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Dating Surfaces and Strata by U-series on Pedogenic Carbonate

Dating Surfaces and Strata by U-series on Pedogenic Carbonate. Warren Sharp Kathryn Fletcher Berkeley Geochronology Center. Therefore:. Accuracy is comparable to measurement precision, commonly 1-2% (2 s ). Strengths of U-series Dating.

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Dating Surfaces and Strata by U-series on Pedogenic Carbonate

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  1. Dating Surfaces and Strata by U-series on Pedogenic Carbonate Warren Sharp Kathryn Fletcher Berkeley Geochronology Center

  2. Therefore: • Accuracy is comparable to measurement precision, commonly 1-2% (2s). Strengths of U-series Dating • Depends only on intrinsic properties of the sample (and well-known decay constants). • Is largely model-independent. • Offers internal checks on its major assumption of closed-system behavior.

  3. The 238U Decay Series 238U 234U 234Pa 234Th 4.5 Gyr 24 d 7 hr a b b 245 kyr a 226Ra 230Th 222Rn 218Po 76 kyr 1.6 kyr 4 d a a a 3 min a 214Pb 214Bi 214Po 210Pb 27 min 20 min 0.2 msec a b b 23 yr b 206Pb 210Po 210Bi 138 d 50 d a b Stable

  4. 230Th/238U evolution with time 230Th 238U Use 232Th as index for contamination Essentially zero initial 230Th in pure, authigenic material

  5. The 230Th/234U/238U Age Equation Must be solved numerically Equation contains only: • Isotope ratios for sample, and • Decay constants (known to ~0.3%)

  6. Then: Samples of 10-100 mg will yield 230Th/U dates with accura-cies at the percent level or better. Basics of U-series Most minerals precipitated directly from near-surface waters contain ppm levels of uranium but (when pure) only ppb levels of thorium. So if these minerals are: • Reasonably free of incorporated detritus (and initial 230Th from other sources); • Have behaved as closed systems; • Are a few hundred to a few hundred thousand years old;

  7. Pedogenic Carbonate • Is widely developed in arid to semi-arid regions. • Accumulates in gravelly sediments in a well-known morphological sequence. • Is highly suitable for precise U-series dating if small, dense, visibly pure samples are analyzed.

  8. Morphology of Carbonate Accumulation in Gravelly Soils Gile et al., 1966 Time

  9. Pedogenic Carbonate:LGM terrace: Wind River Basin, Rocky Mts.

  10. Depth of Carbonate (Bk) versus Mean Annual Precipitation (MAP) MAP (mm) global, post-glacial soils Depth to Bk horizon (cm) Retallack, 2005

  11. U-series Dating of Pedogenic Carbonate • Limitations when applied to landforms • SINCE carbonate accumulation ensues after landform deposition; and • U-series analyses require samples of finite thickness; i.e., > 200 microns, • U-series ages on pedogenic carbonate will be minimumestimates of landform ages. • This “time-lag” is variable and ranges from ~0.5 to 2 ka where determined thus far.

  12. Discordant Cosmogenic and U-series Ages for the Biskra Palms Fan U-series, Soils: Kate Fletcher1, Katherine Kendrick2, Warren Sharp1 New 10Be Dating: Whitney Behr3, Dylan Rood4,Tom Hanks2, Ken Hudnut2, Bob Finkel4 • Berkeley Geochronology Center • U.S. Geological Survey • University of Southern California • Lawrence Livermore National Lab

  13. Biskra Palms fan San Andreas fault system, southern California ~50 mm/yr

  14. Biskra Palms Fan, T2 Surface from van der Woerd et al., 2006

  15. Compare three geochronological datasets for Biskra fan T2 surface: • 10Be on pavement-clasts • (van der Woerd et al., 2006) • 10Be on boulder-tops • (Behr et al., 2007; in prep.) • U-series on pedogenic carbonate • (Fletcher et al., 2007; in prep.)

