Supply routes of suspended particulate Fe, Mn, and Al
To avoid confusion, we will now define the terms biogenic, lithogenic, and
authigenic particles, as they will be used frequently in the following
sections. Biogenic particles refer to suspended organic particles, living
and dead, such as phytoplankton cells. Lithogenic particles comprise mineral
fragments, such as glacial flour and sediment particles. Authigenic particle
include surface-scavenged trace metals and secondary minerals, such as
amorphous FeO(OH) (e.g. goethite), that are formed in seawater and because
of their age are insoluble to weak acid leaches (e.g. 25 % acetic acid
solution).
Characterization of (the two) particulate trace metal fractions
Two different particulate fractions were obtained from samples collected
during JR247: a particulate fraction from suspended particles collected
using 1 µm pore size SAPS filters (P) and a leachable particulate
fraction from unfiltered acidified seawater samples (LPUn) collected at
the same depth. LPUn was calculated following Eq. (1):
LPUn=total dissolvable(TD; unfiltered)-dissolved(D;0.2µmfiltered).
OTE seawater samples: Fe, Mn, and Al concentrations
determined for the dissolved (D) (0.2 µm) and the leachable
particulate fraction (LPUn) (total dissolvable–dissolved) of
unfiltered seawater samples collected during JR247. Additional information
covers sampling date, site (station) ID, event number, and latitude and longitude.
Date
Site ID
Depth
Leach. part. (nmol L-1)
Dissolved (nmol L-1)
lat., long.
(m)
LPUnFe
LPUnMn
LPUnAl
DFe
DMn
DAl
04/01/2011
#9/10 (E95 & E97)
20
20.36
0.95
46.41
5.71
1.83
1.11
50
15.18
0.42
40.86
3.19
1.88
2.27
54.26∘ S, 35.35∘ W
100
9.86
0.23
20.43
1.55
0.92
2.07
130
23.33
0.73
48.91
2.82
0.87
2.68
150
23.71
0.43
46.95
2.35
1.03
0.12
200
27.37
0.62
54.41
2.70
0.89
2.37
05/01/2011
#11/12 (E98 & E101)
20
4.05
0.38
6.68
2.19
0.41
3.57
35
1.52
0.39
7.28
0.41
0.37
–
54.62∘ S, 34.81∘ W
50
9.30
0.60
22.20
7.18
0.64
13.31
75
1.28
0.31
7.85
0.77
0.35
4.56
100
2.02
0.32
3.34
1.09
0.35
1.47
150
1.55
0.38
3.18
1.10
0.45
–
200
13.10
1.31
23.81
1.26
1.17
3.07
300
8.62
0.70
23.25
1.06
0.55
–
400
8.81
0.54
16.54
2.05
0.46
2.69
500
4.51
0.41
11.41
0.72
0.38
0.76
600
2.75
0.37
10.32
0.96
0.36
0.77
700
4.81
0.41
16.85
0.82
0.35
–
06/01/2011
#13 (E105)
20
3.46
0.62
14.68
0.28
0.57
4.53
35
1.00
0.33
7.17
0.10
0.28
2.64
54.53∘ S, 35.27∘ W
50
7.09
0.71
22.62
1.26
0.57
5.77
75
25.03
1.09
61.94
1.23
0.64
5.86
100
34.06
1.30
87.43
0.82
0.74
4.08
07/01/2011
#14 (E113)
20
4.00
0.89
7.87
0.64
0.85
2.57
50
2.23
0.31
7.64
0.27
0.32
1.80
54.56∘ S, 35.59∘ W
75
2.30
0.43
3.58
0.62
0.46
2.42
100
2.26
0.44
3.34
0.35
0.46
0.46
150
23.50
0.94
33.35
0.70
0.62
0.23
200
82.26
2.12
103.11
2.69
0.77
2.31
08/01/2011
#15/16 (E119 & E129)
20
17.66
0.46
26.66
0.99
1.36
–
35
16.60
0.30
13.37
0.96
1.27
–
53.62∘ S, 36.34∘ W
50
16.30
0.23
18.49
1.21
1.40
–
75
23.82
0.56
29.86
0.98
1.28
–
100
8.49
0.10
10.50
0.73
0.56
–
150
1.88
0.03
4.49
2.25
0.40
–
200
2.72
0.02
1.40
0.63
0.44
2.87
300
2.56
0.05
2.40
0.34
0.25
–
400
3.75
0.02
5.28
0.48
0.30
1.17
500
5.28
0.08
9.22
0.43
0.30
–
600
5.50
0.09
11.45
0.53
0.28
1.63
750
5.27
0.06
8.16
0.44
0.30
–
10/01/2011
#17 (E133)
20
10.92
0.22
7.43
2.31
1.20
3.76
35
20.83
0.53
16.22
1.81
1.34
2.56
53.90∘ S, 36.57∘ W
50
34.59
1.00
57.55
2.29
1.42
2.33
75
118.25
2.18
64.36
4.21
1.86
2.19
100
50.71
1.00
77.52
2.48
1.42
1.62
150
112.28
2.23
86.09
3.39
1.41
0.86
Continued.
Date
Site ID
Depth
Leach. part. (nmol L-1)
Dissolved (nmol L-1)
lat., long.
(m)
LPUnFe
LPUnMn
LPUnAl
DFe
DMn
DAl
11/01/2011
#18 (E138)
20
106.71
1.77
95.17
2.75
1.57
3.36
35
83.53
0.00
100.32
1.97
1.33
2.44
54.10∘ S, 36.25∘ W
50
9.67
0.00
18.23
0.74
0.85
–
75
5.65
0.00
8.90
0.62
0.65
–
100
4.50
0.08
23.65
1.25
0.48
5.18
150
7.81
0.11
12.87
1.43
0.49
8.19
12/01/2011
#19/20 (E141 & E143)
20
60.19
2.11
54.29
1.46
1.71
5.30
35
60.17
2.19
87.17
1.34
1.90
8.22
53.54∘ S, 38.11∘ W
50
66.78
2.74
141.75
1.57
1.90
8.73
75
71.69
1.78
79.19
1.61
2.13
11.45
100
10.77
0.25
32.12
0.99
0.67
10.74
150
5.43
0.13
31.35
1.84
0.92
12.00
200
7.92
0.14
27.42
1.45
0.60
9.60
400
5.35
0.00
23.61
1.61
0.45
18.44
600
5.81
0.10
35.99
1.06
0.38
10.74
800
4.26
0.13
35.67
1.07
0.36
11.95
13/01/2011
#21 (E151)
20
44.75
1.54
114.13
0.72
1.38
2.58
35
39.99
1.82
73.37
0.77
0.94
2.29
53.75∘ S, 38.98∘ W
50
48.57
2.03
94.66
1.24
1.36
1.91
75
25.63
0.91
68.56
0.98
1.17
–
100
64.06
1.91
114.03
2.33
1.32
1.51
150
73.04
1.59
62.83
7.70
1.28
12.20
Because of the different sampling approaches (SAPS vs. OTE water samplers),
filter sizes (> 1 µm for SAPS vs. > 0.2 µm for dissolved
seawater), and digestion procedures (aqua regia + HF for
SAPS particles vs. water sample storage at pH 1.7 [22 µmol H+L-1]), concentrations of LPUn and
P differed, but they showed similar
distribution patterns in the water column (Figs. 2 and S1 in the Supplement, Tables 1 and 2).
