SEDIMENTS Text Book – Chapter 5 Why do we care about oceanic sediments?

1. SEDIMENTS • Text Book – Chapter 5 • Why do we care about oceanic • sediments? • Continents are sites of erosion; • Oceans are sites of depositions; • Therefore oceans retain a more-nearly complete and organized record of Earth history.

2. Learning Objectives: Ocean Sediments • 1. Understand the origin and classification of marine sediments. • 2. Explain the factors controlling origin and deposition of sediment on the continental shelf and in the deep ocean. • Questions: • Why don't the oceans have more sediment in them? Where does it all go? Earth is 4.6 billion years old and the oceans should have more sediment in them. • Salt composition of the oceans has not changed for the last 1.5 billion years. Explain why?

3. Marine Sedimentation

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5. Sediments in the Sea Classification of marine sediments can be based upon size or origin. • Size classification divides sediment by grain size into gravel, sand, silt and clay. • Mud is a mixture of silt and clay. • Origin classification divides sediment into four major categories:

6. The 4 main types of sediment • Lithogenous/terrigenous = composed of fragments of pre-existing rock material • Biogenous = composed of hard remains of once-living organisms • Hydrogenous = formed when dissolved materials come out of solution (precipitate) • Cosmogenous = derived from outer space

7. Sediments in the Sea Factors that control sedimentation include particle size and the turbulence of the depositional environment. • Terrigenous sediments strongly reflect their source and are transported to the sea by wind, rivers and glaciers.

8. Streams Rivers

9. Glaciers

10. Landslide (Gravity)

11. Sedimentation in the Ocean ocean environment can be divided into:the shelf – neritic deposits(shallow areas and near a terrigenous source) the deep ocean basin – pelagic deposits(deep areas and far from a terrigenous source)

12. Shelf Versus Basin

13. Sedimentation in the Ocean Shelf sedimentation is strongly controlled by tides, waves and currents, but their influence decreases with water depth. • Shoreline turbulence prevents small particles from settling and transports them seaward where they are deposited in deeper water. • Particle size decreases seaward for recent sediments.

14. Sedimentation in the Ocean Worldwide distribution of recent shelf sediments by composition is strongly related to latitude and climate. • Calcareous biogenic sediments dominate tropical shelves. • River-supplied sands and muds dominate temperate shelves. • Glacial till and ice-rafted sediments dominate polar shelves.

15. Shelf Sedimentation Model

17. Geologic controls of continental shelf sedimentation must be considered in terms of a time frame. • For a time frame up to 1000 years, waves, currents and tides control sedimentation. • For a time frame up to 1,000,000 years, sea level lowered by glaciation controlled sedimentation and caused rivers to deposit their sediments at the shelf edge and onto the upper continental slope. • For a time frame up to 100,000,000 years, plate tectonics has determined the type of margin that developed and controlled sedimentation.

18. Influence of Past Sea Level

20. Case study: The Atlantic Passive Margin

21. Case study: Pacific Destructive/Subduction Margin

22. Carbonate Shelves If influx of terrigenous sediment is low and the water is warm, carbonate sediments and reefs will dominate.

23. Ice Rafting

24. Sedimentation 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

25. Sedimentation in the Ocean • Major pelagic sediments in the ocean are red clay and biogenic oozes. • Authigenic deposits are formed in situ (at place) by precipitation of submarine weathering (e.g. hydrothermal areas, ridges); metal rich sediments and manganese nodules;

27. Biogenic Deposits Calcium Carbonate (e.g. mollusks, corals, foraminifers, some algae) Silica (e.g. diaotms, radiolaria)

28. Tiny animals with calcite shells (CaCO3)

29. Coccoliths Tiny plants with a calcite skeleton (CaCO3)

30. Diatoms SiO2 Radiolaria

31. Silica Sedimentation Siliceous ooze found where it accumulates faster than it dissolves

33. Calcium carbonate Sedimentation CCD – Calcite dissolves beneath the calcite compensation depth (CCD) at ~ 4.5 km

35. Nodules Deposits – hydrogenous sediment from precipitation of metal oxides

37. Distribution of sediments in thedeep ocean 1) latitude2) distance from landmasses3) CCD (carbonate compensation depth) • Glacial marine sediments occur in the high latitudes. • Pelagic clays occur far from land and in the deepest water. • Calcareous oozes occur above the calcium carbonate depth (CCD); while siliceous ooze occur below; • The rate of sedimentation depends on the type of sediment in deep sea.

38. Sedimentation in the Ocean • 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.

39. Sedimentation in the Ocean The Atlantic basin contains a “two-layer-cake” stratigraphy–a thick basal layer of carbonate ooze overlain by a layer of mud. Stratigraphy of the Atlantic Basin

40. Sedimentation in the Ocean The Pacific basin contains a “four-layer-cake” stratigraphy, because unlike the Atlantic its sea floor as it spreads crosses the equator where the CCD is lowered to the ocean bottom.

42. Stratigraphy and Model of Pacific Basin

43. Collecting Marine Sediment There are a number of sampling techniques for obtaining sediment from the ocean bottom. • Bottom dredges scrape the sediment and collect material in a wire or canvas bag. • Grab samplers take a “bite” out of the sediment covering the bottom. • Gravity and piston corers use a weight to drive a core barrel into a soft bottom. A piston corer takes a much longer core than a gravity corer because of the piston in the core barrel.

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49. The Drying Up of the Mediterranean Sea • The Mediterranean basin is located where plates are colliding as Africa moves northward relative to Europe. • Anhydrite and stromatolites of Miocene age indicate that the Mediterranean sea “dried” out between 5 and 25 million years ago. • Two models have been suggested to account for this emptying of the Mediterranean Sea of its water.

50. Refilling the Mediterranean Sea • After drying out, seawater from the Atlantic Ocean cascaded down the face of the Gibraltar Sill, refilling it in about 100 years.