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Manganese and iron in Indian Ocean waters
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
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P. M. Saager, H. J. W, de Baar and P. H. Burkiil REFERENCES
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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
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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.
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
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