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  • View in gallery

    Structure of the GTP sampler.

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    Schematic of the pressure-balanced sampling valve.

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    Schematic illustration of the sampling valve actuator.

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    Functional block diagram of the hardware architecture.

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    GUI of the sampler that enables the scientists to operate the sampler on a laptop.

  • View in gallery

    Collecting fluid samples by using GTP samplers from the bottom of Challenger Deep. GTP samplers (along with other instruments) were deployed by the deep-sea landers (a) Wan Quan and (b) Yan Wei Shi Yan.

  • View in gallery

    Schematic illustration of the subsampling setup.

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Collection of Gas-Tight Water Samples from the Bottom of the Challenger Deep

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  • 1 State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou, China
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Abstract

A new gas-tight pair sampler was designed for the collection of gas-tight fluid samples from the hadal zone. The sampler uses two titanium bottles and one sampling valve to collect two samples at once. The sampler can be deployed in the deepest trenches in the ocean as a result of its ability to resist ultrahigh pressure and its good bidirectional sealing performance. It can be used on manned submersibles, remotely operated vehicles, and deep-sea landers. Three sets of this new sampler were constructed and field tested in the Mariana Trench during the cruise TS-03 from 15 January to 23 March 2017, during which 3 L of water samples were successfully obtained from the bottom of the Challenger Deep.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Can-Jun Yang, ycj@zju.edu.cn

Abstract

A new gas-tight pair sampler was designed for the collection of gas-tight fluid samples from the hadal zone. The sampler uses two titanium bottles and one sampling valve to collect two samples at once. The sampler can be deployed in the deepest trenches in the ocean as a result of its ability to resist ultrahigh pressure and its good bidirectional sealing performance. It can be used on manned submersibles, remotely operated vehicles, and deep-sea landers. Three sets of this new sampler were constructed and field tested in the Mariana Trench during the cruise TS-03 from 15 January to 23 March 2017, during which 3 L of water samples were successfully obtained from the bottom of the Challenger Deep.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Can-Jun Yang, ycj@zju.edu.cn

1. Introduction

The hadal zone refers specifically to ocean depths greater than 6000 m in trench areas (Blankenship-Williams and Levin 2009). The hadal zone includes 37 deep trenches, 9 of which are distributed in the western Pacific Ocean. Trenches account for only 1% of the total area of the global seabed area but represent over 45% of the total vertical depth of the marine environment (Jamieson et al. 2009; Ritchie et al. 2017). They are almost exclusively spatially disjunct environments separated by shallower areas (Jamieson et al. 2010). The Mariana Trench is located between the eastern part of the Philippines Sea plate and the subduction of the Pacific plate (Kato et al. 1998; Taira et al. 2005). The Challenger Deep in the Mariana Trench, with a depth of nearly 11 000 m, is the deepest place in Earth’s oceans (Schmidt and Siegel 2011) and thus has attracted the most attention among all hadal zones worldwide (Luo et al. 2017).

The hadal zone remains one of the least understood and most mysterious habitats on Earth. It is characterized by elevated hydrostatic pressure, cold temperature, low food availability, the absence of natural light, and relatively isolated hydrotopography (Nunoura et al. 2015; Ritchie et al. 2017). Oxygen utilization tests have revealed high microbial activity in surface sediments in this environment, but little is known about the diversity of the microbial community (León- Zayas et al. 2017). For a long time, the hadal zone was considered to be a biological desert, and knowledge of its biology, ecology, and biogeochemistry was very scarce as a result of its inaccessibility (Luo et al. 2017). Despite being considered an “extreme” environment, the hadal zone is host to a diverse range of flora and fauna. The restricted distribution of key taxa to specific trenches underpins the conventional view that hadal trenches are hot spots of species endemism driven by a combination of geographic isolation and potent selection pressures (Ritchie et al. 2017). The study of biological communities, chemical circulation, and life processes is key to understanding the evolution of the hadal zone environment. Today, hadal science remains a major frontier in deep-sea studies.

