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 The laser Raman spectrometer has recently been modified to look at pore-water fluids withdrawn the upper 0.5 meters of sediment.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.

Logbook

Our last foray to the seafloor

We did a morning dive at the San Diego Seep site, which is 42 kilometers (26 miles) due west of La Jolla. The potent fluids and gases seeping there create bacterial mats and chemosynthetic communities, and it made for a convenient last dive for this expedition.

Good things come to those that wait

Today we went back to the alternate Santa Monica Canyon Mound site. This site had a large clam field with bacterial mats as well.

Sampling pore water

The sites south towards San Diego left much to be desired in the chemistry department, therefore, Peter Brewer decided that we should head back to Santa Monica Basin.

You can't always get what you want

Today we conducted two half-day dives. Due west of Oceanside, California, the two sites are areas that were mapped by the autonomous underwater vehicle (AUV), but never explored with a remotely operated vehicle (ROV).

Decision making

Every evening, Chief Scientist Peter Brewer is faced with a decision—what will our mission be for tomorrow? Of course, a cruise plan was made many weeks ago, but when it comes to research expeditions, plans often change.

There and back again

Much of this expedition has entailed exploring new areas of the seafloor. The high-resolution maps made by the mapping autonomous underwater vehicle (AUV) give the scientists a good idea of sites that may be of interest to chemists, geologist, and biologists—but we never really know what we’re going to find until we get there.

Santa Monica Slope

Today we explored another new site based on the maps generated from the autonomous underwater vehicle (AUV) expedition on the Rachel Carson in March.

Santa Monica Mounds

This site truly is a dream come true for deep-sea chemists and microbiologists. The large mound is covered in acres of ghostly undisturbed bacterial mat. The huge swaths of white bacterial mat and occasional patches of bright yellow/orange make the seafloor look otherworldly.

Santa Cruz Basin

The weather improved immensely and we were greeted by sight of the Channel Islands this morning.

Transit to Southern California

Today, we left the dock at 0700 to make our transit to southern California. Although the weather was beautiful in Monterey Bay, as we turned south, we steamed head-on into high winds (well over 30 knots) that are expected to persist all day.