Chemistry Reference
In-Depth Information
mechanistic understanding of the individual effects is therefore needed in order to derive
meaningful mathematical representations of the net effect of climate on the global H
4
SiO
4
weathering flux. This understanding can be obtained through laboratory experimentation
and observations at the catchment level.
Preservation of biogenic silica
The high productivity of diatoms in the oceans requires a very efficient dissolution
of diatom frustules after the organisms die, in order to regenerate H
4
SiO
4
. The high
dissolution efficiency, in turn, is due to the high degrees of undersaturation of seawater
with respect to biogenic silica. This is illustrated in Figure 5, which shows silica
solubilities measured experimentally on fresh phytoplankton, cultured diatoms and
sinking particulate matter collected at different depths of the ocean with so-called
sediment traps. The general increasing tendency with depth reflects a moderate effect of
pressure on the solubility of amorphous silica (Dixit et al. 2001). The measured
Figure 5.
Silica solubilities in seawater, at 4°C. The data points are experimental solubilities measured
in flow-through reactors on sediment trap samples (Gallinari 2002). The pressure correction proposed
by Dixit et al. (2001) was used to account for differences in water depth. Also shown at zero depth is
the range of silica solubilities reported for diatom cultures and open ocean siliceous plankton (Dixit et
al. 2001). The thick broken lines are the pressure-corrected solubility of synthetic silica gel and the
lower limit for plankton and diatom cultures. The thin broken line is the average biogenic silica
solubility as a function of depth. The available data show that solubilities of siliceous materials sinking
through the water column are consistent with solubilities measured on fresh diatoms and phytoplankton.
