Fidelity of the Sr/Ca proxy in recording ocean temperature in the western Atlantic coral Siderastrea siderea

Abstract

Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates < 1.7 mg cm 22 d 21 or < 1.7 mm yr 21 LE returned spuriously high Sr/Ca values, leading to a cold-bias in Sr/Ca-based SST estimates. The threshold-type response curves suggest that extension rate can be used as a quality-control indicator during sample and drill-path selection when using long cores for SST paleoreconstruction. For our corals that passed this quality control step, the Sr/Ca-SST proxy performed well in estimating mean annual temperature across three sites spanning 350 km of the Florida reef tract. However, there was some evidence that extreme temperature stress in 2010 (cold snap) and 2011 (SST above coral-bleaching threshold) may have caused the corals not to record the temperature extremes. Known stress events could be avoided during modern calibrations of paleoproxies.Plain Language Summary Coral skeletons are used to decipher past environmental conditions in the ocean because they live for centuries and produce annual growth bands much like tree rings. Along with measuring coral growth rates in the past, coral skeletons can be chemically sampled to get even more detailed information, like past seawater temperatures. In this study we tested the validity of the strontiumto-calcium (Sr/Ca) temperature proxy in the Massive Starlet Coral (Siderastrea siderea) by sampling 39 corals that were grown in the ocean right next to instruments recording underwater temperature. We found that, as long as corals with very slow growth rates are avoided, the proxy performed well across an extensive region in the western Atlantic.

Quantifying Uncertainty in Sr/Ca‐Based Estimates of SST From the Coral Orbicella faveolata

“…Smith et al () did find a small yet statistically significant relationship between O. faveolata SST residuals and both calcification and extension parameters on an interannual basis; however, they concluded that the potential influence of either calcification rate or linear extension rate explained <10% of the residual SST values, and that on a monthly basis, the relationship was trivial. Kuffner, Roberts, et al () also found a significant relationship between Sr/Ca and linear extension in the coral species, S. siderea , but it was not linear; Sr/Ca showed a cold bias when growth rates dropped below a linear extension threshold of 1.7 mm/year.…”

“…We note that the maximum RMS value in all cases occurred in the summer of 1998 (Figure f), which was a prominent El Niño year. Thermal anomalies associated with El Niño are known to generate coral growth anomalies that may cause especially high variability in coral Sr/Ca (Hetzinger et al, ; Kuffner, Roberts, et al, ). It may, therefore, be best to avoid known temperature stress events in calibration regressions.…”

“…Coral Sr/Ca‐SST reconstructions are most commonly based on geochemical variations derived from a single coral colony and rely on a modern calibration or verification interval over which Sr/Ca is regressed against in situ, local, or gridded records of SST to derive an empirical relationship between Sr/Ca and SST for that colony (e.g., Delong et al, ; Flannery & Poore, ). For example, studies using the western Atlantic coral species S. siderea indicate that there is high reproducibility in mean annual Sr/Ca‐SST estimates among different colonies growing in the Florida Keys (DeLong et al, , ; Kuffner, Roberts, et al, , ), as long as annual growth rates do not fall below a threshold of 1.7 mm/year. Recent work, however, has highlighted the benefit of using both intracolony and intercolony replication to capture the local or regional SST signal (DeLong et al, ).…”

Intercolony δ18O and Sr/Ca variability among Porites spp. corals at Palmyra Atoll: Toward more robust coral‐based estimates of climate

“…Skeletal extension and/or calcification rates have been invoked to explain, and in some cases correct for (Maier et al, ; Goodkin et al, , 2007) intercolony Sr/Ca and δ 18 O offsets in a variety of coral species (e.g., Alibert & McCulloch, ; Cohen & Hart, ; Gagan et al, ; Goodkin et al, ; Kuffner et al, , ; Lough & Barnes, ; Suzuki et al, ). Coral biomineralization models suggest that corals incorporate less Sr 2+ relative to Ca 2+ when they calcify faster, and vice versa (Gaetani & Cohen, ), consistent with experiments that manipulate inorganic aragonite precipitation rates (Holcomb et al, ).…”

“…Extensive empirical data underscores the utility of coral Sr/Ca ratios as a robust proxy for ocean temperature (Beck et al, 1992;DeLong et al, 2013;Inoue et al, 2007;Kuffner et al, 2017;Quinn & Sampson, 2002;Sinclair, 2015;Smith et al, 1979;Weber, 1973), even though a complete mechanistic framework for temperature-dependent incorporation of Sr into coral skeletal aragonite remains elusive (Allison et al, 2011;Gaetani & Cohen, 2006;Gagnon et al, 2012;Sinclair et al, 2006). Paired measurements of coral Sr/Ca and coral δ 18 O allow for the quantification of both SST and δ 18 O sw variations in the past (e.g., Ren et al, 2002;Cahyarini et al, 2008a).…”