南海珊瑚礁铀系年龄及其地质意义

珊瑚礁是铀系方法测定年龄相当理想的对象,在国外已获得十分有价值的成果。主要用于讨论更新世以来海平面变化和构造运动上升速率,研究反映环境的珊瑚生长速度。本文珊瑚礁样品选取于海南岛鹿回头、雷州半岛、西沙和中沙群岛等地。成功地测试了铀系年龄,揭示出与¹⁴C 数据的不一致,初步讨论了构造上升速率和南海海面的变化。

URANIUM-SERIES DATING OF CORAL REEFS FROM SOUTH CHINA SEA AND ITS GEOLOGICAL SIGNIFICANCE

The samples coral reefs collected from Hai Nan Island and Xi Sha Island and Lei Zhou Peninsula in and at the margin of the South China Sea are dated at 6000 -7000 years old by ²³⁰Th/²³⁴U method.The levels of Sr in the coral reef samples are varied between 7.3 and 8.5‰, and are similar to that of coral reefs in the Florida and the Bahamas and the Persian Gulf indicating normal range 7.74 to 8.5‰. The relatively low Sr content for example Sh-06, can indicate the processes of recrystallization into calcite. It may be that Sr concentration and its variation in coral reefs are an additional criterion for cheking closed system.The results obtained by using ²³⁰Th/²³⁴U are compared with ¹⁴C ages for the same specimen. ²³⁰Th/²³⁴U ages for these samples appear to be 1500-2000 years older than ¹⁴C age estimates. It is tempting to conclude that the difference reflects in some measure the ¹⁴C yaers-sideral time discrepancy, because the ¹⁴C variations in the atmosphere are known for the last 8000 years.The apparent rates of vertical diaplacement estimated on the basis of the radio-metric ages including ²³⁰Th/²³⁴U and ¹⁴C slow the local variations within the South China. Sea for the last 7000 years and the average uplift rate was about 1-2 mm/yr.Holocene transgression probably began at the Late Pleistocene and continued rapi dly until about 7000 years ago, after which the rise was more gradual. If the ages determined by ²³⁰Th/²³⁴U method are correct, then it will suggest that, according to Fair-bridge’s sea-level curve, Paleosea levels associated with the early Holocene reefs from the South China Sea stood higher than at present no later than 6000-7000 years ago.