US Dept of Commerce NOAA / OAR / ERL / PMEL

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Hydrothermal Venting Project

Vents Program

Accomplishments in FY 98

In September, 1998, Vents scientists, in a partnership with NSF, Sea Grant, and NURP, began the establishment of a long-term seafloor observatory (the New Millenium Observatory, or, NeMO). The NeMO site includes the entire summit caldera of Axial Volcano, including the area of the eruption. The month-long research cruise to NeMO on the NOAA Ship Ronald H. Brown, which carried the highly sophisticated deep water remotely operated vehicle (ROV), ROPOS, proved to be the most successful international, interdisciplinary Vents seafloor program to date. The January eruption at Axial was verified and intensely surveyed during the cruise. Fluids from hydrothermal vents created by the eruption were sampled and it was discovered that all samples of the fluids contained high-temperature bacteria. A multitude of wide-ranging instruments were deployed at the eruption site to continuously monitor geological, chemical, and biological changes resulting from the eruption cycle.

Other specific accomplishments during the FY 1998 field season included:

• The 23-day ROPOS operations at NeMO included 21 dives consisting of 252 hours of seafloor time. High resolution maps were made with the new Imagenex sonar system mounted on ROPOS and, combined with seafloor video traverses, provided unprecedented views of the relationships between geology, hydrothermal venting, and biology at the eruption site. An array of temperature monitors were left at vents in and around the eruption site which will provide data on the rate of cooling of the system.
• Another year in the unique Vents annual time series plume mapping over the Cleft segment was successfully completed. Plume activity at south Cleft, especially in terms of particle concentrations, remains intense. Plume activity at north Cleft also appeared enhanced compared to the previous year. An extensive plume monitoring program was conducted over the summit and flanks of Axial Volcano during the NeMO cruise. When combined with plume surveys carried out in FY97, Vents scientists now have the best “before and after” plume data of any event yet studied. This year, continuous time-series observations were significantly extended at Axial, including deployment of six MTR(Miniature Temperature Recorder) moorings and three MTR/current meter/MAPR (Miniature Autonomous Plume Recorder) moorings.
• During the NeMO cruise, SUAVE (Submersible System Used to Assess Vented Emissions) was deployed on ROPOS for half of the total dives. SUAVE was engaged in thermochemical surveys of seafloor venting at a variety of sites in the vicinity of the Axial caldera. These measurements will be used to determine temperature and the concentrations of Mn, Fe, and H2S for comparison to historical data for the Axial region, and also to delineate spatial variability in these parameters as it pertains to processes occurring below the seafloor.
• In response to the week-long series of earthquakes at Axial Volcano in January, the NOAA/NSF-sponsored Axial Response Team conducted response cruises in February and again in August and September to map and sample hydrothermal plumes over the region. The particle maxima within the caldera were both higher in the water column and more intense after the eruption than before it. Vertical profiles of particulate Fe and S over the eruption site show very high concentrations from about 1400 m to the bottom. A unique feature of this data set is the very high concentrations of elemental sulfur in the lower 60 m of the water column. This enrichment is highest in September, approximately eight months after the eruption. The concentrations of elemental sulfur, nonvolatile bacterial sulfur filaments in the water column appear to increase over time. The sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time.
• During the Ron Brown cruise, Vent scientists successfully used, for the first time, a new, ROV-mounted, multi-purpose sampler to collect diffuse and focused high-temperature vent fluids and suspended particles with in-line temperature measurement for chemical and microbial analysis. Simultaneous in-situ filtration and fluid collection is opening new avenues of research. The new sampler is well suited to collecting samples in very low flow-rate environments. Combined with discrete fluid sampling using Alvin in July, 1998 and combined with in-situ SUAVE analysis and fluid sampling by ROPOS in September, 1998, Vents scientists now have good spatial coverage and time series information of seafloor vents at the January 1998 eruption area at Axial Volcano. Preliminary results from the water-column sampling showed very large helium-rich plumes over the eastern portion of the Axial Volcano caldera in the vicinity of the new lava flow.
• Vent scientists mounted an expedition to sample hydrothermal vents along the super-fast spreading Southern East Pacific Rise between 5 degrees and 32 degrees south latitude. This cruise included time-series sampling of vent fields previously sampled in 1994 and 1997, as well as sampling of additional previously unexplored sites in the region. The results of this cruise will contribute significantly to understanding global variation in mid-ocean ridge fluid chemistry.
• A survey of Heceta Bank using a multibeam sonar system was conducted in May, 1998. The purpose of the survey was to investigate the geology of groundfish habitats off the Oregon Coast and was conducted in conjunction with the Northwest Fisheries Science Center and Oregon State U. Sea Grant. The data reveal rich details of the geologic structures that form the bank which forms a diverse structural habitat for local groundfish populations. These data are expected to be the first collected in what is envisioned to be a major new research collaboration between NMFS and OAR focused on critical fish habitat.

