Unusually low ²³⁴Th in a hydrothermal effluent plume over the Southwest Indian Ridge

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• 作者：Weifeng Yang ; Xinxing Zhang ; Min Chen ; Yusheng Qiu
• 刊名：Geochemistry, Geophysics, Geosystems
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：17
• 期：9
• 页码：3815-3824
• 全文大小：1.2MB
• ISSN：1525-2027

文摘

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.