Hadal zone research depends greatly on deep-sea technology and equipment. Technical challenges related to sampling at increasing distance from the vessel and at extremely high hydrostatic pressures greatly limit studies of the hadal zone (Linley et al. 2017). Although a variety of in situ measurement instruments are used frequently to study hydrothermal systems, cold seeps, and other deep-sea sites, their use is rarely reported for the study of the hadal zone (White et al. 2005; Ding and Seyfried 2007; Tan et al. 2012; Wu et al. 2013; Jackson et al. 2017). Until recently, understanding of life in the hadal zone relied largely on trawl catches, free vehicle camera and trap work, and a few remotely operated vehicle (ROV) and manned submersible operations (Gerringer et al. 2017). Although several vehicles successfully dove to the Challenger Deep (i.e., Trieste I, Deepsea Challenger, ROV Nereus, and ROV Kaiko) (Barry and Hashimoto 2009; Bowen et al. 2009), studies of the bottom of the hadal zone mainly rely on deep-sea landers as deep-sea equipment platforms. Deep-sea landers are equipped mainly with sampling equipment, such as Niskin bottles and biological traps.

Better understanding of the chemical environment at the bottom of the hadal zone will require study of the dissolved gas components in the bottom seawater, which will entail obtaining gas-tight samples (Dodd et al. 2006). In this study we designed and tested a gas-tight pair sampler for the collection of water samples from the hadal zone. The new sampler uses two titanium bottles and one sampling valve to collect two samples at once. The sampler was used successfully on two deep-sea landers in the Mariana Trench during a recent cruise, and about 3 L of gas-tight water samples were obtained for the first time from the bottom of the Challenger Deep.

2. Methods

a. GTP sampler

The new gas-tight pair (GTP) sampler was designed to be deployed in the deepest ocean trenches to collect two water samples at one time with two titanium bottles. The GTP sampler (Fig. 1) consists of one motor-driven sampling valve (see section 2b for more details) and two sampling cylinders. Each sampling cylinder contains a sample chamber and an accumulator chamber. A custom-designed stop valve for gas precharge is integrated into the accumulator chamber to make the structure more compact. The two sampling cylinders are connected to the sampling valve through high pressure fittings, and they are further fixed by a plastic frame. A sampling tube that connects with the inlet of the sampling valve by a soft polyether ether ketone (PEEK) tube is used to collect the fluid sample from a target point.

Fig. 1.
Fig. 1.

Structure of the GTP sampler.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

Most of the sampler’s components, including the sampling cylinders, sampling valve, and circuit chamber, are made of titanium alloy (Ti6Al4V) in order to withstand high pressure and to reduce weight. The GTP sampler is 52 cm long, 33 cm wide, and 9 cm high without the sampling tube. It weighs approximately 18 kg in the air and 12 kg in seawater. The sample volume of each sampling cylinder is approximately 150 mL.

The working principle of the GTP is similar to that of previous gas-tight samplers (Seewald et al. 2002; Wu et al. 2011). Before deployment, both the sample piston and the accumulator piston are placed at the front of their respective chambers. The sampling tube and the dead volume between the sample piston and the sampling valve are filled with deionized water. Meanwhile, the accumulator chamber is filled with compressed gas (e.g., nitrogen or argon), and its pressure is adjusted to an appropriate value according to deployment depth. When ready to collect fluid samples, a trigger signal is produced manually or automatically (depending on the working mode of the sampler; see section 2c for more details), and the sampling valve is opened by the actuator. Fluids are permitted to fill the sample chamber, and the gas in the accumulator chamber is further compressed until it reaches the pressure balance. The fluid samples are sealed in the sample chamber when the sampling valve is closed.

The GTP sampler can be used on manned submersibles, ROVs, and deep-sea landers. It can be controlled in real time by scientists on board the ship or inside the submersible when it is deployed by ROV or manned submersible. A graphical user interface (GUI) allows the scientists to operate the sampler on a laptop. The sampler can also work as a stand-alone device if it is deployed on a deep-sea lander.

b. Sampling valve and actuator

1) Sampling valve

The sampling valve is one of the most significant components of the GTP sampler (Fig. 2). It is expected to work properly under 115 MPa of hydrostatic pressure to meet the requirement of deployment at full ocean depth. Based on the working principle of the sampler, the sampling valve also must have bidirectional sealing capability to ensure the success of the sampling process. The sampling valve must insulate the sampling cylinders against outside high pressure seawater when the sampler is descending to the seafloor, and it also must adjust to the gradual reduction of ambient pressure to atmospheric pressure when the sampler is ascending to the surface during recovery. The valve also must reliably seal so as not to allow leakage of the fluid samples, thus keeping the samples at nearly in situ pressure and gas tight. Therefore, the sampling valve and the whole sampler must be able to resist ultrahigh pressures from both outside and inside.

