Geologic evolution in action: the GC246 mud volcano
Written 4/10/14 by Mandy Joye; posted 4/15/14 by Mandy Joye
The Gulf of Mexico is home to many diverse and fascinating super-salty (=hypersaline or brine) habitats. One of the most unique brines we’ve ever sampled in the Gulf is the mud volcano in Green Canyon block 246 (GC246) (image 1). GC246 is located approximately 200 km SW of the Mississippi Delta on the flank of the upper-middle continental slope in the Gulf of Mexico. The GC246 block consists of featureless mud plains rising to series of a pockmarked mud mounds at depths around 900 m below the sea surface. It is a truly spectacular site (image 2).
We first sampled this site with the Alvin in November 2010. The main mud volcano and surrounding area, fondly called “Dead Crab Lake” (thank you Ian Macdonald), is a complex terrain of slopes, depressions, and plateaus, punctuated by active brine seeps and flows, brine pools, mud-flows, and massive mud flows. Mottled grey sediments cover much of GC246. These are interrupted occasionally by carbonate outcrops, a variety of colorful mineral crusts (images 3-and fluid flows (image 3) coral communities, black to milky colored brine flows, ochre mud-flows, white to orange Beggiatoa meadows, fields of mussel (Bathymodiolus) and crab carcasses, and communities of live mussels (Bathymodiolus) and snails. In 2010, with the exception of spartan communities of mussels, crabs, bacterial mats, and corals, GC246 is largely devoid of benthic macrofaunal life. Of particular interest at GC246 is Dead Crab Lake, a brine lake approximately 15 to 25 cm deep. DCL is an active brine seep, ringed by orange mineral crusts, dead mussels, and dead crabs. DSV Alvin dive 4651 revealed a small thicket of barite chimneys from which gas was discharging; the chimneys were overlain by reduced sulfur minerals (e.g. arsenic trisulfide) and were surrounded by sulfide-oxidizing mussels. While “sulfide” chimneys are usually associated with hydrothermal activity, they are not usually seen at cold seeps. Cold seep chimneys are usually barite (barium sulfate). The GC246 chimneys suggest exotic subsurface chemistry and surface mineralogy or chemically distinct source fluids.
Both white and orange Beggiatoa meadows were common at GC246 in 2010. Small communities of soft corals (Callagorgia) and brittle stars (Ophiuroidea) inhabit the occasional carbonate hardground outcrops. Bathmodiolus mussels, snails, crabs, sea cucumbers (Benthodytes), and isopods (Bathynomus giganteous) have all been identified in varying degrees of abundance at GC 246, usually in close proximity to active sites of brine seepage.
Oh how things change in the deep sea….
When we revisited this site on April 10, 2014, it was almost unrecognizable. The lake was filled with fluidized mud instead of brine (image 1). And the main brine vent was releasing fluidized mud instead of (clear) brine. The brine lake looked like a bubbling mud pot; they look a lot like the ones that are abundant in surficial hydrothermal environments like Yosemite National Park (http://www.nps.gov/yell/naturescience/mudpots.htm) (images 1 & 7).
We observed a lot of snails and clams on the sediment surface, moving and generating easily visible tracks as they search for snacks (image 8). At areas of apparently high brine flow, a shockingly complex array of minerals – orange, black and white – coat the sediment surface. We collected samples of these materials and will analyze them to determine their chemical composition.
When we originally visited this site, the murkey brine lake was occupied by several dead crabs. Likely the crabs died of osmotic shock after entering the extremely salty solution (7X seawater salinity!). We found only one dead crab on this dive (image 10) but young, vigorous mussels patches were abundant (image 1). Finally, we observed many young crabs running around looking for a meal (image 12).
The change in Dead Crab Lake over the last 3.5 years is profound and suggests rapid changes in its deep subsurface geological plumbing. Given the small, thus young mussels, observed here, the mud flow that filled the lake and turned it from a lake to a bubbling mud pot happened shortly after we visited this site. The observations we have made at this site and the microbiological and geochemical data we will collect will help document how this coupled geobiological ecosystem has – and is – changing over time. These data offer the opportunity for impressive discoveries as we aim to understand what geological factors drove the rapid evolution of this site. This site and the questions we can address here are a great example of the need for long-term deep sea observatory science.