Understanding the Compositional Variability of the Major Components of Hydrothermal Plumes in the Okinawa Trough

Zhigang Zeng,Haiyan Qi,Chen-Tung Arthur Chen,Xiaoyuan Wang

doi:10.1155/2018/1536352

Abstract

Studies of the major components of hydrothermal plumes in seafloor hydrothermal fields are critical for an improved understanding of biogeochemical cycles and the large-scale distribution of elements in the submarine environment. The composition of major components in hydrothermal plume water column samples from 25 stations has been investigated in the middle and southern Okinawa Trough. The physical and chemical properties of hydrothermal plume water in the Okinawa Trough have been affected by input of the Kuroshio current, and its influence on hydrothermal plume water from the southern Okinawa Trough to the middle Okinawa Trough is reduced. The anomalous layers of seawater in the hydrothermal plume water columns have higher K+, Ca2+, Mn2+, B3+, Ca2+/SO42-, and Mn2+/Mg2+ ratios and higher optical anomalies than other layers. The Mg2+, SO42-, Mg2+/Ca2+, and SO42-/Mn2+ ratios of the anomalous layers are lower than other layers in the hydrothermal plume water columns and are consistent with concentrations in hydrothermal vent fluids in the Okinawa Trough. This suggests that the chemical variations of hydrothermal plumes in the Tangyin hydrothermal field, like other hydrothermal fields, result in the discharge of high K+, Ca2+, and B3+ and low Mg2+ and SO42- fluid. Furthermore, element ratios (e.g., Sr2+/Ca2+, Ca2+/Cl−) in hydrothermal plume water columns were found to be similar to those in average seawater, indicating that Sr2+/Ca2+ and Ca2+/Cl− ratios of hydrothermal plumes might be useful proxies for chemical properties of seawater. The hydrothermal K+, Ca2+, Mn2+, and B3+ flux to seawater in the Okinawa Trough is about 2.62–873, 1.04–326, 1.30–76.4, and 0.293–34.7 × 106 kg per year, respectively. The heat flux is about 0.159–1,973 × 105 W, which means that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Highlights

The composition of the major components of hydrothermal plumes and their effect on the thermodynamics and kinetics of ocean processes, including the physical and chemical properties of seawater, have been studied by a number of workers (e.g., [1, 2])
The higher velocity (0.820 m/s) of the hydrothermal plume water column is found in the Yonaguni Knoll IV hydrothermal field (Figure 2(a)), with the lowest salinity (34.2), pH (7.53, 25∘C), B3+ (0.330 mmol/kg), Mg2+ (50.9 mmol/kg), total S (26.9 mmol/kg), K+ (9.70 mmol/kg), Ca2+ (9.02 mmol/kg), Sr2+ (73.2 μmol/kg), Cl− (547 mmol/ kg), and SO42− (26.1 mmol/kg) values in the Iheya North knoll and Clam hydrothermal field (Table 1)
The B3+, total S, and SO42− concentrations of hydrothermal plume water column samples show a positive correlation with Sr2+, Mg2+, Ca2+, and Cl− concentrations in the Iheya North knoll, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields (Figures 3(a), 3(b), 3(c), and 3(d))

Summary

Introduction

The composition of the major components of hydrothermal plumes and their effect on the thermodynamics and kinetics of ocean processes, including the physical and chemical properties of seawater, have been studied by a number of workers (e.g., [1, 2]). The hydrothermal plumes from high temperature vent fluids are devoid of Mg2+, which is related to the formation of magnesium silicates when seawater reacts with subseafloor volcanic rocks such as basalt (e.g., [4, 5]), resulting in the generation of H−, which accounts for the low pH and titration of the alkalinity [3]. Much of the dissolved SO42− in seawater is lost due to heating in the downflow phase of the hydrothermal system, with precipitation of anhydrite (CaSO4) at temperatures of ∼130∘C [3].

Methods

Results

Discussion

Conclusion