A Replication Study on Coral δ11B and B/Ca and Their Variation in Modern and Fossil Porites: Implications for Coral Calcifying Fluid Chemistry and Seawater pH Changes Over the Last Millennium

Boron systematics offer a unique opportunity to reveal coral calcifying fluid (CF) chemistry and seawater pH (pH(sw)). Here, we assess the intercolony differences of skeletal delta B-11 and B/Ca, and examine their variation in modern and fossil Porites spp. collected from the east Hainan Island in the northern South China Sea (SCS), to explore changes in coral CF chemistry and pH(sw) over the last millennium. This enables us to assess whether ocean acidification (OA) has disturbed the ability of corals to control their CF chemistry, and whether splicing coral delta B-11-pH records can trace long-term pH(sw) variability. We demonstrate that coral boron systematics bear remarkable intercolony differences, with mean offset as high as 1.05 parts per thousand for delta B-11 and 183.1 mu mol/mol for B/Ca. With this in mind, we show that fossil corals exhibit no significant difference in their CF carbonate chemistry, but all have systematically higher CF pH (pH(cf), by an average of 0.12 units) and almost equivalent CF dissolved inorganic carbon (DICcf) concentration, compared to modern corals. This suggests greater OA impacts on coral pH(cf) but less noticeable effects on DICcf. In addition, the similar to 0.12 decline in pH(cf) translates to about 0.24 reduction in pH(sw), similar to another coral-based estimate (similar to 0.24) from south Hainan Island, corroborating significant OA in the northern SCS since the industrial era. Nevertheless, we find that pH(sw) in the east Hainan Island has staged a recovery from 1980 to 2010, slowing down the OA pace, highlighting important roles of other local forcing on pH(sw) regulation.

Automated summary

The study reveals significant intercolony differences in coral boron systematics, with fossil corals showing higher CF pH compared to modern corals. Ocean acidification has been evident in the northern South China Sea since the industrial era, but there has been a recovery in seawater pH in east Hainan Island from 1980 to 2010, slowing down the pace of acidification.