The Continental Shelves

1. The Continental Shelves

2. Definition: • Plain extending from the shore to the shelf break. • Geologically part of the continent.

3. Characteristics: • Depth: 10 to 200m Width: 0 - 1,500 km (avg 78 km) • Slope: 0.1° • Shelf edge marked by a distinctive break in slope (135mwd). • Vast deposits of sands, mud, gravels, and vast thicknesses of consolidated rock. • About 90% of the sediment generated by erosion on land is deposited on the continental margins.

4. Question 1: Why are continental shelves that shallow? (10-200m water depth) • Because they have continental crust below: isostatic elevation.

5. Continental margins • Transition between 2 crustal types • Classified as passive or active -> characterizes the shelf morphology: • Passive: wide shelf, high terrigenous input • Active: narrow shelf, low sediment input

6. Shelf sub-regions www.gso.uri.edu

7. Dynamical difference Between Shelf and Deep Ocean: • Shallow water depth (friction) • Presence of coast • Exposure to terrestrial influences (river…) • Vertical water column stratification can be highly variable www.gso.uri.edu

8. Question 2: Why do we care? • Small area (5.3%), but important (compared to open ocean) • Economics • Fishing (90% of the world’s marine food resources) • Mining (aggregates, placers, phosphorites) • Fossil Fuels (20%) • Shipping • Tidal and wave Energy • Environments • Vulnerable (pollution, river, atmosphere, …) • Coastal engineering

9. Continental Shelves: 5.3% of Earth’s surface

10. Shelves distribution: • The broadest shelves occur off the northern coast of Siberia and North America, in the Arctic Ocean. www.mappery.com

11. A new Cold War? • In 2007 Russian scientists sent a submarine to the Arctic seabed at 90° North to gather data in support of Russia's claim that the North Pole is part of the Russian continental shelf. 25% of the world’s undiscovered oil and gas resources lie hidden under certain areas of the Arctic Ocean. http://sever.mvk.ru/en/index.php?idx=18 www.duram.ac.uk/ibru

12. Shelf sedimentation • Factors influencing sedimentation: • Chemistry (produces authigenic minerals) • Sea level fluctuations (controls distribution of relict sediments, barrier reefs etc.) • Climate • Type and intensity of shelf hydraulic regime (“energy”) • Type and rate of sedimentation • Animal-sediment interactions (modifies the substrate)

13. Sea-Level & relict sediments: • Sea-level variations caused by changes in: • Sea floor spreading rates • Ice volumes • Ocean temperature • During the glacial-interglacial cycles, sea-level changed by up to 120m => the coastline migrated back and forth across the shelves.

14. Question 3: How would you recognize relict sediments on the shelf? • Relict deposits are coarse sediment which were originally not deposited in a marine environment.

15. Sea-Level & relict sediments: • During glacials, the continental shelves were exposed above sea-level as continental plains. • Coastal environments migrated back and forth across the shelves. • Rivers and glaciers deposited fluvial and glacial sands on what is now submarine continental shelf. pubs.usgs.gov

16. Sea-Level & relict sediments: • Sable Island sand comes from glacial till — piles of boulders, sand and clay left on the ocean banks by melting glaciers about 19,000 years ago. • You could bury a ten-story apartment building in Sable Island. The nearest solid rock base (bedrock) is about 40 meters straight down. museum.gov.ns.ca

17. Climate: • Controls shelf sedimentation by its effects on the surrounding land mass (siliciclastic source area). • Wet and hot (tropical) intense chemical weathering and clay production. • Cold and dry (temperate) physical weathering and sand/gravel production. www.usouthal.edu

18. Shelf hydraulic regime: • Three dominant processes are responsible for transporting, reworking and sorting sediment on shelves: • Storm and wave dominated Shelves 80% • Tide dominated Shelves 17% • Oceanic current-dominated Shelves 3% A continuum of processes, may vary seasonally

19. Current-dominated Shelves • Onshore winds: pile up water near the coast, pressure gradient, geostrophic coastal currents. • Alongshore winds: wind stress + Coriolis = Ekman transport (upwelling or downwelling).

