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Geofluids
Volume 2018, Article ID 1536352, 20 pages
https://doi.org/10.1155/2018/1536352
Research Article

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

1Seafloor Hydrothermal Activity Laboratory, CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China
3University of Chinese Academy of Sciences, Beijing 100049, China
4Department of Oceanography, National Sun Yat-sen University, Kaohsiung 80424, Taiwan

Correspondence should be addressed to Zhigang Zeng; nc.ca.oidq.sm@gnezgz

Received 27 December 2017; Accepted 22 February 2018; Published 4 April 2018

Academic Editor: Guido Ventura

Copyright © 2018 Zhigang Zeng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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+/, and Mn2+/Mg2+ ratios and higher optical anomalies than other layers. The Mg2+, , Mg2+/Ca2+, and /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 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.

1. 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]). In seafloor hydrothermal fields, chemical circulation at hydrothermal vents leads to a number of elements being added (e.g., Fe2+, Ca2+, Cu2+, Zn2+, Mn2+, and SiO2) or taken out (Mg2+, ) of seawater (e.g., [3, 4]). 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 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]. Na+ can also be lost from the hydrothermal fluid due to Na–Ca replacement reactions in plagioclase feldspars, known as albitization [3]. K+ and the other alkalis are involved in similar types of reaction that can also generate acidity [3]. Cl is the predominant anion in vent fluids, and the precipitation and/or reduction of and the titration of / results in Cl becoming the overwhelming and almost only anion. Most of the cations in vent fluids are present as chlorocomplexes; hence, the levels of Cl in a fluid effectively determine the total concentration of cationic species that can be present [3]. In addition, the major ions are present in seawater in relatively constant ratios (e.g., [4]), but these constant proportions are not maintained in vent fluids [3].

In the Okinawa Trough, the vent fluid chemistry of seafloor hydrothermal systems has been characterized by enrichment in CO2, CH4, , and K+ compared with those in typical sediment-free midocean ridge (MOR) hydrothermal fluids (e.g., [614]). The high K+ content (6.9–79.2 mmol kg−1) in the estimated source fluid is typical for the Iheya North knoll hydrothermal field due to hydrothermal reaction with K-enriched felsic rocks. The high content of B3+ (0.44–2.27 mmol kg−1) and alkalinity of venting fluids are derived from decomposition of sedimentary organic matter [9]. However, the collected venting fluids ( = 328°C) from the Yonaguni Knoll IV hydrothermal field in the southern Okinawa Trough had a wide range of chemical compositions (e.g., Cl 376–635 mmol kg−1; Ca2+ 14.3–26.1 mmol kg−1; Mn2+ 0.70–1.25 mmol kg−1), which is considered as evidence for subseafloor phase separation [14]. Furthermore, Mn2+ has also been studied extensively (e.g., [1517]), and the distribution of Mn2+ in the seawater column is strongly affected by external sources and sinks, hydrothermal processes, and redox condition (e.g., [1724]).

In addition, the Kuroshio current is the largest western boundary current in the north Pacific ocean (e.g., [2527]). It originates from the westward-flowing north equatorial current in the western Pacific [28] and transports warm, saline, generally oligotrophic (especially nitrogen deficient) seawater and enormous amounts of mass (e.g., geochemical materials) and energy (e.g., heat) from the low- to midlatitude regions (e.g., [29, 30]). The current flows east of Taiwan and northwards along the Okinawa Trough (e.g., [31]), with a maximum speed of 1 m/s and a width of 100 km [32]. Seawater transported by the Kuroshio current flows from 19 to 47 Sv (Sverdrup, 1 Sv = 106 m3/s) in the East China Sea [28, 3335] and is an important source of heat for the atmosphere in the global heat balance [36]. Changes in the intensity and volume input of the Kuroshio current can significantly influence seawater character, biogeochemical cycles, and climate in the northwestern Pacific (e.g., [26, 27, 3740]).

Despite these studies, little is known about the influence of the hydrothermal fluid and Kuroshio current environment on the behavior of the major components of hydrothermal plumes in the Okinawa Trough (OT). In this study, we determined the major components of hydrothermal plume water columns in the Okinawa Trough, to understand how they varied and the relationships between major components, physical properties, and current in the hydrothermal plumes in the OT.

