Assessing the magnitude of anthropogenic ocean warming and ocean acidification using the novel Li/Mg-SST and δ11B-pH proxies in the Caribbean coral Siderastrea siderea

Abstract

Coral reefs are highly sensitive ecosystems becoming increasingly damaged by anthropogenic climate change and pollution, particularly in the form of ocean warming, ocean acidification, and chronic sedimentation. In order for us to project how coral reefs will respond to future climate change, it is imperative to determine how they have responded to anthropogenic changes to date. This is a difficult task given that in situ observations of sea surface temperature (SST) and pH (-log10[H]+) are short and sparse prior to recent decades. However, coral skeletons are archives of historic climate and environmental conditions, storing over century-long records at sub-annual resolutions. These archives can be unlocked by calibrating geochemical proxies against modern in situ observations. For instance, the Sr/Ca ratios of tropical coral skeletons are traditionally used to reconstruct historic SST changes, yet the accuracy of this proxy is debated, leading to recent investigations into the Li/Mg-SST proxy which has produced promising results. However, by investigating this proxy for the first time in the Caribbean reef-building coral Siderastrea siderea, it has been found that the Li/Mg ratio of colonies from the forereef zone are three times more sensitive to changes in SST than their backreef coral counterparts. This resulted in accurate reconstructions of SST in the forereef, but unfeasible SSTs in the backreef, suggesting a secondary influence on both the Li/Mg- and Sr/Ca-SST proxies. A combined field and laboratory study confirmed that these ratios are also sensitive to pH change, but the full variability in Li/Mg and Sr/Ca is not described by pH and SST change combined.
Long-term records of seawater pH in Belize reconstructed from the boron isotopic composition (δ11B) of S. siderea showed that forereef corals, which have declining skeletal extension rates, have not been exposed to significant long-term ocean acidification. This contrasts with the ~0.15 pH unit decrease observed in the backreef since the 1920s, despite a lack of evidence of declining extension in this reef zone. Sub-annual δ11B records revealed that seasonal pH cycles in these two reef zones are controlled by two different parameters.
Chronic and increasing sedimentation in the forereef from terrestrial sources implies that nutrification in this region is stimulating macroalgae production, and thus buffering against ocean acidification.
Corals are therefore suitable archives of historic climate that can be used to reconstruct multiple environmental parameters such as sea surface temperature and pH. The structure and longevity of coral skeletons allows changes in these environmental parameters to be assessed at monthly to century-long scales. However, although it is possible to use geochemical proxies from coral, the full extent of additional influences on such proxies are not yet fully quantified and therefore may not be suitable to use in all coral reefs.

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ORCID for Gavin Foster: orcid.org/0000-0003-3688-9668

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