MOLLUSCS AND CRABS IN A PICKLE: A BRINE SEEP KONSERVAT-LAGERSTÄTTE

Sulfide-rich brine lakes in marine settings are suggested to be probable Konservat-Lagerstätte for fish and other organisms. A test of the Lagerstätte potential is in progress at the Flower Garden Banks National Marine Sanctuary, Gulf of Mexico, where the Jurassic Louann salt intersects the sea floor at a depth of 70m. Here the brine has formed a 500m² anoxic lake 30 – 60 cm deep. Salinity in the brine is ~200ppt and the brine is rich in dissolved sulfide and hydrocarbon gases. In 1993, empty molluscan shells, sea urchins, crabs and wood were placed within mesh bags and deployed into the brine pool by submersible. In 1995 and 2001 (two years and eight years after deployment) experiments were recovered and the taphonomic condition of the remains were documented.

Results show a distinct gradient in preservation that follows salinity, oxygen, and sulfide gradients. In the brine pool itself, the blue crab, Callinectes sapidus, was recovered at 2 and 8 years with the soft tissue in excellent condition. The cuticle, however, was softened and decalcified. Down gradient from the brine pool where normal seawater mixes with the brine (lowering salinity to about 60ppt and increasing oxygen levels), crabs have cuticle present, but have lost all soft tissue. Mollusc shells are exceptionally well-preserved in the brine pool, but where brine mixed with normal seawater, shells were severely dissolved. The mixing increases oxygenation and promotes the production of sulfuric acid from the sulfidic brine. These mixing zones also promote thick growth of white filamentous bacterial mats (e.g., Beggiatoa) and the production of elemental sulfur. Bacterial mats have been suggested to act to enhance preservation. However, in the case of brines, the mats mark zones of highly active taphonomic degradation. Therefore, sea floor brine seeps are potentially responsible for exceptional preservation in the fossil record, but are likely ringed by zones of rapid taphonomic loss where sulfidic brines mix with normal sea water.