The IRSL Signal Characteristic Of Known Aged (Modern) Rock Sample Surfaces And It's Significance

Optically Stimulated Luminescence(OSL)age-dating has been successfully applied to determine the exposure ages of rock surfaces.This technique is widely applicable in the fields of geology,geomorphology,climatology,and archaeology.The OSL dating method can accurately measure the age of bedrock and gravel exposed after a particular geological event.And It is important to understand the characteristics and affected factors of Infrared Stimulated Luminescence(IRSL)signal in the surface layer of rock.We selected Reddish-brown argillaceous feldspar sandstone and hoary biotite-granite as samples and designed a series of rock surface bleaching experiments.The light bleaching experiment results show that the bleaching rate of light-colored granite is much higher than that of the opaque dark-colored sandstone.The IRSL signal is bleached in certain depth into the rock surface after a given exposure time,and the longer the bleaching time is,the deeper the bleaching depth.The IRSL signal of granite rock surface can be rapidly bleached during a short time(eg.3 h).This study also reveals that IRSL signal is just originated from a certain depth of the slice(1.2 mm thickness)and the slice is not fully penetrated by infrared light.It is important to study factors that may affect the bleaching rate of the IRSL at 50?signal from a rock surface,including sample lithology,sampling surface orientation,whether the rock surface has a patina(rock varnish)and altitude of the area.To analyze the effects of these factors,we performed a series of rock surface bleaching experiments on reddish-brown argillaceous feldspar sandstone,grey biotite granite,and granite breccia.The results show that the IRSL signal at certain depths beneath sandstone or granite rock surfaces can be quickly reset by exposure to sunlight.The longer the bleaching time,the deeper the bleaching depth.Furthermore,the bleaching rate of the IRSL signal in lighter granite is much higher than in darker sandstone.The sampling surface orientation(i.e.,upward or northward facing)has no significant effect on the resetting depth of the IRSL signal.After sunlight exposure for ca.5114 hours,however,the IRSL luminescence-depth profiles within granite show that the effective luminescence decay rate(?)and the inflection depth from the rock surface of bleaching inflection are higher for an upward-facing surface than for a north-facing surface,thereby indicating that rock surface orientation exerts an influence on age-dating results after ca.5114 h of exposure.For a short exposure time(40 h),the presence of a rock varnish may have a shielding effect and significantly reduce the bleaching rate of the IRSL signal,whereas a rock surface without rock varnish has a deeper bleaching depth.The shielding effect of the rock varnish decreases when the surface exposure time increases to ca.2416 h.Luminescence dating is a potential method which could be applied to determine the burial ages of rock surface.As part of testing the reliability and applicability of this new technique,we sampled a series of modern riverbed cobbles along an endorheic river(Shiyang River)in northwestern part of China.The IRSL signal as a function of depth was measured into two different kinds of cobbles(sandstone and granite).The results show that,(i)the signal bleaching rate for light-coloured granite is faster than the opaque dark-coloured sandstone,shown as granite has been bleached to greater depths than sandstone,(ii)the daylight bleaching depths into cobbles show downstream increasing tread,with almost all bleaching occurring in the upstream section,(iii)despite possible abrasion of the upper surface of granite cobble,the bleaching depth in the upper surface is greater than that in the lower surface,indicating longer exposure time for the upper side.This study advances our understanding of IRSL signal bleaching within rock surfaces,and provides a useful theoretical basis for the selection of appropriate rock surfaces for sampling to improve the accuracy of OSL age-dating.This work has important implications for the dating of fluvial deposits and the potential of luminescence techniques for understanding transport/deposition in fluvial environments.