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Dating Surfaces and Strata by U-series on Pedogenic Carbonate

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Presentation on theme: "Dating Surfaces and Strata by U-series on Pedogenic Carbonate"— Presentation transcript:

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

2 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. Therefore: Accuracy is comparable to measurement precision, commonly 1-2% (2s).

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

4 230Th/238U evolution with time
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 So if these minerals are:
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; Then: Samples of mg will yield 230Th/U dates with accura-cies at the percent level or better.

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 Time 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)
(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 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 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 San Andreas fault system, southern California
Biskra Palms fan ~50 mm/yr

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

15 Compare three geochronological data sets 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
(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 35 ka 10Be ages of pavement clasts from T2 surface,
(van der Woerd et al., 2006) 35 ka

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

21 Highest T2 boulder Hudnut Hanks Behr Platt

22 10Be ages for samples from T2 surface
boulder-tops, Behr et al., 2007 box heights show 2 internal errors Erosion/ Exhumation Inheritance 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 is highly suitable
Pedogenic carbonate from Biskra Palms detrital grain granitic clast carbonate coating Carbonate is highly suitable for U-series dating… median values, n = 21 U ~ 6 ppm 232Th ~ 0.2 ppm (230Th/232Th)Act. ~ 40

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

26 Age (ka) U-series ages for pedogenic carbonate from T2 surface
four sub-samples from a single, ~200 micron-thick clast-coating mean age = 45.3 ± 0.5 ka Age (ka) errors are 2 sigma 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 Lower fan Upper fan minimum age, 45.3 ± 0.5 ka Age (ka) errors are 2sigma ellipses enclose sub-samples from a single clast Fletcher et al., 2007

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

29 youngest pavement-clast
Modeling the effects of rock-erosion on apparent 10Be ages minimum T2 age from U-series 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/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.

32

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

34 stone pavement 0.6 meter depth accretionary mantle basalt bedrock

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

36 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 1 10 100 1000 Time (ka)

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, (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.

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


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