Fig. 6.17 Depth distribution of dissolved Ca2+ and Mg2+ concentrations in sediment pore waters. After Gieskes and Lawrence (1981).

This reaction shows that reverse weathering is exactly opposite to continental weathering reactions, which consume CO2 and liberate HCO- (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 X 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 sediment, particularly fine-grained basaltic volcanic ash. Pore water concentrations of Ca2+, Mg2+ and K+ in deep-sea cores show removal of Mg2+ (and, to a lesser extent, K+) from pore water, mirrored by increases in Ca2+ pore water concentration (Fig. 6.17). These results suggest that basaltic ash is converted to Mg2+ and K+ clay minerals, accompanied by the release of Ca2+ to pore water. The quantitative importance of this mechanism on a global scale is probably small, but good data are sparse.

Degraded aluminosilicate( s) + K +aq) + HCO3(aq) + H4SiO4(aq) ^ K aluminosilicate(s) + CO2(g) + H2O( l)

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