Migration Of Land-ocean Interaction Interface-estuarine Front In Holocene Sedimentary Record Of The Yangtze River Mouth And Its Mechanism

Recently,the accelerated sea level rise induced by polar ice sheet melting combined with sharp decline in Yangtze's sediment load has increased greatly the risk of intensive disasters of the Yangtze delta such as storm surge,coastal flooding,coastal erosion and saltwater intrusion.The Yangtze delta is densely populated and is one of the most important economic centers in China,but the risk of hazards will largely restrict its economic and social development.Therefore,it is urgent to develop effective management stratigies to get prepared for the future challenge.To understand the Holocene change of land-sea interaction in the river mouth area will help understand its present situation and response to the predicted sea level rise.Five sediment cores HZK8,HZK1,HM,NT and TZ were obtained from the paleo-incised Yangtze valley and were analyzed for lithology,grain size,AMS14C ages and organic elemental geochemistry including TOC,TN and ?13C.Foraminiferal analyses were also carried out for some of the cores.We also collected all available data of TOC,TN and ?13C for the suspended sediments in the Yangtze drainage basin and surficial sediments of the Yangtze River mouth and adjacent shelf to reveal the distribution of different types of organic carbon in the river mouth and its linkage to the sediment dynamics in estuary.We then recognized the Holocene changes in organic carbon sources in seven sediment cores(five from this study and two from previous work)from the apex to the subaquous Yangtze delta.We aim to reveal the migration of the Yangtze estuarine front in Holocene and to discuss its linkage to the sea level and monsoon climate change and the delta progradation.The water mass in present Yangtze River mouth can be divided as freshwater,mixed water and plume water,between the latter two is the estuarine front in flood season.The ?13C values of suspended sediments from drainage basin and surficial sediments from ares dominated by estuarine mixed water and freshwater are mostly lower than-24‰,while surficial sediments from area of plume water(ie.prodelta)and shelf are higher than-24‰,suggesting the organic carbon sources of being terrestrial plants freshwater algea and marine algae,respectively.Therefore the estuarine front between mixed water mass and plume water mass is an important boundary for trapping the terrestrial organic carbon within the river mouth.C/V and ?13C in six cores presented a dominant terrestrial organic carbon during c.12-10 cal.ka BP,indicating that estuarine front had not intruded into the paleo-Yangtze estuary at that time.We suggest the low sea level controlled the location of the estuarine front.Further,the increase in river discharge due to the strengthening Asian monsoon might also play important role in the prevailing of freshwater discharge in the narrow-shaped paleoestuary.At c.9 cal.ka BP,terrestrial organic carbon dominated in the sediments of cores NT and HM in present Yangtze delta plain while marine algae dominated in the sediments of core CM97 in Chongming island,core HZK1 near the mouth of north branch and core ZK9 from subaquous Yangtze delta.Thus,location of estuarine front was inferred between HM and CM97,the lower part of the Yangtze delta plain.Such abrupt landward shift of estuarine front suggested the effect of rapid sea level rise during MWP1C which started from c.9 cal.ka BP.The estuarine front stayed between HM and CM97 during the subsequent middle to late Holocene.It migrated seaward to a position between HZK1 and HZK8 at c.1.0 cal ka BP and kept migrating seaward to between HZK8 and ZK9 over the past 400 years.Such phenomenon indicated the interface of land-sea interaction did not further intrude into the paleo-Yangtze estuary although sea level rose another 10 m during c.8-6 cal.ka BP,which was possibly induced by the strong river discharge linked to the intensive Asian monsoon during the Holocene climate optimun.By contrast,no seaward shift of estuarine front happened although sea level changed little over the past 6000 years.We suggested it is related to recession of the Asian monsoon and associated decline in the river discharge.We further suggest that infilling of the paleo-incised valley and progradation of the Yangtze delta explain the seaward migration of the estuarine front over recent 1000 years.Abundance of foraminifera and organic carbon in sediments cores indicated similar migration of estuarine front,inflecting the estuarine front play an important role in bio-geo-chemistry processes in the Yangtze River mouth The characteristics of sedimentary structure from twosides of the Yangtze estuary front are more different.The sediments from outside of the estuary are homogeneous mud and silty clay,while the sediments inside of the estuary mainly consist of very fine sand,fine sand and clayey silt.Besides,the average values of C/N ratio in the Holocene cores range from 2.6 to 23.7,with a minimum value of 3 cores less than 4,indicated that most organic matter had undergone mineralization,remineralization,and humification before being transported to the river mouth and during the early diagenesis.