Using the three thorium isotope toolbox to probe the particle dynamic within an East Pacific Rise hydrothermal plume

The insoluble radiogenic isotopes of thorium (Th) are produced at a known rate in the water column via the decay of soluble uranium (234Th, 230Th) and radium (228Th) isotopes. These three isotopes are radioactive and their half-lives vary from days (234Th) to years (228Th) to tens of thousands of years (230Th). Combining their known production and decay rates with their insolubility makes them excellent tools to study the particle dynamics on a wide range of timescales.

This toolbox was successfully used by Pavia and co-workers (2019, see reference below) to study particle-dissolved exchange within the hydrothermal plume detected during the GEOTRACES GP16 cruise in the southeast Pacific Ocean. The goal of these authors was to unravel how hydrothermal activity affects the different steps characterizing the scavenging processes, i.e. adsorption and desorption onto particles, particle aggregation, sinking, and eventual sedimentation.

Their main conclusions are that: 1) particle aggregation was occurring much more rapidly in the plume, 2) hydrothermal scavenging is partially irreversible, 3) off-axis hydrothermal Th scavenging rate of 0.15yr−1, value deduced from a modelling and 4) 230Th is surprisingly more depleted than the two other isotopes. This likely reflects progressive scavenging in this region of intense hydrothermal activity and underlines the complexity of interpreting the GP16 hydrothermal plume as being solely a local phenomenon.

19 Pavia

Figure: Depletion observed in three thorium isotopes in the hydrothermal plume observed downstream of the East Pacific Rise on the GEOTRACES GP16 section in the South Pacific Ocean. Plots A), B), and C) show the depletion in each thorium isotope at stations 18 (closest to the ridge axis) to station 21 (furthest from the ridge axis). The depletion increases with increasing half-life of thorium isotope, going from 234Th (half-life = 24.1 days) showing the least depletion, followed by 228Th (half-life = 1.91 years), with 230Th (half-life = 75,587 years) the most depleted. D) Shows the map of the study area, with solid white arrows proportional to current speeds at the plume depth of 2500m, and the white dashed arrow displaying the proposed flowpath of the hydrothermal plume observed in the study, along which thorium is progressively removed from the deep ocean. Click here to view the figure larger.

Reference:

Pavia, F. J., Anderson, R. F., Black, E. E., Kipp, L. E., Vivancos, S. M., Fleisher, M. Q., Charette, M. A., Sanial, V., Moore, W. S., Hult, M., Lu, Y., Cheng, H., Zhang, P., Edwards, R. L. (2019). Timescales of hydrothermal scavenging in the South Pacific Ocean from 234Th, 230Th, and 228Th. Earth and Planetary Science Letters, 506, 146–156. DOI: http://doi.org/10.1016/J.EPSL.2018.10.038

Latest highlights

Science Highlights

Specific features characterize the dissolved iron distribution in the North Western Indian Ocean

Venkatesh Chinni and Sunil Kumar Singh propose dissolved iron profiles along two meridional transects realized during spring and fall seasons between the Arabian Sea and the sub-tropical western Indian Ocean…

07.01.2022

Science Highlights

Anthropogenic aerosol has become a dominant source of zinc in the deep water of the Northern South China Sea

Liao and colleagues determined zinc concentrations and isotope compositions in sinking particles collected in the Northern South China Sea…

24.11.2021

Science Highlights

An updated global ocean chromium biogeochemical cycle

Janssen and co-authors present an exhaustive compilation of ocean chromium data…

Science Highlights

Mercury stable isotopes constrain atmospheric pathways to the ocean

The study’s results hold promise that the implementation of anti-pollution measures under the Minamata Convention will likely result in a faster decrease of oceanic mercury levels than previously thought.

18.11.2021

Rechercher