The Orca Basin

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.

Very little is known about the microbiology of the Orca brine or overlying seawater.  The strong geochemical gradients in the basin are certain to drive significant and dramatic changes in microbial community composition and microbial activity.  Over about 150m of depth, the salinity increases from that of normal seawater (about 35 PSU) to over 250 PSU.  Over the same depth interval, temperature increases slightly (from 4 to 5 ºC) and dissolved oxygen concentrations decrease to zero.  The halocline (salnity increase) and oxycline (oxygen concentration decrease) are separated by about 75m.  We were very surprised to observe that the concentration of colored dissolved organic matter increased by over 10 fold over this range, meaning that there is potentially a lot of microbial fuel (dissolved organic matter) in the brine. The halocline was visibly thick with microbial life.  We collected samples across the halocline and oxycline using the ALVIN-mounted brine sampler andthe CTD-rosette and are currently analyzing gas concentrations, chemical composition, and conducting rate assays to evaluate how microbial processes very over the various interfaces.  We are sampling at three sites in the Orca Basin, the Northern sub-basin, the Southern sub-basin, and the middle, most shallow, sub-basin.  We are also collecting sediments from these sites to characterize sediment biogeochemistry and microbiology.  More on that tomorrow…