The concentrations of Fe, Mn, and Al in the LPUn fraction (LPUnFe,
LPUnMn, LPUnAl) were usually slightly lower than the particulate
fraction from suspended particles (PFe, PMn, PAl). The LPUn of
unfiltered seawater samples corresponded to ∼63±4 % of the PFe, 83±11 % of the PAl, and nearly 100±10 %
of the PMn fractions obtained by SAPS. The average LPUn trace metal
ratios (LPUnFe / LPUnMn =33.07±3.45 (1σ) and
LPUnFe / LPUnAl =0.65±0.10 (n=69)) were about half of
the elemental ratios of suspended particles obtained by SAPS (PFe / PMn =68.0±0.6 and PFe / PAl =1.251±0.042 (n=42) (Fig. 3; Tables 1, 2 and 3)).
(a) OTE seawater samples: distribution
of leachable particulate iron (LPUnFe in black), manganese (LPUnMn
in blue), and aluminium (LPUnAl in red) concentrations in the water
column of stations located on the island shelf (125–270 m water depth).
SAPS samples: the particulate Fe (PFe) fraction retrieved by SAPS
is illustrated with open black circles and corresponds to the same axis as
LPUnFe. Concentrations above 120 nmol L-1 are listed in Tables 1
and 2. Error bars represent the standard deviation of the analysis. Density
(σθ) in kilograms per cubic metre is illustrated by the
black dashed line. (b) OTE seawater samples: dissolved iron
(DFe), manganese (DMn), and aluminium (DAl) are represented by the same
colour code as above. Dashed lines illustrate Chlorophyll a (Chl a) content
of the water column recorded by the CTD fluorometer.
Relationship of the entire data set for the particulate
fraction of Fe, Mn, and Al in particulates (P) retrieved using SAPS (a, b) and the leachable particulate fraction (LPUn) estimated from
unfiltered and dissolved seawater samples collected using OTE bottles (c, d). Error bars represent the standard deviation of the analysis. The
linear regression of each relationship is illustrated by a dashed black
line, the formula, and the R2. The grey dashed line in (c) and (d)
represents the linear relationship of particulate trace meals (P) shown in (a) and (b).
The lower concentrations of Fe and Al and the reduced elemental ratios in
the LPUn compared to the P fractions suggest that an unknown fraction
of particulate Fe and Al in seawater was not leached during the
acidification procedure at pH 1.7 over 12 months. However, since P and
LPUn displayed similar trends with depth (Figs. 2 and S1), LPUn was
used in Sect. 3.1.3 and 3.3 as an indicator for the abundance of
particulate trace metals at locations where particulate samples could not be
retrieved by SAPS, e.g. in surface waters collected by the tow fish and
depths greater than 150 m.
SAPS samples: the particulate Fe (PFe), Mn (PMn), and Al
(PAl) concentrations in the top 150 m of the water column at the 14 sites
visited during JR247. The particulate fraction, P, is the sum of leachable
(L) and refractory (R) fractions. Because of low concentrations, the leachable
fraction is indicated in percent of the P fraction. Additional information
covers sampling date, site (station) ID, event number, latitude and
longitude, and water column depth. (Depths marked by * indicate that the
polycarbonate filter was corrupted after retrieving the SAPS.)
Date
Site ID
Depth
Particulate (nmol L-1)
Leachable (% of P)
lat., long.
(m)
PFe
PMn
PAl
LFe
LMn
LAl
25/12/2010
#1/2 (E22)
20
5.17
0.08
4.82
0.37
2.39
1.65
53.70∘ S, 38.21∘ W
50*
9.12
0.14
7.91
0.27
2.61
1.47
(322 m)
150*
76.61
1.09
66.91
6.26
2.74
4.65
26/12/2010
#3 (E31)
20
6.62
0.09
6.64
0.02
3.30
0.79
53.85∘ S, 39.14∘ W
50
267.48
3.85
162.59
1.48
0.79
0.65
(287 m)
150
4.36
0.06
4.26
0.07
1.55
1.93
31/12/2010
#4/5 (E72)
20
8.52
0.12
7.99
0.51
1.68
2.62
53.49∘ S, 37.71∘ W
50
15.15
0.23
12.96
0.56
2.44
2.74
(1917 m)
150
2.33
0.03
2.15
0.65
1.78
2.42
02/01/2011
#6 (E80)
20
85.74
1.11
59.05
1.60
2.28
4.50
53.99∘ S, 36.37∘ W
50
17.76
0.24
8.87
–
–
–
(208 m)
150
137.39
2.02
98.54
3.46
0.91
2.81
03/01/2011
#7/8 (E88)
20
1.95
0.02
0.87
0.13
2.97
4.99
54.10∘ S, 35.46∘ W
50
1.67
0.02
0.92
0.08
4.35
4.24
(330 m)
150
1.23
0.02
0.71
0.19
2.11
5.13
04/01/2011
#9/10 (E96)
20
20.91
0.08
15.74
0.56
5.01
3.24
54.26∘ S, 35.35∘ W
50
19.16
0.27
15.58
0.45
1.22
2.51
(263 m)
150
54.06
0.77
48.10
1.08
1.65
2.08
05/01/2011
#11/12 (E100)
20*
1.49
0.01
0.86
0.18
4.42
2.92
54.62∘ S, 34.81∘ W
50
0.87
0.01
0.60
0.27
6.63
4.20
(747 m)
150
1.76
0.03
1.08
0.37
4.38
3.33
06/01/2011
#13 (E106)
20
2.75
0.03
1.78
0.63
3.13
4.29
54.53∘ S, 35.27∘ W
50
4.11
0.05
3.07
0.44
2.04
2.76
(133 m)
100
10.28
0.15
7.62
0.46
1.70
2.54
07/01/2011
#14 (E114)
20
2.80
0.04
1.84
0.07
1.58
3.29
54.56∘ S, 35.59∘ W
50
1.41
0.02
0.97
0.10
2.57
3.92
(263 m)
150
31.34
0.46
26.92
0.72
1.57
2.28
08/01/2011
#15/16 (E120)
20
24.54
0.37
22.91
0.85
3.95
1.88
53.62∘ S, 36.34∘ W
50
27.72
0.40
23.23
0.43
3.65
1.36
(852 m)
150
4.74
0.07
3.94
0.90
4.31
1.06
10/01/2011
#17 (E134)
20
10.43
0.14
8.09
0.34
1.66
2.41
53.90∘ S, 36.57∘ W
50
43.04
0.60
38.79
1.34
1.07
1.67
(209 m)
150
207.48
3.10
194.88
1.72
0.82
1.50
11/01/2011
#18 (E139)
20
95.52
1.32
88.39
1.39
1.82
1.93
54.10∘ S, 36.25∘ W
50
37.43
0.52
35.33
1.16
1.29
1.85
(276 m)
150
28.00
0.41
23.60
1.26
2.35
2.27
12/01/2011
#19/20 (E142)
20
97.60
1.52
97.10
0.16
1.66
0.33
53.54∘ S, 38.11∘ W
50
90.96
1.42
92.89
0.39
1.98
0.80
(1741 m)
150
7.41
0.12
6.37
0.74
8.25
2.75
13/01/2011
#21 (E152)
20
50.75
0.85
52.78
0.06
2.99
0.12
53.75∘ S, 38.98∘ W
50
59.59
0.93
59.98
0.05
2.15
0.09
(269 m)
150
153.48
2.34
89.63
3.14
1.10
2.94
Suspended particulate trace metals in the water column
Concentrations of PFe, PMn, and PAl in the water column ranged from 0.87 to 267, 0.01 to 3.85, and 0.60 to 195 nmol L-1, respectively
(Fig. 2, Table 2). Concentrations of LPUnFe,
LPUnMn, and LPUnAl ranged from 1 to 118, 0.01 to 100, and 1 to 141 nmol L-1, respectively (Fig. 2, Table 1).
