LONG-TERM DECAY RATES IN ORGANISM REMAINS FROM 15 – 600M DEPTH: PROGRESS REPORT FROM 8-YEAR EXPERIMENTS OF THE SHELF & SLOPE EXPERIMENTAL TAPHONOMY INITIATIVE

The Shelf and Slope Experimental Taphonomy Initiative began a long-term study of taphonomic processes in 1993. Mesh bags containing crabs, urchins, mollusks and wood were placed at depths from 15 – 300 m off the Bahamas carbonate platform and in a variety of depositional environments on the shelf and slope of the Gulf of Mexico (70 – 600m). Upon recovery, each specimen was characterized as to degree of fragmentation, disarticulation, surface condition and infestation by bionts.

After 6 to 8 years, shells within the photic zone are weakened, fragmented or lost. Much of this loss is biologically mediated. Shells deployed below100m are generally unaltered after 8 years, with the exception of discoloration and very minor dissolution in both carbonate and siliciclastic settings. However, taphonomic alteration is proceeding more quickly at petroleum seeps. These sites offer heterogeneous bottom conditions with strongly sulfurous pore waters. Dissolution, breakage, and edge alteration are more common here, but are restricted to regions in the immediate vicinity of the seep.

Urchins and crabs were deployed frozen in 2-mm mesh pouches as part of each experimental array. After eight years all urchins had lost spines and the lantern had disarticulated, but a majority of urchins were recovered with the corona intact, even at 15m depth. Chelipeds of the crab Callinectes were nearly always recovered and, below the photic zone, carapace fragments were also common. The endurance of echinoderm and crab remains despite lack of burial (and thus exposure to microbial activity, if not scavengers) is remarkable. Wood quickly became consumed by Teredo and limnorid isopods, particularly below the photic zone. Shallow deployments (above 100m) show greater resistance to decay. Intermittent burial and overgrowth by encrusters likely slows the activity of the shipworms within the photic zone.

This long-term experimental taphonomy project continues to provide invaluable information on the fate of organism remains. Hard parts exposed at the sediment/water interface within the photic zone will likely not survive more than 10-15 years, depending on species. Deeper shelf/slope settings exhibit a slower rate of taphonomic loss, with the exception of wood that is slightly more durable in shallow shelf settings.