2. Geological Setting

This study used hydrothermal plume water column samples and data from the Iheya North knoll and Clam hydrothermal fields in the middle Okinawa Trough, the Yonaguni Knoll IV, and Tangyin hydrothermal fields in the southern Okinawa Trough (Figure 1). The Okinawa Trough is a back-arc basin in the rifting to spreading stage characterized by the development of normal faulting of transitional crust (atypical crust with mantle-derived material) and frequent magma intrusions, which provides a favorable geological environment for the development of seafloor hydrothermal systems [8, 41, 42].

Figure 1: Location map showing the Okinawa Trough (Bathymetric map and data from http://www.geomapapp.org/index.htm). Bathymetric map showing the tectonic setting and location of the Iheya North, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields (from InterRidge data base; [48, 49]). Red stars indicate the location of hydrothermal plume water column samples, respectively.

As of 2016, there were at least 15 deep-sea hydrothermal fields reported in the Okinawa Trough based on the InterRidge data base, including the Minami-Ensei (e.g., [10]), Iheya North [9, 11, 43], Jade [7, 13], Hakurei [8], Hatoma [4446], Yonaguni Knoll IV [12, 14, 47], and Tangyin hydrothermal fields [48, 49]. The Iheya North knoll hydrothermal field (27°47.2′N, 126°53.9′E) is located at a water depth about 1,000 m along the eastern slope of a small knoll, part of the Iheya North knoll volcanic complex (Figure 1; [8]). About ten active hydrothermal mounds, aligned north to south, are concentrated in a small region [8], with many hosting active fluid venting [8]. A large mound (named North Big Chimney), more than 30 m high and associated with vigorous venting of clear fluid and the highest temperature of 311°C, appears to mark the center of hydrothermal activity [43]. The vent fluids have lower alkalinities (0.5–3.6 mmol l−1), (1.9–2.6 mmol kg−1), and Cl (441–458 mmol kg−1) concentrations, indicating phase separation of the fluid below the seafloor [9].

The Clam hydrothermal field (27°33′N, 126°58′E), located in a small depression on the northern slope along the eastern part of the Iheya Ridge in the middle Okinawa Trough (Figure 1; [8]), was discovered in a deep-tow survey by the R/V NATSUSHIMA in 1988 [7, 51, 52]. Clear hydrothermal fluid with a temperature of up to 220°C was found to be discharging from 1 to 2 m high hydrothermal mounds and fissures (e.g., [8, 13]). The hydrothermal fluids from the Clam hydrothermal field are characterized by anomalously high alkalinity (2.5–10.3 mmol l−1), (0.6–4.4 mmol kg−1), and CO2/3He ratios (e.g., [6, 7, 13, 53]). Based on a temperature of 220°C, the end-member fluid is assumed to contain about 20 mmol kg−1 Mg2+ and 10 mmol kg−1   [6, 53].

The Yonaguni Knoll IV hydrothermal field (24°50.9′N, 122°42.0′E) is situated in an elongated valley with dimensions of ~1,000 × ~500 m (Figure 1; [14]). The valley is mostly covered with muddy sediment, except for the active hydrothermal field and volcanic breccia on its northern slope [14]. Diverse styles of fluid venting were found to occur within the field, including slightly Cl-enriched (614–635 mmol kg−1, compared with the 544 mmol kg−1 of seawater) and Cl-depleted fluid (376–491 mmol kg−1), which were associated with discharge of liquid droplets of CO2 (e.g., [14]).

During three days of investigation on the HOBAB 3 cruise in 2014, we discovered the Tangyin hydrothermal field (Figure 1; 25°4′N, 122°34′E, and 1206 m water depth). Results of our observations and the recovery of biological samples from the field suggest the presence of an active hydrothermal field located on top of a twin seamount named Yuhua Hill [48, 49]. The seamount is approximately 220 m high, extends ~1.5 km from east to west, and consists of two isolated topographic highs that are characterized by felsic volcanic basement with patches of sediment, adjacent to a submarine canyon [49].