Below the isopycnal density layer 27.05 kg m-3, located at
∼50–70 m depth, P and LPUn increased with depth and
showed a maximum near the seafloor of e.g. 207 nmol L-1 for PFe and 112 nmol L-1 for LPUnFe (#17, Table 2). Most sites on the shelf
(bottom depth ≤ 260 m; #9/10, #13, #14, #17, and #21)
showed seafloor maxima, in agreement with other shelf studies. For example,
Milne et al. (2017) reported concentrations of up to 140 nmol L-1 for PFe and 800 nmol L-1 for PAl in bottom waters on the
west African shelf, and Chase et al. (2005) showed bottom
water maxima of up to 400 nmol L-1 for LPUnFe off the Oregon
coast.
The different elemental ratios of the Earth's crust,
sediment, suspended particles (SAPS), faecal pellets, and biogenic particles (average
phytoplankton species).
Particle
Fe / Mn
Fe / Al
Mn / Al
Source
(mol mol-1)
(mol mol-1)
(mol mol-1)
Crust
58.00
0.20
0.0035
a
Sediment
51.50
0.34
0.0066
This study
Suspended part. (SAPS)
68.00
1.25
0.0171
This study
Faecal pellets
70.65
0.48
0.0069
This study
Phytoplankton
1.70
–
–
b
a Wedepohl (1995). b Ho et al. (2003).
Strong linear relationships between elements were observed for suspended
particles (SAPS) obtained from above and below the 27.05 kg m-3
isopycnal, with elemental ratios of PFe / PMn =68.0±0.6, PFe / PAl =1.25±0.04, and PMn / PAl =0.0171±0.0041
(n=42) (Fig. 3, Tables 2 and 3). These elemental ratios were higher than those reported
for the earth's crust (Fe / Mn =58.0, Fe / Al =0.2, Mn / Al =0.0035; Wedepohl, 1995) and sediment samples collected to the south of
the island (mean sediment surface layer of S1, S2, S3; SFe / SMn =51.5±2.4, SFe / SAl =0.34±0.02, SMn / SAl =0.0066±0.0002 (Fig. 4, Tables 3 and 4)), suggesting that the suspended particles
were more enriched in Fe than crustal and lithogenic particles (Table 3).
The Fe / Mn ratios among different phytoplankton species show strong
variations but are typically much lower (Fe / Mn ∼1.7;
Ho et al., 2003), with also lower Fe concentrations
than lithogenic (sediment) particles (cellular Fe concentration of
phytoplankton ∼0.7 mmol kg-1, Ho et
al., 2003; upper crust ∼550 mmol kg-1,
Wedepohl, 1995)). A prevalence of biogenic particles in the
suspended particle pool would be expected to result in reduced PFe / PMn
ratios in our SAPS samples to values less than 51.5, as observed in the
sediments.
(a) OTE seawater samples: from left to
right, concentrations of leachable particulate iron (LPUnFe), aluminium
(LPUnAl), and manganese (LPUnMn) of unfiltered seawater samples
for the two shelf sites (#14, #13) and the shelf edge site (#11/12)
(note different depth scaling). Error bars represent the standard deviation
of the analysis. Water density (σθ) is
shown by the dashed black line. Brown areas represent sediments, and pink
areas the zone of resuspended sediment particles in the water column.
Diagram 14 (left) contains the average LPUnFe / LPUnAl and
LPUnFe / LPUnMn ratio of particles in seawater samples collected
within the pink layers. (b) sediment core samples: diagram
of S1, S2, and S3 displaying the Fe, Mn, and Al content in the three sediment
cores. Shown are average SFe / SAl and SFe / SMn ratios
(mol mol-1) of particles
from the surface layer for sites S1, S2, and S3. Dots on the distance scaling
in the middle represent the distance of each water column station (blue) and
sediment core (brown) station to the nearest shore.
Sediment core samples: particulate iron (SFe), aluminum
(SAl), and manganese (SMn) concentrations in shelf sediments collected
during JC055 in January and February 2011. Pore water data retrieved
additionally from these three cores are listed for Fe (FePW) and Mn
(MnPW). Additional information are event number (MC…),
latitude and longitude, and water column depth.
Station ID
Depth
SFe
SAl
SMn
FePW
MnPW
lat., long.
(cm)
(mol kg-1)
(mol kg-1)
(mmol kg-1)
(µmol kg-1)
(µmol kg-1)
#S1 (MC33)
0.5
0.58
1.77
11.56
3.01
2.29
54.16∘ S, 37.98∘ W
1.5
0.61
1.74
11.52
17.47
0.84
(257 m)
2.5
0.59
1.77
11.78
110.90
0.28
3.5
0.6
1.86
12.05
106.24
0.53
4.5
0.58
1.72
11.82
94.09
0.34
5.5
0.59
1.86
12.04
82.79
0.27
9
0.56
1.72
11.19
32.98
0.00
15
0.55
1.74
11.15
2.44
0.06
25
0.53
1.6
10.81
0.80
0.16
#S2 (MC34)
0.5
0.64
1.77
11.42
1.53
0.87
54.16∘ S, 37.94∘ W
1.5
0.6
1.79
11.73
–
–
(247 m)
2.5
0.58
1.76
11.81
0.97
0.24
6.5
0.59
1.83
12.23
11.19
0.26
10.5
0.58
1.8
11.78
14.28
0.25
14.5
0.54
1.6
10.83
3.59
0.33
16.5
0.56
1.72
11.22
2.27
0.31
#S3 (MC35)
0.5
0.61
1.67
11.42
1.46
0.43
54.15∘ S, 37.97∘ W
1.5
0.59
1.76
11.7
28.94
0.35
(254 m)
2.5
0.58
1.76
11.7
91.52
0.37
3.5
0.59
1.81
12.03
40.16
0.44
5.5
0.57
1.78
11.58
49.37
0.56
8.5
0.59
1.82
11.65
67.92
0.52
17
0.54
1.69
10.8
3.87
0.34
19
0.55
1.67
10.86
1.82
0.12
25
0.55
1.77
11.19
2.73
0.36
29
0.56
1.79
11.19
5.64
0.16
It is most likely that authigenic Fe precipitation (e.g. dissolved Fe (DFe) was scavenged
onto sediment particles) increased the Fe-to-Al (and Fe-to-Mn) ratio of
suspended particles (PFe / PAl =1.25; PFe / PMn =68.0) compared to
sediment particles (SFe / SAl =0.34; SFe / SMn =51.5). At seawater pH
(∼ pH 8), dissolved Fe(III) is rapidly hydrolysed to soluble
Fe(III)(OH)3 (< 0.02 µm), which readily accumulates as
nanometre-sized colloids (0.02–0.2 µm) (Liu and
Millero, 2002) and particles (> 0.2 µm) (own observation).
It has also been shown that both soluble and colloidal Fe are attracted by
charged surfaces (organic and inorganic particle surfaces), a process that
removes DFe and simultaneously increases the amount of particulate Fe in
seawater over time (Schlosser et al., 2011).
A range of mechanisms deliver suspended particles to the surface waters.
These transport mechanisms will be discussed in the following section.
