Surface ocean pH over the last ~300 years: A synthesis and re-evaluation of coral-based reconstructions

Roughly a third of global carbon dioxide emission is absorbed by the ocean, causing an increase in seawater acidity since the pre-industrial era. This phenomenon, known as ocean acidification, not only poses serious threat to the health of many marine ecosystems, but also decreases the ocean’s capacity as a major sink for carbon dioxide which in turn will likely accelerate the increase of carbon dioxide concentration in the atmosphere.

Therefore, to accurately predict ocean’s chemical and biological responses to future ocean acidification, it is essential to understand how various ocean regions have already responded to the atmospheric dioxide increase since the pre-industrial era. This project seeks to use coral skeletons to reconstruct histories of surface ocean acidity over the past ~300-years.

The chemistry of coral skeletons changes with the composition of the seawater in which corals grow and is thus an excellent archive of past changes in ocean acidity. This project provides a much-needed historical perspective on the current and future ocean acidification. It supports the education and training of one graduate student and at least one undergraduate.

It also promotes the education about ocean acidification and coral research in local middle and high schools through planned outreach activities.

Ocean acidification reduces seawater pH and the concentration of carbonate ions in seawater. It threatens the health of many marine ecosystems, especially the survival of marine calcifying organisms that need carbonate ions to produce their calcium carbonate skeletons. Furthermore, it decreases the ocean’s capacity as a major sink for anthropogenic CO2 which in turn will likely accelerate the increase of CO2 concentration in the atmosphere.

However, instrumental records of seawater pH are sparse and at most only span the last ~3-4 decades. Geochemical compositions of the carbonate skeletons of marine calcifying organism vary as a function of the physicochemical conditions of the seawater in which these organisms live, and provide one of the few opportunities to extend our records of past changes in seawater pH.

In particular, the boron isotope composition of long-lived coral skeleton is regarded as a very promising proxy of seawater pH. However, accurate interpretation of coral boron isotope composition in terms of seawater pH is not straightforward, as growing evidence suggests that many factors other than pH, such as seawater temperature and biological regulation, can also influence coral boron isotope composition.

The investigator team will synthesize and re-evaluate coral-based constraints on surface ocean pH, building on recent improved understanding of the mechanisms of coral calcification. The PI team will (1) develop an inverse Bayesian method for accurate seawater pH reconstruction considering all factors influencing coral boron isotope composition; (2) produce robust surface ocean pH records over the last ~300-years by applying the Bayesian method to published coral records; (3) determine the drivers of surface ocean pH variations at different sites on annual to multi-decadal scales.

In addition to providing a much-needed historical perspective on the current and future ocean acidification, this project will produce a suite of numerical methods that are readily to be used by the community for robust coral-based seawater pH reconstruction and for accelerating future experimental studies in this field.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.