3. Sampling and Methods

3.1. Specimen and Data Collection

Samples and data were collected in 2014 during the HOBAB 2 and 3 cruises of the R/V KEXUE from hydrothermal plume water columns in the middle (18 stations) and southern (7 stations) Okinawa Trough (Figure 1). During the occupation of a station, data were collected throughout the water column with a SBE-911 Plus Conductivity-Temperature-Depth (CTD) system coupled to a Seapoint turbidity meter, a SBE 43 dissolved oxygen sensor, and a Lowered Acoustic Doppler Current Profile (LADCP). The probes did not send back to the manufacturer, and they were calibrated by the National Center of Ocean Standards and Metrology (NCOSM) in July 2013. Measurement accuracy was ±0.001°C for temperature, ±0.0003 S/m for conductivity, ±0.015% of 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 full-scale range, 0.001 m/s, and 0.01°, respectively. Turbidity sensitivity was 200 mV/FTU (100x gain, range: 25 FTU). The dissolved oxygen probe was not calibrated in situ besides in the NCOSM. The measurement range for dissolved oxygen was 120% of surface saturation in hydrothermal plume water samples, and the accuracy was ±2% of saturation, with a typical stability of 0.5% per 1,000 hours of deployed time. A total of 640 water samples were taken from different depths with a CTD rosette of 24 Niskin bottles.

The temperature anomaly () of the hydrothermal plume relative to ambient water was calculated using Baker and Lupton’s [54] formula, which is suitable for the Pacific, and the potential density and potential temperature data needed in the formula were obtained by the CTD. However, the entrainment of seawater, which is incorporated in the calculation of the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height [55] and in the calculation of the penetration height of the plume [56], is not considered in this study, so the temperature anomaly value obtained is lower than the actual value, but the trend with depth is meaningful.

3.2. Analytical Methods

In the ship laboratory, the pH of each aqueous sample was determined with a portable pH meter (JENCO 6010, resolution 0.01, automatic temperature compensation) immediately after collection at room temperature about 25°C. The pH meter was calibrated with buffer solutions of pH 4.00 (potassium hydrogen phthalate 0.05 mol L−1) and 6.86 (mixed phosphate 0.025 mol L−1). The samples were filtered into 1 L Nalgene polypropylene bottles (previously soaked in 1 : 1 HNO3 for 48 h, washed to neutral pH with distilled water, and dried) on shore.

Concentrations of Na+, Mg2+, Ca2+, K+, Sr2+, B3+, and total S in the aqueous samples were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (PE 2100DV), with a precision of greater than ±5%, at the Shandong Institute of Geophysical and Geochemical Exploration. Cl and concentrations were measured by ion chromatography (ICS-1100) with an anion exchange resin column (DIONEX AS19) rinsed with 1.8 (mmol L−1) Na2CO3-1.7 (mmol L−1) NaHCO3 at a rinsing rate of 1.0 mL min−1; the precision was ±3%. F concentrations were determined with a fluoride ion selective electrode (PF-1-01) following the method of the National Standard GB 7484-87 of China at the Shandong Institute of Geophysical and Geochemical Exploration. The precision () was ±2%, with 99% recovery. The Mn2+ content of hydrothermal plume samples was determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) (ELEMENT, Thermo Scientific) in ALS Scandinavia AB, Luleå, Sweden, following the method of Rodushkin and Ruth [57]. Reference materials NASS-6 (North Atlantic Seawater) and CASS-5 (Near Shore Seawater) from the National Research Council Canada were used to evaluate the accuracy of Mn2+ determination, with accuracy and precision () both better than 5%.

4. Results

4.1. Major Element Concentrations in Hydrothermal Plume Water Columns

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 (26.1 mmol/kg) values in the Iheya North knoll and Clam hydrothermal field (Table 1). Average current velocity, turbidity, B3+, K+, Sr2+, and Cl values of hydrothermal plume water columns are lower in the Iheya North knoll and Clam than in the Yonaguni Knoll IV and Tangyin hydrothermal fields (Figures 2(b) and 2(c)), while the average dissolved oxygen, pH (25°C), total S, and F values are higher (Table 1). The majority of the Mg2+/Ca2+ ratios of hydrothermal plume water columns in the Iheya North knoll and Clam hydrothermal fields are higher than those in the Yonaguni Knoll IV and Tangyin hydrothermal fields (Table 2). Average Mg2+/Cl, Ca2+/Cl, Mg2+/salinity, and total S/salinity ratios in the Iheya North knoll and Clam hydrothermal fields of the middle Okinawa Trough are higher than in the Yonaguni Knoll IV and Tangyin hydrothermal fields of the southern Okinawa Trough (Table 2). Furthermore, ratios between Ca2+, Mg2+, , Na+, K+, Sr2+, Cl, and salinity in water columns in the Iheya North knoll of Okinawa Trough are more variable, with the largest ranges, than in the Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields (Table 2; Figure 2(d)).

