Southern California 2013, Leg 1: Deep-Sea Chemistry

May 5-14, 2013

During this first leg of the Southern California Expedition, we will be conducting a series of remotely operated vehicle (ROV) dives on sites previously surveyed by MBARI’s mapping autonomous underwater vehicle (AUV) in March of this year. These sites within the Santa Monica Basin and south within the California borderland towards San Diego were selected because of the similarity of their gross topography to other sites where methane mounds and active venting of methane gas and methane rich fluids have been found. Our plan is to visit targets of interest where the high-resolution maps have suggested we will find more active venting sites.

We will be conducting laser Raman studies of pore-water chemistry in the sediments around the active methane gas venting sites and seafloor mounds. Our laser Raman spectrometer bounces a laser beam off of the target of interest—be it a solid, liquid, or gas—providing information about that object’s chemical composition and molecular structure. The laser Raman spectrometer has recently been modified to look at pore-water fluids withdrawn the upper 0.5 meters of sediment. This is a much better method for studying the chemistry of the pore-water fluids in the sediments than collecting a push core, which requires bringing the sediments to the surface, squeezing out the pore-water and then analyzing the chemistry. By processing the samples on-site in real-time we avoid alterations to the pore fluids due to changes in temperature and pressure or contamination of the reducing fluids with oxygen while processing the samples on deck or in the lab.

Also on this leg, Victoria Orphan and her team (postdoctoral fellow Kat Dawson and graduate student Alexis Pasulka) are collaborating with Peter Brewer and Ed Peltzer to characterize active sulfur and methane cycling microorganisms and their isotopic biosignatures within methane seeps along the Southern California Borderland. During the cruise they will collect samples for stable isotope labeling experiments and microbial community characterization in parallel with the laser Raman geochemical analyses and test new ROV-deployable methods for in situ high resolution capture of pore-water sulfides for d34S analysis using secondary ion mass spectrometry. These compatible isotopic and microbial datasets offer valuable context for the laser Raman collected pore-water profiles and, applied in combination, have the potential to advance understanding of the microorganisms and metabolic processes underlying carbon and sulfur cycling at sub-millimeter resolution within deep-sea methane seeps.