Abstract
Next-generation high resolution brightfield microscopy, x-radiography, and microcomputed tomography (microCT) analyses indicate that coral skeleton high density band (HDB) and low density band (LDB) stratigraphic sequences record dynamic changes in coral growth history. HDB-LDB sequences were studied within three small heads of Orbicella annularis, an ecological keystone species in the Caribbean Sea, collected from the leeward fringing reefs on Curaçao. Results indicate that HDB layers are formed by the thickening of exothecal and endothecal dissepiments, costae, and theca located at the margin and external to individual skeletal cups (corallites). Conversely, septa and columellas located inside individual corallites do not change in thickness. HDB-LDB stratigraphic sequences were laterally traced from the center to the margins of individual coral heads, demonstrating that shifts took place in the trajectory of coral skeleton growth. Normal HDB layers in the center of individual coral heads are formed at the same time (age-equivalent) as surfaces of erosion and no skeleton growth (hiatuses) on the margins of the heads. These hiatus surfaces within HDB-LDB stratal geometries indicate that multiple marine ecological and environmental processes affect the orientation, size, shape, and geometry of coral skeletons during coral growth history. The presence of these hiatus surfaces in other large coral heads would strongly impact sclerochronology and the interpretation of multiple environmental factors including sea surface temperature (SST).
Highlights
The calcium carbonate (CaCO3) aragonite skeletons of modern and ancient scleractinian corals have been studied for more than 150 years from a wide variety of perspectives
high density band (HDB) skeletal element thickening is further supported by quantitative analyses of 3D microcomputed tomography (microCT) scans of the volume percent of skeletal aragonite versus pore space (Miller, 2014). These analyses indicate that HDB layers (61 ± 1% skeletal aragonite and 39 ± 1% porosity; n = 20) are ∼5% denser than low density band (LDB) layers (56 ± 2% skeletal aragonite and 44 ± 2% porosity; n = 20; Miller, 2014)
The integrated optical, x-radiography, and microCT microscopy analyses completed in the present study indicate that O. annularis HDB-LDB stratigraphic sequences record changes in the trajectory of coral growth (Figure 11)
Summary
The calcium carbonate (CaCO3) aragonite skeletons of modern and ancient scleractinian corals have been studied for more than 150 years from a wide variety of perspectives. A combination of x-radiography and autoradiography first showed that coral skeletons are composed of highfrequency alternations of high density band (HDB) and low density band (LDB) layers (Knutson et al, 1972) These HDBLDB stratigraphic sequences create a relative time framework (sclerochronology; Helmle and Dodge, 2011) used to better constrain the age and time of growth of coral skeletons for their use as environmental records (proxies) of changes in SST and other environmental factors. It has been demonstrated that HDB and LDB layer formation is strongly influenced by coral and zooxanthellae physiological responses to climate parameters related to SST such as salinity, light availability, lunar cycles, and solar irradiance (Barnes and Lough, 1993; Dávalos-Dehullu et al, 2008; Winter and Sammarco, 2010; DeCarlo and Cohen, 2017)
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