Thermal and Density Stratification in a Seafloor Brine Pool, Northern Gulf of Mexico

Ian R. MacDonald

Brine Pool NR-1 is located on the continental slope of the Gulf of Mexico at a site where a salt diapir has penetrated the near subbottom. The pool sits atop a low cone that rises about 10 m above the surrounding seafloor. It has a distinct sea water-brine interface at a water depth of 650 m. It is about 10 m in diameter and is surrounded by a very dense colony of mussels that possess methanotrophic symbionts. The fluid in the upper layer has a salinity ^approx 120 psu and contains high concentrations of dissolved methane. A continuous stream of methane bubbles vents from a localized region near the center of the pool.

I used a small winch, mounted on submarine Johnson Sea-Link I to lower a CTD (modified for high salinity) into the pool at various points and also used a pole to probe the depth of the pool in the shallower parts. The pool was funnel-shaped, a couple of cm deep in the edges and less than 2 m deep across most of the pool area, but over 33 m deep at the point where the bubbles were venting.

The CTD record shows that temperature increases abruptly at the brine-seawater interface and forms a second thermocline within the brine column. Conductivity also increases abruptly at the interface, consistent with the high salinity of the pool. Remarkably, conductivity decreases at the same brine depth as the thermocline, is erratic for several meters, and then becomes constant to the bottom of the pool. This unusual stratification is maintained by the mixing energy of the bubble stream, which is sufficient to maintain a large volume of suspended sediment. The suspended sediment interferes with the conductivity cell on the CTD to produce the anomalous reading. This stability of this system is determined by the gas venting rate and the shape of the pool and diatreme. Stratification o the fluid may serve to extend the life of the pool.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California