The Potential for Exceptional Preservation at Brine and Hydrocarbon Seeps Tested in situ in the Gulf of Mexico  

Exceptional preservation requires conditions that fall outside those of the typical oxygenated sea floor, such as anoxia, low energy, and/or rapid burial.  In addition to these scenarios, lagerstätten may be the result of unusual geochemical processes ranging from those in highly acidic bog deposits and brines, to early diagenetic processes such as replication of soft tissues (by clays or pyrite). Active seeps provide an opportunity to study decay processes in unusual geochemical environments.  This study is the result of in situ experiments on the longevity of crabs (Calinectes sapidus) and urchins (Echinometra sp. and Heterocentrotus sp.) at two hydrocarbon seeps and a brine seep in the Gulf of Mexico (USA). Crab and urchin carcasses were put in mesh bags and placed on the sediment surface in 1993 with soft parts intact (frozen until deployment), and were observed after 2, 8, and 13 yrs. 

The two active hydrocarbon seep localities (GB425 at 570 m and GC234 at 550 m) support chemosynthetic communities of clams and tubeworms (at GC234 only), with associated arthropods, echinoderms, and grazing gastropods, as well as exhibiting abundant microbial mat development.  Fine siliciclastic mud at these sites is strongly bioturbated, except immediately beneath patches of microbial mat.  Beneath each mat is a black anoxic zone that extends several centimeters into the sediment.  Could these local anoxic zones beneath mats serve to enhance preservation of soft tissue and/or articulation? Our results indicate that this is not the case.  Crabs and urchins collected at GB425 and GC234 were as degraded as, or more degraded than, samples collected at five non-seep localities.  Claws were usually all that remained of the Calinectes samples, and urchins had lost spines and either remained articulated tests or had disarticulated to single plates.  The anoxic zone below patches of bacterial mat is neither pervasive enough nor continuous enough to promote exceptional preservation.  At the anoxic brine (200 ppt) seep, Calinectes carcasses remained articulated, with muscle tissue present and carapace softened, yet intact, and exhibiting original colour, even after 8 years.  The microenvironments bordering the brine pool, possibly corresponding to the exaerobic zone of Savrda and Bottjer (1987), produced either the most efficient destruction (in the brine stream associated with the production of sulfuric acid) or the most exquisite preservation of the entire Gulf of Mexico experiment.  One urchin, from a site tens of meters away from the brine pool that supported thin bacterial mats on very course carbonate sand, was collected with spines attached, and tendons replicated by a mineral crust.  It is clear from our results that taphonomic signatures vary widely even within a single zone of active seepage, from accelerated loss of skeletal material to outstanding preservation.