Latitudinal distributions of 234Th in the upper western Indian Ocean

We conducted an onboard measurement of dissolved- and particulate ²³⁴Th in seawater of upper Indian Ocean. The study region covers the meridional section of upper (<500 m depth) Indian Ocean (3⁰N to 15⁰S at 67⁰E in July 2017, and 5⁰S to 13⁰S at 60⁰E and 5⁰S to 24⁰S 67⁰E in April 2018). Dissolved and particulate (>1.2 μm) ²³⁴Th ranged 0.8 – 2.7 dpm L^-1 and 0.05 – 0.7 dpm L^-1, respectively. In July 2017, the large deficiency of dissolved ²³⁴Th were consistently observed at ~50m depth where the subsurface chlorophyll maximum (SCM) present, along the entire section (5⁰S to 13⁰S). After then, the ²³⁴Th/²³⁸U were almost ~1 in ≥100 m depths. In contrast, in April 2018, the significant deficits of dissolved ²³⁴Th were observed in entire upper water columns, 0 – 200m depths. This difference in distribution patterns between two years appears to be related to the annual-/seasonal- variations of SCM patterns. In 2018, SCM were shown in 70 – 80 m depths near equator (5⁰S degree), and gradually deepens in lower latitude (SCM presents in 130 m depths in 24⁰S). Interestingly, the unusually lowest dissolved ²³⁴Th (and very low particulate ²³⁴Th also,) were observed in 5⁰S 60⁰E, near the Seychelles–Chagos thermocline ridge (SCTR) region. There are two hypotheses to explain this extremely lower concentrations of ²³⁴Th. The one is that the large input of lithogenic particles from SCTR, seems to be due to largest ²³⁴Th removal in the water column of extremely shallow area (<300 m of bottom depth). The other is that unusually strong eastward currents (>1 m/s of zonal velocity, based on ADCP observations) can laterally transport the ²³⁴Th. In this presentation, we will also present the preliminary results of vertical export fluxes of some particulate trace elements (Al, Fe, Mn, Cu, Zn, Ni, Pb, and etc.) in the upper Indian Ocean estimated by using this ²³⁴Th tracer.