3 Isotope Geochemistry of River Water 43
As has been common in such studies, the work of Goldstein and Jacobsen
considered only the influence of dissolved elements in river water when
estimating the marine element budget. In actual fact, some studies have shown
that the marine budget may also be influenced by the suspended load transported
into the oceans by the rivers. We have already discussed that particular matter
carried by the rivers is significantly more enriched in Sm and Nd than filtered
river water.
Table 3.1. Concentrations of the REE in rivers and seawater. (data from Goldstein and
Jacobsen 1988)
sample
La
Ce
Nd
Sm
Eu
Dy_
El"
yl~
susp. load~
39.6 80.9 36.4 6.9
1.43 4.18 1.98 1.6
diss. load'30.8 64.5 40.9 10.8 2.66 1 1 . 5 8.46 6.1
eff. diss. load-'
8.3
25.8 1 6 . 4 4.97 1.04 5.41 5.16 3.5
seawater2
5.3
1.8
3.5
0.65 0.16 1.2
1.2
1.2
in ppm: -' in ppt;
Leaching experiments show that large amounts of REE and Sr are to be found
adsorbed onto particle surfaces or integrated in HCI soluble mineral phases
(Ntigler et al. 1993). Before leaching, the particles have a 87Sr/86Sr ratio of
0.72025 and a Sr concentration of 135 ppm (SL1, Fig. 3.11). After leaching in 1N
strength HCI, the leachate has a Sr isotopic composition of 0.71041 and a Sr
concentration of 410 ppm (LI). This experiment clearly demonstrates that a large
portion of the Sr is situated in exchangeable sites and/or associated with a soluble
phase. The Sr isotopic composition of the leachate is similar to that for the filtered
river water, which gives 0.71026. The dissolved Sr could possibly represent
adsorbed river water, a mixture of this with ancient marine carbonate Sr, or other
Sr-rich components such as phosphates. The particle residue (R) must, according
to mass balance, lie on the straight line. having a more radiogenic signature and a
lower Sr concentration than the untreated particle fraction.
The exchange behavior of the elements adsorbed onto the particles with
elements from the surrounding fluid is made clearer by the following experiment:
Particles in suspension were dried in a beaker, then mixed with seawater and
following that subjected to the same already described experimental procedures.
The non-leached particles now showed a STSr/S6Sr ratio of 0.71825 and Sr
concentration of 154 ppm (SL2). Obviously, exchange with seawater, which had a
87Sr/86Sr ratio of 0.709034, must have served to decrease the Sr isotopic
composition of these particles. The isotopic composition of the leachate (L2) was
that of the seawater. Similar exchange behaviour can be observed with Nd. The
REE are seven times more concentrated at exchangeable sites than within the
particle itself. It is to be expected that these sorts of exchange processes on
particle surfaces must lead to fractionation in seawater.
As has been common in such studies, the work of Goldstein and Jacobsen
considered only the influence of dissolved elements in river water when
estimating the marine element budget. In actual fact, some studies have shown
that the marine budget may also be influenced by the suspended load transported
into the oceans by the rivers. We have already discussed that particular matter
carried by the rivers is significantly more enriched in Sm and Nd than filtered
river water.
Table 3.1. Concentrations of the REE in rivers and seawater. (data from Goldstein and
Jacobsen 1988)
sample
La
Ce
Nd
Sm
Eu
Dy_
El"
yl~
susp. load~
39.6 80.9 36.4 6.9
1.43 4.18 1.98 1.6
diss. load'30.8 64.5 40.9 10.8 2.66 1 1 . 5 8.46 6.1
eff. diss. load-'
8.3
25.8 1 6 . 4 4.97 1.04 5.41 5.16 3.5
seawater2
5.3
1.8
3.5
0.65 0.16 1.2
1.2
1.2
in ppm: -' in ppt;
Leaching experiments show that large amounts of REE and Sr are to be found
adsorbed onto particle surfaces or integrated in HCI soluble mineral phases
(Ntigler et al. 1993). Before leaching, the particles have a 87Sr/86Sr ratio of
0.72025 and a Sr concentration of 135 ppm (SL1, Fig. 3.11). After leaching in 1N
strength HCI, the leachate has a Sr isotopic composition of 0.71041 and a Sr
concentration of 410 ppm (LI). This experiment clearly demonstrates that a large
portion of the Sr is situated in exchangeable sites and/or associated with a soluble
phase. The Sr isotopic composition of the leachate is similar to that for the filtered
river water, which gives 0.71026. The dissolved Sr could possibly represent
adsorbed river water, a mixture of this with ancient marine carbonate Sr, or other
Sr-rich components such as phosphates. The particle residue (R) must, according
to mass balance, lie on the straight line. having a more radiogenic signature and a
lower Sr concentration than the untreated particle fraction.
The exchange behavior of the elements adsorbed onto the particles with
elements from the surrounding fluid is made clearer by the following experiment:
Particles in suspension were dried in a beaker, then mixed with seawater and
following that subjected to the same already described experimental procedures.
The non-leached particles now showed a STSr/S6Sr ratio of 0.71825 and Sr
concentration of 154 ppm (SL2). Obviously, exchange with seawater, which had a
87Sr/86Sr ratio of 0.709034, must have served to decrease the Sr isotopic
composition of these particles. The isotopic composition of the leachate (L2) was
that of the seawater. Similar exchange behaviour can be observed with Nd. The
REE are seven times more concentrated at exchangeable sites than within the
particle itself. It is to be expected that these sorts of exchange processes on
particle surfaces must lead to fractionation in seawater.