sustainability

Article Recent Developments of Exploration and Detection of Shallow-Water Hydrothermal Systems

Zhujun Zhang 1, Wei Fan 1, Weicheng Bao 1, Chen-Tung A Chen 2, Shuo Liu 1,3 and Yong Cai 3,*

1 Ocean College, Zhejiang University, Zhoushan 316000, China; [email protected] (Z.Z.); [email protected] (W.F.); [email protected] (W.B.); [email protected] (S.L.) 2 Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan; [email protected] 3 Ocean Research Center of Zhoushan, Zhejiang University, Zhoushan 316000, China * Correspondence: [email protected]



Received: 21 August 2020; Accepted: 29 October 2020; Published: 2 November 2020


Abstract: A hydrothermal vent system is one of the most unique marine environments on Earth. The cycling hydrothermal fluid hosts favorable conditions for unique life forms and novel mineralization mechanisms, which have attracted the interests of researchers in fields of biological, chemical and geological studies. Shallow-water hydrothermal vents located in coastal areas are suitable for hydrothermal studies due to their close relationship with human activities. This paper presents a summary of the developments in exploration and detection methods for shallow-water hydrothermal systems. Mapping and measuring approaches of vents, together with newly developed equipment, including sensors, measuring systems and water samplers, are included. These techniques provide scientists with improved accuracy, efficiency or even extended data types while studying shallow-water hydrothermal systems. Further development of these techniques may provide new potential for hydrothermal studies and relevant studies in fields of geology, origins of life and astrobiology.

Keywords: hydrothermal fields; shallow water; vent mapping; in-situ observation; water sampler

1. Introduction About forty years ago, the earliest submarine hydrothermal (HT) vent was found on the East Pacific Rise [1], which marks the beginning of HT research. In past decades, detailed studies on HT vent systems have greatly changed our ideas on submarine biological, chemical and geological processes. The extremely hot, acidic and anoxic environment around HT vents gives birth to various novel life forms [2,3]. Such environments provide constant energy, abundant organic elements and large temperature gradients that enable the polymerization of long-chain molecules. This was believed to be one of the possible conditions for the origins of life [4–6]. Massive seafloor sulfides and manganese near active or inactive HT fields were also presumed to be the result of HT activities [7–9]. These minerals not only brought new insight into seabed mineralization mechanisms, but also provided potential resources for commercial mining [9]. Besides, the hot, metal-rich plume discharges from the vents form an important part of the heat and chemical exchange between the lithosphere and the ocean [10], which is essential for global environmental studies. These particular features provide unique research conditions for the specified science fields, and thus have attracted the attention of a growing number of researchers. In past decades, many scientists have reported their work on HT activities and their surrounding environments. But early studies mainly concentrated on HT systems located at deep ocean, e.g., the East Pacific Rise or the Central Indian Ridge [11,12]. Research on shallow-water hydrothermal

Sustainability 2020, 12, 9109; doi:10.3390/su12219109 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 9109 2 of 17

(SHT) systems were seldom found until the end of the 20th century [13]. Tarasov defines shallow-water hydrothermal (SHT) vents as vents located at a depth of 200 m or less considering the degree of obligacy of fauna [14]. According to this definition, 22 SHT zones have been found on Earth (Table1)[ 14]. Their brief locations were indicated in Figure1.

Table 1. The location and depth of known hydrothermal (HT) vent fields [14,15].

Index Depth/m Area References 1 0~? Capo Misseno Golfo di Napoli Gimenez and Marin, 1991 [16] 2 0~10 Santorini Island, Greece, Aegean Sea Dando et al., 1995 [17] 3 0~10 White Point Palos Verdes, California Stein, 1984 [18] 4 0~27 Matupi Harbour, New Britain Island, Papua New Guinea Tarasov et al., 1999; Tarasov, 1999, 2002 [19–21] 5 0~30 Kratemaya Bight Ushishir Voclcano, Kurile Islands Buzhinskaya, 1990; Turpaeva, 2000 [22,23] 6 0~30 Cape Palinur, Italy Southward et al., 1996 [24] 7 0~30 Punta Mita and Punta Banda, Baja California Prol-Ledesma, 2003; Prol-Ledesma et al., 2004 [25,26] 8 0~167 Bay of Plenty, New Zealand Kamenev et al., 1993 [27] 9 1.2~1.5 Kunashir Island, Kurile Islands Kostina, 1991 [28] 10 3~115 Milos Island Greece, Aegean Sea Gamenick et al., 1998 [29] 11 5 Nishino-shina Shintoh, Ogasawara Islands, Japan Takeda and Kubata, 1977; Takeda et al., 1993 [30,31] 12 5 Kita-Iwojina Iwo Islands, Japan Takeda et al., 1993 [31] Tutum Bay, Ambitle Island, Tabar-Feni chain, Papua 13 5~10 Pichler et al., 1999a, 1999b, 2000 [32–34] New Guinea 14 8~20 Kueishan Is. China Taiwan Jeng, 2000; Ng et al., 2000 [35,36] 15 9~13 Bahinde Pozzuoli Gulf of Naples, Italy Gimenez and Marin, 1991 [16] Acunto et al., 1995; Colangelo et al., 1996; 16 10~30 Panarea Eolian Island, Italy Lucila et al., 1996 [37–39] 17Sustainability 16~45 2020, 12, x FOR PEER D. Joao REVIEW de Castro Bank, Azores Avila et al., 2004 [40] 2 of 18 18 23~26 Akusekijina, Tokara Islands, Japan Takeda et al., 1993 [31] 19systems 63~114 were seldom found Esmeralda until Bank,the end W Pacificof the 20th century [13]. Tarasov Turkay defines and Sakai, shallow-water 1995 [41] 20 82~110 Kagoshima Bay, Japan Miura et al., 1997; Miura, 1997 [42,43] 21hydrothermal 100~106 (SHT) vents Kolbeinsey, as vents north located of Iceland at a depth of 200 m or less considering Fricke et al., the 1989 degree [44] of obligacy of fauna [14]. According to this definition, 22 SHT zones have beenVon found Cosel and on Marshall,Earth (Table 2003; 1) 22 200 Macauley Caldera, Northern Kermadec Ridge [14]. Their brief locations were indicated in Figure 1. Smith et al., 2004 [45,46]

Figure 1.FigureThe brief1. The locationbrief location of the of the known known shallow-water shallow-water hydrothermal hydrothermal (SHT) (SHT) vent vent areas. areas. The index The in index in the figurethe is figure listed is inlisted Table in Table1 in detail.1 in detail. Multiple Multiple indexes indexes onon one one symbol symbol show show the theexistence existence of several of several closely distributedclosely distributed vent areasvent areas [14]. [14].

Although SHTTable systems 1. The have location similar and depth inner of structuresknown hydrothermal and fluid (HT) formations vent fields [14,15]. with vents in deep-seas, distinct diffIndexerences Depth/m still exist in various aspects Area such as biomass, temperature, References pH levels and derivatives. 1 0~? Capo Misseno Golfo di Napoli Gimenez and Marin, 1991 [16] Due to its limited2 depth, 0~10 the SHT Santorini plume Island, mayGreece, reach Aegean not Sea only the surrounding Dando et al., region 1995 [17] but also the surface layer of the water3 body, 0~10 which White gives Point risePalos Verdes, to a close California interaction with human Stein, 1984 activities [18] and neighboring Matupi Harbour, New Britain Island, ecological systems.4 0~27 These factors emphasize the necessityTarasov of SHT et al., studies. 1999; Tarasov, 1999, 2002 [19–21] Papua New Guinea Scientists and engineersKratemaya have Bight dedicated Ushishir Voclcano, great Kurile efforts to the detecting, observing and sampling 5 0~30 Buzhinskaya, 1990; Turpaeva, 2000 [22,23] techniques on HT studies. Various equipment,Islands including sensors, in-situ measurement systems or 6 0~30 Cape Palinur, Italy Southward et al., 1996 [24] water/core samplers, was developed.Punta Mita and Most Punta Banda, of them Baja wereProl-Ledesma, successfully 2003; applied Prol-Ledesma in seaet al., trials,2004 and the 7 0~30 others were tested under simulated conditions.California However, most existing[25,26] studies focus on deep-sea 8 0~167 Bay of Plenty, New Zealand Kamenev et al., 1993 [27] 9 1.2~1.5 Kunashir Island, Kurile Islands Kostina, 1991 [28] 10 3~115 Milos Island Greece, Aegean Sea Gamenick et al., 1998 [29] Nishino-shina Shintoh, Ogasawara Islands, Takeda and Kubata, 1977; Takeda et al., 1993 11 5 Japan [30,31] 12 5 Kita-Iwojina Iwo Islands, Japan Takeda et al., 1993 [31] Tutum Bay, Ambitle Island, Tabar-Feni 13 5~10 Pichler et al., 1999a, 1999b, 2000 [32–34] chain, Papua New Guinea 14 8~20 Kueishan Is. China Taiwan Jeng, 2000; Ng et al., 2000 [35,36] 15 9~13 Bahinde Pozzuoli Gulf of Naples, Italy Gimenez and Marin, 1991 [16] Acunto et al., 1995; Colangelo et al., 1996; Lucila 16 10~30 Panarea Eolian Island, Italy et al., 1996 [37–39] 17 16~45 D. Joao de Castro Bank, Azores Avila et al., 2004 [40] 18 23~26 Akusekijina, Tokara Islands, Japan Takeda et al., 1993 [31] 19 63~114 Esmeralda Bank, W Pacific Turkay and Sakai, 1995 [41] 20 82~110 Kagoshima Bay, Japan Miura et al., 1997; Miura, 1997 [42,43] 21 100~106 Kolbeinsey, north of Iceland Fricke et al., 1989 [44] Macauley Caldera, Northern Kermadec Von Cosel and Marshall, 2003; Smith et al., 2004 22 200 Ridge [45,46]

Although SHT systems have similar inner structures and fluid formations with vents in deep- seas, distinct differences still exist in various aspects such as biomass, temperature, pH levels and Sustainability 2020, 12, 9109 3 of 17

HT systems, since the extreme pressure, temperature and depth made it a more challenging mission. This complicated, well-designed equipment was usually designed to be operated on a Remote Operated Vehicle (ROV), not by divers, and thus is not suitable for shallow-water operations. Furthermore, the difference in physical and chemical features makes some of the existing techniques developed for deep-sea HT observations unavailable in shallow-water applications. For example, the yttrium-stabilized zirconia (YSZ) electrode developed by Ding [47–49], which provides excellent performance in researching black smokers, was not suitable for direct use in shallow-waters due to its lower temperature limitation of 150 ◦C since the temperature of fluids in shallow depth is between 10 and 119 ◦C. On the other hand, the shallow depth and relatively low pressure made the SHT environment more accessible, which gave birth to novel approaches that cannot be used in deep-sea environments. In this paper, the exploring techniques including detecting, observing and sampling techniques specified for SHT systems are addressed. Most of them have been tested in sea trials or under equivalent environments. The others have showed their potential for the application in shallow-water environments. Finally, the developing status of SHT exploring techniques concludes the paper and possible future works are discussed.