Glacial outflow and zooplankton feeding activity
While most stations on the shelf showed bottom water maxima of particulate
metals, at three sampling sites located on the shelf (#18) and shelf edge
(#15/16 and #19/20) the particulate trace metal concentrations
featured maxima in the top 100 m of the water column (Figs. 2 and 5). At site
#19/20, ca. 100 km away from the coast with a water depth of 1741 m, the
PFe concentration at 20 m depth was 97 nmol L-1, similar to LPUnFe
(Fig. 5). The elemental ratio PFe / PAl of these samples (e.g. 1.01 for site
#19/20, 20 m depth) was close to the average ratio (PFe / PAl =1.25),
indicating that lithogenic and authigenic Fe dominated the suspended
particulate pool in these surface waters.
(a) OTE seawater samples: distribution
of leachable particulate manganese (LPUnMn in blue), iron (LPUnFe
in black), and aluminium (LPUnAl in red) concentrations in the water
column of the two other stations located on the island shelf edge
(> 700 m water depth). SAPS samples: the particulate Fe
(PFe) is illustrated by black circles and corresponds to the concentration
labels of LPUnFe. Error bars represent the standard deviation of the
analysis. Density (σθ) is illustrated by the black
dashed line. (b) OTE seawater samples: dissolved manganese
(DMn), iron (DFe), and aluminium (DAl) are represented by the same colour
code as for the upper row. Dashed line illustrates the Chl a content of the
water column recorded by the CTD-mounted fluorometer.
The surface water maxima of trace metals could have two supply routes: (1) lateral transport of waters containing lithogenic and authigenic particles
from shallow island shelf sediments, and (2) transport of glacial particles
following melt processes. The reduced salinities (∼33.3 PSU)
recorded in surface waters in Cumberland Bay and ∼50 km
offshore of South Georgia (∼33.8) (Figs. 6c and S2) provide
an indication of glacial outflow, melting of icebergs, and run-off of meltwater streams. Enhanced LPUnFe concentrations of
2.2 µmol L-1 in low-salinity surface waters inside Cumberland Bay are
indicative of a meltwater source (LPUn concentration used as only water
samples from the tow fish available). The LPUnFe concentration
decreased strongly with increasing distance from the coast and exhibited an
abrupt reduction to 1–5 nmol Fe L-1 at the shelf edge
∼100 km offshore. A similar distribution pattern was observed
for LPUnMn (Fig. 6d) and LPUnAl (not shown), for cruises JR247
and JR274. Glacial melt has been reported as an important source of
particulate material in the vicinity of the Antarctic Peninsula
(de Jong et al., 2012). For example, Gerringa et al. (2012) documented elevated total dissolvable Fe
concentration of up to 106 nmol L-1 near the Pine Island Glacier in the
Amundsen Sea, and Raiswell et al. (2008) estimated that per year
1.6 Gmol of nanoparticulate Fe, associated with lithogenic particles, is
delivered to the Southern Ocean by melting ice.
Tow fish seawater samples: concentrations of
leachable particulate Fe (LPUnFe) of unfiltered seawater samples (a),
dissolved Fe (DFe) (b), salinity (c), and leachable particulate Mn
(LPUnMn) in unfiltered seawater samples (d) in surface waters collected
during JR247 (circles) and JR274 (squares) around South Georgia. The highest
LPUnFe concentration was recorded in a single sample in Cumberland
Bay, reaching 2.2 µmol L-1. Because of generally lower
concentrations, we excluded this data point in panel (a). Isobath are represented by grey
lines (GEBCO gridded bathymetry data).
Krill faecal pellets: particulate (P) and leachable (L)
concentrations for Fe, Mn, and Al determined for the 27 individual krill
faecal pellet samples collected during nine krill incubation experiments
on board RRS James Clark Ross (JR247). The particulate fraction, P, is the sum of leachable
(L) and refractory (R) fractions. Because of low concentrations, the leachable
fraction is indicated in percent of the P fraction.
Sample no.
Pellet weight
PFe
PAl
PMn
LFe
LAl
LMn
(mg)
(µg mg-1)
(µg mg-1)
(ng mg-1)
(%)
(%)
(%)
1
4.87
0.88
1.06
12.5
6.33
8.83
13.24
2
2.18
1.33
1.68
16.7
3.02
8.81
8.22
3
4.26
1.07
1.90
17.8
5.37
3.27
11.81
4
1.91
5.19
5.53
76.1
2.15
1.95
5.68
5
1.41
2.70
2.84
39.1
2.46
1.59
3.54
7
7.80
67.1
64.2
998.3
2.93
2.21
3.25
8
0.99
2.71
2.42
35.0
3.76
4.59
5.99
10
1.48
6.42
4.89
71.6
0.29
4.83
0.91
13
2.79
4.13
3.11
50.3
0.36
5.07
1.53
15
0.77
37.3
38.1
531.1
2.03
2.80
6.21
16
1.21
6.35
6.22
81.2
1.24
7.47
3.13
18
12.27
40.0
36.6
582.5
3.95
2.07
4.29
19
2.19
11.2
9.49
146.9
0.15
2.03
1.07
22
2.43
48.1
49.7
721.5
0.81
2.32
0.98
40
3.35
22.8
22.0
337.4
5.51
3.21
5.50
41
8.55
6.91
7.14
103.1
1.11
1.88
4.31
42
3.5
25.7
24.8
376.2
5.09
2.98
5.29
45
0.40
3.96
4.43
43.3
1.27
13.90
1.46
47
7.65
3.63
3.92
52.7
0.34
0.68
3.65
48
0.63
3.06
3.21
34.1
0.05
4.22
0.76
49
4.42
29.6
28.5
438.4
1.65
2.93
1.95
50
7.46
2.31
2.37
34.6
0.36
0.51
2.78
51
5.18
28.0
27.1
431.3
1.85
2.60
2.01
62
1.20
4.63
4.68
68.0
0.31
1.78
0.47
68
2.25
44.0
40.2
667.4
4.84
1.95
4.77
69
1.66
43.6
44.8
663.7
5.66
2.13
5.46
71
3.47
35.3
36.4
557.7
1.50
1.99
1.76
Locally elevated particulate metal concentrations in surface waters may also
be related to production of faecal pellets by swarms of Antarctic krill
(Schmidt et al., 2016). High abundances of Antarctic krill
were estimated from acoustic backscattering observations
(Fielding et al., 2014), and large numbers of faecal
pellets were observed on the SAPS filters during cruise JR247. The stomach
content of Antarctic krill contained up to 90 % sediment particles by
volume, an observation that was attributed to filter feeding by these
organisms on phytoplankton and seabed detritus, with incidental ingestion of
deep-ocean sediments (Schmidt et al., 2011) and
glacial flour (Schmidt et al., 2016). Krill thus take up
lithogenic (sediment) particles and transfer these into the surface ocean
through the egestion of faecal pellets (Schmidt et al.,
2016). The trace metal contents of krill faecal pellets collected during
on-board incubation experiments during JR247 ranged from 0.88 to 67.14 µg Fe mg-1 dry weight (n=27) (Table 5) (Schmidt et al.
