Application of 210Pbex in elucidating contemporary sedimentary dynamics in the water-level fluctuation zone of the Three Gorges Reservoir, China

Abstract

Excess 210Pb (210Pbex) has been widely applied as a proxy indicator for establishing the chronology of sediments in lakes where the sedimentation processes comply with the assumptions for using 210Pbex dating models. However, little is known regarding the applicability of 210Pbex to elucidate contemporary seasonal sedimentary dynamics in reservoir water-level fluctuation zones (WLFZs). In the present study, sedimentary core and bulk sediments were collected from the WLFZ of the Three Gorges Reservoir (TGR) in China, and the physical and chemical properties were determined. Results for 210Pbex activity, total organic carbon, and total nitrogen of bulk sediments indicate pronounced elevation-dependent spatial variation (p < 0.05), but 137Cs activity and particle composition show no systematic trends, allowing 210Pbex activity to be used to differentiate seasonal stratigraphy. For the sedimentary core, therefore, sediment layers with lower 210Pbex activity were inferred to represent sedimentation during wet seasons, when the 210Pbex concentration was diluted by a higher suspended sediment load, whereas sediment layers characterised by higher 210Pbex activity were ascribed to sedimentation during dry seasons, when the tranquil water favoured 210Pbex fallout to be absorbed by suspended sediments and then deposited in the WLFZ. Hereupon, temporal variation is observed in annual sedimentary thickness, with relatively low sedimentation rates during the initial stage (2006–2008) of the TGR impoundment, followed by relatively high sedimentation rates from 2009 to 2013. Over the period from 2014 to 2017, sedimentation rates were low because the suspended sediment was intercepted by upstream cascade reservoirs. Subsequently, sedimentation rates were higher during 2018–2019 as a consequence of an increase in extreme rainfall events. The findings herein highlight that 210Pbex-based seasonal stratification discrimination is effective approach to establish high-resolution sedimentary chronosequences and that net sedimentation rates in WLFZ are controlled by inflow suspended load of the TGR.