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 (delta B-11) in a modern massive Porites coral from Guam Island (NW Pacific). When superimposed onto interannual variability, the data show a slightly decreasing trend of similar to 039%0 (equivalent to similar to 0.05-0.08 pH 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 delta B-11 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. (C) 2013 Elsevier B.V. All rights reserved.