Abstract
Coral skeletal boron isotopes have been established as a proxy for seawater pH, yet it remains unclear if and how this proxy is affected by seawater temperature. Specifically, it has never been directly tested whether coral bleaching caused by high water temperatures influences coral boron isotopes. Here we report the results from a controlled bleaching experiment conducted on the Caribbean corals Porites divaricata, Porites astreoides, and Orbicella faveolata. Stable boron (δ11B), carbon (δ13C), oxygen (δ18O) isotopes, Sr/Ca, Mg/Ca, U/Ca, and Ba/Ca ratios, as well as chlorophyll a concentrations and calcification rates were measured on coral skeletal material corresponding to the period during and immediately after the elevated temperature treatment and again after 6 weeks of recovery on the reef. We show that under these conditions, coral bleaching did not affect the boron isotopic signature in any coral species tested, despite significant changes in coral physiology. This contradicts published findings from coral cores, where significant decreases in boron isotopes were interpreted as corresponding to times of known mass bleaching events. In contrast, δ13C and δ18O exhibited major enrichment corresponding to decreases in calcification rates associated with bleaching. Sr/Ca of bleached corals did not consistently record the 1.2°C difference in seawater temperature during the bleaching treatment, or alternatively show a consistent increase due to impaired photosynthesis and calcification. Mg/Ca, U/Ca, and Ba/Ca were affected by coral bleaching in some of the coral species, but the observed patterns could not be satisfactorily explained by temperature dependence or changes in coral physiology. This demonstrates that coral boron isotopes do not record short-term bleaching events, and therefore cannot be used as a proxy for past bleaching events. The robustness of coral boron isotopes to changes in coral physiology, however, suggests that reconstruction of seawater pH using boron isotopes should be uncompromised by short-term bleaching events.
Highlights
The world’s oceans are simultaneously warming and acidifying at unprecedented pace due to rising atmospheric CO2 concentrations [1], thereby severely threatening marine ecosystems [2,3]
As surface ocean temperatures have already increased by 0.6uC since preindustrial times and are projected to increase by at least another 2.0uC under a business as usual scenario by the year 2100 [1], coral bleaching events are expected to increase in frequency and intensity [11,20,21,22], contributing to the worldwide decline of corals reefs [23,24]
In addition to greenhouse-induced warming, rising levels of atmospheric CO2 concentrations have already caused a drop in surface seawater pH by approximately 0.1 unit compared to preindustrial times [25], and a further decrease of 0.3 pH units by the end of this century has been predicted [1,26]
Summary
The world’s oceans are simultaneously warming and acidifying at unprecedented pace due to rising atmospheric CO2 concentrations [1], thereby severely threatening marine ecosystems [2,3]. In addition to greenhouse-induced warming, rising levels of atmospheric CO2 concentrations have already caused a drop in surface seawater pH by approximately 0.1 unit compared to preindustrial times [25], and a further decrease of 0.3 pH units by the end of this century has been predicted [1,26]. This is of particular concern for marine calcifying organisms including corals because ocean acidification (OA) typically compromises calcification [27,28,29,30,31,32,33]. It is established that coral skeletal d11B does not directly record seawater pH but rather reflects the pH at the site of calcification [34,35,40,42], which is typically elevated by up to 1 unit [35,37,38,39] or more [36] relative to ambient seawater
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