20. Question 4 • Which way does a geostrophic coastal current flow on the eastern margin of North America? • Towards the North, with the Coast (high pressure on its right) • Example, the Davidson Current.

21. Wave-dominated shelves • Dominant wave period = 10 sec / Wave length = 150m • Start to feel the bottom in 75m of water. • Large part of the shelves are affected by the waves. • Fine-grained sediments and small-scale bedforms predominate. www.bigelow.org

22. Wave-dominated shelves • Waves are more variable than currents: their influence depends on the frequency and intensity of storms. • Waves stir up sediment, they do not introduce directionality into transport. • Currents move sediment stirred by waves. • Reworked shelf sediments (resuspension, sorting = winnowing out the finer particles).

23. Storm-dominated shelves • All shelves may be overprinted by storm processes. • Storm waves can affect bottom at 200mwd. • Active sediment transport restricted to intermittent storms. • If frequent, produces wholly storm-dominated regime. • Wind speed and duration affect wave height and period: Stronger and longer storm duration, higher and longer wavelength are resulting waves. www.environmentalgraffiti.com

24. Storm-dominated shelves • A tropical cyclone can cause more change in a coastline (and on the shelf) in one day than fair-weather processes did in 10 years...... • What about the really big storms? (100/500/10,000 year storms) • Although incredibly rare, they would make major impacts on shelf sediments that might be preserved in the rock record. www.ing.pan.pl

25. Tide-dominated shelves • Swept daily by powerful bottom currents. Tidal currents are considerably faster in shelf seas than in the open ocean (amplified as they move into shallow water). • Induce strong bidirectional currents. • Wide variety of bedforms, such as elongated bedforms (ridges). • Frequent sand transport. • Ancient sedimentary sections deposited on tidally-influenced shelves are characterized by herringbone cross-bedding www.scienceblogs.com

26. Tides: brief review • Tides are produced by imbalances in gravitational and centrifugal forces on the surface of the Earth. • Shallow-water waves (speed varies with depth) • When the sun and moon are aligned, tidal range is high, producing “spring tides”. • When the sun and moon are 90º out of phase, tidal range is reduced, producing “neap tides”.

27. Rotary flow in open ocean • Crest (high tide) rotates, tidal waves follow circular paths around amphidromic points • Counterclockwise in Northern Hemisphere • Clockwise in Southern Hemisphere • Cotidal lines World Amphidromic System

28. Tide-dominated shelves • Some bays resonate, producing very high tides. • Bay of Fundy: • Largest tidal range (spring tide max 17 m) • Shape of basin: • Oscillation period close to tidal period • Shoals and narrows to north • Basin oriented toward right (Coriolis…) disc.sci.gsfc.nasa.gov

29. Tidal Energy 1st turbine lowered into Bay of Fundy November 12, 2009 www.washingtonpost.com http://www.cbc.ca/canada/nova-scotia/story/2009/11/12/ns-fundy-turbine.html

30. Tide-dominated shelves • Tidally generated internal waves occur along the thermocline/pycnocline. • Reflection or breaking of these internal waves at the shelf break causes mixing of the water column and brings nutrients to the surface. => High biological production.

31. Shelf sedimentation • Direct sediment supply to and across shelves is pretty negligible, except in areas adjacent to very large rivers and estuary systems. • Most sediment is strung out parallel to shorelines due to long shore drift. • Currently, 50% of the world’s shelves are “relict” artifacts left over from the last sea level low stand. • They are currently sites of reworking and colonization by various benthic fauna, but little to no active sedimentation. www.usouthal.edu

32. Sediment supply • Terrigenous input • terrigenous sediments close to the coast, in particular at high latitude. • Biogenic productivity • Carbonate sediments were conditions are favorable, and terrigenous supply low. • Deposition of sediments depends on: • The rate of supply • Whether it is in suspension • Whether it is bedload • Reworking www.marinesciencetoday.com

33. Question 5: • What are the 4 main mechanisms moving sediment? • Waves • Tides • Currents • Gravity

34. Type and rate of sedimentation • Situation 1: “Normal” • a balance between sediment input and longshore drift resulting in a classic shoreline-hugging sediment wedge. • Situation 2: “High-energy shoreline” • mud is transported from the nearshore across much of the shelf before being deposited. • Situation 3: “very-high sedimentation” • sediment blankets the whole shelf. This only happens adjacent to 12 rivers in the world (including the Mississippi).

