Definition ocean Sediments• Ocean sediments are unconsolidated organic and inorganic particles that accumulate on the ocean floor.• Ocean sediments originate from numerous sources – weathering and erosion of the continents (terrigenous/lithogenous) – volcanic eruptions (volcanogenous) included in terrigenous sediments. – biological activity (biogenous) – chemical processes within the oceanic crust and seawater (Hydrogenous/autigenous) – impacts of extra-terrestrial objects (cosmogenous)
4-2Sedimentation in the Ocean Deep-sea Sedimentation has two main sources of sediment: external- terrigenous material from the land and internal-biogenic and authigenic from the sea. Sedimentation in the Deep Sea
Grain size and current velocity affect the deposition and erosion of sediment. – Smallest and largest particles behave similarly with respect to transportation and erosion. – Sand in the middle of the graph takes the least amount of energy to erode. – Larger particles require more energy to erode because they’re heavy. It takes a stronger current to lift them off the bottom. – Particles smaller than sand also take more energy to erode. Smaller Hjulstrom’s diagram particles (especially clay) tend to be cohesive.12 - 5
Classification• 1. Clasification by origin – a. Terrigenous - erosional products (also volcanics) composed of fragments of pre-existing rock material – b. Biogenous - composed of hard remains of once- living organisms. shells – c. Hydrogenous - formed when dissolved materials come out of solution (precipitate) (in situ precipitation). Desolved materials form as a result weathering – d. Cosmogenous - extraterrestrial (derived from outer space)
Percentage of Sediment type in the Ocean % of all oceanSediment type floor coveredTerrigenous ～ 45%Biogenous ～ 55%Hydrogenous <1%(authigenic)Cosmogenous very small amount
Classified by size according to the Wentworth scale• 2. Clasification based on size – a. Gravel (pebbles, cobbles) = > 2mm – b. Sand = 62 µm - 2 mm – c. Silt = 4 - 62 µm – d. Clay = < 4 µm• Grain sizes are classified by using formula: Φ = -log2d Φ phi is Wentworth scale d = diametre of the grains
• 1. By Constituents – a. Pelagic sediments - open ocean, fine grained • clays & biogenic oozes – b. Hemipelagic - continental margin, coarser grained • muds
Major Sediment Input to the OceansSource Amount (109 tons/yr)Rivers 18.3Glaciers and ice sheets 2.0Wind blown dust 0.6Coastal erosion 0.25Volcanic debris 0.15Groundwater <0.48
Terrigenous (or Lithogenous Sediments):• Derived from weathering of rocks at orabove sea level (e.g., continents, islands)• Two distinct chemical compositions ferromagnesian, or iron-magnesium bearing minerals non-ferromagnesian minerals – e.g., quartz, feldspar, micas• Largest deposits on continental margins(less than 40% reach abyssal plains)• Transported by water, wind, gravity,and ice• Transported as dissolved andsuspended loads in rivers, waves,longshore currents
Sedimentation Processes on the Continental Shelf• Tides, waves, and currents strongly affect continental-shelf sedimentation. – Shoreline turbulence: waves are one of the most notable influences because it keeps particles from settling. Surf and waves carry small particles out to sea. Their affect diminishes further from shore.
• Sediments are also transported to the open- ocean by gravity-driven turbidity currents.• Dense slurries of suspended sediment moved as turbulent underflows• Typically initiated by storm activity or earthquakes• Initial flow often confined to submarine canyons of the continental shelf and slope• Form deep-sea fans where the mouth of the canyon opens onto the continental rise
• Sediment delivered to the open-ocean by Wind wind activity as Blown particulate matter Sand (dust) West• Primary dust source is Africa deserts in Asia and North Africa• Comprise much of the fine-grained deposits in remote open-ocean areas (red clays)• Volcanic eruptions contribute ash to the atmosphere which Pinatubo settles within the June oceans 1991
• Boulder to clay size particles also eroded and transported to oceans via glacial ice• Glacier termination in circum- polar oceans results in calving and iceberg formation• As ice (or icebergs) melt, entrained material is deposited on the ocean floor• Termed ice-rafted debris or diamictites.
