$ Isotopic Composition of Seawater 115
r
3.5" lO-'2[t~t,,]
= c. 7100 years
where
WS
: mass of water in the oceans
CS
: concentration of an element in the ocean
FR
: flux of water from the continents
CR
: concentration of an element in this water
F(Nd) : flux of Nd from the continent.
Goldstein und Jacobsen's (1988) estimated Nd residence time of about 7100 years
is high in comparison with other calculations and does not seem to correspond
with what we already know about the isotopic inhomogeneity of Nd in the oceans:
7100 yr is two to four times the length of the estimated ocean mixing time. It is
possible that the concentration of 'effectively dissolved" load (16.4 ppt) was
highly underestimated (Goldstein and Jacobsen, 1988; Sect. 3.2). Goldstein und
O'Nions (1981) managed to calculate a residence time of 3100 yr and Piepgras et
al. (1979) derived a Nd residence time of about 400-2000 yr by applying the Na
residence time and the Na/Nd ratios in river waters and seawater:
L( Na / Nd)S J
where R stands for river water and S for seawater
Thus, the residence time for Nd is anyway much shorter than for Sr (= 3 - 5 x I0 ~
years). Let us try to derive the time necessary for two oceans (e.g. the Pacific and
the Atlantic ) to become thoroughly mixed. We need to start from the mass balance
equations for both oceans and assume steady state conditions. As the seawater Nd
isotopic ratio strongly reflects input from the continent let us assume for
simplicity's sake that the input from the mantle is negligibly small. The following
factors are important for the construction of the necessary equation system (after
Piepgras and Wasserburg 1980):
1) Input into ocean 1 (flux 1): FAI;
Input into ocean 2 (flux 2): FA2;
kg Nd/year
kg Nd/year
2) Sedimentation rate in Ocean I: FSI ; kg Nd/year
Sedimentation rate in Ocean 2: FS2; kg Nd/year
r
3.5" lO-'2[t~t,,]
= c. 7100 years
where
WS
: mass of water in the oceans
CS
: concentration of an element in the ocean
FR
: flux of water from the continents
CR
: concentration of an element in this water
F(Nd) : flux of Nd from the continent.
Goldstein und Jacobsen's (1988) estimated Nd residence time of about 7100 years
is high in comparison with other calculations and does not seem to correspond
with what we already know about the isotopic inhomogeneity of Nd in the oceans:
7100 yr is two to four times the length of the estimated ocean mixing time. It is
possible that the concentration of 'effectively dissolved" load (16.4 ppt) was
highly underestimated (Goldstein and Jacobsen, 1988; Sect. 3.2). Goldstein und
O'Nions (1981) managed to calculate a residence time of 3100 yr and Piepgras et
al. (1979) derived a Nd residence time of about 400-2000 yr by applying the Na
residence time and the Na/Nd ratios in river waters and seawater:
L( Na / Nd)S J
where R stands for river water and S for seawater
Thus, the residence time for Nd is anyway much shorter than for Sr (= 3 - 5 x I0 ~
years). Let us try to derive the time necessary for two oceans (e.g. the Pacific and
the Atlantic ) to become thoroughly mixed. We need to start from the mass balance
equations for both oceans and assume steady state conditions. As the seawater Nd
isotopic ratio strongly reflects input from the continent let us assume for
simplicity's sake that the input from the mantle is negligibly small. The following
factors are important for the construction of the necessary equation system (after
Piepgras and Wasserburg 1980):
1) Input into ocean 1 (flux 1): FAI;
Input into ocean 2 (flux 2): FA2;
kg Nd/year
kg Nd/year
2) Sedimentation rate in Ocean I: FSI ; kg Nd/year
Sedimentation rate in Ocean 2: FS2; kg Nd/year