University of Groningen
Manganese and iron in Indian Ocean waters
Saager, Paul M.; de Baar, Henricus; Burkill, Peter H.
Published in:
Geochimica et Cosmochimica Acta
DOI:
10.1016/0016-7037(89)90348-7
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to
cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
1989
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Saager, P. M., de Baar, H. J. W., & Burkill, P. H. (1989). Manganese and iron in Indian Ocean waters.
Geochimica et Cosmochimica Acta, 53(9), 2259-2267. DOI: 10.1016/0016-7037(89)90348-7
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 16-06-2017
0016-7037/89/$3.00
Georhimlca n Cosmochimica
AC&I Vol. 53. pp. 2259-2267
copyright 0 1989 Pergamon Pms pk. Printed in U.S.A.
+ 40
Manganese and iron in Indian Ocean waters
PAUL M. SAAGER”*, HEIN J. W. DE BAAR’.+and PETER H. BURIULL’
‘NW0 Laboratorium voor Isotopen Ceologie, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands
2Plyrnouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PLI 3DH, United Kingdom
(Received October 12, 1988; accepted in revised form June 29, 1989)
Abstract-The
first vertical profiles of dissolved Mn and Fe for the (NW) Indian Ocean are reported.
The area is characterized by seasonal upwelling and a broad oxygen minimum zone in intermediate
waters. The dissolved Fe-profile exhibits a maximum (5.1 nM) in the oxygen minimum zone, with low
values both in surface waters (0.3 nM) and deep waters (around 1 nM). Mn concentrations in the surface
waters are elevated (2.0-4.3 nM), and decrease rapidly in an offshore direction. Below the first 25 m,
concentrations decrease dramatically (OS-l.3 nM), indicating removal by oxidation and particle scavenging. Further down, various Mn maxima are observed which can be related to hydrographic features
(sigma-e):
1. Intermediate water originating from the Red Sea lost its dissolved O2 while flowing northward along
the Omani coast and exhibits a strong Mn maximum (4.6-6.5 nM) coincident with the deep Or minimum.
2. At the two inshore stations in the Gulf of Oman this is overlain by relatively modest Mn maxima
(+2.7 nM) related to Arabian Gulf overflow water.
3. Finally the strong Mn maxima (4.4-5.6 nM) in the oxygen minimum zone at the two offshore
stations are related to yet another watermass.
Below these various maxima, concentrations decrease gradually to values as low as 90 pM at 2000
meters depth. Towards the seafloor concentrations increase again, leading to a modest bottom water
maximum (0.7-1.5 nM). The overall vertical distributions of Mn and Fe are strikingly similar, also in
actual concentrations, to those previously reported for the eastern equatorial Pacific, an area also characterized by an extensive 02-minimum zone.
1988). Here we report the first data for dissolved
Mn and Fe in the Indian Ocean.
Continental inputs of Mn and Fe into the oceans occur
via both fluvial (MARTIN and MEYBECK, 1979) and atmospheric pathways (HODGE et al., 1978; DUCE, 1986; STATHAM
and CHESTER, 1988). However, flocculation of Fe during the
early stages of estuarine mixing often effectively removes virtually all Fe (>90%) and much of the Mn from river water
(SHOLKOVITZ,1976, 1978; GOLDSTEINand JACOBSEN,1988).
Emanations from submarine hydrothermal vents cause elevated Fe and Mn ‘concentrations which in the case of Mn
can be traced for many miles away from the ridge crest before
mixing and scavenging eventually erodes this signal (KLINKHAMMERef al., 1986; HUDSON ef al., 1986). Otherwise the
hydrothermal source only affects its direct surroundings, as
reflected in the ferromanganese deposits on the flanks of the
ridge (HEATH and DYMOND, 1977).
Both Mn and Fe are essential elements for phytoplankton
growth (BRANDet al., 1983). Under special oceanic conditions
of ample nutrient (nitrate and phosphate) availability as encountered in the Southern Ocean, the subarctic North Pacific
and upwelling areas, Mn and Fe may sometimes affect productivity (COALE and BRULAND, 1989; MARTIN and GORDON, 1988). Within the euphotic zone photochemical reduction of oxyhydroxides may maintain Mn and Fe in solution, available for phytoplankton uptake (SUNDA el nl.,
1983; SUNDA and HUNTSMAN, 1988).
Throughout the oceanic water column Fe and Mn are removed via oxidative scavenging by biogenic or organically
coated particles (BALISTRIERI er al., 1981; MARTIN and
INTRODUCIION
GORDON,
IN THE PASTDECADEour understanding
of Mn in ocean waters has increased considerably. Distribution and fate in the
Atlantic Ocean are dominated
by atmospheric input
(KREMLING, 1985; STATHAM and BURTON, 1986), intense
scavenging at mid-depth (BENDER et al., 1977) and fluxes
from reducing shelf and slope sediments (KREMLING, 1983,
1985). The same processes influence the distribution and fate
of Mn in the Pacific Ocean. In addition, the suboxic waters
of the east equatorial Pacific are characterized by an intense
dissolved Mn maximum (KLINKHAMMERand BENDER,1980;
LANDINGand BRULAND,1980, 1987; MARTIN and KNAUER,
1982, 1983, 1984, 1985; MARTIN et al.. 1985).
For Fe similar maxima are reported for the same east
equatorial Pacific region (GORWN
et al., 1982; LANDING
and BRULAND, 1987). In Pacific surface waters the very low,
subnanomolar Fe levels appear to correlate with nutrients,
hinting at the possible role of Fe as a limiting micronutrient
(MARTIN and GORDON, 1988). For the north Atlantic, SYMES
and RESTER (1985) report higher levels, typically increasing
with depth to about 10 nM in bottom waters. The aforementioned overall dataset for Fe in the open ocean is still
very limited due to contamination problems during sampling
and analysis (LANDING and BRULAND, 1987; MARTIN and
* Present address: Free University, Department of EarthSciences,
De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands.
t Presenf address: Netherlands Institute for Sea Research, P.O.
Box 59, 1790 AB Den Burg, Texel, The Netherlands.
2259
2260
KNAUER,
P. M. Saager, H. J. W. de Baar and P. H. Burkiii
1983; CRAIG, 1974), where FeMn coatings on bac-
teriahave also heen reported(COWENand BRULAND,1985).
