March 6, 2003: Leg 2, Day #5

One of the newer tools that we are using in the Guaymas Basin dives brings together two in situ chemical sensors. The In-Situ Ultraviolet Spectrophotometer (ISUS) that uses the fluid's absorption spectra to determine the quantity of bisulfide (HS-). Because the high temperature vents contain large amounts of hydrogen sulfide (H₂S) we would expect to measure high levels of bisulfide in the mixed peripheral plumes. The plumes also contain large amounts of methane, derived from either thermal or microbial degradation of organic material. This combination of methane and hydrogen sulfide would significantly impact the oxidation/reduction potential (Eh) of the local fluid due to the addition of these two highly reduced constituents. 

A month or so before the ship sailed for La Paz, we saw an opportunity to combine an in situ Eh sensor from Ko-ichi Nakamura from AIST in Japan with the ISUS developed by Ken Johnson's group at MBARI (http://www.mbari.org/chemsensor).

The "Plume Sniffer" has a snorkel that sucks fluid past the platinum electrodes of the Eh Meter, through the optics of the UV Spectrophotometer, and then past a thermistor to measure the fluid temperature. This entire system, completed during the port stop in La Paz, is mounted on the ROV Tiburon swing-arm. 

During the first two dives, we have run the pump and measured the Eh and bisulfide continuously. Although there have been small problems with logging and keeping mud out of the inflow system, the data—available in real time in the control room—has been very consistent. For both low temperature diffusive sites, as well as the large focused hydrothermal sites, the bisulfide and Eh change continuously beginning several 10's of meters from the vent. The bisulfide increases as the Eh decreases showing that the fluid is becoming more reduced with increase in bisulfide and/or methane. We observed changes even over small microbial mats with no observable venting! This suggests to us that this Plume Sniffer is more sensitive than humans looking for flow and would be especially useful in the overlying water column.

Below is a sample of data from Dive T522 that shows the variations we observed working around the vent areas.