Ocean acidification trend in the tropical North Pacific since the mid-20th century reconstructed from a coral archive

Ocean acidification caused by anthropogenically elevated CO₂ concentration in the atmosphere can pose a critical threat to calcifying marine organisms and coral reef ecosystems. However, because of temporally and spatially limited instrumental pH records, little is known about the actual long-term trend and natural variability of seawater pH during the past century. We present an annually resolved time series of a pH proxy record for 1940-1999 using boron-isotope composition (δ¹¹B) in a modern massive Porites coral from Guam Island (NW Pacific). When superimposed onto interannual variability, the data show a slightly decreasing trend of ~0.39‰ (equivalent to ~0.05-0.08pH units for surface water pH) in the northwestern tropical Pacific since the mid-20th century. This first reported, coral-based reconstruction of long-term open ocean pH is a unique archive for ocean acidification trend in the North Pacific Ocean for the past, which, along with δ¹¹B records from South Pacific corals, can be an important key to ascertaining the extent and rapidity of actual acidification in the Pacific Ocean in the future.