<p><strong>Abstract.</strong> Quantifying the saturation state of aragonite (&#937;<sub><i>Ar</i></sub>) within the calcifying fluid of corals is critical for understanding their biomineralisation process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry allow determination of calcifying fluid pH and [CO<sub>3</sub><sup>2&#8722;</sup>], but direct quantification of &#937;<sub><i>Ar</i></sub> (where &#937;<sub><i>Ar</i></sub> =[CO<sub>3</sub><sup>2&#8722;</sup>][Ca<sup>2+</sup>]/<i>K<sub>sp</sub></i>) has proved elusive. Here we test a new technique for deriving &#937;<sub><i>Ar</i></sub> based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of &#937;<sub><i>Ar</i></sub> from 10 to 34, and found a strong dependence of Raman peak width on &#937;<sub><i>Ar</i></sub> that was independent of other factors including pH, Mg/Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid &#937;<sub><i>Ar</i></sub> available for comparison. However, Raman analysis of the international coral standard JCp-1 produced &#937;<sub><i>Ar</i></sub> of 12.3&#8201;&#177;&#8201;0.3, which we demonstrate is consistent with published skeletal Sr/Ca, Mg/Ca, B/Ca, <i>&#948;</i><sup>44</sup>Ca, and <i>&#948;</i><sup>11</sup>B data. Raman measurements are rapid (&#8804;&#8201;1&#8201;s), high-resolution (<&#8201;1&#8201;&#956;m), precise (derived &#937;<sub><i>Ar</i></sub> &#177;1 to 2), and require minimal sample preparation; making the technique well suited for testing the sensitivity of coral calcifying fluid &#937;<sub><i>Ar</i></sub> to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of &#937;<sub><i>Ar</i></sub> over multiple years of growth in a <i>Porites</i> skeleton from the Great Barrier Reef, and we evaluate the response of &#937;<sub><i>Ar</i></sub> in juvenile Acropora cultured under elevated CO<sub>2</sub> and temperature.</p>
Abstract. The boron isotopic (δ11Bcarb) compositions of long-lived Porites coral are used to reconstruct reef-water pH across the central Great Barrier Reef (GBR) and assess the impact of river runoff on inshore reefs. For the period from 1940 to 2009, corals from both inner as well as mid-shelf sites exhibit the same overall decrease in δ11Bcarb of 0.086 ± 0.033‰ per decade, equivalent to a~decline in seawater pH (pHsw) of ~ 0.017 ± 0.007 pH units per decade. This decline is consistent with the long-term effects of ocean acidification based on estimates of CO2 uptake by surface waters due to rising atmospheric levels. We also find that compared to the mid-shelf corals, the δ11Bcarb compositions for inner shelf corals subject to river discharge events, have higher and more variable values and hence higher inferred pHsw values. These higher δ11Bcarb values for inner-shelf corals are particularly evident during wet years, despite river waters having lower pH. The main effect of river discharge on reef-water carbonate chemistry thus appears to be from higher nutrients driving increased phytoplankton productivity, resulting in the drawdown of pCO2 and increase in pHsw. Increased primary production therefore has the potential to counter the more transient effects of low pH river water (pHrw) discharged into near-shore environments. Importantly however, inshore reefs also show a consistent pattern of sharply declining coral growth that coincides with periods of high river discharge. This occurs despite these reefs having higher pHsw values and hence higher seawater aragonite saturation states, demonstrating the over-riding importance of local reef-water quality on coral reef health.
<p>Coral reefs are increasingly threatened by climate change and mass bleaching events. Predicting how corals will respond to rapid ocean warming requires a better understanding of how they have responded to environmental change in the past &#8211; information that can be reconstructed from coral skeletal records. However, significant knowledge gaps remain in our understanding of how coral biomineralization and the incorporation of geochemical tracers is impacted by heat stress and bleaching, particularly since the physiological status of corals used for reconstruction of past stress events is often unknown. Using boron-based geochemical tracers (&#948;<sup>11</sup>B, B/Ca), we investigated how heat stress caused by a marine heatwave impacted the carbonate chemistry of the coral calcifying fluid as well as skeletal trace element composition in the branching coral <em>Acropora aspera</em>. Importantly, we recorded in situ temperature and coral health status during the bleaching event and after 7 months of recovery. We show that heat-stressed <em>Acropora</em> corals continued to upregulate the pH of their calcifying fluid (cf); however, dissolved inorganic carbon upregulation inside the cf was significantly disrupted by heat stress. Similarly, we observed suppression of the typical seasonality in the temperature proxies Sr/Ca, Mg/Ca, Li/Ca and Li/Mg, likely due to a combination of reduced growth rates, disruption of key enzymes involved in calcification and Rayleigh fractionation. Anomalies in TE/Ca ratios were still observed 7 months after peak bleaching, even though symbiont densities and chlorophyll a concentrations were fully restored at this point. Interestingly, the response to heat stress did not differ between the thermally variable intertidal and the thermally more moderate subtidal environments whose coral populations are known to have a different heat tolerance, nor between colonies with varying degrees of bleaching. Our findings suggest that coral biomineralization mechanisms are highly sensitive to heat stress, and that the biogeochemical stress response of branching <em>Acropora</em> corals is remarkably consistent with that of massive <em>Porites</em> corals.</p>
Quantifying the saturation state of aragonite (Ar) within the calcifying fluid of corals is critical for understanding their biomineralization process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry enable the determination of calcifying fluid pH and [CO 2− 3 ], but direct quantification of Ar (where Ar = [CO 2− 3 ][Ca 2+ ]/K sp) has proved elusive. Here we test a new technique for deriving Ar based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of Ar from 10 to 34, and we found a strong dependence of Raman peak width on Ar with no significant effects of other factors including pH, Mg/Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid Ar available for comparison. However, Raman analysis of the international coral standard JCp-1 produced Ar of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Mg/Ca, Sr/Ca, B/Ca, δ 11 B, and δ 44 Ca data. Raman measurements are rapid (≤ 1 s), high-resolution (≤ 1 µm), precise (derived Ar ± 1 to 2 per spectrum depending on instrument configuration), accurate (±2 if Ar < 20), and require minimal sample preparation, making the technique well suited for testing the sensitivity of coral calcifying fluid Ar to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of Ar over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of Ar in juvenile Acropora cultured under elevated CO 2 and temperature.
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