<p>While a number of studies have investigated bleaching of the optical signals of sediments in rivers and deltas, unified trends and mechanisms for bleaching in these settings remain unresolved. Here, we explore the bleaching of the optically stimulated luminescence (OSL) signal of quartz sediments in a large fluviodeltaic system across time and space, by comparing residual doses of sand and silt from the modern Mississippi River channel with estimated residual doses of sand isolated from Late Holocene Mississippi Delta mouth bar and overbank deposits. Further insight is obtained from a comparison of burial ages of paired quartz sand and silt of Mississippi Delta overbank deposits. Contrasting some previous investigations, we find that the bleaching of the OSL signal is at least as likely for finer sediment as for coarser sediment of the meandering Mississippi River and its delta. In addition we find an unexpected spatiotemporal pattern in OSL bleaching of mouth bar sand deposits. We suggest this may be caused by changes in upstream pathways of the meandering channel belt(s) within the alluvial valley, or by distributary channel and coastal dynamics within the delta. Our study demonstrates that the degree of OSL bleaching of sand in a large delta can be highly time- and/or space-dependent. Silt is shown to be generally sufficiently bleached in both the modern Mississippi River and associated paleo-deposits regardless of age, and may provide a viable option for obtaining OSL chronologies in megadeltas. In addition to informing dating approaches, our work contributes to initiatives to use luminescence signals to fingerprint sediment pathways within river channel networks and their deltas.</p>