biology chemistry geology instruments
Lost City Expedition: Science

Sampling Gases From the Seafloor: Rock-Microbe Linkages

During this mission, we are extremely interested in learning about the gas compositions of the vent fluids. We believe that because of the novel environment that Lost City is formed in, that the gases dissolved in the warm vent fluids will be extremely different than those in black smoker systems. Results from studying mantle-hosted hydrothermal fields on land, and from some systems in the ocean, combined with results of experiments in the laboratory, lead us to suspect that the gases in the Lost City fluids will be rich in methane (CH4) and hydrogen (H2). These gases may be very important for energy and cell building in the organisms, and they may provide hints as to what some of the gases were like in hydrothermal systems operating very early on in the Earths history.

We will be using specialized titanium “gas-tight” samplers to collect hydrothermal vent fluids for gas extraction and analyses. These samplers are evacuated prior to use to minimize the contamination by air in any sample and they remain sealed after sampling to prevent loss of gas if the fluids degas on ascent for some reason (cooling and/or shrinking occur with high-temperature hydrothermal vent samples). In the past, we have found that traditional vent sampling syringes (“major samplers”) can lose gas as they depressurize on ascent. In the lab the samplers are connected to a vacuum extraction line and the fluids are dumped from the sampler into a flask where all gases are “boiled” (low temperature vacuum extraction) out of the fluid. This “whole” gas sample is dried (of water vapor) and moved to calibrated volumes for a pressure measurement. That allows calculation of a “total gas” number for the sample which is used with later component analyses to determine dissolved concentrations for the various gas species. The gas mix is then sub-sampled into glass ampules (small glass tubes that are sealed by melting the glass using a very hot flame after the gas is transported into them) for later analyses onshore. The water is saved for elemental analyses by the fluid chemists.

Back in the shore-based laboratories a number of different analyses take place on the gas sub-samples. A measurement is made of the different components in the gas mix by gas chromatography. We are able to analyze for Ne,H2,N2,O2,Ar,CO,CH4 and CO2 routinely and generally estimate H2S as the unanalyzed remainder. Samples are also sent to Dr. John Lupton of NOAA/PMEL in Newport, Oregon for analysis of helium isotopes by mass spectrometry- these are important indicators of magmatic processes happening below the sea-floor.

Onshore will also be using isotope mass spectrometry to look at Carbon 13 and Carbon 14 values in the dissolved gases from these vents- this can provide some age and process information to the study. An additional interest of this program will be to measure for the presence of higher order organic volatiles such as ethane, propane and butane, hypothesized because these are likely products of fluid-rock interactions associated with serpentinization.