Estimating the age of desert alluvial surfaces with spaceborne radar data

We present a new dating approach for desert alluvial surfaces that utilizes radar image data to obtain regional-scale correlations between dated surfaces and surfaces of unknown ages. The study was carried out along the Arava segment of the Dead Sea Transform north of the Gulf of Elat (Aqaba) using Advanced Land Observation Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data with 6–12 m/pixel resolutions. Nineteen independently dated Quaternary morphostratigraphic alluvial units (5 to 1910 ka) from eight different sites with variable lithologies (carbonate/magmatic/chert) were examined in order to develop, test and validate the proposed approach. Expanding on previous studies that established the characteristic smoothing of desert alluvial surface through time and the capability of measuring such roughness variations with radar, we employed a pixel-aggregation approach that yielded a robust (R² = 0.95) power-law relation between the average radar backscatter value from the surface (‘AR’) and the abandonment age (‘T’) of the unit as measured with independent methods such as luminescence and/or cosmogenic radionuclide dating. We found that: 1) This AR-T correlation can be inverted to obtain calibrated radar-based T estimates as old as ~ 1.5 Ma with 35% uncertainty; 2) ALOS PALSAR 6 m/pixel images acquired at incidence angles of 34–38° yielded optimal results; 3) a sample of at least 70 contiguous radar pixels was required to capture the natural roughness variability of the surfaces examined, and 4) surface lithology did not exert a primary effect on T inversions. Validation experiments yielded radar-based ages of 3 ± 1, 91 ± 32, 484 ± 169 and 1604 ± 561 ka that compare to Holocene, 56 ± 14, 540 ± 60 and 1590 ± 250 ka ages previously determined for the same surfaces, respectively. We propose that with region-specific calibrations, spaceborne radar data can be used to quantitatively estimate the age of abandoned alluvial units across regional scales.