Cold Bias in Coral-based Seawater Temperature Reconstructions

Cold bias in coral-based seawater temperature reconstructions

Corals are small invertebrate organisms which typically live in colonies and produce skeletons which form the basis of coral reef structures. The chemistry of the skeletons, in particular the balance of two elements, strontium (Sr) and calcium (Ca),is affected by seawater temperature and  the analysis of fossil skeletons can indicate past climate. Sr/Ca coral climate records have been pivotal in understanding past changes in global climate e.g. for studying the frequency and severity of El Nino in the past, and for testing global climate models for predicting 21st century climate change. However the Sr/Ca of many fossil corals that grew around the time of the last glacial maximum yields seawater temperatures that are significantly cooler than predicted from climate models or from analysis of other climate proxies, a phenomenon termed ‘cold bias’.

Now an international team, led by researchers at the University of St. Andrews, has found that variations in the seawater carbonate system can induce changes in coral skeletal Sr/Ca which mimic the effects of cooling temperatures. Seawater pCO₂ (the dissolved CO₂ concentration) oscillated over glacial-interglacial time scales in response to changes in atmospheric CO₂. The team grew corals over a range of seawater pCO₂ but at the same seawater temperature to investigate how past changes in seawater pCO₂ affected skeletal chemistry. They built a unique coral culturing system designed which uses computer feedback and automated chemical monitoring to control the pCO₂ chemistry of the seawater within very tight limits. They found that coral skeletal Sr/Ca increased significantly in 2 of the 3 genotypes studied in response to changes in seawater pCO₂. The magnitude of the variation equates to errors in reconstructed SST of up to -5°C. Changes in past seawater pCO₂ over glacial-interglacial time scales may explain the cold bias observed in some fossil corals. This result provides an explanation for the inconsistencies observed in some coral climate records and allows coral climate data to be used more effectively to understand climate change.

The research is published this week in Nature Scientific Reports.

(link: http://www.nature.com/articles/srep26888)

Cole, C, Finch AA, Hintz C, Hintz K and Allison N, Understanding cold bias: Variable response of skeletal Sr/Ca to seawater pCO₂ in acclimated massive Porites corals. Scientific Reports. 6, 26888; doi: 10.1038/srep26888 (2016).