Pelagic lithogenous sediments• Sources of fine material: – Volcanic ash (volcanic eruptions) – Wind-blown dust – Fine grained material transported by deep ocean currents - Abyssal clay (red clay) – Oxidized iron
Composition of Red Clay• Clay minerals: montmorillonite, illite, chlorite, kalonite, and mixed-layer derivatives• Lithogenous minerals: feldspar, pyroxene, quartz• Hydrogenous (or authigenic) minerals: zeolite and ferromanganese oxides and hydroxides.
Distribution of Clay MineralsThe clay mineral which are most abundant in deep sea clayare montmorillonite and illiteFig.8.8 Clay mineral distribution on the ocean floor. The map shows the dominant mineralin the fraction less than 2 ㎛ . Mixture indicates that no one clay mineral exceeds 50% of
Hemipelagic Sediments• Characteristic of the continental slope & rise• Muds carried across shelf by wave & tide energy as slightly dense plumes – extend out from slope at depth where denser water is encountered• Relatively fast sedimentation rate• Hemipelagic mud is generally gray or green from the presence of sulfides or magnetite
Biogenous Sediments:• composed primarily of marine microfossil remains• shells of one-celled plants and animals, skeletal fragments• median grain size typically less than 0.005 mm (i.e., silt or clay size particles)• characterized as CaCO3 (calcium carbonate) or SiO2 (silica) dominated systems• sediment with biogenic component less than 30% termed calcareous, siliceous clay• calcareous or siliceous oozes if biogenic component greater than 30%
planktonic foraminifera• Oozes consist of biogenous minterals: shells of planktonic foraminifera, radiolarians, coccolithophores, and diatoms.• About one half of the deep sea radiolarians floor is covered by oozes.• The most important factors controlling the composition of biogenous deep sea sediments are fertility and depth. coccolithophores• Fertility controls the supply of plankton remains, while depth controls the dissolution of carbonate (through pressure and water mass chemistry). diatoms
Controlling FactorsFig.8.4 Distribution of major facies in a depth-fertility frame, based on sedimentpatterns in the eastern central Pacific. Numbers are typical sedimentation rates inmm/1000 yr(which is the same as m/million yr). [Source as for Fig.8.2]
Calcareous oozes• Consist of foraminifera, coccolithophores and pteropods which cover ~50% of the ocean floor – distribution controlled largely by dissolution processes – cold, deep waters are undersaturated with respect to CaCO3 – deep water is slightly acidic as a result of elevated CO2 concentrations – solubility of CaCO3 also increases in colder water and at greater pressures – CaCO3 therefore readily dissolved at depth• level below which no CaCO3 is preserved is the carbonate compensation depth• typically occurs at a depth of 3000 to 4000 m
• Calcium carbonate dissolves better in colder water, in acidic water, and at higher pressures. In the deep ocean, all three of these conditions exist. Therefore, the dissolution rate of calcium carbonate increases greatly below the thermocline. This change in dissolution rate is called the lysocline. Below the lysocline, more and more calcium carbonate dissolves, until eventually, there is none left. The depth below which all calcium carbonate is dissolved is called the carbonate compensation depth or CCD.
Patterson (1542) showed adrastic increase of dissolutionrates below 3500 m in thecentral Pacific.
Dissolution patterns in the deep sea• The CCD is the particular depth level at any one place in the ocean where the rate of supply of calcium carbonate to the sea floor is balanced by the rate of dissolution, so that there is no net accumulation of carbonate. Generalized diagrams illustrating the relative position of calcite and aragonite• ACD (Aragonite solubility profiles in the modern tropical Compensation Depth) ocean and the variation in temperature with depth. The major zones of• CCD (Calcite digenesis are plotted to the right. Compensation Depth)
Lysocline• Another CCD-like level which can be mapped to describe dissolution patterns is the lysocline.• The concept of the lysocline was introduced to denote a contour- following boundary zone between well-preserved and poorly-preserved foraminiferal assemblages on the floor of the central Atlantic Ocean and on that of the South Pacific.• The lysocline marks the top of the Antarctic Bottom Water.