2. Detecting and Mapping Techniques for SHT Vent Fields

2.1. Surveying HT Plume with Multi-Sensors Active HT vent fields continuously discharge hot vent fluids, which have been gradually diluted by the ambient seawater and form the mixing, buoyant turbulence known as HT plumes. HT plumes can spread from tens to thousands of kilometers under the influence of the ocean current [50,51]. The signature of the vent fluid in temperature, turbidity and chemical ingredients causes anomalies in the water column that can be used to detect the existence of HT plumes. By tracing and mapping HT plumes, the position of the HT vents can be further located. One traditional way of searching for HT plumes in vast waters is surveying with vessels or ROVs equipped with towed multi-sensor systems (Figure2). Usually a combination of physical and/or chemical sensors integrated in a protective shell is deployed to the depth where the HT plume may occur. The sensor bundle is connected to the vessel with cables to ensure real-time communication. Thus, when the sensors get in touch with the HT plume during navigation, the feature anomalies can be instantly recognized. To further improve the chance of encountering an HT plume, sometimes the deployment depth of the towed sensors is continuously adjusted to forms a “w” shaped trace, or a chain of sensors deployed discretely at different depths is applied (Figure1). When the existence of an HT plume is confirmed, a refined survey at a lower depth or an up close survey such as optical observationSustainability will 2020 be, 12 performed, x FOR PEER REVIEW to further locate the position of HT vents. 4 of 18

FigureFigure 2. Vessels 2. Vessels search search HT HT plume plume using using integrated integrated multi-sensors multi-sensors[ 52[52].]. TheThe chance of of encountering encountering the HT plumethe HT can plume be enlarged can be enlarged by adjusting by adjusting the depth the dept of theh of towed the towed sensors sensors during during the the survey, survey, or byor by using a verticalusing array a vertical of sensors. array of sensors.

VariousVarious physical physical and and chemical chemical features features havehave been been used used to to confirm confirm the the existence existence of HT of plumes. HT plumes. TheseThese features features can can be roughlybe roughly divided divided into into three three aspects:aspects: physical physical characteristics characteristics (temperature (temperature and and 33 turbidity),turbidity), chemical chemical characteristics characteristics (( He, H H2,2 CO, CO2, CH2, CH4, H42S,,H Fe,2S, Mn Fe, and Mn Oxidation-reduction) and Oxidation-reduction) and andmovement movement characteristics characteristics (rise height, (rise height,advection advectionof the non-buoyant of the layer) non-buoyant [10,53,54]. However, layer) [10 the,53 ,54]. sensitivity of these features in detecting HT vents differs. For example, turbidity anomalies can be detected tens of kilometers away, but the precision in locating the source is less than a few kilometers. Temperature anomalies can provide a location precision of a few hundred meters, but are difficult to detect. Oxidation-reduction has similar location precision to temperature and is sensitive to both high temperature vents and low temperature vents, which makes it a good choice for real-time surveys. Tao et al. compared the efficiency of the present vessel exploration methods and concluded that a combination of turbidity and oxidation-reduction potential sensors offers the best efficiency [53]. In recent years, researchers tend to use multiple sensors to search HT plumes to avoid false alarms. The existence of HT plumes is confirmed only when the potentials of different features coincide. Surveying HT plumes with towed multi-sensors has led to the discovery of many deep-sea HT vents [53,55]. However, detailed reports on SHT detecting remain scarce. Nakatani et al. Nakatani et al. discovered an SHT vent area at a depth of approximately 200 m in Kagoshima Bay, Japan using a newly developed ROV TUNA-SAND (Figure 3) [56]. The vent was further observed by video camera. Liu et al. Liu surveyed the HT vent area south-east of the Kueishan Island on a vessel equipped with a sensor chain containing 4 chemical sensors, 1 turbidity sensor and 1 hydrophone [57]. Figure 4 shows the navigation track of the vessel. The survey indicated several HT vents that had not been reported.

Figure 3. TUNA-SAND and its survey in Kagoshima Bay, Japan. The ROV was released from the vessel and surveyed at a depth of approximately 200 m, searching for evidence of undiscovered HT vents [56]. Sustainability 2020, 12, x FOR PEER REVIEW 4 of 18

Figure 2. Vessels search HT plume using integrated multi-sensors [52]. The chance of encountering the HT plume can be enlarged by adjusting the depth of the towed sensors during the survey, or by Sustainability 2020using, 12 a, vertical 9109 array of sensors. 4 of 17

Various physical and chemical features have been used to confirm the existence of HT plumes. However,These the features sensitivity can be ofroughly these divided features into in three detecting aspects: physical HT vents characteristics differs. For (temperature example, and turbidity 3 anomaliesturbidity), can be chemical detected characteristics tens of kilometers ( He, H2 away,, CO2, butCH4, the H2S, precision Fe, Mn and in locatingOxidation-reduction) the source and is less than movement characteristics (rise height, advection of the non-buoyant layer) [10,53,54]. However, the a few kilometers. Temperature anomalies can provide a location precision of a few hundred meters, sensitivity of these features in detecting HT vents differs. For example, turbidity anomalies can be but aredetected difficult tens to detect.of kilometers Oxidation-reduction away, but the precision has insimilar locating locationthe source precisionis less than a to few temperature kilometers. and is sensitiveTemperature to both high anomalies temperature can provide vents a location and low precis temperatureion of a few hundred vents, whichmeters, makesbut are difficult it a good to choice for real-timedetect. surveys.Oxidation-reduction Tao et al. has compared similar location the e precisionfficiency to of temperature the present and vesselis sensitive exploration to both high methods and concludedtemperature that vents a combination and low temperature of turbidity vents, andwhich oxidation-reduction makes it a good choice potential for real-time sensors surveys. off ers the Tao et al. compared the efficiency of the present vessel exploration methods and concluded that a best efficiency [53]. In recent years, researchers tend to use multiple sensors to search HT plumes to combination of turbidity and oxidation-reduction potential sensors offers the best efficiency [53]. In avoid falserecent alarms. years, researchers The existence tend to ofuse HT multiple plumes sens isors confirmed to search HT only plumes when to avoid the potentialsfalse alarms. ofThe di fferent featuresexistence coincide. of HT plumes is confirmed only when the potentials of different features coincide. SurveyingSurveying HT plumes HT plumes with with towed towed multi-sensors multi-sensors ha hass led led to tothe the discovery discovery of many of manydeep-sea deep-sea HT HT vents [53vents,55]. [53,55]. However, However, detailed detailed reports reports on on SHT SHT detecting detecting remainremain scarce. scarce. Nakatani Nakatani et al. et Nakatani al. Nakatani et et al. discoveredal. discovered an SHT an vent SHT area vent atarea a depthat a depth of of approximately approximately 200 200 m m in inKagoshima Kagoshima Bay, Japan Bay, Japanusing a using a newly developed ROV TUNA-SAND (Figure 3) [56]. The vent was further observed by video camera. newly developed ROV TUNA-SAND (Figure3)[ 56]. The vent was further observed by video camera. Liu et al. Liu surveyed the HT vent area south-east of the Kueishan Island on a vessel equipped with Liu et al.a sensorLiu surveyed chain containing the HT vent4 chemical area south-eastsensors, 1 turbidity of the Kueishan sensor and Island 1 hydrophone on a vessel [57]. equipped Figure 4 with a sensor chainshows containingthe navigation 4 chemical track of the sensors, vessel. The 1 turbidity survey indicated sensor andseveral 1 hydrophone HT vents that [ 57had]. not Figure been4 shows the navigationreported. track of the vessel. The survey indicated several HT vents that had not been reported.

Figure 3.FigureTUNA-SAND 3. TUNA-SAND and itsand survey its survey in Kagoshima in Kagoshima Bay, Bay, Japan. Japan. The The ROV ROV waswas releasedreleased from from the the vessel vessel and surveyed at a depth of approximately 200 m, searching for evidence of undiscovered HT andSustainability surveyed 2020 at a, 12 depth, x FOR of PEER approximately REVIEW 200 m, searching for evidence of undiscovered HT vents5 of 18 [56 ]. vents [56].

Figure 4.FigureThe 4. navigation The navigation track track of theof the survey survey south-eastsouth-east of of Kueishan Kueishan Island Island [57]. [57The]. sensor The sensorchain chain containingcontaining 6 sensors 6 sensors was was released released and and towed towed duringduring the the survey. survey. Data Data suggests suggests the existence the existence of of undiscoveredundiscovered SHT ventsSHT vents in the in the surveying surveying area. area.