Fig. 2.
Fig. 2.

Schematic of the pressure-balanced sampling valve.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

To meet the bidirectional sealing requirement and to achieve a compact configuration, we used a custom-made sampling valve. A special feature of the sampling valve is its pressure-balanced valve poppet. The sampling valve employs two sliding O-ring seals and a cone seal consisting of a titanium valve poppet and a PEEK seat (Fig. 2). Because the two sliding O-ring seals and the valve seat have the same diameter, the pressure forces on the valve poppet are always equal and opposite no matter which port is subjected to high pressure. A return spring provides an initial contact sealing force between the valve poppet and seat. The value of return spring force was determined by finite element analysis and laboratory experiments. Finite element analysis based on the titanium poppet–PEEK seat contact model was established to study the sealing performance of the sampling valve. The specific method of finite element analysis has been presented in reference (Wu et al. 2010). Finally, the spring force was set to be 800 N in this case to guarantee a reliable sealing under high pressure.

The previous sampling valve (Wu et al. 2011) adopted a self-tightening design that made use of the ambient pressure to produce an extra sealing force on the valve poppet. It can make the valve a more reliable closure under high pressure. However, it also needs more driving force to open the valve. The axial force on the valve poppet would be over 4000 N at the bottom of the Challenger Deep if we still adopt the previous design, which makes opening the valve very difficult. By contrast, the poppet of the GTP’s sampling valve is always pressure balanced no matter which port is subjected to high pressure. One obvious benefit is that the sealing force does not change with the pressure of the samples. Therefore, the valve actuator needs only to overcome the spring force and a slight friction force to move the poppet and open the valve.

2) Actuator

The actuator uses a screw drive to convert the rotary motion of a direct current (dc) electric motor to linear motion, which can provide large thrust to control the sampling valve. The motor chamber has four through grooves in the axial direction, which are in alignment with the grooves of the motor support sleeve (Fig. 3). The motor, which is fixed with a support sleeve, can move only in the horizontal direction as a result of the presence of steel balls embedded in the grooves. The motor shaft drives the screw to move forward and backward to open and close the sampling valve, respectively. Two switches are connected to the motor to provide position information to the control system so that the actuator can control the accurate stroke of the sampling valve.

Fig. 3.
Fig. 3.

Schematic illustration of the sampling valve actuator.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

In the current design, a Maxon dc motor (DCX22L) with a planetary gear box (GPX22HP) is used for the actuator. Its output can reach about 4-KN linear thrust at the rated speed. The actuator offers small volume, large force, and good controllability. It may prove to have important applications in other deep-sea equipment.

c. Sampler control and operation

1) Electronics

The main functions of the electronic control system are to control the valve actuator and to record the measurements of electric current, voltage, and temperature. An MSP430 microcontroller unit is used for the circuit design because of its abundant on-chip peripherals and ultralow power consumption (Fig. 4). Two additional important functional modules of the circuit board are the motor controller and the signal conditioning module, which drive the dc motor and process the feedback switch signals and the current, voltage, and temperature, respectively. The circuit board also has a 32-Mb flash memory card for data storage and uses an inductively coupled link interface to communicate with the control software.

Fig. 4.
Fig. 4.

Functional block diagram of the hardware architecture.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

2) Software

The program code for the microcontroller accommodates two working modes of the sampler. In the remote control mode, the microcontroller operates completely according to the commands sent by the host computer. For stand-alone autonomous operation, the microcontroller automatically records all in situ data periodically and drives the motor to open and close the sampling valve based on the preset time. A GUI was developed using LabVIEW software to facilitate control of the sampler with the host computer by common users (Fig. 5). This GUI is used to send commands to the sampler and to monitor the feedback data. The operation parameters of the sampler can be easily reconfigured by the GUI for autonomous working mode.

Fig. 5.
Fig. 5.

GUI of the sampler that enables the scientists to operate the sampler on a laptop.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

3) Operation

In the current design, the GTP sampler can be operated either by remote control via real-time communication with a submersible or in autonomous mode as a stand-alone device. Therefore, it can be deployed by a variety of vehicles, such as ROVs, manned submersibles, and deep-sea landers. However, only deep-sea landers are currently available to investigate the hadal zone at depths beyond 7000 m. When deployed by a lander, the sampler will work as a stand-alone device in autonomous mode and will take samples according to a preset procedure. Parameters that must be set prior to deployment include the start and end times of the sampling process, and these are based on when the lander is expected to arrive and depart from the seafloor.

