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The oceans are warming and coral reefs are bleaching with increased frequency and severity, fueling concerns for their survival through this century. Yet in the central equatorial Pacific, some of the world’s most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16 was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30% bleaching) and two moderate (, Hannah Barkley and Anne Cohen and colleagues used bleaching signatures in coral skeletons to examine the Jarvis Island coral community response to multiple El Niño heatwaves. They find the historically productive ecosystem experienced 10 bleaching events in the past 60 years and its recovery provides insights into coral reef resilience under ocean warming.

International audience; Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8-21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ω cf , pH cf , and DIC cf). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa.

High-latitude coral reefs provide natural laboratories for investigating the mechanisms and limits of coral calcification. While the calcification processes of tropical corals have been studied intensively, little is known about how their temperate counterparts grow under much lower temperature and light conditions. Here, we report the results of a long-term (2-year) study of seasonal changes in calcification rates, photo-physiology and calcifying fluid (cf) chemistry (using boron isotope systematics and Raman spectroscopy) for the coral Turbinaria reniformis growing near its latitudinal limits (34.5° S) along the southern coast of Western Australia. In contrast with tropical corals, calcification rates were found to be threefold higher during winter (16 to 17° C) compared with summer (approx. 21° C), and negatively correlated with light, but lacking any correlation with temperature. These unexpected findings are attributed to a combination of higher chlorophyll a, and hence increased heterotrophy during winter compared with summer, together with the corals' ability to seasonally modulate pH(cf), with carbonate ion concentration [Image: see text] being the main controller of calcification rates. Conversely, calcium ion concentration [Ca(2+)](cf) declined with increasing calcification rates, resulting in aragonite saturation states Ω(cf) that were stable yet elevated fourfold above seawater values. Our results show that corals growing near their latitudinal limits exert strong physiological control over their cf in order to maintain year-round calcification rates that are insensitive to the unfavourable temperature regimes typical of high-latitude reefs.

Ocean acidification threatens the persistence of biogenic calcium carbonate (CaCO 3 ) production on coral reefs. However, some coral genera show resistance to declines in seawater pH, potentially achieved by modulating the chemistry of the fluid where calcification occurs. We use two novel geochemical techniques based on boron systematics and Raman spectroscopy, which together provide the first constraints on the sensitivity of coral calcifying fluid calcium concentrations ( ) to changing seawater pH. In response to simulated end-of-century pH conditions, Pocillopora damicornis increased to as much as 25% above that of seawater and maintained constant calcification rates. Conversely, Acropora youngei displayed less control over , and its calcification rates strongly declined at lower seawater pH. Although the role of in driving calcification has often been neglected, increasing may be a key mechanism enabling more resistant corals to cope with ocean acidification and continue to build CaCO 3 skeletons in a high-CO 2 world.

Figure 2 in the original article has been updated with this figure 2 due to discrepancies related to incorrect mapping with one of the islands.

The U/Ca ratio of aragonite coral skeleton exhibits coherent patterns of seasonal and interannual variability. In field-sampled corals and those grown in controlled culture experiments, strong correlations have been found between coral skeleton U/Ca and water temperature, pH, carbonate ion concentration, and salinity. However, the mechanism(s) underlying these different correlations remain unclear. We performed abiogenic precipitation experiments designed to evaluate the sensitivity of U partitioning between aragonite and seawater to temperature, pH, and the concentration of carbonate ion in seawater. Aragonite was precipitated from seawater by addition of carbonate alkalinity at rates set to maintain stable carbonate chemistry during precipitation. Experiments were conducted at 20–40 °C, with pH 7.8–9.0 and carbonate ion concentrations of 600–2600 μmol kg−1. U/Ca ratios of the bulk precipitate and fluid were determined by inductively coupled plasma mass spectrometry. Our results show that the U/Ca ratio of aragonite precipitated from seawater decreases with increasing carbonate ion concentration, and is independent of pH and temperature. We use these results as a framework to interpret the skeletal composition of coral aragonite precipitated from a calcifying fluid that is semi-isolated from the external seawater environment. Accordingly, coral U/Ca ratios are consistent with calcifying fluid carbonate ion concentrations that are several times greater than those of ambient seawater. Correlations between coral U/Ca ratios and seawater temperature, carbonate chemistry, and other environmental variables arise indirectly, via the impacts of these variables on the carbonate ion concentration of the coral calcifying fluid.

Internal waves (IWs) generated in the Luzon Strait propagate into the Northern South China Sea (NSCS), enhancing biological productivity and affecting coral reefs by modulating nutrient concentrations and temperature. Here we use a state-of-the-art ocean data assimilation system to reconstruct water column stratification in the Luzon Strait as a proxy for IW activity in the NSCS and diagnose mechanisms for its variability. Interannual variability of stratification is driven by intrusions of the Kuroshio Current into the Luzon Strait and freshwater fluxes associated with the El Nino-Southern Oscillation. Warming in the upper 100m of the ocean caused a trend of increasing IW activity since 1900, consistent with global climate model experiments that show stratification in the Luzon Strait increases in response to radiative forcing. IW activity is expected to increase in the NSCS through the 21st century, with implications for mitigating climate change impacts on coastal ecosystems.

Author Posting. © The Author(s), 2014]. This is the author's version of the work. It is posted here by permission of Geological Society of America for personal use, not for redistribution. The definitive version was published in Geology 43 (2015): 7-10, doi: 10.1130/G36147.1.

