Briney-mud volcanoes and fluidized mud avalanches – oh what a sight!
Written by Mandy Joye, 20 April 2014; posted by Mandy Joye, 21 April 2014.
As you can probably tell, the Brine Microbial Observatory Project is all about extreme environments and the microorganisms that thrive there. Today, the story is about extreme eruptions. Back in about 2005, we found an interesting site in Garden Banks Lease Block 697 (GB697). We observed massive discharges of fluidized mud and Ian MacDonald, my good colleague from Florida State University, coined the name “The Hot Site”; because, usually these types of eruptive mud discharges are a little hot (~40 ºC). We visited the area with Alvin back in 2010 and discovered that the erupting fluids were in fact cold (about 5 ºC), which was perplexing. Still, the terrain was astonishing and extremely interesting, so to us it’s still a hot site. The area is characterized by active salt tectonics and gas, fluidized mud, and oil discharges along the flanks of a central salt diaper (image-2). Gas fluxes abound all around the flanks of the structure. The diamonds are sites of water column gas plumes identified by NOAA’s Okeanos Explorer (OE) in early April 2014. Yellow circles are where we sampled during our one day at this site; we packed in a lot. We also ran a seismic line and saw numerous additional plumes along the lower (S) edge of the feature and up the SW edge below the area imaged by the OE.
The seafloor is characterized by numerous topographic highs and mud volcanoes (image-3). Where we dove, a small (2 x 2 m), highly active crater discharges copious amounts of gas and fluidized mud (images 1 & 4). When looking down into the crater (image-1), it’s like flying above clouds in an airplane. It was completely mesmerizing. When we parked at the crater, we could not see the bottom, only muddy brine fog (image-4). Fine mud particles are suspended in the dense brine and it floats, looking like clouds or smoke. But it is a fluid – and the fluid salinity is approximately 210‰ (6 times normal seawater) and the temperature is cold, about 4.97 ºC. This active fluidized mud/brine flow is considered geologically young because we do not find extensive locales of authigenic carbonate deposition. But we cannot pinpoint the age of this feature based on the data we have in hand.
A number of small spider crabs were running around looking for lunch (image-5); they appeared to be grazing on the microbial mats that carpeted the crater edge. After collecting a lot of high definition video, we started collecting sediment samples along the crater rim. We do this by carefully inserting polycarbonate tubes fitted with special “T” handles (so that the Alvin’s manipulator hand can hold on to them) into the sediment. We usually collect 6 replicate cores per site (image-6). Cores from such highly active areas can be saturated with gas so it’s best to collect more than you need in case one or two rockets into space during recovery.
During the hour we coring and collecting brine and water samples, something changed, perhaps the current or the tide shifted. But, the discharge switched from a fairly equal mixture of mud and gas with the gas bubbles being barely visible to an incredible release of gas. The bubbles were rising so fast and furiously that they could not be photographed as the circular bubbles they were, rather they looked like oblong extended strips (image-7). When we flew over the crater to try and capture pictures of the gas expulsion, all we could see was brine fog below us but some small gas bubbles were trapped on the camera lens (image-8). When we were finished working at the top of the crater, we flow downslope to see where the brine was flowing. What we saw there is one of the coolest things I’ve ever seen: a muddy brine avalanche (image-9; thank you pilot Bruce Strickrott for nailing what we should call this area). Here, the brine is tumbling downslope, like snow down a mountain- side during an avalanche; it’s an incredible sight to see.
From the main vent, we moved to the NE in search of microbial mats and other brine-influenced habitats. We found numerous patches of brine-influenced sediments that have a distinct coloration and texture, making it easy for a trained eye to pick them out (image-10). At first we saw only small spots (~5 cm patches) of mats (image-11) but then we started seeing large areas of mats (>1.5 m across) (image-12). Here, we sat down to collect sediment cores, as this was one of the nicest mats we’ve seen during the cruise. Once the cores were in place, we always photograph them so that there is a record of which core came from which hole (i.e. their relative placement with respect to each other; image-13). After retrieving the cores and placing them into their quivers for transport to the surface, the holes left behind quickly filled with cloudy brine and gas was slowly trickling out (image-14).
Finally, we moved to search for mussels and carbonate rocks. We spent a lot of time looking for live mussels and carbonates but to no avail. All we saw were lots of mud mounds and a lot of macro-biology. We saw a giant sea cucumber (image-15). These organisms are detrital feeders, scooping up sediment and sorting out the delicious portions while discharging the residual material as feces. And as our search for mussels and carbonate, ended, we came upon a thick microbial mat atop a brine flow and we sampled it. By that time, we were out of battery and it was time to surface.
This is a place we want to come back to and visit again – we’ve barely scratched the surface of the microbiological and geochemical dynamics of the area.