Temporal And Spatial Distribution Of Magnetic Characteristics Of Sediments In The Pearl River Delta And Their Implications For Sedimentary Environment Evolution And Early Diagenesis

The early diagenesis of magnetic minerals is an important process after the burial of sediments.The process changes the original magnetic mineral assemblage and affects the interpretation of mineral magnetic proxies for the sedimentary environment.Therefore,the in-depth study of early diagenesis can improve the application of this technique.This study carries out the sedimentology and environmental magnetism analyses using the surficial sediments collected from the channel network of the Pearl River delta and three Holocene drilling cores(MZ,HP and LY).By comparing the magnetic properties of the surficial and borehole sediments,this study aims to investigate the implications of magnetic properties for the sedimentary processes in the modern river mouth,to identify the stages of early diagenesis in the buried sediments,and to discuss the formation mechanism of early diagenetic products.The following main conclusions are obtained.1)The surface sediments of the river bed of the Pearl River Delta in the flood and dry season all present that coarse particles in the top area of the delta plain and fine particles in the mouth area,which reflects the dynamic difference between the river action in the top and the tide action near the mouth.The magnetic variation of surface sediments is closely related to grain size of sediments and hydrodynamic conditions.The high values of?and?_fd%mainly appear near the mouth in the dry season,indicating that magnetic minerals are mainly concentrated in fine-grained sediments,which are exported to the sea with runoff in the flood season,and imported into the river network with tidal current and saline intrusion in the dry season.Even in the fluvial-dominated distributary channel,the runoff has strong seasonality,and the tidal effect is strengthened in the dry season,which has obvious influence on the braided channel deposition.The stronger the effect of runoff in estuary,the greater the difference of magnetic in flood and dry seasons.Therefore,combined with the comparison of?and?_fd%,it can well reflect the changes of hydrodynamic conditions of the river and the estuary.2)The Holocene depositional environment of the three cores changed significantly.Core MZ from bottom to top is tidal channel of early Holocene,estuary bay of early-middle Holocene,and delta accretion sequence of late Holocene.Core HP is composed of estuarine bay and delta sequences dating from the Middle Holocene.Core LY is mainly composed of distributary channel and delta plain facies strata in late Holocene.3)The magnetism of the sediments in each cores generally shows a tendency of weakening from the top down.The magnetic properties of core HP and core MZ are strong in the last 2000 years,which is consistent with the rapid increase of population in the Pearl River Basin since the Han Dynasty.It is speculated that the rapid increase in the population of the basin led to increased human activities such as deforestation,land reclamation and mining,which intensified the erosion of rock and soil in the basin,thus increasing the content of ferromagnetic minerals transported to the estuary.4)The magnetic characteristics of the cores sediments generally lack the relationship with the changes of sedimentary facies,and there is no correlation with the grain size composition of the sediments,and the magnetism of the middle and lower strata is generally very weak,which is speculated to be strongly influenced by early diagenesis.According to room temperature magnetic parameters and thermal magnetic curve characteristics to recognize magnetic mineral combination of different depths,which appear the original magnetic minerals found at the top of the core,and reduced downward with the emergence of iron-sulfur authigenic mineral,shows that the early diagenesis in the core stratum of the pearl river delta is given priority to with magnetic mineral dissolution and pyrite mineralization process.The sulfate-methane transition zone(SMTZ)in MZ core was identified based on magnetic characteristics and magnetic mineral assemblages,and the stratum of extremely weak magnetic properties in core is in anaerobic oxidation of methane.5)Authigenic greigite,an intermediate mineral in pyritization process,was found in some sections of delta front and estuary bay facies strata of HP and MZ cores.According to the magnetic mineral assemblage and vertical distribution characteristics,it can be inferred that the gerigite in delta front facies was formed in the sulfate reduction stage of organic matter,and the greigite in estuary bay facies was formed in the anaerobic oxidation stage of methane.The authigenic mechanism of the two greigites essentially reflects the difference in the supply of reactive organic carbon and sulfate ions in different sedimentary environments.