Table 1: Physical properties and major component compositions of hydrothermal plume water columns in the middle and southern Okinawa Trough.
Table 2: Major component ratios of hydrothermal plume water columns in the middle and southern Okinawa Trough.
Figure 2: Major element concentrations in hydrothermal plume water columns from the Iheya North (IN), Clam, Tangyin to the Yonaguni Knoll IV (YKI) hydrothermal field in the Okinawa Trough: (a) current velocity; (b) turbidity; (c) B3+ concentration; and (d) Ca2+/salinity. Average seawater data from Turekian [50] and Millero [4].
4.2. Major Component Correlations in Hydrothermal Plume Water Columns

The B3+, total S, and 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)). The salinity, B3+, total S, and K+ of hydrothermal plumes also show positive correlations with Sr2+/Ca2+, Ca2+/Cl, Ca2+/, and Mg2+/Cl ratios in the Okinawa Trough (Figure 4). Furthermore, the Sr2+ concentrations of hydrothermal plume water column samples show a negative correlation with 1/B ratio in the Iheya North knoll, Clam, and Tangyin hydrothermal fields (Figure 4(d)).

Figure 3: Major component correlations for hydrothermal plume water columns in the Okinawa Trough: (a) Sr versus B concentration; (b) Ca versus total S concentration; (c) total S versus Mg concentration; and (d) concentration versus Cl concentration.
Figure 4: Major component correlations for hydrothermal plume water columns in the Okinawa Trough: (a) Sr2+/Ca2+ ratio versus B3+ concentration; (b) Ca2+/Cl ratio versus total S concentration; (c) Ca2+/ ratio versus total S concentration; and (d) 1/B ratio versus Sr2+ concentration.

5. Discussion

5.1. Variable Current Velocity in Hydrothermal Plume Water Columns

The current velocity in hydrothermal plume water columns in the southern Okinawa Trough is significantly more variable, and of higher magnitude (0.016 to 0.963 m s−1), than that in the middle Okinawa Trough (Table 1; Figure 2(a)); this trend is consistent with Kuroshio current velocity patterns (e.g., [58]). When the maximum velocity of the Kuroshio current is located within the top 80 m of the water column, it ranges from 0.36 to 2.02 m s−1; when the maximum velocity is below 80 m, it ranges from 0.31 to 1.11 m s−1 [33]. Thus, the velocity of the Kuroshio current reduces from the southern Okinawa Trough to the middle Okinawa Trough (e.g., [58]), suggesting that the current variations of hydrothermal plume water in the Okinawa Trough are controlled by spatial variations in the intensity and position of the Kuroshio current (e.g., [33, 58, 59]).

5.2. Variable Major Components in Hydrothermal Plume Water Columns

In the Iheya North knoll hydrothermal field, concentrations of K+, Ca2+, and B3+ in anomalous layer in hydrothermal plume water columns are higher than in average seawater [4, 50] (Figures 5 and 6) and are lower or higher than in hydrothermal fluids (K+ 6.9–82 mmol kg−1; Ca2+ 0–25 mmol kg−1; B3+ 0.44–2.27 mmol kg−1;   −7–15 mmol kg−1) in the area [9, 60], and those of Mg2+ and are lower than in average seawater [4, 50] (Figure 7). The Ca/ ratios in the anomalous layers are higher, and the Mg2+/Ca2+ ratios are lower than in average seawater [50, 60, 61] (Figure 8), suggesting that they result from active hydrothermal discharge (e.g., [1, 2, 62]).