In some other studies, though conducted in deep-sea HT areas, the navigation height of the towed sensors is in the scope of 5 to 250 m above the seafloor [58–60]. These studies somehow indicated the potential of applying similar detecting methods in SHT research.

2.2. SHT Vents Exploration with Bubble Cluster Detection The gas (such as methane) that discharges from HT vents forms bubble clusters in the upper water column. The acoustic signature in reflection and scattering make it possible for the recognition of bubble clusters and their sources. Gas discharging is much more common in SHT vents compared to deep-sea HT vents [61]. The gas mixed with HT fluids can originate from the atmosphere or volcanic activities [62], and thus provides potential feasibility for bubble cluster detection. Lin proposed an acoustic method for the remote detection of HT vents (Figure 5a) [63]. In his study, a transducer was fixed to the bottom of a vessel which cruised around HT areas. Sound waves were directly emitted downward to the seafloor. The anomalies in reflection tells the existence of possible HT vents and their positions. An acoustic HT detection system based on the abovementioned scheme was developed and tested in Song Hua Lake, Jilin Province, China. A fenestrated hose with one end connected to an air pump was deployed to the bottom of the lake to generate bubble clusters. A vessel equipped with the transmitter surveyed in an eight shaped track on the surface while emitting sound waves straightly downward at a carrier frequency of 20/200 Hz. The depth of the experimental area was about 40 m. Through analysis of the received signal, the depth and spreading range of bubble clusters were obtained (Figure 5b). Sustainability 2020, 12, 9109 5 of 17

In some other studies, though conducted in deep-sea HT areas, the navigation height of the towed sensors is in the scope of 5 to 250 m above the seafloor [58–60]. These studies somehow indicated the potential of applying similar detecting methods in SHT research.

2.2. SHT Vents Exploration with Bubble Cluster Detection The gas (such as methane) that discharges from HT vents forms bubble clusters in the upper water column. The acoustic signature in reflection and scattering make it possible for the recognition of bubble clusters and their sources. Gas discharging is much more common in SHT vents compared to deep-sea HT vents [61]. The gas mixed with HT fluids can originate from the atmosphere or volcanic activities [62], and thus provides potential feasibility for bubble cluster detection. Lin proposed an acoustic method for the remote detection of HT vents (Figure5a) [ 63]. In his study, a transducer was fixed to the bottom of a vessel which cruised around HT areas. Sound waves were directly emitted downward to the seafloor. The anomalies in reflection tells the existence of possible HT vents and their positions. An acoustic HT detection system based on the abovementioned scheme was developed and tested in Song Hua Lake, Jilin Province, China. A fenestrated hose with one end connected to an air pump was deployed to the bottom of the lake to generate bubble clusters. A vessel equipped with the transmitter surveyed in an eight shaped track on the surface while emitting sound waves straightly downward at a carrier frequency of 20/200 Hz. The depth of the experimental area was about 40 m. Through analysis of the received signal, the depth and spreading range of bubble Sustainabilityclusters were 2020 obtained, 12, x FOR PEER (Figure REVIEW5b). 6 of 18

(a) (b)

FigureFigure 5. 5. AcousticAcoustic detection detection of of bubble bubble clusters clusters associated associated with with HT HT vents. vents. (a (a) )The The sketch sketch map map of of detectingdetecting bubble bubble clusters; clusters; ( (bb)) the the reflection reflection signal signal received received after after filtering. filtering. The The smaller smaller peak peak indicates indicates thethe position position of of bubble bubble clusters clusters.. The The larger larger peak peak indicates indicates the the depth depth of of the the lake lake [63]. [63].

ComparedCompared with with HT HT plume plume tracking, tracking, the the method method of of acoustic acoustic bubble bubble cluster cluster detection detection has has obvious obvious advantagesadvantages in in sensitivity sensitivity and and mapping mapping accuracy. accuracy. It It responds responds immediately immediately to to the the bubble bubble clusters clusters encounteredencountered and and is is applicable applicable to to all all kinds kinds of of HT HT vents that discharge gases. Since Since bubbles bubbles are are less less affectedaffected by ocean currentscurrents whilewhile risingrising to to the the surface, surface, the the precision precision in in mapping mapping the the corresponding corresponding HT HTvent vent is generally is generally tens oftens meters of meters to hundreds to hundreds of meters, of meters, which can which be further can be improved further improved while conducted while conductedin extremely in shallowextremely waters. shallow waters. SoSo far, far, research research on on field field experiments of of HT exploration in in shallow-waters shallow-waters by by bubble bubble cluster cluster detectiondetection remains remains scarce. scarce. However, However, some some sea sea trials trials conducted conducted in in deep-sea deep-sea HT HT vent vent fields fields demonstrate demonstrate encouragingencouraging results. Michio etet al.al. reportedreported acoustic acoustic water water column column anomalies anomalies (AWA) (AWA) revealed revealed by aby two a twofrequency frequency Multi Multi Beam Beam Sounder Sounder (MBES) (MBES) rising rising from from HT ventHT vent fields fields in the in mid-Okinawathe mid-Okinawa Trough Trough [64]. [64].The The AWA AWA detected detected was was attributed attributed to theto the existence existence of of droplets droplets of of carboncarbon dioxidedioxide and HT HT plume. plume. KasayaKasaya et al. furtherfurther appliedapplied a a survey survey method method using using a hull-mounted a hull-mounted MBES MBES and aand high-frequency a high-frequency MBES MBES mounted on AUV to the HT exploration conducted at the North Iheya Knoll [65]. The existence of two unidentified HT activity zones was first discovered by the hull-mounted MBES. Later the detailed positions of the newly discovered HT activity zones were further obtained using the AUV- mounted MBES. These achievements demonstrated that the acoustic survey of HT activity related bubble clusters is one useful approach for HT exploration, especially in deep waters. However, considering the complexity of acoustic channels in shallow waters, further studies are needed to verify the effectiveness of acoustic survey for HT exploration in shallow waters.

2.3. Mapping SHT Vents Using Satellite Imagery The limited depth of SHT vents makes it possible for the application of some remote sensing techniques, such as satellite imagery, which is not available for deep-sea HT studies. The potential of mapping and monitoring active HT systems with satellite images was reported by Martelat and his colleagues [66]. Satellite data spanning several years were involved in their study, some of them were proprietary WorldView-2 images which were taken with spectral bands that can penetrate water depths of up to 30 m. Fifteen SHT vent fields worldwide were identified in the available data, and two of them (Milos at Greece and Kueishantao at Taiwan) were further analyzed in detail. Figure 6 shows the satellite data image of HT outflow zones at Milos, Greece, at depths of up to 30 m (Figure 6a) and the image of turbidity due to HT plumes viewed at the sea surface at Kueishan Island, Taiwan (Figure 6b) [66]. Sustainability 2020, 12, 9109 6 of 17 mounted on AUV to the HT exploration conducted at the North Iheya Knoll [65]. The existence of two unidentified HT activity zones was first discovered by the hull-mounted MBES. Later the detailed positions of the newly discovered HT activity zones were further obtained using the AUV-mounted MBES. These achievements demonstrated that the acoustic survey of HT activity related bubble clusters is one useful approach for HT exploration, especially in deep waters. However, considering the complexity of acoustic channels in shallow waters, further studies are needed to verify the effectiveness of acoustic survey for HT exploration in shallow waters.

2.3. Mapping SHT Vents Using Satellite Imagery The limited depth of SHT vents makes it possible for the application of some remote sensing techniques, such as satellite imagery, which is not available for deep-sea HT studies. The potential of mapping and monitoring active HT systems with satellite images was reported by Martelat and his colleagues [66]. Satellite data spanning several years were involved in their study, some of them were proprietary WorldView-2 images which were taken with spectral bands that can penetrate water depths of up to 30 m. Fifteen SHT vent fields worldwide were identified in the available data, and two of them (Milos at Greece and Kueishantao at Taiwan) were further analyzed in detail. Figure6 shows the satellite data image of HT outflow zones at Milos, Greece, at depths of up to 30 m (Figure6a) and the image of turbidity due to HT plumes viewed at the sea surface at Kueishan Island, Taiwan Sustainability(Figure6b) 2020 [66]., 12, x FOR PEER REVIEW 7 of 18

Figure 6. SatelliteSatellite imagery imagery of of SHT SHT vents. vents. (a) (Seafloora) Seafloor outflow outflow observed observed in Milos in Milos associated associated with with HT activities;HT activities; (b) high (b) high turbidity turbidity HT HTplumes plumes observed observed in Kueishan in Kueishan Tao Tao[66]. [66 ].

The result of their study shows that satellite im imageryagery can not only be used to discover unknown SHT vents, but that it is also eeffectiveffective in analyzing the spatial-temporal change of the vent activities and its corresponding features, such as HT plumes plumes,, micro-bacterial mats mats and and discharged discharged particles. particles. Compared toto traditional traditional field field surveys, surveys, the approachthe approa of satellitech of satellite imagery imagery is moree ffiis cientmore and efficient economical. and economical.It is especially It is suitable especially for largesuitable scale for observations large scale ob suchservations as distribution such as distribution of discrete vents,of discrete HT plumevents, HTactivities plume and activities turbidity and variation turbidity associated variation with associated HT activities. with TheHT methodactivities. is widelyThe method applicable is widely since applicablemost of the since satellite most data of the involved satellite in data this pilotinvolved study inare this publicly pilot study available. are publicly The limitation available. of The the limitationapproach generallyof the approach lies in thegenerally quality lies of images in the quality and the of nature images of and theHT the vents.nature The of the former HT vents. was easily The formeraffected was by various easily affected factors suchby various as light factors illumination, such as weatherlight illumination, conditions orweat clouds,her conditions which might or clouds, reduce whichthe resolution might reduce of the satellitethe resolution images of or the even satellite make images them unavailable. or even make The them later unavailable. generally determines The later generallythe visibility determines of an HT ventthe visibility in the satellite of an image. HT vent HT in vents the thatsatellite discharge image. reflective HT vents particles that discharge or exhibit reflectivevisible HT particles features or such exhibit as HT visible precipitates HT features or large-scale such as HT micro precipitates bacterial or mats large-scale are easier micro to identify. bacterial mats are easier to identify.