35. Question 6: www.usouthal.edu

36. Complex sedimentation environment Nittrouer and Wright, 1994, Rev. Geophys.

37. Sediment distribution Early Conceptual Model (Marr, 1929) • ‘Belt of Variables’: Inshore region characterized by variable but generally coarse (> 63 μm) sediment sizes. • Mid-shelf ‘Mud Belt’ with mean sediment < 63 μm because of decreasing energy offshore. • ‘Organic Belt’: pelagic biogenic deposits . This outer transition is caused by the depletion of suspended terrigenous sediment.

38. Cross-shelf sediment transport • Diffusion? • Shoreline = sediment source • Shelf break = sediment sink • cross shelf gradient in sediment concentration. => Produce a diffusive flux. • Advection? • by surface plumes and currents • by nearbed currents • by nearbed density underflows Swift (1970)

39. Mud Belt formation • Mud rapidly removed from advective buoyant plumes. • 3 different mechanisms proposed for seaward transport of muds that have sunk from the plume: • wave-generated diffusion (Swift, 1970). • advection in oceanographic currents – bbl, bottom boundary layer (McCave, 1972). • wave supported, gravity-driven underflows (Moore, 1969).

40. Gravity driven flows • Moore’s idea: • Sediment accumulates in wave boundary layer. • Sediment-rich nearbed layer flows under influence of gravity. • Problems: • Need very high concentrations (> 10 kg m-3) to induce sediment transport on shelf (low gradient) • How is sediment suspension maintained? -> By waves • At first widely rejected, because slope too small to sustain gravity flows, and sediment concentrations cannot grow large enough to cause suspensions to flow down gradient. • No previous near seabed measurements to confirm model predictions. Only recently did technology improve to the point where this latter mechanism was recognized.

41. The Ascent of Tripods • Tripods measure waves, currents and sediment near seabed • Tripod technology advanced rapidly starting in ’70’s • Holy grail was measurements very near the bed (within cm’s) http://soundwaves.usgs.gov/2000/08/buoy.jpg

42. Tripod Time Series Ogston et al. (2000) Wave orbital velocity Mean along shelf current Mean across shelf current Suspended sediment concentration Temperature

43. Sand

44. Mud

45. Sand Mid-shelf Mud “Relict” Outer-shelf Mud Sand-Mud Transition

46. Biogeochemistry of the Shelves: • Bioturbation: burrowing animals can also play an important role in reworking sediments. They can resuspend a significant amount of sediments to be carried away by currents. • Sediment denitrification: occurs in anaerobic environments. • Methane release? Clathrates http://epocaarctic2009.wordpress.com

47. Methane hydrates: • A permafrost 'lid' on the sub-sea sediments on the Siberian shelf caps and holds massive reservoirs of shallow methane deposits in place. • Growing evidence for release of methane in this inaccessible region suggests that the permafrost lid is starting to melt/get perforated and thus leak methane. www.awi.de www.scientificamerican.com

48. Continental Shelves: Summary • Economically critical zone. • More dynamic than the open ocean (coast, tides, river…). • Climate determines the sediment composition (gravel, sand, mud...). • Sedimentary processes dominated by the hydraulic regime: • tides, waves, storms and/or ocean currents. • Sedimentary environment dominated by reworking more than direct transport or deposition (except for tide or current). • Shelf environment ultimately controlled by sea level.

49. Coastal Environments

50. Coastal region • Shore: between low tide and highest elevation affected by storm waves • Coast: from shore to farthest inland ocean features