Global sea surface salinity and temperature impacts on trace elements, crystallography, and organics in massive Porites coral skeletons from a museum collection

Shallow-water scleractinian corals are critical coral-reef architects that grow in especially saline ocean waters. While coral skeleton-based trace element proxies are the standard for evaluating past sea surface temperature (SST), global sea surface salinity (SSS) has been overlooked as a variable that may also be linked with coral skeletal geochemistry and mineralogy. The underpinnings of coral skeleton formation and its response to local environmental conditions are important when evaluating trace element-based paleoproxies and predicting coral health under future climate. Here, we systematically survey massive Porites species from the National Coral Collection that grew across a wide range of SSS (~ 31–41 PSU) from around the world. Some paleoproxy-relevant trace element (TE)/Ca ratios (B, 87 Sr, U, Li) positively correlate with SSS, but only U/Ca exhibits correlations with aragonite crystal unit cell parameters. The only clear correlation between SSS and unit cell parameters exists for b -axis lengthening for a subset of the samples growing at typical SSSs (30–36 PSU). As expected, SST is negatively correlated with SSS and expected TEs ( 87 Sr, Li, Li/Mg) across the full dataset. Our quantitative crystallographic measurements also provide calcite phase % contributions to the aragonite coral skeletons (< 3%), which are negatively correlated with Li/Mg, Li/Ca, and 87 Sr/Ca. Relative organic contents across the samples reveal positive correlations with Mn/Ca (full dataset) and a -axis length (for 30–36 PSU samples). Our survey of SSS suggests that coral organisms exercise great control over their internal fluid geochemistry such that their skeletons remain similar to one another, despite growing under very different salinities. Moreover, the correlations with TE/Ca ratios suggest that in addition to SST, future paleoproxy studies should still consider the influence of major SSS shifts and calcite contents when screening for samples, and when interpreting skeletal geochemistry for reconstructing other environmental variables.