Marine sediments. Announcements. Virtually all the ocean floor is covered in sediment What are the main sources of marine sediment?

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1 Announcements Homework #1 due Thursday Homework 2 assigned Thursday; will require out of class work Due Feb 6 (1.5 weeks) Marine sediments Virtually all the ocean floor is covered in sediment What are the main sources of marine sediment? Where is each kind mainly found Examples of each kind Patterns of global and regional sediment distribution Deep sea Margins Later in the semester - paleoceanography (exploiting the oceans memory preserved in sediments) Sediment accumulating on fresh lava Soft sediments with brittlestar tracks 1

2 Sediments with ripples from deep-sea currents Most sediments are in margin environments (despite greater area of deep ocean floor) Margin sediments also accumulate most rapidly 2

3 Classification by size Size classification divides sediment by grain size into gravel, sand, silt and clay. Mud is a mixture of silt and clay Large particles can only be moved by strong currents; small are easily transported and only settle out when seawater motions are weak. Sources of sediment Terrigenous from land Biogenous formed biologically in the ocean Hydrogenous formed chemically in the ocean Cosmogenous from outer space Terrigenous sediments Terrigenous sediments: rivers Dominant on continental margins Accumulates rapidly - near the source Also found in deep ocean Accumulates slowly - far from source Mainly quartz and clays Colorado River Mississippi River Continental sediments are the products of erosion on land - by rivers, glaciers, wind, and various chemical and physical processes. They are transported to the ocean by rivers and by the atmosphere. Rivers are source of most sediment on continental margins 3

4 River input of sediment to ocean Terrigenous seds: examples Volcanic ash - can see these in ocean sediments Terrigenous seds: more examples Terrigenous seds: more examples Glacial-Marine sediments ice-rafted debris Deposited by sediment-laden icebergs as they move away from ice sheets and melt Windblown dust from dry regions 4

5 Biogenic sediments Formed by marine organisms Made of calcium carbonate (CaCO3) or silica (SiO4) Mainly microscopic shells (1mm or less); some fragments of larger things Dominant sediment type in mid-depth ocean: oozes : sediments that are mostly biogenic Calcareous: made of calcium carbonate Siliceous: made of silica Biogenic sediments: calcium carbonate examples Close to shore: reef rubble from a storm Biogenic sediments: calcium carbonate examples These are foraminifera - microscopic animals Found in open (deep) ocean as well as near shore Biogenic sediments: silica examples All of these are diatoms - the most abundant marine plant. 5

6 Oozes: Common in mid-depths (1-5km) Hydrogenous sediments Form chemically in the ocean Specific environments (not common everywhere) Sediments of mainly biogenic origin Can be silicic oozes, calcareous oozes or mixed Hydrogenous sediments: manganese nodules Deep ocean floor Formation not well understood Economic value? Hydrogenous sediments: evaporites Shallow water, dry climate Evaporate seawater ===> precipitate salts Gulf of California Middle East 6

7 Found at ridges Hydrogenous sediments: Hydrothermal vents Hydrothermal vents ( black smokers ) emit hot water When it cools, dissolved metals precipitate forming the smoke - settles out as metallic sediments Video Fell from outer space Rare! (but 16,000-33,000 tons/yr fall) Example: microtektites Cosmogenous sediments: microtektites Shelf sedimentation Processes have specific time scales: 1000 years, waves, currents and tides control sedimentation. 1,000,000 years, sea level lowered by glaciation controls sedimentation and cause rivers to deposit their sediments at the shelf edge and onto the upper continental slope. 100,000,000 years, plate tectonics determines the type of margin that develops and controls sedimentation. Hjulström s Diagram graphs the relationship between particle size and energy for erosion transportation deposition 7

8 Cont. Shelf Sedimentation Shelf sedimentation is strongly controlled by: Tides Waves Currents Their influence decreases with water depth. Shoreline turbulence prevents small particles from settling in the shallow water Particle size decreases seaward for recent sediments. Cont. Shelf Sedimentation Worldwide distribution of recent shelf sediments by composition is strongly related to latitude and climate. Calcareous biogenous sediments dominate tropical shelves. River-supplied sands and muds dominate temperate shelves. Glacial till and ice-rafted sediments dominate polar shelves. Sediment transport from margins to deep sea Turbidity current Slumps and slides along slopes Turbidity currents Turbidity currents are underwater avalanches of sediments that erode deep canyons in the slope and transport sediments from the shelf to the deep sea. They can be very destructive, e.g. to undersea cables. 8

9 Submarine canyons on continental slopes Deep sea sedimentation - Sediment distribution on ocean floor Ridge = high carbonate Deep basins = low carbonate New Jersey low high California Carbonate as % of total sed in Atlantic Why no carbonate in deepest basins? Deep-sea sedimentation Carbonate sediment dissolves in deep water (> ~ 5km) Why? Deep water is more acidic and has lower carbonate ion concentration (undersaturated) Depth below which no carbonate = Carbonate compensation depth External terrigenous material from the land Internal biogenous and hydrogenous from the sea. 9

10 Deep-Sea Sedimentation Deep-Sea Sedimentation The distribution of sediments in the deep ocean reflects: Latitude distance from landmasses the calcium carbonate compensation depth Siliceous oozes found where siliceous organisms live Calcareous oozes found where CaCO3 doesn t dissolve Terrigenous seds dominate margins Deepest basins - clays (small terrigenous particles) Glacial-marine sediments near ice sheets (highest lats) Deep-sea stratigraphy Refers to the broad-scale layering of sediments that cover the basaltic crust. The stratigraphy of the deep sea is strongly influenced by sea-floor spreading. As plates move, they encounter varying water depths and varying sediment inputs Atlantic stratigraphy Two main layers - a thick basal layer of carbonate ooze overlain by a layer of mud (reflects transition to depths >CCD) 10

11 Pacific stratigraphy (general) Pacific stratigraphy The Pacific basin contains a four-layer stratigraphy. It crosses the equator where the CCD is lowered to the ocean bottom. Pacific stratigraphy 11