Paired Density and Geochemistry Records Demonstrate the Combined Impact of Skeletal Density and Architecture on the Geochemistry of Modern and Sub-Fossil Corals from the Galápagos Islands

Coral geochemistry is widely applied to develop paleoclimate reconstructions, but the fidelity of geochemical proxies can be sensitive to coral growth rates and skeletal architecture. Extension rate—here defined as the linear distance between successive Sr/Ca minima (SST maxima)—is frequently measured to screen for potential growth impacts. However, extension rate is not synonymous with coral growth rate (i.e., calcification rate, the product of both annual extension and skeletal density). Furthermore, geochemical abnormalities, such as those found in converging corallite fans, can occur even in the absence of extension anomalies. In contrast, coral density represents a higher-resolution and higher-precision metric for assessing growth impacts on geochemistry, and allows the measurement of coral calcification rate. This study compares the efficacy of extension, density, and calcification rate for detecting growth impacts on geochemical proxies.

We present paired records of geochemistry (Sr/Ca, Mg/Ca, and partial Ba/Ca, δ¹⁸O, and δ¹³C) and growth (density, extension, and calcification) from four sub-fossil and modern Porites spp. corals from Wolf Island, Galápagos, spanning approximately 1700 to 1800 and 1960 to 2010. We find that density is more sensitive than either extension or calcification as an indicator of growth-related impacts on geochemistry, and that geochemical proxies vary in their sensitivity to coral growth. We find statistically significant correlations between density and geochemistry from sampling paths with suboptimal skeletal architecture, but no such relationship in optimal transects. We also explore growth rate anomalies during major pre-industrial and modern El Niño events. Our results demonstrate that density measurements can improve coral paleoclimate reconstructions.