干旱区生态环境知识资源中心

Alluvial fans are important depositional landforms that offer valuable records of terrestrial sedimentation history if their surfaces can be mapped and dated accurately. Unfortunately, as this often depends on detailed field mapping and intensive absolute dating techniques, it can be a challenging, expensive and time-consuming exercise. In this study, we demonstrate that quantitative information about the ages of alluvial fan surfaces in Owens Valley, California, is recorded by Landsat-8 multispectral satellite imagery. We show that systematic changes in the wavelength-dependent brightness of fan surfaces occur gradually over a timescale of similar to 100 kyr in this semi-arid setting, and are highly correlated with known deposit ages. Using spectro-radiometry and X-ray diffraction analysis of sediment samples collected in the field, we interpret that surface reflectance evolves primarily in response to the in-situ production of secondary illite and iron oxide by weathering in this landscape. Furthermore, we demonstrate that first-order predictions of absolute fan surface age can be derived from multispectral imagery when an initial age calibration is available. These findings suggest that multispectral imagery, such as Landsat data, can be used (i) for preliminary mapping of alluvial fans prior to detailed field work and before choosing sampling sites for conventional dating techniques, and (ii) to extend age models to undated neighbouring surfaces with equivalent physical properties, once an age-brightness calibration has been established.