May 6th, 2003; Leg 5, Day #16

Today we revisited the bubbling methane vent site we located during our previous leg, known informally as Pinkie’s Vent. The plan for the day included two dives: the morning dive for collecting push cores, gas samples, and heat flow measurements; and the afternoon dive for further work with the laser Raman instrument. 

Suites of cores were collected for pore water analysis and foraminiferal and microbiological sampling. The seafloor where these cores were collected is covered with dense patches of trochid gastropods (see left) and small, presumably juvenile, chemosynthetic clams. The trochid gastropods, which are cousins of land snails, are heterotrophs that graze on organic matter found on the surface of marine sediment. They are often found around chemosynthetic vent communities and are one step down the food chain from the clams. They make their living by eating the castoffs from the clams, which are the primary producers at vent communities. 

We recovered the vehicle at noon, and the pilots quickly reconfigured the hardware for the afternoon dive. During this dive, the laser Raman probe head was mounted vertically in an aluminum frame that we planned to place on the seafloor. This rather large frame was secured to the front of the vehicle—on the “front porch,” as we like to call it (see right). 

The afternoon progressed quickly as we made several attempts to capture either solid hydrate chunks from the seafloor or gas from the vent area. Gas was trapped beneath a gas collection funnel and placed in a slot in the aluminum frame. The laser beam was projected through the sample. When collecting laser Raman spectra, the vehicle lights are turned off to reduce spectral interference. Because only a very small amount (~1%) of the laser light is shifted by the Raman effect, we have to record the light spectrum for 15-20 minutes per experiment (see below). Sheri White was rather pleased to get strong methane signals from the experiments conducted this afternoon. 

– Bill Ussler and Peter Walz

Trapping gas with the gas collection funnel (above left) to be analyzed using the lasar Raman spectrometer (above right).

The answer to last night’s mystery item: Lynne Christianson, in stylish purple gloves, is holding a sediment squeezer body. This plastic container is filled with sediment extruded from a sediment core, then covered with a thin rubber dental dam, and placed into a clamping jig that allows air pressure to push the water that fills the pore spaces around sediment particles into a collection syringe. This is the way in which we obtain water and dissolved gas samples for chemical analysis in Bill Ussler’s on-board chemistry van.