1 OVERVIEW OF NeMO'99 - Bob Embley

During the NeMO99 cruise, ROPOS recovered more than 260 geologic, biologic and chemical samples, retrieved 29 experiments and and instruments, and deployed 25 experiments. ROPOS made 18 successful dives totaling 204 hours of bottom time in 21 days on site. Despite major problems with the hydraulic systems on the ROPOS, which caused us considerable downtime, the hardworking ROPOS team and the efforts of the science party were able to recover to make this year's cruise rival the success of NeMO98.

The chemical and biological effects of the 1998 diking event are still apparent on the eastern side of the caldera where the lava flow was erupted. Vigorous hydrothermal venting continues in many places (e.g. Marker 33, Cloud, and Magnesia), whereas there has been reduction or cessation in others. Although the amount of visually apparent microbial activity (such as the white floc in the water column) decreased, "snowblower" activity continued at some vents (e.g. Magnesia Vent) and hyperthermophilic microbes were again found in abundance at most of the diffuse (low temperature) sites, (whose temperature is well below their optimal growth rate). These observations show that there is still enhanced subsurface microbial growth more than a year and a half after the eruption. Of particular interest was the discovery of a hyperthemophile growing optimally at 102C.

Colonization of the new vents continued. Tubeworms had arrived at Marker 33 and were present in greater densities at sites that only contained small patches in 1998 (e.g., Nascent Vent). Analyses of the water samples, biological grabs, and video observation should tell us much about the evolution of the chemical and biological systems on the summit of Axial.

Mapping of the new lava flow and associated hydrothermal system continued in 1999. We located the (probable) southern end of the lava flow about 6 kilometers south of the caldera along a spectacular fissure system that was followed for hundreds of meters. The eruption was entirely contained in the 1 to 2 meter wide fissure until it overflowed its fissure and erupted in the large mound of lava that was found in 1998 at 4552'N. Farther north towards the caldera, the trace of the 1998 eruption became more confused as the slightly older eruptions mixed with the 1998 lava. Several new vents (Bag City, Crevice, Coquille) were found south of Marker 113 (the southernmost point investigated in 1998). The vent fluid sampler recovered 113 water and filter samples from all the (still active) major vents.

The ASHES vent field was only visited on two dives in 1999 because of the early downtime of ROPOS, and part of one dive was again spent at the CASM vent field in the northern part of the caldera.

Active hydrothermal venting was only found on the summit of Axial, probably where the underlying zone of molten lava permanently resides. The summit area contains hydrothermal systems formed during several different volcanic or tectonic (earthquake generated) events, providing a natural laboratory for studying colonization and evolution of these unique chemosynthetic ecosystems.