珊瑚生长率重建西沙海域中晚全新世海温变化

珊瑚生长率是记录海洋表层海水温度(SST)的重要指标之一。本文在测量西沙永兴岛海域1个现代滨珊瑚(Porites lutea)和4个中-晚全新世滨珊瑚样品生长率基础上, 定量重建了中晚全新世5个特征时段的SST:1887~2007A.D.(27.27℃)、1993~1936a B.P.(26.73℃)、2985~2904a B.P.(26.5℃)、4004~3962a B.P.(26.8℃)和5514~5398a B.P.(27.0℃)。这些结果表明5.5~1.9ka B.P. 期间西沙海域SST总体上呈下降趋势, 与中国大陆的平均温度基本同步变化。西沙海域中晚全新世SST的降低趋势受东亚夏季风强度减弱的控制, 也可能与这一时期逐渐活跃的ENSO存在联系, 而岁差引起的北半球太阳辐射量的减少可能是SST变化的根本驱动因素。

SEA SURFACE TEMPERATURE VARIATIONS DURING THE MID-LATE HOLOCENE RECONSTRUCTED BY Porites CORAL GROWTH RATES IN THE XISHA ISLANDS

South China Sea is an important source region, providing su mmer monsoon rainfall throughout mainland China, which accounts for more than 80% total precipitation in the region. Yongxing Island (Woody Island, 16°50'N, 112°20'E) is the largest reef island within the Xisha Islands located in the middle of the South China Sea. Observational record of Xisha Sea Station show that the annual mean SST of Yongxing Island waters is 27.4℃, the annual mean minimum SST is 24.7℃, occurred from December to February of next year, the annual mean maximum SST is 29.8℃ appeared from May to September. In this sea area, annual mean salinity is 33.6 ‰. The lowest salinity is 33.3 ‰ occurred in October and the high salinity is 33.9 ‰ in February. The hydrological conditions of Xisha Islands waters are very suitable for coral co mmunities.Porites lutea is an important reef building coral, which can form continuous annual growth bands composed of high-low density sub-bands in skeletal growth, providing a clear annual chronology, much like tree rings. In this paper, four fossils and one modern Porites coral are drilled from Yongxing Island, Xisha Islands. Core samples of coral skeleton were cut into 8-mm-thick slices along the growth axis. X-radiograph of each skeletal slices were taken using a HITACHI medical X-ray machine (58kV, 50mA, 0.06s) after the skeletal slices were rinsed and dried. An X-ray photo of the slice was then scanned to generate a digital image of gray using a professional film scanner (EPSON 4990). Then entire core sample of corals was obtained by splicing together fragments of X-ray photos in Photoshop. Skeletal images show distinct annual growth bands composed of dark high-density sub-bands alternates with pale low-density sub bands.Making use of software of CoralXDS and measuring tool of Photoshop, the annual growth rate of samples was measured. The skeletal growth rate of Porites lutea is sensitive to surroundings, which is an important indicator to record sea surface temperatures (SST). By linear regression method, the linear relationship between annual bandwidth of modern Porites corals and SSTs in Yongxing Island waters from 1961 to 2008 was constructed. According to the bandwidth thermometer, we reconstructed SSTs for five characteristic periods in the Mid-late Holocene:1887~2007A.D.(27.3℃), 1993~1936a B.P.(26.7℃), 2985~2904a B.P.(26.5℃), 4004~3962a B.P.(26.8℃) and 5514~5398a B.P.(27.0℃). These results put together to show a general decreasing trend in SST from 5.5ka B.P. to 1.9ka B.P., which kept pace with changes in mean temperature of the Chinese mainland basically. The decreasing trend of SSTs in Xisha sea area may be controlled by the strength of the East Asian su mmer monsoon during the mid-late Holocene, which also relates to gradually active ENSO events during this period. The decline of su mmer radiation in the Northern Hemisphere caused by precession periodicity may be the primary forcing to drive SST changes in the Xisha Islands during the Mid-late Holocene.