Post-glacial small delta process uncovered by luminescence and radiocarbon chronology of core sediments from coastal South China Sea

Abstract

Reliable chronology is crucial for reconstructing delta processes. The past decades witnessed a boost of dating works on the most economically influential large deltas (i.e., subaerial area >1000 km²), but chronology remains lacking on many small deltas (i.e., subaerial area <1000 km²) that are also densely populated and economically active. Luohe River Delta (LRD) in the coastal South China Sea is such a small delta, whose evolution concerns hundreds of thousands of people, and literally no reliable dating results have been reported to support research of its processes. Herein, dating work combining radiocarbon (¹⁴C) on mollusk shells and quartz Optically Stimulated Luminescence (OSL) methods were performed on core LFZK06 from LRD, to test applicability of both methods by age comparison and to establish a chronological framework, using Bayesian age-depth models, of LRD for the first time. ¹⁴C ages are systematically c. 0.5–1 ka younger than OSL ages from the same depths. Such young bias of ¹⁴C ages in LRD contrasts with previously observed overestimated ¹⁴C results in the nearby Pearl River Delta (PRD), likely due to the distinction in bedrock types between Luohe River (granites and sandstones only) and Pearl River (limestone prevalence causing hardwater effect) drainages. Ages of core LFZK06 span from 57 ± 7 ka to 3.42 ± 0.05 cal ka BP and contain a hiatus between 57±7–11.9 ± 1.7 ka. Holocene deltaic sequence of the core shows three-stage sedimentary processes: (1) rapid deposition of prodelta/delta front sediments at ${10.2}_{‐0.4}^{+0.5}$ ka–${7.7}_{‐0.3}^{+0.4}$ ka related to rapid sea level rise, (2) hiatus during ${7.7}_{‐0.3}^{+0.4}$ ka–${5.7}_{‐0.8}^{+0.5}$ ka likely due to reduced sediments input or river channel migration, (3) rapid accumulation of delta plain sediments during ${5.7}_{‐0.8}^{+0.5}$ ka–${3.42}_{‐0.85}^{+0.42}$ ka reflecting depocenter shift toward core location. Moreover, changes of quartz OSL sensitivity were detected, indicative of sediment provenance transition. A change in sediment source from nearby granite weathering towards fluvial long-transported materials occurred at a depth of 24.7 m, with low quartz OSL sensitivity in the gravelly layer below (11.9 ± 1.7 ka) and one magnitude higher quartz OSL sensitivity in prodelta deposits above (${10.2}_{‐0.4}^{+0.5}$ ka).