CHANGES IN HOLOCENE DEPOSITIONAL ENVIRONMENT OF QIN'AO EMBAYMENT ON NAN'AO ISLAND INFERRED FROM SEDIMENT GRAIN-SIZE AND LOSS-ON-IGNITION

Based on the data of loss on ignition(LOI),grain size and sediment lithology in combination with AMS¹⁴C dates,we reconstructed the environmental evolution history of the Qing'ao Embayment of the Nan'ao Island in southern China since 8 500 cal.aBP.The results suggest that changes in organic carbon indicated by LOI have a relationship with grain size,which indicates that hydrodynamic conditions are related to the enrichment of terrestrial organic carbon in the coastal area.For example,the organic carbon is more likely to enrich in the subtidal and lagoonal silt deposit.In contrast,the content of organic carbon is lower in sandy deposit which is under influence of high-energy wave.The results show that the changes in depositional environment are mainly affected by the Holocene sea-level changes.The embayment has three depositional phases:marine clayey silt,coastal sandy sediment dominated by fine to medium sand and lagoonal silt.The environmental evolution was mainly driven by the Holocene sea level change.Between 8 500 and 5 500 cal.aBP,as the sea level rose rapidly,the embayment inundated by the sea water and created enough accommodation space for the sediment to accumulate.Complete mollusk fossils are widely found and the content of organic carbon was high during this period.Between 5 500 and 3 000 cal. aBP,the sea level was relatively stable or declined slightly,close to the modern sea level.The barrier started to form under water.During 3 000 and 1 300 cal.aBP,sea level became stable.The barrier started to fill up most of the embayment basin,and lagoonal environment began to form.Since 1 300 cal.aBP,the lagoon have developed maturely and transformed into a closed one.The area became alluvial plain in the modern time.