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 SharpKathryn FletcherBerkeley 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).
4 230Th/238U evolution with time Use 232Th as index for contaminationEssentially zero initial 230Thin pure, authigenic material
5 The 230Th/234U/238U Age Equation Must be solved numericallyEquation contains only:Isotope ratios for sample, andDecay constants (known to ~0.3%)
6 So if these minerals are: Basics of U-seriesMost 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., 1966Time
9 Pedogenic Carbonate: LGM terrace: Wind River Basin, Rocky Mts.
10 Depth of Carbonate (Bk) versus Mean Annual Precipitation (MAP) (mm)global, post-glacial soilsDepth to Bk horizon (cm)Retallack, 2005
11 U-series Dating of Pedogenic Carbonate Limitations when applied to landformsSINCE carbonate accumulation ensues after landform deposition; andU-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 Sharp1New 10Be Dating: Whitney Behr3, Dylan Rood4, Tom Hanks2, Ken Hudnut2, Bob Finkel4Berkeley Geochronology CenterU.S. Geological SurveyUniversity of Southern CaliforniaLawrence Livermore National Lab
13 San Andreas fault system, southern California BiskraPalmsfan~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 faultswalebar
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 show2 sigma internal errors10Be 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 show2 sigma internal errors10Be Age(ka)mean age =35.5 ± 2.5 kainterpretation of van der Woerd et al. (2006)
22 10Be ages for samples from T2 surface boulder-tops,Behr et al., 2007box heights show2 internal errorsErosion/ExhumationInheritancemean ageof cobbles10Be Age(ka)for all 10Be ages,MSWD = 11.3probability of fit = 0.000pavement-cobbles,van der Woerdet al., 2006
23 U-series dating of pedogenic carbonate Fletcher et al. (2007)soil profile,T2 alluvium2 mdatedhorizonKendrick
24 Carbonate is highly suitable Pedogenic carbonatefrom Biskra Palmsdetrital graingraniticclastcarbonatecoatingCarbonate is highly suitablefor U-series dating…median values, n = 21U ~ 6 ppm232Th ~ 0.2 ppm(230Th/232Th)Act. ~ 40
25 Age (ka) U-series ages of T2 carbonate Lower fan Upper fan errors are Mid.fanLower fanUpper fanAge(ka)errors are2sigmaellipses enclosesub-samplesfrom a single clastFletcher et al., 2007
26 Age (ka) U-series ages for pedogenic carbonate from T2 surface four sub-samples from asingle, ~200 micron-thickclast-coatingmean age =45.3 ± 0.5 kaAge(ka)errors are2 sigmaFletcher et al., 2007MSWD = 1.3No excess scatter
27 Age (ka) U-series ages of T2 carbonate Lower fan Upper fan Mid.fanLower fanUpper fanminimum age,45.3 ± 0.5 kaAge(ka)errors are2sigmaellipses enclosesub-samplesfrom a single clastFletcher et al., 2007
28 Ages for T2 surface Age (ka) boulder-tops, Behr et al., 2007 box heights showexternal errorsminimum depositional ageof T2 from U-series, 45 kaErosion/ExhumationAge(ka)boulder-tops,Behr et al., 2007pavement-cobbles,van der Woerdet al., 2006
29 youngest pavement-clast Modeling the effects of rock-erosionon apparent 10Be agesminimum T2 agefrom U-series10BeModelAge(yr)youngest boulder-topexternal errors,1syoungest pavement-clastErosion rate (mm/ka)
30 Soil profile, Indio Hills, courtesy of Tom Rockwell ~20-cm thick remnant A-layerAt 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 ConclusionsU-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.
36 Secular variation in slip rates within the San Andreas system? (Bennett, Friedrich, & Furlong, 2004)San Andreas30Biskra Palms,this studySlipRate20(mm/yr)10San Jacinto1101001000Time (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/yrBehr et al. and Fletcher et al. (in prep.): < 15 mm/yrGeodetic Rates (Coachella Valley, past 30 yr)Fay & Humphreys (2005): ± 0.5 mm/yrMeade & Hager (2005): ± 0.5 mm/yr