Chemistry Reference
In-Depth Information
Figure 6.
Asymptotic pore water concentration of silicic acid versus the detrital to opal ratio of the
sediment. The different symbols correspond to a variety of ocean basins (for detailed site description
and original references, see Dixit and Van Cappellen 2003). The observed inverse relationship is the
result of early diagenetic interactions between biogenic silica remains and aluminum solubilized from
detrital aluminosilicates.
ultimately result in the precipitation of new (authigenic) silicate minerals (Michalopoulos
and Aller 1995). Formation of authigenic silicates, a process also known as “reverse
weathering,” is a largely overlooked sink for reactive silica in the oceans. Whether
through Al incorporation into biogenic opal or via reverse weathering reactions, increased
aluminum availability in the oceans decreases the silica recycling efficiency.
The early diagenetic interactions between Al and biogenic silica explain the
observed inverse relationship between the asymptotic pore water concentration of silicic
acid and the %detrital:%opal ratio of marine sediments (Fig. 6). This relationship
provides a striking illustration of the strong coupling between the benthic cycles of silica
and aluminum. This coupling has potentially far-reaching consequences for our
understanding of regional patterns and global variability of biosiliceous production in the
oceans. In particular, the supply of detrital aluminosilicates mobilized by soil erosion and
mechanical rock weathering on land may modulate the preservation efficiency of
biogenic silica in the oceans and, therefore, by virtue of Equation (13), the biosiliceous
productivity of the oceans. It is important to note in this respect that the responses of
mechanical erosion and chemical weathering to changes in tectonic regime, climate or
vegetation may be very different. For instance, continental glaciation is likely to increase
mechanical erosion, but decrease chemical weathering.
In addition to the role of Al, biogenic silica surfaces age with time (Van Cappellen
1996). This aging further protects the biosiliceous fragments from dissolution and, hence,
enhances their preservation in the sedimentary record. Preliminary acid-base titrations of
freshly cultured diatom shells and older sedimentary biosilicious oozes suggest that aging
