Unusually low234Th in a hydrothermal effluent plume over the Southwest Indian Ridge

Abstract

Particle dynamics in hydrothermal plumes are crucial to understanding the cycling of carbon and trace elements in the global oceans, but this subject area has been poorly studied. We investigated radioactive 234Th in a hydrothermal plume of a recently discovered vent over the Southwest Indian Ridge (SWIR). Above the plume, total 234Th was in equilibrium with 238U, showing the typical characteristic of general deep water. However, there was a 234Th deficit within the plume, with 234Th/238U ratios in the 0.77–0.91 range. Particulate 234Th accounted for ∼10% of the total 234Th, contrasting with 4% in the overlying water. On average, the scavenging and removal rates of 234Th were 17.5 ± 2.5 dpm m−3 d−1 and 11.8 ± 2.5 dpm m−3 d−1. The residence time of dissolved 234Th (avg. 108 ± 8 d) was much higher than particulate 234Th (avg. 19 ± 1 d), indicating that scavenging of 234Th from dissolved to particulate phase dominated its residence time scale. Particulate organic carbon (POC) increased 15% within the plume, compared with the overlying water. Combining the removal of 234Th and the ratio of POC to particulate 234Th, the POC removal flux was 9.3 ± 0.6 mmol m−2 d−1. Similarly, 2.2 ± 0.6 mmol m−2 of particulate nitrogen (PN) was removed per day from the plume. The magnitude of POC and PN removal implied an important role of the hydrothermal plume in delivering organic matter to the seafloor. This study thus reveals the different particle dynamic characteristics within the hydrothermal plume over the SWIR compared to the ordinary deep oceans.