Geology Reference
In-Depth Information
relative abundance of 5 Pt:1 Pd agreeing with a factor of five difference
in ß
4
. Pt is enriched in ferromanganese nodules following oxidation to a
stable (IV) state, behaviour not observed for Pd. Rh exists predomi-
nantly in the heptavalent state, but accumulates in reducing sediments as
lower valence sulfides. Au and Ag are present predominantly in solution
as monovalent forms. Ag, but not Au, accumulates in anoxic coastal
sediments.
4.3 SUSPENDED PARTICLES AND MARINE SEDIMENTS
4.3.1 Description of Sediments and Sedimentary Components
The sediments represent the major sink for material in the oceans. The
main pathway to the sediments is the deposition of suspended particles.
Such particles may be only in transit through the ocean from a conti-
nental origin or be formed in situ by chemical and biological processes.
Sinking particles can scavenge material from solution. Accordingly, this
section introduces the components found in marine sediments, but
emphasises processes that occur within the water column that lead to
the formation and alteration of the deposited material.
Marine sediments cover the ocean floor to a thickness averaging
500m. The deposition rates vary with topography. The rate may be
several millimetres per year in nearshore shelf regions, but is only from
0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal
material is formed along spreading ridges and moves outwards eventu-
ally to be lost in subduction zones, the major trenches in the ocean.
Because of this continual movement, the sediments on the seafloor are
no older than Jurassic in age, about 166 million years.
The formation of marine sediments depends upon chemical, biolog-
ical, geological and physical influences. There are four distinct processes
that can be readily identified. Firstly, the source of the material obvi-
ously is important. This is usually the basis for classifying sediment
components and will be considered below in more detail. Secondly, the
material and its distribution on the ocean floor are influenced by its
transportation history, both to and within the ocean. Thirdly, there is
the deposition process that must include particle formation and altera-
tion in the water column. Finally, the sediments may be altered after
deposition, a process known as diagenesis. Of particular importance are
reactions leading to changes in the redox state of the sediments.
The components in marine sediments are classified according to
origin. Examples are given in Table 6. Lithogenous (or terrigenous)
material comes from the continents as a result of weathering processes.