Figure 5: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) K+ anomalies in the Iheya North and Clam hydrothermal fields; (b) K+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) Ca2+ anomalies in the Iheya North and Clam hydrothermal fields; (d) Ca2+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields. Average seawater data from Turekian [50] and Millero [4].
Figure 6: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) turbidity anomalies in the Iheya North and Clam hydrothermal fields; (b) turbidity anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) B3+ anomalies in the Iheya North and Clam hydrothermal fields; (d) B3+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields. Average seawater data from Turekian [50] and Millero [4].
Figure 7: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) Mg2+ anomalies in the Iheya North and Clam hydrothermal fields; (b) Mg2+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) anomalies in the Iheya North and Clam hydrothermal fields; (d) anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields. Average seawater data from Turekian [50] and Millero [4].
Figure 8: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) Ca2+/ anomalies in the Iheya North and Clam hydrothermal fields; (b) Ca2+/ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) Mg2+/Ca2+ anomalies in the Iheya North and Clam hydrothermal fields; (d) Mg2+/Ca2+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields. Average seawater data from Turekian [50] and Millero [4].

In the Clam hydrothermal field, concentrations of K+, Ca2+, Mn2+, and B3+ in anomalous layers in hydrothermal plume water columns are higher than in average seawater [4, 50] (Figures 5, 6, and 9), those of Mg2+ and are lower than in average seawater [4, 50] (Figure 7), and those of K+, Ca2+, Mn2+, B3+, Mg2+, and in anomalous layers in hydrothermal plume water columns are lower or higher than those (K+ 50–60 mmol kg−1; Ca2+ ~20 mmol kg−1; Mn2+ 400–500 mol kg−1; B3+ 5-6 mmol kg−1; Mg2+ 33.4–53.1 mmol kg−1;   ~10 mmol kg−1) of hydrothermal fluids in the Clam hydrothermal field [6, 53]. The Ca2+/ ratios are higher, and the Mg2+/Ca2+ ratios are lower than those in average seawater [4, 50] (Figure 8), indicating that the Mg2+/Ca2+ and Ca2+/ ratios of seawater columns can be used to describe the chemical anomalies of hydrothermal plumes in the Clam hydrothermal field.

Figure 9: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) Mn2+ anomalies in the Clam hydrothermal field; (b) Mn2+ anomalies in the Yonaguni Knoll IV hydrothermal field. Average seawater data from Turekian [50], and Millero [4].

In the Yonaguni Knoll IV hydrothermal field, the concentrations of K+, Ca2+, Mn2+, and B3+ in anomalous layers in hydrothermal plume water columns are higher than in average seawater [4, 50] (Figures 5, 6, and 9), those of Mg2+ and lower than in average seawater [4, 50] (Figure 7), and those of K+, Ca2+, Mn2+, B3+, Mg2+, and are lower or higher than those (K+ 55.3–90.1 mmol kg−1; Ca2+ 14.3–26.1 mmol kg−1; Mn2+ 700–1250 mol kg−1; B3+ 2.93–4.31 mmol kg−1; Mg2+ 0 mmol kg−1; 0 mmol kg−1) of hydrothermal fluids in the Yonaguni Knoll IV hydrothermal field [14]. The Mn2+/Mg2+ and Ca2+/ ratios are higher, and the Mg2+/Ca2+ and /Mn2+ ratios are several orders of magnitude lower than those of average seawater [50, 61] (Figures 8 and 10), suggesting the Mg2+/Ca2+, Mn2+/Mg2+, and Ca2+/ ratios of seawater columns can also be used to trace seafloor hydrothermal activity in the Yonaguni Knoll IV hydrothermal field.

Figure 10: Hydrothermal plume profiles in the Yonaguni Knoll IV hydrothermal field: (a) Mn2+/Mg2+ anomalies; (b) /Mn2+ anomalies. Average seawater data from Turekian [50] and Millero [4].

In the Tangyin hydrothermal field, concentrations of K+, Ca2+, and B3+ in anomalous layers in water columns are higher than in average seawater [4, 50] (Figures 5 and 6), those of Mg2+ and are also lower than in average seawater [4, 50] (Figure 7), like Mn2+ and turbidity anomalies of hydrothermal plumes in the Clam and Yonaguni Knoll IV hydrothermal fields (Figures 6 and 9). The B3+/Mg2+ and Ca2+/ ratios are all higher and the Mg2+/Ca2+, /B3+, and /K+ ratios are lower than those in average seawater [53, 61] (Figures 8 and 11), implying that hydrothermal plumes in the Tangyin hydrothermal field result from high K, Ca, and B and low Mg and fluid discharge in the southern Okinawa Trough.

