Instability of sea water pH in the South China Sea during the mid-late Holocene; evidence from boron isotopic composition of corals

We used positive thermal ionization mass spectrometry (PTIMS) to generate high precision δ ¹¹B records in Porites corals of the mid-late Holocene from the South China Sea (SCS). The δ ¹¹B values of the Holocene corals vary significantly, ranging from 22.2‰ to 25.5‰. The paleo-pH records of the SCS, reconstructed from the δ ¹¹B data, were not stable as previously thought but show a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values. The latter is likely caused by the large amount of anthropogenic CO ₂ emissions since the Industrial Revolution but variations of atmospheric pCO ₂ cannot explain the pH change of the SCS before the Industrial Revolution. We suggest that variations of monsoon intensity during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene. Results of this study indicate that the impact of anthropogenic atmospheric CO ₂ emissions may have reversed the natural pH trend in the SCS since the mid-Holocene. Such ocean pH records in the current interglacial period can help us better understand the physical and biological controls on ocean pH and possibly predict the long-term impact of climate change on future ocean acidification.