Single-grain post-IR IRSL signals of K-feldspars from alluvial fan deposits in Baja California Sur, Mexico

Abstract

Single grains of K-feldspar from alluvial fan units are dated using a more time-stable signal, the post-infrared infrared stimulated luminescence, or, ‘post-IR IRSL’. A quick measurement protocol is discussed, ‘fast post-IR IRSL,’ that stimulates first with the IR diodes at the lower temperature and then measures grain-by grain at the higher temperature. A criterion is offered for rejecting outlying grains based on hierarchical clustering. Single-grain fading rates are found to diverge from single aliquot fading values, and the fading rates from the brightest subset of grains correspond well with an infinite age cobble and independent age control. Age comparison with a cosmogenic depth-profile age shows agreement at 1σ. The depositional chronology suggests that the climate responsible for regionally-extensive, upper-regime floods which aggraded the older units, transitioned into a climate producing weaker channelized floods around the Late Pleistocene–Holocene transition.