po ratios at the corresponding depths
(Table 2, Fig. 7). The POC fluxes calculated from only the deficit term and
the total term ranged from negligible to 7 mmol C m-2 d-1 and
from negative to 10 mmol C m-2 d-1, respectively. This is in good
agreement with the SS fluxes derived via the 210Po/210Pb method
ignoring advection in other regions of the world ocean (negligible to 8.5 mmol C m-2 d-1) (e.g., Shimmield et al., 1995; Sarin et al.,
1999; Kim and Church, 2001; Stewart et al., 2007; Verdeny et al., 2008;
Roca-Martí et al., 2016; Subha Anand et al., 2017).
POC fluxes derived from 210Po for the mixed-layer depth (MLD),
the base of the euphotic zone (Z1%), the base of the primary
production zone (PPZ), and the 234Th-238U equilibrium depth
(ThEq). (a) POC fluxes derived from the 210Po fluxes that were
calculated from the deficit alone; (b) POC fluxes derived from the sum of
the 210Po fluxes that were calculated from the 210Po deficit and
vertical advective flux. Note that the > 1 µm particles
were used to calculate the POC/210Po ratios. The stations were plotted
from west to east.
The highest-estimated POC fluxes (Table 2) along the transect were observed
at most of the investigated depths in the Labrador Sea and at the Greenland
Shelf, whereas the lowest export was in the Iberian and West European
basins. An exception to this pattern was found at station 26, where POC flux
was actually similar in magnitude to the flux at stations 64 and 69. Station
26 was located in the middle of the Subarctic Front (SAF), a cold and fresh
anomaly originating from subpolar water (Zunino et al., 2017). The
hydrographic properties associated with the SAF appear to promote high
primary production (174±19 mmol C m-2 d-1, Table 1) and
subsequently high carbon export (Kemp et al., 2006; Rivière and
Pondaven, 2006; Guidi et al., 2007; Waite et al., 2016). While stations on
the Greenland Shelf (stations 60 and 64) had the greatest estimated carbon
export at the depth of ThEq (5–10 mmol C m-2 d-1), station 60
at the depth of Z1% had the lowest POC flux (-12.5 to -8.4 mmol C m-2 d-1).
The negligible deficit of 210Po at the MLD and Z1% seen at
stations 21, 38 and 44 leads to negligible 210Po-derived POC fluxes at
those depths and stations (Table 2, Fig. 7). The relatively low POC
export (negligible – 1.7 mmol C m-2 d-1) at stations 1 and 13, on
the other hand, resulted from low particulate POC/210Po ratios (Table 2). In fact, the Iberian Basin had the lowest measurements of particulate
POC/210Po ratios in both the small- and large-size fractions relative to
the other four basins along the transect (Fig. 4). This basin was also the
only region along the transect where the phytoplankton community was not
dominated by diatoms but by smaller phytoplankton, in particular
coccolithophores. Smaller phytoplankton cells could scavenge more 210Po
(higher particulate 210Po activity relative to the large particles) due
to larger surface area per unit of volume, lowering their ratio of POC
concentration to 210Po activity.
POC export efficiency
The POC export flux calculated from the total 210Po flux at the depth
of the PPZ was compared to the satellite-derived NPP over ∼138 days (see Sect. 2.5) at each station, and the ratio was reported as
the POC export efficiency.
The export efficiencies in this study were below 10 % at 10 out of 11
stations, averaging 6±4 % (n=10, excluding the negative value
at station 60, Fig. 8). Export efficiencies < 10 % observed here
were similar to those found in the equatorial Pacific, the Arabian Sea, and
at the BATS site (Buesseler, 1998; Subha Anand et al., 2017),
but lower than those reported at high-latitude sites
(> 25 %),
such as the Arctic (Gustafsson and Andersson, 2012; Moran et al.,
1997; Roca-Martí et al., 2016), the Bellingshausen Sea (Shimmield
et al., 1995), and the Antarctic Polar Front (Rutgers van der Loeff et
al., 1997).