3. Results

Three sets of the GTP sampler were constructed and field tested in the Mariana Trench during the cruise TS-03 from 15 January to 23 March 2017, which was organized by the Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences. The samplers were deployed 10 times by two deep-sea landers (Wan Quan and Yuan Wei Shi Yan) in the Challenger Deep (Fig. 6). During the deployments, each sampler was set to work as a stand-alone device to collect samples automatically at the seafloor, and they were recovered together with the deep-sea landers. As a result, 3 L of water samples were successfully obtained from the bottom of Challenger Deep, which was indicated by the pressure measurement on board the ship.

Fig. 6.
Fig. 6.

Collecting fluid samples by using GTP samplers from the bottom of Challenger Deep. GTP samplers (along with other instruments) were deployed by the deep-sea landers (a) Wan Quan and (b) Yan Wei Shi Yan.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

Table 1 shows that most of the samples were maintained at nearly in situ pressure, although some samples experienced a slight pressure drop as a result of the expansion of the sample cylinder. The obvious pressure loss of the sample collected by GTP-A is believed to be attributed to the slight leakage of the sampler, which was confirmed by onboard examination of the sampler afterward. The discrepancy in pressure between GTP-B and GTP-C was mainly due to the different precharged pressure of the gas in the accumulator prior to deployment.

Table 1.

Usage records of the GTP sampler in Challenger Deep.

Table 1.

Because we were unable to analyze the gas components of the fluid samples on board the ship during this cruise, it was necessary to transfer the samples to gas-tight bottles for analysis on land. Once the sampler was recovered, we connected the sampling cylinder to the subsampling setup (Fig. 7). Before connecting the gas-tight syringe, we let a small amount of the sample bleed from the tee connector to make sure there was no air trapped inside the tubing connected to the sample chamber. Extraction of the fluid sample was achieved by gently opening the microflow control valve. Water was simultaneously pumped into the accumulator chamber to maintain constant pressure within the sampler. After a 50-mL aliquot of fluid was withdrawn, the syringe extracted 10 mL of helium gas, which was then placed in the refrigerator for 12 h. Afterward, the headspace gas of the syringe was injected into a vacuum bottle for onshore laboratory analysis.

Fig. 7.
Fig. 7.

Schematic illustration of the subsampling setup.

Citation: Journal of Atmospheric and Oceanic Technology 35, 4; 10.1175/JTECH-D-17-0170.1

Bottom seawater samples were also collected with Niskin bottles mounted on the landers to allow a direct comparison. Quantitative chemical analysis of part of the samples has been completed. Measured concentrations of H2 and CH4 in fluids collected by the GTP sampler were one order of magnitude higher than values determined in fluids collected using Niskin bottles (Table 2). This discrepancy demonstrates the need for gas-tight samples from the hadal zone for a better understanding of the hadal environment. It also illustrates the effectiveness of the GTP sampler.

Table 2.

Comparison of the gas concentrations in fluid samples collected by the GTP and Niskin bottles.

Table 2.

4. Conclusions

A new gas-tight pair sampler capable of collecting two fluid samples from a depth up to 11 000 m was designed and tested on two deep-sea landers. Gas-tight fluid samples were successfully obtained with the new sampler from the bottom of the Challenger Deep. Although the GTP is currently being used to study the dissolved gas components of the water samples, this sampler will be a powerful tool for collecting microorganism samples for subsequent cultivation after the thermal insulation of the sampling cylinder is improved. We believe the sampler will have wide application in research of the environment and life processes of the hadal zone.

Acknowledgments

We are grateful to the crews of R/V Tan Suo Yi Hao and the deep-sea landers Wan Quan and Yan Wei Shi Yan for their support during the sea trial. We are also grateful to Dr. Ji-wei Li, Shun Chen, and Meng-ran Du for their help during subsampling and analysis of the samples. This work was supported by the National Key Research and Development Program of China (2016YFC0300500) and the Hadal Trench Research Program (XDB06040300) of the Chinese Academy of Sciences.

REFERENCES

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    • Crossref
    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
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    • Crossref
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    • Crossref
    • Search Google Scholar
    • Export Citation
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    • Crossref
    • Search Google Scholar
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