Element Input from Atmospheric Dust in the Southern Ocean

As discussed in Chapter 3, the Southern Ocean cannot be considered a single functional unit, but a mosaic of distinct subsystems with specific physical and biological processes. These processes affect the spatio-temporal distribution of major nutrients and trace elements, especially Fe, which is essential to respiratory electron transport chains, synthesis of chlorophyll and production of amino acids in phytoplankton organisms (Raven 1990). Dissolved Fe2+ was probably easily bioavailable during the first stages of biological evolution in the primordial ocean but, as photosynthesis and an oxygenated ocean developed, the metal began to precipitate into solid Fe3+ phases. Thus, although in marine organisms Fe is involved in many essential metabolic pathways, in some oceanic areas it has become a limiting factor for life. This generally occurs in open-ocean waters, especially in the Southern Ocean where there is negligible fluvial or aeolian input of Fe and other lithophilic elements from continents. Very low concentrations of total and dissolved Fe have been measured in surface waters of the Pacific region of the Southern Ocean, in the Drake Passage and in the Antarctic Circumpolar Current (Martin et al. 1990; Westerlund and Öhman 1991a; Löscher et al.

Table 9. Typical concentrations of dissolved trace metals (nmol l-1; Co, pmol l-1) at different depths in offshore waters of the Southern Ocean

Depth (m) Fe Mn Cd Co

Cu

Ni

Zn

Sampling period

South Drake Passage (60° 46'S, 63° 26'W>

30 0.16 0.08 0.28 25

0.97

-

0.63

March 1989

110 0.10 0.21 0.56 26

1.12

-

1.49

300 0.26 0.25 0.81 29

1.52

-

4.74

550 0.40 0.29 0.77 27

1.68

-

5.25

1,420 0.76 0.31 0.66 21

2.07

-

5.67

Weddell Sea (65° 20'S, 15° 27'W)b

50 0.79 0.25 0.58 -

0.65

-

4.00

22 Dec. 1988

200 0.90 0.31 0.84 -

0.52

-

5.50

600 1.05 0.25 0.81 -

0.70

-

5.80

1,000 1.41 0.18 0.78 -

1.00

-

6.25

73° 18'S, 39° 59'W (close to the Filchner Shelf)

50 1.92 0.36 0.60 29

1.95

5.80

4.70

25 Jan.1989

200 4.17 0.25 0.64 27

2.05

7.30

5.90

400 2.63 0.25 0.65 32

2.15

7.20

6.95

800 4.06 0.16 0.60 27

2.60

7.05

5.95

Weddell Sea (ice-covered, 60° 59'S, 49° 03'W)c

10 - - 0.56 -

3.80

6.12

-

Dec. 1988

20 - - 0.53 -

3.54

6.46

9.17

80 - - 0.60 -

3.20

5.91

4.43

200 - - 0.60 -

4.91

6.01

11.6

300 - - 0.55 -

4.01

-

10.6

Scotia Sea (57° 03'S, 48° 51'W)

10 - - 0.17 -

2.44

8.52

6.27

Dec. 1988

30 - - 0.23 -

1.53

6.47

1.68

60 - - 0.43 -

1.48

7.50

1.83

100 - - 0.57 -

1.81

6.13

3.21

300 - - 0.89 -

1.81

7.67

7.34

Antarctic Confluence (59° 22'S,48° 44'W)

10 - - 0.41 -

4.69

4.77

4.74

Dec. 1988

30 - - 0.48 -

4.17

5.11

5.05

60 - - 0.55 -

2.80

-

5.20

100 - - 0.64 -

2.91

5.45

6.73

300 - - 0.61 -

4.47

4.60

9.48

Depth (m) Fe Mn Cd Co Cu Ni Zn Sampling period

Remote Pacific waters of the Southern Ocean (63° 59'S,89° 31'W)d

50 100 200 400 800

25 April 1995

50 100 200 400 800

18 April 1995

a Martin et al. (1990) b Westerlund and Öhman (1991a, b) c Nolting and de Baar (1994) d de Baar et al. (1999)

1997; de Baar et al. 1999; Table 9). Only over or near continental and Antarctic island shelves are the concentrations of dissolved Fe (e.g. Martin et al. 1990; Westerlund and Öhman 1991a; Löscher et al. 1997; Fitzwater et al. 2000; Grotti et al. 2001; Table 10) in the same range as those usually reported for other seas (e.g. Martin and Gordon 1988). Studies on relationships between concentrations of bioavailable Fe and those of nutrients generally show that Fe influences phytoplankton uptake of N, P and Si. Waters with higher biomasses and statistically significant decreases of nutrient concentrations are usually more affected by Fe derived from continental sources. Tables 9 and 10 specify the water sampling period, as the effects of sea-ice melting and phytoplankton activity must be considered when comparing concentrations of dissolved Fe in surface waters. When the sea-ice cover disappears, possible atmospheric inputs of soluble lithophile elements are removed by phy-toplankton and incorporated into organic particulate matter. In surface waters from the Terra Nova Bay shelf, for instance, Grotti et al. (2001) found that the dynamics of metal concentrations followed those of primary production, which was characterised by algal blooms in the second half of December and in late summer. In the absence of water-column stratification, the profile of dissolved Fe and Mn in intermediate and deep waters was rather constant until January; then, the progressive removal of these metals by sinking materials determined a decrease in concentrations.