Plot of POC export flux derived from the 210Po method
(210Po–POC) versus satellite estimates of net primary production (NPP).
The NPP values were averaged for the previous 138 days (210Po
half-life) prior to the sampling date. The sum of the 210Po fluxes
calculated from the 210Po deficit and vertical advective flux, and the
POC/210Po ratios in the > 1 µm particles were used to
derive POC fluxes. The 210Po–POC fluxes were integrated within the
primary production zone (PPZ). Lines of export efficiency (EF) of 10 %,
5 %, and 1 % are drawn in the plot. The numbers in the plot are station numbers. The colored dots of the stations correspond to the
basins.
Export efficiencies ranged from 0.5 % to 2.5 % in the Iberian Basin, while
the values in the Irminger Basin (3±3 %, excluding station 60)
were similar to the export efficiencies in the Western European Basin (5±5 %) and in the Iceland Basin (6±6 %). The export
efficiencies, in contrast, were larger in the Labrador Basin (10±3 %). The lowest export efficiencies observed in the Iberian Basin were
consistent with the dominance of smaller phytoplankton species there
(coccolithophores and cyanobacteria; Fig. 2). Indeed, small cells are
usually slow-sinking particles that are likely more prone to degradation
(Villa-Alfageme et al., 2016), leading to lower
export efficiencies. Conversely, the higher export efficiencies at other
stations, all generally dominated by diatoms (Fig. 2), support the idea that
diatoms may be more efficient in exporting POC than smaller phytoplankton
(Buesseler, 1998). Differences in export efficiencies between the
basins dominated by diatoms suggest that other factors may also play some
role (e.g., temporal decoupling between production and export).
The POC export efficiency could also vary widely within the same basin.
Taking the two stations in the Western European Basin for instance, export
efficiency at station 26 was ∼5-fold greater than that
estimated at station 21, likely consistent with a lower contribution of
diatoms and a higher contribution of smaller phytoplankton at station 21
relative to those at station 26 (Fig. 2). But overall the time-series
composition of the phytoplankton community at the two stations was similar
(Fig. 2). The site-specific environment may have impacted the export of the
same cell type to different degrees (Durkin et al., 2016).
Station 26 was in the middle of the SAF, and the mesoscale physical
processes (i.e., turbulence and mixing) at the front can introduce nutrients
into the local euphotic zone (Lévy et al., 2012).
Large phytoplankton species generally dominate in these nutrients-rich
waters and can promote massive episodic particle export (e.g., Kemp et
al., 2006; Guidi et al., 2007; Waite et al., 2016).
Comparison of 210Po and 234Th derived POC fluxes
The measurements of 234Th/238U disequilibrium to estimate POC
export flux were simultaneously carried out during the GEOVIDE cruise
(Lemaitre et al., 2018). The authors discussed the influence of
vertical advection on 234Th export flux and concluded it can be
neglected. In the present study, estimates of 234Th-derived POC
(234Th-POC) flux were compared to 210Po-derived POC
(210Po–POC) flux. To avoid discrepancies, both the 234Th-POC and
210Po–POC flux estimates were calculated at the depth of ThEq using the
POC / radionuclide ratio in total particles > 1 µm (TPF) and
large particles > 53 µm (LSF), and both methods ignored
physical transport and assumed steady state, where any deviation from
secular equilibrium was created by sinking particles with an adsorbed and/or
absorbed excess of the short-lived daughter isotope.
210Po flux vs. 234Th flux
The integrated 210Po and 234Th fluxes at the depth of ThEq were
compared (Fig. 9). There was a spatial trend of 234Th flux, but not
210Po flux along the transect; 234Th fluxes at stations 1 to 38
(eastern section, 1580±430 dpm m-2 d-1) were
significantly greater (Wilcoxon rank sum test, p value < 0.002) than
the fluxes at stations 44 to 77 (western section, 710±230 dpm m-2 d-1). The means of the 210Po fluxes in the western and
eastern sections were not statistically different from each other (Wilcoxon
rank sum test, p value =0.3). However, the flux of 210Po and
234Th correlated with each other better in the western (n=5,
R2=0.6) than in the eastern (n=6, R2=0.01) sections.