FeMn oxyhydroxides on settling biogenic particles are important carriers (BALISTRIERI
et (zl., 198 1) for othertraceelements like Cu, Zn, Cd, REE, and Co (BALI~TRER~
and MWRRAY, 1982; HEGGIEand LEWIS, 1984; DE BOAR a al., 1988;
Jacobs et a!., 1987). Regenerative fluxes from reducing sediments again contribute dissolved Mn to the overlying waters,
resulting in higher Mn levels towards the ocean margins
(MURRAYand GILL, 1978; FROELICW A al., 1979; TREFRY
and PRESLEY, 1982; MARTINand KNAUER,1984; MARTIN
et al., 1985; HEGGIE et al., 1987). Rapid oxidation largely
prevents the build-up of similar lateral gradients for dissolved Fe.
In anoxic basins, reductive dissolution and oxidative precipitation occur below and above the anoxic/oxic interface.
High levels of dissolved Fe and Mn are reported for the Black
Sea (SPENCER
and BREWER,197 1; HARALJXSON
and WESTERLUND,1988; LANDIFJG
and LEWIS, 19881, the Cariaco
Trench (JACOBSet ai., 1987), Framvaren Fjord (JACOBS
ef
al., 1985) and hypemaline Tyro and Bannock basins (DEBAAR
et al., 1987b).
Theoretical thermodynamic considerations predict very
low solubilities for both Mn and Fe (subnanomolar) under
standard marine conditions, i.e., oxygen saturation and pH
of about 8.2 (STUMMand MORGAN, 198 1). Under reducing
conditionsand lowerpH, Mn and Fe are considerably more
soluble. Kinetics of both reduction and oxidation are often
microbially mediated (EMERSONet al., 1982; NEALSON,
1983a,b) where the apparent oxidation of Mn appears to be
much slower than that of Fe. The exact physico-chemical
state of dissolved Fe and Mn (BYRNEef al., 1988; STUMM
and MORGAN, 1981) is beyond the scope of this study. Preliminaryresultsof this study and our complementary work
on Cu, Ni, Zn, Cd as well as REE have been summarized
previously (DE BAAR et al., 1987a; GERMAN et al., 1987:
SAAGERet al., 1987).
SAMPLE AREA
The northwest Indian Ocean, or more specifically the Arabian Sea, is characterized by strong seasonal upwelling driven
by the SW monsoons (April-September)
(SLATER and
KROOPNKX,1982; SENGUFTAand NAQVI, 1984). The Arabian Sea is a semi-enclosed basin due to the near presence
of the Asian continent, while the Carl&erg Ridge has also
been suggested to restrict circulation (SLATERand KRCXXNICK, 1982). The entire Arabian Sea has a broad oxygen
minimum zone extending between roughly 100 and 1200 m
depth with concentrations as low as 1 FM (DEUSERet al.,
1978; SLATERand KROOPNICK,1982; SEN GUPTA and
NAQVI,
1984). Intense denitri&ation
and ensuing nitrate
anomalies have been reported (NAQW, 1987), but complete
anoxia does not occur in the water column (DEuSERet al.,
1978). Curiously enough the highest rates of denitrification
are reported in more open waters where primary production
is much lower than in the Omani coastal upwelling area
(NAQVI, 1987; WMASUNDARand NAQVI, 1988).
The ample supply of nutrients caused by the strong up
welling leads to an enormous primary production averaging
twice that of the world’s oceans @LATERand K&OOPNICK,
1982; IITEKOTef al., 1987). Intense mineralization in the
intermediate waters drives the efficient consumption
of
available dissolved oxygen. The Arabian Sea oxygen minimum is driven by excessive consumption combined with
oxygen depletion in the renewal water before it ever reaches
the Arabian !%a (SWALLOW,1984). Modern estimatesofthe
residence time of the oxygen depleted layer are around 2-4
years (NAQVI,1987; SOMASUNDAR
and NAQVI,1988), instead
of earlier estimates of 30 years (SLATERand KROOPMCK,
1982; SEN GU~TA and NAQV~,1984). Such rapid circulation
prevents complete anoxia (NAQVI, 19871,
SAMPLING
AND METHODS
The NERC/IMER cruise aboard U.K.. R.V. CXuries Darwin took
place in ttrefallof 1984. It ran northward &ng 67*E, into the Arabian
Sea (&&ions l-6), then we&ward into the Gulf’ of Oman (stations
6-I I ) (Fig. 1f_Here we report resuhs from stations 5-9, thus pnrviding
a section going from open ocean waters towards near-shore wate~j
off the omani coast.
Samples were collected using a new tinless steel CID-rosette
sampler with modified (BRuLAND et ui., 19791, precleaned Idiitre
Teflon+x&xl GoFIo bott& (Gene& 0xanics). The rosette sa.mph~
was first sent down in order to obtain real-time hydmgraphic data
and to flushtheGoFlo bottles.Upon recovery the samfles, taken
during the upcast, were pressure filtered through acid cleaned Cl.4pm
Nuwre
filters, using in-tine, all Teflon filtratior! units and acidified
with 0.25 ml quartz distilled HCl (Q-HCl) to pH 2-3 and stored in
hot-&d chned 250 ml polyethylene bottles. Separate aiiquots were
coBeot& and made av&ble to the Cambri& group for REE analysis
(Gv
d al., 1987). The filters were not kept fur analysis. The
data thus only iadude distoiwgl
Mn anddissolved ‘Feas operationally
defined by the 0.4 pm filter. Trace metals were preconcentrated ashore.
using Chclex- ! 00(BioRad) ion cllclaaw chromatography (KINGSTOI\J
et al.. 1979) modified a&r DE tin
t 1983). Mill&Q water (Millipore)
and Qdist#d
qents
were us& Concentrates were analyzed with
a Perkin Elmer 5UOOatomic abrsorprion spctro&otometer
equipped
with Zeeman background correction (FERNANDEZ c-f
al.,1980) and
an AS 40 autosampler. The complete procedure is given elsewhere
(SAAGER,
1987,available
uponrequest
from the first author}. Initial
reagentblanks were assess4 beforehand and found to be ne&ible
or undetectable. Recovery tests (r = 0.996) confirmed the previously
reportad yield for Chelex-100 (KINGSTON et al.,1979: DE BAAR,
1983). 0verall procedural blanks were zsesd
by extracting MilliQ water and were ~0.03 nM for Mn and ~0.1 nM for the one Fe
profile, mspectiveiy. Both at sea and ashore analytical procedures
were perfbrmed inside a class-100 iaminar flow bench situated in a
clean air v3n or laboratory.