Other major activities and accomplishments included:

• The utilization of the U.S. Navy’s SOSUS arrays has enabled Vents scientists to monitor, in real-time, the low-level seismicity of the North Pacific and the very low level volcanic seismicity of the northeast Pacific spreading center systems (including the Juan de Fuca ridge system and Axial volcano). The Vents acoustic detection and location activity is now fully operational and includes continuous analysis of the real-time data. This monitoring has provided the critical information to enable rapid responses when eruptions, e.g., the 1998 Axial Volcano eruption, are detected. Seismicity of the equatorial east Pacific also continues to be monitored in a non-real-time mode using an array of six autonomous hydrophone instruments moored on the east Pacific Rise.
• Effects of time-variable currents on the transport of heat from a vent were examined using a combined modeling and field-data analysis approach. Tidal currents, particularly, cause water to pass back and forth over vents with the result that venting results in irregular, patchy hydrothermal plume distributions in the water column. This model study helps to quantify, for example, errors in estimates of vent field heat flux based on such undersampled plume data.
• The influence of ridge crest topography in steering flow was evaluated using a three-dimensional time-dependent numerical model of baroclinic circulation. Oscillatory currents are rectified and intensified near the sea floor by ridge topography. The model shows that tidal forcing cannot be responsible for the observed mean circulation that characteristically flows in opposite directions on each side of the ridge. Instead oscillatory forcing at lower frequencies seems to be the cause, the most probable forcing mechanism being variable pressure gradients caused by the passage of atmospheric storms over the site. During the year, 18 papers were published in refereed journals resulting from Vents research efforts.

Vents Program

Plans for FY 99

• When volcanic events along northeast Pacific seafloor spreading centers occur, Vents scientists will respond in order to continue studying the physical, thermal, and chemical characteristics of hydrothermal plumes as well as hydrothermal fluids venting on the seafloor. A major goal of event response is also to survey and sample microorganisms which are now know to be associated with venting from new eruption sites.
• The second annual NeMO cruise will take place in the summer of FY 1999. Activities will include deployment of geophysical and hydrothermal instrument systems, sampling and mapping of thermochemical, suspended matter, and water column properties, and continued geological mapping of the Axial eruption site.
• A state-of-the-art, and first of its kind, real-time buoy will be deployed at NeMO. The buoy will make it possible to access video, chemical, and biological sensors in near real-time.
• Deployment of a North Atlantic autonomous hydrophone array, developed by Vents, in the North Atlantic. Acoustic data will continue to be collected from autonomous hydrophones deployed in the eastern equatorial Pacific.
• Physical, chemical, and biological time series data will continue to be collected at the NeMO observatory in the Axial Volcano caldera. Analysis and interpretation of previously collected samples will continue.
• A Vents/NSF collaboration to develop an acoustic horizontal deformation system will be deployed at NeMO and a site along the southern Juan de Fuca Ridge. This system is designed to provide information about crustal deformation associated with volcanic events.
• Data from the southern East Pacific Rise Study will be analyzed and interpreted in a collaboration with scientists from the University of Hawaii. The data will be used to compare chemical processes and hydrothermal source strengths on the Juan de Fuca Ridge with those of the Southern East Pacific Rise. This study is part of an effort to compare the volcanic and hydrothermal characteristics of a medium rate seafloor spreading center (the Juan de Fuca Ridge) and those of the highest rate spreading center region in the world, the Southern East Pacific Rise.
• Moorings at NeMO are providing measurements of temperature variability of both high-temperature and diffuse heat sources of hydrothermal heat. These measurements will be used with current-flow data in an inverse calculation that will attempt to pinpoint new vent locations.
• The three-dimensional baroclinic circulation model will continue to be used to study effects of topography on flow above ridge topography.