111••Distribution of sediments on the sea Distribution of sediments on the sea floorfloor••Seabed ResourcesSeabed ResourcesSedimentsare particles of organic or inorganic matte that accumulate in a loose, unconsolidated form. Record of geologic/oceanographic history••Types (Classification)Types (Classification)••Location or distribution of sedimentsLocation or distribution of sediments••Rates of Deposits/AccumulationRates of Deposits/AccumulationChapter 5 Chapter 5 --SedimentsSediments221. Sediments and volcanism are the most important agents of physical change on the deep-ocean floor2. Study of sediments is important to ocean’s chemistry, morphology and history as well as to Earth’s climate (paleoclimate) The position and nature of sediments provide important clues to Earth's recent history, and valuable resources can sometimes be recovered from them. The ages of portions of the ocean floor can be determined by studying core samples of sediments just above the basalt seabed,(see Fig. 5.26 for an example). The youngest sediments are foundnear the ridges and rises and the oldest close to the trenches.Study of Sediments is important to oceanography because: 33Sediment ClassificationSediment Classification••Particle Size (Grain Size)Particle Size (Grain Size)••Location (where the grains are deposited)Location (where the grains are deposited)••Source and ChemistrySource and Chemistry44Large (L)Medium (M)Small (S)Grain Size ClassificationGrain Size Classification55Sediments May Be Classified By Particle SizeSediments May Be Classified By Particle SizeThe velocities of currents The velocities of currents required for erosion, required for erosion, transportation, and transportation, and deposition deposition (sedimentation) of (sedimentation) of sediment particles of sediment particles of different sizes.different sizes.To dislodge and carry a To dislodge and carry a particle of size A, the particle of size A, the speed of a current must speed of a current must exceed 20 centimeters exceed 20 centimeters per second (8 inches per per second (8 inches per second). When the second). When the current falls below 1 current falls below 1 centimeter per second centimeter per second (1/2 inch per second), (1/2 inch per second), the particle will be the particle will be deposited.deposited.66Sediment can be classified by particle size. Waves and currents generally transport smaller particles farther than larger particles.How far sediments go horizontally and how long it takes to get to bottom of sea depends on size. Shape is also important to how sediments go around and settle in the bottom. LMS277Poorly SortedPoorly SortedWell SortedWell Sortedwell sorted: uniform grain sizepoorly sorted: variable grain size88Sediment Size ClassificationWentworth grain-size scale (Wentworth, 1922). 99Bluff ErosionOffshore Glacially Deposited Sand Ridges, Relict Ebb ShoalsSources of Sand For Littoral Transport 2 mTide Dominated &RiverineWave DominatedMixed EnergyGravelSandBarrier IslandCliff or Bluff Coast101011111212Maximum Amount of Material Derived From Bluff Erosion•Historic estimates 81,100 yd3/yr to 132,100 yd3/yr•The bluffs at Montauk Point are receding at 1 ft/yr •This recession rate has been well documented due to endangerment of the historic Montauk Light House constructed in 1796.31313•Analysis of the bluff composition and historic rates of recession have determined Montauk (Ronkonkoma Moraine) bluffs could not account for all of the material contained within the littoral system. •Based on sieve analysis data •63-percent of the size fraction (by weight) is similar in composition (fine to medium sand) to the barrier beaches to the west 1414•Littoral Transport reaches a maximum rate of 463,015 to 601,657 yd3/yr at Democrat Point (Fire Island Inlet) 15156 to 29 % of Longshore transport at Fire Island Inlet. 1616109,868 to 517,948 yd3/yr of sediment may be coming from offshore, however the exact mechanism for the material transport into the littoral zone has not been determined (Schwab et al., 1999) 1717Classification Based on Location (where sediments are found) Neritic: near continental margins & islands Pelagic: deep sea floor181841919Marine Sediments Are Usually Combinations of Terrigenous (from rocks) and Biogenous (organic) DepositsThe sediment of The sediment of continental shelves continental shelves is called is called neriticneriticsediment, and sediment, and contains mostly contains mostly terrigenousterrigenousmaterial.material.Sediments of the Sediments of the slope, rise, and slope, rise, and deepdeep--ocean floors ocean floors are are pelagicpelagicsediments, and sediments, and contain a greater contain a greater proportion of proportion of biogenousbiogenousmaterial.material.2020Key Turbidity currentsSubmarine canyonDeep-sea fansDistance from shelf edge50 150kmmiles050100Continental slopeContinental riseAbyssal plainContinental shelfSediment slump massesGraded beds of turbiditesUnderlying basaltic crust100 Storm windsSea levelWave baseSubstrate liquefied by wave activityTurbidity current2121Classification Based on Source & ChemistryType SourceTerrigenous pre-existing rock(or Lithogenous) all land derived materialBiogenous living organismsHydrogenous precipitation from sea waterCosmogenous space2222LithogenousFrom rocks, wood, waste sludge, volcanic stuffFrom rocks, wood, waste sludge, volcanic stuffResults from erosion by air & water Transported by Results from erosion by air & water Transported by winds, water, ice and gravity. Also by glaciers and winds, water, ice and gravity. Also by glaciers and icebergsicebergs••dominates the dominates the neriticneriticsediments because it is the sediments because it is the largest source for theselargest source for these••Pelagic Pelagic lithogenouslithogenoussediments sediments ÆÆabyssal clay (about abyssal clay (about 75% of clay), very slow accumulation, rich in Fe 75% of clay), very slow accumulation, rich in Fe ÆÆred clayred clay2323The Sediment Cycle.Over geological time, mountains rise as lithospheric (crustal) plates collide, fuse, and subduct. Water and wind erode the mountains and transport resulting sediment to the sea. The sediments are deposited on the seafloor, where they travel with the plate and are either uplifted or subducted. Thus, the material is eventually made into mountains again.2424BiogenousOozes – sediment containing at least 30%