Sinking fluxes of 210Po (blue circles) and 234Th (red
squares) integrated to the depth at which 234Th activity returned to
equilibrium with 238U activity (ThEq), assuming steady state and
negligible physical transport along the GA01 transect. Note that the
stations are plotted from west to east, and the transect was separated into
the western (stations 44–77) and eastern (stations 1–38) sections.
These relationships may be related to both the stage of the bloom and
different half-lives of the two isotopes. Indeed, 234Th fluxes
integrate the conditions that occurred days to weeks prior to the sampling
date, while the 210Po method integrates the flux over the past few
months. Within the Iberian Basin, stations 1 and 13 were sampled weeks to
months after the bloom development (Fig. 6). The moderate to relatively high
234Th fluxes are thus surprising. Lemaitre et al. (2018) argue that
the greater fluxes there might be related to the proximity of the Iberian
margin, where particle dynamics were intense and lithogenic particles were
numerous (Gourain et al., 2018). A temporal decoupling
between production and export could be an alternative possibility. The
Western European and Icelandic basins were sampled during bloom development,
and the NPP peaks occurring just before sampling may have promoted the high
fluxes. In fact, these basins have been characterized by the presence of
fast-sinking particles during the bloom
(Villa-Alfageme et al., 2016), likely also
explaining the high export. In contrast, the lower export observed in the
western section may be due to the fact that the sampling occurred during the
decline of the bloom, probably with a decoupling between production and
export in the Labrador Basin, or during a particle retention event in the
Irminger Basin.
Unlike the observations of higher 234Th export flux in the eastern than
western sections, there were no significant differences in 210Po export
flux between the two sections. This observation supports the argument that
the 210Po deficit tends to smooth out episodic events due to
integration over longer time periods. The 210Po deficit records
seasonal changes in export fluxes, whereas the 234Th deficit represents
more recent changes in the water column (Verdeny et al., 2009; Hayes et
al., 2018). Indeed, the 210Po deficit integrates the flux over months
that include a period of lower flux prior to the bloom along the GA01
transect, whereas the 234Th deficit integrates the flux only over weeks
that include the bloom itself at most of the stations (Fig. 6). Therefore,
the specific stage of the bloom shortly prior to the sampling date appears
to have less influence on the 210Po-derived than the 234Th-derived
export flux along this transect.
Plot of the POC flux derived from 210Po (210Po–POC)
versus the POC flux derived from 234Th (234Th-POC) at 11 stations
along the GA01 transect. Both the fluxes of 210Po and 234Th were
calculated from the deficit term alone, assuming steady state and negligible
physical transport. The POC / radionuclide ratios on (a) total particulate
fraction (TPF, > 1 µm) and (b) large-size fraction (LSF,
> 53 µm) were used to calculate the POC flux. The fluxes
were integrated down to the depth at which the 234Th activity returned to
equilibrium with 238U activity (ThEq). The numbers in the plot are
station numbers. The colored dots of the stations correspond to the basins.
