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Abstract The geochemistry of tropical coral skeletons is widely used in paleoclimate reconstructions. However, sub‐aerially exposed corals may be affected by diagenesis, altering the aragonite skeleton through partial dissolution, or infilling of secondary minerals like calcite. We analyzed the impact of intra‐skeletal calcite on the geochemistry (δ18O, Sr/Ca, Mg/Ca, Li/Mg, Li/Ca, U/Ca, B/Ca, Ba/Ca, and Mn/Ca) of a sub‐aerially exposedPoritessp. coral. Each micro‐milled coral sample was split into two aliquots for geochemistry and X‐ray diffraction (XRD) analysis to quantify the direct impact of calcite on geochemistry. We modified the sample loading technique for XRD to detect low calcite levels (1%–2%; total uncertainty = 0.33%, 2σ) in small samples (∼7.5 mg). Calcite content ranged from 0% to 12.5%, with higher percentages coinciding with larger geochemical offsets. Sr/Ca, Li/Mg, Li/Ca, and δ18O‐derived sea‐surface temperature (SST) anomalies per 1% calcite were +0.43°C, +0.24°C, +0.11°C, and +0.008°C, respectively. A 3.6% calcite produces a Sr/Ca‐SST signal commensurate with local SST seasonality (∼1.5°C), which we propose as the cut‐off level for screening calcite diagenesis in paleo‐temperature reconstructions. Inclusion of intra‐skeletal calcite decreases B/Ca, Ba/Ca, and U/Ca values, and increases Mg/Ca values, and can therefore impact reconstructions of paleoclimate and the carbonate chemistry of the semi‐isolated calcifying fluid in corals. This study emphasizes the importance of quantifying fine‐scale calcite diagenesis to identify coral preservation levels and assure robust paleoclimate reconstructions.
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This content will become publicly available on May 27, 2027
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, 87Sr, 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 (87Sr, 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 87Sr/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.
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- PAR ID:
- 10686640
- Publisher / Repository:
- Coral Reefs
- Date Published:
- Journal Name:
- Coral reefs
- ISSN:
- 0722-4028
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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