2016). The molar ratios of the faecal pellets (PFe / PAl =0.48±0.07 and PMn / PAl =0.0069±0.001) were similar to those for sediment
particles (SFe / SAl =0.34±0.02 and SMn / SAl =0.0066±0.001; Tables 1, 2, 3, and 4), indicating that Fe in krill faecal pellets was
predominately associated with lithogenic (sediments) and/or glacial flour
particles, as also reported by Schmidt et al. (2016). In contrast, the molar
ratio of faecal pellets (PFe / PMn =70.65±8.22) was higher than that
of sediments (SFe / SMn =51.5±2.4) but just slightly higher than
that of suspended SAPS particles (PFe / PAl =68.0±0.6). The observed
variability in the PFe / PAl and PFe / PMn ratio in the various particle pools
is therefore a consequence of different amounts of lithogenic and authigenic
particles.
Supply routes of dissolved Fe, Mn, and Al
Concentrations of DFe, DMn, and DAl in the water column showed strong
variations and ranged from ∼0.1 to 25.9, 0.3 to 19.6, and 0.1 to 18.4 nmol L-1, respectively (Figs. 2, 5
and 7), with highest values in the surface waters in Cumberland Bay and
lowest beyond the shelf break (Fig. 6). Dissolved Fe concentrations from
this study are in agreement with reported DFe near the Antarctic Peninsula
(0.6–14.6 nmol L-1; de Jong et al., 2012) and
Crozet Islands (0.1–2.5 nmol L-1;
Planquette et al., 2007). Sources and sinks of
dissolved trace metals, and their distribution in the water column are
discussed in the following sections.
(a) OTE seawater samples: from left to
right, concentrations of dissolved iron (DFe), aluminium (DAl), and
manganese (DMn) for the two shelf sites (#14, #13) and the shelf edge
site (#11/12). Note different depth scaling. Error bars represent the
standard deviation of the analysis. Pink areas represent the zone of
resuspended sediments in the water column. The DFe / DMn ratios of the
seawaters collected within the pink zone are indicated. (b) sediment core samples: diagram of S1, S2, and S3 displaying the Fe
(black) and Mn (blue) content in pore waters of the three sediment cores.
Values off-axis can be found in Table 4. Shown are average
FePW / MnPW ratios (mol mol-1) of top surface layer (1 cm) for
sites S1, S2, and S3. Dots on the distance scaling in the middle represent the
distance of each water column station (blue) and sediment core (brown)
station to the nearest shore.
Supply from sediment pore waters
Elevated pore water concentrations of Fe and Mn (FePW and MnPW)
were observed in sediments from the southern shelf sites at water depths of
around 250 m and ranged from 0.5 to 110 µmol L-1 for Fe and
0.1 to 2 µmol L-1 for Mn (Fig. 7 and Table S2 in the Supplement). The down-core
distributions of FePW and MnPW were consistent with microbial
dissimilatory Mn and Fe reduction during organic matter oxidation
(Canfield and Thamdrup, 2009), and thus concentrations were elevated
at defined depth horizons controlled by their redox potential (Eh)
(Bonneville et al., 2009; Raiswell and Canfield, 2012).
The FePW and MnPW concentrations near the sediment–seawater
interface were used to calculate fluxes of Fe and Mn to bottom waters
following diffusion of reduced Fe and Mn species across an oxygenated layer
in surface sediments. These calculations were performed following Boudreau
and Scott (1978) and Homoky et al. (2012),
and are described in detailed in the Sect. S3 and Table S1. We
are aware that our calculated fluxes represent minimum estimates of pore
water efflux, which under natural conditions is supplemented by advection
due to bioirrigation, bioturbation, and bottom water currents
(Homoky et al., 2016). In addition, sediment cores were collected
on the southern shelf, while seawater and particulate samples were collected
on the northern shelf side. The benthic Fe fluxes for the southern shelf
may be lower than those on the northern shelf, as an elevated primary
productivity and enhanced particle export on the northern side will result
in enhanced bacterial respiration, which reduces Eh and promotes the
dissolution of Fe oxides with subsequent release of Fe into bottom waters.
Notwithstanding the above issues, we calculated substantial benthic fluxes
from sediment pore waters to bottom waters on the southern shelf for
FePW of < 0.1 to 44.4 µmol m-2 d-1 and MnPW
of 0.6 to 4.1 µmol m-2 d-1. The upper flux values for Fe are
comparable to those reported for dysoxic and river-dominated continental
margins (3.5–55 µmol m-2 d-1;
Homoky et al., 2012), seasonal maxima of temperate
and oxic shelf seas (23–31 µmol m-2 d-1;
Klar et al., 2017), and shelf sediments
off the Antarctic Peninsula
(1.3–15.5 µmol m-2 d-1;
de Jong et al., 2012). The Mn fluxes were relatively low
for shelf environments, with for example fluxes of 70–4450 µmol m-2 d-1 reported for Baltic and Black Sea
sediments (Pakhomova et al., 2007). The Fe pore water fluxes from
the South Georgia shelf sediments, which extend over an area of ca.
40 000 km2, indicate that these may serve as an important year-round source to
overlying waters, totalling 4 to 1728 kmol DFe d-1 and 25 to
164 kmol DMn d-1.
Benthic release of trace-metal-enriched pore waters shaped the distributions
of dissolved trace metals in bottom waters on the shelf. Concentrations of
DFe, DMn, and DAl were enhanced at isopycnals > 27.05 kg m-3
(e.g. DFe up to 7.70 nmol L-1 at site #21, Table 1) compared to
surface waters (e.g. DFe as low as 0.30 nmol L-1 at site #13, Table 1;
Figs. 2 and 7). Trace-metal-enriched bottom waters were also observed at
sites #13, #14, #17, and #18 (Fig. 2). The molar DFe / DMn ratios
in oxygenated bottom waters varied between 1.1 and 3.5 and were thus similar
to pore waters near the sediment–seawater interface (0–1 cm depth;
FePW/MnPW=2.2±1.0; Fig. 7). The similar trace metal
ratios suggest that Fe and Mn in enriched pore waters crossed the
sediment–bottom water interface and accumulated in shelf bottom waters.
To determine the vertical DFe fluxes from near bottom to surface waters, we
employed a method outlined by de Jong et al. (2012) and
calculated both the advective and diffusive flux terms, which are not
affected by the benthic Fe and Mn fluxes. We included the advective term in
our calculations, because it has been shown that internal waves that cross
shallow topographies and wind shear stress produce strong turbulence that
facilitate Eckman upwelling (vertical advection) on the shelf (Kurapov et
al., 2010; Moore, 2000; Wolanski and Delesalle, 1995). Applying literature
values from the Southern Ocean for vertical diffusivity (KZ=1×10-4 m2 s-1; Charette et al., 2007) and
advective velocity (w=1.1×10-6 m s-1; de Jong
et al., 2012), an average vertical DFe flux on the shelf of 0.41±0.26 µmol m-2 d-1 from subsurface waters into the surface
mixed layer was estimated (Sect. S4). The surface mixed-layer
depth was determined by a density criterion (∼0.03 kg m-3;
de Boyer Montégut et al., 2004) and was located at
∼50 m depth. About 38 % of the DFe flux was related to
Ekman upwelling (advective term) and 62 % to the diffusive flux. This
vertical flux is at the lower end of the calculated benthic flux from this
study (FePW < 0.1 to 44.4 µmol m-2 d-1) and
agrees with values reported for other Southern Ocean shelf regions near the
Antarctic Peninsula (within 20–70 km from the coast: ∼2.7±3.4 µmol m-2 d-1; de Jong et al.,
2012) and the Crozet Islands (only diffusive flux of 0.06 µmol m-2 d-1; Planquette et al., 2007).
