Abstract:

Porites corals are dominant in the South China Sea (SCS) coral reef systems. Porites have been widely used in the coral-based high-resolution paleo-climate reconstructions, based on their seasonal skeletal density bands. There were local difference and contradictions on the seasonal skeletal density bands of Porites attributing to the single sample site in the previous studies. In this paper, gray value and geochemical proxies (Sr/Ca and δ 18 O) were analyzed by using digital image processing, inductively coupled plasma atomic emission spectrometry and gas isotope ratio mass spectrometer in 4 living Porites collected from four typical coral reef sites in the northern, middle and southern SCS respectively, including Wenchang, Yongxing Island, Panshiyu Reef, and Meiji Reef. As compared with the sea surface temperatures, the skeletal density showed significant seasonal variations in all samples, but the seasonal pattern of skeletal density band varied within the different sites. The high and low skeletal density bands formed separately in summer and winter in the Porites sample from Wenchang in the northern SCS; the high and low density bands developed separately in autumn and spring in the skeletal samples from Yongxing Island and in spring and autumn in the sample from Panshiyu Reef respectively in the middle of the SCS; and the high and low density bands of the skeletal sample occurred separately in winter and summer at Meiji Reef in the southern SCS. The Generalized Additive Mixed Model was applied to reveal the relationship of skeletal density to sea surface temperature, sea surface salinity and net shortwave radiation, respectively. Regardless of the random effects from sample individuals and sites, the results showed the different relationship between skeletal gray values and environmental factors in four sites: sea surface temperature and net shortwave radiation had negative correlation to the skeletal gray values in Wenchang; while skeletal gray values in Meiji Reef had strongly positive correlation with sea surface temperature and negative with net shortwave radiation; in Yongxing Island, significant positive correlation existed between skeletal gray values and sea surface salinity; and there was positive correlation between skeletal gray values and sea surface temperature in Panshiyu Reef, showing the distinctly different responses of skeletal density to the environmental factors in the four sites. However, when the random effects of the sample individuals and sites were taken into account, non-linear relationship was found between the skeletal density and the sea surface temperature and net shortwave radiation on the whole, but there was no positive or negative relationship between salinity and skeletal gray values. It means that over the large spatial scale in the SCS, the temperature and radiation were the possible major influences on seasonal skeletal density of coral Porites samples. Coral skeletal density increased with the rising sea surface temperature, when the temperature was higher than 27℃, the slope of density/SST decreased. In addition, the optimal radiation for the Porites was found to be 150 W/m 2 , coral density decreased when the radiation was higher than the optimum, in contrast, the density increased with the increase of the radiation when the radiation was lower than the optimum.

Key words: Porites, coral density, geochemical proxies, environmental factors