3. In-Situ Observation Techniques for SHT Fluid

3.1. Direct Measurement Techniques Direct measurement techniques include all kinds of chemical and physical sensors or probes that come into touch with the HT plume during observation. With the development of sensor technology, most of the sensors or measuring instruments needed for HT observation have been commercialized and can be directly applied in field surveys. However, the developing of new types of sensors is still of great significance to overcome the extreme acid, heat and corrosive environment in an HT flow. Until now, few studies were found reporting sensors specially designed for SHT applications. But some studies have verified the performance of their newly developed sensors in SHT environments. Pan et al. developed an improved IrO2 electrode for pH measurement with various promising properties including satisfying reproducibility and sensitivity, small open circuit potential drift, long life time and remarkable agreement with respect to potential/pH slopes and apparent standard electrode potentials of electrodes from the same batch [67,68]. The electrode can work steadily at pressures of up to 50 MPa and temperatures of up to 90 °C, and could be potentially used at temperatures of up to 150 °C. Such range perfectly covers the need for SHT measurement. The developed IrO2 electrode, integrated with one Eh electrode and one Ag/Ag2S, was tested in the lab and later applied in several marine investigations conducted in Kueishan Tao HT vent areas, including HT vents mapping and 3D HT plume structure detection (Figure 7). Sustainability 2020, 12, 9109 7 of 17

3. In-Situ Observation Techniques for SHT Fluid

3.1. Direct Measurement Techniques Direct measurement techniques include all kinds of chemical and physical sensors or probes that come into touch with the HT plume during observation. With the development of sensor technology, most of the sensors or measuring instruments needed for HT observation have been commercialized and can be directly applied in field surveys. However, the developing of new types of sensors is still of great significance to overcome the extreme acid, heat and corrosive environment in an HT flow. Until now, few studies were found reporting sensors specially designed for SHT applications. But some studies have verified the performance of their newly developed sensors in SHT environments. Pan et al. developed an improved IrO2 electrode for pH measurement with various promising properties including satisfying reproducibility and sensitivity, small open circuit potential drift, long life time and remarkable agreement with respect to potential/pH slopes and apparent standard electrode potentials of electrodes from the same batch [67,68]. The electrode can work steadily at pressures of up to 50 MPa and temperatures of up to 90 ◦C, and could be potentially used at temperatures of up to 150 ◦C. Such range perfectly covers the need for SHT measurement. The developed IrO2 electrode, integrated with one Eh electrode and one Ag/Ag2S, was tested in the lab and later applied in several marine investigations conducted in Kueishan Tao HT vent areas, including HT vents mapping and 3D HT Sustainability 2020, 12, x FOR PEER REVIEW 8 of 18 plume structure detection (Figure7).

(a) (b)

Figure 7. The IrO2 electrode applied in SHT studies in Kueishan Tao HT vent areas. (a) The integrated Figuresensors; 7. (Theb) pH IrO2 and electrode temperature applied measured in SHT instudies the HT in plume Kueishan of a Tao yellow HT vent. vent Theareas. three (a) The pH curvesintegrated refer sensors;to the pH (b measured) pH and temperature at depths of 3, measured 5 and 7 m, in respectively. the HT plume The of letters a yellow A, B, vent. C and The D mark three the pH variation curves refercycle to of the the pH temperature. measured at When depths the of temperature 3, 5 and 7 m, rises, respectively. the values The of pH letters and A, Eh B, drop, C and indicating D mark thatthe variationthe area iscycle under of thethe influence temperature. of HT When fluids [the68]. temperature rises, the values of pH and Eh drop, indicating that the area is under the influence of HT fluids [68]. Ding proposed an all-solid type dissolved H2S microelectrode optimized for its measurement 4 7 range.Ding The proposed lower limit an all-solid of detection type wasdissolved extended H2S frommicroelectrode 10− mol/L optimized to 10− mol for/L its by measurement direct current range.carrier The electroplating. lower limit Theof detection sensor was was also extended successfully from tested10−4 mol/L in the to HT 10− vent7 mol/L field by in direct Kueishan current Tao carrier(Figure electroplating.8)[69]. The sensor was also successfully tested in the HT vent field in Kueishan Tao (Figure 8) [69].

Figure 8. The H2S microelectrode integrated in a multi-parameter chemical sensor [69].

Frank and his colleagues studied the micro-profiles of O2, pH, H2S and temperature of a shallow water HT vent system at Milos, Greece using a miniaturized version of a profiling instrument. Data obtained were used to analyze the microenvironment and microcirculation of the vent [70]. In summary, though not much isolated research was reported, the development of SHT direct measuring technique continues with the development of sensor techniques. The trend of future development may lie in the measurement range for higher measurement accuracy under extreme conditions, or in the stability and durability of the sensors to obtain a better performance in long- term observation.

3.2. Remote Sensing Techniques Direct measurement techniques offer reliable, accurate data for HT studies. But the data collected are usually discrete, which provides only limited information when studying the continuous distribution of a certain parameter, e.g., temperature field or flow velocity field. A sensor array might extend the information collected [71], but the blocking effect of sensors to the HT fluid also prevents scientists from obtaining the original distribution of the desired parameter. Furthermore, the Sustainability 2020, 12, x FOR PEER REVIEW 8 of 18

(a) (b)

Figure 7. The IrO2 electrode applied in SHT studies in Kueishan Tao HT vent areas. (a) The integrated sensors; (b) pH and temperature measured in the HT plume of a yellow vent. The three pH curves refer to the pH measured at depths of 3, 5 and 7 m, respectively. The letters A, B, C and D mark the variation cycle of the temperature. When the temperature rises, the values of pH and Eh drop, indicating that the area is under the influence of HT fluids [68].

Ding proposed an all-solid type dissolved H2S microelectrode optimized for its measurement range. The lower limit of detection was extended from 10−4 mol/L to 10−7 mol/L by direct current Sustainabilitycarrier electroplating.2020, 12, 9109 The sensor was also successfully tested in the HT vent field in Kueishan8 ofTao 17 (Figure 8) [69].

Figure 8. The H 2SS microelectrode microelectrode integrated integrated in a multi-parameter chemical sensor [69]. [69].

Frank and his colleagues studied the micro-profilesmicro-profiles ofof OO22, pH, H2SS and and temperature temperature of a shallow water HT HT vent vent system system at at Milos, Milos, Greece Greece using using a miniaturized a miniaturized version version of a profiling of a profiling instrument. instrument. Data Dataobtained obtained were wereused usedto analyze to analyze the microenviron the microenvironmentment and microcirculation and microcirculation of the of vent the vent[70]. [70]. In summary, thoughthough notnot muchmuch isolatedisolated research waswas reported,reported, the developmentdevelopment of SHT direct measuring technique continues with the developmentdevelopment of sensorsensor techniques.techniques. The The trend of future development may lie in the measurement range forfor higher measurement accuracy under extreme conditions, or or in in the the stability stability and and durability durability of the of thesensors sensors to obtain to obtain a better a better performance performance in long- in long-termterm observation. observation.

3.2. Remote Sensing Techniques Direct measurement techniques ooffefferr reliable,reliable, accurateaccurate datadata forfor HTHT studies.studies. But the data collected are usuallyusually discrete, discrete, which which provides provides only limitedonly limit informationed information when studying when thestudying continuous the distributioncontinuous ofdistribution a certain parameter,of a certain e.g., parameter, temperature e.g., temperature field or flow field velocity or flow field. velocity A sensor field. array A sensor might array extend might the informationextend the information collected [ 71collected], but the [71], blocking but the e ffblockingect of sensors effect of to sensors the HT to fluid thealso HT fluid prevents also scientistsprevents fromscientists obtaining from theobtaining original distributionthe original of distributi the desiredon parameter.of the desired Furthermore, parameter. the measurementFurthermore, error the caused by sulfur deposition and the drifting effect during long-term measurement has long been a problem in the in-situ measurement of HT fluid [72]. To overcome the above shortcomings, a series of remote sensing approaches were developed to study the 3D structure, temperature distribution, flow field or heat flux field of the HT fluid. Taking photos or videos using underwater cameras is a traditional remote sensing approach. It intuitively records the appearance of the HT plume and the surrounding terrain or biological features, which is important in further analysis. It was widely used in the exploration of both deep-sea HT vents and SHT vents in past decades [69,73]. In recent years, the high-resolution photography and advanced algorithms provided by modern photogrammetry makes it possible to obtain even more information through photos. Gerdes et al. proposed their study on the 3D reconstruction approach of HT vent fauna based on video imagery. The vent was located at the southeastern Indian Ridge in the Indian Ocean. They used a motion photogrammetry structure to build a high-resolution 3D reconstruction model of one side of a newly discovered active HT chimney complex (Figure9)[ 74]. Based on the random forest model, they predicted that accuracy on faunal assemblage distribution of the model has reached 84.97%. This technique provides scientists with the possibility to identify ecological distributions and terrain features on a larger scale. Though the data were collected in a deep-sea HT vent, the technique has the potential to work in various situations in marine monitoring, including SHT observations. Sustainability 2020, 12, x FOR PEER REVIEW 9 of 18 measurement error caused by sulfur deposition and the drifting effect during long-term measurement has long been a problem in the in-situ measurement of HT fluid [72]. To overcome the above shortcomings, a series of remote sensing approaches were developed to study the 3D structure, temperature distribution, flow field or heat flux field of the HT fluid. Taking photos or videos using underwater cameras is a traditional remote sensing approach. It intuitively records the appearance of the HT plume and the surrounding terrain or biological features, which is important in further analysis. It was widely used in the exploration of both deep-sea HT vents and SHT vents in past decades [69,73]. In recent years, the high-resolution photography and advanced algorithms provided by modern photogrammetry makes it possible to obtain even more information through photos. Gerdes et al. proposed their study on the 3D reconstruction approach of HT vent fauna based on video imagery. The vent was located at the southeastern Indian Ridge in the Indian Ocean. They used a motion photogrammetry structure to build a high-resolution 3D reconstruction model of one side of a newly discovered active HT chimney complex (Figure 9) [74]. Based on the random forest model, they predicted that accuracy on faunal assemblage distribution of the model has reached 84.97%. This technique provides scientists with the possibility to identify ecological distributions and terrain features on a larger scale. Though the data were collected in a deep-sea HT Sustainabilityvent, the technique2020, 12, 9109 has the potential to work in various situations in marine monitoring, including9 of 17 SHT observations.

