Cruise Log: August 6, 2008
Brief Update: You may be wondering what we’ve been doing the past 48 hours. The winch that lowers Jason has been malfunctioning. After the last lowering of Jason, when it reached the bottom, the winch was no longer able to effectively ‘reel in’ Jason. So we have been spending the past many hours, very slowly and painfully bringing Jason back to the surface. It finally, much to the relief of everybody, reached the surface this morning at 5AM. The engineers have been working around the clock to try and figure out what the problem is, called the manufacturer etc. but don’t have a solution yet to what has caused the failure. With much seafloor time lost, and the uncertainty of what is wrong with the winch, we have resolved to conclude our dive schedule and will head back to Miami later this evening. The scientific secrets of Snake Pit vents will have to remain hidden until another scientific expedition visits this remote part of the Northern Atlantic Ocean. Keep checking the updates, as on the transit we will still be analyzing data and growing new microorganisms; and providing you with vignettes of life at sea on the R/V Revelle… You may have noticed the reference “Ghost Buster sensor” in our updates. These sensors were developed by Drs. Kang Ding and Bill Seyfried. Here Bill describes some of the aspects of these sensors. |
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In-situ Chemistry of Hydrothermal Vent Fluids It is well known that redox components and pH of hydrothermal vent fluids potentially provide fundamental constraints on mass transport processes at depth in the ocean crust. Moreover, these species can also be linked to metabolic pathways of microbial communities inhabiting near vent environments, underscoring the need to develop technologies that allow redox and pH measurement and monitoring at a wide range of temperatures and pressures. Since the discovery of deep-sea hydrothermal vents more than 30 years ago, numerous efforts have been made to constrain the pH and redox conditions of so-called end-member hydrothermal fluid venting from chimney structures on the seafloor. In almost all cases, however, this has required quenching fluid samples and then correcting for temperature and pressure using available thermodynamic data. Unfortunately, the lack of thermodynamic data, especially in the near critical region of seawater has effectively precluded unambiguous interpretation of these geochemically critical parameters. Thus, motivated by this need, the research group from the University of Minnesota participated in the cruise to test the performance of a new generation of solid-state electrochemical sensors for hydrothermal application. |
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In general, three different chemical sensor systems were used during the cruise. For short-term measurements, an in-situ sensor package operated directly in real-time from within the Jason 2 van was utilized (Fig. 1). The second sensor system deployed during the cruise, however, was optimized for longer-term monitoring studies. The data-logging in-situ sensor system was deployed at Rainbow, Lucky Strike and Lost City. It is important to emphasize the difficulty encountered in locating substrates suitable for deployment of the chemical sensor and associated data-logger. The tall chimney structures rising from inclined basal mounds that dominated hydrothermal vents at virtually all sites proved unusually challenging for effective deployment. That these challenges were met successfully was due in large part to the considerable skills of the Jason pilots and their support team. The third chemical sensor system that was deployed during the cruise involved a recently developed auto-calibrating system for pH monitoring in diffuse-flow hydrothermal vent fluids. Previous studies (lab and field-based) by the UM group have indicated the need for calibration cycles, even when relatively stable solid-state Ir electrochemical cells are utilized for pH measurement. The auto-calibrating system was initially tested on the wire-line at an intermediate depth to check automation schemes involving the optimum timing of standardization, pH measurement, and seawater rinse cycles. This pre-test was successful. Thus, subsequently, the system was deployed for approximately one day in diffuse flow vent fluids in the vicinity of Marker 6 at the Lost City Hydrothermal System. Data indicate successful performance of in-situ standardization and measurement cycles. The observed difference between the two on-board pH buffers revealed values consistent with theoretical predictions, while the measured pH of the vent fluid is estimated to be slightly less than 8 at vent conditions (~11-12˚C). |
Figure 1: In-situ redox and pH measurements being made at Rainbow vent (Site X3) where highly reducing and low pH hydrothermal fluids are indicated. The low in-situ pH of the vent fluids is surprising in light of the ultramafic lithologies that hosts the hydrothermal system
Figure 2: In situ chemical datalogger and pH calibrator deployed at Lost City. |
It is becoming increasingly clear that geological, geochemical and biological processes at MORs (mid-ocean ridges) are inherently dynamic, underscoring the need for the development and testing of novel in-situ instrumentation. Objectives that were significantly furthered during the present cruise. |
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