the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Aug 2025
Deconvolving the biogeochemical controls on coral Sr/Ca and Ba/Ca proxies: New perspectives from paired stable Ca, Sr and Ba isotope compositions
Abstract. This study introduces a novel approach to disentangle the biogeochemical controls on Sr/Ca and Ba/Ca signatures in coral skeletons using paired stable Ca, Sr and Ba isotopes to assess their specific uptake dynamics during coral biomineralization. The observed seasonal variations in stable Ca (δ44/42Ca) and Sr isotopes (δ88/86Sr) underscore the capability of corals to actively mediate the transport of Ca2+ and Sr2+ ions to the calcifying fluid prior to aragonite precipitation. We suggest that while the individual concentrations of Ca and Sr in the calcifying fluid vary seasonally, the Sr/Ca ratio of the fluid is likely comparable to that of seawater due to similar ion uptake dynamics. In contrast, the observed coral stable Ba isotope compositions (δ138/134Ba) remain essentially constant, suggesting a passive transport mechanism of Ba2+ ions, possibly through direct seawater leakage. The contrasting ion transport behaviours of Ba and Ca elucidate the underlying cause of the temperature-dependent variations in coral Ba/Ca records. By evaluating the uptake dynamics of Ca, Sr and Ba via their respective isotope systems, this study provides useful implications for the accurate application of coral Sr/Ca and Ba/Ca as proxies for paleoclimate reconstructions.
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Status: open (until 03 Dec 2025)
- RC1: 'Comment on egusphere-2025-4126', Anonymous Referee #1, 21 Oct 2025 reply
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Comment
50
Sr/Ca has been widely shown to have significant vital effects impacting its accuracy (DeCarlo et al. 2016, Paleoceanography)
120/130
Please report the the long-term reproducibility (2sd) for Sr and Ba isotopes, as is done for C and Ca isotopes.
Fig 3
Please report 2SD shading on δ13C
Typo in caption: ( e ) currently reads δ138/134Ca
225
The findings of Gussone and others (2003) are misstated: rather than an increase in temperature leading to higher δ44Ca values, an increase in temperature pushes the selection preference of 42Ca over 44Ca (fractionation factor) toward unity, such that δ44Ca in the theoretical solid are indistinguishable from δ44Ca in the fluid.
235
In terms of calcium ion dehydration prior to passage through a channel, I appreciate that the work of Hoffman and others (2018) shows that water-ion exchange reactions are temperature dependent, such that the resultant magnitude of kinetic isotope fractionation is affected. That said the manuscript uses this mechanism to suggest that the fractionation factor (⍺Ca) is likely changing, although no calculation of ⍺Ca is presented. With only the δ44Ca values, as presented in Fig 4a, it’s difficult to follow how this mechanistic interpretation relates to the data.
Notably, the findings of the cited work by Mejía an others (2018) suggested that decreasing temperatures lead to modelled increases in the magnitude of ⍺Ca, allowing more selective uptake of the light isotope.
Aside from this point, it would be helpful to clarify why the corals studied by Böhm and others (2006) or Inoue and others (2015) would not also be susceptible to more selective uptake of light Ca through the same dehydration mechanism invoked for the corals measured within this study. In Inoue and others (2015), one out of three coral colonies showed a decrease in δ44Ca with increasing temperature.
It would be helpful to show δ13C v. SST and δ13C v. δ44Ca to clarify the suggested connection between SST, photosynthetic activity/δ13C, and the resultant indirect relationship between δ44Ca and SST.
260
While the work of Müller and others (2018) finds a negative correlation between δ88Sr and temperature, they are clear than this is opposite the common trend seen between δ44Ca and temperature. It would be helpful for the reader to contextualize this point by Müller as it relates to the interpretation of Shannon (1976), which suggests that both Sr and Ca are transported and fractionated via a similar biologically-mediated pathway and is used to explain the similarity between changes in δ88Sr and δ44Ca observed in this study.
285
How was the magnitude of Ba isotope fractionation calculated?
Figure 4
It is noted in the test that the correlations between SST and ∂44Ca (and ∂88Sr) are statistically significant. What type of statistical method was used? In particular, was a Deming type regression used, as is needed for data with uncertainty in both the x and y variables?
I ask partially because when browsing the supplementary data, simple linear regressions (2-tailed paired t-test) show statistically significant correlation between ∂138Ba and SST.
Throughout the text, please clarify the statistical methods used.
320
The author’s mention that “temperature-sensitive partitioning of Sr and Ca into coral aragonite is likely the primary driver of seasonal variations in skeletal Sr/Ca records”, however the analysis throughout the text discusses mechanistic drivers of these signatures as related to ion transport into the extracellular calcifying fluid. Can you clarify what is meant?