Orca Basin

For the past four days, we have been sitting atop a site named Orca Basin.  This is a large crater-like feature on the seafloor that has accumulated dense, salty brine fluid inside.  This brine layer has been observed up to 250 m thick, and sampling it has been rather exciting.  We lower a CTD rosette with 23 Niskin bottles (10 L each) on a wire from the ship into this layer.  The Niskin bottles are positioned vertically on the rosette (see Rich posing with the rosette below), so as the rosette descends, the water flows up through the bottles.  We can then remotely trigger each of the Niskin bottles to close and bring the trapped water inside it back to the surface where we can analyze it. We can also monitor water quality parameters (temperature, salinity, dissolved oxygen, organic matter content) as the rosette descends.  Once the rosette enters the brine layer, the salinity jumps from normal ocean water levels (~35 ppt) to about 250 ppt.    This added salt content in the water gives it a texture similar to mineral oil.  Back in our lab, everything that this water touches reveals itself with little salt crystals after it dries. 

At the water depths here in the Orca Basin (2300 m), it takes roughly 3 hours for the CTD rosette to go to the bottom and back to the surface.  For the past 3 nights, Rich and I (together with Melitza Crespo-Medina from UGA) have made 2 CTD casts per night.  Since leaving port on Nov. 8th, Rich and I have measured a total of 150 samples for radium isotopes, and 175 samples for radon.  See Mandy Joye’s blog (http://gulfblog.uga.edu/) for a concise description of how we will use these data to determine discharge rates into these sites.

Another group on the ship (Andreas Teske’s group from UNC – see the Teske blog at http://teskelab2010.wordpress.com/) is working with sediment cores from these sites.  They deploy a multi-corer from the ship that lands on the bottom and fires 8 core barrels into the sediment.  Once these cores come back to the ship, it always makes for an exciting time to see what the cores look like.  Last night’s multi-core, for example, came back with bright red sediments inside (see below).  We’ve also seen cores come back filled with hydrocarbons – oil naturally seeps out of the seabed throughout the Gulf, and we have identified several of these sites.  A new discovery from our water sampling for radium and radon is that these tracers are also identifying these oil-rich layers throughout the water column. 

We leave the Orca Basin site tonight after Alvin comes back out of the water and head for a site closer to land.  From there, we will meet up with a supply ship that will deliver additional scientific and media personnel.  Media groups from NPR, National Geographic, and ABC will be joining our cruise to report on our findings as they relate to the brine seeps as well as the BP oil spill. 

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