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

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16. Sediment Transport Across the Continental Shelf and Lead-210 Sediment Accumulation Rates OCEAN/ESS 410 Lecture/Lab Learning Goals • Know the terminology of and be able to sketch 16. Sediment Transport passive continental margins • Differences in sedimentary processes between active Across the Continental Shelf and passive margins • Know how sediments are mobilized on the and Lead-210 Sediment continental shelf • Understand how lead-210 dating of sediments works Accumulation Rates • Application of lead-210 dating to determining sediment accumulation rates on the continental shelf and the interpretation of these rates - LAB William Wilcock Terminology Passive Margins Shelf Break Abyssal Plain Continental Shelf - Average gradient 0.1° Shelf break at outer edge of shelf at 130-200 m depth (130 m depth = sea Transition from continental to oceanic crust level at last glacial maximum) with no plate boundary. Continental slope - Average gradient 3-6° Formerly sites of continental rifting Continental rise (typically 1500-4000 m) - Average gradient 0.1-1° Abyssal Plain (typically > 4000 m) - Average slope <0.1° 1 Active Margins Sediment transport differences Plate boundary (usually convergent) Narrower continental shelf Plate boundary can move on geological time Active margins - narrower shelf, typically have a higher sediment supply, scales - accretion of terrains, accretionary prisms earthquakes destabilize steep slopes. Sediment Supply to Continental Shelf Sediment Mobilization - 1. Waves • Rivers • Glaciers • Coastal Erosion Sediment Transport across the Shelf Once sediments settle on the seafloor, bottom currents are required to mobilize them. • Wave motions The wave base or maximum depth of wave motions is about one half the • Ocean currents wave length 2 Sediment Mobilization 2. Bottom Currents Shallow water waves • 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 Wave particle orbits flatten out in shallow water continental shelf are Wave generated bottom motions” often strongest near • strongest during major storms (big waves) outer margins • extend deepest when the coast experiences long wavelength swell from local or distant storms Aguihas current off east coast of southern Africa. The current flows south and the contours are in units of cm/s Sediment Distribution on the Upcoming lab Continental shelf In the lab following this lecture you are Coarse grained sands - require strong going to calculate a sedimentation rate for currents to mobilize, often confined to muds on the continental shelf using shallow water where wave bottom radioactive isotope Lead-210 and you are interactions are strongest (beaches) going to interpret a data set collected off Fine grained muds - require weaker the coast of Washington. currents to mobilize, transported to deeper water. 3 Activity - Definition and equations Radioactive decay - Basic equation Activity is the number of disintegrations in A unit time per unit mass (units are decays The number or atoms of an unstable isotope elements per unit time per unit mass. For 210Pb the decreases with time usual units are dpm/g = decays per dN minute per gram ) ! " N N - Number of atoms of an C - detection coefficient, a value between unstable isotope dt A = c!N 0 and 1 which reflects the fraction of the dN λ - radioactive decay constant is disintegrations are detected (electrically or ! = "N the fraction of the atoms that photographically) dt decay in unit time (e.g., yr-1) dA Obtained by multiplying both sides of the ! = " A middle equation on the previous slide by dt the constant cλ ln2 T1/2 - half life is the time for half T1 = the atoms to decay 2 ! 238U Decay Series 210Pb or Pb-210 is an isotope of lead that forms as part of a decay sequence of Uranium-238 238U 234U …230Th 226Ra Half Life 4.5 Byr Half life 1600 yrs, Rocks eroded to sediments 222Rn…210Pb…206Pb Gas, half life Half life, Stable 3.8 days 22.3 years 4 210-Pb in sediments Pb-210 concentrations in sediments 210 Sediments contain a background level of Pb that is A Pb-210 activity “supported” by the decay of 226Ra (radium is an alkali B metal) which is easily eroded from rocks and incorporated Surface mixed layer - bioturbation into sediments. As fast as this background 210Pb is lost by radioactive decay, new 210Pb atoms are created by the Measured Pb-210 activity decay of 226Ra. “ ” Excess Pb-210 activity Young sediments also include an excess or unsupported (measured minus concentration of 210Pb. Decaying 238U in continental rocks Region of radioactive decay. background) 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. Background Pb-210 levels from 210 decay of Radon in sediments Measurements of how the excess Pb decreases with Depth, Z (“supported” Pb-210) depth can be used to determine rates. (or age) Excess Pb-210 concentrations Solving the equation - 1 Excess Pb-210 activity A2 A1 dA The equation relating activity to the ! = " A radioactive decay constant dt t A2 t2 1 For a constant dA sedimentation rate, S Integrating this with the limits of ! = # dt integration set by two points (cm/yr), we can " A " A1 t1 t replace the depth 2 A t axis with a time axis "! ln A$ 2 = & "t$ 2 # % A1 # %t1 Work with data in thisregion Work z = St A z ln A ln A ln 1 t t ! 2 + 1 = = "( 2 ! 1 ) Age of t = A 2 sediments, t S A relationship between age and activity 5 Solving the equation - 2 Pb-210 sedimentation rates A1 ln t t Plot depth against natural logarithm of Pb-210 activity = !( 2 " 1 ) A ln(A) 2 Ignore data in mixed layer z z Substitute in the relationship between ( 2 ! 1 ) t ! t = age and depth ( 2 1 ) S ! z " z A1 ( 2 1 ) ln = A S Depth, z 2 S Slope = ! z z " !( 2 " 1 ) S = An expression for A1 the sedimentation ln rate A Ignore data with background levels 2 Summary - How to get a sedimentation rate 1. Identify the background (“supported”) activity A - the value B Limitations of A at larger depths where it is not changing with depth. 2. Subtract the background activity from the observed activities at shallower depths and take the natural logarithm to get ln • Assumption of uniform sedimentation (A)=ln(Aobserved-AB) rates. Cannot use this technique 3. Plot depth z against ln(A). where sedimentation rate varies with 4. Ignore in the points in the surface mixed region where ln(A) time (e.g., turbidites). does not change with depth. 5. Ignore points in the background region at depth • Assumption of uniform initial and (Aobserved = AB). background Pb-210 concentrations 6. Measure the slope in the middle region (take it as a positive (reasonable if composition is constant). value). 7. Multiply the slope by the radioactive decay constant (λ = 0.0311 yr-1) to get the sedimentation rate. 6 .
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