Geology Reference
In-Depth Information
micrometeorites or cosmic spherules. The composition is either magnetite
or a silicate matrix including magnetite. They are ubiquitous but scarce,
with relative contributions to the sediments decreasing with an increase in
sedimentation rate. Finally, there are anthropogenic components, notably
heavy metals and Sn, which can have a significant influence on sediments
in coastal environments.
As noted in Section 4.1.1, the principal modes of transport of partic-
ulate material to the ocean are by rivers or via the atmosphere. Within
the oceans, distribution is further affected by ice rafting, turbidity
currents, organisms and oceanic currents. Turbidity currents refer to
the turbid and turbulent flow of sediment-laden waters along the sea-
floor caused by sediment slumping. They are especially important in
submarine canyons and can transport copious amounts of material,
including coarse-grained sediments, to the deep sea. Ice rafting can also
transport substances to the deep sea. Although, ice rafting is presently
confined to the polar latitudes (401N and 551S), there have been con-
siderable variations in ice limits within the geologic record. Organisms
are notable not only for biogenous sedimentation, but also they can
influence fine-grained lithogenous material that becomes incorporated
into faecal pellets consequently accelerating the settling rate. Ocean
currents are important for the distribution of material with a long
residence time. Major surface currents are zonal and tend to reinforce
the pattern of aeolian supply. On the other hand, deepwater currents are
of little consequence as velocities are slow relative to the settling rates.
4.3.2 Surface Chemistry of Particles
4.3.2.1 Surface Charge. Parcles in seawater tend to exhibit a negative
surface charge. There are several mechanisms by which this might arise.
Firstly, the negative charge can result from crystal defects (i.e., vacant
cation positions) or cation substitution. Clay minerals are layered
structures of octahedral AlO
6
and tetrahedral SiO
4
. Either substitution
of Mg(II) and Fe(II) for the Al(III) in octahedral sites or replacement of
Si(IV) in tetrahedral location by Al(III) can cause a net negative charge.
Secondly, a surface charge can result from the differential dissolution of
an electrolytic salt such as barite (BaSO
4
). A charge will develop
whenever the rate of dissolution of cations and anions differs. Thirdly,
organic material can be negatively charged due to the dissociation of
acidic functional groups.
Adsorption processes can also lead to the development of a negatively
charged particle surface. One example is the specific adsorption of
anionic organic compounds onto the surfaces of particles. Another