The ubiquity of Fe makes this element extremely dif%xit to measure without contamination (blWNG and BRULANLI,i987; MAK~~N
and GORDON, 1988) duringsarrr@iag
and proc&ng. This explains
the limited number of open ocean .I%profiles. 14 retrospect the shipboard operations appearto havebeen satisfactory. Despite al1 above
rigorous pnx&utions we did hcnvever encounter suious contamination
pmbkmsin the shore laboratory for Fe in the first sample sets. Detection limits were estimated as twice the Stan&d deviation of the
blank, yieIding 0.05 nM for Mn and 0.15 nM fur Fe, respectively.
Precision of the measurements was estimated 2% for Mn and 5% for
Fe at the 1 nM level.
RESULTS
AND MSCUSSIDFU*
Hydrography
The most prominent feature in the vertical distributions
of salinity and oxygen (Fig. 2) is the broad oxygen minimum,
extending between xlme 100 meters and 1200 meters depth.
Oxygen concentrations as low as 6 PM have been measured
2261
Dissolved Fe and Mn protiles in the Indian Ocean
3d
2d
id
0’
;*
.
,
IO”
I
60'
5d
I
70*
FIG. 1. Sampling locations in the northwest Indian Ocean: station 5 (14”3O’N, 67”E); station 6 (19”N, 67”E); station
7 (21’ 16’N, 63”22’E); station 8 (22”3O’N,60’4O’E);station 9 (23”3O’N,59’E). Also shown are stations 416 of GEOSECS
and 1958 of DANIELSSON(~~~~).
(Table
I). Nitrate
reduction
was indicated
at the offshore
depth (Fig. 2 for station 9), also in good agreement with earlier
observations (SENGUFTAand NAQVI, 1984).
stations by the presence of NO; in the oxygen minimum
zone (Fig. 3).
Mn and Fe
The influence of Arabian Gulf overflow waters is very clear
Dissolved Mn. All vertical profiles of dissolved Mn exhibit
the same general trends, but there are some subtle differences
at stations 7, 8 and 9. Temperature (not shown), salinity and
oxygen content all show sharp maxima at 160-500 meters
station6
0
34.7
A
station
60
35.2
120
357
9
100
36.2
depth
(d bar)
500-
1000-
2500salinity
3000-
FIG. 2. Vertical CID-profiles of salinity and dissolved O2 at stations 6 and 9.
P. M. Saager, H. J. W. de Baar and P. H. Burkill
2262
Table
Depth
(da)
ation
Sal
Psu
1. Concentrations
of dissolved oxygen, Mn and Fe together with other hydrographlc
parameters as a function of depth for stations 5 to 9. Dissolved oxygen was
analysed on board ship by triplicate Wlnkler titrations.
1 = duplicate analysis.
u-a
Pot.temp.
8,OC
5 (14O30'N,
0,
PM
Mn
nM
Fe
nM
Depth
(da)
Sal
Psu
ii
36.631
36.659
36.057
35.81
35.605
35.544
35.602
35.606
35.576
35.565
35.511
35.491
35.372
35.021
34.832
34.775
34.75
34.741
Station
6 (19ON
2.857
1.999
1.577
1.374
205.3
199.4
43.5
11.1
8.0
10.5
14.1
9.9
16.7
32.6
12.6
17.2
56.3
105.9
131.8
148.8
151.3
35.4
35.247
34.994
34.847
34.778
a.68
7.272
4.676
2.979
2.008
27.483
27.575
27.708
27.765
27.793
15.3
20.0
58.8
94.5
118.8
34.746
1.49
27.807
134.2
3200
27.229
7 (21016.N,
36.389
36.335
36.254
36.17
36.196
36.083
36.17
36.237
36.414
35.932
35.649
35.523
35.368
35.223
34.969
34.845
34.774
34.744
2.57
1.45
1.36
2.37
3.98
5.62
3.59
1.50
1.35
1.05
0.!4
0.73
0.75
0.27
0.34
0.23
0.39
0.31
3
5
10
67OE)
26.407
26.316
25.335
22.436
21.309
19.504
18.364
17.018
14.543
13.388
12.168
11.434
10.129
1000
1200
1600
2000
2500
4
20
50
a0
100
127
175
200
285
400
600
800
1000
1200
1600
2000
2500
3363
5.099
23.891
24.008
25.128
25.524
25.809
26.107
26.703
26.924
27.073
27.165
27.259
27.344
27.484
27.68
27.764
27.792
27.804
27.811
206.8
207.9
209.2
206.1
161.1
74.8
55.9
37.1
17.4
15.4
a.8
24.8
7.5
12.6
6.3
36.685
36.681
36.599
36.62
36.547
36.305
36.204
36.043
35.97
35.966
35.882
35.81
35.655
35.606
35.534
Station
27.245
26.814
21.627
19.453
17.704
16.293
13.761
12.698
ii.814
11.278
10.53
10.046
a.535
23.913
24.114
24.158
24.41
25.087
25.327
25.689
25.926
26.252
26.753
26.942
27.066
27.168
27.347
10
20
30
40
70
90
120
150
175
204
300
400
500
600
800
0-e
02
uM
Mn
nM
67OE)
Station
125
150
175
200
300
400
500
600
700
a00
1000
1500
2000
2500
3000
4000
Pot.temp.
