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

Abstract

AbstractBoron systematics offer a unique opportunity to reveal coral calcifying fluid (CF) chemistry and seawater pH (pHsw). Here, we assess the intercolony differences of skeletal δ11B 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 pHsw 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 δ11B‐pH records can trace long‐term pHsw variability. We demonstrate that coral boron systematics bear remarkable intercolony differences, with mean offset as high as 1.05‰ for δ11B and 183.1 μmol/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 (pHcf, 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 pHcf but less noticeable effects on DICcf. In addition, the ∼0.12 decline in pHcf translates to about 0.24 reduction in pHsw, similar to another coral‐based estimate (∼0.24) from south Hainan Island, corroborating significant OA in the northern SCS since the industrial era. Nevertheless, we find that pHsw 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 pHsw regulation.