74 SEDIMENTARY ROCKS/Deep Ocean Pelagic Oozes coccolithophores, pteropods, diatoms, and radiolaria 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 produced 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, calcareous oozes, the composition of which is dominated 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 calcareous 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 (termed ‘foraminiferal ooze’) (Figure 4) or by the remains of planktonic plants (coccolithophores; termed ‘nannofossil ooze’) In either type of calcareous ooze, the other component will often be the second most important 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 minerals, detrital carbonate, zeolites, volcanic glass, and cosmic spherules Minor biogenic components may include benthonic (bottom-dwelling) foraminifera, ostracods, echinoid remains, radiolaria, silicoflagellates, 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 carbonate 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 overwhelmingly 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 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