The chemistry of natural waters - Geochemistry: An Introduction - page 153

δ

13 C (per mil)

0 1 2

0

1

13 C

PO 4

δ

2

3

4

Bottom

5

0 1 2 3

PO 4 (mmol kg − 1 )

Figure 7.13

13 C isotopic composition of carbon
dissolved in seawater by depth. Surface biological activity depletes the water in nutrients and in
12 C. Dissolution of dead debris and biological excreta below the thermocline produces the
opposite effect. When primary organic productivity at the surface increases, these contrasts
between surface water and deep water also intensify (after Broecker, 1994 ) .

Characteristic distribution of a nutrient (phosphate) and of the δ

40 14

NO 3

35

12

30

10

25

8

20

6

δ

15 N

15

4

10

2

5

0 0

0 1000 2000 3000 4000 5000 6000

Depth (m)

Figure 7.14

15 N content in the waters of the Southern Ocean: intense surface
biological activity depletes the dissolved nitrate and, as with carbon ( Fig. 7.13 ), preferentially
removes the light isotope, here 14 N(afterSigman et al. , 1999 ).

Variation in nitrate and δ