TOWARD A POPULATION-BASED LICHENOMETRIC DATING METHOD FOR GLACIAL LANDFORMS

Dating glacial landforms using a population-based approach to lichenometry offers advantages to traditional lichenometry techniques, which typically consider only a small portion of the largest tail of the lichen diameter distribution. First, a population provides a more robust statistic; and second, the error can be quantified.

Our strategy in sampling the lichen population for a single landform follows the Fixed-Area Largest Lichen (FALL) method set out by Bull and Brandon (1998) for multiple age landforms (rockfall deposits). This strategy records the largest lichen individual in a given sample area, with 100 sample areas generally targeted per landform. To date, our primary emphasis has been to use available historic documents and photographs in order to reconstruct the Little Ice Age retreat chronology for the major valley glaciers of New Zealand (Mueller, Hooker, Tasman, Murchison, Classen, and Godley) in combination with lichenometry data to generate a calibration curve for Rhizocarpon Geographicum. Furthermore, by focusing on both moraine and channel landforms, we can isolate the importance of post-depositional instability.

We find that a population-based approach generates a calibration curve, which transitions from an initial phase of rapid growth into a period of much slower, steady growth. Due to this flattening of the curve, there exists an upper age limit past which lichenometry ceases to be effective. Also, an ongoing problem centers on what statistical value best describes a lichen population. An assumption of a normal distribution yields scatter among samples of the same age, with lower error; whereas, an assumption of an extreme distribution yields less scatter, with larger confidence limits, thus yielding increased agreement in ages among different landforms.