Seeking enlightenment of fluvial sediment pathways by optically stimulated luminescence signal bleaching of river sediments and deltaic deposits
2019
Abstract. Reconstructing sediment pathways in fluvial and deltaic systems
beyond instrumental records is challenging due to a lack of suitable
methods. Here we explore the potential of luminescence methods for such
purposes, focusing on bleaching of the optically stimulated luminescence
(OSL) signal of quartz sediments in a large fluviodeltaic system across time
and space. We approach this 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.
In contrast to 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. We attribute this to the
differences in light exposure related to transport mode (bedload vs.
suspended load). 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 signal 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 silt may therefore provide a
viable option for obtaining OSL chronologies in megadeltas. Our work
contributes to initiatives to use luminescence signals to fingerprint
sediment pathways within river channel networks and their deltas and also
helps inform luminescence dating approaches in fluviodeltaic environments.
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