Table 10. Typical concentrations of dissolved trace metals (nmol l-1; Co, pmol l-1) at different depths in neritic waters of the Southern Ocean

Depth (m) Fe Mn Cd Co Cu Ni Zn Sampling period

Gerlache Strait (64° 55'S, 63° 19'W)a

15

7.4G

5.G5

G.56

82

2.24 -

4.9G

5G

4.7G

4.19

G.6G

59

2.G9 -

5.1G

2GG

6.85

3.86

G.7G

82

2.16 -

5.9G

Gerlache Inlet (Terra Nova Bay, 74° 43'S, 164°

11'E)b

4

1.5

123

-

-

7.1 -

-

5G

1.3

2.2G

-

-

2.8 -

-

1GG

1.1

G.86

-

-

2.7 -

-

38G

1.2

1.G4

-

-

4.3 -

-

2

1.G

G.31

-

-

1.1 -

-

3G

G.6

G.41

-

-

1.G -

-

11G

G.8

G.85

-

-

1.3 -

-

38G

G.6

G.74

-

-

G.8 -

-

4

4.1

4.G5

-

-

11.6 -

-

25

1.6

2.G2

-

-

5.G -

-

1GG

3.3

1.33

-

-

2.6 -

-

38G

1.8

1.15

-

-

1.1 -

24 Nov. 1997

7 Jan.1998

5G 1GG 2GG 3GG

23 Jan.1989

12

1.26

5.42

G.71

Jan.1990

28

2.15

6.75

4.92

26

2.G8

6.66

5.G1

38

2.16

6.78

5.1G

26

1.92

6.23

2.95

Jan.1990

26

1.91

6.3G

4.GG

25

2.1G

6.G8

5.2G

22

2.21

6.G8

5.26

a Martin et al. (199G) b Grotti et al. (2GG1) c Westerlund and Öhman (1991a) d Fitzwater et al. (2GGG)

Although open-ocean waters receive very few lithogenic materials from melting ice and icebergs or remobilised marine sediments, de Baar et al. (1995) suggest that increased productivity along the Polar Front downcurrent of the Drake Passage was due to the upwelling of continentally derived Fe. Hiscock et al. (2003) suggest that a winter recharge of upwelled Fe-rich Upper Circumpolar Deep Water (UCDW) within the Antarctic and Southern Antarctic Circumpolar Current (ACC) zones provides enough Fe to support a diatom bloom. This bloom annually propagates polewards to the Southern Boundary of the ACC. However, other scientists such as Measures and Vink (1999) sustain that upwelled water usually has insufficient Fe to match upwelled N species, and external sources of Fe are required for the full utilisation of nutrients. Aeolian dust is the only external source of lithogenic elements in open-ocean waters, but the current atmospheric dust load over the Southern Ocean is one of the lowest in the world (Duce and Tindale 1991).

Aluminium is one of the most valuable tracers of soil and rock dust input in ocean surface waters because it is a relatively invariant component (about 7-8 %) of the Earth' crust. It has a relatively short residence time on the ocean surface (3-5 years), and is removed from the water column with little biological recycling. Average concentrations of dissolved Al in the Weddell Sea (about 3 nmol l-1; Moran et al. 1992; Sanudo-Wilhelmy et al. 2002) are about 20 times lower than those measured in equatorial Atlantic waters by Vink and Measures (2001). Like those of Al and Fe, concentrations of dissolved Mn in surface waters from the open Southern Ocean (Table 9) are often lower than those usually reported for other seas, especially in the Northern Hemisphere where continents are much more extensive. Based on the low concentrations of dissolved Mn (0.08 nmol l-1) measured in surface waters from the South Drake Passage, Martin et al. (1990) suggest that Mn deficiency may be another factor contributing to the limitation of phytoplankton growth in the Southern Ocean.

Concentrations of dissolved Al, Fe and Mn are usually much lower than those in suspended matter; Westerlund and Ohman (1991a) found that par-ticulate Fe/Al and Fe/Mn ratios in the Weddell Sea, compared to ratios in the Pacific Ocean, were much closer to the terrestrial ratio. On the Ross Sea shelf, below a depth of 125 m, Fitzwater et al. (2000) found that particulate Fe/Al and Mn/Al ratios (0.40 and 0.020 respectively) were higher than average upper crustal ratios. Like in other oxygenated shelf areas, the enrichment of the two metals was probably due to the re-suspension of shelf sediments containing Fe and Mn oxides.

Was this article helpful?

0 0

Post a comment