EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Latitudinal distributions of 234Th in the upper western Indian Ocean

Suk Hyun Kim, Intae Kim, and Hyunmi Lee
Suk Hyun Kim et al.
  • Korea Institute of Ocean Science and Technology, Marine Environmental Research Center, Busan, Korea, Republic of (

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.

How to cite: Kim, S. H., Kim, I., and Lee, H.: Latitudinal distributions of 234Th in the upper western Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21824,, 2020