Figure 9.9. SouthSouth sideside ofof reconstructed reconstructed 3D 3D textured textured dense dense point point cloud cloud and and derived derived terrain terrain variables variables [74]. ([74].B) shows (B) shows the aspect the aspect of chimney; of chimney; colors on colors (C) show on (C the) show different the roughness;different roughness; (D) shows ( theD) inclinationshows the ofinclination slope, with of slope, a maximum with a maximum value of 90 value degrees. of 90 (E degrees.) shows ( theE) shows Gaussian the surfaceGaussian curvature. surface curvature. (F,G) are detailed(F,G) are views detailed of boxviews 1 and of box box 1 2 and in figure box 2 (inA), figure respectively. (A), respectively.

Although photogrammetry provides us with abundant information on plume appearance, terrain signature and biological distribution, some invisible features, such as temperature and flow flow velocity, cannot be obtained. Besides, Besides, the the high high tu turbidityrbidity of of water water around around HT HT vents vents often often exerts exerts a anegative negative influence influence on onphoto photo qualit quality.y. Thus, Thus, approaches approaches based based on acou onstic acoustic methods methods were raised. were raised. Rona Ronaand his and colleagues his colleagues were were among among the theearliest earliest to re tosearch research HT HT vents vents using using acoustic acoustic methods methods [75–77]. [75–77]. During the dive in November 1990, they scanned an HT plume in arcuate sectors at increasing angular increments from the vent orificesorifices up to 90 °◦ usingusing a a sonar sonar system system mounted mounted on on the the submersible. submersible. Using the data collected, the 3D structure of the HT plume with a height of up to 100 m above the seafloorseafloorSustainability was 2020 successfully, 12, x FOR PEER reconstructed REVIEW (Figure(Figure 1010).). 10 of 18

(a) (b)

FigureFigure 10.10. TheThe front front (a ()a and) and back back (b) (viewsb) views of the of reconstructed the reconstructed 3D image 3D image of two of adjacent two adjacent HT plumes HT plumes[76]. The [76 grey]. The part grey in the part figure in the shows figure the shows patterns the patternsof the HT of plumes. the HT The plumes. black The part black is the part ellipsoids is the ellipsoidsfitted to show fitted the to showroughness the roughness of a plume. of a plume.

TheThe 3D3D imagingimaging approachapproach proposed proposed by by Rona Rona waswas basedbased onon thethe scatteringscattering featurefeature ofof soundsound beamsbeams onon suspendedsuspended particlesparticles thatthat focusedfocused onon thethe visualizationvisualization ofof the the plume plume structure. structure. FanFan etet al.al. proposedproposed anan acousticacoustic tomographytomography methodmethod toto measuremeasure thethe 2D2D temperaturetemperature distributiondistribution aroundaround seafloorseafloor HTHT ventsvents [ 72[72,78,79].,78,79]. The The measurement measurement plain plain was coveredwas covered by 96 acoustic by 96 pathsacoustic generated paths by generated 16 hydrophones by 16 hydrophones distributed on the edge of the plane (Figure 11a). Each hydrophone was used as the transmitter and the receiver at the same time. Signal correlation analysis was performed to obtain the time of flight (ToF) between two hydrophones, which reflects the average value of temperature distributed along the corresponding acoustic path. Based on all the ToFs collected, the reconstruction of the 2D temperature field was performed by the total least-squares method (Figure 11b). They tested their proposed method both in a swimming pool and around a hot spring beneath the Lake Qiezishan, Yunnan Province, China (Figure 12a,b). Satisfying results were obtained in both trials. The reconstructed temperature field (Figure 12c) in the Lake Qiezishan trial showed good accuracy with a maximum error of approximately 3.5 °C compared with the measurement results obtained by thermocouples. The Lake Qiezishan trial revealed its potential for the application in SHT observation.

(a) (b)

Figure 11. The reconstruction of 2D temperature field using acoustic tomography. (a) The distribution of hydrophones and corresponding acoustic paths on the measuring plane; (b) the meshed plane for the reconstruction of 2D temperature distribution using the total least-squares method [72]. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 18

(a) (b)

Figure 10. The front (a) and back (b) views of the reconstructed 3D image of two adjacent HT plumes [76]. The grey part in the figure shows the patterns of the HT plumes. The black part is the ellipsoids fitted to show the roughness of a plume.

The 3D imaging approach proposed by Rona was based on the scattering feature of sound beams on suspended particles that focused on the visualization of the plume structure. Fan et al. proposed Sustainability 2020, 12, 9109 10 of 17 an acoustic tomography method to measure the 2D temperature distribution around seafloor HT vents [72,78,79]. The measurement plain was covered by 96 acoustic paths generated by 16 distributedhydrophones on distributed the edge of on the the plane edge (Figure of the 11 planea). Each (Figure hydrophone 11a). Each was hydrophone used as the was transmitter used as andthe thetransmitter receiver and at the the same receiver time. at Signalthe same correlation time. Signal analysis correlation was performed analysis was to obtainperformed the time to obtain of flight the (ToF)time of between flight two(ToF) hydrophones, between two which hydrophones, reflects the which average reflects value ofthe temperature average value distributed of temperature along the correspondingdistributed along acoustic the corresponding path. Based on acoustic all the path. ToFs collected,Based on theall the reconstruction ToFs collected, of the the 2D reconstruction temperature fieldof the was 2D performedtemperature by field the totalwas performed least-squares by method the total (Figure least-squares 11b). They method tested (Figure theirproposed 11b). They method tested boththeir inproposed a swimming method pool andboth around in a aswimming hot spring beneathpool and the around Lake Qiezishan, a hot springYunnan beneath Province, the ChinaLake (FigureQiezishan, 12a,b). Yunnan Satisfying Province, results China were (Figure obtained 12a,b). in Satisfying both trials. results The reconstructedwere obtained temperaturein both trials. field The (Figurereconstructed 12c) in temperature the Lake Qiezishan field (Figure trial showed 12c) in good the Lake accuracy Qiezishan with a trial maximum showed error good of approximatelyaccuracy with 3.5a maximum◦C compared error with of approximately the measurement 3.5 results °C compar obtaineded with by thermocouples. the measurement The Lakeresults Qiezishan obtained trial by revealedthermocouples. its potential The Lake for the Qiezishan application trial inrevealed SHT observation. its potential for the application in SHT observation.

(a) (b)

Figure 11. TheThe reconstruction reconstruction of of 2D 2D temperatur temperaturee field field using acoustic tomography. ( a) The The distribution distribution of hydrophones and corresponding acoustic paths on the measuring plane; ( b) the meshed plane for the reconstruction ofof 2D2D temperaturetemperature distributiondistribution usingusing thethe totaltotal least-squaresleast-squares methodmethod [[72].72].

Based on similar principles, Pan et al. developed a device for measuring the heat flux rising from an HT vent. Multiple layers of measurement planes were included in the approach to obtain temperature distribution at different heights [80]. The distribution of velocity field inside the measurement plane was estimated by the measurement of ToFs among the hydrophone in different layers. Using both the temperature field and velocity field reconstructed, the heat flux inside the measuring plane can be estimated. The remote sensing techniques have the ability to observe continuous distribution of parameters around the HT vents. The remote sensing keeps them free from corrosion and sulfur deposition, thus it is ideally suitable for long-term in-situ observation. However, most of the existing approaches require complex algorithms to reconstruct the observing target from the measured data, which made the approaches less robust and sensitive. Though most of the remote sensing techniques reported were suitable for SHT studies, the wide application was believed to be limited by the complexity and cost of the measurement systems. More engineering practice is still needed to evaluate the performance of remote sensing methods in SHT observations. SustainabilitySustainability2020 2020, 12, 12, 9109, x FOR PEER REVIEW 11 of11 18 of 17

(a)

(b) (c)

FigureFigure 12. 12.The The Lake Lake Qiezishan Qiezishan trial trial of of the the acoustic acoustic tomography tomography to to reconstruct reconstruct the the 2D 2D temperature temperature field aroundfield around a hot spring a hot locatedspring locate at thed floor at the of floor Lake of Qiezishan, Lake Qiezis Yunnanhan, Yunnan Province, Province, China. (China.a) The ( schematica) The diagramschematic of diagram the experimental of the experimental set-up. The set-up. vent The orifice vent wasorifice covered was covered by a chimneyby a chimney to form to form a sharp a temperaturesharp temperature gradient. gradient. The hydrophone The hydrophone array was a deployedrray was fromdeployed two raftsfrom that two work rafts collaboratively. that work (b)collaboratively. The deployment (b) ofThe the deployment hydrophone of array;the hydrophone (c) the reconstructed array; (c) the 2D reconstructed temperature 2D field. temperature The red area nearfield. the The center red area indicates near the center highest indicates temperature the highest in the temperature measurement in the plane measurement [72]. plane [72].

