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U-series Disequilibrium Part 2 9/11/12

U-series Disequilibrium Part 2 9/11/12. Lecture outline: 1) 210 Pb excess and residence time 2) 231 Pa - fraternal twin to 230 Th a. 231 Pa/ 230 Th concordia b. oceanic and sedimentary Pa and Th. photo of box coring device w/ core. 210 Pb Excess. Over what age range is

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U-series Disequilibrium Part 2 9/11/12

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  1. U-series Disequilibrium Part 2 9/11/12 • Lecture outline: • 1) 210Pb excess and residence time • 2) 231Pa - fraternal twin to 230Th • a. 231Pa/230Th concordia • b. oceanic and sedimentary • Pa and Th photo of box coring device w/ core

  2. 210Pb Excess Over what age range is 210Pb dating useful? 226Ra 222Rn 210Pb 210Bi t1/2=22.26y 238U-series Intermediate t1/2=1600y gas that escapes into atmosphere 210Pb removed from atmosphere by precipitation and/or dry deposition, becomes incorporated into lake sediments, ice sheets, ocean surface, etc. Like 230Thex, this “unsupported”, or “excess” 210Pb can be used to date sediments: What happens if sediment contains 226Ra?

  3. 210Pb Excess and residence time Phenomenon: 210Pb ~ 0.5(226Ra) in bottom waters, always Explanation? From previous lecture steady state assumption Activity parent Activity daughter physical removal from system If we assume no change in the amount of 210Pb, then we can calculate k = 0.032 yr-1 And mean residence time for 210Pb in deep ocean (Rt) equals 1/k, or ~30y Similar calculations can be applied to calculate Rt’s for many other parent-daugther systems, assuming your measurements represent steady state.

  4. 231Pa - fraternal twin of 230Th Parent half-life Rt complication 230Th 234U 75,000y ~30y 234U excess 231Pa 235U 32,000y ~110y none So wherever you apply 230Th, you can apply 231Pa: 1. sedimentation rates 2. carbonate dating These two geo-chronometers better agree (yield concordant ages), otherwise…. what has happened? From Cutler et al., 2003 Who should we point the finger at? and why?

  5. 231Pa/230Th concordia and open system behavior Unlike the U/Pb concordia, 230Th activity varies with 234U, so concordia change with 234U/238U, usually expressed in “delta”234U: What is the δ234U of seawater? from Cheng et al, 1998 Because Th and Pa are both “sticky”, U mobility is the most common cause of open system behavior. upper intercept = true age of deposit lower intercept = ambiguous, depends on δ234U of added U, discrete vs. continuous alteration What would a discordia for constant U gain look like?

  6. 231Pa/230Th ratios in the ocean: back of the envelope • Given: • - • where ‘t’ is the time between production • of 230Th and its incorporation into sediment • (t for Th and Pa is ~300y in the ocean) • dividing the 230Th and 231Pa equations: • - fixed U isotopic ratios in seawater • (235U/238U)atomic = 137.88 • (234U/238U)activity = 1.15 • We calculate that the (230Th/231Pa)A = 10.8 • or (231Pa/230Th)A = 0.093

  7. 231Pa/230Th ratios in modern sediments What’s going on here? Observation: The sediments do not reflect theoretical seawater values. This observation implies that Pa and Th are scavenged differentially. Which is scavenged more efficiently? How can we understand values in the Southern Ocean which are greater than 0.093? Yu, 1996

  8. 231Pa/230Th as a tracer of scavenging and water mass circulation Kerr, 1988

  9. 231Pa/230Th as a tracer of scavenging and water mass circulation Schmitz, 1996

  10. 231Pa/230Th as a tracer of scavenging and water mass circulation Boyle, 1996

  11. 231Pa/230Th ratios in modern vs LGM sediments LGM MODERN How can we interpret these panels?

  12. 231Pa/230Th ratios in paleoceanography More recently, paleoceanographers have taken advantage of the fractionation of 231Pa and 230Th associated with scavenging to reconstruct: 1) scavenging intensities and/or 2) deep-water advection strength (given constant scavenging) - so increase in 231Pa/230Th would indicate a slowdown in deep ocean circulation, given constant scavenging (cannot go greater than 0.093) From Henderson, 2002

  13. 231Pa/230Th ratios in a North Atlantic core (33°N, 57°W) note: axis inverted NADW strong NADW weak • key points: • LGM values 0.013 higher than Holocene  30% reduction in MOC? • deglacial characterized by large, abrupt changes in ratio  • implies ocean circulation key to abrupt climate changes in ice cores • - reaches magic number 0.093 during Heinrich  complete collapse of MOC? From McManus et al., 2004

  14. oxygen isotopes of ice for paleo-temperature oxygen isotopes in calcite for paleo-temperature 231Pa/230Th for paleo-circulation 14C dating for age control 230Th-normalized sedimentation rates 238U-234U-230Th dated coral terraces for paleo-sea level

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