Two main types are recognized, cal-careous oozes, the composition of which is domin-ated by the remains of calcareous plankton, and siliceous oozes, which are dominated by the remains of
Trang 1coccolithophores, pteropods, diatoms, and
radi-olaria In the upper water column, these remains are
biologically ‘packaged’ and ‘repackaged’ into larger
particles, which hastens their descent to the seafloor
(e.g., as faecal pellets or phytoplankton aggregates)
Indeed, most of the organic and skeletal matter
pro-duced in the euphotic zone is consumed and only a
fraction is exported, and a fraction of this reaches the
deep seafloor, where more is destroyed by dissolution
The distribution of biogenic oozes is strongly depth
controlled due to dissolution of calcium carbonate
with depth Two main types are recognized,
cal-careous oozes, the composition of which is
domin-ated by the remains of calcareous plankton, and
siliceous oozes, which are dominated by the remains
of siliceous plankton Siliceous oozes lithify into
radiolarites, diatomites, and cherts, whereas
calcar-eous oozes lithify into pelagic chalks and limestones,
and examples are well known from the geological
record, well-documented examples occurring in the
Troodos Massif, Cyprus, and the Ligurian Apennines
in Italy
Calcareous Oozes
Calcareous oozes may be dominated either by the
tests and test debris of planktonic foraminifera
(ter-med ‘foraminiferal ooze’) (Figure 4) or by the remains
of planktonic plants (coccolithophores; termed
‘nan-nofossil ooze’) In either type of calcareous ooze, the
other component will often be the second most
im-portant constituent In the modern world ocean,
50% of the seafloor is blanketed by foraminiferal
ooze (Table 1) Calcareous oozes commonly also
contain a terrigenous fraction (which may amount
to 10–15%), composed mainly of quartz and clay minerals, but may contain trace amounts of pyrite, iron and manganese precipitates, mica, chert, rock fragments, glauconite, feldspar, ferromanganese min-erals, detrital carbonate, zeolites, volcanic glass, and cosmic spherules Minor biogenic components may include benthonic (bottom-dwelling) foraminifera, ostracods, echinoid remains, radiolaria, silicoflagel-lates, diatoms, sponge spicules, pteropod shells and shell debris (in shallow water), phosphatic vertebrate remains and fish teeth
Pteropods (pelagic gastropods) are relatively common zooplankton, especially in warm-water latitudes, and some forms secrete delicate aragonitic shells Pteropod shells may range up to 30 mm in length, although most are in the range 0.3 to
10 mm Aragonite is unstable and dissolves as ocean waters become undersaturated in respect to carbon-ate with depth Consequently, pteropod-rich oozes are only found at depths shallower than 2500 m in the Atlantic Ocean and shallower than 1500 m in the Pacific Ocean
Foraminifera comprise a group of protozoans characterized by a test of one to many chambers composed of secreted calcite or agglutinated grains Test sizes are generally in the range 0.05–1 mm Forms with agglutinated tests are typically benthonic (bottom-dwelling) and make only a very minor contribution to pelagic sediments, which are over-whelmingly dominated by the remains of globular planktonic forms Modern species show clear latitudinal distribution patterns related to water temperature Oxygen isotope analysis of planktonic foraminifera tests can provide estimates for past
Figure 4 Illustration showing the three main types of pelagic sediments as seen under the microscope in plane polarized light Left: Calcareous ooze from the North Atlantic Ocean, comprising mainly planktonic foraminifer tests and test fragments The larger complete foraminifer tests are about 0.1 mm across Centre: Siliceous ooze from the South Atlantic Ocean, comprising mainly silica sponge spicules (tubular forms), radiolaria (high relief bell shaped and circular forms, right of centre), and broken centric diatom frustules (lower left and centre) Two planktonic foraminifera can be seen in the upper centre field The foraminifera are about 0.05 mm across Right: North east Atlantic Ocean pelagic red clay containing rhomboid dolomite crystals The red colour is due to the presence
of amorphous or poorly crystalline iron oxide minerals and grain coatings The largest dolomite rhomb (upper right) is about 0.01 mm across.
74 SEDIMENTARY ROCKS/Deep Ocean Pelagic Oozes