5 Isotopic Composition of Seawater 113
60
t.O
20
0
60
~ ~0
e-(3- 2O
0
40
20
Atlantic ocean
~
Seawater
N-15
1"-'7
~
Metal-rich sediments
~
R ~ deep sea ctay
_N22Z2
Pacific ocean
i
N=23
R/ ' /
In~ian ocean
E ~
05120
0.5125
0.5130
l'-}Nd/..,-Nd
Fig. 5.16. Present day Nd isotopic composition in the oceans
The first study that looked at the Nd isotopic composition of the Pacific was
carried out by DePaolo and Wasserburg (1977) on phosphate fish bone splinters,
These yielded a ENd value of -9,2, Today we know however that the actual value
should lie between 0 and -5 and is therefore far higher than that determined by
DePaolo and Wasserburg. It is possible that these authors analyzed continental
detritus along with fish phosphate or that diagenetic processes had altered the
isotopic composition of the bone by exchanging with detritus. If this pioneering
study did indeed measure unaltered bone phosphate, then a further interesting
possibility presents itself: perhaps the fish grew up in the Atlantic ocean
incorporating therefore an Atlantic isotopic signature but had died in the Pacific,
O'Nions et at. (1978) were able to determine relatively homogeneous Nd
isotopic ratios from metal-rich sediments and ferromanganeses nodules from the
floor of the Pacific ocean (~Nd: -4.3 to - 1.6). Piepgras et al. (1979) then went on