Instruments and Methods Identifying annual peaks in dielectric profiles with a selection curve

A novel 'selection curve' method is developed to interpret annual layers in the West Antarctic ice sheet (WAIS) Divide ice core based on dielectric properties (DEP). Because dielectric measurements are non-contact and represent the integrated response of the ice volume, they are particularly useful for the brittle zone of the core. Seasonal differences in ice chemistry create an annual signal in DEP, though multiple peaks of varying strength within a year may complicate the interpretation of annual layers. The selection curve algorithm uses a spline curve whose shape selects successive annual peaks in plots of DEP. This spline curve was scaled to the average annual-layer thickness at a given depth, where the layer thickness was best estimated using the fast Fourier transform (FFT) power spectrum within a sliding 10 m window. To explore the accuracy and stability of the method, several spline curves were generated from varying lengths of calibration data taken from multiple depths in the WAIS core. Using 50 m of manually interpreted calibration data, the selection curve method matched a manual interpretation throughout the entire 1200 m dataset to within 2% root-mean-square error (RMSE). This method is equally applicable to glaciochemical and other time/depth series measurements.