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.

Utilizing 210Po deficit to constrain particle dynamics in mesopelagic water, western South China Sea

“…The residence times of particulate 210 Po at the slope stations (i.e., 105A and 91) ranged from 0.11 to 0.16 year within the mesopelagic layer (averaging 0.13 ± 0.04 year), comparable to the residence times of TPM in a hydrothermal plume over the Southwest Indian Ridge [ Yang et al ., ]. Such a time scale allows particulate matter to move over a very long range as proved in general deep water [ Siegel et al ., ] and in the mesopelagic or bathypelagic hydrothermal plumes [ Wang et al ., ; Estapa et al ., ].…”

“…In this study, the k values of 210 Po averaged 0.0020 ± 0.0015 day −1 in mesopelagic water (Table ), comparable to 0.0029 ± 0.0009 day −1 estimated based on the data sets ( t test, p > 0.10) in the northern SCS [ Wei et al ., ]. The scavenging constant illustrates the intensity of particle‐reactive nuclide adsorption on particulate matter in seawater [ Yang et al ., ]. Higher k values correspond to stronger affinity of particle with 210 Po.…”

“…Combining all the residence times and removal rates of 210 Po, it is clear that particle cycling is very active in the mesopelagic layer in the SCS. Based on the currently limited studies, particulate matter has been found to play important roles in trace elements [ Rigaud et al ., ], nutrients [ Kadko et al ., ] and carbon cycling [ Hu et al ., ; Yang et al ., ] in deep water with active particle dynamics. Our study provides an opportunity to unravel the particle cycling in mesopelagic water in the SCS.…”

“…Such deficiencies in the total 210 Geochemistry, Geophysics, Geosystems 10.1002/2017GC006899 the mesopelagic layer. In general, the predominant removal of 210 Po is sinking after being adsorbed onto particulate matter [Yang et al, 2013;Rigaud et al, 2015;Wei et al, 2017] in a manner similar to the particlereactive thorium isotopes [Yang et al, 2016]. Since 210 Po reached equilibrium with 210 Pb at the top mesopelagic layer for all stations (Figure 3), the deficits of 210 Po in mesopelagic water could not result from the sinking particles out of the euphotic zone.…”

Mineralogy and Geochemistry of Deep-Sea Sediments from the Ultraslow-Spreading Southwest Indian Ridge: Implications for Hydrothermal Input and Igneous Host Rock

“…Al 2 O 3 versus SiO 2 of 120 biogenic sediments, from 46 • E to 52 • E, on SWIR. Data are from[5,13,25,28,65,66] and reference therein.…”