PHOTOSENSITIVE SILVER CHLORIDE: A PROXY FOR COSMOGENIC NUCLIDES IN EXPERIMENTAL LANDSCAPE MODELING?

Cosmogenic nuclides are commonly used to infer erosion rates on geologic timescales. Experimental models of erosional systems could test implicit assumptions, particularly the effects of stochastic sediment delivery and transport, and degree of sediment mixing. A potential proxy for cosmogenic nuclide ingrowth is silver chloride (AgCl), a photosensitive compound commonly used in photography. AgCl darkens under exposure to UV light similarly to cosmogenic nuclides produced from cosmic rays: AgCl molecules closest to the surface darken most, while AgCl molecules located a few microns below the surface darken to a lesser degree. This pattern of decreasing darkening with depth mimics the exponential decrease in cosmogenic nuclide production with depth. Also, increased exposure time to UV light results in a darker surface, similar to higher concentrations of cosmogenic nuclides.

AgCl can potentially provide a highly visual survey of spatial and temporal fluctuations in erosion, transport, and sediment mixing. The darkest areas of the landscape, where the darkest AgCl has formed or is in temporary storage, represent the areas of highest cosmogenic nuclide concentrations, from which low erosion rates are inferred. Conversely, the lightest areas of the landscape correlate with short residence times.

Initial research focused on quantifying the effects of UV light on AgCl, to determine its suitability as a cosmogenic nuclide proxy and calibrate the use in experimental models. Parameters investigated include 1) darkening rate; 2) degree of darkening, measured in greyscale and three color bands; 3) different UV light intensities; 4) effect of water on darkening; 5) effect of different sediment:AgCl mixing ratios on darkening. We encountered an additional challenge in reducing AgCl to a fine powder suitable for use in an erosional experimental model. Additional research included investigation into a suitable experimental erosional model. An initial erosional experiment was performed in an existing one meter diameter tank at St. Anthony Falls Laboratory. The sediment blend was 95% fine silica and 5% kaolin, eroded by lightly misting sprinklers. A slowly lowering gate (8 cm / hour) simulated an uplifting system. The sediment – water outlet was a centimeter-wide incision in the gate. The model simulated a complex, stochastic landscape with migrating ridges and valleys.