Records Of Sea Level Changes Over The Mid-Holocene From U-Series Ages Of Shore Reefsin Tanmen,Hainan Island

Microatolls and in situ massive corals in coral reefs are ideal indicators for recording the past sea level positions.Reconstructing the history of sea level is of great scientific significance for understanding the variation and trend of sea level.On the basis of detailed geomorphological investigation of coral reefs in the waters of Tanmen Town,Qionghai City,eastern Hainan Island(the following abbreviation is “Tanmen shore reefs”),we drilled cores from 9 microatolls and20 in situ massive Porites in the geomorphological section perpendicular to the coastline.Its elevation and position were measured by GPS,and the structure of microatolls and in situ massive Porites were described on site.We sliced,cleaned and X-rayed coral slices in the laboratory,and then measured the annual growth rate with the software of DICOM explorer.Finally,40 coral samples were selected for high precision U-series dating according to the X-ray photographs.Based on the U-series age of corals and the relationship between coral and sea level,we reconstructed the sea level fluctuation history of Tanmen in the mid-Holocene.We also analyzed the relationship between sea level and climate changes in the mid-Holocene according to the relationship between coral growth rate and sea surface temperature.The main conclusions are as follows:(1)The U-series age of Tanmen coral reefs is 6143～3013 a BP(a BP:relative to 1950 CE).6143～4368 a BP is the main development stage of coral reefs,which basically formed the modern geomorphology of Tanmen shore reefs.3220～3013 a BP is the second development stage of Tanmen coral reefs,but it is only sporadically developed in low-lying areas of the outer reef-flat.The coral samples during 4368～3220 a BP were missing.(2)The height difference between sea level indicators in different periods represents the height of sea level changes.The study on the elevation of in situ massive Porites and the U-series age in Tanmen shows that the relative sea level in Tanmen shore reefs shows a significant downward trend during 6000～3000 a BP.6000～5000,4700～4400 and 3200～3000 a BP are the three relatively high sea level stages in this area since Holocene,and the elevation of sea level was1.8～2.4 m,1.5～1.8 m and 1.3 m higher than present,respectively.(3)Microatolls development during 6143-4384 a BP.According to their age,elevation and relationship with sea level,it is concluded that the sea level in Tanmen shore reefs had at least three cycles of fluctuations with amplitudes up to 0.40～0.69 m on century scales.(4)During the mid-Holocene,the growth rate of Tanmen coral ranged from0.81 to 1.51 cm/a,with an average of 1.11 cm/a.Based on the relationship between coral growth rate and sea surface temperature,we conclude that the temperature of Tanmen shore reefs has a downward trend during 6100～3000 a BP.A relatively warm period can be found during 6100～5500 a BP,while relatively cold periods during 5500～4400 and 3200～3000 a BP.(5)The temperature recorded by corals in Tanmen shore reefs is similar to the trend of sea level,and the relatively high sea level stages are corresponded to the relatively warm period or warming period.Therefore,it is speculated that climate fluctuations have a regulatory effect on sea level changes.