REVEL
Will we discover a new vent field?

During the University of Washington ALVIN dive program in the summer of 1995, a water column temperature anomaly, dubbed "Mothra", was located during the nighttime hydrographic program on the Endeavour Segment (see T1 on Mothra Map). The plume, located approximately 3 km south of the Main Endeavour Field (MEF), was surveyed during 13 "tow-yo" operations using a CTD package towed in a sawtooth (up and down) pattern through the water column along and across the hydrothermally active axial valley. These surveys allowed real time characterization of the water mass overlying the ridge axis indicating an extensive hydrothermal plume that was distinct from the MEF plume to the north. Based on results from this survey, a generalized target located at 47o55.46'N and 129o06.51'W was the most likely location of a possible fourth hydrothermal vent field on the Endeavour Segment (see T1 on Mothra Map).

During the REVEL cruise, CTD operations are being conducted complementary to ROPOS operations during down times of the ROV. The main goals of the CTD study are to provide accurate dive coordinates for ROPOS to visit the source of the Mothra anomaly, to characterize the size of the venting area, and to characterize the chemical and thermal output of the source(s). Based on previous work, a series of 23 vertical water casts spaced 50 meters apart, and 2 tow-yo surveys (up-down cycling of the CTD package while the ship is underway) have been completed in the vicinity of the 1995 target to delineate the Mothra field.

Precision maneuvering of the Thompson and excellent long base line navigation allowed superb definition of the spatial relationships within the area of the Mothra anomaly. During the initial operations, two strong transmission and potential temperature anomalies were consistently detected at water depths of ~1900 m and 2045 m. The anomalies showed an increase in hydrothermal particulates in the water column and a corresponding increase in the temperature difference between hydrothermal fluids in the plume and background ocean water. The upper temperature anomaly at ~1900 m is believed to reflect input from both the southward migration of neutrally buoyant hydrothermal fluids from the MEF, and possibly from the Mothra source. Characteristics of the deeper plume are consistent with an intense heat and particulate source located near the initial target identified in the summer of 1995.

On August 17, during a tow-yo survey (RVLT08) initiated north of the MEF and continuing south of the potential Mothra target, an incredibly intense temperature anomaly was detected (0.35oC) at a water depth of ~2225 m, ~17 m above the seafloor. This anomaly is one of the largest anomalies detected to date. We believe it reflects the buoyant rise of hydrothermal fluid directly below the position of the CTD and strongly indicates actively venting structure(s) nearby.

Based on the position of the August 17 anomaly, a tight N-S trending tow-yo survey (RVLT09) was conducted during the morning of August 18). To accurately delineate the location of the temperature anomaly source, the Thompson was precisely maneuvered at 0.2-0.3 knots into a succession of positions and the CTD package was vertically cycled at 30 meters per minute in the interval of the anomalous temperatures. Two additional sharp temperature excursions were detected indicating an actively venting Structure(s) within less than ~50 m of 47o55.52'N and 129o06.43'W (see T2 on Mothra Map).

The survey results provide an excellent target for a follow-on ROPOS dive to investigate the source; our hope is that we will discover a major new vent field! The results also show that the CTD package provides a powerful tool for locating and refining the location and character of unexplored sources of hydrothermal effluent. The timing of a Mothra dive will be determined by weather and the demands of other portions of the science program. The results will be reported here on the day following the dive.