CHANGES IN HOLOCENE DEPOSITIONAL ENVIRONMENT OF QIN'AO EMBAYMENT ON NAN'AO ISLAND INFERRED FROM SEDIMENT GRAIN-SIZE AND LOSS-ON-IGNITION
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摘要: 通过对粤东南澳岛青澳湾NA6、NA9号钻孔剖面的岩性、粒度和烧失量特征进行综合分析,结合AMS 14C测年结果,讨论了该地区8 500 cal.aBP以来沉积环境的演变过程。结果显示,烧失量反映的有机碳变化与沉积物粒度组分有十分密切的关系,表明海岸带水动力条件的改变与陆源有机碳的富集有关。潮下带淤泥沉积以及滨岸的!湖沉积较有利于有机碳的富集。相反,砂质堆积的有机碳含量较低,与高能的海浪作用有关。钻孔研究表明,该区沉积环境演变主要受全新世海平面变化的控制:8 500~5 500 cal.aBP为海平面快速上升期,研究区被海水所淹没为潮下带沉积,沉积物中发现许多完整的近岸海相软体动物,且有机碳含量较高;5 500~3 000 cal.aBP为砂质沉积,潮间带砂坪表明海平面稳定或略有下降;3 000~1 300 cal.aBP水下沙坝-沙丘沉积,海平面基本稳定在现今高度;1 300 cal.aBP以后为潟湖沉积,最后在洪积物冲积的堆积作用和海退下形成平原。Abstract: 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.
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Key words:
- transgression /
- Loss-on-Ignition /
- barrier-lagoon system /
- sedimentary environment /
- Nan'ao Island /
- Holocene
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