Every day at sea usually provides at least one surprise.The Orca Basin has provided us with quite a few big surprises, the most striking of which was the discovery of red and pink (!) sediments in the middle of the basin. In the Northern and Southern mini-basins, we collected cores of black, extremely sulfidic mud. When we sent the multiple-corer over the side in the central Orca Basin, we expected to retrieve something similar but much to our surprise, the mud recovered from the more shallow (2000 vs. 2200m) central basin was red (see image gallery below for more photos). Red and pink deep sea sediment…where in the world does that come from?

The Orca Basin occupies a large (~150 km2) depression (roughly SE  to NW) along the continental slope in the Northern Gulf ofMexico.  At the basin bottom, a hypersaline brine (260‰, about 7 times the salinity of seawater) fills depressions below roughly 2200m.  There are two deep depressions, one to the north and another to the south end of the basin (the dark purple colors on the map above) are the focus of our work here.  We will also sample in the middle of the basin, where it is less deep but where previous studies documented active brine flows. The brine layer is about 250m thick and it derives from dissolution of Jurassic age salt that underlies and surrounds the basin.  This basin is very different from the other brine lakes/pools and mud volcanoes we are studying because brine seeps from the canyon walls into the basin rather than venting up through the seafloor.  The  Orca basin could be more similar to brine basins in the Mediterranean than to mud volcanos and upward-advection-derived brine lakes and pools in the Gulf of Mexico.   We aim to make this comparison.

When fluids seep out of the seafloor, they provide chemical substrates to fuel both microbial and macro-biological communities.  Even hyper-salty brine seeps are “oases” of life along the seafloor.  While the salt content of some seafloor brines can create challenges for macro-organisms, microbial life thrives in brines.

One of the most significant challenges faced when working at areas with active fluid venting is to quantify the fluid flux rate from the deep reservoirs to the overlying water column.  A primary objective of this project is to link microbial community composition and activity to differences in fluid chemistry and fluid flux, so we have to quantify the fluid flux and evaluate whether, and if so how, the flux changes over time. To the eye, fluid fluxes at these sites appear high:  this image shows the strong (yellow) sonar reflection of gas bubbles being released along a fault line at the GB425 mud volcano. The wall of bubbles was about 50 meters long and about 3 meters wide (see image gallery below to view the bubble wall).

Every cruise on board the R/V Atlantis with the DSV ALVIN has one thing in common – amazing discoveries and fun initiation rituals. Each day begins around 6:30, with final checks of the sub and ultimately loading the science observers of the day into the ALVIN (Melitza Crespo is shown below).  We have several scientists who’ve never been in ALVIN before and for them there is a special treat upon returning to the surface, you can look forward to a bath in lots of 4ºC water.  Brrrr.  There’s even a special throne where one sits while receiving this special treat (shown here is Dr. Kirsten Habicht’s initiation from two days ago).