4. WaterBased Samplers on similar for principles, SHT Vents Pan et al. developed a device for measuring the heat flux rising from an HT vent. Multiple layers of measurement planes were included in the approach to obtain Although the fast development of in-situ measurement techniques provides scientists with temperature distribution at different heights [80]. The distribution of velocity field inside the instant and convenient ways of investigating HT features, limitations still exist when measuring measurement plane was estimated by the measurement of ToFs among the hydrophone in different organic parameters or biomass due to the lack of effective sensors. The analysis of water samples is layers. Using both the temperature field and velocity field reconstructed, the heat flux inside the stillmeasuring an important plane approachcan be estimated. in HT studies. Various advanced tools have been developed to collect water samples,The remote microbial sensing matstechniques or cores have from the deep-seaability to HTobserve vents continuous and keep distribution them unaltered of parameters during their transportationaround the HT to vents. the laboratory The remote [ sensing81]. These keeps tools them significantly free from corrosion accelerate and the sulfur progress deposition, of deep-sea thus HTit studies.is ideally suitable for long-term in-situ observation. However, most of the existing approaches requireCompared complex with algorithms deep-sea to HT reconstruct studies, thethe developmentobserving target of samplingfrom the measured tools for SHT data, exploration which made lags farthe behind. approaches One reason less robust may and be that sensitive. the SHT Though vents aremost easier of the to remote explore. sensing When techniques samples can reported be easily obtained by scuba divers using glass syringes, plastic bags or bottles, which are actually the most Sustainability 2020, 12, 9109 12 of 17 commonly used methods even in recent years [10], the expensive, specially designed sampling tools seem unnecessary. Furthermore, most of the complex, heavy tools designed for accurate, automatic sampling were not suitable for scuba divers to operate. However, with the increasing demand in sampling accuracy and efficiency, sampling techniques targeted at SHT studies have begun to attract the attention of researchers. Wu and his colleagues developed a handheld sampler for collecting organic samples from SHT vents (Figure 13a) [82]. The sampler utilizes a syringe-like sampling bottle made of titanium to collect organic samples. The bottle was sealed with a Teflon fluorinated ethylene propylene encapsulated O-ring to achieve perfect gas-tight performance. Two major improvements were made to ensure its performance in SHT sampling. First, a temperature logger was integrated into the sampler, which measures the temperature at the inlet of the snorkel using a thermocouple and displays the result on an LED display in real-time. The reading displayed by a red LED half hidden in a titanium housing can be clearly recognized even in high turbidity waters (Figure 12c). The real-time temperature measurement helps the diver to find the point with the highest temperature in the HT plume where the mixing of HT fluid and surrounding seawater were believed to be the weakest [83]. This feature helps in improving the purity of the HT samples collected. Second, an orifice or an annular gap between the piston rod and the end cap was used to regulate the flow rate of the sampler. A properly regulated flow rate will balance the purity of samples with the sampling efficiency. The sampler was tested in the Kueishan Tao HT vent field. The comparative test based on DOC concentration and salinity analysis shows that water samples with higher purity compared with glass-bottle-collected samples Sustainability 2020, 12, x FOR PEER REVIEW 13 of 18 were successfully obtained.

(a)

(b) (c)

FigureFigure 13. 13.The The handheld handheld sampler sampler developed developed for SHTfor SHT sampling. sampling. (a) The (a) structureThe structure diagram diagram of the of sampler; the (b)sampler; collecting (b) organic collecting samples organic from samples the orifice from the of a orifice yellow of vent a yellow in the vent Kueishan in the TaoKueishan vent field;Tao vent (c) the real-timefield; (c) measurement the real-time measurement while sampling while [82 ].sampling [82].

5. Discussion and Conclusions Studies focused on SHT systems have been gradually given attention since the end of the last century. Being motivated by the increasing demand in scientific research, techniques that focus on SHT exploration are experiencing a gradual increase. Some of the detecting, observing and sampling methods are mentioned in this paper. Most of them have been successfully conducted at selected sites, such as the acoustic imaging of temperature fields and the satellite imagery of SHT vents. The others were tested in similar environments, such as the bubble cluster detection method, which showed the potential of engineering applications. Although most of the newly developed HT vent exploring techniques were still in the experimental stage, the impact that they might bring to the fundamental science can be seen. With the specially designed sensors and samplers, the measurement data and samples collected will be more dependable. Remote sensing techniques, such as satellite imagery, provides scientists with a fast and economic way of studying SHT activities, which might greatly reduce labor and time on field tests. The acoustic tomography method provides scientists with means of measuring temperature fields and heat flux, as well as the 3D structure of the flume, data which has hardly been obtained before. These new tools bring more possibilities in the fundamental science on HT studies, and thus might reveal new research fields in the near future. However, the development of exploring techniques for SHT vents was still in its infant stage. But the techniques are evolving at an increasing rate. Researchers have begun to put their eyes on the diversity that lies between SHT systems and deep-sea HT systems when developing new exploring Sustainability 2020, 12, 9109 13 of 17

5. Discussion and Conclusions Studies focused on SHT systems have been gradually given attention since the end of the last century. Being motivated by the increasing demand in scientific research, techniques that focus on SHT exploration are experiencing a gradual increase. Some of the detecting, observing and sampling methods are mentioned in this paper. Most of them have been successfully conducted at selected sites, such as the acoustic imaging of temperature fields and the satellite imagery of SHT vents. The others were tested in similar environments, such as the bubble cluster detection method, which showed the potential of engineering applications. Although most of the newly developed HT vent exploring techniques were still in the experimental stage, the impact that they might bring to the fundamental science can be seen. With the specially designed sensors and samplers, the measurement data and samples collected will be more dependable. Remote sensing techniques, such as satellite imagery, provides scientists with a fast and economic way of studying SHT activities, which might greatly reduce labor and time on field tests. The acoustic tomography method provides scientists with means of measuring temperature fields and heat flux, as well as the 3D structure of the flume, data which has hardly been obtained before. These new tools bring more possibilities in the fundamental science on HT studies, and thus might reveal new research fields in the near future. However, the development of exploring techniques for SHT vents was still in its infant stage. But the techniques are evolving at an increasing rate. Researchers have begun to put their eyes on the diversity that lies between SHT systems and deep-sea HT systems when developing new exploring techniques. Some research fields are raising increasing demand on the development of new techniques. For example, the need in studying HT vent microbiology is demanding a sampling method with higher sample densities in order to study the activity of micro-organisms in sharp temperature and chemical gradients. The same demand might also be raised by the origins of life studies. Future work in SHT exploring techniques may involve measuring techniques with higher reliability and accuracy, or with totally new measuring parameters.

Author Contributions: Writing—Original draft, Z.Z.; project administration, W.F.; resources/writing-review & editing, W.B.; supervision, C.-T.A.C.; writing—review and editing, S.L.; investigation, Y.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Funds of China grant No. 41976199 and No. U1805242. And the APC was funded by the National Natural Science Funds of China grant No. 41976199. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Francheteau, J.; Needham, H.D.; Choukroune, P.; Juteau, T.; Séguret, M.; Ballard, R.D.; Fox, P.J.; Normark, W.; Carranza, A.; Cordoba, D. Massive deep-sea sulphide ore deposits discovered on the east pacific rise. Nature 1979, 277, 523–528. [CrossRef] 2. Elderfield, H.; Schulz, A. Mid-ocean ridge hydrothermal fluxes and the chemical-composition of the ocean. Annu. Rev. Earth Planet. Sci. 1996, 24, 191–224. [CrossRef] 3. Leveille, R.J.; Juniper, S.K. Deep-sea hydrothermal vents and cold seeps. In Biogeochemistry of Marine Systems; Black, K., Shimmield, G.B., Eds.; Biological Science Series; Blackwell Publishing: Oxford, UK, 2003; pp. 238–292. 4. Longo, A.; Damer, B. Factoring origin of life hypotheses into the search for life in the solar system and beyond. Life 2020, 10, 52. [CrossRef][PubMed] 5. Preiner, M.; Asche, S.; Becker, S.; Betts, H.; Boniface, A.; Casas, E.C.; Chandru, K.; Erastova, V.; Garg, S.G.; Khawaja, N.; et al. The future of origin of life research: Bridging decades-old divisions. Life 2020, 10, 20. [CrossRef][PubMed] 6. Martin, W.; Baross, J.; Kelley, D.; Russell, M. Hydrothermal vents and the origin of life. Nat. Rev. Microbiol. 2008, 6, 805–814. [CrossRef] Sustainability 2020, 12, 9109 14 of 17