Quantifying the saturation state of aragonite (ΩAr) 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 [CO32−], but direct quantification of ΩAr (where ΩAr =[CO32−][Ca2+]/Ksp) has proved elusive. Here we test a new technique for deriving ΩAr based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of ΩAr from 10 to 34, and found a strong dependence of Raman peak width on ΩAr 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 ΩAr available for comparison. However, Raman analysis of the international coral standard JCp-1 produced ΩAr of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Sr/Ca, Mg/Ca, B/Ca, δ44Ca, and δ11B data. Raman measurements are rapid (≤ 1 s), high-resolution (

A 2 °C increase in global temperature above pre-industrial levels is considered a reasonable target for avoiding the most devastating impacts of anthropogenic climate change. In June 2015, sea surface temperature (SST) of the South China Sea (SCS) increased by 2 °C in response to the developing Pacific El Niño. On its own, this moderate, short-lived warming was unlikely to cause widespread damage to coral reefs in the region, and the coral reef “Bleaching Alert” alarm was not raised. However, on Dongsha Atoll, in the northern SCS, unusually weak winds created low-flow conditions that amplified the 2 °C basin-scale anomaly. Water temperatures on the reef flat, normally indistinguishable from open-ocean SST, exceeded 6 °C above normal summertime levels. Mass coral bleaching quickly ensued, killing 40% of the resident coral community in an event unprecedented in at least the past 40 years. Our findings highlight the risks of 2 °C ocean warming to coral reef ecosystems when global and local processes align to drive intense heating, with devastating consequences.

Ocean acidification is a threat to the continued accretion of coral reefs, though some undergo daily fluctuations in pH exceeding declines predicted by 2100. We test whether exposure to greater pH variability enhances resistance to ocean acidification for the coral Goniopora sp. and coralline alga Hydrolithon reinboldii from two sites: one with low pH variability (less than 0.15 units daily; Shell Island) and a site with high pH variability (up to 1.4 pH units daily; Tallon Island). We grew populations of both species for more than 100 days under a combination of differing pH variability (high/low) and means (ambient pH 8.05/ocean acidification pH 7.65). Calcification rates of Goniopora sp. were unaffected by the examined variables. Calcification rates of H. reinboldii were significantly faster in Tallon than in Shell Island individuals, and Tallon Island individuals calcified faster in the high variability pH 8.05 treatment compared with all others. Geochemical proxies for carbonate chemistry within the calcifying fluid (cf) of both species indicated that only mean seawater pH influenced pH cf . pH treatments had no effect on proxies for Ω cf . These limited responses to extreme pH treatments demonstrate that some calcifying taxa may be capable of maintaining constant rates of calcification under ocean acidification by actively modifying Ω cf .

Coral reef community responses vary but bioerosion increases under natural ocean acidification., Ocean acidification threatens the survival of coral reef ecosystems worldwide. The negative effects of ocean acidification observed in many laboratory experiments have been seen in studies of naturally low-pH reefs, with little evidence to date for adaptation. Recently, we reported initial data suggesting that low-pH coral communities of the Palau Rock Islands appear healthy despite the extreme conditions in which they live. Here, we build on that observation with a comprehensive statistical analysis of benthic communities across Palau’s natural acidification gradient. Our analysis revealed a shift in coral community composition but no impact of acidification on coral richness, coralline algae abundance, macroalgae cover, coral calcification, or skeletal density. However, coral bioerosion increased 11-fold as pH decreased from the barrier reefs to the Rock Island bays. Indeed, a comparison of the naturally low-pH coral reef systems studied so far revealed increased bioerosion to be the only consistent feature among them, as responses varied across other indices of ecosystem health. Our results imply that whereas community responses may vary, escalation of coral reef bioerosion and acceleration of a shift from net accreting to net eroding reef structures will likely be a global signature of ocean acidification.

International audience; In preparing calcium carbonate samples for the measurement of various geochemical proxies, it is often necessary to remove contaminating phases while leaving the phase of interest altered as little as possible. Here we evaluate the effects of some common cleaning protocols (rinsing (H2O), bleach (~ 3% NaOCl), hydrogen peroxide (30%), sodium hydroxide (0.006–0.1 M NaOH), and acid leaching (0.05 N HNO3)) on the elemental (Li, B, Na, Mg, Sr, Ba, Pb, and U) and boron isotope composition of both biogenic and synthetic calcium carbonates formed in marine environments.In untreated samples, the presence of elevated concentrations of Na and Mg, the most abundant cations in seawater, can be reduced with minimal cleaning (e.g. rinsing). Cleaning protocols that cause partial dissolution are problematic, especially for samples that are compositionally heterogeneous because the remaining sample may be biased towards particular phases with distinctive elemental or isotopic compositions. We show that the use of either acid or unbuffered hydrogen peroxide can lead to partial dissolution which was associated with an increase in the U/Ca ratio of the remaining sample. Bleaching or rinsing with water did not result in significant sample dissolution, suggesting that these cleaning techniques may be safely used on heterogeneous samples. Cleaning treatments, other than those resulting in significant dissolution of heterogeneous samples, had no significant effect on δ11B, suggesting that boron isotopes are generally robust to the effects of sample pre-treatment.