Chronology And Environmental Evolution Of The Pearl River Delta Since The Middle Pleistocene

The estuary delta has witnessed the“alternation of land and sea”and recorded the geomorphological landscape,sedimentary environment,tectonic activity,hydrodynamic conditions,sediment flux,biological assemblage,human activities and other kinds of information of the area in different periods.It has become a key area for studying global change.The Pearl River delta（PRD）is located in South China,facing the South China Sea.Compared with other delta in China,the bedrock height of the PRD is higher and the Quaternary thickness of it is thinner.The Quaternary deposits are composed of sediments that formed under two sets of marine transgression-regression cycle.Meanwhile,the fault zone of this area is developed.The PRD is a complex delta,with multiple sedimentary centers.These characteristics make the sedimentary environment and delta construction process of the PRD have both similarities with other estuarine deltas and its own particularities.With decades of research,the sedimentary process of the late Quaternary strata in the PRD and the mechanism behind it are well known.But all along,the chronological issue has been one of the thorny problems.The time of re-starting sedimentation in the plain area during the Quaternary,and the time of the first transgression event are still inconclusive.These problems limit the complete understanding of delta evolution model and are the key problems to be solved in the research.As an absolute dating technology,luminescence dating has relatively high accuracy and reliability,and a wide range of dating.The required materials,including quartz and feldspar,are very rich in delta sediments,which is expected to be an important way to solve the chronological problem.This study takes the sediment cores in the PRD as the study objects,including cores HPQK01 and HPQK02 which were drilled from the hinterland of delta plain,and core YJ01 which was drilled from coastal area that near the Lingdingyang estuary.Integrated application of quartz Optically stimulated luminescence（OSL）,feldspar post-infrared stimulated luminescence（post-IR IRSL/p IRIR）and AMS¹⁴C dating methods were used to establish the chronology framework of these three cores.Combined with the lithological characteristics of sediments and micropaleontological assemblages,including foraminifera,ostracods and diatom,the late Quaternary sedimentary history of the PRD was established,and the environmental evolution of the area since the late Quaternary were reconstructed.The role of tectonic activities,sea level changes and human activities on these processes was discussed.Our dating results are the oldest at about 250 ka and the latest to 0.24 ka.The feldspar p IR₅₀IR₂₅₀ corrected ages show that the deposit time of lower continental strata in PRD is between 250 ka and 120 ka.The age of lower marine strata and the mottled clay layer is between 123 ka and 85 ka.The upper fluvial strata have a quartz OSL age of 53±5 ka.The quartz OSL ages of the upper marine strata and the top delta plain sediments are between 9.01 ka and 0.24 ka,and the AMS¹⁴C ages of these units are between 7.97 cal ka BP and 1.11 cal ka BP.By systematically applying the feldspar p IRIR dating method to the lower-cycle strata of the PRD,we have clarified the absolute chronology framework of this strata,and determined the sedimentary time of the lower marine unit at MIS（Marine isotope stages）5,and the starting development time of Quaternary drainage system in the plain area at MIS 7.Based on the AMS¹⁴C age,it is believed that the first transgression event occurred during MIS 3 to MIS 2.Our results now show that this cognition is problematic.Combined with the information provided by the three sediment cores in this study and previous work on the late Quaternary strata in the PRD,we rebuild the history of sedimentary environment changes and the resulting geomorphologic landscape changes in the study area since the late Middle Pleistocene.At MIS 7,the whole study area was in a state of denudation,the riverbed facies sediments were deposited where only near the fault zone,or near the river estuary.During the MIS 6 period,the river system development tended to be mature,forming the lower terrestrial strata.During MIS 5,influenced by global sea level rise,the late Pleistocene marine transgression event occurred in the study area,forming the lower marine strata.During MIS 4-MIS 2,the geomorphic pattern showed as valley and interfluve alternative distribution,and the upper terrestrial strata were developed.In details,the old marine strata were incised by river to form small valleys;and the interfluve blocks were deeply weathered and developed a widespread paleosol on the top,which is called as mottled clay today.During MIS 1,the marine transgression event occurred during early Holocene,and the upper marine strata was developed in the PRD.During the mid-late Holocene,the delta plain gradually expanded and matured,and the delta plain sedimentary facies were developed.In these processes,the driving force for the change of sedimentary environment and landscape is different in different periods.At the early stage of the development of the PRD,which is in the middle Pleistocene,runoff basically developed along the fault zone,and mainly controlled by the fault activity and tectonic subsidence.After entering the late Pleistocene,the basement ground sank to a certain height,combined with the sea level rose during MIS 5,the PRD experiencing the first marine transgression event.Since the late Pleistocene,the evolution of the regional sedimentary environment and the construction of the landscape have little connection to do with the regional tectonic activities,but the sea level changes have played a major role in controlling.Since 2 ka,especially since modern times,the influence of human activities on the sedimentary environment have become more and more obvious.