DIGITAL IMAGE ANALYSIS OF PHOSPHATIC CEMENTATION OF CORAL CALCARRENITE IN THE SOUTH CHINA SEA AND ITS ENVIRONMENTAL IMPLICATIONS

A digitization method was established using the principles of digital image to study the microlaminae and textures of the guano phosphatic cementation of coral calcarrenite. By this high-resolution method, some characters of the microlaminaes such as width, brightness and color were described and analyzed quantificationally. The results showed that the deposition of guano-derived phosphate onto the surface of carbonate bioclasts layer by layer might have been controlled by the cementation process of the coral calcarrenite. The digital characteristics for the phosphatic cements of three sedimentation layers indicated that the cemented time of coral calcarrenite was obviously different, and at least three episodes could be distinguished. Since the phosphate cementation on the surface of coral calcarrenite was closely associated with the superficial coating of avian guano on this island, the cementation processes with different times implied that there were at least three flourish periods of seabird population and vegetation distribution on the Dongdao Island between 2 900 and 5 700 aBP. Furthermore, the phosphate cements formed at different stages had different digital character assemblages of microlaminae width and brightness which might have recorded the continuous information of historical climate and ecology change. Comparing the change curves of seabird population and the climate on Dongdao Island at three stages of 2 900 aBP, 5 000 aBP and 5 700 aBP, we can find that the relatively stable climate condition might be advantageous for the increase of seabird population. This paper provides a new approach to studying the texture characteristics of guano phosphate cements found in numerous islands of tropical regions and their eco-environmental implications.