Chunyang Tan

Development of an In Situ pH Calibrator in Deep Sea Environments

The pH Calibrator is an instrument for in situ calibration and measurement of chemical species (pH) in aqueous fluids at elevated pressure. The calibration device consists of two interactive systems: a data-logging sensor system and an automated fluid de- livery system. The solid-state pH sensor array is sealed in a flow cell to decrease measurement response time. The automated fluid de- livery system consists of a pump and process control valves, which intermittently deliver seawater samples for measurement. Following measurement cycles, the pH buffer solutions are drawn into the sensor cell to perform two-point calibration. Here, we describe the initial steps in its conversion from a battery-powered instrument for short-term application into a unit that can be used to take full advantage of cabled-powered observatories that will soon be coming online at a number of locations in marine environments. Accordingly, we made use of the continuous power supply and TCP/IP network capability intrinsic to the Monterey Accelerated Research System (MARS)-cabled ocean observatory, Monterey Bay, CA, to reconfigure the unit in anticipation of future seafloor deployments. The flow through system has been further optimized to reduce dead volume effects in the calibration process, while the mechanical system can now better tolerate increases in fluid discharge pressure and flow rate for long-term operations. Furthermore, the sensor head is now equipped with a novel valve to enhance operations in hydrothermal diffuse flow environments. To gain full access to the MARS-cabled observatory, the dc-dc power converter module and Ethernet to serial module are integrated to the electronics. The pH Calibrator has passed the wet node simulator tests in a seawater tank at the Monterey Bay Aquarium Research Institute, moving the project closer to fulfilling long-term objectives for marine studies.

PHYSICAL AND CHEMICAL CHANGES FROM BASALT-CO2-RICH FLUID INTERACTION DURING FLOW-THROUGH EXPERIMENTS AT 150°C AND 150 BAR

To further our understanding of the relationship between environmental change and hominin evolution, at an important archeological and paleontological locality, XRD bulk analysis was completed on 1,183 core samples from a drilling core collected in West Turkana, Kenya. The core itself covers about 1.354 -1.85 Ma. Most minerals present are detrital feldspars, mica, and quartz. The authigenic minerals present are mostly carbonates, zeolites, and sulfur bearing minerals such as gypsum and pyrite. The few metal oxides present may be derived from paleosols in the core. This particular study is focused almost entirely on the data gathered from the XRD analysis of clay samples. We analyzed 70 samples from the submicron fraction for clay mineralogy. Both oriented and randomly oriented analyses were completed in order to characterize the clay minerals present.

Accurate pH measurement and determination in deep-sea environments by an in-situ pH sensor calibration device

An in-situ calibration method to accurately and precisely constrain the pH of deep sea environments is reported. This method involves the pre-evaluation of the Ir-IrOx|Ag-AgCl electrode in the laboratory, followed by the in situ calibration using pH buffers at the ocean bottom. The standard potential E° and in situ slope are derived by the calibration process. This calibration method was implemented in an in situ calibration device (pH Calibrator) and used in the seawater column and hydrothermal diffuse flow measurements. The precision, accuracy and stability of the pH value determined are sufficient for many geochemical processes.

An in-situ chemical sensor system for cabled ocean observatory applications at hydrothermal vents

An in-situ chemical sensor system was developed for long term monitoring of hydrothermal vent fluids as part of the US NSF Ocean Observatory Initiative (OOI) Regional Scale Nodes (RSN) in the Northeast Pacific Ocean. The sensor wand design is based on Yittria-stabilized zirconia (YSZ) ceramic membrane electrode in conjunction with Ag|AgCl reference electrode, Au or Pt redox electrode and Ag|Ag2S (H2S) electrode. The YSZ ceramic electrode was constructed with a titanium tube for the long term deployment application. To achieve high measurement accuracy, the high impedance signal of the YSZ ceramic electrode was converted to the low impedance signal by a pre-amplifier. The instrument utilizes the power supply and bandwidth of the fiber optic ocean cable to conduct continuous monitoring of the hot vent fluids. The instrument was deployed on Regional Scale Nodes (RSN) Axial Seamount site via MJ03C mid-power junction box. A modified version of the instrument was also deployed multi times by DSV Alvin during AT26-17 cruise on Axial Seamount and Main Endeavour Field (MEF) vent field, Juan De Fuca Ridge. This integrated electrochemical system has successfully measured pH, dissolved H2 and H2S of hydrothermal vent fluids at 300 °C and 220 bars.

Deployment of an in-situ pH calibrator on MARS cabled ocean observatory

A recently developed in situ pH measurement and calibration system (pH Calibrator) has been deployed on MARS (Monterey Accelerated Research System) cabled ocean observatory located at Monterey Bay, California. The deployment took place at the depth 880 m, and lasted for more than two months. A key aspect of the instrument is user-defined auto-calibration, enhancing the accuracy of pH measurements in harsh, high-pressure environments, over long intervals, as reported here. Thus, the instrument takes full advantage of cable power and internet connectivity permitting real-time monitoring of pH, with calibration scheme optimized to achieve specific scientific objectives in response to local environmental factors. Here we report recent modifications to the instrument, which enhanced its effectiveness during the MARS cabled ocean observatory deployment.