An Introduction to Environmental Chemistry

eqn. 6.19

This reaction shows that reverse weathering is exactly opposite to continental
weathering reactions, which consume CO 2 and liberate HCO 3 - (see Section 4.4.3).
Experimental studies with Amazon delta sediments in the mid 1990s have provided
the first evidence that reverse weathering reactions occur naturally. The
K+sink in Amazon continental shelf sediments alone is calculated to be 6.8¥
1012 gyr-^1 , representing about 10% of the annual global K+river flux to the oceans.
Some removal of ions from seawater occurs through permanent burial in
sediment pore water. The total removal of major ions by this process is small,
less than 2% of the river input for all elements except Na+and Cl-. The burial
flux may be significant for Na+and Cl-(20–30% of the river flux), but the data
are uncertain.
Seawater buried in marine sediment may react with components of the sedi-
ment, particularly fine-grained basaltic volcanic ash. Pore water concentrations
of Ca^2 +, Mg^2 +and K+in deep-sea cores show removal of Mg^2 +(and, to a lesser
extent, K+) from pore water, mirrored by increases in Ca^2 +pore water concen-
tration (Fig. 6.17). These results suggest that basaltic ash is converted to Mg^2 +
and K+clay minerals, accompanied by the release of Ca^2 +to pore water. The quan-
titative importance of this mechanism on a global scale is probably small, but
good data are sparse.

De K HCO H SiO

CO H O

aq aq aq

gl

graded aluminosilicate

K aluminosilicate

s

s

() ()+ -() ()

() () ()

++ +

Æ++

344

22

The Oceans 215

Concentration (mmol l–1)

00

100

200

300

400

500

20 40 60 80

Mg2+

Depth (m) Ca2+

Fig. 6.17Depth distribution of dissolved Ca^2 +and Mg^2 +concentrations in sediment pore
waters. After Gieskes and Lawrence (1981).