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 MarineMineral 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


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., Fe 2+ , Ca 2+ , Cu 2+ , Zn 2+ , Mn 2+ , and SiO 2 ) or taken out (Mg 2+ , SO 4 2− ) of seawater (e.g., [3,4]).The hydrothermal plumes from high temperature vent fluids are devoid of Mg 2+ , 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 SO 4 2− in seawater is lost due to heating in the downflow phase of the hydrothermal system, with precipitation of anhydrite (CaSO 4 ) 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 SO 4  2− and the titration of HCO 3 − /CO 3 2− 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 addition, the Kuroshio current is the largest western boundary current in the north Pacific ocean (e.g., [25][26][27]).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 = 10 6 m 3 /s) in the East China Sea [28,[33][34][35] 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,[37][38][39][40]).
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.

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 backarc 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].
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].

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 (Δ) 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.

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 HNO 3 for 48 h, washed to neutral pH with distilled water, and dried) on shore.
Concentrations of Na + , Mg 2+ , Ca 2+ , K + , Sr 2+ , B 3+ , 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 SO 4 2− concentrations were measured by ion chromatography (ICS-1100) with an anion exchange resin column (DIONEX AS19) rinsed with 1.8 (mmol L −1 ) Na 2 CO 3 -1.7 (mmol L −1 ) NaHCO 3 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 ( = 9) was ±2%, with 99% recovery.The Mn 2+ content of hydrothermal plume samples was determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) (ELE-MENT, 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 Mn 2+ determination, with accuracy and precision ( = 5) both better than 5%.

IN
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 Mg 2+ /Ca 2+ 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 (  more variable, with the largest ranges, than in the Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields (Table 2; Figure 2(d)).

Major Component Correlations in Hydrothermal Plume
Water Columns.The B 3+ , total S, and SO Trough (Figure 4).Furthermore, the Sr 2+ 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)).

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]).
In the Tangyin hydrothermal field, concentrations of K + , Ca 2+ , and B 3+ in anomalous layers in water columns are higher than in average seawater [4,50] (Figures 5 and 6), those of Mg 2+ and SO 4 2− are also lower than in average seawater [4,50] (Figure 7), like Mn 2+ and turbidity anomalies of hydrothermal plumes in the Clam and Yonaguni Knoll IV hydrothermal fields (Figures 6 and 9).The B 3+ /Mg 2+ and Ca 2+ /SO 4 2− ratios are all higher and the Mg 2+ /Ca 2+ , SO 4 2− /B 3+ , and SO 4 2− /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 SO 4 2− fluid discharge in the southern Okinawa Trough.
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 Sr 2+ /Ca 2+ , Mg 2+ /Cl − , and Ca 2+ /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.

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 equations where   is the specific heat capacity (J g −1 K −1 ),  is the potential density in the hydrothermal plume, V is the current velocity of the hydrothermal plume, Δ is the temperature anomaly of the plume relative to ambient seawater,   flux  Turekian [50] and Millero [4].
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  3).
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     to seawater is about 0.159-1,973 × 10 5 W, 2.62-873, 1.04-326, 1.30-76.4,0.293-34.7,−2.81-−374, and −1,377-−10,785 × 10 6 kg per year, respectively.Using the heat flux of 0.159-1,973 × 10 5 W calculated above and a total oceanic heat flux of 32 × 10 12 W [67], this suggests that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

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, B 3+ , Mg 2+ , total S, K + , Ca 2+ , Sr 2+ , Cl − , and SO 4 2− concentrations of the hydrothermal plume water columns in the Iheya North knoll and Clam hydrothermal field are the lowest, and the B 3+ , total S, and SO 4 2− concentrations of hydrothermal plume water column samples show a positive correlation with Sr 2+ , Mg 2+ , Ca 2+ , and Cl − concentrations in the Iheya North knoll, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields.The majority of the SO 4 2− /B 3+ and Mg 2+ /B 3+ 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 B 3+ /Mg 2+ , K +/ SO 4 2− , and Ca 2+ /SO 4 2− ratios of anomalous layers in the hydrothermal plume water columns, like Mn 2+ , turbidity, and temperature anomalies, are higher than in other layers, which indicates hydrothermal input.The Mg 2+ /Ca 2+ , Mg 2+ /K + , SO 4 2− /B 3+ , and SO 4 2− /Mn 2+ ratios of anomalous layers are lower than in other layers and are consistent with the low Mg 2+ and SO 4 2− concentration of vent fluids in the Okinawa Trough.During dilution of the hydrothermal plume by seawater, Ca 2+ and Mn 2+ show similar variations.In the Iheya North knoll, Clam, Yonaguni Knoll IV, and Tangyin hydrothermal fields, salinity, B 3+ , total S, and K + of hydrothermal plumes show positive correlations with Sr 2+ /Ca 2+ , Ca 2+ /Cl − , Ca 2+ /SO 4 2− , and Mg 2+ /Cl − ratios, and Sr 2+ concentrations and 1/B ratio show negative correlation, and the high K, Ca, and B and low Mg and SO 4 2− concentration of vent fluid influences chemical variation of hydrothermal plume in Tangyin hydrothermal field.In addition, the element ratios (e.g., Sr 2+ /Ca 2+ , Ca 2+ /Cl − ) in the hydrothermal plume water column of the Okinawa Trough are similar to those in seawater, indicating that Sr 2+ /Ca 2+ and Ca 2+ /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 × 10 5 W, and the calculated mass fluxes of K + , Ca 2+ , Mn 2+ , B 3+ , Mg 2+ , and SO 4 2− are up to 0.873, 0.326, 0.076, 0.034, −0.374, and −10.8 × 10 9 kg per year.These results mean that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Figure 1 :
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.

)Figure 3 :
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) SO 4 2− concentration versus Cl − concentration.

Figure 5 :
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) Ca 2+ anomalies in the Iheya North and Clam hydrothermal fields; (d) Ca 2+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields.Average seawater data from Turekian[50] and Millero[4].

Figure 6 :
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) B 3+ anomalies in the Iheya North and Clam hydrothermal fields; (d) B 3+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields.Average seawater data from Turekian[50] and Millero[4].

Figure 7 :
Figure 7: Hydrothermal plume profiles in the middle and southern Okinawa Trough: (a) Mg 2+ anomalies in the Iheya North and Clam hydrothermal fields; (b) Mg 2+ anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields; (c) SO 4 2− anomalies in the Iheya North and Clam hydrothermal fields; (d) SO 42− anomalies in the Tangyin and Yonaguni Knoll IV hydrothermal fields.Average seawater data from Turekian[50] and Millero[4].

Table 2 )
. Average Mg 2+ /Cl − , Ca 2+ /Cl − , Mg 2+ /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 (Table2).Furthermore, ratios between Ca 2+ , Mg 2+ , SO 4 2− , Na + , K + , Sr 2+ , Cl − , and salinity in water columns in the Iheya North knoll of Okinawa Trough are

Table 2 :
Major component ratios of hydrothermal plume water columns in the middle and southern Okinawa Trough.