Quantifying skeletal density at high temporal resolution in massive scleractinian corals by using two-dimensional grid-scanning gamma densitometry

Abstract

The quantification of skeletal density in massive scleractinians is necessary for a better understanding of skeletal growth in reef-forming corals. However, skeletal density is difficult to quantify and requires sophisticated analytical techniques. In this study, two-dimensional grid-scanning gamma densitometry is used for the first time, to quantify skeletal density fluctuations at higher temporal (intra-annual) resolution as compared to previous annual bulk densities determined with this approach. For testing this application and to evaluate its use for being a tool in coral sclerochronology, a colony of the widespread Atlantic massive coral Orbicella faveolata from the central Belize Barrier Reef (Central America) is herein investigated. In the studied coral, temporal resolution of individual density values corresponds to an approximately bi-weekly resolution. A long-term decline in (intra-)annual skeletal density is observed combined with reduced calcification rates. This indicates a limitation in the capability for skeletal formation in O. faveolata corals within the central Belize Barrier Reef, expressed in reduced skeletal carbonate accretion. In general, time series analyses and statistical correlations of the obtained high-resolution density datasets with skeletal growth patterns (linear extension rates, calcification rates) and geochemical (δ13C, δ18O) data reveal a complex interplay of environmental parameters, which might have controlled the skeletal density in the studied coral.

Reef‐wide and long‐term skeletal growth records of the mountainous star coral (Orbicella faveolata) from Belize barrier and atoll reefs (Central America)

“…However, these shorter growth periodicities were not observed continuously throughout the complete skeletal growth time. Gischler and Oschmann (2005) and Zoppe et al (2022) discussed quasi‐decadal periodicities in δ 18 O isotope and skeletal density records from the cores BZE‐2, BZE‐6 and BZE‐M3, in the context of the Atlantic SST dipole variation. However, the gradual connection to cycle lengths of up to 32 years in BZE‐M3 may also indicate some influence of the multidecadal variations ( ca 20–30 years) of the Pacific Decadal Oscillation (Mantua et al, 1997), which was suggested by Gischler et al (2009) to have influenced Pleistocene Orbicella corals from the Florida Reef Tract.…”

“…The latter authors only analysed BZE‐GR for 5‐year running means; therefore, skeletal density values at an annual resolution are lacking. Zoppe et al (2022) used BZE‐2 explicitly to study density‐band formation in O. faveolata and evaluated the methodical approach of two‐dimensional 241 Am grid‐scanning gamma densitometry for analysis of skeletal density records at high temporal (sub‐annual) resolution. They also measured the annual linear extension rate, bulk density and calcification rate for BZE‐2, which are compared herein with the other studied skeletal growth records.…”

“…The quantification of skeletal (bulk) density for each growth increment was obtained by two‐dimensional grid‐scanning 241 Am gamma densitometry (Deveaux et al, 2017; Zoppe et al, 2022) at the Institut für Kernphysik at Goethe‐Universität, Frankfurt, Germany. Individual core slabs were measured following a grid‐scanning of the sample along the growth axis under a radioactive‐decaying 241 Am isotope source.…”

“…Each grid point corresponds to a single calibrated measurement with a spatial resolution of 0.5 × 0.5 mm. Emitted X‐ray beams of the 241 Am source penetrated the core slabs and were detected by a two‐step detection system (for details on the measurement procedure and calibration of the gamma densitometer, see Deveaux et al (2017) and Zoppe et al (2022)). A web‐based program was developed by Deveaux et al (2017) as a graphical user interface to obtain density measurements based on the generated scans of the gamma densitometer (Figs –).…”

“…Long‐term trends are indicated by regression lines with corresponding statistics figured in boxes on the right. Data for core BZE‐2 are from Zoppe et al (2022). [Color figure can be viewed at wileyonlinelibrary.com]…”