Geology Reference
In-Depth Information
regenerated at the O
2
minimum. Elements (i.e., Zn) associated with the
skeletal material may exhibit a smoothly increasing trend with depth.
The third case pertains to elements, notably Mn, that have a substantial
input from hydrothermal waters. These are released into oceanic waters
from spreading ridges. Ocean topography is such once these waters are
advected away from such regions towards the abyssal plains, they are
then found at some intermediate depth. Bottom enrichment is observed
for elements (Mn) that are remobilised from marine sediments. The
behaviour of Al combine features outlined above, resulting in a mid-
depth minimum concentration. Surface enrichment evident in mid (411
N) but not high (
601 N) latitudes in the North Atlantic results from
the solubilisation of aeolian material. Removal occurs via scavenging
and incorporation into siliceous skeletal material. Subsequent regener-
ation by shell dissolution increases deepwater Al levels.
B
4.2.5 Physico-Chemical Speciation
Physico-chemical speciation refers to the various physical and chemical
forms in which an element may exist in the system. In oceanic waters, it
is difficult to determine chemical species directly. Whereas some indi-
vidual species can be analysed, others can only be inferred from ther-
modynamic equilibrium models as exemplified by the speciation of
carbonic acid in Figure 9. Often an element is fractionated into various
forms that behave similarly under a given physical (e.g., filtration) or
chemical (e.g., ion exchange) operation. The resulting partition of the
element is highly dependent upon the procedure utilised, and so known
as operationally defined. In the following discussion, speciation will be
exemplified with respect to size distribution, complexation characteris-
tics, redox behaviour and methylation reactions.
Physico-chemical speciation determines the environmental mobility of
an element, especially with respect to partitioning between the water and
sediment reservoirs. The influence can be manifested through various
mechanisms as summarised in Figure 13. Settling velocities, and by
implication the residence time, are controlled by the size of the particle.
Thus, dissolved to particulate interactions involving adsorption, precip-
itation or biological uptake can readily remove a constituent from the
water column. The redox state can have a comparable influence. Mn and
Fe are reductively remobilised in sediments. Following their release in
reduced forms from either hydrothermal sources or interstitial waters,
they rapidly undergo oxidation to form colloids, which then are quickly
removed from the water column. Speciation also determines the bio-
availability of an element for marine organisms. It is generally accepted