7. Herzig, M.P.; Hannington, D.M. Polymetallic massive sulfides at the modern seafloor—A review. Ore Geol. Rev. 1995, 10, 95–115. [CrossRef] 8. Marumo, K.; Hattori, K.H. Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough: Mineralogy geochemistry and isotope characteristics. Geochim. Cosmochim. Acta 1999, 63, 2785–2804. [CrossRef] 9. Miller, K.; Thompson, K.; Johnston, P.; Santillo, D. An overview of seabed mining including the current state of development, environmental impacts, and knowledge gaps. Front. Mar. Sci. 2018, 4, 1–24. [CrossRef] 10. Chen, C.A.; Wang, B.J.; Huang, J.F.; Lou, J.; Kuo, F.; Tu, Y.; Tsai, H. Investigation into extremely acidic hydrothermal fluids off Kueishan Tao, Taiwan, China. Acta Oceanol. Sin. 2005, 24, 125–133. 11. Von Damm, K.L.; Edmond, J.M.; Grant, B.; Measures, C.I.; Walden, B.; Weiss, R.F. Chemistry of submarine

hydrothermal solutions at 21◦ N, East Pacific Rise. Geochim. Cosmochim. Acta 1985, 49, 2197–2220. [CrossRef] 12. Gamo, T.; Chiba, H.; Yamanaka, T.; Okudaira, T.; Hashimoto, J.; Tsuchida, S.; Ishibashi, J.I.; Kataoka, S.; Tsunogai, U.; Okamura, K.; et al. Chemical characteristics of newly discovered black smoker fluids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge. Earth Planet. Sci. Lett. 2001, 193, 371–379. 13. Hua, L.C.; Bo, Y.X. Introduction to Modern Shallow-submarine Hydrothermal Activity. Adv. Earth Sci. 2006, 21, 918–924. 14. Tarasov, V.G.; Gebruk, A.V.; Mironov, A.N.; Moskalev, L.I. Deep-sea and shallow-water hydrothermal vent communities: Two different phenomena. Chem. Geol. 2005, 224, 5–39. [CrossRef] 15. Mironov, A.N.; Gebruk, A.V.; Moskalev, L.I. Biogeography of Hydrothermal Vent Communities and Obligate Hydrothermal Taxa; Biology of Hydrothermal Systems; KMK Press: Moscow, Russia, 2002; pp. 410–455. 16. Giménez, F.; Marín, A. Los anélidos poliquetos de una solfatara submarina en el Golfo de Nápoles. An. De Biol. 1991, 17, 143–151. 17. Dando, P.R.; Hughes, J.A.; Thiermann, F. Preliminary observations on biological communities at shallow hydrothermal vents in the Aegean Sea. Hydrothermal Vents Process. 1995, 87, 303–317. [CrossRef] 18. Stein, J.L. Subtidal gastropods consume sulphur-oxidizing bacteria: Evidence from coastal hydrothermal vents. Science 1984, 223, 696–698. [PubMed] 19. Tarasov, V.G.; Gebruk, A.V.; Shulkin, V.M.; Kamenev, G.M.; Fadeev, V.I.; Kosmynin, V.N.; Malakhov, V.V.; Starynin, D.A.; Obzhirov, A.I. Effect of shallow-water hydrothermal venting on the biota of Matupi Harbor (Rabaul Caldera, New Britain Island, Papua New Guinea). Cont. Shelf Res. 1999, 19, 79–116. 20. Tarasov, V.G. The Coastal Ecosystems and Shallow-Water Hydrothermal Venting; Dalnauka Press: Vladivostok, Russia, 1999. 21. Tarasov, V.G. Environment and Biota of Shallow-Water Hydrothermal Vents of the West Pacific; Biology of Hydrothermal Systems; KMK Press: Moscow, Russia, 2002; pp. 264–319. 22. Buzhinskaya, G.N. Polychaetes (Polychaeta) of coastal waters of Yankich Island (Ushishir Islands of the Kurile Ridge). Truduiy ZIN AN SSSR 1990, 218, 18–35. 23. Turpaeva, E.P. A new species of marine spiders (Pycnogonida) from Far East Seas of Russia. Zool. Zhurnal 2000, 79, 626–630. 24. Southward, A.J.; Kennicutt, M.C.; Alcala-Herrera, J.; Abbiati, M.; Airoldi, L.; Cinelli, F.; Bianchi, C.N.; Mori, C.; Southward, E.C. On the biology of submarine caves: Appraisal of 13C/12C ratios as a guide to trophic relations. J. Mar. Biol. Assoc. U. K. 1996, 76, 265–285. [CrossRef] 25. Prol-Ledesma, R.M. Similarities in the chemistry of shallow submarine hydrothermal vents. Geothemics 2003, 32, 639–644. [CrossRef] 26. Prol-Ledesma, R.M.; Canet, C.; Torres-Vera, M.A.; Forrest, M.J.; Armienta, M.A. Vent fluid chemistry in Bahia Conception coastal submarine hydrothermal system, Baja California Sur, Mexico. J. Volcanol. Geotherm. Res. 2004, 137, 311–328. [CrossRef] 27. Kamenev, G.M.; Fadeev, V.I.; Selin, N.I.; Tarasov, V.G. Composition and distribution of macro—And meiobenthos around hydrothermal vents in the Bay of Plenty, New Zealand. N. Z. J. Mar. Freshw. Res. 1993, 27, 407–418. [CrossRef] 28. Kostina, E.E. Macrobenthos of littoral of Kraternaya Bight and Goryachii Plyazh (Kuriles). Biota 1991, 2, 4–36. 29. Gamenick, I.; Abbiati, M.; Giere, O. Field distribution and sulphide tolerance of Capitella capitata (Annelida: Polychaeta) around shallow water hydrothermal vents off the Milos (Aegean Sea). A new sibling species? Mar. Biol. 1998, 130, 447–453. [CrossRef] Sustainability 2020, 12, 9109 15 of 17

30. Takeda, M.; Kubata, Y. Crabs of the Ogasawara Islands: IV. A collection made the new volcanic island, Nishino-shina-shinto, in 1975. Bull. Natl. Sci. Mus. A Zool. 1977, 3, 91–111. 31. Takeda, M.; Takeuchi, H.; Suganuma, H. Occurrence of Xenograpsus novaeinsularis Takeda et Kubata (Crustacea: Decapoda: Brachyura) in the Tokara and two Islands. Natl. Environ. Sci. Res. 1993, 6, 59–64. 32. Pichler, T.; Giggenbach, W.F.; McInnes, B.A.; Buhl, D.; Duch, B. Fe-sulfide formation due to sea water-gas-sediment interaction in a shallow-water hydrothermal system at Lihir Island, Papua New Guinea. Econ. Geol. 1999, 94, 281–288. [CrossRef] 33. Pichler, T.; Veizer, J.; Hall, G.E.M. The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater. Mar. Chem. 1999, 64, 229–252. [CrossRef] 34. Pichler, T.; Heikoop, J.M.; Risk, M.J.; Veizer, J.; Campbell, I.L. Hydrothermal effect on isotope and trace element records in modern reef corals: A study of Porites lobata from Tutum Bay, Ambite Island, Papua New Guinea. Palaios 2000, 15, 225–234. [CrossRef] 35. Jeng, M.S. The undersea wonders of Kueishan Island. Nat. World 2000, 68, 38–45. 36. Ng, N.K.; Huang, J.F.; Ho, P.H. Description of a New Species of hydrothermal Crab Xenograspus testudinatus (Crustacea: Decapoda: Brachyura: Grapsidae) from Taiwan. Natl. Taiwan Mus. Spec. Publ. Ser. 2000, 10, 191–199. 37. Acunto, S.; Maltagliati, F.; Cinelli, F. Osservazioni sui popolamenti bentonici di un’area interessata da attività idrotermale nei pressi dell’Isola di Panarea (Isole Eolie). Biol. Mar. Medit. 1995, 3, 434–436. 38. Colangelo, M.A.; Dall’Olio, P.; Pointi, M.; Ceccherelli, V.U. Prime osservazioni sul meiobenthos associato a fondali interessati da fenomeni vulcanismo scondario nell’Isola di Panarea (Me). In Caratterizzazione Ambientale Marina del Sistema Eolie e dei Bacini Limitrofi di Cefalù e Gioia (EOCUMM95); Faranda, F.M., Povero, P., Eds.; Data Report; CoNISMa: Genova, Italy, 1996; pp. 369–374. 39. Lucila, M.; Pomar, C.A.; Guiffre, G. Pico-, nano- and micro- plankton communities in hydrothermal marine coastal environments of the Eolian Islands (Panarea and Vulcano) in the Mediterranean Sea. J. Plankton Res. 1996, 18, 715–730. [CrossRef] 40. Avila, S.P.; Cardigos, F.; Santos, R.S. D. Joao de Castro Bank, a shallow water hydrothermal-vent in the Azores: Checklist of the marine mollusks. Arquipelago 2004, 21A, 75–80. 41. Turkay, M.; Sakai, K. Decapod crustaceans from a volcanic hot spring in the Marianas. Senckenbergiana Marit. 1995, 26, 25–35. 42. Miura, T.; Tsukahara, J.; Hashimoto, J. Lamellibranchia satsuma, anew species of vestimentiferan worms (Annelida: Pogonophora) from a shallow hydrothermal vent in Kagoshima Bay, Japan. Proc. Biol. Soc. Wash. 1997, 110, 447–456. 43. Miura, T. Two new species of the genus Ophryotrocha (Polychaeta Iphitimidae) from Kagoshima Bay. Bull. Mar. Sci. 1997, 60, 300–305. 44. Fricke, H.; Giere, O.; Stetter, K.; Alfredsson, G.A.; Kristjansson, J.K.; Stoffers, P.; Svavarsson, J. Hydrothermal vent communities at the shallow subpolar mid-Atlantic ridge. Mar. Biol. 1989, 102, 425–429. [CrossRef] 45. Von Cosel, R.; Marshall, B.A. Two new species of large mussels (Bivalvia: Mytilidae) from active submarine volcanoes and a cold seep off the astern North Island of New Zealand, with description of a new genus. Nautilus 2003, 117, 31–46. 46. Smith, P.J.; McVeagh, S.M.; Won, Y.; Vrijenhoek, R.C. Genetic heterogeneity among New Zealand species of hydrothermal vent mussels (Mytilidae: Bathymodiolus). Mar. Biol. 2004, 144, 537–545. [CrossRef]

47. Ding, K.; Seyfried, W.E.Direct pH Measurement of NaCl-Bearing Fluid with an in Situ Sensor at 400 degrees ◦C and 40 Megapascals. Science 1996, 272, 1634–1636. [CrossRef]

