[1] The Pacific Decadal Oscillation (PDO) has been shown to have significant climatic and environmental impacts across the Pan-Pacific Basin; however, there are no records of PDO activity from the South China Sea (SCS), the largest marginal sea in the northwest Pacific Ocean. This study suggests that a series of geochemical profiles obtained from a modern coral in the northern SCS records annual PDO activity dating back to 1853. These geochemical data are significantly correlated with the PDO index, and their patterns of variation closely match those of the PDO index over the last century. The relationship between the PDO and coral geochemistry may be related to the influence of the PDO on rainfall on Hainan Island. Rainfall patterns influence the volume of terrestrial runoff, which, in turn, is a primary determinant of

The double-spike method with multi-collector inductively coupled plasma-mass spectrometry was used to measure the Mo mass fractions and isotopic compositions of a set of geological reference materials including the mineral molybdenite, seawater, coral, as well as igneous and sedimentary rocks. The long-term reproducibility of the Mo isotopic measurements, based on two-year analyses of NIST SRM 3134 reference solutions and seawater samples, was ≤ 0.07‰ (two standard deviations, 2s, n = 167) for d Molybdenum (Mo) is one of the most abundant transition metals in the oceans and is important for a range of biological and geochemical processes (Manheim and Landergren 1974, Colodner et al. 1995, Morford and Emerson 1999. It has seven stable isotopes with relative abundances ranging from 9% to 25%, covering a mass range of about 8% (Anbar 2004). Molybdenum isotopes are increasingly being used in Earth sciences. Of particular interest is the highly redox-sensitive behaviour of Mo, which is primarily used to investigate the oxygenation of Earth's ocean and atmosphere (Barling et al.

Long‐term seawater pH records are essential for evaluating the rates of ocean acidification (OA) driven by anthropogenic emissions. Widespread, natural decadal variability in seawater pH superimposes on the long‐term anthropogenic variations, likely influencing the OA rates estimated from the pH records. Here, we report a record of annual seawater pH estimated using the δ11B proxy over the past 159 years reconstructed from a Porites coral collected to the east of Hainan Island in the northern South China Sea (SCS). By coupling this time series with previously reported long‐term seawater pH records in the West Pacific, the decadal variability in seawater pH records and its possible driving mechanisms were investigated. The results indicate that large decadal variability in seawater pH has occurred off eastern Hainan Island over the past 159 years, in agreement with previous records. The Qiongdong upwelling system, which controls nutrient supplies, regulates surface water productivity, and is driven by the East Asian summer monsoon, is the primary control of this decadal variability, while terrestrial inputs appear not influence significantly. Meanwhile the impacts of the Pacific Decadal Oscillation (PDO) and the El Nino and Southern Oscillation (ENSO) systems on seawater pH off eastern Hainan Island is likely limited. In contrast, the PDO is the main factor to influence the decadal seawater pH variability offshore the East Australia, while the mechanism controlling the decadal seawater pH variability in Guam is not clear yet. Meanwhile, The rate of decrease in seawater pH estimated from coral records are significantly different in different regions and over different time spans, which may reflect a combination of natural decadal variability in seawater pH and long‐term variations. Therefore, understanding the mechanisms driving natural variability in seawater pH is important for improving estimates of ocean acidification rates driven by anthropogenic emissions.