Geology Reference
In-Depth Information
tribution of pelagic sediments on the sea bottoms is
controlled by the different dissolution of biogenic
grains, produced in the surface waters of the oceans.
Most dissolution of siliceous tests (radiolaria, diatoms)
occurs in highly silica-undersaturated shallow surface
waters.
In contrast, most dissolution of carbonate tests takes
place on the sea floor as bottom waters become more
undersaturated in calcium carbonate. Therefore, sili-
ceous biogenic sediments reflect the biological fertil-
ity of the surface waters, whereas carbonate biogenic
deep-sea sediments chiefly reflect the preservation po-
tential of carbonate at various depths.
The proportions of siliceous and carbonate-rich
oozes depend on bottom-water circulation which con-
trols both dissolution patterns, and, through upwelling,
fertility patterns (Berger 1970, 1974, 1976). Differences
in the composition of pelagic calcareous plankton re-
flect differences in oligotrophic, mesotrophic and
eutrophic nutrient environments (Henrich et al. 2001).
2.4.5.5 Carbonate Plankton and Carbonate
Oozes
Carbonate oozes cover about 50% of the modern ocean
floor. They are deposited on deep-sea plains, subma-
rine rises, ridges and plateaus. Modern carbonate oozes
consisting of the shells of minute drifting planktonic
organisms comprise foraminiferal ooze, nannofossil
ooze and pteropod ooze: Foraminiferal oozes are domi-
nated by tests of sand-sized planktonic foraminifera (Pl.
73/10) occurring within carbonate mud consisting of
calcareous nannofossils. Nannofossil oozes or cocco-
lith oozes are very fine-grained muds composed pre-
dominantly of remains of planktonic algae (Chryso-
phyta: Coccolithophorida; Pl. 7/3-5). Pteropod oozes
are restricted o shallow tropical areas in depths
< 3 000 m. They consist of aragonitic shells of plank-
tonic gastropods (pteropods and heteropods). The car-
bonate production is greatest in surface waters with high
biological productivity caused by upwelling. Produc-
tion and growh rates of pelagic foraminifera and coc-
colithophorids are differnt.
2.4.5.4 Resedimentation (Allochthonous
Carbonates)
Plankton contributes to the sedimentary record by
(1) settling of individual tests, (2) fecal pelletization,
and (3) the deposition of various biogenic materials (in-
cluding 'marine snow', aggregates, flocks and particu-
late organic matter, POM). Individual settling may play
a role in the deposition of larger microfossils (e.g. fora-
minifera, radiolarians), but fecal pelletization and sea-
sonally varying sinks of detrital aggregates (e.g. phy-
todetritus displaying an important food resource in the
deep sea, Thiel et al. 1988) play the greatest role in the
transportation of smaller tests (e.g. coccolithophorids;
Honjo 1976).
Downslope gravity transport along deep-marine slopes
occurs by various processes including rockfall, slump-
ing, sliding, and sediment gravity flows.
Subaqueous rockfalls occur in areas with very steep
topography producing talus and scree deposits at the
base of slopes.
A slump is a downslope translocation of a sediment
parcel along discrete shear planes. In slides, large blocks
move on only a few slippage planes. Both slumping
and sliding are controlled and triggered by high rates
of sedimentation creating oversteep slopes and differ-
ential compaction, structural changes (e.g. effect of salt
domes) as well as by earthquakes. In sediment gravity
flows, the sediment is transported downwards under
the influence of gravity.
The sediments are transported by different mecha-
nisms: In 'grain flows', the sediment is supported dur-
ing transport by direct grain to grain interactions. In
'debris flows' larger grains are supported by a matrix
consisting of a mixture of fine sediment and interstitial
fluid.
In 'turbidity currents', the sediment is supported
mainly by the upward component of fluid turbulence.
The resulting deposits differ in grain size, sorting, tex-
ture and sedimentary structures. Because turbidity cur-
rents are episodic, the resulting turbidites are interbed-
ded with finer pelagic sediments representing the back-
ground sedimentation.
-> A cautionary note: Comparable calcareous plank-
ton is not known from pre-Mesozoic records and com-
parisons with modern analogues work for pelagic Me-
sozoic-Tertiary carbonates, but not for Paleozoic slope
and basin carbonates (see Sect.15.8).
2.4.5.6 Preservation Potential and Dissolution
Levels
In modern oeans carbonate sediments are largely
absent below depth of about 4500 m. High hydrostatic
pressure, low water temperature and high partial pres-
sure of CO
2
lead to dissolution and significant difer-
ences in the carbonate preservation potential of deep-
sa sediments. These differences have been used to dis-
tinguish two levels:
