Open AccessUnusually low 234 T h in a hydrothermal effluent plume over the S outhwest I ndian R idge
Author(s) -
Yang Weifeng,
Zhang Xinxing,
Chen Min,
Qiu Yusheng
Publication year - 2016
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2016gc006580
Subject(s) - plume , particulates , geology , hydrothermal circulation , scavenging , panache , ridge , water column , oceanography , mineralogy , geochemistry , environmental chemistry , chemistry , paleontology , biochemistry , physics , organic chemistry , thermodynamics , antioxidant
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 234 Th in a hydrothermal plume of a recently discovered vent over the Southwest Indian Ridge (SWIR). Above the plume, total 234 Th was in equilibrium with 238 U, showing the typical characteristic of general deep water. However, there was a 234 Th deficit within the plume, with 234 Th/ 238 U ratios in the 0.77–0.91 range. Particulate 234 Th accounted for ∼10% of the total 234 Th, contrasting with 4% in the overlying water. On average, the scavenging and removal rates of 234 Th 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 234 Th (avg. 108 ± 8 d) was much higher than particulate 234 Th (avg. 19 ± 1 d), indicating that scavenging of 234 Th 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 234 Th and the ratio of POC to particulate 234 Th, 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.