G, nc
2.43
2.18
2.70
2.29
1.63
0.96
0.85
2.40
3.23
4.06
4.39
3.88
2.29
1.91
2.66'
2.53'
0.66
0.47
0.40
0.28
1.14'
1.181
1.34
26.477
23.216
21.181
20.647
19.285
la.291
17.839
16.71
13.74
11.494
9.931
a.325
6.965
4.394
2.923
1.974
1.462
23.811
23.893
24.824
25.336
25.502
25.777
26.097
26.261
26.67
26.963
27.19
27.373
27.514
27.6
27.72
27.769
27.793
27.807
233.7
222.0
131.0
41.4
28.2
13.9
14.7
17.7
12.7
10.5
14.1
15.1
21.5
62.6
90.8
117.0
127.4
1.94
1.66
1.79
0.92
1.24
1.10
1.10
0.96
1.19
6.23
6 44
3.73
0.91
0.55
0.37
0.25
0.34
0.65
::
30
50
75
100
125
150
175
200
240
400
600
a00
1000
1200
1600
2000
2500
25.671
25.661
25.666
24.445
21.947
20.774
19.394
18.269
17.33
16.577
16.022
15.746
15.196
12.86
11.277
10.043
a.664
7.149
4.566
2.944
1.965
24.325
,24.326
24.328
24.466
24.884
25.233
25.542
25.792
26.002
26.177
26.354
26.465
26.632
27.019
27.216
27.36
27.485
27.587
27.713
27.767
27.791
3000
34.744
1.589
27.798
4
9
:r;
24
30
35
0.30
1.65
2.33
0.76
1.45
1.10
0.57
1.40
3.93
3.16
5.21
2.39
2.07
1.78
1.60
0.95
1.10
1.80
60°40'E)
36.581
36.572
36.57
36.574
36.263
35.855
35.888
35.813
35.765
35.737
35.731
35.794
35.856
35.911
35.77
35.63
35.531
35.4
35.239
34.985
34.845
34.77
Station
63022'E)
26.861
0 (2Z030'N,
::
90
120
141
170
200
300
400
600
a00
1000
:200
1600
2000
2300
2750
9 (23030'N.
36.875
36.799
36.62
36.448
36.169
36.188
36.207
36.228
36.112
36.078
36.057
36.137
36.22
36.208
36.481
35.857
35.664
35.534
35.4
35.245
34.989
34.848
34.794
34.752
118.7
2.47
2.51
2.23
2.33
2.10
1.63
1.52
1.45
1.82
2.93
1.57
1.38
1.44
1.22
0.74
4.64
4.02
2.16
1.16
0.51
0.64
0.26'
0.24'
0.36
216.2
222.2
207.7
110.7
45.8
29.0
21.9
18.0
12.7
14.8
10.1
12.2
13.7
15.2
13.5
7.4
19.6
6.3
15.2
50.8
85.9
102.0
93.3
4.32
2.86
3.43
3.77
2.51
2.05
2.12
1.88
1.33
1.42
1.20
1.03
2.60
1.89
0.88
0.52
4.59
3.20
2.37
0.80
0.49
0.09
0.15
0.63
219.0
222.2
221.5
221.7
203.9
159.1
109.3
48.1
11.4
16.1
12.7
16.3
a.1
16.5
10.9
12.6
13.0
24.4
55.1
88.6
111.0
59OE)
29.251
23.39
28.701
23.518
22.373
21.43
21.22
20.861
20.024
19.21
la.215
17.952
17.392
16.805
16.458
13.409
11.661
10.1
a.682
7.203
4.618
2.996
2.294
1.701
25.002
25.281
25.354
25.469
25.605
25.792
26.029
26.156
26.357
26.489
26.781
26.974
27.171
27.352
27.482
27.583
27.711
27.764
27.782
27.796
Dissolved Fe and Mn proties in the Indian Ocean
St
St8
9
;j_’
st 7
r
!
1
st 5
st 6
b?
b?
bian Gulf overflow waters. Adverse political conditions prevented us from further investigating this source. However,
higher Mn concentrations are to be expected for a relatively
small, land-enclosed basin such as the Arabian Gulf.
In the oxygen minimum zone, concentrations increase
dramatically over only 100 to 200 meters. Maximum concentrations were found at station 7 (6.4 nM). Below these
maxima, concentrations gradually decrease downward where
they reach values typical for the deep ocean (about 0.2 nM,
e.g., LANDING and BRULAND, 1987; MARTIN et al.. 1985).
There is a striking resemblance between vertical profiles reported here and those observed in the east Pacific (MARTIN
et al., 1985; LANDINGand BRULAND, 1987), where a similar
oxygen regime exists. For comparison the profile at Pacific
station VERTEX-II is inserted in the profile at our NW10
station 6. Not only do the shapes of the profiles look alike,
actual concentrations are virtually the same as well. Although
the maximum concentration values do not show an unambiguous trend away from the coast, the actual shape of the
Mn maximum does. The Mn maximum becomes narrower
and sharper and shoals in an offshore direction. At inshore
stations 8 and 9 maximum concentrations are found at 600
meters depth and the Mn maximum extends over almost
800 meter. At offshore station 5 the maximum is found at
200 meters depth and is limited to a much narrower zone.
Without data on vertical fluxes and suspended particulate
Mn, it is very difficult to assess the relative importance of in
situ dissolution of FeMn oxyhydroxides within the oxygen
minimum zone and horizontal advection from reducing shelf
sediments. Horizontal advection is likely to be important.
Both Martin and coworkers (MARTIN and KNAUER, 1982,
1983, 1984, 1985; MARTIN et al., 1985) and LANDING and
NO;(MI
FIG. 3. Dissolved NO; concentrations for the upper water column
(Data courtesy R. Howland, PML, UK).
between stations (Fig. 4, Table 1). Surface water concentrations are elevated, showing a marked decrease from coastal
(4.3 nM, station 9) to more open ocean waters (2-2.5 nM,
stations 5-8). The strong decrease in an offshore direction
clearly hints at a continentally derived source. Both aeolian
and fluvial inpits, but also diffusion out of mildly reducing
nearshore sediments, can cause these surface water enrichments. The net evaporation probably hides all visible fresh
water influences of fluvial inputs but several major rivers
drain into or near this part of the ocean (Indus; Euphrates
and Tigris in the Arabian Gulf). Photoreduction of labile Mn
oxyhydroxides as proposed by SUNDA et al. (1983) might
maintain Mn in solution. The relative importance of aeolian
deposition has not been assessed but will probably be large
because of the proximity of the arid Arabian subcontinent.