  16. Relict bar and swale, Biskra T2 fan surface van der Woerd et al. (2006) San Andreas fault swale bar

  17. 10Be dating of pavement cobbles; typical sample F (van der Woerd et al., 2006)

  18. 10Be ages for pavement cobbles from T2 surface box heights show 2 sigma internal errors 10Be Age (ka) data from van der Woerd et al. (2006)

  19. 10Be ages of pavement clasts from T2 surface, (van der Woerd et al., 2006) 35 ka

  20. mean age = 35.5 ± 2.5 ka 10Be ages for pavement cobbles from T2 surface box heights show 2 sigma internal errors 10Be Age (ka) interpretation of van der Woerd et al. (2006)

  21. Highest T2 boulder Hudnut Hanks Behr Platt

  22. boulder-tops, Behr et al., 2007 Inheritance Erosion/ Exhumation 10Be ages for samples from T2 surface box heights show 2 internal errors mean age of cobbles 10Be Age (ka) for all 10Be ages, MSWD = 11.3 probability of fit = 0.000 pavement-cobbles, van der Woerd et al., 2006

  23. U-series dating of pedogenic carbonate Fletcher et al. (2007) soil profile, T2 alluvium 2 m dated horizon Kendrick

  24. carbonate coating granitic clast Pedogenic carbonate from Biskra Palms detrital grain Carbonate is highly suitable for U-series dating… median values, n = 21 U ~ 6 ppm 232Th ~ 0.2 ppm (230Th/232Th)Act. ~ 40

  25. U-series ages of T2 carbonate Mid. fan Upper fan Lower fan Age (ka) errors are 2sigma ellipses enclose sub-samples from a single clast Fletcher et al., 2007

  26. mean age = 45.3 ± 0.5 ka U-series ages for pedogenic carbonate from T2 surface four sub-samples from a single, ~200 micron-thick clast-coating Age (ka) errors are 2 sigma Fletcher et al., 2007 MSWD = 1.3 No excess scatter

  27. minimum age, 45.3 ± 0.5 ka U-series ages of T2 carbonate Mid. fan Upper fan Lower fan Age (ka) errors are 2sigma ellipses enclose sub-samples from a single clast Fletcher et al., 2007

  28. Erosion/ Exhumation Ages for T2 surface box heights show external errors minimum depositional age of T2 from U-series, 45 ka Age (ka) boulder-tops, Behr et al., 2007 pavement-cobbles, van der Woerd et al., 2006

  29. minimum T2 age from U-series Modeling the effects of rock-erosion on apparent 10Be ages 10Be Model Age (yr) youngest boulder-top external errors,1s youngest pavement-clast Erosion rate (mm/ka)

  30. Soil profile, Indio Hills, courtesy of Tom Rockwell ~20-cm thick remnant A-layer • At Biskra Palms, the A-layer is missing and was likely eroded, thereby exhuming partially shielded cobbles. • Stripping of ~50 cm from the fan surface at the Pleistocene/Holoceneclimate transition, for example, would yield the observed array of pavement-cobble ages of 25-45 ka.

  31. Conclusions • U-series dating of pedogenic carbonate can provide reliable and precise minimum landform ages. • Biskra Palms T2 surface is > 45 ka, rather than 35±2.5 ka, with attendant reduction in slip rate. • Effects of erosion/exhumation on cosmogenic surface exposure ages must be considered even on well-preserved, late Pleistocene landforms.

  32. a b c 1 mm age averaging results in scatter towards younger ages Clast > 3 mm3 of sample needed to yield ages precise to 1-2%

  33. stone pavement accretionary mantle 0.6 meter depth basalt bedrock

  34. 100 pavement flow surface 75 cone scoria 3He age (ka) 50 25 0 stratigraphic age Wells et al. (1995) pavement-clast ages

  35. Secular variation in slip rates within the San Andreas system? (Bennett, Friedrich, & Furlong, 2004) San Andreas 30 Biskra Palms, this study Slip Rate 20 (mm/ yr) 10 San Jacinto 0 1 10 100 1000 Time (ka)

  36. Geochemistry of Uranium and Thorium in the Near-Surface Environment • Uranium in oxygenated water (such as almost all natural, near-surface water) readily complexes to form the uranyl ion, (UO2)2+; hence uranium and has appreciable solubility at low temperature. • Thorium occurs only in the +4 oxidation state in natural waters and does not generally complex; hence thorium is extremely insoluble at low temperature.

  37. Southern San Andreas Slip Rates • Geological Rate (Biskra Palms, past 50 kyr) • van der Woerd et al. (2006): 13-19 mm/yr • Behr et al. and Fletcher et al. (in prep.): < 15 mm/yr Geodetic Rates (Coachella Valley, past 30 yr) • Fay & Humphreys (2005): 21.4 ± 0.5 mm/yr • Meade & Hager (2005): 23.2 ± 0.5 mm/yr

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