EXCEPTIONAL PRESERVATION IN A GULF OF MEXICO BRINE SEEP: LAGERSTÄTTE IN THE MAKING?

There is increasing interest in the role that atypical marine geochemical environments play in producing exceptionally preserved assemblages. Recently, brines have been suggested as a possible setting for Burgess Shale type preservation at the type locality (Powell, 2005). A modern brine seep and shallow lake occur at the East Flower Garden Banks at 70m in the Gulf of Mexico that can be used to test this hypothesis. In 1993 the Shelf and Slope Experimental Taphonomy Initiative (SSETI) placed crab remains, urchins, bivalves, gastropods, and wood samples in and around this brine lake to observe decay rates. The samples were placed in mesh bags attached to a PVC rod to be easily deployed and retrieved by submersible. Sample arrays were collected after two years, eight years, and in August 2006, SSETI will collect a thirteen-year sample. The results show differential preservation of organisms, depending on organism type and location with respect to the brine. In the brine pool, mollusk shell is pristine, including color retention. Away from the brine, shell shows minor dissolution and color loss, but in the transition zone where brine mixes with normal sea water, shells are severely dissolved. Urchin tests show a similar pattern, with the addition of some soft tissue preservation within the brine. Crabs have excellent soft tissue preservation and articulation within the brine, with a softening of the cuticle and loss of carapace. Outside the influence of the brine, disarticulated crab claws are all that are preserved. In the transition zone, only crab claw was recovered, but claws are often articulated. Wood shows excellent preservation in the brine pool and in the transition zone, but is completely lost to wood-eating clams and arthropods away from the brine. These results suggest a distinct pattern of preservation that might be associated with brine preservation. One would expect to find exceptional preservation in the brine itself, surrounded by a narrow zone of complete taphonomic loss where brine mixes with normal seawater, and then a return to more “normal” seafloor preservation in surrounding sediments.

Powell, W. 2005. Mineralogical and lithological features associated with brine seeps in the Burgess Shale, with implications for the interpretation of the Kinzers Shale, PA, GSA Abst. with Prog., Salt Lake City, p 547.