Characterisation of ocean acidification over decadal timescales

Despite the recognised need to improve awareness and monitoring of ocean acidification, huge portions of the ocean have a severely limited, or no, pH monitoring capacity. To help address this need, state-of-the-art autonomous seawater sensors were installed through the CME Programme in Fiji, providing the first high-quality time series of seawater pH, temperature, salinity and dissolved oxygen measurements for the region. However, changes in the alkalinity and dissolved inorganic carbon (DIC) content of seawater caused by reef biology, water residence time and depth can cause the pH of seawater on a coral reef to vary by up to ±0.5 units over daily, monthly, and seasonal time scales. As a result, it can take several months, or years, of continual measurements to produce the baseline trends against which the impacts of anthropogenic ocean acidification may be resolvable in such dynamic environments. 

The ability to generate historical pH records from coral cores using the boron isotope geochemical proxy enables data produced by modern pH sensors to be rapidly placed into a historic context without the need to wait for several years of operation. This, together with the absence of any equivalent historical pH records for the entire South Pacific, meant that an assessment of long-term changes in seawater pH and temperature was urgently needed to help resolve the impacts of climate change on marine environments in the region.  

Working in partnership with the University of the South Pacific, the University of Southampton and Cardiff University, this project combined world-leading expertise in the geochemical techniques used to reconstruct seawater pH (the boron isotope proxy), temperature (trace-metal ratios such as Li/Mg) and ecosystem health (carbon and oxygen isotopes), with expert understanding of coral physiology/growth mechanisms and other active seawater monitoring programmes in the region (e.g. ReefTEMPS) to provide the first long-term record of combined environmental changes for the region. All of the data and scientific information generated through this project represented a significant contribution to our understanding of long-term changes in ocean acidification in Fiji and the South Pacific region, and the project marked the first time that such historical records were combined with high-resolution autonomous analyses to contextualise the extent of variation being recorded by modern sensors.

Outputs

• Determined elemental and isotopic trends from a coral core collected close to the location of the autonomous ocean acidification sensors in Fiji.
• Seawater pH and temperature to be reconstructed over a ~60 year period and providing background context for the modern seawater measurements being made at that site.
• Trained collaborators from the University of the South Pacific in the analytical techniques and data interpretation processes.