Just below the surface, concentrations drop off sharply (0.91.4 nM) as a result of oxidative removal and/or particle scavenging. At stations 8 and 9 this decrease is interrupted by
local small maxima at about 125-175 m depth (2.9 nM and
2.6 nM at stations 8 and 9, respectively), coinciding with the
elevated salinity, oxygen and temperature indicative of Ara-
(23456
0
0
I
z
3
4
5
0
t
2
3
4
5
6
7
.
‘=..-._.
depth
r--
idborl
2263
.’
i
,
FIG. 4. Vertical profiles of dissolved Mn for Stations 5-9. Also shown in this figure is the Mn profile at Station
VERTEX-II (2) in the east equatorial Pacific (from LANDINGand BRULAND,1987). The solid bar at the depth axis
denotes the oxygen minimum zone.
P. M. Srliiger,H. J. W. de Baar and P. H. Burkill
2264
Mn
for temperature, oxygen and salimty (Fig. .31and secondi>
because the earlier reported values for sigma-0 are only an
estimate for the Arabian Sea as a whole and have not beer?
plotted in detail for this region.
Secondly, there is the Mn maximum III the oxygen rninrmum zune. This maximum is located at sigma-t, :- 27.2 a~
stations 7,8 and 9. The small anomaly at 800 meters depth
at station 6 could also be related to this isopycnal, although
it is situated at sigma-l = 27.35. The most likely explanation
for the Mn distribution at this isopycnal is that Red Sea water
(sigma-8 = 27.2, present at 500-800 meters depth; W‘L’RIKI.
197 1; SEN GUIY~Aand NAQVI, 1984), which is already iam
in dissolved oxygen due to the high salinity and temperature
in the Red Sea, lost virtually all its oxygen while flowing
northward in the upwelling waters along the coast of Oman
(D. B. OLSON, pers. commun., 1988; WYRTKI, 1971; SF:
GUPTA and NAQVI, 1984). Maximum Mn concentratlonz
decrease from station 7 to station 9. It IS probable that Mn
diffused from reducing sediments where the oxygen minimum
intersects the shelf is horizontally transported northward with
the Red Sea water mass. Horizontal processes thus appear
to be of considerable importance at stations 7 to 9. ‘Thirdly
the Mn maxima at offshore stations 5 and 6 are situated a:
sigma-8 = 26.4 + 0.3 and belong to yet another watermass
(or masses). The Mn maxima at stations 5 and 6 are cleariv
distinct from the maxima at stations 7-9 tt is not clear I:$
which watermass these maxima belong. Oi si;rc reduction 0;
FeMn oxyhydroxides may be the more important process
here. It must be noted that only at stations .F and 6 is then:
a deep nitrite maximum, related to the oxygen minimum
At stations 7 to 9 there is only a nitrite maximum in the
euphotic zone, which is related to nit&cation. Primary prc?duction, however, is highest at these stations and Ihe oxygen
(nmol.ng-‘I
1s
l
i4
-0
FIG. 5. Dissolved Mn against sigma-e,
for explanation
see text
BRULAND (1987) attribute a large part of the Pacific Mn
maximum to horizontal mixing. The importance of horizontal advection in the northwest Indian Ocean has been
demonstrated for denitrification, export of the nitrate deficit
and mixing ofwater masses (SWALLOW,1984; NAQVI, 1987).
However, from the relation between Mn concentrations and
the density of the water, valuable information is obtained
with regard to horizontal,mixing. From a plot of Mn versus
sigma-8 three distinct water masses can be recognized (Fig.
5). First of all there is the aforementioned subsurface maximum at stations 8 and 9, the peak of which corresponds to
sigma-8 = 26.3 & 0.1. This sigma-l value is slightly lower
than the reported value for the Arabian Gulf overflow water,
which is 26.6 + 0.3, situated at a depth of 300 meter (WYRTIU,
197 1). We do, however, believe that the subsurface maximum
is related to the high salinity Arabian Gulf overtlow waters,
first because of the concurrence of the Mn maxima with those
I ” :”
zoooji
/
j
/
!
I
3000
/’
;.
i
*
i,
!
1
FIG. 6. Vertical profile of dissolved Fe at StatIon 7. Also showr; %i
the Fe profile at VERTEX-II (2) in the east equatorial Pacific (data
from LANDINGand BRULAND,1987). The solid bar at th? \,-axi:
denotes the oxygen minimum.
Dissolved Fe and Mn profiles in the Indian Ocean
demand for the decomposition of organic matter would thus
be largest. This is in accordance with the results of NAQVI
( 1987), although no explanation was given for this observation. It has been proposed that in situ reduction might be
more important where nitrate reduction is evident (MARTIN
and KNAUER, 1982; KLINKHAMMERand BENDER, 1980).
Mn maxima in the oxygen minimum of the east Pacific were
especially high where nitrate reduction occurred (KLINKHAMMERand BENDER, 1980). In our case, however, highest
Mn maxima are found where productivity is highest, but
where denitrification is not. More information is needed to
solve this problem.
The very low values in deep waters are the result of extensive oxidative removal and particle scavenging. Very low
concentrations were measured (90 pM at 2000 meters depth
at station 9), confirming the reactivity of this metal and its
short residence time in the ocean. At a few stations Mn concentrations increase slightly towards the bottom, leading to
a modest bottom water maximum. These elevated values are
most likely driven by a flux from the sediments (HEGGIE et
al., 1987).
Dissolved Fe. The one profile of dissolved Fe at station 7
is reported in Fig. 6 and Table 1. The profile bears great
similarity to those reported for the east Pacific (GORDON et
al., 1982; LANDINGand BRULAND, 1987; MARTIN and GORDON, 1988). For comparison the profile at Station VERTEXII (2) (from LANDING and BRULAND, 1987) is inserted in
Fig. 6. Earlier estimates for Fe in the Indian Ocean only considered unfiltered, total dissolvable Fe (dissolved plus particulate) and appear to be affected by contamination. Fe profiles
reported by DANIELSSON(1980) are a factor 5 to 10 higher
than our values. The profiles measured in the late sixties
(TOPPING, 1969) are higher by as much as three orders of
magnitude and appear to be mainly of historical interest.