DFe supply from the leachable fraction of particles
The analytical protocol for the analysis of SAPS-collected particulate
material allows separate estimation of the refractory and leachable
fractions of trace elements (R and L, respectively). The R fraction of the
suspended matter is considered to include silicates and aged oxide minerals,
while the L fraction represents predominantly fresh oxyhydroxides, biogenic
material, and loosely bound surface-associated elements which are readily
remobilized using leaching procedures (Berger et al., 2008).
Concentrations of LFe, LMn, and LAl in the water column showed strong
variations, ranging from a few picomoles to several nanomoles per litre
(Table 2). On average, LFe and LAl concentrations at 150 m depth
(∼1.3 nmol LFe L-1 and ∼0.95 nmol LAl L-1) were significantly higher than at 20 and
50 m: LFe =0.3 nmol L-1 (Student t test: t[0.95;28]=1.725[1.703]) and LAl =0.43 nmol L-1 (Student t test: t[0.90;28]=1.383[1.313]). The LMn concentrations did not vary strongly and remained near
constant throughout the top 150 m (LMn =8.9 pmol L-1; Student t test: t[0.65;28]=0.400[0.390]). The average
contribution of L to the particulate pool P (R+L) was low: 0.83±1.13 % for Fe, 2.55±1.58 % for Mn, and 2.42±1.32 % for
Al (Table 2). A study conducted in the North Pacific near the Columbia River
outflow reported considerably higher L fractions (e.g. 6.6±3.0 %
of Fe, 78.7±14.0 % of Mn, 6.3±2.0 % of Al; Berger et al., 2008), which was attributed to enhanced
biogenic particle levels in the low-salinity waters of the river
(Berger et al., 2008). In contrast, results from our study
showed that particulate trace metals predominately had a refractory
component (R), indicating that Fe, Mn, and Al were mainly incorporated in
mineral structures unaffected by a weak acid leach (e.g. aged oxyhydroxides
and alumosilicates).
A weak linear relationship between R and L was observed for Fe
(R2=0.57, n=41), Mn (R2=0.64, n=41), and Al (R2=0.63, n=41) (Figs. S3 and S4), indicating that the L fraction
included mainly lithogenic and authigenic (e.g. scavenged) Fe, Mn, and Al and
that not much LFe was incorporated in biogenic particles. The scavenging of
dissolved trace metals by charged particle surfaces is established
(Homoky et al., 2012; Koschinsky et al., 2003), but how well Fe and other
trace metals can be remobilized from marine particle surfaces and which
process may modify their availability over time are not yet well constrained
(Achterberg et al., 2018; Fitzsimmons and Boyle, 2014; Milne et al.,
2017).
For instance, scavenged Fe is reported to exchange with DFe in the water
column of the tropical and high-latitude North Atlantic (Achterberg et
al., 2018; Fitzsimmons and Boyle, 2014; Milne et al., 2017). In addition,
recent work has concluded that zooplankton grazing and the production of
faecal pellets remobilize DFe from lithogenic and biogenic particles
(Giering et al., 2012; Schmidt et al., 2016). In contrast, freshly
produced inorganic Fe(III) oxyhydroxide (FeOOH × nH2O) precipitates in
seawater are accessible but are subject to chemical and structural
conversions that lead to less leachable Fe with time (Yoshida et
al., 2006).
DFe supply from Antarctic krill
Elevated dissolved trace metal concentrations in the top 200 m of the water
column coincided with elevated particulate trace metal concentrations at
sites #11/12, #15/16, #18, and #19/20 (Figs. 2, 5, and 7). The
SAPS filters from these stations contained a high load of krill faecal
pellets. To elucidate the relationship between dissolved trace metal
concentrations and the local abundance of Antarctic krill and their faecal
pellets, krill were caught and incubated on board the vessel as described in
Schmidt et al. (2016).
Antarctic krill excretion rates of DFe were variable, relating positively to
the extent of recent ingestion of diatoms. However, on average krill
released ∼2.0±1.9 nmol DFe individual-1 d-1
(Schmidt et al., 2016). When a mean krill abundance
of 465±588 individuals m-2, estimated from acoustic
backscattering measurements (Fielding et al., 2014), was applied,
krill excreted 1.1±2.2 µmol DFe m-2 d-1 into the
top 300 m of the water column (Schmidt et al., 2016). In
addition, krill produced ca. 1.8±1.6 mg of faecal pellets per
individual per day, containing ca. 0.30±0.33 µmol Fe mg-1. Particle leaches performed on those faecal pellet samples with
25 % acetic acid showed that on average 2.5±2.1 % (9.3±13.3 nmol Fe mg-1) of the total Fe in these pellets could be
remobilized (Table 5), which would equate to a production of 14±24 nmol LFe individual-1 d-1. By multiplying the mean LFe by the ambient
krill density used above, we calculate a LFe flux of 6.5±8.2 µmol m-2 d-1 from the faecal pellets to the water column.
Since krill are mobile animals, questions remain over where the major part
of the LFe flux occurs and what the fate of this Fe source is. The highest
krill abundances have generally (but not exclusively) been recorded in the top
100 m layer (Fielding et al., 2014), and hence a large
proportion of this LFe flux from krill is likely to occur in the upper
waters. Notwithstanding our current uncertainties over the depths of origin
and fate, the LFe flux from krill faecal pellets and the release of DFe were
on average an order of magnitude higher than estimated vertical diffusive
and advective DFe fluxes, with other grazers, such as copepods and salps,
adding to the recycled-flux estimates. This illustrates the importance of
zooplankton-mediated Fe cycling, in agreement with previous studies
(Hutchins and Bruland, 1994; Sato et al., 2007).
The experimental set-up did not allow us to establish the origin of the Fe
released by krill, being both “recycled” Fe from biogenic material and
“new” Fe from lithogenic (and authigenic) material. However, Schmidt et al. (2016) concluded that zooplankton gut passage mobilizes
lithogenic Fe, and they showed that there are strong spatial patterns in the
organic and lithogenic make-up of faecal pellets. This included an
exponential decline in the quantity of lithogenic particles in krill
stomachs with distance from sources of glacial flour on the northern South
Georgia coast. For instance, the lithogenic content at one site on the shelf
contributed ∼90 % of stomach content volume, suggesting that
a large quantity of the accessible Fe was remobilized from those inorganic
particles.
Offshore transport of trace-metal-enriched water masses
Along a W–E transect (Fig. 1; #14 via # 13 to #11/12), lateral
water mass transport carried suspended particles offshore. Because of the
small size of the SAPS particulate data set (two data points), we considered
the LPUn fraction for this transect (Fig. 1). Indeed, elevated
concentrations of the P and LPUn metal fractions were observed in
subsurface waters that had been in recent contact with the shelf. These
metal-enriched waters, detected at the eastern shelf edge site #11/12
between 200 and 400 m water depth (Figs. 1 and 4), exhibited similar
temperature and salinity signatures to shelf bottom waters. Furthermore, the
elemental ratios of the LPUn fraction in these waters were similar to
the particles in the surface sediments (S1, S2, and S3) and the resuspended
particles in the bottom boundary layer (#13 and #14) on the shallow
shelf (Fig. 4). A similar distribution was also found for the P fractions,
but limited to sites #13 and #14, as SAPS were not deployed below 150 m
at the shelf edge site #11/12.
