pH and pCO2 at the site of calcification in Scleractinian corals: evidence from boron-based proxies

E.J. Hendy, S Krief, M Fine, R Yam, A Meibom, A Shemesh

Research output: Chapter in Book/Report/Conference proceedingConference Contribution (Conference Proceeding)

Abstract

Anthropogenic CO2 uptake by the oceans has potentially serious consequences for calcifying organisms such as those that are the basis of coral reef ecosystems. Determining the impact of future seawater pH and carbonate chemistry changes on coral reef ecosystems requires a detailed understanding of the process of biomineralization in scleractinian coral. One gap in our knowledge is the extent to which coral physiology modifies and controls pH at the site of calcification. In our study, two ecologically important reef-building coral species, massive Porites sp. and Stylophora pistilata, were cultured for up to 14 months under controlled pCO2 experimental conditions corresponding to seawater pH values of 8.2, 7.6, and 7.3, respectively. The response of both species to the three different seawater pH conditions was monitored using skeletal δ11 B and B/Ca, in addition to a suite of other skeletal tracers and physiological measures. It is well established that the relative concentration of the two main boron species in the ocean, boric acid [B(OH)3 ] and the borate ion [B(OH)4 - ], is pH dependent and that a 27 permil difference in δ11 B occurs between these two chemical species. This study applies the recent innovations in δ11 B analysis of carbonates by multi- collector inductively coupled mass spectrometry (MC-ICPMS) that have revolutionised the measurement of this palaeo-pH proxy in other biogenic carbonates [3]. It is evident from this study that both S. pistillata and massive Porites sp. significantly modify the pH at site of calcification relative to the external seawater. All the coral fragments in our long-term culturing experiment survived and added new skeleton, even under conditions where the ambient seawater was undersaturated with respect to aragonite. The skeletal δ11 B values plot above seawater δ11 B-pH borate fractionation curves calculated using either the theoretically derived αB value of 0.981 [1] or the empirical αB value of 0.974 [2]. This offset can be reconciled with the ambient experimental seawater pH values by invoking a shift in pH at the site of calcification towards more alkaline conditions. The magnitude of this pH shift increases significantly in coral grown under higher pCO2 . The δ11 B-pH estimates, and the magnitude of the internal pH modification within the coral’s tissue, are within the range predicted from physiological processes occurring within symbiont-bearing coral and measured in microsensors studies. A shift in the ratio of skeletal material laid down during dark and light calcification and/or greater manipulation of internal pH by the coral polyp via ion-transport enzymes can explain the increased pH shift at higher pCO2 . Between coral species it is apparent that fast calcifying, branched species typically have lower δ11 B for a given external pH than slower calcifying, massive forms. Massive Porites sp. are important palaeoclimate archives. Although bulk coral δ11 B is recording pH at the site of calcification rather than ambient seawater pH, the internal coral δ11 B-pH response to the external seawater pH can be established. As a result, the skeletal δ11 B values of massive Porites sp. may be used to reconstruct paleo-pH using an effective fractionation value, β B , of 0.8915 in place of αB . [1] Kakihana et al., 1977. Bull. Chem. Soc. Japan 50, 158-163. [2] Klochko et al., 2006. Earth Planet. Sci. Lett. 248, 261-270. [3] Foster et al., 2008. Earth Planet. Sci. Lett. 271, 254-266.
Translated title of the contributionpH and pCO2 at the site of calcification in Scleractinian corals: evidence from boron-based proxies
Original languageEnglish
Title of host publicationEGU General Assembly 2010, Vienna
Volume12
Publication statusPublished - 2010

Bibliographical note

Conference Proceedings/Title of Journal: Geophysical Research Abstracts
Conference Organiser: European Geophysical Union

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