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

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Presentation on theme: "Dating Surfaces and Strata by U-series on Pedogenic Carbonate Warren Sharp Kathryn Fletcher Berkeley Geochronology Center."— Presentation transcript:

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

2 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. 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. Strengths of U-series Dating Therefore: Accuracy is comparable to measurement precision, commonly 1-2% (2  ).

3 238 U The 238 U Decay Series 234 Th 234 U 234 Pa 230 Th 226 Ra 222 Rn 218 Po 214 Bi 214 Pb 214 Po 210 Pb 210 Bi 210 Po 206 Pb 4.5 Gyr24 d 76 kyr 1.6 kyr 4 d 27 min20 min0.2 msec Stable 138 d 7 hr          23 yr  245 kyr 3 min  50 d

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

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

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

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. 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) Depth to Bk horizon (cm) global, post-glacial soils 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 minimum estimates 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 Fletcher 1, Katherine Kendrick 2, Warren Sharp 1 New 10 Be Dating: Whitney Behr 3, Dylan Rood 4, Tom Hanks 2, Ken Hudnut 2, Bob Finkel 4 U-series, Soils: Kate Fletcher 1, Katherine Kendrick 2, Warren Sharp 1 New 10 Be Dating: Whitney Behr 3, Dylan Rood 4, Tom Hanks 2, Ken Hudnut 2, Bob Finkel 4 1.Berkeley Geochronology Center 2.U.S. Geological Survey 3.University of Southern California 4.Lawrence Livermore National Lab 1.Berkeley Geochronology Center 2.U.S. Geological Survey 3.University of Southern California 4.Lawrence Livermore National Lab

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

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

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

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

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

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

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

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

21 Highest T2 boulder Hanks Behr Hudnut Platt

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

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

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

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

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

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

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

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

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/Holocene climate transition, for example, would yield the observed array of pavement-cobble ages of ka. 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/Holocene climate transition, for example, would yield the observed array of pavement-cobble ages of 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. 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

33 age averaging results in scatter towards younger ages a bc Clast 1 mm > 3 mm 3 of sample needed to yield ages precise to  1-2%

34 0.6 meter depth stone pavement accretionary mantle basalt bedrock

35 stratigraphic age 3 He age (ka) pavement flow surface cone scoria Wells et al. (1995) pavement-clast ages

36 Secular variation in slip rates within the San Andreas system? (Bennett, Friedrich, & Furlong, 2004) (mm/ yr) San Andreas San Jacinto Time (ka) Slip Rate Biskra Palms, this study

37 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, (UO 2 ) 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. Uranium in oxygenated water (such as almost all natural, near-surface water) readily complexes to form the uranyl ion, (UO 2 ) 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.

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


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