A high-precision gamma densitometer for quantifying skeletal density in coral skeletons: Physical background and first results

Abstract

Measuring density in coral skeletons with high precision is challenging and forms the dominant source of experimental uncertainty in related studies. To reduce this uncertainty, we developed a precise and easy-to-handle γ-densitometer. This instrument illuminates coral samples with a close-to-monochromatic γ-ray beam and measures the attenuation in the material by means of single photon counting. Knowing the thickness of the sample, the density can be extracted from the attenuation. After calibration, we obtained a precision of 3.7% for absolute and 0.6% for relative density measurements. The spatial resolution is 0.5 mm. The detector system has been tested with the genus Porites, typically used in paleoclimate studies in the Indo-Pacific, and with the genus Orbicella (formerly Montastraea), which has been commonly used in the Atlantic. A record (a.d. 1965–1999) from an Orbicella faveolata from the Belize Barrier Reef exhibits a decrease in skeletal density and in calcification rate; extension rates increase over time. A record from an O. faveolata from the offshore Glovers Reef Atoll, Belize, exhibits a density increase during a.d. 1900–2005 while skeletal extension rate decreased; calcification rate shows a decline over time. Skeletal density in a Porites lutea from the Maldives decreases from a.d. 1917–2007 whereas skeletal extension rate and calcification rate exhibit increasing trends. These first results along with those from other studies suggest that coral density data from numerous corals in a region are needed to establish robust trends in coral calcification over time, and, that susceptibility to ocean acidification apparently might vary among coral taxa.