The LPUnFe concentration decreased offshore with distance from the island:
from site #14 at 200 m depth (LPUnFe =82.26 nmol L-1; water depth =255 m) to
site #13 at 100 m depth (LPUnFe =34.06 nmol L-1; water depth =130 m) to site #11/12 between
200 and 400 m depth (LPUnFe ∼10.18 nmol L-1; water
depth =750 m) (Fig. 4 and Table 1). A similar decrease was observed for
the SAPS Fe data: from site #14 at 150 m depth (PFe =31.12 nmol L-1) to site #13 at 100 m
depth (PFe =10.23 nmol L-1). The
decrease of PFe and LPUnFe with increasing distance to the coast is in
agreement with previous observations for the western subarctic Pacific
(Lam and Bishop, 2008), which reported elevated LFe
concentrations in the range of 0.6 to 3.8 nmol L-1 in subsurface waters
between 100 and 200 m depth along the Kamchatka shelf and related this
observation to offshore water mass transport. However, we assume that
particles in the deep particulate Fe maximum are not transported over very
large distances, due to their tendency to sink, and thus do not
significantly contribute to the offshore Fe supply (Sect. 3.4).
Consistent with the observed P and LPUn distributions, elevated
dissolved metal concentrations at depths between 200 and 400 m at site
#11/12 indicated that trace-metal-enriched shelf bottom waters were
transported offshore (Fig. 7). However, dissolved trace metal concentrations
were more variable than P and LPUn, and in the case of DMn they were highest at
depths at shelf edge site #11/12. Notwithstanding the above issue, for
horizontal flux calculations we used the entire DFe data set for water
depths between 100 and 400 m. Average DFe concentrations in this depth range
were highly variable and did not follow an exponential or power law function
with distance from the coast (Fig. S5), which is necessary to
determine scale length and horizontal diffusivity (Kh)
(de Jong et al., 2012). As a result, horizontal flux
calculations from the data could not be executed.
The distribution of dissolved trace metals in surface waters indicated that
there was a limited transfer of DFe beyond the shelf break into the bloom
region. Surface samples showed that DFe concentrations were strongly
enriched in surface waters on the shelf (0.3–25.9 nmol L-1, Fig. 6b), while DFe concentrations beyond the shelf break decreased abruptly to
concentrations < 0.2 nmol L-1 (Fig. 6b). This indicates that
DFe was quickly removed from ACC surface waters following passage of the
island. However, previous studies in the region suggest DFe transfer beyond
the shelf break of South Georgia (Borrione et al., 2014; Nielsdóttir
et al., 2012). Nielsdóttir et al. (2012) reported
surface waters downstream of the island shelf with up to 2 nmol DFe L-1,
with seasonal variations and highest concentrations during austral summer in
January/February 2008. Dissolved Fe data from JR247 (2011) and JR274 (2012)
were also obtained during the summer season but indicated rapid reduction
in concentrations through mixing with DFe-depleted ACC water, biological
uptake, and/or particle scavenging (authigenic precipitation).
Iron budget in the bloom region
Large seasonal phytoplankton blooms downstream of South Georgia recorded by
Earth-observing satellites are initiated by Fe supplied by the South Georgia
island–shelf system during the passage of ACC waters (Fig. 1) (Borrione
et al., 2014; Nielsdóttir et al., 2012). Based on our study, the main
DFe sources during this passage of the ACC were benthic release and vertical
mixing, release of DFe from krill and krill faecal pellets, and supply of
particles from run-off and glacial meltwater. In the following sections we
will discuss the strength of each DFe source in the bloom region ca. 1250 km downstream of the island and estimate how much DFe is required to
stimulate the elevated primary productivity in that region. Because of the
lack of observational data for the region, this part of the study combines
literature values from different Southern Ocean studies. This approach
contains large uncertainties, which are discussed in detail in Sect. 3.4.6.
Phytoplankton Fe requirements in the phytoplankton bloom region
The surface ocean in the vicinity of South Georgia during the austral summer
features strongly elevated biomass production (Gilpin et
al., 2002) and represents the largest known CO2 sink in the ACC
(12.9 mmol C m-2 d-1; Jones et al., 2012). The Fe
requirements of the phytoplankton community in austral summer within the
bloom that reaches several hundred kilometres downstream of the island were
determined by combining satellite-depth-integrated net primary production
data derived from a phytoplankton pigment adsorption (αph)-based model (62±21 mmol
C m-2 d-1; Ma et al., 2014) over the period of 2003–2010 with an average
intracellular Fe / C ratio obtained from five Southern Ocean diatom species
(5.23±2.84 µmol Fe mol-1 C; Strzepek et
al., 2011). This approach yielded an approximate Fe requirement of 0.33±0.11 µmol DFe m-2 d-1 for the phytoplankton
community (Fig. 8). For a more detailed description of the applied values
and calculations see Sect. S4.
Sketch of DFe fluxes on the shelf, in the transition zone,
and in the downstream blooming region, separated by the red dashed lines.
(left sketch) The dissolved Fe fluxes on the shelf that together
generate Fe rich biogenic and lithogenic particles (dark green). These are
transferred offshore (light green arrows) following the ACC to open-ocean
sites (sketch in the middle). Iron-enriched particles (dark green) in the
transition zone are recycled and supplement DFe requirements of the
phytoplankton community in the transition zone. During each cycle of
recycling and uptake an unknown Fe fraction is lost by vertical export.
(right sketch) The dissolved Fe fluxes in the blooming zone.
Horizontal and vertical mixing
De Jong et al. (2012) reported that horizontal and vertical
advective, diffusive (diapycnal), and deep winter mixing downstream (1250–1570 km) of the Antarctic Peninsula (between 51 and
59∘ S) supplied DFe to the surface waters in quantities that
exceeded the DFe requirement of primary producers during austral summer
(0.13±0.04 µmol DFe m-2 d-1). In their study
region, de Jong et al. (2012) determined that ∼0.30±0.22 µmol DFe m-2 d-1 was supplied by
horizontal and vertical fluxes, of which 91 % of the vertical flux was
attributed to Ekman upwelling (advective term), and 43 % of the entire DFe
flux was supplied by deep winter mixing. Tagliabue et al. (2014) reported similar model estimates for the region that is
located south of the PF and characterized by strong Ekman upwelling
and winter entrainment.
For the bloom region downstream of South Georgia, model calculations by
Tagliabue et al. (2014) indicated that less than 0.0003 µmol DFe m-2 d-1 was supplied by diapycnal mixing and
that ∼-0.0027 µmol DFe m-2 d-1 was removed by
Ekman downwelling. For the vertical flux component, this yields an overall
loss of DFe of ∼-0.002 µmol DFe m-2 d-1
(0.0003+(-0.0027)) in the bloom region north of South Georgia (Fig. 8).
Because the sampling in our study was not suitable for calculations of the
horizontal flux, we applied the horizontal flux estimates from de Jong et al. (2012) for our own Fe budget. For a region ca. 1250 km
downstream of a source, calculations according to de Jong et al. (2012) suggest that ca. 0.11±0.03 µmol DFe m-2 d-1
is supplied to the bloom region by horizontal advection
and diffusion (Fig. 8).
Deep winter mixing
The entrainment of new DFe during winter represents an important Fe source
to surface waters in the Southern Ocean (de Jong et al., 2012; Tagliabue
et al., 2014). Elevated DFe concentrations in subsurface waters support
primary production in the austral spring following entrainment by deep
winter mixing. Model estimates showed that DFe supplied by winter mixing,
together with diapycnal mixing, matches the Fe requirements at most
low-productivity sites in the Southern Ocean. However, deep winter mixing at the
highly productive sites north of South Georgia supplies only ∼0.011 µmol m-2 d-1 (Tagliabue et al., 2014)
(Fig. 8). Later in the season primary productivity in surface waters is
considered to rely strongly on Fe derived from recycling of biogenic
material (Boyd et al., 2015).
