Bomb-produced radiocarbon dynamics and salinity-water isotopes relationship at two stations in the tropical South Pacific Ocean and implications for upper ocean processes
摘要
We report results of radiocarbon (14C) of dissolved inorganic carbon together with oxygen and hydrogen isotope compositions (δ18O and δD) in water samples collected from two stations along the World Ocean Circulation Experiment Hydrographic Program transect P17 in the South Pacific in March and April 2018. We quantify the changes of bomb-produced radiocarbon (bomb-14C) in the seawater and compare them with data previously reported for the stations around. The penetration depth of the bomb-14C at both stations remains above 500 m over the period of 1991–2018, pointing to limited vertical mixing due to the isopycnal circulation in the upper ocean. The estimated inventory in the entire water column at two stations has decreased to an averaged value of 8 × 109 atoms cm−2 over the 27 years. The maximum reduction in Δ14C values occurs at ~ 200 m in the upper part of the thermocline and is attributed to the migration of the subtropical water northward to the tropics. A relatively shallow slope in the relationship between salinity and water isotopes is also observed in the seawater samples of 100–300 m depth from the two stations, which is interpreted as resulting from the sinking of the subtropical surface seawater after strong evaporation. Such isotope properties constrained by the combined 14C, δ18O and δD data characterize the water that feeds the Equatorial Undercurrent.