Holocene Depositional Environment Evolution And Human Activity Records In The Shunde Plain,Pearl River Delta

The evolution of the Holocene sedimentary environment in the delta region is resulted from the superposition of natural factors and human activities at different temporal and spatial scales.As one of the large deltas in the world,the Pearl River Delta has a unique net-river and estuary system.At the same time,the Pearl River Delta region is densely populated and economically developed,facing many opportunities and challenges in the context of global changes.Therefore,by studying the Holocene evolutionary processes of the sedimentary environment in the Pearl River Delta,identifying the land-sea interaction and the records of human activities in the sediments,it is beneficial to better understand the mechanism of the delta evolution driven by natural factors and human activities.It provides scientific basis and theoretical guidance for predicting environmental evolution,coordination between human activities and ecological environment and sustainable development of economy and society in delta area.In this thesis,we collected three boreholes(LY,HP and MZ seaward in arrangement)near the Hongqili Channel in the Shunde Plain,the Pearl River Delta,and the three boreholes were analyzed for sedimentology,geochronology and geochemistry.Meanwhile,the C/N and?¹³C data of surface sediments,soil and plant samples in the Pearl River Basin and river mouth and borehole data of chronologies and?¹³C values in the delta region were collected and compared.This study reconstructed the evolution of sedimentary environment in the Shunde Plain of the Pearl River Delta during the Holocene and the records of human activities in the delta sedimentary strata.Main results were obtained as follows:1)The Shunde plain of the Pearl River Delta experienced the filling of incised valley,embayment accumulation and deltaic progradation.The Holocene sediment thickness is 9.93 m in LY,based by mottled clay.Piedmont alluvial-diluvial sediments overlie the mottled clay.The sedimentary environment changed from flood jet channel to subaqueous distributary channel and delta plain successively over past ca.1400 years.The Holocene sediment thickness is 17.70 m and44.29 m,in HP and MZ respectively,based by gravelly channel lag.Core site HP was drowned as embayment at ca.6.2 cal.kyr BP and the deltaic suqeucene started to form at ca.4.0 cal.kyr BP.Tidal river accumulation occurred at core site MZ in the early Holocene,and the site was drowned as embayment at ca.9.8 cal.kyr BP.Detaic sequence started to accumulate there at ca.2.8 cal.kyr BP.2)The changes of alkaline earth metals in present cores reflect the early Holocene transgression and mid-to late Holocene deltaic progradation.By contrast,the variability of organic carbon compositions lacks the response to climate and sea level changes,and terrestial organic carbon prevailed in the early Holocene.These results may indicate that the diluted water prevailed,while the high-salinity shelf water is weak in the semi-closed embayment during the early-middle Holocene.An alternative explaination is that the tidal mixing is strong in the embayment due to the broken and complex topographic boundary,which prohibited the water stratification and associated high primary productivity.The?¹³C in the borehole MZ increased slightly at ca.4900 cal.yr BP,and the C/N decreased significantly at ca.2800 cal.yr BP,which indicates the increasing input of marine-sourced sediments induced by the enhanced tidal action caused by the topographic changes and net-river diversion.3)The?¹³C of the sediments in the Pearl River Delta increased significantly in the past 2000years,which reflects the increased contribution of C₄ plants in the Pearl River Basin and is consistent with the increasing sugarcane planting in the Basin from the Han Dynasty and the introduction of corn from the late Ming and early Qing Dynasty.4)The sedimentary strata in the boreholes record the reclamation activities near the mouth of the Pearl River Delta from the Ming and Qing Dynasties:the reclamation by riprap and embankment weakened the scour effect of the river flow,and accelerated the trapping of fine-grained sediments within the embankment,thus accelerating the landforming process of the delta.