Figure 11: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) /B3+ anomalies in the Iheya North and Clam hydrothermal fields; (b) /B3+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) Mg2+/B3+ anomalies in the Iheya North and Clam hydrothermal fields; (d) Mg2+/B3+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields. Average seawater data from Turekian [50] and Millero [4].

The majority of potential density/salinity ratios and turbidity in hydrothermal plume water columns decrease from the Yonaguni Knoll IV, Clam, and Tangyin to the Iheya North knoll hydrothermal fields (Table 1), while pH mostly increases (Table 1). These patterns are consistent with variations in temperature, and salinity of Kuroshio seawater from the southern to the middle Okinawa Trough [63, 64], suggesting that the physical and chemical influence of the Kuroshio current on hydrothermal plume water from the southern Okinawa Trough to the middle Okinawa Trough is reduced.

In addition, the Sr2+/Ca2+, Mg2+/Cl, and Ca2+/Cl ratios in the hydrothermal plume water column samples of the Okinawa Trough are similar to those average seawater (Table 2; [4, 50]), indicating that these ratios might be harnessed as a proxy of seawater chemical properties.

5.3. Quantifying the Heat and Major Element Flux of Vent Fluids to Seawater

Hydrothermal plumes in the Iheya North knoll, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields have been mapped by near-bottom CTD casts and sampled for major element compositions using rosette-mounted Niskin bottles. The heat and mass flux of the plume relative to ambient seawater of the same potential density is calculated using the equationswhere is the specific heat capacity (J g−1 K−1), is the potential density in the hydrothermal plume, is the current velocity of the hydrothermal plume, is the temperature anomaly of the plume relative to ambient seawater, is the element (e.g., K+, Ca2+, Mn2+, B3+, Mg2+, ) flux of the hydrothermal plume, and is the element concentration anomaly of the plume relative to ambient seawater.

Assuming a specific heat capacity for the hydrothermal plume of 4.0 J g−1 K−1 (3–5°C, [65]), (1) were applied to the hydrothermal plume samples to calculate the heat and mass flux output from the vents and vent fields. The flux of heat, K+, Ca2+, Mn2+, B3+, Mg2+, and to seawater is about 0.212–2,631 J m−2 s−1, 1.11–369 × 10−6, 0.441–138 × 10−6, 0.549–32.3 × 10−9, 0.124–14.7 × 10−6, −1.19–−158 × 10−6, and −1.16–−639 × 10−6 kg m−2 s−1, respectively (Table 3).

Table 3: Flux of hydrothermal heat, K+, Ca2+, Mn2+, B3+, Mg2+, and to seawater in the middle and southern Okinawa Trough.

Assuming the discharge of hydrothermal plumes is stable and persistent, and the area of plumes equals the area of the hydrothermal field, as in the Jade hydrothermal field (100 × 50 m) [66], the total area of the 15 hydrothermal fields in the middle and southern Okinawa Trough is about 7.5 × 104 m2, and the flux of heat, K+, Ca2+, Mn2+, B3+, Mg2+, and to seawater is about 0.159–1,973 × 105 W, 2.62–873, 1.04–326, 1.30–76.4, 0.293–34.7, −2.81–−374, and −1,377–−10,785 × 106 kg per year, respectively. Using the heat flux of 0.159–1,973 × 105 W calculated above and a total oceanic heat flux of 32 × 1012 W [67], this suggests that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

6. Conclusions

The current velocity (0.820 m/s) of the hydrothermal plume water column in the Yonaguni Knoll IV hydrothermal field is higher than in other fields, and the salinity, pH values, B3+, Mg2+, total S, K+, Ca2+, Sr2+, Cl, and concentrations of the hydrothermal plume water columns in the Iheya North knoll and Clam hydrothermal field are the lowest, and the B3+, total S, and 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. The majority of the /B3+ and Mg2+/B3+ ratios of hydrothermal plume water columns in the Iheya North, Clam, and Tangyin, are higher than those in the Yonaguni Knoll IV hydrothermal field. From the Yonaguni Knoll IV, Clam, and Tangyin to the Iheya North knoll hydrothermal fields, the majority of potential density/salinity ratios and turbidity in hydrothermal plume water column samples tend to decrease, while pH mostly increases, suggesting that 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.

