Insights into the skeletal growth of massive corals from the central Belize Barrier Reef (Central America): Results from high-resolution skeletal density variations in Orbicella faveolata

Massive reef-forming corals (Scleractinia) are biological and palaeoclimatological archives, based on their skeletal growth chronology that is characterized by annual density bandings (analogue to dendrochronology). A limitation in coral sclerochronology is the quantification of skeletal density due to the need of precise analytical techniques. This study deals with the application of two-dimensional grid-scanning gamma densitometry (241Americium-radiation beam technique) to quantify skeletal density fluctuations at higher spatiotemporal (intra-annual) resolution. An advantage of this method is a reduction of local influences along the coral growth axis (e.g., pore spaces, calyx architecture), comparative to previous one-dimensional track selection techniques with gamma densitometry. The widespread Atlantic coral Orbicella (ex Montastraea) faveolata was used as a representative massive coral species. A colony from the central Belize Barrier Reef (BBR, Central America) was analysed, yielding 1005 calibrated density values corresponding to a skeletal growth period of 38 years (1962-1999 CE). An average of 27.6±3.5 measurements per growth increment translates into a roughly biweekly resolution for selected grid point rows. The colony shows a long-term decline in skeletal density, accompanied by reduced rates in carbonate accretion. This implies limited capability for skeletal formation in O. faveolata corals within the central BBR. Annual density bandings are likely connected to intra-annual fluctuations in δ13C isotope composition, suggesting a relationship to their mixotrophic lifestyle with a carbon uptake either by heterotrophic feeding and/or photosynthetic activity of their endosymbiotic zooxanthellae. However, combined data suggest a more complex influence of environmental and biological controls on skeletal density variations in the present coral.