Accuracy of Cosmogenic Ages for Moraines Jaakko Putkonen and Terry Swanson Presented by Andrea Hatsukami ESS 433

Background There is a common assumption that ages of surface boulders are the same as the age of the landform Average age range of ~38% between oldest and youngest boulders from each moraine Analysis shows that 85% of all moraine boulders predate the dated moraine

Purpose Analyze uncertainty in moraine ages found with cosmogenic dating techniques Use numerical simulations to analyze moraine surfaces and how they change over time Determine the best method to follow in order to obtain the most accurate values

Assumptions Published boulder ages are accurate Duration of exposure of boulders is assumed to be equal to the moraine’s age Moraine formation is instantaneous

Methods (1) Formula: dz/dt = -dq/dx Variables: Application: dz/dt  rate of change of the local surface elevation q  divergence of sediment flux x  horizontal distance in a plane perpendicular to the moraine axis Application: Any diffusive sediment-transport process Motion restricted down the path that follows the steepest slope

Methods (2) Formula: q = -κ*dz/dx Variables: Application: κ  volume of sediment moving down slope per unit time and per unit length dz/dx  slope inclination Application: Expresses the general increase in soil flux with slope steepness For short slopes, typically ranges between 10^-2 and 10^ -4 m2 yr-1 over a wide spectrum of geographic areas, climates and alluvial substrates For longer slopes, as high as 20 m on erosional scarps in alluvium, increases linearly with distance downslope

Methods (3) Formula: κ = α + βx Variables: Applications: α  site-specific constant (m2/yr) β  site-specific constant (m/yr) Applications: Topographic diffusivity

Methods (4) Combine the transport equation, expressions for conservation of material and for κ to produce Formula: dz/dt = d/dx (κdz/dx) = κd2z/dx2 + βdz/dx Variables: dz/dx  slope inclination dz/dt  rate of change of the local surface elevation κ  volume of sediment moving down slope per unit time and per unit length β  site-specific constant, units m/yr Application: Generalized diffusion equation for topographic evolution Generalized diffusion equation for topographic evolution One more equation that does not tie into the diffusion equation but is important: The normalized age range is calculated by dividing (normalizing) the maximum and minimum boulder ages in a set from a single moraine by the maximum boulder age. The difference between these numbers is the normalized age range

Application Topographic evolution through time, including the surface lowering rate at the moraine crest. Determination of the rate that new boulders are exposed at the moraine surface

Stages of determining cosmogenic age Three separate stages: Length of prior exposure before boulder was incorporated into matrix Length of the burial and related decay of derived isotopes Length of re-exposure at the surface

Results Small moraines (<20 m initial height) have degraded little Old and tall moraines ( >40,000 yr, >50 m) degradation is a serious problem Desirable to sample large, completely exhumed boulders and smallest moraines that have experienced minimal crest lowering All cosmogenic exposure data should be published, even if age range is large Largest boulders (diameter 10 m) may also be rockfall deposits rather than products of basal erosion of the ice.