Dust deposition
Dissolved Fe supplied by the deposition of aeolian dust is considered to be
an important source to the Southern Ocean (Conway et al., 2015; Gabric et
al., 2010; Gassó and Stein, 2007). Aeolian flux model estimates,
supplied by Borrione et al. (2014) using a regional South Georgia
model, suggested that up to 8 µmol Fe m-2 d-1 is delivered
to the bloom regions downstream of South Georgia by dry and wet deposition.
However, reliable dry- and wet-deposition estimates for the Southern Ocean
are limited. Data from the South Atlantic along 40∘ S,
∼1000 km north of South Georgia, showed that rather low
levels of DFe (∼0.002 µmol m-2 d-1) are
supplied by dry deposition (Chance et al., 2015). In
addition, ∼1.0±1.2 µmol DFe m-2 d-1
are delivered sporadically to the 40∘ S area by wet deposition
(Chance et al., 2015). However, even when assuming that
similar wet-deposition fluxes occur north of South Georgia, fertilization
with DFe is temporally and spatially limited. Furthermore, it is very
unlikely that such sporadic events could cause long-lasting and
far-extending phytoplankton blooms strictly constrained between the PF and the
SACCF.
Luxury Fe uptake on the shelf
Our conservative estimate of DFe supply to the bloom region by
vertical/horizontal mixing, deep winter entrainment, and dust deposition
(< 0.12 µmol Fe m-2 d-1) covers only
∼30 % of the estimated phytoplankton requirements
(∼0.33 µmol Fe m-2 d-1) (Fig. 8). We
hypothesize that the missing supply of ∼0.21 µmol DFe m-2 d-1 is supplied to the bloom region through the offshore
advection of phytoplankton cells that are enriched in labile Fe. It has been
demonstrated that Fe-rich biogenic particles can be created by luxury iron
uptake of diatoms (Iwade et al., 2006; Marchetti et al., 2009). Using
bottle incubation experiments, Iwade et al. (2006) showed
that under Fe-replete conditions the coastal diatom Chaetoceros sociale stores more
intracellular Fe than needed for the production of essential enzymes and
proteins. We therefore hypothesize that phytoplankton cells that grew under
excess nutrient supply on the South Georgia shelf stored more Fe than needed
for their metabolic processes. Due to subsequent cycles of grazing, lysis or
bacterial decomposition, this iron can be remobilized in surface waters and
made available for renewed phytoplankton uptake.
High Fe recycling efficiencies, described by the fe ratio
(Boyd et al., 2005), are
required to maintain the cycle of remineralization and uptake in the
euphotic zone. This counteracts the loss of particulate Fe by vertical
export. Boyd et al. (2015) reported the highest
recycling efficiencies of ∼90 % for subantarctic
DFe-depleted waters such as downstream of South Georgia. Further, these
workers showed that the degree of recycling is controlled by the abundance
of prokaryotes with a high Fe quota, such as cyanobacteria, and particularly
by grazing zooplankton. The waters off South Georgia feature among the
highest biomasses worldwide of metazoan grazers (Atkinson
et al., 2001). These large grazers, chiefly copepods and Antarctic krill,
are able to efficiently ingest large diatoms, including species that are
known to store luxury iron (Atkinson, 1994; Hamm et al., 2003), thereby
disintegrating cell membranes and releasing trace metals.
In recent years it has become apparent that the recycling of biogenic
particles in the euphotic zone is a critical mechanism that maintains
primary production, especially when the dissolved nutrient pools become
exhausted (Boyd et al., 2015; Tagliabue et al., 2014). However,
uncertainties remain over the degree to which Fe is lost during each cycle
of uptake and remineralization. Thus more research is needed, especially
fieldwork that encompasses the community structures (bacteria,
phytoplankton, zooplankton, and higher predators;
Ratnarajah et al., 2017; Wing et al.,
2014), the degree of recycling for macro- and micro-nutrients in the
euphotic zone, and loss of Fe through vertical export.
An alternative explanation to our suggestion that recycling of
luxury-iron-enriched biota contributes to the downstream bloom is that iron is adsorbed
directly onto particles that are advected directly offshore. For example
freshly precipitated Fe(III) oxyhydroxides (FeOOH × nH2O) may be
adsorbed onto biogenic and non-biogenic material. Iron freshly absorbed onto
biogenic and non-biogenic material can be released and incorporated by
phytoplankton and bacteria. However, the bioavailability of adsorbed and
inorganic Fe changes over time. Both Wells et al. (1991) and
Chen and Wang (2001) demonstrated that the bioavailability of
freshly precipitated FeOOH and Fe adsorbed onto colloids/inorganic particles
decreases over time. This is primarily due to the dehydration of the loosely
packed structure that is subsequently transferred into amorphous FeOOH in
the mineral structure goethite. Because of this we suggest that the majority
of Fe from inorganic FeOOH or Fe adsorbed onto particles must be released
and utilized in an early stage of the voyage, mainly on the shelf or shortly
after the shelf break.
Budget uncertainties
Estimates for Fe budgets are challenging and often contain large
uncertainties. This is primarily due to the lack of site- and time-specific
flux data. Moreover, the mean annual estimates necessary for reliable
supply calculations reach a high level of accuracy only after the same
region has been monitored multiple times to cover seasonal and annual
anomalies. In the following, we will discuss the uncertainty of the
different Fe fluxes in the blooming region north of South Georgia.
We identified three main processes that together account for ∼98 % of the total Fe flux in the blooming region and thus contribute
largest uncertainties: the horizontal flux, dry/wet deposition, and winter
entrainment. Horizontal flux estimates of this study rely on literature
values that were collected offshore of the Antarctic Peninsula. In contrast,
South Georgia is an island with a confined shelf region, and thus horizontal
DFe fluxes may differ greatly. Furthermore, we showed not only that dry-deposition
dust fluxes are generally low but also that the Fe flux can
be supplemented strongly by sporadic wet-deposition events (∼1.0±1.2 µmol DFe m-2 d-1)
(Chance et al., 2015). Atmospheric fluxes are variable,
illustrated by the large standard deviation of the wet-deposition Fe fluxes
obtained at 40∘ S. Furthermore, to determine the magnitude of the
seasonal DFe winter entrainment reliable estimates of the winter mixing
layer depth (WMLD) and the ferrocline are required. Even though the WMLD can
be estimated very precisely using Argo float data, the depth of ferrocline
in Tagliabue et al. (2014) is based on 140 unique
observations distributed over the entire Southern Ocean. Due to this,
regional anomalies are not captured. In addition to the DFe fluxes in the
blooming region, we also assume that the biological Fe demand estimated for
the phytoplankton community contributes a large error. The biological Fe
requirements were determined using satellite-derived net primary production
data and an average intracellular Fe / C ratio derived from five different diatom
species native to the Southern Ocean. Both parameters are not well
constrained, and because of the lack of observational data we applied the
lowest intracellular Fe / C ratio available in the literature
(Strzepek et al., 2011). However, we found that even small
changes of both parameters change the estimated Fe availability in the
bloom region strongly. Nevertheless, flux estimates even with large
uncertainties can help us understand the degree of the nutrient supply vs.
consumption by organisms and help to pinpoint the limitation of the
estimates made. To ultimately reduce the level of uncertainty and to improve
our biogeochemical models, more observational data from the bloom region
north of South Georgia are required.