Geology Reference
In-Depth Information
secondary material may be formed as components of continental or
volcanic origin become altered by low-temperature reactions in seawa-
ter, a mechanism known as halmyrolysis. Halmyrolysis reactions can
occur in the estuarine environment, being essentially an extension of
chemical weathering of lithogenous components. Such processes con-
tinue at the sediment-water interface. Accordingly, there are consider-
able overlaps between the terms weathering, halmyrolysis and
diagenesis. Owing to the importance that surface chemistry has on the
final composition, authigenic precipitation and halmyrolysis are con-
sidered further in Section 4.3.2.
Biogenous (or biotic) material is produced by the fixation of mineral
phases by marine organisms. The most important phases are calcite and
opaline silica, although aragonite and magnesian calcite are also deposited.
As indicated in Table 7, several plants and animals are involved, but the
planktonic organisms are the most important with respect to the World
Ocean. The source strength depends upon the biological species compo-
sition and productivity of the overlying oceanic waters. For instance,
siliceous oozes are found in polar latitudes (diatoms) and along the equator
(radiolaria). The relative contribution of biogenous material to the sedim-
ents depends upon its dilution by material from other sources and the
extent to which the material can be dissolved in seawater. As noted
previously, both calcareous and siliceous skeletons are subject to consid-
erable dissolution in the water column and at the sediment-water interface.
There are two sources that give rise to minor components in the marine
sediments. Cosmogenous material is that derived from an extra-terrestrial
source. Such material tends to comprise small (i.e.,
o
0.5 mm) black
Table 7 Quantitatively important plants and animals that secrete calcite, arag-
onite, Mg-calcite, and opaline silica
Mineral
Plants
Animals
Coccolithophorids
a
Foraminifera
a
Molluscs
Bryozoans
Calcite
Aragonite
Green algae
Molluscs
Corals
Pteropods
a
Bryozoans
Mg-calcite
Coralline (red) algae
Benthic foraminifera
Echinoderms
Serpulids (tubes)
Diatoms
a
Radiolaria
a
Sponges
Opaline silica
a
Planktonic organisms
Source: adapted from Berner and Berner, 1987.
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