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16. Sediment Transport Across the Continental Shelf and Lead-210 Sediment Accumulation Rates William Wilcock

Lecture/Lab Learning Goals. Know the terminology of and be able to sketch passive continental marginsDifferences in sedimentary processes between active and passive marginsKnow how sediments are mobilized on the continental shelfUnderstand how lead-210 dating of sediments worksApplication of lea

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16. Sediment Transport Across the Continental Shelf and Lead-210 Sediment Accumulation Rates William Wilcock

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    1. 16. Sediment Transport Across the Continental Shelf and Lead-210 Sediment Accumulation Rates William Wilcock

    2. Lecture/Lab Learning Goals Know the terminology of and be able to sketch passive continental margins Differences in sedimentary processes between active and passive margins Know how sediments are mobilized on the continental shelf Understand how lead-210 dating of sediments works Application of lead-210 dating to determining sediment accumulation rates on the continental shelf and the interpretation of these rates - LAB

    3. Passive Margins Transition from continental to oceanic crust with no plate boundary. Formerly sites of continental rifting

    4. Terminology

    5. Active Margins Plate boundary (usually convergent) Narrower continental shelf Plate boundary can move on geological time scales - accretion of terrains, accretionary prisms

    6. Sediment transport differences

    7. Sediment Supply to Continental Shelf Rivers Glaciers Coastal Erosion

    8. Sediment Mobilization - 1. Waves

    9. Shallow water waves Wave particle orbits flatten out in shallow water Wave generated bottom motions strongest during major storms (big waves) extend deepest when the coast experiences long wavelength swell from local or distant storms

    10. Sediment Mobilization 2. Bottom Currents The wind driven ocean circulation often leads to strong ocean currents parallel to the coast. These interact with the seafloor along the continental shelf and upper slope. The currents on the continental shelf are often strongest near outer margins

    11. Sediment Distribution on the Continental shelf Coarse grained sands - require strong currents to mobilize, often confined to shallow water where wave bottom interactions are strongest (beaches) Fine grained muds - require weaker currents to mobilize, transported to deeper water.

    12. Upcoming lab In the lab following this lecture you are going to calculate a sedimentation rate for muds on the continental shelf using radioactive isotope Lead-210 and you are going to interpret a data set collected off the coast of Washington.

    13. Radioactive decay - Basic equation

    14. Activity - Definition and equations

    15. 238U Decay Series 238U ?234U ?230Th ?226Ra ?222Rn?210Pb?206Pb

    17. 210-Pb in sediments Young sediments also include an excess or unsupported concentration of 210Pb. Decaying 238U in continental rocks generates 222Rn (radon is a gas) some of which escapes into the atmosphere. This 222Rn decays to 210Pb which is then efficiently incorporated into new sediments. This unsupported 210Pb is not replaced as it decays since the radon that produced it is in the atmosphere. Measurements of how the excess 210Pb decreases with depth can be used to determine rates.

    18. Pb-210 concentrations in sediments

    19. Excess Pb-210 concentrations

    20. Solving the equation - 1

    21. Solving the equation - 2

    22. Pb-210 sedimentation rates

    23. Summary - How to get a sedimentation rate Identify the background (supported) activity AB - the value of A at larger depths where it is not changing with depth. Subtract the background activity from the observed activities at shallower depths and take the natural logarithm to get ln(A)=ln(Aobserved-AB) Plot depth z against ln(A). Ignore in the points in the surface mixed region where ln(A) does not change with depth. Ignore points in the background region at depth (Aobserved = AB). Measure the slope in the middle region (take it as a positive value). Multiply the slope by the radioactive decay constant (? = 0.0311 yr-1) to get the sedimentation rate.

    24. Limitations Assumption of uniform sedimentation rates. Cannot use this technique where sedimentation rate varies with time (e.g., turbidites). Assumption of uniform initial and background Pb-210 concentrations (reasonable if composition is constant).

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