Keywords:17O-excess, the triple isotopic composition, precipitation
A high-precision analysis of the δ17O of water has provided a promising new tracer 17O-excess (Barkan and Luz, 2005). The changes in 17O-excess is caused by molecular diffusion process during evaporation, which is basically similar to d-excess (=δD - 8×δ18O). In fact, a positive correlation between 17O-excess and d-excess of water vapor was observed in the ocean (Uemura et al., 2010). However, much lower correlations were found in Antarctica (Landais et al., 2008; Touzeau et al., 2016) and in the United States (Li et al., 2012), probably because of kinetic fractionations. Here, we measured the isotopic composition in precipitation on Okinawa jima (Japan) to reveal seasonal 17O-excess variation in East Asian monsoon region. Precipitation samples were collected every week over 2 years in Okinawa, Japan. The δ18O, δ17O and δD of water were measured using a cavity ring-down spectroscopy (L2140-i, Picarro). The 17O-excess in precipitation shows lower values in summer and higher values in winter. The seasonal variation is similar to that of d-excess, which was interpreted as changes in relative humidity in the moisture source ocean (Uemura et al., 2012). The results suggest that the precipitation in Okinawa is mainly controlled by isotope fractionation during the evaporation in the ocean.