White Cliffs of Dover Formation of calcareous deposits • composed largely of foraminifera and coccolithophores http://en.wikipedia.org/wiki/White_cliffs_of_Dover
Carbonate Shelves• Carbonate sediments and reefs form in warm shallow water regions where the influx of terrigenous materials is low.
Plate stratigraphy• Developed at the mid-oceanic ridge• The axial rift valley is flank with hosts which covered by biogenic sediments• As the spreading continues the hosts subsides below the CCD the biogenicsediments are overlain by pelagic red clay.•The stratigraphy of the plate consists of Basalt at the bottom, and is overain bybiogenic sediments and finally red clay.
Siliceous Ooze• Distribution, production, and dissolution patterns of the siliceous deposits• Remains of diatoms, silicoflagellates and radiolarians, and sponge spicules, all of which are made of opal, a hydrated form of amorphous silicon oxide.• Diatom oozes are typical for high latitudes, diatom muds for pericontinental regions, and radiolarian oozes for equatorial areas.
The siliceous deposits typically occur in areas ofhigh fertility; that is, in regions of surface waterwith relatively high phosphate values.Fig.8.15 Flux of siliceous fossil to the sea floor.[W. H. Berger, J. C.Herguera, in P. G. Falkowski. A. D. Woodhead. eds, 1992, Primaryproductivity and biogeochemical cycles in the sea. Plenum Press, New
Siliceous ooze• Seawater undersaturated with silica• Siliceous ooze commonly associated with high biologic productivity in surface ocean
Distribution of neritic and pelagic marinesediments
Controlling factors• The formation of siliceous rocks is controlled by • the rate of production of siliceous organisms in the overlying waters • the degree of dilution by terrigenous, volcanic, and calcareous particles • the extent of dissolution of the siliceous skeletons
• There is a distinct negative correlation between silica and calcite distributional patters.• increasing fertility leads to decreasing preservation of calcite, but increasing accumulation of silica.• A similarly opposing trend is indicated for depth relationships, with silica corrosion being greatest in upper waters, that of carbonate being greatest at depth
Deep sea cherts• Silicified sediments cemented by cryptocrystalline and microcrystalline quartz• appears to proceed from mobilization and reprecipitation of opal, generating a disordered cristobalite (=fibrous quartz) which eventually alters toward a quartzitic rock with mostly quartz-replaced and quartz-filled fossils as diagenesis progresses.
Hydrogenous (or Authigenic) Sediments• produced by chemical processes in seawater• essentially solid chemical precipitates of several common forms• Non-biogenous carbonates – form in surface waters supersaturated with calcium carbonate – common forms include short aragonite crystals and oolites
• Phosphorites – phosphate crusts (containing greater than 30% P2O5) occurring as nodules – formed as large quantities of organic phosphorous settle to the ocean floor – unoxidized material is transformed to phosphorite deposits – found on continental shelf and upper slope in regions of high productivity
• Manganese nodules – surficial deposits of manganese, iron, copper, cobalt, and nickel – accumulate only in areas of low sedimentation rate (e.g., the Pacific) – develop extremely slowly (1 to 10 mm/million years)
• evaporites (salt deposits) – occur in regions of enhanced evaporation (e.g., Isolated seas, Red Sea. Persian Gulf and Dead Sea) – evaporative process Dead Sea Jordan, removes water and leaves a salty brine – Consist of gypsum, anhidrite, halite.
• The term evaporites is used for all deposits, such as salt deposits, mainly chemical sediments that are composed of minerals that precipitated from saline solutions concentrated by evaporation. Evaporite deposits are composed dominantly of varying proportions of halite (rock salt) (NaCl), anhydrite (CaSO4) and gypsum (CaSO4.2H2O). Evaporites may be classified as chlorides, sulfates or carbonates on the basis of their chemical composition (Tucker, 1991).
Cosmogenous Sediments:• sediments derived from extraterrestrial materials• includes micrometeorites and tektites• tektites result from collisions with extraterrestrial materials – fragments of earths crust melt and spray outward from impact crater – crustal material re-melts as it falls back through the atmosphere – forms glassy tektites