More people die on dusty days (Thorax, 2013)

How do we date landscapes and measure erosion rates?

slope profile evolution by diffusion erosion convex upwards curvature concave upwards curvature deposition Applied to dated fault scarps Talus wedge VE = 2

Scarp diffusion methods: 1) midpoint method

Erf:

Scarp diffusion methods: 1) full-fit method (Pelletier et al., 2006)

Which scarp is younger? What are the approximate ages of the scarps? Do they represent one earthquake or several? Please read: Pearthree and Calvo (1987)

Radio-isotope chronometers

“Exposure” chronometers

Cosmo Isotope production versus depth Gosse and Phillips, 2001

The case of glacial erosion

TCN Accumulation Stable TCN – linear increase Radioactive TCN – initial increase to steady state Concentration (atoms/g) Time 3 He, 21 Ne

N=concentration P=production rate =decay constant T=time Exposure dating requires:

(1) TCN Production increases with latitude. (2) TCN production increases/decreases with changes in geomagnetic field.

(3) TCN Production increases with elevation. Sea Level 50,000 m

Shielding of cosmic rays by surrounding topography

Production Rates - Production rates vary with TCN and mineral type. - Target atoms have to be bigger than spallation product. TCN Prod. Rate (at/g/yr)target 3 He 115olivine 21 Ne21, 38Qtz, Ol 10 Be 6quartz 14 C15quartz 26 Al37quartz 36 Cl67, 137Ca, K

Production (and accumulation) of TCN also affected by: (1)self-shielding (2)Topographic shielding (3)Erosion (4)Burial Uncertainties in TCN dating: (1)Calibration/measurement of production rates. (1)Changes in geomagnetic field over time, particularly Holocene. (2)Previous exposure.

With constant exposure ratio of isotope production eventually decreases

Upon burial or shielding ratio decreases below the constant exposure line

Sampling Strategies : -surface stability (i.e., desert pavements, desert varnish). -Highest, flattest surface on deposit. -Largest, flattest boulder on deposit. Sample Preparation -crush rocks -Physical and chemical mineral-separation processes. - 3 He, 21 Ne: melt mineral at 1400 C under vacuum, measure gas on mass spectrometer. -Radioactive TCN: chemical processes to extract element of interest. Isotopic ratios measured on AMS.

Applications of TCN: dating surfaces, estimate rates of geomorphic processes. (1) Estimating Fault Displacement Rates.

Displacement Rates on the Toroweap and Hurricane faults

Thermoluminescence / Optically stimulated luminescence Background

TL/OSL measurement

TL ‘saturation’

Uranium-series dating I U-238 Po-210Pb-206Pb-210 U-234 Rn-222 Th-230Ra-226 (stable) 4.5 x 10 9 years days years days 2.5 x x x 10 3 years U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium

Uranium-series dating II U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead; U-235Pa-231 Pb-207 Th-227 Ra-223 (stable) 7.1 x 10 8 years 3.2 x days 11

Blisniuk and Sharp (2003)