CRYSTALLINE STRUCTURE AND DIAGENESIS OF HIGH AND LOW DENSITY BANDS IN PORITES CORAL SKELETONS, MYRMIDON REEF, GREAT BARRIER REEF, AUSTRALIA

Corals grow CaCO₃ (aragonite) skeletons with high- and low-density bands (HDBs and LDBs) that are composed of changing isotopic compositions used in paleoclimate reconstructions. This study demonstrates that HDBs and LDBs are strongly affected by physical, chemical, and biological alteration during and after deposition (seafloor diagenesis). Four colonies of Porites growing at 4-24 m water depth were cored on Myrmidon Reef, Great Barrier Reef, Australia. A total of 35 polished 25 µm-thick thin sections distributed throughout the length of each core were analyzed at 250 nm-scale resolution with brightfield, polarization, phase contrast, and ring aperture contrast microscopy. Crystalline structure of HDB and LDB in original skeletons was observed to have minimal diagenesis in the top 6 cm of each core. These HDBs consistently have 28% lower porosity and thicker skeletal walls than LDBs. Other components including the abundance of centers of calcification, sclerodermites, and skeletal debris in pores showed no major HDB-LDB differences. Comparison of HDB and LDB from top-to-bottom within cores and across bathymetry indicates that diagenesis increases with age and water depth. HDBs consistently have 32% lower porosity than LDBs and significant down-core increases in aragonite needle cementation. Several other diagenetic products were also observed, including skeletal replacement, seafloor borings, skeletal debris, hi-Mg calcite cementation, and dissolution and reprecipitation at the margin of sclerodermites. These results indicate that seafloor diagenesis can significantly alter HDB and LDB patterns to change, relocate, and increase skeleton density primarily through replacement and cementation. Combined with porosity occlusion by seawater aragonite cements, this will impact the positioning and interpretation of isotopic and trace element analyses. These observations open a fundamentally new understanding of the magnitude and range of processes to which seafloor diagenesis can impact coral isotopic analyses used for paleoenvironmental reconstructions.