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Week 1: Exposure age dating and basic erosion rate studies

Week 1: Exposure age dating and basic erosion rate studies. Kata Juta (“the Olgas”), NT, Australia. Some Processes and Impacts that we don’t really have a handle on yet: D = debris flows; G = glacial erosion; B = bedrock failure; R = river inc./dep.; C = climate. B. B. G. G. C.

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Week 1: Exposure age dating and basic erosion rate studies

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  1. Week 1: Exposure age dating and basic erosion rate studies Kata Juta (“the Olgas”), NT, Australia

  2. Some Processes and Impacts that we don’t really have a handle on yet: D = debris flows; G = glacial erosion; B = bedrock failure; R = river inc./dep.; C = climate B B G G C D D C C Rinc Rdep

  3. Form = f(process)? Linear transport: Qs = KdS? Incision: KaAmSn? BRK = E > production Nonlinear Qs? Cliffs: incision >> production; strength Near Guge, sw Tibet

  4. Outback erosion in brief… 0.9 m/Ma 4.2 & ~10 2.1 m/Ma Bottom line, it still is Slow Down Under…

  5. EROSION OF ROCKY LANDSCAPES - I: FLINDERS RANGES Erosion by slabby blockfall; clasts break down in transit,brief transit time on slope Sampling question: has a surface been recently exposed, or long-exposed ?

  6. Flinders Ranges examples Blocky quartzite: 7.4 m/Ma Avg. fr. quartzite: 5.5 m/Ma Quartzite incision: 4.3 m/Ma Quartzite sand: 3.7 m/Ma

  7. A proposed conceptual framework for the world…

  8. Fate of a primary charged galactic cosmic ray particle ‘a’. Influence on a proton trajectory of a geomagnetic dipole field that is tilted from Earth's spin axis shown as the western half of the field in a meridian plane ‘e.p.' is equatorial plane. Outer shaded region is forbidden to particles of a given insufficient rigidity. Trajectories 1, 2, and 3, arriving from infinity, must pass through the “jaws” into inner allowed region and ultimately follow a dipole field line down the “horn” to atmosphere. Trajectory 4 is impossible due to the opacity of the earth. Distribution of allowed main cone ‘a', Stormer and shadow forbidden cones ‘f ', and the penumbra ‘p' are approximated from for a 10GV positively charged particle at mid-latitude. (Gosse and Phillips, 2001)

  9. Major components of a cosmic-ray extensive cascade showingsecondary particle production inatmosphere and rock. Numbers in rock refer to in situ cosmogenic nuclide interactions:1) 35Cl(n, )36Cl; 2) 16O(n, 4p3n)10Be; 3) 28Si(n, p2n)26Al

  10. From Dunne et al., 1999

  11. From Dunne et al., 1999

  12. Simplest applications: Exposure Age dating and erosion • Major Assumptions: • No “inheritance” of nuclide concentrations • Steady state erosion • Simple exposure history (e.g. no shielding) • Production rate can be constrained • Major Geomorphic Questions tackled: • Exposure age of a surface • Exposure age of terraces (bedrock and deposits) • Erosion rate of exposed bedrock • Soil production rates

  13. Two-isotope diagrams for interpreting 10Be and 26Al measurements. (A) Isotopic trajectory of non-eroding sample exposed continuously at the surface. Numbers to right of curve are exposure ages corresponding to circular symbols. Trajectory ends at saturation where in situ production is equal to decay. In reality, saturation is rarely reached as nuclides are lost by surface erosion.

  14. Exposure histories represented in A and B define a banana-shaped window into which measured samples will plot if they have been continually exposed at the surface. Samples that have been shielded will plot in the gray shaded area below the line of steady-state erosion end points shown in B. Abundances of 10Be normalized to sea level,high latitude. Isotopic trajectories of steadily eroding samples exposed at the surface. Numbers below octagons are rates of erosion in m/Myr. Octagons are end points representing steady-state nuclide abundances resulting from the different rates of erosion. The dashed line represents the continuum of steady-state erosion end points resulting from the spectrum of possible rates of erosion. Erosion acts as an effective decay constant, causing saturation to occur at lower nuclide abundances than in A.

  15. - the simplest application - EXPOSURE AGE DATING cosmic rays rock surface

  16. - And, of key interest to us - long-term EROSION RATES can be inferred cosmic rays eroding surface

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