In the hydrothermal fields of the Okinawa Trough, the B3+/Mg2+, K+/, and Ca2+/ ratios of anomalous layers in the hydrothermal plume water columns, like Mn2+, turbidity, and temperature anomalies, are higher than in other layers, which indicates hydrothermal input. The Mg2+/Ca2+, Mg2+/K+, /B3+, and /Mn2+ ratios of anomalous layers are lower than in other layers and are consistent with the low Mg2+ and concentration of vent fluids in the Okinawa Trough. During dilution of the hydrothermal plume by seawater, Ca2+ and Mn2+ show similar variations. In the Iheya North knoll, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields, salinity, B3+, total S, and K+ of hydrothermal plumes show positive correlations with Sr2+/Ca2+, Ca2+/Cl, Ca2+/, and Mg2+/Cl ratios, and Sr2+ concentrations and 1/B ratio show negative correlation, and the high K, Ca, and B and low Mg and concentration of vent fluid influences chemical variation of hydrothermal plume in Tangyin hydrothermal field. In addition, the element ratios (e.g., Sr2+/Ca2+, Ca2+/Cl) in the hydrothermal plume water column of the Okinawa Trough are similar to those in seawater, indicating that Sr2+/Ca2+ and Ca2+/Cl ratios might be harnessed as a proxy of seawater chemical properties. The calculated heat flux to seawater in the Okinawa Trough is about 0.159–1,973 × 105 W, and the calculated mass fluxes of K+, Ca2+, Mn2+, B3+, Mg2+, and are up to 0.873, 0.326, 0.076, 0.034, −0.374, and −10.8 × 109 kg per year. These results mean that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Additional Points

Research Highlights. (i) Hydrothermal plume has been affected by Kuroshio current, (ii) influence of Kuroshio on hydrothermal plume from SOT to MOT is reduced, (iii) high K, Ca, and B and low Mg and fluid influences chemical variation of plume in Tangyin, (iv) up to 0.0006% of ocean heat is supplied by plumes in the Okinawa Trough, and (v) hydrothermal K, Ca, B, and Mn fluxes are up to 8.73, 3.26, 0.347, and 0.764 × 108 kg p.a.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors would like to thank the crews of the R/V KEXUE during the HOBAB 2 and 3 cruises for their help with sample collection. They are grateful to Professor Fei Yu and Dr. Xinyuan Diao of Institute of Oceanology, Chinese Academy of Sciences, for providing the LADCP and performing the data processing. They are most grateful for the detailed and constructive comments and suggestions provided by Professor Alfonso Mucci, which greatly improved an earlier version of the manuscript. Elsevier’s Language Editing Services helped to improve the quality of language in the manuscript. This work was supported by the National Natural Science Foundation of China (Grant nos. 41325021, 414776044), the National Key Basic Research Program of China (Grant no. 2013CB429700), the National Programme on Global Change and Air-Sea Interaction (Grant no. GASI-GEOGE-02), the International Partnership Program of Chinese Academy of Sciences (Grant no. 133137KYSB20170003), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA11030302), the Special Fund for the Taishan Scholar Program of Shandong Province (Grant no. ts201511061), the AoShan Talents Program Supported by Qingdao National Laboratory for Marine Science and Technology (Grant no. 2015ASTP-0S17), the Innovative Talent Promotion Program (Grant no. 2012RA2191), National Special Fund for the 13th Five-Year Plan of COMRA (Grant no. DY135-G2-1-02), the Scientific and Technological Innovation Project financially supported by Qingdao National Laboratory for Marine Science and Technology (Grants nos. 2015ASKJ03, 2016ASKJ13), the National High Level Talent Special Support Program, the CAS/SAFEA International Partnership Program for Creative Research Teams, and the Qingdao Collaborative Innovation Center of Marine Science and Technology.

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