September 5th, 2010: Sometimes, I get a feeling that the day is going to offer some surprises. This morning, I had a feeling. We’ve spent a lot of time in the Southwest quadrant over the past two weeks searching for oil and gas. We’ve seen mostly weak signals. The sediments at the sites we visited during that time were oxidized and did not contain a lot of gas or oil.
Until we sampled at a site about 20 miles offshore from Mississippi, we did not see oil along the seafloor. At that station, we saw a thin layer (couple of mm) of what looked like sedimented oil. We won’t know the oil content (or source) until we do detailed analyses after the cruise but oil has a distinct feel and this sediment felt oily. We got a glimpse of what we had expected to see. Today, at a site about 16 nautical miles from the wellhead, we dropped the multicorer into a valley. When the instrument returned from the bottom, it contained something we had not seen before: a layer of flocculent, sedimented material that was cm’s thick. The top, apparently recent layer, contained some fraction of oil.
At a natural oil seep, the entire sediment column is saturated with oil. Cores of sediment collected from natural seeps are oil-stained top to bottom and often the water overlying the sediment core has a thick (mm to cm) layer of crude oil floating at the top. Natural oil seep sediments are distinctive. The photos of cores shown from GC185 here are extreme examples (they are VERY oily!) but the point is that the entire sediment column is oil stained at a natural seep. At the site we visited today, the oil obviously came from the top (down from the water column) not the bottom (up from a deep reservoir).
What we found today is not a natural seep. We collected control sediments in a region to the south east of the wellhead that was never overlain by the blowout oil slick. Those sediments consisted of fine grained sediment mixed with calcareous ooze. There was no hint of oil in the control sediments. The near shore sediments contained grayish muddy clay and a thin layer of orange-brown oil at the surface. The sediments we collected today were similar at the bottom -- gray muddy clay -- but the upper few cm consisted of oil floc -- we call it “oil aggregate snow”, because it settled down to the water column to the seafloor just like snow falls from the sky to the ground.
If you take a close look at the snow layer, small oil aggregates are visible. Also visible are (much larger) pteropod shells (which must have been recently deposited because the shells dissolve rapidly) and remnants of zooplankton (skeletons) and benthic
infauna (dead worms and their tubes). Microbial aggregates are visible and abundant but the normal invertebrate fauna you’d expect to see in these sediments were not present.
We will determine how much oil is in this layer and evaluate the rates of microbial metabolism in the sediments when we return to UGA. We want to know whether and how much of this material lies along the seafloor at other sites. So, tomorrow, we will go to a site about 12 nautical miles northwest of the wellhead and run a full station there. We’ll see what the sediments look like there and with that knowledge, we’ll decide where to go next.