From a low surface water concentration of 0.3 nM, values
increase with some scatter to about 1-2 nM in the upper 150
meters. These observations are similar to those reported for
the Atlantic Ocean (SYMESand RESTER, 1985) whereas Pacific surface waters generally exhibit lower values (LANDING
and BRULAND, 1987; MARTIN and GORDON, 1988). Below
175 meters concentrations clearly increase downward, where
they reach a maximum in the suboxic zone (5.1 nM at 600
meters depth) as was the case for Mn. Below this maximum,
concentrations gradually decrease downward to uniformly
low values in deep waters (1-2 nM). The data indicate a
slight increase towards the seafloor, possibly as a result of
diffusion from sediments or resuspension of bottom sediments. Within and below the suboxic zone, Fe concentrations
are very similar to those reported for the Pacific Ocean; Fe
levels in the deep Atlantic Ocean are considerably higher.
It is thought that the low surface water concentrations are
the result of uptake by phytoplankton (e.g., MARTIN and
GORDON, 1988). The decomposition of organic matter sinking out of the surface waters would release Fe back into the
water column. Correlations between Fe and NOT have recently been reported for some surface waters (MARTIN and
GORDON, 1988) yet at our station no significant correlation
was found (not shown). In this regard the behaviour of Fe is
different from that of Mn. There are a few explanations why
surface waters are depleted in Fe and not in Mn, since Mn
2265
too is an essential nutrient. First, both fluvial and atmospheric
inputs could be larger for Mn. Second, photoreduction of Fe
(RNDEN et al., 1984) might not be sufficient in keeping Fe
in solution, where photoreduction of Mn might. Because the
oxidation of Mn proceeds slower than that of Fe, it is possible
that Mn diffused from near-coastal sediments and laterally
transported offshore is metastably kept in solution. Third, it
is possible that the planktonic demand for Fe is much larger
than for Mn. Finally, the Fe maximum in the suboxic zone
may also be an advective feature derived from Fe mobilized
in reducing shelf sediments.
In deep oxygenated waters the dissolved MnfFe-concentration ratio is less than I (Mn/Fe = 50.2) whereas in the
suboxic zone the ratio is equal to or larger than 1. This was
also found in the suboxic east equatorial Pacific (LANDING
and BRULAND, 1987) where in the oxygenated deep waters
Mn/Fe < 1 and in the suboxic zone Mn/Fe > 1. Also, in the
Cariaco Trench, Mn/Fe < 1 in the oxygenated subsurface
waters and Mn/Fe > 1 in the anoxic zone, although in anoxic
waters new equilibria are established between Fe-sulfide and
Mn-carbonate (JACOBSet al., 1987). Apparently Mn is more
readily mobilized in a low oxygen environment than is Fe.
CONCLUSIONS
The similar distributions of Mn and Fe in the oceans suggest that the distributions of these elements are largely driven
by regional sources and sinks, despite their known involvement in biological processes. This contrast with the nutrient
type trace metals (e.g., Cd, Zn, Ni; BRULAND, 1983) is the
result of the high reactivity of Mn and Fe and their own
redox chemistry and also underlines the short oceanic residence time of these elements.
The distributions of Mn and Fe in the NW Indian Ocean
appear to be governed largely by horizontal advection, in
keeping with earlier observations in the East Equatorial Pacific. This confirms the results for the east equatorial Pacific,
where horizontal mixing along isopycnals is also responsible
for the observed distribution of Mn and Fe.
Acknowledgements-We are most grateful to officers and crew of the
NERC RV Charles Damin. Dr. R. F. C. Mantoura and colleagues
at NERC/IMER (currently PML) kindly allowed our participation
in a very successful and pleasant cruise. Upon advice of Drs. Burton
and Statham and ourselves, NERC kindly provided the sampling
aear. Prof. Dr. H. N. A. Priem at the NW0 Laboratorv for Isotope
Geology, Amsterdam, is thanked for allowing us to make use of the
excellent clean laboratory facilities. Coos van Belie is thanked for his
assistance and advice in the laboratory. Theo van Zessen is thanked
for his help with the analyses. R. Howland (PML) is thanked for the
nitrite data. Critical comments of Drs. Y. Sato and W. Helder, L.
Gerringa, J. Middelburg, R. F. Nolting and one anonymous reviewer
considerably improved the manuscript. This project was realized with
support and advice from Pietemel Montijn and grants from the U.K.
Natural Environment Research Council (grant GR3/6010) and the
Ministerie van Onderwijs and Wetenschappen and the Nederlandse
Raad voor Zeeonderzoek (The Netherlands Department of Science
and Education and the Netherlands Marine Research Foundation),
respectively.
Editorial handling: R. G. Bums
2266
P. M. Saager, H. J. W, de Baar and P. H. Burkiil
REFERENCES
BALISTRIERJ
L., BREWERP. G. and MURRAYJ. W. ( 198I) Scavenging
residence times of trace metaIs and surface chemistry of sinking
particles in the deep sea. DeepSea Res. 28. 10 I- I2 1.
BALISTRIERI
L. S. and MURRAY J. W. (1982) The adsorption of Cu.
Pb, Zn, and Cd on goethite from major ion seawater. Geachim.
Cosmochim. Acto 46, 1253-1265.
BENDERM. L., KLINKHAMMER G. P. and SPENCERD. W. (1977)
Manganese in seawater and the marine manganese balance. Deep
Sea Res. 24, 799-8 12.
BRAND L. E., SUNDAW. G. and GUILLARDR. R. L. (1983) Limitation of marine phytoplankton reproductive rates by zinc, manganese and iron. Limnol. Oceanogr. 28, 1182-I 198.
BRULANDK. W. (1983) Trace elements in Sea Water. In Chemicul
Oceunogruphy(eds. J. P. RILEYand R. CHESTER),
Vol. 8. pp. 157221. Academic Press.
BRWLANDK. W., FRANKS R., KNAUER G. A. and MARTINJ. H.
(1979) Sampling and analytical methods for the determination of
copper, cadmium, zinc and nickel at the nanogram per liter level
in sea water. Anal. Chim. Acta 105, 233-245.
BYRNER. H., KUMP L. R. and CANTRELLK. J. ( 1988) The influence
of temperature and pH on trace metal speciation in seawater. &far.
Chem. 25,163-181.
