Modern Sedimentary Characteristics Of Lingding Bay,pearl River Estuary And Its Implications For Human Activities

Intensive human activities at the coast have increasingly impacted fluvial sediment source-to-sink processes and associated geomorphological characteristics in estuarine,coastal,and continental shelf environments.Therefore,many deltas around the world are facing system regime shifts.Human activities in catchment and delta since the1970s,including dam construction,sand mining,channel dredging,and land reclamation,directly perturb hydro-morpho-dynamics and sedimentary processes in the Pearl River Delta(PRD).Studies of human activities'impacts on the sediment dynamics and geomorphologic processes using sedimentary records in the PRD would provide scientific guidance for predicting the evolution of the PRD and sustainable development of the Guangdong-Hong Kong-Macao Greater Bay Area(GBA).In this study,250 surficial and 15 short core sediments collected in December 2016(dry season)and August 2017(flood season)in the Lingding Bay(LDB),PRD were used to analyze grain size,environmental magnetism,organic geochemistry,alkaline-earth metals,and radionuclides activities.Together with compilation of previous studies,this study tends to identify the sedimentary records of human activities and to discuss the human impacts on sediment transport,sedimentary pattern and mechanisms in the LDB.The main results obtained are as follows:1.The grain size analysis of surficial sediments collected in the LDB showed that the anomalous patches of sandy and gravelly deposits were mainly distributed in the navigation channels and on adjacent shoals,indicating that human activities such as channel dredging and sand mining have strongly perturbed the composition of surficial sediments.The grain size end-member and grain size trend analysis of surficial sediments showed that there are two convergences of fine-grained sediments located in the northeastern and southwestern of LDB,and fine-grained deposits increased significantly in the flood season.2.The distribution characteristics of strontium(Sr)and barium(Ba)in the surficial sediments indicated that the runoff and tidal action were strengthened in the flood season,so it was speculated that the convergences of fine-grained sediments in NE LDB was the result of the strengthened fluvial-tide interaction.In recent ten years,the increasing of terrigenous organic carbon contribution in LDB may indicated the enhancement of runoff input.However,the extremely weak magnetism indicated that the early diagenesis of the surficial sediments were significant in the east of LDB,which showed that the surficial sediments were mainly from marine-sourced imported by tidal current.Therefore,it was speculated that human activities such as sand mining and channel dredging have re-exposed the older sediments on the present seabed,thus causing terrigenous organic carbon dominated,but significant early diagenesis of the surficial sediments in the east of LDB.The excessive ²¹⁰Pb(²¹⁰Pb_ex)analysis further showed that the top layer in the core located in the convergence of NE LDB was perturbed by human activities and consists of‘older'sediments.3.The formation of the convergence of fine-grained sediments in the SE LDB should be related to the changes in hydro-morpho-dynamics in the western channel of Qi'ao Island in past few decades,which has shifted from flood-dominated in 1970s to ebb-dominated in 2010s,reflecting the influences of reclamation and shoaling of West Shoal.Vertical sedimentary sequences of short cores located in the convergence of SW LDB also confirmed that natural deposition dominated.4.The ²¹⁰Pb_ex activities in the short cores near the shoal of outer Lingding Channel were strongly perturbed,and the deposits were obviously coarser or even gravelly deposits at the top layer in the cores,reflecting that the channel dredging and dumping activities caused the alternate distribution of the‘older'sediments with the‘younger'modern natural sediments in the strata.5.The short cores in the continental shelf outside the LDB record an increase in the terrestrial sediments input,which may be related to the shoaling of LDB.The seasonal changes in compositions of surficial sediments further indicated that part of fine-grained sediments deposited in the flood season were re-suspended and transported to the continental shelf in the dry season.