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

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 + , Ca 2+ , Mn 2+ , B 3+ , Ca 2+ /SO 4 2− , and Mn 2+ /Mg 2+ ratios and higher optical anomalies than other layers. The Mg 2+ , SO 4 2− , Mg 2+ /Ca 2+ , and SO 4 2− /Mn 2+ 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 + , Ca 2+ , and B 3+ and low Mg 2+ and SO 4 2− fluid. Furthermore, element ratios (e.g., Sr 2+ /Ca 2+ , Ca 2+ /Cl − ) in hydrothermal plume water columns were found to be similar to those in average seawater, indicating that Sr 2+ /Ca 2+ and Ca 2+ /Cl − ratios of hydrothermal plumes might be useful proxies for chemical properties of seawater. The hydrothermal K + , Ca 2+ , Mn 2+ , and B 3+ flux to seawater in the Okinawa Trough is about 2.62-873, 1.04-326, 1.30-76.4, and 0.293-34.7 × 10 6 kg per year, respectively. The heat flux is about 0.159-1,973 × 10 5 W, which means that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Dispersion and Intersection of Hydrothermal Plumes in the Manus Back-Arc Basin, Western Pacific

“…During the MANUS cruise of R/V KEXUE in 2015, 18 hydrocast stations were set up in the water column above the PACMANUS (9 stations) and Desmos (9 stations) hydrothermal fields, among which the seawater and hydrothermal plume samples were collected at 8 stations (Figures 1(c) and 1(d)). We followed the methods of Zeng et al [38]. The seawater and hydrothermal plume samples were collected at different depths with a Conductivity-Temperature-Depth (CTD) alu-minum rosette (Seabird) containing twenty-four 10 L Niskin bottles.…”

“…The measurement accuracies were ±0.001°C for temperature, ±0.0003 S/m for conductivity, ±0.015% of the full-scale range for pressure, ±0.005 m/s for velocity (0.5% of the water velocity relative to LADCP), and ±2°5′ for direction, with resolutions of ±0.0002°C, ±0.0003 S/m, ±0.0015% of the full-scale range, 0.001 m/s, and 0.01°, respectively. The turbidity sensitivity was 200 mV/FTU (100x gain; range of 25 FTU) [38].…”

“…The pH meter was calibrated with buffer solutions of pH 4.00 (0.050 mol/L of potassium hydrogen phthalate) and 6.86 (0.025 mol/L of mixed phosphate). The aqueous samples were filtered through a 47 mm Merck Millipore 0.10 μm nitrocellulose membrane into 1 L Nalgene polypropylene bottles (previously soaked in 1 : 1 HNO 3 for 48 h and washed to neutral pH with deionized water and ultrapure water) within the ship laboratory immediately after collection from the Niskin bottles [38]. Filtered water samples were acidified to a pH of 1.8 using 2 M ultrapure HNO 3 (J. T. Baker), capped tightly, and resealed with a Parafilm sealing membrane.…”

“…For the determination of Na, Mg, Ca, and K, the filtered and acidified samples were diluted 10 times by pure water and the concentrations of dissolved Na, Mg, Ca, and K in the aqueous samples were measured by inductively coupled plasma optical emission spectrometry (PE 2100DV) with a precision greater than ±5% at the Shandong Institute of Geophysical and Geochemical Exploration [38]. For the determination of Cl and SO 4 2-, the samples were diluted 200 times by pure water and the Cl and SO 4 2concentrations were measured by ion chromatography (ICS-1100) with an anion exchange resin column (Dionex AS19) rinsed with a solution of 1.8 mmol/L of Na 2 CO 3 and 1.7 mmol/L of NaHCO 3 at a rinsing rate of 1 mL/min and precision of ±3% [38]. The accuracy was controlled by the recovery; for Na, Mg, Ca, K, Cl, and SO 4 2-, the recovery rates were 99.5%, 99.6%, 99.8%, 99.7%, 99.7%, and 100.1%, respectively.…”

“…The dFe and dMn contents of the seawater and plume samples were determined by inductively coupled plasma 3 Geofluids sector field mass spectrometry (Element, Thermo Scientific) at ALS Scandinavia AB, Luleå, Sweden, in accordance with the method described by Rodushkin and Ruth [40]. Reference materials NASS-6 (North Atlantic Seawater) and CASS-5 (Nearshore Seawater) from the National Research Council of Canada were used to evaluate the accuracy of the dFe and dMn analyses, and accuracies and precision (n = 5) greater than 5% were achieved for both [38]. (Tables 1 and 2).…”