COALEK. H. and BRULANDK. W. ( 1989) Effects of iron, manganese.
copper and zinc enrichments on productivity in the Subarctic Pacific. (submitted).
COWENJ. P. and BRULANDK. W. ( 1985) Metal deposits associated
with bacteria: implications for Fe and Mn marine biogeochemistry.
Deep-Sea Res. 32, 253-272.
CRAIG H. (1974) A scavenging model for trace elements In the deep
sea. Earth Planet. Sci. Lelt. 16, 149-159.
DANIELSON L-G. (1980) Cadmium, cobalt. iron, lead, nickel and
zinc in Indian Ocean waters. Mar. Chem. 8, I 99-2 I 5,
DE BAARH. J. W. (1983) The marine geochemistry of the rare earth
elements. Ph.D. dissertation. WHOI-83-45, 278~.
DE BAAR H. J. W., SAAGERP. M. and HOWLAND R, J. (1987a)
Nutrient type distributions of Ni, Cu, Zn and Cd in the Northwest
Indian Ocean (abstr.). Eos 68, 1755.
DE BAAR H. J. W., SAAGERP. M. and DE JONC J. 1‘. M. ( 1987b)
Dissolved Mn in the east Mediterranean and its hypersaline anoxic
basins. Eos 68, 1682.
DEBAARH. J. W., GERMANC. R.. ELDER~ELDH. and VANGAAN~
P. (I 988) Rare earth element distributions in anoxic waters of the
Cariaco Trench. Geochim. Cosmochim. Acta 52, 1203-1219.
DEUSERW. G., Ross E. H. and MLODZINSKAZ. J. (1978) Evidence
for and rate of denitrification in the Arabian Sea. Deep-Sea Res
25,43 l-445.
DUCER. A. (1986) The impact of atmospheric mtrogen, phosphorus
and iron species on marine biological productivity. In The Roie
of Air-Sea Exchange in Geochemicai Cyciing ted. P. BUAT-MC.
NARD),pp. 497-529. Reidel, Dordrecht.
EMERSONS., KALHORNS., JACOBSL., T~so 3. M., NEALSON K. H.
and ROSSONR. A. (1982) Environmental oxidation rate of manganese(II): Bacterial catalysis. Geochim. Cosmochim. Acla46, 10731079.
FERNANDEZF. J., MYERSS. A. and SLAVINW. (I 980) Background
correction in atomic absorption utilizing the Zeeman effect. Anal.
Chem. 52,74 l-746.
FINDEND. A. S., TIPPINGE., JAWORSWG. H. M. and REYNOLDS
C. S. (1984) Light-induced reduction of natural iron(iii) oxide and
its relevance to phytoplankton. Nuture 309, 783-784.
FROELICHP. N., KLINKHAMMERG. P., BENDERM. L., LUEDTK~
N. A., HEATHG. R., CULLEND., DAUPHINP., HAMMONDD.,
HARTMANB. and MAYNARDV. (1979) Early oxidation oforganic
matter in plagic sediments of the eastern equatorial Atlantic: Sub.
oxic diagenesis. Geochim. Cosmochim. Ada 43, 1075-1090.
GERMANC. R., DE BAAR H. J. W. and ELDERFIELDH. (1987) REE
in the NW Indian Ocean: Biogeochemical cycling in a suboxic
marine environment (abstr.). Ens 68(50), p. 1755.
GOLDSTEINS. J. and JACOBSENS. B. ( 1988) REE in the Great Whale
river estuary, northwest Quebec. Earth Planet. Sri. Lett. 88, 241252.
GORWN R. M., MARTINJ. H. and KNAUERi;. A. (1YtlZ1Iron t!:
northeast Pacific waters. Nuture 299.6 11-b 12.
HARALDSXIN C. and WE.STERLUND
S. (1988) Trace metis in tht:
water columns of the Black Sea and Framvaren Fjord. Mar. Chfwi
23,4 17-424.
HEATH G. R. and DYMONDJ. (1977) Genesis and transformatlen
of metaIIiferous sediments from the East Pacific Rise, Bauer Deep
and Central Basin, northwest Nazca Plate. &cl]. SM. .&ner Euli
83,723-733.
HEGGIED., KLMKHAMMER
G. P. and CULLEND. (1987) Mangana
and copper tluxes from continental margin sediments. Geochim
Cosmochim. Acta 51,1059-1070.
HEGGIE D. and LEWIST. (1984) Cobalt in pore waters rn marine
sediments. Nafure 311,435-455.
HODGEV., JOHNSONS. R. and GOLDBERGE. D. (1978) InUuencr
of atmospherically transported aerosols on surface ocean water
composition. Geochem. J. 12, 7-20.
HUDSONA., BENDERM. L. and GRAHAM D. W. (1986) iron enrichments in hydrothermal plumes over the East Pacific Rise. &ri!:
Planet. Sci. Lett. 79,250-254.
ITTEKOTV., MANGANINIS. J., GU~TA M. L. S., DECENS B. p;
DEGENSE. T. and HONJO S. (1987) Particle fluxes in the ArabIan
Sea (abstr.). Eos 68(50), p. 1772.
JACOBSL., EMERSONS. and SKEIJ. (1985) Partitioning and transpon
of metals across the 02/HzS interface in a permanently anoxic
basin: Framvaren Fjord, Norway. Geochim. (‘nsmwhim. .Arro49,
1433-1444.
JACOBSL., EMERSONS. and HUESTEDS. (1987) ‘I race metai gc~f.
chemistry in the Cariaco Trench. Deep-Sea Res. 34,965-98 1”
KINGSTON H. M., BARNESI. L., BRADY T. J., RAM T. C. and
CHAMPM. A. (1979) Separation of eight transition elements from
alkali and alkaline earth elements in estuarine and seawater with
chelating resin and their determination with graphite furnace
atomic absorption spectrometry. Anal. Chem 50. na. 14. 20642070.
K.LINKHAMMER
G. P. and BENDERM. L. i f980) The distribution
of manganese in the Pacific Ocean. Eurrh Piunet. Sci Lcrr 46,
361-384.
KLINKHAMMER
G. P., ELDERRELDH., GREAVU
M., KUNA P. and
NEL.SENT. (1986)Manganese geochemistry near high-temperature
vents in the Mid-Atlantic Ridge rift valley. .&&I Planet. Sci I,tvt
80,230-240.