POC/210Po vs. POC/234Th ratio
In order to calculate POC export flux, one needs both the export of the
daughter nuclide at a defined depth as well as the particulate
POC / radionuclide ratio on the sinking particles. In situ pump filtered
particles, either operationally defined as small (1–53 µm, SSF),
large (> 53 µm, LSF), or total (> 1 µm,
TPF) particles, may all represent a combination of sinking and non-sinking
particles. In the present study, the particulate POC / radionuclide ratio on
the TPF and LSF were examined and used to calculate POC export flux. The
POC/210Po and POC/234Th ratios in the particles at the depths of
ThEq were derived from the power law functions in each basin (TPF POC/210Po
in Table 2, LSF POC/234Th in Lemaitre et al., 2018; LSF POC/210Po and TPF POC/234Th not shown). The TPF
POC/210Po and TPF POC/234Th ratios had very similar spatial trends
(n=11, R2=0.8, p value < 0.0001) along the transect,
with the lowest POC / radionuclide ratios in the Iberian and Western European
basins and the highest ratios in the Labrador Sea. In contrast, the LSF
POC/210Po ratios were not correlated with LSF POC/234Th ratios
(n=11, R2=0.3, p value =0.07). The correlation of values within
the TPF but not the LSF suggests that the composition of large particles was
different from that of the total particles, and that the difference in
particle association between POC and 210Po and 234Th was greater in
large particles than in total particles.
210Po-derived POC vs. 234Th-derived POC
When the radionuclide fluxes were multiplied by the POC / radionuclide values,
the range of the calculated POC fluxes were negligible to 7 mmol C m-2 d-1 and negligible to 12 mmol C m-2 d-1 via the 210Po
method using the TPF and LSF POC/210Po ratios, respectively, and from
2.5 to 13 mmol C m-2 d-1 and from 1 to 12 mmol C m-2 d-1 via the 234Th method using the TPF and LSF POC/234Th ratios
(Fig. 10). The 234Th-POC and 210Po–POC fluxes agreed
within a factor of 3 along the transect, with higher POC estimates derived
from the 234Th method in 9 out of 11 stations. This was consistent with
previous studies that have typically found higher-estimated POC flux via the
234Th method (e.g., Shimmield et al., 1995; Stewart et al., 2007;
Verdeny et al., 2009).
When using the total particle POC / radionuclide ratios, only stations 26 and
60 were characterized by slightly higher 210Po-derived POC flux
estimates than 234Th-derived estimates (0.2 and 1.5-fold,
respectively). In contrast, at station 1 the difference between the methods
was greatest, with the 210Po-derived POC flux negligible and the
234Th-POC flux of about 7 mmol C m-2 d-1. At stations 13, 21,
and 44, the 234Th-POC fluxes were greater than 210Po–POC estimates
by almost 1-fold, whereas in the Iceland and Labrador basins, the
234Th-POC fluxes were larger than 210Po–POC estimates by 3- and
2-fold, respectively. When using the large particle POC / radionuclide ratios,
only in the Irminger Basin was there higher 210Po-derived POC flux than
234Th-derived POC flux (> 0.2 to 3-fold). In the Iberian
Basin, the greatest difference between the methods was found at station 1,
where 210Po-derived POC flux was negligible, while 234Th-derived
POC flux was the highest along the transect but with a large uncertainty (12±22 mmol C m-2 d-1), and at station 13 the 234Th-POC
flux was greater than the 210Po–POC estimate by 4-fold. In the Western
European, Iceland, and Labrador basins, the 234Th-POC fluxes were
larger than 210Po–POC estimates by 5, 4, and 2-fold.
Wilcoxon rank sum tests revealed that the 234Th-POC estimates were
significantly greater than 210Po-derived POC export at the stations
from the Iberian Basin to the Iceland Basin (n=6, TPF: p value < 0.01, LSF: p value < 0.02), but not at the stations from the
Irminger Basin to the Labrador Basin (n=5, TPF: p value > 0.1, LSF: p value =1). Since the ratios of POC to radionuclides for total
particles had very similar spatial trends along the transect, the
discrepancy between TPF 234Th-POC and TPF 210Po–POC flux estimates
must be driven primarily by the discrepancy between the SS estimates of
234Th and 210Po fluxes, discussed in Sect. 4.5.1. In contrast,
the discrepancy between the POC to isotope ratios in the large particle may
have led to some degree of discrepancy between the LSF 234Th-POC and
LSF 210Po–POC flux estimates.