48. Ding, K.; Seyfried, W.E.; Tivey, M.K.; Bradley, A.M. In situ measurement of dissolved H2 and H2S in high-temperature hydrothermal vent fluids at the Field, Main Endeavour Ridge, Juan De Fuca. Earth Planet. Sci. Lett. 2001, 186, 417–425. [CrossRef] 49. Ding, K.; Zhang, Z.; Seyfried, W.E.; Bradley, A.M.; Zhou, Y.; Yang, C.J.; Chen, Y. Integrated in-situ chemical sensor system for submersible deployment at deep-sea hydrothermal vents. In Proceedings of the Oceans 2006, Boston, MA, USA, 18–21 September 2006; IEEE: New York, NY, USA, 2006; pp. 951–956. 50. Saito, M.A.; Noble, A.E.; Tagliabue, A.; Goepfert, T.J.; Lamborg, C.H.; Jenkins, W.J. Slow-spreading submarine ridges in the South Atlantic as a significant oceanic iron source. Nat. Geosci. 2013, 6, 775–779. [CrossRef] Sustainability 2020, 12, 9109 16 of 17

51. Resing, J.A.; Sedwick, P.N.; German, C.R.; Jenkins, W.J.; Moffett, J.W.; Sohst, B.M.; Tagliabue, A. Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean. Nature 2015, 523, 200–203. [CrossRef] 52. Li, J. Study on the Application of Partially Observable Markov Decision Process in Autonomous Hydrothermal Vent Prospecting. Ph.D. Thesis, Harbin Engineering University, Harbin, China, 2013. 53. Tao, C.; Chen, S.; Baker, E.T.; Li, H.; Liang, J.; Liao, S.; Chen, Y.J.; Deng, X.; Zhang, G.; Gu, C. Hydrothermal plume mapping as a prospecting tool for seafloor sulfide deposits: A case study at the Zouyu-1 and Zouyu-2 hydrothermal fields in the southern Mid-Atlantic Ridge. Mar. Geophys. Res. 2017, 38, 3–16. [CrossRef] 54. Tivey, M.K. Generation of seafloor hydrothermal vent fluids and associated mineral deposits. Oceanography 2007, 20, 50–65. [CrossRef] 55. Wheeler, A.J.; Murton, B.J.; Copley, J.; Lim, A.; Morris, K. Moytirra: Discovery of the first known deep-sea hydrothermal vent field on the slow-spreading mid-atlantic ridge north of the azores. Geochem. Geophys. Geosystems 2013, 14, 4170–4184. [CrossRef] 56. Nakatani, T.; Ura, T.; Ito, Y.; Kojima, J.; Tamura, K.; Sakamaki, T.; Nose, Y. AUV “TUNA-SAND” and its Exploration of hydrothermal vents at Kagoshima Bay. In Proceedings of the OCEANS 2008—MTS/IEEE Kobe Techno-Ocean, Kobe, Japan, 8–11 April 2008; pp. 1–5. 57. Liu, J. Wavelet Processing and Analysis of in-Situ Observation data on its Periodicity of Hydrothermal Vents on the Seafloor of Kueishan Island. Master’s Thesis, Hangzhou Dianzi University, Hangzhou, China, 2008. 58. Yoerger, D.; Bradley, A.; Jakuba, M.; German, C.; Shank, T.; Tivey, M. Autonomous and remotely operated vehicle technology for hydrothermal vent discovery, exploration, and sampling. Oceanography (Washington, DC) 2007, 20, 152–161. [CrossRef] 59. Connelly, D.P.; Copley, J.T.; Murton, B.J.; Stansfield, K.; Wilcox, S. Hydrothermal vent fields and chemosynthetic biota on the world’s deepest seafloor spreading centre. Nat. Commun. 2012, 3, 620. [CrossRef] 60. Cai, Z.; Luis, A.J.M.; Han, J.; Wang, K.; Ye, Y. A multi-channel chemical sensor and its application in detecting hydrothermal vents. Acta Oceanol. Sin. 2019, 38, 128–134. [CrossRef] 61. Liu, C.H. The Formative Cause of Native Sulfur Chimney Near Kueishan Island off Northeastern Taiwan. Ph.D. Thesis, Institute of Oceanology, Chinese Academy of Sciences, Beijing, China, 2007. 62. Brown, L.K. Gas Geochemistry of the Volcanic Hydrothermal Systems of Dominica and St. Lucia, Lesser Antilles: Implications for VolcanicMonitoring. Unpublished Senior Honors Thesis, University of New Mexico, Albuquerque, NM, USA, 2002. 63. Lin, F. Research on the Method of Acoustic Detetion of Underwater Hydrothermal. Master’s Thesis, Harbin Engineering University, Harbin, China, 2010. 64. Tanahashi, M.; Ueda, S.; Kondo, M.; Kaji, T. What does multibeam echo-sounder of two frequencies reveal?—Possibility of new exploration method for hydrothermal vent field. Butsuri Tansa 2014, 67, 17–24. 65. Kasaya, T.; Machiyama, H.; Kitada, K.; Nakamura, K. Trial exploration for hydrothermal activity using acoustic measurements at the north iheya knoll. Geochem. J. 2015, 49, 597–602. [CrossRef] 66. Martelat, J.E.; Escartin, J.; Barreyre, T. Terrestrial shallow water hydrothermal outflow characterized from out of space. Mar. Geol. 2020, 422, 106–119. [CrossRef] 67. Pan, Y.; Seyfried, W.E. Experimental and theoretical constraints on pH measurements with an iridium oxide

electrode in aqueous fluids from 25 to 175 ◦C and 25 MPa. J. Solut. Chem. 2008, 37, 1051–1062. [CrossRef] 68. Pan, Y. Development and Sea Trial Study of Electrochemical Sensors for Seafloor hydrothermal vent. Ph.D. Thesis, Zhejiang University, Hangzhou, China, 2012. 69. Ding, Q. Microelectrodes in the Research of Ocean Acidification and Hydrothermal Vents: Developments and Applications. Master’s Thesis, Zhejiang University, Hangzhou, China, 2014. 70. Frank, W.; Holby, O.; Glud, R.N.; Nielsen, H.K.; Gundersen, J.K. In situ microsensor studies of a shallow water hydrothermal vent at milos, greece. Mar. Chem. 2000, 69, 43–54. 71. Wu, H.C.; Chen, Y.; Yang, C.J.; Zhang, J.F.; Zhou, H.Y.; Peng, X.T. Mechatronic integration and implementation of in situ multipoint temperature measurement for seafloor hydrothermal vent. Sci. China Ser. E Technol. Sci. 2007, 37, 438–445. [CrossRef] 72. Fan, W. Research on Acoustic Measurement Techniques for Determing the Temperature Distribution Around Seafloor Hydrothermal Vents. Ph.D. Thesis, Zhejiang University, Hangzhou, China, 2010. Sustainability 2020, 12, 9109 17 of 17

73. Butterfield, D.A.; Holden, J.F.; Shank, T.M.; Tunnicliffe, V.; Sherrin, J.; Herrera, S.; Baker, E.T.; Lovalvo, D.; Makarim, S.; Malik, M.A.; et al. Video observations by telepresence reveal two types of hydrothermal venting on Kawio Barat Seamount. In Proceedings of the 2010 Fall Meeting, AGU, San Francisco, CA, USA, 13–17 December 2010; pp. 13–17. 74. Gerdes, K.; Arbizu, P.M.; Schwarz-Schampera, U.; Schwentner, M.; Kihara, T.C. Detailed mapping of hydrothermal vent fauna: A 3d reconstruction approach based on video imagery. Front. Mar. Sci. 2019, 6, 1–21. [CrossRef] 75. Rona, P.A.; Palmer, D.R.; Jones, C.; Chayes, D.A.; Czarnecki, M.; Carey, E.W.; Guerrero, J.C. Acoustic imaging

of hydrothermal plumes, East Pacific Rise, 21◦ N, 109◦ W. Geophys. Res. Lett. 1991, 18, 2233–2236. [CrossRef] 76. Rona, P.A.; Bemis, K.G.; Silver, D.; Jones, C.D. Acoustic imaging, visualization and quantification of buoyant hydrothermal plumes in the ocean. Mar. Geophys. Res. 2002, 23, 147–168. [CrossRef] 77. Bemis, K.G.; Rona, P.A.; Jackson, D.; Jones, C.D. Time-averaging fluctuating seafloor hydrothermal plumes: Measurements by remote acoustic sensing. Geophys. Res. Abstr. 2003, 5, 6–11. 78. Fan, W.; Chen, Y.; Pan, H.C.; Mao, J.; Pan, Y.W. Acoustic tomography for estimating temperature of deep-sea hydrothermal vents. In Proceedings of the 4th IFAC International Workshop on Bio-Robotics, Champaign, IL, USA, 10–11 September 2009. 79. Fan, W.; Chen, Y.; Pan, H.C.; Ye, Y.; Cai, Y.; Zhang, Z.J. Experimental study on underwater acoustic imaging of 2-D temperature distribution around hot springs on floor of Lake Qiezishan, China. Exp. Therm. Fluid Sci. 2010, 34, 1334–1345. [CrossRef] 80. Pan, H.; Chen, Y.; Mao, J.; Fan, W.; Wu, M. Progress in developing a device for measuring heat flux from the hydrothermal vent in deep ocean using acoustic method. Oceans 2008, 2008, 1–5. 81. Breier, J.A.; Gomez-Ibanez, D.; Reddington, E.; Huber, J.A.; Emerson, D. A precision multi-sampler for deep-sea hydrothermal microbial mat studies. Deep Sea Res. Part I Oceanogr. Res. Pap. 2012, 70, 83–90. [CrossRef] 82. Wu, S.J.; Yang, C.J.; Arthur Chen, C.T. A handheld sampler for collecting organic samples from shallow hydrothermal vents. J. Atmos. Ocean. Technol. 2013, 30, 1951–1958. [CrossRef] 83. Fornari, D.; Bradley, A.; Humphris, S. Inductively coupled link (ICL) temperature probes for hot hydrothermal fluid sampling from ROV Jason and DSV Alvin. RIDGE Events 1997, 8, 26–30.

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