KREMLINGK. (1983) Trace metal fronts in European shelf waterx.
Nature 303,225-227.
KREMLING K. (1985) The distribution of cadmium, copper. nickef.
manganese, and aluminium in surface waters of the open Atlantic
and European shelf area. Deep-Sea Res. 32, 53 I.-555.
LANDINGW. M. and BRULANDK. W. (1980) Manganese I$>~fi~
north Pacific. Earth Planet. Sci. Left. 49, 45-M.
LANDINGW. M. and BRULANDK. W. (1987) The contrasttng bji,geochemistry of iron and manganese in the Pacific Ocean. &&rim
Cosmochim. Acta 51, 29-43.
LANDING W. M. and LEWISB. L. (1988) ‘Trace metal speciation ic
the Black Sea: Mn and Cu results from the 1988 NSF Black Sea
Expedition. Eos 69, 1239.
MARTINJ-M. and MEYBECKM. (1979) Elemental mass balance o?
material carried by major world rivers. Mat. Chem. 7, 373-206.
MARTINJ. H. and GORDON R. M. (1988) Northeast Pacific iron
distributions in relation to phytoplankton productivity Dears-.%&
Res. 35, 177-196.
MARTIN J. H. and I(NAUER G. A. (1982) Manganese cycbng 111
northeast Pacific equatorial waters. J. Mar. Res. 40, 12 13-1225.
MARTIN J. H. and KNAUER G. A. (1983) VERTEX: Manganese
transport with CaCOJ. Deep-Sea Res. 30,411-4X.
MARTIN J. H. and KNAUERG. A. (1984) VERTEX: Manganese
transport through oxygen minima. Eurth Planet. Sci Lefr. 67. ?C;41.
MARTINJ. H. and KNAUER G. A. (1985) Lateral transpon ot Mn
in the north-east PacificGyre oxygen minimum. hilltrtre314,524-526.
MARTINJ. H., KNAUERG, A. and BROENKOWW. W. (1985) VERTEX: the IateraI transport of manganese in the northeast PacifiL.
Deep-Sea Res. 32, 1405-1427
Dissolved Fe and Mn profiles in the Indian Ocean
MURRAY J. W. and GILL G. (1978) The geochemistry
of iron in
Puget Sound. Geochim. Cosmochim. Acta 42, 9-19.
NAQVI S. W. A. (1987) Some aspects of the oxygen deficient conditions and denitrification in the Arabian Sea. J. Mar. Res. 45,
1049-1072.
NEALSONK. H. ( 1983a) The microbial iron cycle. In Microbial Geechemistry (ed. W. E. I‘&JMBEIN); Ch. 6. Blackwell, Oxford.
NEALSONK. H. ( 1983b) The microbial manganese cycle. In Microbial
Geochemistry (ed. W. E. KRUMBEIN);Ch. 7. Blackwell, Oxford.
SAAGER P. M. (1987) Trace metals in the Indian Ocean. M.Sc. thesis,
Univ. Utrecht, The Netherlands, 89~.
SAAGERP. M., DE BAARH. J. W. and ELDEW~ELDH. (1987) Manganese in the Indian Ocean (abstr.). Eos 68, p. 1754.
SENGUPTA R. and NAQVIS. W. A. (1984) Chemical oceanography
of the Indian Ocean, north of the equator. Deep-Sea Res. 31, nos.
6-8a, 67 I-706.
SHOLKOVITZE. R. ( 1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geechim. Cosmochim. Acta 40,831-X45.
SHOLKOVITZE. R. (1978) The flocculation of dissolved Fe, Mn, Al,
Cu, Ni, Co and Cd during estuarine mixing. Earth Planet. Sci.
Len. 41, 77-86.
SLATERR. H. and KROOPNICKP. (1982) Controls on dissolved oxygen distribution and organic carbon deposition in the Arabian
Sea. In Marine Geology and Oceanography of Arabian Sea and
Coastal Pakistan (eds. B. U. HAG and J. D. MILLIMAN),Ch. 18.
SOMASUNDAR
K. and NAQVI S. W. A. (1988) On the renewal of the
denitrifying layer in the Arabian Sea. Oceanol. Acta 11, 167- 172.
SPENCERD. W. and BREWERP. G. (197 1) Vertical advection diffusion
2267
and redox potential as controls on the distribution of manganese
and other trace metals dissolved in waters of the Black Sea. .I
Geophys. Res. 76,5877-5892.
STATHAMP. J. and BURTONJ. D. (1986) Dissolved manganese in
the north Atlantic Ocean. Earth Planet. Sci. Lett. 79, 55-65.
STATHAMP. J. and CHESTERR. (1988) Dissolution of manganese
from marine atmospheric particulate-s into seawater and rainwater.
Geochim. Cosmochim. Acta 52,2433-2437.
SUNDA W. G. and HUNTSMANS. A. (1988) Effect of sunlight on
redox cycles of manganese in the southwestern Sargasso Sea. Deep
Sea Res. 35, 1297- I3 17.
SUNDA W. G., HUNTSMANS. A. and HARVEYG. R. (1983) Photoreduction of manganese oxides in sea water and its geochemical
and biological implications. Nature 301, 234-236.
STUMMW. and MORGANJ. J. (1981) Aquaric Chemistry. J. Wiley
& Sons. 780p.
SWALLOWJ. C. (1984) Some aspects of the physical oceanography
of the Indian Ocean. DeepSea Rex 3f, nos. 6-f& 639-650.
SYMESC. J. and KESTERD. R. (1985) The distribution of iron in
the northwest Atlantic. Mar. Chem. 17, 57-74.
TOPPINGG. J. ( 1969) Concentrations of Mn, Co, Cu, Fe and Zn in
the northern Indian Ocean and Arabian Sea. J. Mar. Res. 27,3 18326.
TREFRY J. H. and PRESLEYB. J. (1982) Manganese fluxes from
Mississippi Delta sediments. Geochim. Cosmochim. Acra 46, 17151726.
WYRTK~K. ( I97 1) Oceanographic Atlas of the International Indian
Ocean Expedition. National Science Foundation, Washington DC.,
53lp.