2.2 CHEMISTRY
2.2a Ron Brown, ROPOS 2000 Cruise Report -Dave Butterfield
The vent fluid sampling program has two main goals this year: 1) to re-sample the diffuse vents along the 1998 eruption zone and adjacent older lavas and 2) to re-sample hot vents within the ASHES vent field. The samples collected will allow us to evaluate sub-seafloor reaction processes, contribute to our understanding of the nature of different sub-seafloor hydrothermal reservoirs, and relate the chemistry of hydrothermal fluids to the geologic surroundings and the microbial community structure. We are tracking the evolution of fluid composition at Axial to understand long-term and post-eruption changes in the hydrothermal system. The fluid chemistry data provide important environmental constraints for the study of macrofaunal and microbial communities.
The primary sampling tool for vent fluids is once again the Hydrothermal Fluid and Particle Sampler, commonly known as The Beast. The Beast has a ½" outer diameter fluid flow pathway of titanium and teflon, 6 PVC pistons (5 used for gas analysis and 1 for general chemistry), 8 bag samplers (Kynar inert plastic). There are 10 filter sets to collect particles without water, and the default filter setup for this cruise has been four 0 .45 micron polycarbonate membrane XRF filters, one 0.2 micron lipid filter, 2 double in-line 3 micron/sterivex combination filters, two 0.2 micron membrane filters for Fluorescent In-Situ Hybridization (FISH), and one sterivex filter for DNA extraction. In addition, two of the bag samples had in-line 0.45 micron polycarbonate filters. The Beast was transferred to the Brown on July 4th from the Atlantis, along with Dave Butterfield, Kevin Roe, and Ben Larson. Chris Meinig and Nick Delich provided important shipboard support. The Beast provides samples for fluid chemistry (Dave Butterfield, Kevin Roe), particle chemistry (Jim Gendron/Richard Feely), microbiology (Julie Huber, Mausmi Mehta), rare gas chemistry (Leigh Evans/John Lupton), shipboard methane/hydrogen/nitrous oxide measurement by GC (Ben Larson/Marv Lilley), carbon isotopes (Kim Juniper), and various other analyses. Samples are divided and preserved for extensive shore-based analysis.
Shipboard analysis this year included gas chromatography for methane/hydrogen and nitrous oxide, pH and alkalinity, hydrogen sulfide, ammonia, and chloride. Once again, we sampled vents along most of the hydrothermally active portion of the 1998 lava flow. The northernmost active site this year was marker N3, south of Magnesia, which appeared totally inactive. Output at marker 33 appeared somewhat diminished from 1999, and the marker 108 area was dying out. Joystick was also much less active, and temperatures were less than 6 degrees C. A new site venting 10 degree C fluids between Joystick and Bag City, dubbed Iron City, was sampled and had H2S below the detection limit (< 1 micromol/liter). A preliminary look at the shipboard data indicates no striking changes in chemistry from 1999, but further analysis is needed for a meaningful comparison.
2.2b Gas Sampling - Leigh Evans
The goal of this year's gas sampling is to build time series observations with NeMO '98 and '99 at as many vents as possible. These include several diffuse venting sites from the eastern part of the volcano's caldera and high temperature vents at Ashes vent field in the west. In some cases, a lack of venting at the precise sites from previous years necessitated sampling vents in the same neighborhood.
Vent fluids were sampled by two methods, gastight bottles and the PMEL Hot Fluid Sampler. Their gas contents were extracted into glass ampules for chemical analysis on land. These include helium concentration and isotope ratio, hydrogen, methane, carbon dioxide and others. The liquids leftover will be analyzed for magnesium and possible silica to determine the degree of dilution by seawater.
2.2c Studies of Dissolved Gasses from Hydrothermal Fluids - Ben Larson
Our research involves the study of dissolved gasses from hydrothermal vent systems. The measurements essential to this study were accomplished on this cruise via an SRI 8610C gas chromatograph using samples obtained from Dave Butterfield's vent fluid sampler. Concentrations of methane, hydrogen, and nitrous oxide were measured in fluid samples ranging in temperature from 4C at Joystick vent to 302.2C at Inferno vent. Dissolved methane and hydrogen are an important diagnostic tool for the microbial activity occurring within the hydrothermal vent community as hydrogen is consumed and methane is produced in the microbial process known as methanogenesis. The methane and hydrogen concentrations from this cruise data will be analyzed in the context of the ongoing data set acquired from previous cruises to this region. In addition to methane and hydrogen, a new dissolved gas, nitrous oxide, was measured as well. Our current method of N2O measurement precludes measurement of this gas in fluids having high CO2 concentrations such as those fluids from high temperature vents, however, we were able to collect N2O data on several of the diffuse flow vents. N2O concentrations are interesting because N2O is an intermediate in the denitrification process that removes dissolved nutrients from the water, thus making them unavailable for biological uptake. The N2O data collected on this cruise will be further analyzed to determine what conclusions, if any, can be made regarding denitrification in diffuse flow environments.
2.2d NeMO 2000 Sample Report - Jim Gendron
During the NeMO 2000 cruise I collected 18 filters for XRF analysis, and 15 filters for SEM analysis back at PMEL. I also analyzed 17 vent fluid samples for pH, and 25 for alkalinity on board ship.